Round Vesicles Research Articles

Distribution and structure of identified tonic and phasic synapses between L-neurones in the locust ocellar tract.

The ultrastructures and distributions of the discrete anatomical synapses which constitute two distinct types of output connections made by individual ocellar L-neurons, L1-3, are described. Outputs to neurones L4-5 are excitatory and transmit tonically, whereas reciprocal connections among the three L1-3 neurones are inhibitory and incapable of transmission for longer than a few milliseconds. The tonically transmitting synapses are located in the lateral ocellar tract and are made between the axons of L1-3, which do not receive inputs, and short branches of L4-5, which make no outputs. Each excitatory connection is composed of a few hundred discrete anatomical synapses, each characterised by a bar-shaped presynaptic density which is 0.15-1.5 microns in length and associated with a large number of round synaptic vesicles. Two postsynaptic profiles are apposed to each presynaptic density. Associated with tonic synapses are abundant invaginations of the presynaptic membrane. Synapses of the reciprocal, inhibitory, phasic connections occur in the protocerebral arbors of L1-3, among numerous output synapses of these neurones. Each phasic connection is composed of a few tens of discrete anatomical synapses. Each bar-shaped presynaptic density is associated with two postsynaptic profiles, and is 0.1-1.0 microns long. Compared with the tonic, excitatory connection, there are fewer vesicles and fewer invaginations of the presynaptic membrane associated with each synapse.

Ultrastructural studies of the primate parabigeminal nucleus: electron microscopic autoradiographic analysis of the tectoparabigeminal projection in Galago crassicaudatus

The normal ultrastructure of the parabigeminal nucleus and the morphology and synaptic relationships of tectoparabigeminal terminals have been examined. Five different morphological types of terminals have been observed within the parabigeminal nucleus. Three of these profiles contain round vesicles and make asymmetrical synapses, while two contain pleomorphic vesicles and make symmetrical synapses. Electron microscopic autoradiographic data indicate that labeled tectoparabigeminal terminals represent only one of the three profiles containing round vesicles. Such terminals are primarily presynaptic to dendritic shafts, and several labeled profiles have been observed presynaptic to the same dendrite.

Distribution of the piriform cortical terminals to cells in the central segment of the mediodorsal thalamic nucleus of the rat

A Golgi electron microscopic study was undertaken to investigate the distribution of terminals from the piriform cortex that synapse on identified dendrites of neurons in the central segment of the mediodorsal thalamic nucleus of the rat. The piriform cortical terminals were identified as degenerating terminals following lesions in the cortex. They consisted of two types, i.e., large (LR type) and small (SR type) presynaptic terminals, both of which had round synaptic vesicles and formed asymmetric synaptic contacts. SR boutons terminated preferentially onto distal dendrites and never synapsed on primary dendrites. LR terminals synapsed preferentially on proximal dendrites, but were also found on more distal dendritic segments.

Ultrastructural development of the medial superior olive (MSO) in the ferret.

When ferrets are born, four weeks before the onset of hearing, few synapses are evident in the medial superior olive (MSO). The synapses present are immature and almost exclusively found in the neuropil. The MSO somata are virtually devoid of synaptic contacts but are contacted by fine glial processes that increasingly ensheathe the somata during the first postnatal week. By P12, somatic synaptogenesis in the MSO is evident. Initially the terminals contain vesicles of irregular shape, size, and distribution. The glial lamellae appear to withdraw as the synaptic contacts form but continue to cover the asynaptic portions of the cell surface. The lamellae frequently extend from ensheathing the soma to encapsulate the immature terminals. During the next two weeks, synaptic density and terminal encapsulation proceed until the somata is surrounded by encapsulated synaptic terminals as in the adult ferret MSO. While most immature terminals contain round vesicles, during the first postnatal week some terminals with nonround vesicles can be distinguished. The first distinction between types of nonround vesicle-containing terminals, i.e., pleiomorphic and ovoid, is in the second postnatal week. This distinction becomes increasingly clear and by the end of the first postnatal month, terminal types can be reliably categorized. These observations indicate that: (1) synapses are present in the MSO neuropil one month prior to the onset of hearing, (2) the major period of synaptogenesis begins approximately two weeks prior to the onset of hearing, and (3) glial lamellae ensheathe MSO somata prior to the onset of somatic synaptogenesis, withdraw as synapses form, and subsequently re-extend to encapsulate newly formed synapses.

Ultrastructural examination of enkephalin and substance P input to cholinergic neurons within the rat neostriatum

Enkephalin and substance P-containing inputs to cholinergic perikarya were examined in the rat neostriatum using an ultrastructural immunocytochemical double-labeling protocol. Sections of rat neostriatum were double-labeled for either choline acetyltransferase (ChAT) and substance P or ChAT and enkephalin using silver intensified colloidal gold and peroxidase as labels. Regions containing both ChAT-positive neurons and peroxidase reaction product were identified in the light microscope prior to sectioning for electron microscopy. Substance P-containing terminals which contained round synaptic vesicles and made symmetrical synaptic contacts were commonly observed in the neostriatum. Substance P synapses onto ChAT-positive perikarya and dendrites were frequently observed: up to 5 synaptic contacts were observed onto a ChAT-positive dendrite. Enkephalin labeling was also seen in a population of axon terminals containing round synaptic vesicles and exhibiting symmetrical synaptic specializations. In contrast to substance P-containing terminals, relatively few synaptic contacts were observed onto ChAT-positive labeled perikarya and dendrites although enkephalin-labeled terminals were seen in frequent contact with perikarya and dendrites of unlabeled spiny neurons. Since enkephalin and substance P are contained within different populations of striatal spiny neurons, the results of the present study suggest that these two types of neurons differ in their intrinsic striatal connections.

Enrichment of glutamate in zinc-containing terminals of the cat visual cortex.

The presence of glutamate and GABA was examined in zinc-containing terminals of the cat visual cortex using a post-embedding immunogold method. The surface density of immunogold-labelling was also evaluated in morphologically defined ultrastructural elements, namely terminals having round synaptic vesicles and making asymmetrical synapses (RA boutons), terminals with flat vesicles and symmetrical synapses (FS) and glial cell processes. Glutamate immunoreactivity was highest in RA terminals and in zinc-containing boutons. It was lower in FS terminals and lowest in glial cell processes. GABA immunoreactivity was highest in FS terminals and low in all other ultrastructural elements analysed, including zinc-containing terminals. Therefore, zinc-containing terminals show an enrichment of glutamate and they are likely to use this amino acid as their neurotransmitter. Moreover, the fact that many RA terminals that are negative for zinc show an enrichment of immunoreactive glutamate suggests that zinc-containing fibres represent a subpopulation of the glutamate axonal network.

Ultrastructural and immunocytochemical study of lamina II islet cells in rat spinal dorsal horn

In order to compare the ultrastructure of GABA-immunoreactive and nonimmunoreactive islet cells in lamina II of the rat dorsal horn, a combined ultrastructural and immunocytochemical study of nine Golgi-stained neurones was performed. Cell bodies of these neurones were tested with antiserum to GABA, and in most cases with antiserum to glycine, while parts of the cell body and dendritic tree were examined with the electron microscope. Four of the neurones had cell bodies that were immunoreactive with GABA antiserum, and 2 of these were also glycine-immunoreactive, while 2 were not. Cell bodies of the remaining five neurones were not immunoreactive with GABA antiserum, nor, in the 3 cases tested, with glycine antiserum. Three of the GABA-immunoreactive cells possessed vesicle-containing dendrites and were presynaptic at dendrodendritic synapses, whereas no vesicles were observed in the dendrites of any of the neurones that were not GABA-immunoreactive. The axon of one of the nonimmunoreactive cells was found with the electron microscope. It gave rise to boutons that contained round agranular vesicles and a few dense-cored vesicles. Three synapses formed by this axon were identified and all were asymmetric. No obvious differences were detected in the types of profile that were presynaptic to GABA-immunoreactive and nonimmunoreactive cells. These results suggest that GABAergic islet cells are a source of presynaptic dendrites in lamina II of the rat and that some presynaptic dendrites contain GABA and glycine, while others contain GABA without glycine. The nonimmunoreactive islet cells presumably represent a distinct functional class of neurones and some of these may release an excitatory amino acid transmitter, possibly in addition to one or more neuropeptides.

Cytoarchitectural organization of the electromotor system in the electric catfish (Malapterurus electricus)

The cytoarchitectural organization of the electromotor system of the electric catfish (Malapterurus electricus) was investigated in order to obtain insight into the neuronal reorganization accompanying the functional transition of a presumptive previous motor system to an electromotor system eliciting electric organ discharge. The electric catfish possesses two giant electromotoneurons situated within the rostral spinal cord. Intracellular dye injections have revealed the enormous extension of the dendritic tree of electromotoneurons. About 50 primary dendrites span the entire lateral funicle and intermediate grey matter, and reveal an extensive contralateral projection. The giant dendritic tree (1.2 mm in rostrocaudal direction) presumably receives inputs from all ascending and descending pathways of the spinal cord. Electromotoneurons and motoneurons receive the same type of fibre inputs, and electromotoneurons and interneurons are connected through common presynaptic elements. The innervation pattern of the electromotoneurons and spinal motoneurons is similar. Synaptic terminals with round synaptic vesicles often reveal chemical contacts and gap junctions. Furthermore, dendrites of the two electromotoneurons form juxtapositions (ephapses) with each other and also with spinal interneurons. Our results suggest that the two electromotoneurons are homologous to median (primary) spinal motoneurons and are the central structures of the electromotor system within the central nervous system of the electric catfish. A high capability of information processing can be attributed to the giant dendritic trees from functional considerations. This presumably enables the electromotoneurons to elicit an electric organ discharge in different behavioural contexts with a minimum of functional reorganization.

Glutamate-immunoreactive terminals synapse on primate spinothalamic tract cells.

Glutamate has been shown to excite spinothalamic tract (STT) neurons and has been localized to primary afferent neurons, spinal cord projection neurons, and interneurons in the spinal cord dorsal horn. The likelihood that glutamate-immunoreactive (GLU-IR) terminals directly innervate STT neurons was investigated. For these studies three lamina IV or V STT cells in the lumbar spinal cords of three monkeys (Macaca fascicularis) were identified electrophysiologically and characterized. Two were identified as high threshold neurons and one as a wide dynamic range neuron. Following intracellular injection of the cells with HRP and reaction to give the cells a Golgi-like appearance, the tissues were processed for electron microscopy. Postembedding immunogold methods with antibodies specific for glutamate were used to identify GLU-IR terminals apposing the somata and dendrites of the STT neurons, including dendrites that extended into laminae IV and III. The GLU-IR terminals were numerous and constituted a mean of 46% of the population counted that appose the STT soma and 50% of the profiles apposing the dendrites. Fifty-four percent of the somatic and 50% of the dendritic surface length was contacted by GLU-IR terminals. Most terminals contained round clear vesicles and some contained a variable number of large dense core vesicles. For one of the three cells examined it was determined that 45% of the terminals apposing the soma were GLU-IR and 30% of the terminals were gamma aminobutyric acid-immunoreactive (GABA-IR). In an additional monkey, a lamina I cell retrogradely labeled from the ventral posterolateral nucleus of the thalamus was found to be ensheathed in glial processes.(ABSTRACT TRUNCATED AT 250 WORDS)

Cholinergic and non-cholinergic neurons in the rat pedunculopontine tegmental nucleus.

Choline acetyltransferase immunhistochemistry was employed at light and electron microscopic levels in order to determine the distribution of cholinergic neurons in two subdivisions of the rat pedunculopontine tegmental nucleus that were previously defined on cytoarchitectonic grounds, and to compare the synaptic inputs to cholinergic and non-cholinergic somata in the subnucleus dissipatus, which receives major input from the substantia nigra. Large cholinergic neurons were found in both the pars compacta and the pars dissipata of the pedunculopontine nucleus. However, they were intermingled with non-cholinergic neurons and did not respect the cytoarchitectural boundaries of the nucleus. Ultrastructural study showed that all cholinergic neurons in the subnucleus dissipatus exhibited similar features. The majority had large somata (largest diameter greater than or equal to 20 microns) containing abundant cytoplasmic organelles and nuclei displaying a few shallow invaginations. Synaptic terminals on the cholinergic cell bodies were scarce and unlabeled boutons containing spherical synaptic vesicles and establishing asymmetric synaptic junctions were the dominant type. In contrast, the non-cholinergic neurons presented prominent differences in the size of their somata as well as in the distribution of axosomatic synapses. Two almost equally represented classes of non-cholinergic neurons which are referred to as large (largest diameter greater than or equal to 20 microns) and small (largest diameter less than 20 microns) were recognized. Large non-cholinergic cell bodies were ultrastructurally similar to the cholinergic ones, but they received rich synaptic input by unlabeled nerve terminals which contained pleomorphic vesicles and were engaged in symmetric synaptic junctions. Small non-cholinergic cell bodies were characterized by deeply invaginated nuclei surrounded by a narrow rim of cytoplasm, and were often found near or in direct apposition to the cholinergic somata. Their major input consisted of axosomatic boutons containing round synaptic vesicles. These results demonstrate that cells in the pedunculopontine tegmental nucleus are differentiated with regard to their axosomatic synaptic inputs which may influence their firing properties. Furthermore, they support previous suggestions that nigral afferents may be preferentially distributed to a subpopulation of the pedunculopontine neurons.

Evidence for GABAergic interneurons in the red nucleus of the painted turtle.

Immunocytochemical and electrophysiological evidence supporting the presence of GABAergic interneurons in the turtle red nucleus is presented. Injections of HRP into the spinal cord produced labeling of large neurons in the contralateral red nucleus. The peroxidase-antiperoxidase (PAP) method revealed smaller cells immunoreactive to an antibody against glutamate decarboxylase (GAD), the synthetic enzyme for the inhibitory neurotransmitter GABA, that were interspersed among larger immunonegative neurons. Similar small neurons were densely immunostained by antibodies to GABA-glutaraldehyde conjugates obtained from different sources and applied according to pre-embedding and postembedding protocols. Rubrospinal neurons retrogradely labeled with HRP measured 16 and 27 microns in mean minor and major cell body diameters, while GABA-like immunopositive neurons situated within the red nucleus measured 7 and 13 microns. There was very little overlap in soma size between the two cell populations. Therefore, we suggest that the GAD- and GABA-positive neurons may be local inhibitory interneurons. This notion is further supported by observations of pre-embedding immunostaining for GAD and postembedding immunostaining for GABA showing that the turtle red nucleus is amply innervated by immunoreactive axon terminals. These puncta are closely apposed to cell bodies and dendrites of both immunonegative large neurons and immunopositive small neurons. Moreover, immunogold staining at the electron microscopic level demonstrated that GABA-like immunoreactive axon terminals with pleomorphic synaptic vesicles formed symmetric synapses with cell bodies and dendrites of the two types of red nucleus cells. These ultrastructural features are commonly assumed to indicate inhibitory synapses. A moderately labeled bouton with round vesicles and asymmetric synapses was also observed. In addition, the two types of red nucleus neurons received asymmetric axosomatic and axodendritic synapses with GABA-negative boutons provided with round vesicles, features usually associated with excitatory functions. To obtain electrophysiological evidence for inhibition, intracellular recordings from red nucleus neurons were conducted using an in vitro brainstem-cerebellum preparation from the turtle. Small, spontaneous IPSPs were recorded from 7 out of 14 red nucleus cells studied. These morphological and physiological results provide strong support for concluding that the turtle red nucleus, like its mammalian counterpart, contains GABAergic inhibitory interneurons. While we have not identified the main source of input to these interneurons, in view of the scarce development of the reptilian cerebral cortex, this input is unlikely to come from the motor cortex as it does in mammals.(ABSTRACT TRUNCATED AT 400 WORDS)

Electron microscopic identification of lamina I axon terminations in the nucleus submedius of the cat thalamus

Ascending lamina I axons were labeled with Phaseolus vulgaris leucoagglutinin and the synaptic connections of their terminals in nucleus submedius ( Sm) were studied in the electron microscope. The terminals were large, contained rounded synaptic vesicles, and were involved in complex synaptic aggregations with pre- and postsynaptic dendrites. It was observed that clustered large boutons from a single axon could contact a single dendritic shaft. These observations support a sensory role for lamina I input to Sm.

Prefrontal cortical efferents in the rat synapse on unlabeled neuronal targets of catecholamine terminals in the nucleus accumbens septi and on dopamine neurons in the ventral tegmental area.

Physiological and pharmacological studies indicate that descending projections from the prefrontal cortex modulate dopaminergic transmission in the nucleus accumbens septi and ventral tegmental area. We investigated the ultrastructural bases for these interactions in rat by examining the synaptic associations between prefrontal cortical terminals labeled with anterograde markers (lesion-induced degeneration or transport of Phaseolus vulgaris leucoagglutinin; PHA-L) and neuronal processes containing immunoreactivity for the catecholamine synthesizing enzyme, tyrosine hydroxylase. Prefrontal cortical terminals in the nucleus accumbens and ventral tegmental area contained clear, round vesicles and formed primarily asymmetric synapses on spines or small dendrites. In the ventral tegmental area, these terminals also formed asymmetric synapses on large dendrites and a few symmetric axodendritic synapses. In the nucleus accumbens septi, degenerating prefrontal cortical terminals synapsed on spiny dendrites which received convergent input from terminals containing peroxidase immunoreactivity for tyrosine hydroxylase, or from unlabeled terminals. In single sections, some tyrosine hydroxylase-labeled terminals formed thin and punctate symmetric synapses with dendritic shafts, or the heads and necks of spines. Close appositions, but not axo-axonic synapses, were frequently observed between degenerating prefrontal cortical afferents and tyrosine hydroxylase-labeled or unlabeled terminals. In the ventral tegmental area, prefrontal cortical terminals labeled with immunoperoxidase for PHA-L were in synaptic contact with dendrites containing immunogold reaction product for tyrosine hydroxylase, or with unlabeled dendrites. These results suggest that: (1) catecholaminergic (mainly dopaminergic) and prefrontal cortical terminals in the nucleus accumbens septi dually synapse on common spiny neurons; and (2) dopaminergic neurons in the ventral tegmental area receive monosynaptic input from prefrontal cortical afferents. This study provides the first ultrastructural basis for multiple sites of cellular interaction between prefrontal cortical efferents and mesolimbic dopaminergic neurons.

Immunocytochemical evidence for in vitro release of glutamate and GABA from separate nerve terminal populations in the rat pontine nuclei.

A quantitative electron microscopic immunocytochemical method was used to study the synaptic handling of glutamate and GABA in slice preparations from the rat pontine nuclei. Slices were subjected to a depolarizing stimulus (55 mM K+, 20 min) in the presence of a physiological or low Ca(2+)-concentration. Depolarization at physiological [Ca2+] evoked a depletion of glutamate-like immunoreactivity from nerve terminals that contain round vesicles and establish asymmetric synaptic contacts. When depolarization was induced in the presence of only 0.1 mM Ca2+ (10 mM Mg2+ added), the loss of glutamate was significantly reduced or abolished, indicative of a Ca(2+)-dependent component of glutamate release. By means of a double-labeling immunocytochemical method we could identify a population of nerve terminals that displayed strong GABA-like immunoreactivity, and a level of glutamate-like immunoreactivity that was low but yet clearly above background level. This type of terminal contains elongated or pleomorphic vesicles and establishes symmetric synaptic contacts. In these terminals, depolarization evoked a Ca(2+)-dependent depletion of GABA-like immunoreactivity, but failed to change the level of glutamate-like immunoreactivity. The present study demonstrates that two different types of nerve terminal in the rat pontine nuclei contain releasable pools of glutamate and GABA, respectively, and that the GABA-releasing terminals also contain a non-releasable pool of glutamate. The glutamate of the latter pool could act as precursor of GABA.

Ultrastructural studies of retinal, visual cortical (area 17), and parabigeminal terminals within the superior colliculus of Galago crassicaudatus.

The morphology and synaptic relationships of anterogradely labeled retinal, visual cortical (area 17), and parabigeminal terminals have been analyzed within the superficial gray (stratum griseum superficiale) of Galago crassicaudatus. Our data regarding the retinocollicular projection reveal two populations of terminals based upon size. The population of smaller terminals are found in clusters, while the larger occur in isolation. Both populations of retinocollicular terminals form synapses primarily with dendritic spines, but synapses upon pale vesicle filled (PVF) profiles and dendritic shafts also occur. Corticotectal terminals contain round vesicles and make asymmetrical synapses, primarily onto dendritic spines; few form synapses with PVF profiles. Our findings suggest the possibility that there are two populations of corticotectal terminals based upon differences in size and morphology. Parabigeminotectal profiles contain densely packed round vesicles and make asymmetrical synapses. These terminals, which are exclusively cholinergic in Galago, are presynaptic to dendrites of various sizes. Convergence of retinal and cortical terminals has been observed. This convergence occurs on distinctly separate regions of the postsynaptic membrane. In contrast, convergence of retinal and parabigeminal terminals occurs on the same region of the postsynaptic cell(s).

Ultrastructural study of ascending projections to the lateral mammillary nucleus of the rat.

We examined the synaptic organization of ascending projections from the pars ventralis of the dorsal tegmental nucleus of Gudden (TDV) and the laterodorsal tegmental nucleus to the lateral mammillary nucleus (LM). The LM neuropil consists of terminals containing pleomorphic synaptic vesicles and forming symmetric synaptic contact, and terminals containing round synaptic vesicles and forming asymmetric synaptic contact. They make up 63% and 37%, respectively, of all axodendritic terminals. All axosomatic terminals contain pleomorphic vesicles and make symmetric contact. Following injection of WGA-HRP into the TDV, many anterogradely labeled terminals and retrogradely labeled cells are found in the LM. Labeled terminals contact mainly proximal (more than 2 microns diameter) and intermediate (1-2 microns diameter) dendrites. Serial ultrathin sections of the LM show that 55% of axosomatic terminals are labeled anterogradely. Following injection of WGA-HRP into the laterodorsal tegmental nucleus, many anterogradely labeled terminals are found in the LM, but no retrogradely labeled cells are present. Labeled terminals contact mainly distal (less than 1 micron diameter) and intermediate dendrites as well as somata. In the LM neurons, 46% of axosomatic terminals are labeled anterogradely. All labeled terminals from these nuclei contain pleomorphic vesicles and make symmetric synaptic contact. These results indicate that almost all axosomatic terminals come from the TDV and the laterodorsal tegmental nucleus, which send inhibitory inputs to the lateral mammillary nucleus.

Fine distribution of substance P-like immunoreactivity in the dorsal nucleus of the vagus nerve in cats

The ultrastructure of substance P (SP)-immunoreactive elements in the cat dorsal motor nucleus of the vagus nerve was examined using pre-and post-embedding immunocytochemical procedures. Substance P-like immunoreactivity was observed in axon terminals and axon fibres which were mostly unmyelinated. Quantitative data showed that at least 16% of axon terminals contained SP. Their mean diameter was larger than that of their non-immunoreactive counterparts. Most (83%) SP-containing terminals were seen to contact dendrites but some were observed adjoining soma or entirely embedded in the cytoplasm of vagal neurons (4.5%). Only 0.5% were observed to contact soma of interneurons. A few immunoreactive axon terminals (4%) were observed in contact with non-immunoreactive axon terminals. Round agranular vesicles and numerous dense core vesicles were visible in most SP-containing axon terminals (84.6%). The immunogold procedure showed the preferential subcellular location of SP to be dense core vesicles. In 32.4% of cases, SP-containing terminals were involved in synaptic contacts that were generally of the asymmetrical Gray type 1 and mainly apposed dendrites. The theoretical total of synaptic contacts was 74.5% and this suggests the existence of weak non-synaptic SP innervation involving approximately 25% of SP-containing axon terminals. No axo-axonic synapses were observed in the dorsal vagal nucleus. These results support the hypothesis that SP found in the dorsal vagal nucleus originates partly from vagal afferents and is involved in direct modulation of visceral functions mediated by vagal preganglionic neurons.

Morphology of intracellularly stained spiny neurons in rat striatal grafts

Two to six months after implantation of fetal striatal primordia into the kainic acid-lesioned neostriatum of adult rats, spiny neurons in the grafts were stained intracellularly with biocytin. To determine whether the spiny neurons in the grafts differentiate morphologically as in the host neostriatum, the intracellularly stained spiny neurons in the grafts were studied with light and electron microscopy and compared with that of spiny neurons in the host neostriatum. The spiny neurons in the grafts had ovoid or polygonal cell bodies with dendrites radiating in all directions. The somata were smooth and the dendrites, except for their most proximal portions, were rich in spines. All these features resembled the appearance of spiny neurons in the intact neostriatum. However, quantitative studies showed that the somata of spiny neurons in the grafts were larger than those in the host neostriatum (projected cross-sectional areas of230 ± 64.6μm 2 in the grafts and158 ± 28.9μm 2 in the host) and the spine density of graft neurons was lower than that of host neurons. Cells near the border of the grafts had dendrites extending both into the graft and into the host neostriatum. In these cells, the dendrites in the grafts had fewer spines than the dendrites in the host tissue. The axons of spiny neurons in the grafts had very large and dense intrastriatal collateral arborizations, which occupied a much larger volume than that of the dendritic domain of the parent cells. The local axonal arborizations of each of these cells filled almost the entire graft. In some cells, axonal branches were traced outside the grafts and were seen to enter the internal capsule fascicles. Unlike spiny neurons in the normal adult neostriatum, the spiny cells of the graft could have nuclear indentations. With this exception, the ultrastructural features of spiny neurons in the grafts were very similar to those in the hosts. Many unlabeled boutons made synapses on identified spiny neurons in the grafts. Terminals with small round vesicles made synaptic contacts on dendritic shafts and dendritic spines, while terminals with flattened or pleomorphic vesicles contacted somata, dendrites, and dendritic spines. Labeled axon collaterals of graft neurons made symmetrical synapses on somata, dendrites and spines in the grafts and in the host neostriatum. In the grafts, more than 60% of the axon terminals contacted dendritic shafts. The proportion of axosomatic and axospinous synapses varied substantially from cell to cell. These results show that the spiny neurons in striatal grafts possess the general somatodendritic features of spiny neurons in the adult neostriatum, except for their lower dendritic spine density. The unusually dense intrastriatal collateral arborizations of spiny neurons may account for the unusually strong inhibitory responses exhibited by these neurons.

Chapter 11 The organization of GABAergic neurons in the mammalian superior colliculus

GABA is an important inhibitory neurotransmitter in the mammalian superior colliculus. As in the lateral geniculate nucleus, GABA immunoreactive neurons in SC are almost all small and are distributed throughout the structure in all mammalian species studied to date. Unlike the LGN, GABA-labeled neurons in SC have a variety of morphologies. These cells have been best characterized in cat, where horizontal and two granule cell morphologies have been identified. Horizontal cells give rise to one class of presynaptic dendrite while granule C cells give rise to another class of spine-like presynaptic dendrite. Granule A cells may be the origin of some GABAergic axon terminals. GABA containing synaptic profiles form serial synapses, providing a possible substrate for disinhibition. The distribution of GABAA and GABAB receptor subtypes appears similar to that of GABA neurons, with the densest distribution found within the superficial gray layer. However, antibody immunocytochemistry of the beta 2 and beta 3 subunits of the GABAA receptor reveals that it is located at both synaptic and non-synaptic sites, and may be associated with membrane adjacent to terminals with either flattened or round vesicles. A few GABA containing neurons in SC colocalize the pentapeptide leucine enkephalin or the calcium binding protein calbindin. However, none appear to co-localize parvalbumin, a situation different from GABA containing interneurons in the LGN and visual cortex. The diversity of GABA neurons in SC rivals that found in visual cortex, although unlike visual cortex, the pattern of co-occurrence does not distinguish GABA cell types in SC. The superior colliculus also differs from both LGN and visual cortex in that GABA and calbindin immunoreactivity is not altered by either long-term occlusion and/or short-term enucleation in adult Rhesus monkeys. No consistent differences have been found in the optical density of GABA labeling in either cells or neuropil. To conclude, GABA neurons in the superior colliculus share some properties like those in LGN and others like those in visual cortex. In other properties, they differ from GABA neurons in both the LGN and visual cortex. The GABA systems in the superior colliculus are similar in all mammalian species studied, suggesting that they are phylogenetically conserved systems which are not amenable to plastic alterations, a situation different to that in the geniculostriate system.

Fine structure of GABA-labeled axonal endings in the inferior colliculus of the cat: Immunocytochemistry on deplasticized ultrathin sections

Antisera to GABA conjugates and postembedding techniques were used to identify GABA-containing axonal endings at the electron microscopic level in the inferior colliculus. Over 90% of the GABA-labeled axonal endings had a similar morphology. They contained pleomorphic synaptic vesicles and made symmetrical synapses. The exceptional endings contained round vesicles and made symmetrical synaptic contacts or had pleomorphic vesicles with asymmetrical contacts. The majority of GABA-labeled axonal endings synapsed on dendrites; however, a few labeled axosomatic synapses were also found. Potential sources for these GABAergic synapses are neurons intrinsic to the inferior colliculus or from the dorsal nucleus of the lateral lemniscus. These findings suggest a basic similarity for most GABAergic endings in the inferior colliculus despite their possible origin from different cell types.