Local Axon Collaterals Research Articles

Inhibitory interactions between spiny projection neurons in the rat striatum.

The spiny projection neurons are by far the most numerous type of striatal neuron. In addition to being the principal projection neurons of the striatum, the spiny projection neurons also have an extensive network of local axon collaterals by which they make synaptic connections with other striatal projection neurons. However, up to now there has been no direct physiological evidence for functional inhibitory interactions between spiny projection neurons. Here we present new evidence that striatal projection neurons are interconnected by functional inhibitory synapses. To examine the physiological properties of unitary inhibitory postsynaptic potentials (IPSPs), dual intracellular recordings were made from pairs of spiny projection neurons in brain slices of adult rat striatum. Synaptic interactions were found in 9 of 45 pairs of neurons using averages of 200 traces that were triggered by a single presynaptic action potential. In all cases, synaptic interactions were unidirectional, and no bidirectional interactions were detected. Unitary IPSPs evoked by a single presynaptic action potential had a peak amplitude ranging from 157 to 319 microV in different connections (mean: 277 +/- 46 microV, n = 9). The percentage of failures of single action potentials to evoke a unitary IPSP was estimated and ranged from 9 to 63% (mean: 38 +/- 14%, n = 9). Unitary IPSPs were reversibly blocked by bicuculline (n = 4) and had a reversal potential of -62.4 +/- 0.7 mV (n = 5), consistent with GABA-mediated inhibition. The findings of the present study correlate very well with anatomical evidence for local synaptic connectivity between spiny projection neurons and suggest that lateral inhibition plays a significant role in the information processing operations of the striatum.

Dopamine D2 receptor mediated presynaptic inhibition of striatopallidal GABA A IPSCs in vitro

The modulation of GABA release within the globus pallidus (GP) by dopamine was studied using whole-cell patch clamp recordings from visually identified neurones. In sagittal slices, single shock electrical stimulation in the striatum evoked GABA A inhibitory postsynaptic currents (IPSCs), which were inhibited by dopamine in a dose-dependent manner (0.3–30 μM) with an IC 50 value of 0.7 μM. The inhibition was accompanied by an increase in paired pulse facilitation, indicative of a presynaptic effect. In coronal slices, stimulation within the GP adjacent to the recording site evoked GABA A IPSCs which were relatively unaffected by dopamine indicating the lack of modulation of GABA release from terminals of local GP axon collaterals. No consistent changes in holding current, membrane potential, firing rate or the frequency of spontaneous IPSCs was observed. Tetrodotoxin-resistant miniature (m)IPSCs were recorded in chloride-loaded cells. Dopamine (3–30 μM) reduced the frequency of mIPSCs, but was without effect on mIPSC amplitude, confirming a presynaptic effect. The addition of the ‘D2 like’ agonist quinpirole (3 μM), but not the ‘D1 like’ agonist SKF 38393 (10 μM), mimicked these effects. The ‘D2 like’ antagonist sulpiride (10 μM), while having no effect alone, blocked the action of dopamine. In contrast the dopamine D4 selective antagonist L745, 870 (1 μM) or D1 antagonist SCH 23390 (10 μM) were without effect. These results indicate that dopamine acts on presynaptic D2 receptors on striatopallidal terminals to reduce the release of GABA in the GP. Attenuation of this mechanism following the depletion of dopamine may contribute to the changes in GP neuronal activity observed in animal models of Parkinson's disease.

Morphology of local axon collaterals of electrophysiologically characterised neurons in the rat medial septal/ diagonal band complex.

Neurons in the medial septal/diagonal band complex (MS/DB) in vivo exhibit rhythmic burst-firing activity that is phase-locked with the hippocampal theta rhythm. The aim was to assess the morphology of local axon collaterals of electrophysiologically identified MS/DB neurons using intracellular recording and biocytin injection in vitro. Cells were classified according to previous criteria into slow-firing, fast-spiking, regular-spiking, and burst-firing neurons; previous work has suggested that the slow-firing neurons are cholinergic and that the other types are GABAergic. A novel finding was the existence of two types of burst-firing neuron. Type I burst-firing neurons had significantly longer duration after hyperpolarisation potentials when held at -60 mV, and at -75 mV, type I neurons exhibited a low-threshold spike with more rapid activation and inactivation kinetics than those of type II neurons. We have, also for the first time, described the main features of the local axon collaterals of the five neuron types. All filled neurons possessed a main axon that gave forth 1-12 local primary axon collaterals. All electrophysiological types, except for the type I burst-firing neuron, had a main axon that coursed toward the fornix. Myelination of the main axon was a prominent feature of all but the slow-firing neurons. Branching of the primary axon collaterals of the fast-spiking and type I burst-firing neurons was more extensive than that of the other cell types, with those of the slow-firing neurons exhibiting the least branching. All cell types possessed axon collaterals of the en passant type, and some in addition had twiglike or basketlike axon terminals. All cell types made synapses on distal dendrites; a proportion of the fast-spiking and burst-firing cells in addition had basketlike terminals that made synaptic contacts on proximal dendrites and on somata. Two morphological types of somata were postsynaptic to the basket cells: large (20-30-microm) oval cells with dark cytoplasm, and large oval cells with paler cytoplasm, often with an apical dendrite. The presence of lamellar bodies in the large dark neurons suggests that they may be cholinergic neurons, because previous work has localised these structures in some neurons that stain for choline acetyltransferase. Our work suggests therefore that there may be GABAergic neurons in the MS/DB that form basket synaptic contacts on at least two types of target cell, possibly cholinergic and GABAergic neurons, which means that the basket cells could play a key role in the generation of rhythmic activity in the MS/DB.

Intrastriatal GABA A receptor blockade does not alter dopamine D 1/D 2 receptor interactions in the intact rat striatum

The purpose of this study was to investigate the effects of intrastriatal blockade of GABA A receptors on dopamine D 1/D 2 receptor interactions in the intact rat striatum. Muscarinic receptors mediate the ability of the D 2 receptor antagonist, eticlopride, to block an increase in striatonigral neuropeptide messenger RNA stimulated by the full D 1 agonist, SKF-82958. However, because D 2 receptor antagonists activate striatopallidal neurons, it is possible that increased GABA release from local medium spiny axon collaterals also contributes to the ability of eticlopride to block the effects of SKF-82958. This hypothesis was addressed by infusing the GABA A receptor antagonist, bicuculline, into the dorsal striatum in rats treated with eticlopride and SKF-82958. In contrast to the actions of the muscarinic antagonist, scopolamine, bicuculline did not affect the increase in behaviors induced by SKF-82958 or the ability of eticlopride to block them. Quantitative in situ hybridization demonstrated that bicuculline did not significantly affect basal preprodynorphin messenger RNA, nor did it affect the ability of eticlopride to decrease SKF-82958-induced preprodynorphin messenger RNA. However, the level of the preprodynorphin hybridization signal in bicuculline plus SKF-82958-treated rats was significantly lower than in saline plus SKF-82958-treated rats. In contrast, bicuculline, eticlopride or SKF-82958 by themselves increased basal preproenkephalin messenger RNA. However, there was no significant interaction among bicuculline, eticlopride and SKF-82958 on preproenkephalin messenger RNA levels. These data indicate that blockade of striatal GABA A receptors has only a subtle effect on acute dopamine agonist-induced changes in gene expression. These results are discussed in the context of local intrastriatal interactions.

Synaptic targets of the intrinsic axon collaterals of supragranular pyramidal neurons in monkey prefrontal cortex.

The principal axons of supragranular pyramidal neurons in the cerebral cortex travel through the white matter and terminate in other cortical areas, whereas their intrinsic axon collaterals course through the gray matter and form both local and long-distance connections within a cortical region. In the monkey prefrontal cortex (PFC), horizontally oriented, intrinsic axon collaterals from supragranular pyramidal neurons form a series of stripe-like clusters of axon terminals (Levitt et al. [1993] J Comp Neurol 338:360-376; Pucak et al. [1996] J Comp Neurol 376:614-630). The present study examined the synaptic targets of the intrinsic axon collaterals arising from supragranular pyramidal neurons within the same stripe (local projections). Approximately 50% of the within-stripe axon terminals in monkey PFC area 9 targeted dendritic spines. In contrast, for both the intrinsic axon collaterals that travel between stripes (long-range projections), and the axon terminals that project to other PFC areas (associational projections), over 92% of the postsynaptic structures were dendritic spines (Melchitzky et al. [1998] J Comp Neurol 390:211-224). The other 50% of the within-stripe terminals synapsed with dendritic shafts. Dual-labeling studies confirmed that these within-stripe terminals contacted gamma-aminobutyric acid-immunoreactive dendritic shafts, including the subpopulation that contains the calcium-binding protein parvalbumin. The functional significance of the differences in synaptic targets between local and long-range intrinsic axon collaterals was supported by whole-cell, patch clamp recordings in an in vitro slice preparation of monkey PFC. Specifically, the small amplitude responses observed in layer 3 pyramidal neurons during long-range, low-intensity stimulation were exclusively excitatory, whereas local stimulation also evoked di/polysynaptic inhibitory responses. These anatomic and electrophysiological findings suggest that intrinsic connections of the PFC differ from other cortical regions and that within the PFC, feedback (within-stripe) inhibition plays a greater role in regulating the activity of supragranular pyramidal neurons than does feedforward inhibition either between stripes or across regions.

Images in neuroscience. Neural networks: neural systems IV: prefrontal cortex.

Back to table of contents Previous article Next article Images in NeuroscienceFull AccessNeural NetworksNeural Systems IV: Prefrontal CortexDavid A. Lewis, M.D., David A. LewisSearch for more papers by this author, M.D., Pittsburgh, Pa.Published Online:1 Nov 2000https://doi.org/10.1176/appi.ajp.157.11.1752AboutSectionsPDF/EPUB ToolsAdd to favoritesDownload CitationsTrack Citations ShareShare onFacebookTwitterLinked InEmail Different cerebral regions in the human brain have specific patterns of local neuronal circuitry and structure that are designed to carry out the distinct functions of the region. The prefrontal cortex is involved in mediating aspects of cognition, particularly tasks that involve working memory or the temporal integration of information. This kind of information processing depends not only on long-range connections between the prefrontal cortex and other cortical areas, but also on local axon collaterals linking neighboring noncontiguous cell groups in the prefrontal cortex with each other. The figure depicts a basic functional unit of prefrontal cortex circuitry that could serve as the anatomic substrate for the types of dynamic neuronal interactions that occur in relation to the behavioral events subserved by the prefrontal cortex. The intrinsic axon collaterals of pyramidal neurons in layers 2 and 3 of the prefrontal cortex, which travel horizontally through the cortical gray matter, exhibit divergence, convergence, and reciprocity of connections across discrete prefrontal cortex cell groups. The divergence of axon collaterals permits a spatially restricted input to recruit an array of neurons whose coordinated activity is required to generate a particular response. The convergence of axon collaterals enables multiple discontinuous points across the cortical surface to provide input to a single point. This arrangement allows divergent neuronal inputs to be relayed to the same location, facilitating the integration of their information content for a specific task. The reciprocity of axonal connections provides for bidirectional influences among different groups of neurons. The presence of reciprocal excitatory connections among spatially segregated locations in prefrontal cortical neuronal sets may contribute to producing sustained neuronal activity of the kind that is seen during the performance of certain psychological tasks, like delayed response or working memory tasks. It is of interest that the interconnected groups of neurons in the prefrontal cortex appear to be larger than in other cortical association regions, perhaps reflecting the complexity of the psychological tasks that the prefrontal cortex subserves.Address reprint requests to Dr. Tamminga, Maryland Psychiatric Research Center, University of Maryland, P.O. Box 21247, Baltimore, MD 21228. Image adapted with permission from Melchitzky DS, Sesack SR, Pucak ML, Lewis DH: Synaptic targets of pyramidal neurons providing intrinsic horizontal connections in monkey prefrontal cortex. J Comp Neurol 1998; 390:211–224, Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc. Figure The schematic diagram on the left illustrates the synaptic connections of supragranular pyramidal neurons in the prefrontal cortex of the monkey. Pyramidal neurons in layers 2–3 of the monkey prefrontal cortex are clustered into stripe-like arrays (average dimensions: 275 μm by 1.8 mm) that are reciprocally connected by intrinsic axon collaterals. Intrinsic axon terminals from pyramidal neurons in one stripe form excitatory synapses on the dendritic spines (a) of pyramidal neurons (p) located in other intrinsic clusters. These pyramidal neurons in turn send intrinsic axon collaterals (b) to the first stripe. Intrinsic axon terminals from supragranular pyramidal neurons also form asymmetric synapses on dendritic shafts (c). Many of these dendritic shafts have the morphological characteristics of local circuit neurons and are hypothesized to include the wide-arbor (WA) class of γ-aminobutyric acid (GABA) cells. The size of the axon arbors of this class of GABA neurons suggests that they extend into the gap or interstripe region, where they may provide inhibitory input to the cell bodies of pyramidal neurons (d). Pyramidal neurons that furnish intrinsic axon collaterals also provide associational principal axon projections (e) to other areas of the prefrontal cortex. The axon terminals of these associational principal axon projections form excitatory synapses with the dendritic spines of other pyramidal neurons (f). Note that the branching axons and axon terminals of intrinsic stripes are confined to layers 1–3, whereas those of the associational stripes span all six cortical layers. On the right, the cluster of labeled intrinsic axon terminals arises from an injection site in the prefrontal cortex located approximately 6 mm medial to (left of) the cluster. Note that the dense array of labeled axons and boutons is confined to layers 1–3 and that this cluster is associated with horizontally oriented axons that travel through the gray matter (see arrows). FiguresReferencesCited byDetailsCited ByDivided attention capacity in adults with autism spectrum disorders and without intellectual disability1 May 2009 | Autism, Vol. 13, No. 3Journal de Gynécologie Obstétrique et Biologie de la Reproduction, Vol. 35, No. 1La Revue Sage-Femme, Vol. 5, No. 2 Volume 157Issue 11 November 2000Pages 1752-1752 Metrics PDF download History Published online 1 November 2000 Published in print 1 November 2000

Synaptic targets of the intrinsic axon collaterals of supragranular pyramidal neurons in monkey prefrontal cortex

The principal axons of supragranular pyramidal neurons in the cerebral cortex travel through the white matter and terminate in other cortical areas, whereas their intrinsic axon collaterals course through the gray matter and form both local and long-distance connections within a cortical region. In the monkey prefrontal cortex (PFC), horizontally oriented, intrinsic axon collaterals from supragranular pyramidal neurons form a series of stripe-like clusters of axon terminals (Levitt et al. [1993] J Comp Neurol 338:360-376; Pucak et al. [1996] J Comp Neurol 376:614-630). The present study examined the synaptic targets of the intrinsic axon collaterals arising from supragranular pyramidal neurons within the same stripe (local projections). Approximately 50% of the within-stripe axon terminals in monkey PFC area 9 targeted dendritic spines. In contrast, for both the intrinsic axon collaterals that travel between stripes (long-range projections), and the axon terminals that project to other PFC areas (associational projections), over 92% of the postsynaptic structures were dendritic spines (Melchitzky et al. [1998] J Comp Neurol 390:211-224). The other 50% of the within-stripe terminals synapsed with dendritic shafts. Dual-labeling studies confirmed that these within-stripe terminals contacted gamma-aminobutyric acid-immunoreactive dendritic shafts, including the subpopulation that contains the calcium-binding protein parvalbumin. The functional significance of the differences in synaptic targets between local and long-range intrinsic axon collaterals was supported by whole-cell, patch clamp recordings in an in vitro slice preparation of monkey PFC. Specifically, the small amplitude responses observed in layer 3 pyramidal neurons during long-range, low-intensity stimulation were exclusively excitatory, whereas local stimulation also evoked di/polysynaptic inhibitory responses. These anatomic and electrophysiological findings suggest that intrinsic connections of the PFC differ from other cortical regions and that within the PFC, feedback (within-stripe) inhibition plays a greater role in regulating the activity of supragranular pyramidal neurons than does feedforward inhibition either between stripes or across regions.

Long-range inhibition within the zebra finch song nucleus RA can coordinate the firing of multiple projection neurons.

The zebra finch forebrain song control nucleus RA (robust nucleus of the archistriatum) generates a phasic and temporally precise neural signal that drives vocal and respiratory motoneurons during singing. RA's output during singing predicts individual notes, even though afferent drive to RA from the song nucleus HVc is more tonic, and predicts song syllables, independent of the particular notes that comprise the syllable. Therefore RA's intrinsic circuitry transforms neural activity from HVc into a highly precise premotor output. To understand how RA's intrinsic circuitry effects this transformation, we characterized RA interneurons and projection neurons using intracellular recordings in brain slices. RA interneurons fired fast action potentials with steep current-frequency relationships and had small somata with thin aspinous processes that extended throughout large portions of the nucleus; the similarity of their fine processes to those labeled with a glutamic acid decarboxylase (GAD) antibody strongly suggests that these interneurons are GABAergic. Electrical stimulation revealed that RA interneurons receive excitatory inputs from RA's afferents, the lateral magnocellular nucleus of the anterior neostriatum (LMAN) and HVc, and from local axon collaterals of RA projection neurons. To map the functional connections that RA interneurons make onto RA projection neurons, we focally uncaged glutamate, revealing long-range inhibitory connections in RA. Thus these interneurons provide fast feed-forward and feedback inhibition to RA projection neurons and could help create the phasic pattern of bursts and pauses that characterizes RA output during singing. Furthermore, selectively activating the inhibitory network phase locks the firing of otherwise unconnected pairs of projection neurons, suggesting that local inhibition could coordinate RA output during singing.

GABAA-mediated local synaptic pathways connect neurons in the rat suprachiasmatic nucleus.

The suprachiasmatic nucleus (SCN) in mammals functions as the biological clock controlling circadian rhythms, but the synaptic circuitry of the SCN is largely unexplored. Most SCN neurons use the neurotransmitter gamma-aminobutyric acid (GABA), and anatomic studies indicate many GABAergic synapses and local axon collaterals; however, physiological evidence for synaptic communication among SCN neurons is indirect. We have used three approaches to investigate local circuitry in the SCN in acute hypothalamic slices from rat. First, tetrodotoxin was used to block action-potential-dependent synaptic release, which resulted in a decrease in the frequency of spontaneous synaptic currents in SCN neurons, suggesting that spontaneously active neurons in the slice connect synaptically to SCN neurons. Postsynaptic currents in SCN neurons were also evoked by the selective stimulation of other SCN neurons with glutamate, which avoids direct activation of axons that might originate outside the SCN. Two different methods of glutamate microapplication (i.e., pressure ejection and ultraviolet photolysis of caged glutamate) indicated that SCN neurons receive GABAA-receptor-mediated synaptic input from other SCN neurons. In contrast, glutamate-receptor-mediated synaptic connections between SCN neurons were not detected. The GABAergic synapses that comprise the network described here could conceivably be a substrate for the synchronization and amplification of the circadian rhythm of SCN firing. Alternatively, this circuitry might mediate other aspects of clock function such as the integration of environmental and physiological information.

Dendrodendritic and Axoaxonic Synapses in the Thalamic Reticular Nucleus of the Adult Rat

Currently, it is believed that cell-cell communications occur in the thalamic reticular nucleus (RT) during thalamocortical operations, but the anatomical substrate underlying these intrinsic interactions has not been characterized fully in the rat yet. To further our knowledge on this issue, we stained juxtacellularly rat RT neurons with biocytin or Neurobiotin and examined their intrinsic axon collaterals and "axon-like processes" at both light and electron microscopic levels. Of 111 tracer-filled RT cells for which the axon could be followed from its origin up to the thalamus, 12 displayed short-range, poorly ramifying varicose local axon collaterals, which remained undistinguishable from parent distal dendrites, raising the question as to whether their varicosities were presynaptic terminals. Correlated light and electron microscopic observations of the proximal part of these intrinsic varicose axonal segments revealed that their varicosities and intervaricose segments were, in fact, postsynaptic structures contacted by a large number of boutons that, for the most, formed asymmetric synapses and were nonimmunoreactive for GABA. Similarly, the so-called "axon-like processes" stemming from the soma or dendrites also were identified as postsynaptic structures. Two unexpected observations were made in the course of this analysis. First, the hillock and initial segment of some RT axons were found to receive asymmetric synaptic inputs from GABA-negative terminals. Second, examination of serial ultrathin sections of dendritic bundles cut in their longitudinal plane revealed the existence of several short symmetric dendrodendritic synapses and numerous puncta adhaerentia between component dendrites. In conclusion, dendrodendritic junctions might be a prominent anatomical substrate underlying interneuronal communications in the RT of the adult rat. Furthermore, excitatory axoaxonic synapses on the axon hillock, initial segment, and local axon collaterals might represent a powerful synaptic drive for synchronizing the firing of RT neurons. Future studies are essential to verify whether excitatory axoaxonic synapses with the axon hillock are a general feature in the RT.

GABAB receptor-mediated responses in GABAergic projection neurones of rat nucleus reticularis thalami in vitro.

1. Whole-cell voltage-clamp recordings were obtained from GABAergic neurones of rat nucleus reticularis thalami (NRT) in vitro to assess pre- and postsynaptic GABAB receptor-mediated responses. Presynaptic inhibition of GABA release was studied at terminals on local axon collaterals within NRT as well as on projection fibres in the somatosensory relay nuclei. 2. The GABAB receptor agonist (R)-baclofen (10 microM) reduced monosynaptically evoked GABAA-mediated inhibitory postsynaptic currents (IPSCs) in NRT and somatosensory relay cells to 11 and 12% of control, respectively. 3. Action potential-independent miniature IPSCs (mIPSCs) were observed in both cell types. Mean mIPSC amplitude was 20 pA in both NRT and relay cells at a holding potential of 0 mV. The mean mIPSC frequencies were 0.83 and 2.2 Hz in NRT and relay cells, respectively. Baclofen decreased mIPSP frequency by about half in each cell type without affecting amplitude. 4. Paired-burst inhibition of evoked IPSCs was studied in relay and NRT cells by applying pairs of 100 Hz stimulus bursts separated by 600 ms. The mean ratio of second to first peak IPSC amplitudes was 0.77. 5. In NRT cells baclofen induced a linear postsynaptic conductance increase of 0.82 nS with an associated reversal potential of -121 mV. A small (0.14 nS) GABAB component of the evoked IPSC was detected in only a minority of NRT cells (3 of 18). 6. All pre- and postsynaptic effects of baclofen, as well as PBI, were largely reversed by the specific GABAB receptor antagonist CGP 35348 (0.5 mM). 7. We conclude that activation of GABAB receptors in NRT leads to presynaptic autoinhibition of IPSCs in both NRT and relay cells, and to direct activation of a small linear K+ conductance. In addition our experiments suggest that reciprocal connectivity within NRT can be partially mediated by a small GABAB inhibitory event.

Axonal sprouting of CA1 pyramidal cells in hyperexcitable hippocampal slices of kainate-treated rats.

CA1 pyramidal cells become hyperexcitable following hippocampal kainate lesions. To examine if axonal sprouting contributes to this epileptiform activity, the local axonal arborization of CA1 pyramidal cells was examined after intracellular labelling with biocytin in hippocampal slices from control rats and in hyperexcitable slices obtained from rats treated with kainate (bilateral intracerebroventricular injections) 2-4 weeks previously. Biocytin-labelled cells with an axon that could be followed from the soma to the alveus were drawn and reconstructed with a camera lucida (15 cells from control slices and 14 cells from hyperexcitable slices). Local axonal arborizations were more extensive in cells of hyperexcitable slices. This increase in axon collaterals was generally seen in the alveus and in stratum oriens, but changes were more prominent in the latter. In stratum oriens, cells from hyperexcitable slices showed a significant increase in mean total axon length (1035 versus 373 mu m in control), in mean number of branching points (6.50 versus 0.67 in control) and in mean number of segment orders per axon (3.07 versus 1.47 in control). Their first-order axon segments were similar in length to those of control cells (236 versus 338 pm in control), but with significantly more branching points (2.86 versus 0.53 in control). Their second-order axon segments were significantly longer (381 versus 63 mu m in control) and also showed more branching points (2.71 versus 0.13 in control). Their third- and fourth-order axon segments were also longer and with more branching points. Under high-power light microscopic examination, biocytin-labelled axonal varicosities in cells of hyperexcitable slices were often seen in close apposition with their own dendrites, presumably making synaptic contact (five of nine cells examined). No such appositions were seen in any of the control cells (seven cells examined). These results indicate that, following kainate lesions, there is sprouting of local axon collaterals of CA1 pyramidal cells in stratum oriens and in the alveus. This local increase in axon collaterals may contribute to the epileptiform activity in the CA1 area by providing recurrent excitation via newly formed synaptic, and perhaps even autaptic, contacts with pyramidal cell dendrites.

The secretory trypsin inhibitor like-peptide, PEC-60 increases dopamine D 2 receptor agonist induced inhibition of GABA release in the dorsolateral neostriatum of the awake freely moving rat. An in vivo microdialysis study

The effects of local perfusion with the secretory trypsin inhibitor like-peptide, PEC-60 on dompamine and γ-aminobutyric acid (GABA) release in the dorsolateral neostriatum and GABA release in the globus pallidus were studied using in vivo microdialysis in the awake freely moving rat. Local perfusion with PEC-60 (500 nM and 1 μM) increased dopamine release in the dorsolateral neostriatum while the highest (1 μM) concentration of PEC-60 decreased striatal but not pallidal GABA release. An inactive form of the peptide, S-carboxyamidomethylated PEC-60 (1 μM) failed to influence either striatal dopamine and GABA or pallidal GABA release. In addition, when PEC-60, at a dose which did not affect striatal and pallidal GABA release (100 nM), was co-perfused together with the dopamine D 2 receptor agonist pergolide (500 nM), a potentiation in the ability of pergolide to reduce GABA release in the dorsolateral neostriatum was observed and this effect was counteracted by co-perfusion with the selective dopamine D 2 receptor antagonist raclopride (1 μM). In contrast, the pergolide induced inhibition of striatal dopamine release was unaffected by PEC-60 (100 nM). These data indicate that PEC-60 differentially regulates dopamine and GABA release in the dorsolateral neostriatum by a selective and facilitory interaction with the postsynaptic dopamine D 2 receptor possibly involving high-affinity PEC-60 like-peptide binding sites located on local axon collaterals of a discrete subpopulation of efferent GABA neurons and/or on GABA interneurons.

Glutaminase-positive and glutaminase-negative pyramidal cells in layer VI of the primary motor and somatosensory cortices: a combined analysis by intracellular staining and immunocytochemistry in the rat

Pyramidal neurons in layer VI of the primary motor and somatosensory cortices were examined by a combined method of intracellular recording, biocytin injection, and immunocytochemistry using in vitro slice preparations of rat brain immunofluorescence staining revealed that biocytin-injected pyramidal cells in layer VI were separated into glutaminase (PAG)-immunopositive and PAG-immunonegative cells. Although the two groups of pyramidal cells showed no statistically significant differences in passive membrane properties and spike characteristics, a clear difference was found in spike afterpotentials. Ten of 12 PAG-positive pyramidal cells showed no or a small fast afterhyperpolarization (fAHP), whereas 10 of 11 PAG-negative pyramidal cells displayed a large fAHP. Depolarizing afterpotentials were observed only in PAG-positive pyramidal cells than in PAG-negative cells. In contrast, the arborization of basal dendrites was more developed in PAG-positive pyramidal cells than in PAG-negative cells. The main axons of all the pyramidal cells entered the subcortical axons of all the pyramidal cells entered the subcortical white matter. The local axon collaterals of PAG-positive pyramidal cells were widely spread in the horizontal direction, whereas those of PAG-negative cells were distributed vertically along the dendritic tree. Since PAG is considered to be a marker of glutamatergic neurons in the cerebral cortex, the present results indicate that layer VI pyramidal cells are separated into glutamatergic and nonglutamatergic neurons that have different electrical properties and input-output organizations. Thus, cortical outputs from layer VI are suggested to use at least two distinct systems.

Synaptic Input of Horizontal Interneurons in Stratum Oriens of the Hippocampal CA1 Subfield: Structural Basis of Feed‐back Activation

The synaptic input of interneurons with horizontal dendrites in stratum oriens of the CA1 region was investigated, with particular attention to the portion of synapses originating from local pyramidal cells. Most of these GABAergic interneurons are known to contain somatostatin, and terminate on pyramidal dendrites in conjunction with entorhinal afferents in stratum lacunosum-moleculare. A smaller number of horizontal cells in this layer are immunoreactive for calbindin, and project to the medial septum. Selective ischaemic degeneration was used to label local axon collaterals of CA1 pyramidal cells, and immunostaining for mGluR1 or calbindin to visualise somatostatin- and calbindin-containing horizontal interneurons, respectively, at the stratum oriens-alveus border. The number of degenerating and intact synaptic boutons was counted on mGluR1- as well as on calbindin-positive dendrites and somata, whereas in another group of animals the proportion of GABA-immunoreactive synapses was estimated on calbindin-positive dendrites. On average, > 60% of the total presynaptic elements of both cell types were degenerating, i.e. originated from CA1 pyramidal cells, whereas GABA-positive boutons, which are known to survive ischaemia, are likely to account for a large proportion of non-degenerating boutons. Thus the vast majority of presumed excitatory synapses on somatostatin- and calbindin-containing horizontal neurons derives from local collaterals of CA1 pyramidal cells. The remaining GABA-negative synapses surviving ischaemia may also originate from CA1 pyramidal cells, e.g. from those in the ventral hippocampus, which are rarely damaged by global forebrain ischaemia. Alternative sources may include subcortical afferents known to innervate interneurons, or ipsi- and contralateral CA3 pyramidal cells, which, according to the present results, may account only for a negligible number of synapses on these interneurons types. We conclude that somatostatin-containing neurons at the oriens-alveus border of CA1, which are likely to mediate an inhibitory control of the efficacy and/or plasticity of entorhinal synapses on pyramidal cell dendrites, are driven primarily in a feed-back manner. The source of afferent excitation for calbindin-containing horizontal neurons in this region is very similar, suggesting that the GABAergic hippocamposeptal feed-back is also activated by local pyramidal cell collaterals.

Structure-function relationships in rat brainstem subnucleus interpolaris: XII. neonatal deafferentation effects on cell morphology.

In the developing whisker-barrel neuraxis, it is known that pattern formation, receptive fields, axon projections, and even cell survival are under the control of peripheral signals transmitted through the infraorbital nerve. However, afferent influences upon the development of single-cell morphologies have not received thorough study. Intracellular recording, antidromic activation, receptive field mapping, dye injection, and computer-assisted cell reconstruction methods were used to assess the morphology of trigeminal (V) brainstem neurons in adult rats whose infraorbital nerves were transected at birth. Projection and local-circuit neurons in the spinal V subnucleus interpolaris (SpVi; n = 43) and local-circuit neurons in the adjacent subnucleus caudalis (SpVc; n = 11) were compared with similar cell types in normal control rats, as well as with spinal V neurons located outside of the deafferented region in experimental rats. SpVi cells displayed abnormally convergent and discontinuous receptive fields that included greater-than-normal numbers of vibrissae and other receptor organs. However, their morphologies did not differ significantly from normal on any quantitative measure, including soma size, number of proximal dendrites, or dendritic tree area, perimeter, or shape. Moreover, SpVi cells near deafferented brainstem territories did not display dendritic tree polarity toward or away from the deafferented region. In SpVc, laminae I-V cells had responses and morphologies that were indistinguishable from those of controls. Thus, (1) altered receptive fields of neonatally deafferented SpVi neurons are not attributable to changes in their morphology; (2) SpVc cells are resilient following deafferentation; and (3) the development of SpV dendrites and local axon collaterals is controlled by factors other than those directly conveyed by primary afferents.

D2 dopamine receptor protein location: Golgi impregnation-gold toned and ultrastructural analysis of the rat neostriatum.

The neostriatal distribution of D2 dopamine receptor protein has been assessed using subtype-selective polyclonal antibodies generated against three unique polypeptide sequences of the receptor. The experimental tissues were processed by peroxidase based immunohistochemical procedures for routine light microscopy, Golgi impregnation-gold toned morphological characterization, and correlative light/electron microscopy. The results demonstrated a regional gradient of D2-like dopamine receptor expression in the neostriatum, where lateral portions in the nucleus exhibited more reactive cell bodies than medial portions. D2-like expression was detected in the three populations of neostriatal neurons, i.e., the medium-sized spiny projection neurons, and the medium- and large-sized aspiny interneuron types. Morphometric measurements of labeled neurons verified that medium and large diameter neurons expressed the D2-like receptor subtype. D2-like immunoreactivity was distributed throughout the cytoplasm in dendritic processes, and in presynaptic terminal boutons. Immunoreactivity for the receptor protein was also detected in small, thinly myelinated axons, suggesting the possibilities of anterograde transport of the receptor from cell bodies in the substantia nigra to their neostriatal terminal fields, as well as from local axon collaterals of neostriatal projections neurons. These findings provide evidence of widespread distribution of the D2-like receptor protein in neostriatal neurons, and showed that the presynaptic D2 receptors contain analogous epitopes to the postsynaptic receptor subtype.

GABA synapses formed in vitro by local axon collaterals of nucleus accumbens neurons

GABAergic medium-spiny neuron axons not only form the principal projections of the nucleus accumbens (nAcc) but also branch locally in a dense network overlapping their own dendrites, suggesting that their recurrent synapses mediate the major information processing functions of the nAcc. We used postnatal nAcc cultures to study these synapses individually. In culture, as in the intact nAcc, medium-spiny neurons account for over 95% of the cells and are GABAergic. Strikingly, these neurons showed a spike afterhyperpolarization (AHP) that was largely blocked by the GABAA antagonist bicuculline. The bicuculline-sensitive AHP occurred without or with latency, and met criteria for monosynapticity; consistent with this, dye fills showed the presence of recurrent axons and a low incidence of dye coupling. Blockade of Ca2+ influx eliminated this autaptic PSP, while TTX almost completely eliminated it, indicating that it is due to exocytic GABA release principally at axodendritic contacts. While blocking GABAB receptors had no direct effect on the autaptic PSP, activating these receptors with baclofen produced presynaptic inhibition, as well as directly mediated hyperpolarization; together, these actions increased the signal-to-noise ratio in the cellular response to synaptic inputs. Bicuculline also increased the signal-to-noise ratio; in addition, it induced burst firing and depolarization inactivation. In contrast, the indirect GABA agonist flurazepam and the GABA uptake blocker nipecotic acid each enhanced autaptic PSPs. Since autapses formed in vitro appear to be functionally equivalent to synapses between neighboring medium-spiny neurons that receive similar inputs, these results bear on the function of intrinsic GABA synapses in the intact nAcc. Thus, intrinsic GABA synapses are likely to regulate the signal-to-noise ratio in nAcc information processing and may be important targets for the modulatory actions of endogenous neurotransmitters and drugs.

Synapses on axon collaterals of pyramidal cells are spaced at random intervals: a Golgi study in the mouse cerebral cortex.

In this study we investigated the arrangement of synapses on local axon collaterals of Golgi-stained pyramidal neurons in the mouse cerebral cortex. As synaptic markers we considered axonal swellings visible at high magnification under the light microscope. Such axonal swellings coincide with synaptic boutons, as has been demonstrated in a number of combined light and electron microscopic studies. These studies also indicated that, in most cases, one bouton corresponds precisely to one synapse. Golgi-impregnated axonal trees of 20 neocortical pyramidal neurons were drawn with a camera lucida. Axonal swellings were marked on the drawings. Most swellings were 'en passant'; occasionally, they were situated at the tip of short, spine-like processes. On axon collaterals, the average interval between swellings was 4.5 microns. On the axonal main stem, the swellings were always less densely packed than on the collaterals. Statistical analysis of the spatial distribution of the swellings did not reveal any special patterns. Instead, the arrangement of swellings on individual collaterals follows a Poisson distribution. Moreover, the same holds to a large extent for the entire collection of pyramidal cell collaterals. This suggests that a single Poisson process, characterized by only one rate parameter (number of synapses per unit length), describes most of the spatial distribution of synapses along pyramidal cell collaterals. These findings do not speak in favour of a pronounced target specificity of pyramidal neurons at the synaptic level. Instead, our results support a probabilistic model of cortical connectivity.

Identification and characterization of striatal cell subtypes using in vivo intracellular recording and dye‐labeling in rats. III. Morphological correlates and compartmental localization

In the first two reports of this series, in vivo intracellular recording techniques were used to characterize the electrophysiological properties of two types of striatal neurons that had been identified by their distinct response patterns to stimulation of corticostriatal afferents. In this paper, we examined whether cells showing Type I or Type II response patterns also differed with respect to their morphology or compartmental localization by combining intracellular recording and Lucifer yellow staining with immunocytochemical localization of calbindin 28 kd immunoreactivity. In the majority of cases, both Type I and Type II neurons exhibited similar morphological characteristics, with 80% of the Type I cells (13/16) and all of the Type II cells (n = 40) being small or medium spiny neurons. In each case where the morphological class of the cell was different than the spiny cell class, the cell exhibited a Type I response pattern. These spiny neurons had somata that averaged 17.1 +/- 1.3 microns in diameter and gave rise to between four and eight primary dendrites. The axons typically arose from cell bodies (7/13 for Type I and 25/40 for Type II cells) and emitted extensive local axonal collaterals. However, the axons of Type I cells more frequently originated from the dorsal surface of the somata (9/13; 69%), whereas Type II axons more frequently arose from the ventral surface of the somata (25/35; 71%), which may account for their different extracellular waveforms. In coronally sectioned tissue (n = 18), the axons always projected laterally when the somata were located in the medial striatum and projected medially when the somata were in the lateral striatal region. In a subset of experiments (N = 22), Lucifer yellow-stained neurons were localized with respect to their position within the patch and matrix compartments of the striatum using subsequent staining for calbindin 28 kd immunoreactivity. Of the 20 labeled medium spiny neurons examined (Type II: N = 13; Type I: N = 7), 19 were located in the calbindin-positive matrix compartment. The only neuron localized to the patch compartment was a medium spiny cell that exhibited a Type II paired impulse response pattern. In addition, of the two aspiny neurons from this group with beaded dendrites, one was localized to the border between adjacent patch and matrix compartments, whereas the other was located completely within the matrix compartment. Therefore, despite their distinct paired impulse response patterns, the majority of both Type I and Type II neurons were medium spiny cells located in the matrix compartment of the striatum.(ABSTRACT TRUNCATED AT 400 WORDS)