Synaptic Triads Research Articles

A schizophrenia risk factor induces marked anatomical deficits at GABAergic-dopaminergic synapses in the rat ventral tegmental area: Essential evidence for new targeted therapies.

To develop new therapies for schizophrenia, evidence accumulated over decades highlights the essential need to investigate the GABAergic synapses that presynaptically influence midbrain dopaminergic neurons. Since current technology restricts these studies to animals, and evidence accumulated in recent decades indicates a developmental origin of schizophrenia, we investigated synaptic changes in male rat offspring exposed to maternal immune activation (MIA), a schizophrenia risk factor. Using a novel combination of lentiviruses, peroxidase-immunogold double labeling, three-dimensional serial section transmission electron microscopy and stereology, we observed clear anatomical alterations in synaptic inputs on dopaminergic neurons in the midbrain posterior ventral tegmental area (pVTA). These changes relate directly to a characteristic feature of schizophrenia: increased dopamine release. In 3-month-old and 14-month-old MIA rats, we found a marked decrease in the volume of presynaptic GABAergic terminals from the rostromedial tegmental nucleus (RMTg) and in the length of the synapses they made, when innervating pVTA dopaminergic neurons. In MIA rats in the long-term, we also discovered a decrease in the volume of the postsynaptic density (PSD) and in the maximum thickness of the PSD at the same synapses. These marked deficits were evident in conventional GABA-dopamine synapses and in synaptic triads that we discovered involving asymmetric synapses that innervated RMTg GABAergic presynaptic terminals, which in turn innervated pVTA dopaminergic neurons. In triads, the PSD thickness of asymmetric synapses was significantly decreased in MIA rats in the long-term cohort. The extensive anatomical deficits provide a potential basis for new therapies targeted at synaptic inputs on midbrain pVTA dopaminergic neurons, in contrast to current striatum-targeted antipsychotic drugs.

Synaptic Remodeling in the Cone Pathway After Early Postnatal Horizontal Cell Ablation.

The first synapse of the visual pathway is formed by photoreceptors, horizontal cells and bipolar cells. While ON bipolar cells invaginate into the photoreceptor terminal and form synaptic triads together with invaginating horizontal cell processes, OFF bipolar cells make flat contacts at the base of the terminal. When horizontal cells are ablated during retina development, no invaginating synapses are formed in rod photoreceptors. However, how cone photoreceptors and their synaptic connections with bipolar cells react to this insult, is unclear so far. To answer this question, we specifically ablated horizontal cells from the developing mouse retina. Following ablation around postnatal day 4 (P4)/P5, cones initially exhibited a normal morphology and formed flat contacts with OFF bipolar cells, but only few invaginating contacts with ON bipolar cells. From P15 on, synaptic remodeling became obvious with clustering of cone terminals and mislocalized cone somata in the OPL. Adult cones (P56) finally displayed highly branched axons with numerous terminals which contained ribbons and vesicular glutamate transporters. Furthermore, type 3a, 3b, and 4 OFF bipolar cell dendrites sprouted into the outer nuclear layer and even expressed glutamate receptors at the base of newly formed cone terminals. These results indicate that cones may be able to form new synapses with OFF bipolar cells in adult mice. In contrast, cone terminals lost their invaginating contacts with ON bipolar cells, highlighting the importance of horizontal cells for synapse maintenance. Taken together, our data demonstrate that early postnatal horizontal cell ablation leads to differential remodeling in the cone pathway: whereas synapses between cones and ON bipolar cells were lost, new putative synapses were established between cones and OFF bipolar cells. These results suggest that synapse formation and maintenance are regulated very differently between flat and invaginating contacts at cone terminals.

Fluoxetine-induced recovery of serotonin and norepinephrine projections in a mouse model of post-stroke depression

Chronic treatment with fluoxetine (FLX) is required for its antidepressant effects, but the role of serotonin (5-HT) axonal plasticity in FLX action is unknown. To address this, we examined mice with a stroke in the left medial prefrontal cortex (mPFC) resulting in persistent anxiety-like and depression-like behaviors and memory deficits as a model of post-stroke depression. Chronic treatment with FLX (but not exercise) completely reversed the behavioral phenotype and partially reversed changes in FosB-labeled cells in the mPFC, nucleus accumbens, septum, hippocampus, basolateral amygdala (BLA), and dorsal raphe. In these regions, 5-HT or norepinephrine (NE) innervation was quantified by staining for 5-HT or NE transporters, respectively. 5-HT synapses and synaptic triads were identified as synaptophysin-stained sites on 5-HT axons located proximal to gephyrin-stained or PSD95-stained spines. A week after stroke, 5-HT innervation was greatly reduced at the stroke site (left cingulate gyrus (CG) of the mPFC) and the left BLA. Chronically, 5-HT and NE innervation was reduced at the left CG, nucleus accumbens, and BLA, with no changes in other regions. In these areas, pre-synaptic and post-synaptic 5-HT synapses and triads to inhibitory (gephyrin+) sites were reduced, while 5-HT contacts at excitatory (PSD95+) sites were reduced in the CG and prelimbic mPFC. Chronic FLX, but not exercise, reversed these reductions in 5-HT innervation but incompletely restored NE projections. Changes in 5-HT innervation were verified using YFP staining in mice expressing YFP-tagged channelrhodopsin in 5-HT neurons. Thus, FLX-induced 5-HT axonal neuroplasticity of forebrain projections may help mediate recovery from brain injury.

Mapping the connectivity of serotonin transporter immunoreactive axons to excitatory and inhibitory neurochemical synapses in the mouse limbic brain

Serotonin neurons arise from the brainstem raphe nuclei and send their projections throughout the brain to release 5-HT which acts as a modulator of several neuronal populations. Previous electron microscopy studies in rats have morphologically determined the distribution of 5-HT release sites (boutons) in certain brain regions and have shown that 5-HT containing boutons form synaptic contacts that are either symmetric or asymmetric. In addition, 5-HT boutons can form synaptic triads with the pre- and postsynaptic specializations of either symmetrical or asymmetrical synapses. However, due to the labor intensive processing of serial sections required by electron microscopy, little is known about the neurochemical properties or the quantitative distribution of 5-HT triads within whole brain or discrete subregions. Therefore, we used a semi-automated approach that combines immunohistochemistry and high-resolution confocal microscopy to label serotonin transporter (SERT) immunoreactive axons and reconstruct in 3D their distribution within limbic brain regions. We also used antibodies against key pre- (synaptophysin) and postsynaptic components of excitatory (PSD95) or inhibitory (gephyrin) synapses to (1) identify putative 5-HTergic boutons within SERT immunoreactive axons and, (2) quantify their close apposition to neurochemical excitatory or inhibitory synapses. We provide a 5-HTergic axon density map and have determined the ratio of synaptic triads consisting of a 5-HT bouton in close proximity to either neurochemical excitatory or inhibitory synapses within different limbic brain areas. The ability to model and map changes in 5-HTergic axonal density and the formation of triadic connectivity within whole brain regions using this rapid and quantitative approach offers new possibilities for studying neuroplastic changes in the 5-HTergic pathway.

Presynaptic gating of excitation in the dorsal raphe nucleus by GABA

The dorsal raphe nucleus (DR) controls forebrain serotonin neurotransmission to influence emotional states. GABA neurotransmission in the DR has been implicated in regulating sleep/wake states and influencing anxiety and aggression. To gain insight into how GABA regulates DR activity, we analyzed the organization of both GABA and glutamate axons in the rat DR using a high-resolution immunofluorescence technique, array tomography, as well as EM. This analysis revealed that a third or more of GABA-containing axons are organized in synaptic triads with a glutamatergic axon and a common postsynaptic target. Electrophysiological recordings showed that GABA has the capacity to presynaptically gate glutamate release in the DR through a combination of GABA-A and GABA-B receptor-mediated effects. Thus, GABA-glutamate synaptic triads are a common feature of the network architecture of the DR with the potential to regulate excitation of the nucleus.

Distinct synaptic mechanisms create parallel S-ON and S-OFF color opponent pathways in the primate retina

Anatomical and physiological approaches are beginning to reveal the synaptic origins of parallel ON- and OFF-pathway retinal circuits for the transmission of short (S-) wavelength sensitive cone signals in the primate retina. Anatomical data suggest that synaptic output from S-cones is largely segregated; central elements of synaptic triads arise almost exclusively from the "blue-cone" bipolar cell, a presumed ON bipolar, whereas triad-associated contacts derive primarily from the "flat" midget bipolar cell, a hyperpolarizing, OFF bipolar. Similarly, horizontal cell connectivity is also segregated, with only the H2 cell-type receiving numerous contacts from S-cones. Negative feedback from long (L-) and middle (M-) wavelength sensitive cones via the H2 horizontal cells elicits an antagonistic surround in S-cones demonstrating that S versus L + M or "blue-yellow" opponency is first established in the S-cone. However, the S-cone output utilizes distinct synaptic mechanisms to create color opponency at the ganglion cell level. The blue-cone bipolar cell is presynaptic to the small bistratified, "blue-ON" ganglion cell. S versus L + M cone opponency arises postsynaptically by converging S-ON and LM-OFF excitatory bipolar inputs to the ganglion cell's bistratified dendritic tree. The common L + M cone surrounds of the parallel S-ON and LM-OFF cone bipolar inputs appear to cancel resulting in "blue-yellow" antagonism without center-surround spatial opponency. By contrast, in midget ganglion cells, opponency arises by the differential weighting of cone inputs to the receptive field center versus surround. In the macula, the "private-line" connection from a midget ganglion cell to a single cone predicts that S versus L + M opponency is transmitted from the S-cone to the S-OFF midget bipolar and ganglion cell. Beyond the macula, OFF-midget ganglion cell dendritic trees enlarge and collect additional input from multiple L and M cones. Thus S-OFF opponency via the midget pathway would be expected to become more complex in the near retinal periphery as L and/or M and S cone inputs sum to the receptive field center. An important goal for further investigation will be to explore the hypothesis that distinct bistratified S-ON versus midget S-OFF retinal circuits are the substrates for human psychophysical detection mechanisms attributed to S-ON versus S-OFF perceptual channels.

Combination of group I mGlu receptors antagonist with dopaminergic agonists strengthens the synaptic transmission at corticostriatal synapses in culture

Restoring synaptic plasticity in neurodegenerative diseases could prevent neuronal degeneration, as well as motor and cognitive disorders. In Parkinson’s disease, synaptic plasticity at corticostriatal synapses is altered. Dendrites of striatal medium spiny neurons (MSNs) receive dopaminergic inputs from the substantia nigra and glutamatergic cortical afferents. Because both glutamate and dopamine are required to induce and sustain MSNs plasticity, the particular molecular mechanisms involved at this synaptic triad are difficult to understand. In the present work, we established a convenient in vitro model of the corticostriatal synapse to study synaptic plasticity. We focused on long-term depression involving group I metabotropic glutamate (mGlu) receptors. We found that in striatal neurons co-cultured with cortical neurons, the absence of dopaminergic stimuli favored the excess of glutamatergic drive from cortical neuron terminals, thus resulting in a constitutive depression of the corticostriatal glutamatergic transmission. Indeed, concomitant blockade of group I mGlu receptors and activation of dopaminergic receptors stably reduced the depression of the synaptic transmission. Thus the dependence on glutamate and dopamine balance of the corticostriatal synapse responsiveness validates the accuracy of this manageable in vitro model to depict the molecular pathways involved in the plasticity at corticostriatal synapses and to test restorative therapeutic approaches in Parkinson’s disease.This article is part of a Special Issue entitled ‘Metabotropic Glutamate Receptors’.

Retinoblastoma (Rb) regulates laminar dendritic arbor reorganization in retinal horizontal neurons

Neuronal differentiation with respect to the acquisition of synaptic competence needs to be regulated precisely during neurogenesis to ensure proper formation of circuits at the right place and time in development. This regulation is particularly important for synaptic triads among photoreceptors, horizontal cells (HCs), and bipolar cells in the retina, because HCs are among the first cell types produced during development, and bipolar cells are among the last. HCs undergo a dramatic transition from vertically oriented neurites that form columnar arbors to overlapping laminar dendritic arbors with differentiation. However, how this process is regulated and coordinated with differentiation of photoreceptors and bipolar cells remains unknown. Previous studies have suggested that the retinoblastoma (Rb) tumor suppressor gene may play a role in horizontal cell differentiation and synaptogenesis. By combining genetic mosaic analysis of individual synaptic triads with neuroanatomic analyses and multiphoton live imaging of developing HCs, we found that Rb plays a cell-autonomous role in the reorganization of horizontal cell neurites as they differentiate. Aberrant vertical processes in Rb-deficient HCs form ectopic synapses with rods in the outer nuclear layer but lack bipolar dendrites. Although previous reports indicate that photoreceptor abnormalities can trigger formation of ectopic synapses, our studies now demonstrate that defects in a postsynaptic partner contribute to the formation of ectopic photoreceptor synapses in the mammalian retina.

In VivoDevelopment of Outer Retinal Synapses in the Absence of Glial Contact

Astroglia secrete factors that promote synapse formation and maintenance. In culture, glial contact has also been shown to facilitate synaptogenesis. Here, we examined whether glial contact is important for establishing circuits in vivo by simultaneously monitoring differentiation of glial cells and local synaptogenesis over time. Photoreceptor circuits of the vertebrate retina are particularly suitable for this study because of the relatively simple, laminar organization of their connectivity with their target neurons, horizontal cells and bipolar cells. Also, individual photoreceptor terminals are ensheathed within the outer plexiform layer (OPL) by the processes of one type of glia, Müller glia cells (MGs). We conducted in vivo time-lapse multiphoton imaging of the rapidly developing and relatively transparent zebrafish retina to ascertain the time course of MG development relative to OPL synaptogenesis. The emergence of synaptic triads, indicative of functional photoreceptor circuits, and structural association with glial processes were also examined across ages by electron microscopy. We first show that MG processes form territories that tile within the inner and outer synaptic layers. We then demonstrate that cone photoreceptor synapses are assembled before the elaboration of MG processes in the OPL. Using a targeted cell ablation approach, we also determined whether the maintenance of photoreceptor synapses is perturbed when local MGs are absent. We found that removal of MGs had no appreciable effect on the stability of newly formed cone synapses. Thus, in contrast to other CNS circuits, contact from glia is not necessary for the formation or immediate stabilization of outer retinal synapses.

GABA- and glycine-immunoreactive synapses in spinal cord of frog Rana temporaria

Postembedding immunogold method was used to examine the distribution of gamma-aminobutyric acid- and glycine-immunoreactives synapses on the motoneurons and primary afferent axons in frog spinal cord. Analysis of all labeled boutons on dendrites and somata of motoneurons showed that 7% were labeled for GABA, 23% only for glycine and approximately 70% were immunoreactive for both GABA and glycine. These results confirm the predominant role of glycine in postsynaptic inhibition of motoneuronal activity. Three populations of synaptic boutons were found on primary afferent axons: GABA-immunoreactive (25%), glycine-immunoreactive (5%) and the majority of the immunoreactive synapses exhibited colocalization of two inhibitory transmitters. Greater proportion of axo-axonal synases was organized in synaptic triads. The possible roles of glycine in the axo-axonal synapses on the primary afferent fibers are discussed.

Observations on the synaptic ribbon morphology in retinas of two human subjects at autopsy

Retinal photoreceptor and bipolar cell axon terminals possess synaptic ribbons (SR) that aid in the trafficking of synaptic vesicles at active zones. In rodents, besides SR, a number of other synaptic elements [e.g., synaptic spheres (SS)] are known to appear when exposed to altered ambient illumination. Here, we report changes of ribbon shape in photoreceptor and bipolar cell axon terminals in retinas of two persons at autopsy who suffered from brain hemorrhage. In both subjects, retinal hemorrhage was present in the outer and inner nuclear layers. SR were bent or swollen and transformed into SS. A count revealed that about 54-60% of the photoreceptor axon terminals over the nasal to temporal retina possessed SS. They were associated with synaptic triads or remained floating in cytoplasm. The bipolar cell axon terminals possessed either SR or sphere-like bodies. As these features were not seen in control retinas of donors who died of other causes, we assume that in hemorrhagic subjects, SR underwent transformation into SS, in which perhaps ischemia (caused due to vascular obstructions by hemorrhage) played a leading role.

Synaptic circuitry in the retinorecipient layers of the optic tectum of the lamprey (Lampetra fluviatilis). A combined hodological, GABA and glutamate immunocytochemical study

The ultrastructure of the retinorecipient layers of the lamprey optic tectum was analysed using tract tracing techniques combined with GABA and glutamate immunocytochemistry. Two types of neurons were identified; a population of large GABA-immunonegative cells, and a population of smaller, highly GABA-immunoreactive interneurons, some of whose dendrites contain synaptic vesicles (DCSV). Five types of axon terminals were identified and divided into two major categories. The first of these are GABA-immunonegative, highly glutamate-immunoreactive, contain round synaptic vesicles, make asymmetrical synaptic contacts, and can in turn be divided into AT1 and AT2 terminals. The AT1 terminals are those of the retinotectal projection. The origin of the nonretinal AT2 terminals could not be determined. AT1 and AT2 terminals establish synaptic contacts with DCSV, with dendrites of the retinopetal neurons (DRN), and with conventional dendritic (D) profiles. The terminals of the second category are GABA-immunoreactive and can similarly be divided into AT3 and AT4 terminals. The AT3 terminals contain pleiomorphic synaptic vesicles and make symmetrical synaptic contacts for the most part with glutamate-immunoreactive D profiles. The AT4 terminals contain rounded synaptic vesicles and make asymmetrical synaptic contacts with DRN, with DCSV, and with D profiles. A fifth, rarely observed category of terminals (AT5) contain both clear synaptic vesicles and a large number of dense-core vesicles. Synaptic triads involving AT1, AT2 or AT4 terminals are rare. Our findings are compared to these of previous studies of the fine structure and immunochemical properties of the retinorecipient layers of the optic tectum or superior colliculus of Gnathostomes.

Fine structure of the visual dorsolateral anterior thalamic nucleus of the pigeon (Columba livia): A hodological and GABA‐immunocytochemical study

The ultrastructure of the lateroventral subcomponent of the visual dorsolateral anterior thalamic nucleus of the pigeon (DLLv) was analyzed using hodological techniques and GABA-immunocytochemistry. Two types of GABA-immunonegative hyperpalliopetal neurons and a single type of strongly GABA-immunoreactive (-ir) interneuron were identified, the latter displaying long dendrites with some containing synaptic vesicles (DCSV). Ten types of axon terminal were identified and divided into two categories. The first, GABA-immunonegative and making asymmetrical synaptic contact, contain round (RT1, RT2, RT3) or pleiomorphic synaptic and many dense-core vesicles (DCT). RT1 terminals are retinothalamic and RT2 terminals hyperpalliothalamic; both mainly contact dendrites of projection neurons (72% and 78% respectively), less frequently dendrites of interneurons and sometimes DCSV; RT1 terminals are rarely involved in synaptic triads. The second category are consistently GABA-immunopositive. Four types (PT1-4), distinguished by their pleiomorphic synaptic vesicles, make symmetrical synaptic contact essentially with dendrites of projection neurons, more rarely on dendrites of interneurons (PT2). PT1 terminals are very probably those of interneurons, whereas the rare PT4 terminals are of retinal origin. A fifth type (RgT) contains round synaptic vesicles and makes asymmetrical synaptic contact with dendrites of projection neurons and interneurons. PT2 and RgT terminals occasionally contact DCSV of interneurons, which are sometimes involved in synaptic triads. Two final subcategories (DCgT1-2) contain many dense-core vesicles. Our findings are compared with those of previous studies concerning the fine structure and neurochemical properties of the GLd of reptiles and mammals, with special reference to the origin of the extraretinal and extracortical projections to this structure.

The synaptic microcircuitry associated with primary afferent terminals in the interpolaris and caudalis of trigeminal sensory nuclear complex

Previous ultrastructural studies indicating a higher number of axoaxonic contacts on individual low-threshold mechanoreceptive afferents in the principalis (Vp) than in the oralis (Vo) of cat trigeminal sensory nuclear complex (TSNC) suggest that the synaptic microcircuitry associated with primary afferents manifests unique differences across the sensory nuclei of TSNC. To address this issue, we analyzed synaptic microcircuits associated with fast adapting vibrissa afferent terminals in the interpolaris (Vi) and caudalis (Vc, laminae III/IV) by using intraaxonal injections of horseradish peroxidase (HRP) in cats. Forty-two and 65 HRP-labeled boutons were analyzed in the Vi and Vc, respectively. The labeled boutons contained clear, spherical vesicles. They most frequently formed asymmetric axodendritic synapses and were commonly postsynaptic to unlabeled axon terminals containing pleomorphic vesicles (p-endings) with symmetric junctions. The examination of synaptic contacts over the entire surface of individual boutons indicated that the afferent boutons made contacts with an average of two postsynaptic targets in the Vi and Vc. In contrast, axoaxonic contacts, and labeled boutons participating in synaptic triads, where p-endings contacted both the boutons and their postsynaptic targets, were, on average, higher in the Vi than in the Vc. These results suggest that the output of sensory information conveyed through low-threshold mechanoreceptive afferents is more strongly controlled at the level of the first synapse by presynaptic and postsynaptic mechanisms in the Vi responsible for sensory discriminative functions than in the Vc for sensorimotor reflexive functions.

GABA and glycine in synaptic microcircuits associated with physiologically characterized primary afferents of cat trigeminal principal nucleus

Previous studies suggest that sensory information conveyed through trigeminal afferents is more strongly controlled at the level of the first synapse by GABA-mediated presynaptic mechanisms in the trigeminal principal sensory nucleus (Vp) than other sensory nuclei. However, it is unknown if such a mechanism is common to functionally different classes of primary afferent in the same nucleus or across the nuclei. To address these issues, the present study focused on synaptic microcircuits associated with slowly adapting (SA) mechanosensory afferents innervating the periodontal ligaments in the cat Vp and attempted to examine GABA, glycine, and glutamate immunoreactivity in axon terminals involved in the circuits. Afferents were physiologically characterized before injection of horseradish peroxidase (HRP) and preparation for electron microscopy. HRP-labeled afferent boutons were serially sectioned and immunostained with antibodies against GABA, glycine, and glutamate using a postembedding immunogold method. All the afferent boutons examined contacted non-primary dendrites and they were frequently postsynaptic to unlabeled axons (p-endings). Axodendritic and axoaxonic contacts per afferent bouton were 1.3 (46/35) and 2.0 (70/35), respectively. Most p-endings were immunoreactive for GABA (63/70) and also glycine was co-stained in the majority of the p-endings (49/63). Thirty percent of p-endings with the colocalization of GABA and glycine participated in synaptic triads where a p-ending formed a synapse with the same dendrite as the afferent bouton. None of the p-endings was immunoreactive for glutamate. Most afferent boutons were enriched with glutamate but were immunonegative for GABA and glycine. This study provides evidence suggesting that transmission from SA afferents is strongly controlled presynaptically by GABAergic interneurons with colocalized glycine, and that a proportion of these interneurons, involved in synaptic triads, may also have postsynaptic inhibitory actions on target neurons of the SA afferents.

Synaptic interactions between the terminals of slow-adapting type II mechanoreceptor afferents and neurones expressing gamma-aminobutyric acid- and glycine-like immunoreactivity in the rat spinal cord.

The object of this study was to analyse the synaptic interactions of slow-adapting type II (SAII) afferent terminals in laminae III-V of the rat spinal cord. The axons of SAII afferents were physiologically characterized by intracellular recording before injection with neurobiotin and preparation for electron microscopy. Axon terminals were serially sectioned and immunolabelled with antibodies against gamma-aminobutyric acid (GABA) or glycine by using a postembedding immunogold procedure. Computer-aided reconstruction was used to reveal the relative distribution of different types of synapses on terminal and en passant synaptic boutons. Eighty-nine percent of boutons received axoaxonic synaptic contacts, the mean number of contacts per bouton being 3.5. Fifty-nine percent of presynaptic axons were immunoreactive for both GABA and glycine and 45% for GABA alone. Most boutons (95%) made axodendritic contacts, and the mean number of dendrites contacted was 1.6. More than half of the postsynaptic dendrites were greater than 1 microm in diameter. Twenty-three percent were immunoreactive for glycine, and 71% were not immunoreactive for either antibody. Synaptic triads in which an axon presynaptic to the afferent was also in contact with a dendrite postsynaptic to the afferent were seen at 63% of boutons. These results are discussed in the light of similar studies of other low-threshold mechanoreceptive afferent terminals in the rat and cat and in the context of what is known of the sensory interneurones carrying information from slow- and rapid-adapting mechanoreceptors.

GABA- and glycine-like immunoreactivity in axons and dendrites contacting the central terminals of rapidly adapting glabrous skin afferents in rat spinal cord.

The object of the present study was to determine the nature and distribution of synaptic contacts on the terminals of rapidly adapting mechanosensory afferents innervating the glabrous skin of the rat foot. Afferents were physiologically characterized by intracellular recording, before injection with neurobiotin and preparation for electron microscopy. Axon terminals were serially sectioned and immunolabeled with antibodies against GABA and glycine using a postembedding immunogold method. Afferent boutons in lamina III were often surrounded by several presynaptic axons and postsynaptic dendrites (thus forming type II glomeruli), while boutons in laminae IV-V had only simple, nonglomerular interactions. In both regions triadic synaptic arrangements where presynaptic interneurons contact both afferent boutons and their postsynaptic dendrites were present in 50-75% of boutons. Approximately three-quarters of presynaptic axons were immunoreactive for both GABA and glycine and most of the remainder for GABA alone. Most postsynaptic dendrites were not immunoreactive. Comparisons are made with information from similar studies of other rat and cat afferents conducting in the Aalphabeta range. This demonstrates that although the principles of control may be similar for cutaneous afferents of this type there are significant differences between cutaneous and 1a muscle afferents in the rat. There are also differences in detail between the interactions of afferents of the same modality in rat and cat; in the rat there are greater numbers of presynaptic axons per bouton and a greater proportion of boutons receive axo-axonic contacts and are involved in synaptic triads.

Synaptic organization of monosynaptic connections from mesencephalic trigeminal nucleus neurons to hypoglossal motoneurons in the rat.

Synaptological characteristics of synapses between axonal boutons of the trigeminal mesencephalic nucleus (Vme) neurons and the hypoglossal nucleus (XII) motoneurons (MNs) were studied using biotinylated dextran amine (BDA) anterograde labeling combined with horseradish peroxidase (HRP) retrograde transport in the rat. BDA was initially iontophoresed into Vme unilaterally and 7 days later HRP was injected into the anterior two-thirds of the ipsilateral tongue. After histochemical reactions, BDA anterogradely labeled boutons were seen to appose closely to somata and dendrites of HRP retrogradely labeled MNs in XII by light microscopy. A total of 212 BDA-labeled Vme boutons were examined ultrastructurally, which had an average diameter of 1.3 +/- 0.4 microm and contain small clear spherical vesicles. Eighty-eight percent of Vme boutons (187/212) synapsed on dendrites of HRP-labeled XII MNs. Twenty-five Vme boutons (25/212, 12%) made synapses with somata of HRP-labeled XII MNs. Thirty-five percent (74/212) of BDA-labeled Vme boutons were also contacted by unlabeled P-type terminals. Presynaptic P-type terminals contained spherical (47%, 35/74), pleomorphic (43%, 32/74), and flattened (10%, 7/74) synaptic vesicles. Thus, P-type terminals (as a presynaptic element), BDA-labeled Vme boutons, and XII MNs constitute axoaxodendritic and axoaxosomatic synaptic triads. There are four types of synaptic microcircuits in XII neuropil: synaptic convergence, synaptic divergence, presynaptic inhibition synaptic circuits, and feedforward regulation circuits. This detailed ultrastructure examination of the synaptic organization between Vme neurons and XII MNs provides insights into the synaptic mechanisms of the trigeminal proprioceptive afferents involved in the jaw-tongue reflex and coordination during oral motor behaviors.

Ultrastructural features of synapse from dorsal parvocellular reticular formation neurons to hypoglossal motoneurons of the rat

The dorsal parvocellular reticular formation (PCRt) receives projection of the trigeminal mesencephalic nucleus neurons. It contains the dorsal group of interneurons that integrate and coordinate activity of the oral motor nuclei. Ultrastructural features of synaptic connection from the dorsal PCRt neurons to the motoneurons of the hypoglossal nucleus (XII) were examined at both the light and electron microscopic levels in rats. Biotinylated dextran amine (BDA) was initially iontophoresed into the dorsal part of PCRt unilaterally. Seven days later horseradish peroxidase (HRP) was injected into the body of the tongue. After histochemical reaction for visualization of HRP and BDA, the BDA-labeled fibers and terminals were seen distributing bilaterally in XII with ipsilateral predominance. BDA-labeled terminals were closely apposed upon HRP retrogradely labeled somata and dendrites of the XII motoneurons. A total of 1408 BDA-labeled boutons were examined ultrastructurally, which had mean size of 1.22±0.37 μm in diameter. Five hundred-ninety three of these boutons in both the ipsilateral ( n=401) and contralateral ( n=192) XII were seen to synapse on both the dendrites and somata of HRP-labeled motoneurons. The vast majorities of synapses were axodendritic (98%, 580/593), while 2% of them were axosomatic. Of the 1408 BDA-labeled boutons, 69.6% of them were S-type boutons containing small clear and spherical synaptic vesicles and 30.4% of them were PF-type boutons containing pleomorphic and flattened synaptic vesicles. Approximately 64% of synapses between BDA-labeled boutons and HRP-labeled motoneurons were asymmetric, and 33% of synapses were symmetric. No axoaxodendritic or axoaxosomatic synaptic triad was observed. The present study illustrated the anatomical pathway and synaptological characteristics of neuronal connection between the dorsal PCRt premotor neurons and the XII motoneurons. Its functional significance in coordinating activity of XII motoneurons during oral motor behaviors has been discussed.

A neural network theory of proportional analogy-making

A neural network model that can simulate the learning of some simple proportional analogies is presented. These analogies include, for example, (a) red-square:red-circle∷yellow-square:?, (b) apple:red∷banana: ?, (c) a:b∷c:?. Underlying the development of this network is a theory for how the brain learns the nature of association between pairs of concepts. Traditional Hebbian learning of associations is necessary for this process but not sufficient. This is because it simply says, for example, that the concepts “apple” and “red” have been associated, but says nothing about the nature of this relationship. The types of context-dependent interlevel connections in the network suggest a semilocal type of learning that in some manner involves association among more than two nodes or neurons at once. Such connections have been called synaptic triads, and related to potential cell responses in the prefrontal cortex. Some additional types of connections are suggested by the problem of modeling analogies. These types of connections have not yet been verified by brain imaging, but the work herein suggests that they may occur and, possibly, be made and broken quickly in the course of working memory encoding. These working memory connections are referred to as differential, delayed and anti-Hebbian connections. In these connections, one can learn transitions such as “keep red the same”; “change red to yellow”; “turn off red”; “turn on yellow,” and so forth. Also, included in the network is a kind of weight transport so that, for example, red to red can be transported to a different instance of color, such as yellow to yellow. The network instantiation developed here, based on common connectionist building blocks such as associative learning, competition, and adaptive resonance, along with additional principles suggested by analogy data, is a step toward a theory of interactions among several brain areas to develop and learn meaningful relationships between concepts.