Plasticity of GABA- and glutamate-containing terminals in the mouse thalamic ventrobasal complex deprived of vibrissal afferents: an immunogold-electron microscopic study

Glutamate, but not aspartate, is enriched in trigeminothalamic tract terminals and associated with their synaptic vesicles in the rat nucleus submedius

To examine the possible roles of glutamate and aspartate as neurotransmitters in the nucleus submedius (Sm) of rats, the distributions of these amino acids were examined by electron microscopic immunogold labeling. High levels of glutamate were detected in trigeminothalamic tract terminals anterogradely labeled with horseradish peroxidase conjugates. These terminals also displayed a positive correlation between the densities of synaptic vesicles and gold particles signaling glutamate. In contrast, aspartate levels in such terminals were low and displayed no correlation with the density of synaptic vesicles. Terminals of presumed cortical origin contained the highest estimated levels of glutamate, but the positive correlation between glutamate signal and synaptic vesicle density did not reach statistical significance, presumably due to technical factors. The latter terminals also contained relatively high levels of aspartate, though without any correlation to synaptic vesicle density. The present findings provide strong support for glutamate, but not aspartate, as a trigeminothalamic tract neurotransmitter responsible for the fast synaptic transmission of nociceptive signals to neurons in the rat nucleus submedius. Aspartate presumably serves metabolic roles in these terminals. With respect to terminals of presumed cortical origin, our data are not at odds with the notion that also these terminals use glutamate as their neurotransmitter. Our findings do not support a neurotransmitter role for aspartate in the latter terminals, although such a role cannot be entirely refuted.

Corticothalamic inhibition in the thalamic reticular nucleus

Mutual inhibition between the GABAergic cells of the thalamic reticular nucleus (RTN) is important in regulating oscillations in the thalamocortical network, promoting those in the spindle range of frequencies over those at lower frequencies. Excitatory inputs to the RTN from the cerebral cortex are numerically large and particularly powerful in inducing spindles. However, the extent to which corticothalamic influences can engage the inhibitory network of the RTN has not been fully explored. Focal electrical stimulation of layer VI in the barrel cortex of the mouse thalamocortical slice in vitro resulted in prominent di- or polysynaptic inhibitory postsynaptic currents (IPSCs) in RTN cells under the experimental conditions used. The majority of cortically induced responses consisted of mixed PSCs in which the inhibitory component predominated or of large IPSCs alone, implying inhibition of neighboring cells by other, cortically excited RTN cells. Within the mixed PSCs, fixed and variable latency components could commonly be identified. IPSCs could be blocked by application of ionotropic glutamate receptor antagonists or of GABA(A) receptor antagonists, also indicating their dependence on corticothalamic excitation triggering disynaptic or polysynaptic inhibition. Spontaneous GABA(A) receptor-dependent IPSCs were routinely observed in the RTN and, taken together with the results of cortical stimulation, indicate the existence of a substantial network of intrareticular inhibitory connections that can be effectively recruited by the corticothalamic system. These results suggest activation of cortical excitatory inputs triggers the propagation of inhibitory currents within the RTN and support the view that activation of the RTN from the somatosensory cortex, although focused by the topography of the corticothalamic projection, is capable of disynaptically engaging the whole inhibitory network of the RTN, by local and probably by reentrant GABA(A) receptor-based synapses, thus spreading the corticothalamic influence throughout the RTN.

The light and electron microscopic characterisation of identified striato-ventrotegmental projection neurons in the domestic chick (Gallus domesticus)

A major projection of the medial striatum (lobus parolfactorius, LPO) of birds is the striato-ventrotegmental pathway projecting to the area ventralis tegmentalis. In the present study, we investigated the morphology and connectivity of striato-ventrotegmental neurons in the medial LPO. The neurons were identified by injecting the fluorescent retrograde tracer fast blue (FB) into the area ventralis tegmentalis. FB-labelled neurons in the LPO were targeted and iontophoretically injected with lucifer yellow (LY) in paraformaldehyde fixed slices. The fluorescent LY label in the filled neurons was then photoconverted, and the ultrastructure of cells was investigated. According to our results, the soma of striato-ventrotegmental neurons is rich in organelles, in particular rough and smooth endoplasmic reticula and they possess a large, unindented and slightly eccentric nucleus. The LY-labelled cells possess relatively few, sparsely spiny dendrites, and represent a type of medium-sized spiny projection neuron characteristic of the striata of birds. Axospinous synapses on the labelled cells are asymmetric and correspond morphologically to the glutamatergic excitatory type of terminals described previously in the LPO. Both symmetric and asymmetric axodendritic and axosomatic synapses were detected. Some symmetric synapses were GABA immunolabelled, whereas some asymmetric synapses were immunopositive to glutamate. Axon collaterals of labelled cells formed symmetric or asymmetric axodendritic synapses. Direct contact without interposing glial processes was observed between one of the FB-labelled neurons and an adjacent neuronal soma. There was also a microneuron attached to one of the labelled cells, which we identified as a neurogliaform 'dwarf' cell.

Nonsynaptic noradrenaline release in neuro-immune responses

Evidence has recently been obtained that the branches of the autonomic nervous system, mainly, the sympathetic [25], regulate cytokine production. Not only the primary (thymus, bone marrow) and secondary (spleen, tonsils, and lymph nodes) lymphoid organs, but also many other tissues are involved in immune responses and are heavily influenced by noradrenaline (NA) derived from varicose axon terminals of the sympathetic nervous system [25, 100]. Besides NA released from nonsynaptic varicosities of noradrenergic terminals [92], circulating catecholamines (adrenaline, dopamine, NA) are also able to influence immune responses, the production of pro- and anti-inflammatory cytokines by different immune cells. The sympathetic nervous system (catecholamines) and the hypothalamic-pituitary-adrenal (HPA) axis (cortisol) are the major integrative and regulatory components of different immune responses. In our laboratory convincing evidence has been obtained that NA released non-synaptically [90, 92] from sympathetic axon terminals and enhanced in concentration in the close proximity of immune cells is able to inhibit production of proinflammatory (TNF-alpha, IFN-gamma, IL-12, IL-1) and increase antiinflammatory cytokines (IL-10) in response to LPS [25, 91], indicating a fine-tuning control of the production of TNF-alpha and other cytokines by sympathetic innervation under stressful conditions. This effects are mediated via beta2-adrenoceptors expressed on immune cells and coupled to cAMP levels.

Evidence for a role of GABA interneurones in the cortical modulation of midbrain 5-hydroxytryptamine neurones

Recent electrophysiological studies demonstrate that the ventral medial prefrontal cortex has a powerful inhibitory influence on 5-hydroxytryptamine (5-HT) neurones in the dorsal raphe nucleus. Here we utilised a combination of anatomical and electrophysiological methods to characterise the cellular substrate underlying this effect.Anterograde tracing (Phaseolus vulgaris leucoagglutinin) using electron microscopy demonstrated a pathway from the ventral medial prefrontal cortex that makes neuronal contacts throughout the dorsal raphe nucleus. These contacts were predominantly asymmetrical synapses adjoining GABA immunoreactive dendrites and spines. In vivo extracellular recordings were made in the dorsal raphe nucleus of the anaesthetised rat from a subpopulation of non-5-HT neurones. These neurones were fast-firing, irregular and with short spike width, properties strongly reminiscent of immunochemically identified GABA interneurones in other brain regions. Recordings of classical 5-HT neurones were also included. Electrical stimulation of the ventral medial prefrontal cortex elicited a rapid onset (16 ms latency), orthodromic excitation of the non-5-HT neurones (13/25 neurones). This stimulation also caused a pronounced inhibition of most 5-HT neurones tested, with a longer latency (30 ms), and this was partially blocked by locally applied bicuculline. These data provide the first evidence that the ventral medial prefrontal cortex influences the activity of large numbers of raphe 5-HT neurones by targeting a local network of GABA neurones. This circuitry predicts that physiological and pathological changes in the ventral medial prefrontal cortex will impact on significant parts of the forebrain 5-HT system.

Effects of prenatal exposure to ethanol on systems matching: the number of neurons in the ventrobasal thalamic nucleus of the mature rat

Following prenatal exposure to ethanol, rats have a 1/3 fewer neurons in the second order (principal sensory nucleus of the trigeminal nerve) and fourth order neurons (somatosensory cortex) of the trigeminal-somatosensory pathway than do controls. Based on the numerical matching hypothesis, we predict that the number of third-order neurons (in the ventrobasal nucleus of the thalamus; VB) also will show a similar effect of prenatal ethanol exposure. Stereological methods were used to determine the total number of neurons in the VB on postnatal day 30. Surprisingly, prenatal exposure to ethanol had no effect on the VB volume or on the number of VB neurons. Thus, prenatal exposure to ethanol induces numerical imbalances within the trigeminal-somatosensory system.

An in vitro electrophysiological and Co2+-uptake study on the effect of infraorbital nerve transection on the cortical and thalamic neuronal activity

Changes of neuronal membrane characteristics in somatosensory barrel cortex and barreloid thalamus were investigated in rats following unilateral transection of the infraorbital nerve. Kainate induced Co2+-uptake method and image analysis were used to assess the Ca2+ permeability of non-NMDA (N-methyl-D-aspartate) glutamate receptors. Changes in some biophysical parameters of the affected cortical neurons were also investigated by intracellular recording in slice experiments. The altered neuronal activity was measured on days 1, 5 and 14 after surgery. Kainate induced Co2+ uptake increased markedly reflecting enhanced Ca2+ permeability of alpha-amino-3-hydroxy-5-methyl-isoxazole-4-propionate/kainate (AMPA/KAIN)-type receptors. Changes were more pronounced in the cortex than in the thalamus and peaked on the first day following nerve transection. After that, parameters gradually returned to the normal level. However, a small enhancement was still detectable in the cortex at the end of the 2-week-long observation period. In parallel with the increased Co2+-uptake, moderate membrane potential changes, stronger spiking activity and enhanced excitability were characteristic for cortical neurons. The observed alterations in neuronal characteristics underlie the reorganization and regeneration processes following injuries or surgeries. We can conclude that immediate change of the receptive field in the barrel cortex following unilateral nerve transection is based on changes in biophysical parameters of the neurons. Altered peripheral activation evokes changes in the neuronal activity, thus providing opportunity for a quick synaptic rearrangement. AMPA/KAIN-type glutamate receptors have a decisive role in the regulation of these processes. This kind of synaptic plasticity is more significant in the cortex than in the thalamus.

Striato-telencephalic and striato-tegmental circuits: relevance to learning in domestic chicks

Memory formation for a passive avoidance task in the domestic chick is likely to involve a hyperstriatum ventrale (IMHV)-archistriatum-lobus parolfactorius (LPO) arc. The present study summarises previous findings, relevant to this neural system, and is also supplemented with some recent data from our laboratory. Projections from the IMHV on the archistriatum, as well as from the archistriatum on the LPO, have been characterised using a combination of anterograde pathway tracing (Phaseolus lectin), and post-embedding GABA and glutamate immunocytochemistry. The majority of IMHV efferents have been found to synapse with dendritic spine heads and necks of densely spiny projection neurons of the ventral archistriatum, and the ultrastructure of synapses suggested a potent excitatory input. Similar synaptic connections of the excitatory type were ultrastructurally verified between ventral archistriatal afferent terminals and dendrites or spines of the LPO, suggesting an involvement of the medium sized spiny neurons, which are typical of the striatum. Although some of the IMHV boutons terminating in the archistriatum were immunoreactive to glutamate, this was not observed in the archistriatal-LPO pathway. Tegmental connections of the basal ganglia, in particular LPO, are also likely to play a role in processing of the avoidance response. We have demonstrated reciprocal connections between the LPO and dopaminergic (TH-positive) neurons of the substantia nigra and ventral tegmentum. Dopamine D1 receptors were upregulated bilaterally in the LPO following avoidance learning and this response was not accompanied by significant changes in the level of dopamine or its metabolites (HVA, DOPAC), as revealed by HPLC chromatography of brain samples dissected from the LPO of control and trained chicks. The dopamine receptor-related phosphoprotein DARPP-32 was localised in dendritic elements of the LPO, often forming asymmetric synapses with glutamate immunoreactive axon terminals. The findings are consistent with a scenario in which the striatum acts as a suppressor of natural pecking behaviour. Learned visual association with the target (bead) occurs in the IMHV and is relayed to the basal ganglia via the limbic archistriatum (amygdala equivalent), the latter introducing a motivational element (aversion, fear). Suppression of a brainstem pecking centre is likely to involve activation of the nigrostriatal (tegmentostriatal) dopaminergic circuit.

Hippocampal synaptic density and glutamate immunoreactivity following transient cerebral ischaemia in the chick

A transient ischaemic episode of 10 min duration was induced in 1-day-old chicks. After a 1-week survival period, synapse density was assessed in the ventral hippocampus using the 'disector' technique. A significant decrease was observed in asymmetric synapses, markedly greater than that observed previously in the dorsal hippocampus. Because the effect occurred mainly on excitatory synapses, the distribution of glutamate in the ventral hippocampus was also assessed by a postembedding immunogold labelling technique. The density of gold particles was significantly greater in both boutons and neuropil in the ischaemic group compared to controls, lending support to the theory of excitotoxicity as an explanation for ischaemic neural degeneration.

Brainstem inputs to the ferret medial geniculate nucleus and the effect of early deafferentation on novel retinal projections to the auditory thalamus

Following specific neonatal brain lesions in rodents and ferrets, retinal axons have been induced to innervate the medial geniculate nucleus (MGN). Previous studies have suggested that reduction of normal retinal targets along with deafferentation of the MGN are two concurrent factors required for the induction of novel retino-MGN projections. We have examined, in ferrets, the relative influence of these two factors on the extent of the novel retinal projection. We first characterized the inputs to the normal MGN, and the most effective combination of neonatal lesions to deafferent this nucleus, by injecting retrograde tracers into the MGN of normal and neonatally operated adult ferrets, respectively. In a second group of experiments, newborn ferrets received different combinations of lesions of normal retinal targets and MGN afferents. The resulting extent of retino-MGN projections was estimated for each case at adulthood, by using intraocular injections of anterograde tracers. We found that the extent of retino-MGN projections correlates well with the extent of MGN deafferentation, but not with extent of removal of normal retinal targets. Indeed, the presence of at least some normal retinal targets seems necessary for the formation of retino-MGN connections. The diameters of retino-MGN axons suggest that more than one type of retinal ganglion cells innervate the MGN under a lesion paradigm that spares the visual cortex and lateral geniculate nucleus. We also found that, after extensive deafferentation of MGN, other axonal systems in addition to retinal axons project ectopically to the MGN. These data are consistent with the idea that ectopic retino-MGN projections develop by sprouting of axon collaterals in response to signals arising from the deafferented nucleus, and that these axons compete with other sets of axons for terminal space in the MGN.

Plastic changes in glycine and GABA release and uptake in adult brain stem auditory nuclei after unilateral middle ear ossicle removal and cochlear ablation

[i] In young adult guinea pigs, the effects of unilateral ossicle removal and unilateral cochlear ablation were determined on [14C]glycine or [14C]GABA release and uptake measured in subdivisions of the cochlear nucleus (CN), the superior olivary complex, and the auditory midbrain, after 2 or 5, 59, and 145 postlesion days. Activities were compared to those of age-matched, unlesioned controls. [ii] [14C]Glycine release declined bilaterally in the anteroventral and dorsal CN after ossicle removal and in the dorsal CN after cochlear ablation. [iii] Transient elevations of release occurred at 59 days in the ipsilateral posteroventral CN ([14C]glycine) and bilaterally in the ventral nucleus of the lateral lemniscus ([14C]GABA) after ossicle removal, and bilaterally in the medial superior olive ([14C]glycine) after cochlear ablation. [iv] In the medial nucleus of the trapezoid body, [14C]GABA release was depressed bilaterally 5 days after ossicle removal, but was elevated at 145 days contralaterally after ossicle removal and ipsilaterally after cochlear ablation. [v] In the contralateral central nucleus of the inferior colliculus, [14C]GABA release was elevated persistently after ossicle removal. After cochlear ablation, release was elevated at 5 days, near the control at 59 days, and elevated again at 145 days. [vi] After both lesions, [14C]glycine uptake was elevated bilaterally in the CN and medial superior olive. [14C]GABA uptake became depressed by 59 or 145 days bilaterally in the auditory midbrain. [vii] These changes may stem from regulation and may contribute to mechanisms that generate symptoms such as loudness recruitment and tinnitus, which often accompany hearing loss.

Physiology and pharmacology of corticothalamic stimulation-evoked responses in rat somatosensory thalamic neurons in vitro

Whole cell current- and voltage-clamp recording techniques were employed in a rat thalamocortical slice preparation to characterize corticothalamic stimulation-evoked responses in thalamic neurons. Three types of corticothalamic stimulation-evoked responses were observed in thalamic neurons. Of thalamic neurons, 57% responded to corticothalamic stimulation with purely excitatory synaptic responses, whereas 27% had inhibitory synaptic responses and 16% had mixed excitatory/inhibitory responses. This suggested corticothalamic activation of multiple distinct synaptic circuits, presumably involving both nucleus reticularis thalami (NRT) and thalamus, because the rat ventrobasal complex is virtually devoid of GABAergic interneurons. Corticothalamic-stimulation-evoked excitatory postsynaptic currents (EPSCs) were predominantly slow rising currents that showed nonlinear voltage dependence, characteristics of an N-methyl-D-aspartate (NMDA)-receptor-mediated synaptic current. These slow rising EPSCs were blocked by the NMDA antagonist 2-amino-5-phosphonovaleric acid (APV). A minority of corticothalamic EPSCs had faster kinetics, and were blocked by 6-cyano-7 nitroquinoxaline-2,3-dione (CNQX). Corticothalamic stimulation of varying frequency optimally activated burst responses in thalamic neurons at low frequencies (3-6 Hz). The optimal 3- to 6-Hz response was reduced by ethosuximide, by APV, and by detaching the neocortex from the thalamocortical slice, suggesting that T current, NMDA receptors, and neocortical properties all contributed to generation of this 3- to 6-Hz frequency preference. In contrast to corticothalamic EPSCs, medial-thalamic-stimulation-evoked responses consisted of fast CNQX-sensitive EPSCs that were predominantly voltage insensitive, with no 3- to 6-Hz frequency preference. In thalamic neurons in which corticothalamic stimulation evoked predominantly inhibitory synaptic responses, this inhibitory postsynaptic potential (IPSP) had early and late phases, often followed by a rebound burst. The early IPSP reversed at -95 mV and was bicuculline sensitive, whereas the late IPSP reversed at -113 mV and was blocked by the gamma-aminobutyric acid-B (GABA(B)) antagonist 3-N[1-(S)-(3,4-dichlorophenyl)ethyl]amino-2-(S)-hydroxypropyl-P-benzy lphoshinic acid (CGP-55845A). In thalamic neurons in which corticothalamic stimulation evoked a mixed excitatory postsynaptic potential (EPSP)/IPSP response, repetitive corticothalamic stimulation rapidly reduced IPSPs and enhanced EPSPs at higher frequencies. This resulted in burst firing being triggered in these mixed response neurons at frequencies >6 Hz. Corticothalamic feedback onto thalamic relay neurons activated diverse responses due to differing relative activation of NRT and "feedforward" inhibitory responses. These multiple in vitro corticothalamic responses differ from responses encountered in other in vitro thalamic preparations lacking a synaptically connected neocortex, but are similar to results evident in thalamic neurons in response to cortical stimulation in vivo. In addition, the thalamocortical 3- to 6-Hz frequency preference was conserved, suggesting that many factors critical for this emergent property of the thalamocortical system are maintained in vitro.

Synaptic terminals immunolabelled against glutamate in the lobus parolfactorius of domestic chicks (Gallus domesticus) in relation to afferents from the archistriatum

The lobus parolfactorius (LPO) has been implicated in memory formation associated with passive avoidance training of young posthatch domestic chicks. The anatomical circuitry underlying memory formation in the chick is likely to involve the intermediate medial hyperstriatum ventrale-archistriatum-LPO arc. In the present work, we attempted to combine an ultrastructural characterisation of archistriatal afferent terminals in LPO with a description of the synaptic structure of LPO, in particular those elements that are immunoreactive to glutamate and GABA. Ventral archistriatal regions of 7-day-old domestic chicks were iontophoretically injected with Phaseolus vulgaris leucoagglutinin and the anterograde transport of the tracer was detected in the LPO. Selected samples from these birds, and also from other day-old chicks, were resin-embedded and reacted for L-glutamate or GABA, using the postembedding immunocytochemical method. Glutamate was abundant in the neuropil of LPO and typically seen in axodendritic or axospinous terminals with asymmetrical junctions, often multiple or perforated postsynaptic appositions. Conversely, GABA was often present in aspinous dendrites, probably representing GABAergic local circuit neurons or (putative striatonigral) projection neurons. Archistriatal efferents terminating in LPO formed small en passant or terminal varicosities, with infrequent asymmetrical axospinous synapses. Glutamate was not detected in these boutons. The findings imply that the functional state of LPO, based on powerful glutamatergic excitation, may be modified by a non-glutamatergic archistriatal input.

Distribution of cholinergic, GABAergic and serotonergic neurons in the medial medullary reticular formation and their projections studied by cytotoxic lesions in the cat

As part of a larger study concerning the role of neurons in the medial medullary reticular formation in sleep-wake states, the distribution and projections of cholinergic, GABAergic and serotonergic neurons were studied within the lower brainstem of the cat. Cells were plotted with the aid of an image analysis system through the medullary reticular formation and raphe in adjacent sections immunostained for choline acetyltransferase, glutamic acid decarboxylase and serotonin. Immunostained fibres and varicosities were examined and quantified by microdensitometry in regions of the medulla, pons and upper spinal cord in normal and quisqualate-injected animals to assess the loss of local and distant projections following cytotoxic destruction of neurons in the medial medullary reticular formation. Choline acetyltransferase-immunoreactive neurons are unevenly and sparsely distributed, though none the less in significant numbers (estimated at approximately 9080 in total), through the medial medullary reticular formation, and are present in all tegmental fields, including the gigantocellular (approximately 3700) and magnocellular (approximately 1760) rostrally and the ventral (approximately 3240) and paramedian (approximately 380) caudally, and are absent in the midline raphe. Glutamic acid decarboxylase-immunoreactive neurons are more evenly and densely distributed in large numbers (estimated at approximately 18,720) through the medial medullary reticular formation, being present in the gigantocellular (approximately 5960), magnocellular (approximately 8260), ventral (approximately 2280) and paramedian (approximately 2220) tegmental fields, and are also numerous within the raphe magnus and pallidus-obscurus nuclei (approximately 3880). Serotonin-immunoreactive cells are sparse in the medial medullary reticular formation (estimated to total approximately 1540), where they are mainly located in the magnocellular tegmental field (approximately 1340), and are concentrated in larger numbers within the raphe nuclei (approximately 8060). Cholinergic varicose fibres were moderately densely distributed through the medial medullary reticular formation, as well as through more distant lateral, rostral and caudal brainstem and upper spinal regions. After cytotoxic lesions focussed in the gigantocellular and magnocellular tegmental fields, a loss of approximately 55% of the cholinergic neurons in the medial medullary reticular formation was associated with a minor decrease (approximately 35% in optical density measures) of local cholinergic fibres. Small and variable reductions in varicose fibres (and their optical density measures) were detected in distant structures (including the pontine lateral, gigantocellular and subcoerular tegmental fields and the caudal spinal trigeminal nucleus), that were none the less correlated with the number of intact medial medullary cholinergic cells, suggesting that these cells may project to distant brainstem targets, in addition to providing a minor proportion of the local cholinergic innervation of the medial medullary reticular formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Ultrastructural and immunocytochemical characterization of primary afferent terminals in the rat cuneate nucleus

The cuneate nucleus is a relay center for somatosensory information by receiving tactile and proprioceptive inputs from primary afferent fibers that ascend in the dorsal funiculus. The morphology, synaptic contacts, and neurochemical content of primary afferent terminals in the cuneate nucleus of rats were investigated by combining anterograde transport of horseradish peroxidase conjugated to wheat-germ agglutinin or to cholera toxin (injected in cervical dorsal root ganglia) with postembedding immunogold labeling for glutamate and GABA. Both tracers gave similar results. Two types of terminals were labeled: type I terminals were irregularly shaped, had a mean area of 4.0 microns 2, synapsed on several dendrites, and were contacted by other terminals, some of which were GABA positive. Type II terminals were dome-shaped, had a mean area of 2.18 microns 2, and made synaptic contact on a single dendrite. All the anterogradely labeled terminals (interpreted as endings of primary afferents) were enriched in glutamate but not in GABA. The finding that identified primary afferent terminals are enriched in glutamate with respect to other tissue profiles strongly suggests a neurotransmitter role for glutamate in this afferent pathway to the rat cuneate nucleus.

Enrichment of glutamate immunoreactivity in lemniscal terminals in the ventropostero lateral thalamic nucleus of the rat: an immunogold and WGA-HRP study

BACKGROUND

The ventropostero lateral nucleus (VPL) is a thalamic somatosensory center receiving inputs from limbs and trunk; some of this input is via terminals of the dorsal column medial lemniscal pathway. These fibers convey non-noxious somesthesic information.

METHODS

In this study the neurochemical content of lemniscal afferents in VPL of rats was investigated at the electron microscopic level by combining anterograde transport of horseradish peroxidase conjugated to wheat germ agglutinin, injected in the dorsal dorsal column nuclei, with postembedding immunogold labeling for glutamate (Glu).

RESULTS

Anterograde labeling in VPL was detected only in myelinated axons and in large terminals containing round synaptic vesicles, interpreted as lemniscal afferents. Quantitative evaluation of gold particle density showed enrichment of Glu immunolabeling in the identified lemniscal terminals with respect to other neuronal profiles. Observation of serial sections immunoreacted for Glu demonstrated consistency of labeling, whereas in alternate sections immunoreacted for Glu and for the inhibitory amino acid GABA these two antigens were always present in distinct types of terminals.

CONCLUSIONS

These findings are in agreement with several lines of evidence, obtained with different experimental approaches, supporting the hypothesis that Glu plays a major role in conveying sensory stimuli to the thalamus from second order neurons in the dorsal column nuclei.

Neurotransmitters in subcortical somatosensory pathways

Investigations during recent years indicate that many different neuroactive substances are involved in the transmission and modulation of somesthetic information in the central nervous system. This review surveys recent developments within the field of somatosensory neurotransmission, emphasizing immunocytochemical findings. Increasing evidence indicates a widespread role for glutamate as a fast-acting excitatory neurotransmitter at different levels in somatosensory pathways. Several studies have substantiated a role for glutamate as a neurotransmitter in primary afferent neurons and in corticofugal projections, and also indicate a neurotransmitter role for glutamate in ascending somatosensory pathways. Other substances likely to be involved in somatosensory neurotransmission include the neuropeptides. Many different peptides have been detected in primary afferent neurons with unmyelinated or thinly myelinated axons, and are thus likely to be directly involved in primary afferent neurotransmission. Some neurons giving rise to ascending somatosensory pathways, primarily those with cell bodies in the dorsal horn, are also immunoreactive for peptides. Recent investigations have shown that the expression of neuropeptides, both in primary afferent and ascending tract neurons, may change as a result of various kinds of peripheral manipulation. The occurrence of neurotransmitters in intrinsic neurons and neurons providing modulating inputs to somatosensory relay nuclei (the dorsal horn, the lateral cervical nucleus, the dorsal column nuclei and the ventrobasal thalamus) is also reviewed. Neurotransmitters and modulators in such neurons include acetylcholine, monoamines, GABA, glycine, glutamate, and various neuropeptides.

Effects of neonatal transection of the infraorbital nerve upon the structural and functional organization of the ventral posteromedial nucleus in the rat

The present study examined the way in which an indirect partial deafferentation of the medial portion of the ventrobasal complex (VPM/VPL) induced by neonatal transection of the infraorbital nerve (ION) altered the structural and functional properties of its constituent neurons. This manipulation significantly reduced the volume of the contralateral VPM/VPL. In addition, cell counts in Nissl-stained material revealed a significant reduction of the number of VPM/VPL neurons contralateral to neonatal ION transection. We also analyzed the effect of neonatal ION transection on the soma-dendritic morphology of individual neurons in the ventral posteromedial nucleus of the thalamus (VPM) by intracellular injection of horseradish peroxidase (HRP) in vivo and Lucifer yellow in fixed slices. Neonatal transection of the ION resulted in increased dendritic length, area, and volume of VPM neurons in both preparations; however only the changes observed in fixed slices reached statistical significance. Alterations in the functional characteristics of VPM neurons were also observed following neonatal nerve damage. There was a significant decrease in the percentage of vibrissae-sensitive neurons and a corresponding increase in the percentages of neurons responsive to guard hair deflection or that were unresponsive to peripheral stimulation. Neonatal nerve damage also resulted in significantly longer latencies of VPM cells after stimulation of either trigeminal nucleus principalis or subnucleus interpolaris. The present results indicate that the development of normal response properties and soma-dendritic morphology of VPM neurons is dependent upon intact afferent input during development. Indirect partial deafferentation of VPM/VPL by neonatal transection of the ION results in reduced neuron number, which may result in decreased competition among the dendrites of these neurons. This proposal is consistent with observations of increased dendritic dimensions of VPM neurons contralateral to neonatal ION damage.

Large GABA cells of chick ectostriatum: anatomical evidence suggesting a double GABAergic disinhibitory mechanism. An electron microscopic immunocytochemical study

In an extension of our previous light microscopic observations, a type of neuron which shows GABA-like immunoreactivity was identified and described in the ectostriatal core of young domestic chicks, using pre- and postembedding electron microscopic immunocytochemistry. Large GABA immunopositive (GABA+) cells are characterized by an ovoidal or polygonal soma of 12-16 micron diameter, uniformly distributed nuclear chromatin, a prominent Golgi apparatus and an abundance of rough endoplasmic reticulum. In addition to axodendritic terminals, large GABA neurons receive numerous axosomatic synapses of both symmetrical and asymmetrical types covering a substantial part of their perikaryal surface. Axosomatic terminals with symmetrical junctions are usually immunoreactive to GABA whereas the boutons with asymmetrical synaptic specialization are immunonegative. GABA+ boutons also synapse with dendritic spine necks presumably belonging to projection neurons. These terminals usually contain loosely packed synaptic vesicles without any marked accumulation near the synaptic cleft. Large GABA+ terminals with densely packed vesicles were found to synapse with axon hillocks. Based on known descriptions of ectostriatal cytoarchitecture and synaptology, it is suggested that the GABA+ cells of chick ectostriatum represent inhibitory interneurons which may be equivalent to GABAergic non-pyramidal neuronal types of mammalian visual cortex. GABA+ axosomatic synapses afferent to large GABA cells are likely to form the structural basis for a disinhibitory mechanism in the avian ectostriatum.