Expression of Nerve Growth Factor,p75, and trk in the Somatosensory and Motor Cortices of Mature Rats: Evidence for Local Trophic Support Circuits

Neurotrophins such as nerve growth factor (NGF) are critical for the maintenance of CNS neurons. We determined the expression of NGF and the neurotrophin receptors p75 and trk in the somatosensory and motor cortices of mature rats with immuno-histochemical techniques. Sections of mature rat cortex were processed immunohisto-chemically with primary antibodies directed against NGF, p75, or trk. The distribution of immunoreactive elements was examined, and stereological techniques were used to determine the density and size of immunoreactive cell bodies. Some sections processed for trk immunoreactivity were examined with an electron microscope.

From the size and morphology of the labeled cells, it appeared that only neurons in the gray matter were NGF-positive. NGF was detected in one-third of the neurons in layers II-III, V, and VI of both somatosensory cortex and motor cortex; however, fewer than 1 in 12 of the layer IV neurons was NGF-positive. With the notable exception of layer V, few cell bodies (2–10% of the total population) were p75– or trk-immunoreactive. Layer Vb was replete with receptor-positive cell bodies; more than one-third of the layer Vb neurons were p75– or trk-positive. All labeled cells appeared to be pyramidal neurons. The distribution of p75 labeling with the two anti-p75 antibodies was indistinguishable. In addition, the neuropil in the supragranular laminae was p75– or trk-positive. Electron microscopy showed that trk immunoreactivity was also expressed by dendrites. Only rarely were immunoreactive axons detected.

In summary, NGF is expressed by cortical neurons throughout cortex, and neurotrophin receptors are widely produced by postsynaptic targets. Thus, NGF appears to participate in an intracortical autoregulatory system. The strong expression of neurotrophin receptors by pyramidal neurons in layer Vb (the origin of brainstem and spinal cord projections) suggests that the neurotrophins are especially critical for the regulation of corticofugal projection systems.     

Gap junction proteins in inhibitory neurons of the adult barrel neocortex

Recent studies indicate that electrical coupling among cortical neurons may persist throughout development; electrophysiological recordings made in cortical slices from young rats reveal that numerous GABAergic neurons are electrically coupled. To determine whether these in vitro findings reflect an inhibitory neural circuit that could be functionally relevant in vivo in adult rodents, we sought to identify whether inhibitory, parvalbumin-containing neurons of the mature cortex express gap junction proteins. Immunohistochemistry was used to examine the laminar distribution of the gap junction-forming proteins connexin 32 (Cx32), connexin 36 (Cx36) and connexin 43 (Cx43) in the somatosensory cortex of the adult mouse. Double labeling immunofluorescence identified Cx32, Cx36 and Cx43 in cortical neurons that were immunoreactive (-ir) for the neuronal markers neurofilament 145 kDa and neuronal nuclei (NeuN). Parvalbumin-ir neurons throughout the cortical laminae were labeled with Cx32-ir, Cx36-ir and Cx43-ir. Stereological methods were used to quantify the extent of parvalbumin colocalization with connexins. Analysis indicated that approximately 40% of parvalbumin-ir neurons were double labeled with either Cx32-ir or Cx43-ir, and approximately 50% of parvalbumin-ir neurons were double labeled with Cx36. These findings establish an anatomical substrate for widespread electrical coupling of neurons in somatosensory cortex and suggest that gap junctions among inhibitory interneurons may persist into adulthood, providing an important mechanism for neuronal communication.     

The Intrinsic Organization of the Ventroposterolateral Nucleus and Related Reticular Thalamic Nucleus of the Rat: A Double-Labeling Ultrastructural Investigation with γ-Aminobutyric Acid Immunogold Staining and Lectin-Conjugated Horseradish Peroxidase

An electron-microscopic investigation of the synaptic organization of the rat's ventroposterolateral nucleus (VPL) and of a reticular thalamic nucleus (RTN) area related to somatosensory thalamic nucleus was performed. In a group of 11 rats, wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) was injected either in the first somatosensory area of cortex (SI) or in the dorsal column nuclei (DCN). The retrogradely and/or anterogradely transported enzyme was visualized using paraphenylenediamine-pyrocatechol (PPD-PC) as substrate. In a second series of six experiments, an immunocytochemical procedure using a specific anti-γ-aminobutyric acid (anti-GABA) was employed. Postembedding localization of GABA was performed for ultrastructural observation by means of the colloidal gold immunostaining procedure. Thin sections of recognized VPL and RTN areas from WGA:HRP-injected animals were further processed for immunocytochemistry in order to localize simultaneously, at the electron-microscopic level, the transported enzyme and GABA.

The results obtained with this procedure demonstrated that HRP-labeled terminals from DCN contacted the soma and proximal dendrites of VPL neurons, while the terminals labeled after SI cortical injections were predominantly localized to the distal portion of the dendrites. The same cortical injection also determined the presence of labeled synaptic boutons contacting the soma, and both proximal and distal dendrites of RTN neurons. GABA-immunolabeled terminals were observed in VPL in a number larger than those observed with other methods, since not only typical F terminals were labeled but also terminals containing round and/or pleomorphic vesicles. GABA-ergic terminals contacted the soma and the proximal and distal dendrites of VPL neurons, while in RTN cells they made synaptic contact mainly with the soma and proximal dendrites. In the double-labeling experiments, terminals containing both HRP and specific immunogold GABA staining were never observed.

The present data provide a direct demonstration of the presence of a strong inhibitory input from RTN upon VPL neurons and of the existence of autoinhibition within RTN neurons.     

Areal distributions of cortical neurons projecting to different levels of the caudal brain stem and spinal cord in rats

Distributions of corticospinal and corticobulbar neurons were revealed by tetramethylbenzidine (TMB) processing after injections of wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) into the cervical or lumbar enlargements of the spinal cord, or medullary or pontine levels of the brain stem. Sections reacted for cytochrome oxidase (CO) allowed patterns of labeled neurons to be related to the details of the body surface map in the first somatosensory cortical area (SI). The results indicate that a number of cortical areas project to these subcortical levels: (1) Projection neurons in granular SI formed a clear somatotopic pattern. The hindpaw region projected to the lumbar enlargement, the forepaw region to the cervical enlargement, the whisker pad field to the lower medulla, and the more rostral face region to more rostral brain stem levels. (2) Each zone of labeled neurons in SI extended into adjacent dysgranular somatosensory cortex, forming a second somatotopic pattern of projection neurons. (3) A somatotopic pattern of projection neurons in primary motor cortex (MI) paralleled SI in mediolateral sequence corresponding to the hindlimb, forelimb, and face. (4) A weak somatotopic pattern of projection neurons was suggested in medial agranular cortex (Agm), indicating a premotor field with a rostromedial-to-caudolateral representation of hindlimb, forelimb, and face. (5) A somatotopic pattern of projection neurons representing the foot to face in a mediolateral sequence was observed in medial parietal cortex (PM) located between SI and area 17. (6) In the second somatosensory cortical area (SII), neurons projecting to the brain stem were immediately adjacent caudolaterally to the barrel field of SI, whereas neurons projecting to the upper spinal cord were more lateral. No projection neurons in this region were labeled by the injections in the lower spinal cord. (7) Other foci of projection neurons for the face and forelimb were located rostral to SII, providing evidence for a parietal ventral area (PV) in perirhinal cortex (PR) lateral to SI, and in cortex between SII and PM. None of these regions, which may be higher-order somatosensory areas, contained labeled neurons after injections in the lower spinal cord. Thus, more cortical fields directly influence brain stem and spinal cord levels related to sensory and motor functions of the face and forepaw than the hindlimb. The termination patterns of corticospinal and corticobulbar projections were studied in other rats with injections of WGA:HRP in SI.(ABSTRACT TRUNCATED AT 400 WORDS)     

Convergence across Tactile Afferent Types in Primary and Secondary Somatosensory Cortices

Integration of information by convergence of inputs onto sensory cortical neurons is a requisite for processing higher-order stimulus features. Convergence across defined peripheral input classes has generally been thought to occur at levels beyond the primary sensory cortex, however recent work has shown that this does not hold for the convergence of slowly-adapting and rapidly-adapting inputs in primary somatosensory cortex. We have used a new analysis method for multi-unit recordings, to show convergence of inputs deriving from the rapidly-adapting and Pacinian channels in a proportion of neurons in both primary and secondary somatosensory cortex in the anaesthetised cat. We have validated this method using single-unit recordings. The secondary somatosensory cortex has a greater proportion of sites that show convergence of this type than primary somatosensory cortex. These findings support the hypothesis that the more complex features processed in higher cortical areas require a greater degree of convergence across input classes, but also shows that this convergence is apparent in the primary somatosensory cortex.     

Expression of nerve growth factor and its receptors in the somatosensory-motor cortex of Macaca nemestrina

The present study was designed to examine the nerve growth factor (NGF) system (ligand and receptor-expressing neurons) in the somatosensory (areas 1, 3a, and 3b) and motor (area 4) cortices of the mature macaque. Light and electron microscope immunohistochemistry was used to assess the distribution and identity of NGF-, p75-, and trk-expressing elements. In each cortical area examined, NGF-positive neuronal somata were distributed through all laminae; most immunolabeled neurons were in layers II, III, and V. Based upon light microscope criteria (e.g., the morphology of proximal dendrites), both pyramidal and stellate neurons expressed NGF. Of the identifiable NGF- immunoreactive cells, 92% were pyramidal neurons and the remainder was stellate neurons. The electron microscope study showed that most (88%) NGF-positive somata formed symmetric synapses, whereas the others formed both symmetric and asymmetric synapses. As the somata of pyramidal neurons form only symmetric synapses and those of inhibitory stellate neurons form both symmetric and asymmetric somatic synapses, the ultrastructural data support the light microscopic analyses. In contrast, neurotrophin receptors, p75 and trk, were expressed chiefly by the cell bodies of layer V pyramidal neurons and the supragranular neuropil. At the ultrastructural level, receptor-positive profiles were post-synaptic elements (e.g., dendritic shafts and spines) and the concentration of immunoreactivity was greatest in the vicinity of post-synaptic densities. Thus, NGF regulatory systems parallel excitatory and inhibitory neurotransmitter systems. Cortex contains the morphological framework by which pyramidal and/or inhibitory stellate neurons can affect the activity of post-synaptic pyramidal neurons via anterograde and autocrine/paracrine NGF systems.     

Areal Distributions of Cortical Neurons Projecting to Different Levels of the Caudal Brain Stem and Spinal Cord in Rats

Distributions of corticospinal and corticobulbar neurons were revealed by tetramethylbenzidine (TMB) processing after injections of wheatgerm agglutinin conjugated to horseradish peroxidase (WGA:HRP) into the cervical or lumbar enlargements of the spinal cord, or medullary or pontine levels of the brain stem. Sections reacted for cytochrome oxidase (CO) allowed patterns of labeled neurons to be related to the details of the body surface map in the first somatosensory cortical area (SI). The results indicate that a number of cortical areas project to these subcortical levels: (1) Projection neurons in granular SI formed a clear somatotopic pattern. The hindpaw region projected to the lumbar enlargement, the forepaw region to the cervical enlargement, the whisker pad field to the lower medulla, and the more rostral face region to more rostral brain stem levels. (2) Each zone of labeled neurons in SI extended into adjacent dysgranular somatosensory cortex, forming a second somatotopic pattern of projection neurons. (3) A somatotopic pattern of projection neurons in primary motor cortex (MI) paralleled SI in mediolateral sequence corresponding to the hindlimb, forelimb, and face. (4) A weak somatotopic pattern of projection neurons was suggested in medial agranular cortex (Ag_m), indicating a premotor field with a rostromedial-to-caudolateral representation of hindlimb, forelimb, and face. (5) A somatotopic pattern of projection neurons representing the foot to face in a mediolateral sequence was observed in medial parietal cortex (PM) located between SI and area 17. (6) In the second somatosensory cortical area (SII), neurons projecting to the brain stem were immediately adjacent caudolaterally to the barrel field of SI, whereas neurons projecting to the upper spinal cord were more lateral. No projection neurons in this region were labeled by the injections in the lower spinal cord. (7) Other foci of projection neurons for the face and forelimb were located rostral to SII, providing evidence for a parietal ventral area (PV) in perirhinal cortex (PR) lateral to SI, and in cortex between SII and PM. None of these regions, which may be higher-order somatosensory areas, contained labeled neurons after injections in the lower spinal cord. Thus, more cortical fields directly influence brain stem and spinal cord levels related to sensory and motor functions of the face and forepaw than the hindlimb.

The termination patterns of corticospinal and corticobulbar projections were studied in other rats with injections of WGA:HRP in SI. Injections in lateral SI representing the face produced dense terminal label in the contralateral trigeminal complex. Injections in cortex devoted to the forelimb and forepaw labeled the contralateral cuneate nucleus and parts of the dorsal horn of the spinal cord. The cortical injections also demonstrated interconnections of parts of SI with some of the other regions of cortex with projections to the spinal cord, and provided further evidence for the existence of PV in rats.     

Imaging the spatio-temporal dynamics of supragranular activity in the rat somatosensory cortex in response to stimulation of the paws

We employed voltage-sensitive dye (VSD) imaging to investigate the spatio-temporal dynamics of the responses of the supragranular somatosensory cortex to stimulation of the four paws in urethane-anesthetized rats. We obtained the following main results. (1) Stimulation of the contralateral forepaw evoked VSD responses with greater amplitude and smaller latency than stimulation of the contralateral hindpaw, and ipsilateral VSD responses had a lower amplitude and greater latency than contralateral responses. (2) While the contralateral stimulation initially activated only one focus, the ipsilateral stimulation initially activated two foci: one focus was typically medial to the focus activated by contralateral stimulation and was stereotaxically localized in the motor cortex; the other focus was typically posterior to the focus activated by contralateral stimulation and was stereotaxically localized in the somatosensory cortex. (3) Forepaw and hindpaw somatosensory stimuli activated large areas of the sensorimotor cortex, well beyond the forepaw and hindpaw somatosensory areas of classical somatotopic maps, and forepaw stimuli activated larger cortical areas with greater activation velocity than hindpaw stimuli. (4) Stimulation of the forepaw and hindpaw evoked different cortical activation dynamics: forepaw responses displayed a clear medial directionality, whereas hindpaw responses were much more uniform in all directions. In conclusion, this work offers a complete spatio-temporal map of the supragranular VSD cortical activation in response to stimulation of the paws, showing important somatotopic differences between contralateral and ipsilateral maps as well as differences in the spatio-temporal activation dynamics in response to forepaw and hindpaw stimuli.     

A Comparison of the Ipsilateral Cortical Projections to the Dorsal and Ventral Subdivisions of the Macaque Premotor Cortex

The cortical connections of the dorsal (PMd) and ventral (PMv) subdivisions of the premotor area (PM, lateral area 6) were studied in four monkeys (Macaca fascicularis) through the use of retrograde tracers. In two animals, tracer was injected ventral to the arcuate sulcus (PMv), in a region from which forelimb movements could be elicited by intracortical microstimulation (ICMS). Tracer injections dorsal to the arcuate sulcus (PMd) were made in two locations. In one animal, tracer was injected caudal to the genu of the arcuate sulcus (in caudal PMd [cPMd], where ICMS was effective in eliciting forelimb movements); in another animal, it was injected rostral to the genu of the arcuate sulcus (in rostral PMd [rPMd], where ICMS was ineffective in eliciting movements). Retrogradely labeled neurons were counted in the ipsilateral hemisphere and located in cytoarchitectonically identified areas of the frontal and parietal lobes. Although both PMv and PMd were found to receive inputs from other motor areas, the prefrontal cortex, and the parietal cortex, there were differences in the topography and the relative strength of projections from these areas.

There were few common inputs to PMv and PMd; only the supplementary eye fields projected to all three areas studied. Interconnections within PMd or PMv appeared to link hindlimb and forelimb representations, and forelimb and face representations; however, connections between PMd and PMv were sparse. Areas cPMd and PMv were found to receive inputs from other motor areas—the primary motor area, the supplementary motor area, and the cingulate motor area—but the topography and strength of projections from these areas varied. Area rPMd was found to receive sparse inputs, if any, from these motor areas. The frontal eye field (area 8a) was found to project to PMv and rPMd, and area 46 was labeled substantially only from rPMd. Parietal projections to PMv were found to originate from a variety of somatosensory and visual areas, including the second somatosensory cortex and related areas in the parietal operculum of the lateral sulcus, as well as areas 5, 7a, and 7b, and the anterior intraparietal area. By contrast, projections to cPMd arose only from area 5. Visual areas 7m and the medial intraparietal area were labeled from rPMd. Relatively more parietal neurons were labeled after tracer injections in PMv than in PMd. Thus, PMv and PMd appear to be parts of separate, parallel networks for movement control.     

Corticofugal projections to the periaqueductal gray matter in the cat midbrain

Stereotaxic microinjections of horseradish peroxidase (HP) were made into different parts of the rostral and caudal periaqueductal gray (PAG) in cats to study corticofugal projections to the PAG. The method of retrograde axonal transport of HP demonstrated labeled neurons in the I and II somatosensory areas, frontal, cingular and insular cortex of the brain. It was shown that the II somatosensory cortex projects to all the areas of the rostral and caudal PAG. The frontal cortex projects to the dorsolateral quadrant of the PAG. The findings obtained enabled the detection of the morphological substrate of the corticofugal effects on one of the antinociceptive brain structures--the PAG.     

Responses in the First or Second Somatosensory Cortical Area in Cats during Transient Inactivation of the Other Ipsilateral Area with Lidocaine Hydrochloride

Simultaneous recordings were obtained from the primary and secondary somatosensory cortical areas (SI and SII) in cats anesthetized with ketamine or pentobarbital. A total of 40 individual neurons were studied (29 in SII and 11 in SI) before, during, and following injections of microliter quantities of lidocaine hydrochloride in the other ipsilateral cortical area. Activity in the cortex injected with the local anesthetic was monitored with single-neuron, multi-neuron, or evoked potential responses to determine the time course of inactivation within 0.5-2 mm of the injection sites. Recording sites in both cortical locations were in the representations of the distal forelimb. Responses were elicited by transcutaneous electrical stimulation across the receptive fields with needle electrodes. Short-latency responses were synchronously activated, and, in those circumstances where single neurons were isolated in both areas, no overall differences in latency were noted. Anesthetization of either cortical area never blocked access of somatosensory information to the intact area, even when the injected cortex was completely silenced in the vicinity of the injection mass. In 15 SII neurons and 7 SI neurons, changes were seen in short-latency evoked responses to stimulation of their receptive fields or in background activity following local anesthesia of the other area through several cycles of injection and recovery. In 7 of these 15 SII cells, changes were noted in the timing and/or firing rates of the short-latency responses; changes were noted in the short-latency responses of 2 of these 7 SI cells while SII was silenced. In 11 SII and 6 SI cells, “background” activity that was recorded during the interstimulus intervals either increased (most cases) or decreased during local anesthesia of the other area. The results are discussed in reference to the hypothesis that primary sensory cortical areas feed information forward to secondary areas, and these feed back modulatory controls to the primary regions.     

Structural Types of Spinal Cord Marginal (Lamina I) Neurons Projecting to the Nucleus of the Tractus Solitarius in the Rat

The structural types of spinal cord marginal (lamina I) neurons projecting to the nucleus of the tractus solitarius (NTS) were studied. Upon injections of cholera toxin subunit B (CTb) into the caudal part of the NTS, including its lateral and medial portions, labeled cells occurred bilaterally in laminae I, IV-VII, and X, and the lateral spinal nucleus (LSN). After injections into the lateral portion alone, only a few cells were labeled in laminae V, VII, and X, and the LSN, and none in the superficial dorsal horn. Of 1882 labeled marginal cells, 38% belonged to the flattened type, 37% to the pyramidal type, and 25% to the fusiform type. Flattened and pyramidal cells were labeled in considerably greater numbers than those reported when other supraspinal targets of these cells were injected with CTb. Since cells in the NTS are known to be under marked 7-aminobutyric acidergic (GABA-ergic) inhibition, it is possible that only strong input conveyed by great numbers of flattened and pyramidal cells is capable of overcoming that barrier. Fusiform cells were labeled in numbers similar to those observed previously after tracer injections into the two other targets of this neuronal type, the parabrachial nuclei and the lateral reticular nucleus. Considering that these regions, as well as the NTS, control cardiovascular and respiratory functions, it is suggested that fusiform cells transmit noxious input that will influence autonomic reflexes processed in the three nuclei.     

Characteristics of GABAergic neurons and their synaptic relationships with intrinsic axons in the cat motor cortex

The intrinsic circuitry of the motor cortex comprises a complex network of connections whose synaptic relationships are poorly understood. This study was designed to determine the characteristics of subsets of GABAergic neurons containing the calcium-binding proteins parvalbumin (PV) and calbindin (CB), and their relationships with intrinsic axons in motor cortex. Immunohistochemically identified PV-containing neuronal profiles were more evenly distributed across cortical laminae (38% in II-III, 32% inV, 30% in VI) and more numerous (2.1/1) than CB-containing neuronal profiles (71% in II-III, 17% in V, 12% in VI). Relationships between neurons and axons intrinsic to motor cortex were visualized with fluorescent markers using the laser scanning confocal microscope. Similar percentages of PV (43%) and CB-immunoreactive (IR) (40%) neurons formed sparsely distributed appositions (1-5/neuron) with anterogradely labeled axons. The mean distances of such appositions from the somata were significantly different for the two groups (PV, mean = 22 microm, range = 1.6-93 microm; CB, mean = 32 microm, range = 6.2-132 microm). PV-IR neurons had a lower ratio of axosomatic/axodendritic appositions (1/99) compared with CB-IR neurons (14/86). Ultrastructural studies confirmed these findings. Fifty-seven percent of CB-IR neurons and 38% of PV-IR neurons formed synapses with intrinsic axons. Both populations received sparse input (1-6 synapses/neuron). Nearly all appositions between labeled terminals and postsynaptic profiles formed one synapse. Postsynaptic dendrites of PV-IR neurons (mean = 1.4 microm diameter) were larger than those of CB-IR neurons (mean = 1.1 microm), indicating more proximal synapses. Distinct input patterns of intrinsic axons to the two populations of neurons suggest unique roles in cortical processing.     

Characteristics of GABAergic neurons and their synaptic relationships with intrinsic axons in the cat motor cortex

The intrinsic circuitry of the motor cortex comprises a complex network of connections whose synaptic relationships are poorly understood. This study was designed to determine the characteristics of subsets of GABAergic neurons containing the calcium-binding proteins parvalbumin (PV) and calbindin (CB), and their relationships with intrinsic axons in motor cortex. Immunohistochemically identified PV-containing neuronal profiles were more evenly distributed across cortical laminae (38% in II-III, 32% in V, 30% in VI) and more numerous (2.1/1) than CB-containing neuronal profiles (71% in II-III, 17% in V, 12% in VI). Relationships between neurons and axons intrinsic to motor cortex were visualized with fluorescent markers using the laser scanning confocal microscope. Similar percentages of PV (43%) and CBimmunoreactive (IR) (40%) neurons formed sparsely distributed appositions (1-5/neuron) with anterogradely labeled axons. The mean distances of such appositions from the somata were significantly different for the two groups (PV, mean =22 mum, range = 1.6-93 mum; CB, mean = 32 mum, range = 6.2-132 mum). PV-IR neurons had a lower ratio of axosomatic/ axodendritic appositions (1/99) compared with CB-IR neurons (14/86). Ultrastructural studies confirmed these findings. Fifty-seven percent of CB-IR neurons and 38% of PV-IR neurons formed synapses with intrinsic axons. Both populations received sparse input (1-6 synapses/neuron). Nearly all appositions between labeled terminals and postsynaptic profiles formed one synapse. Postsynaptic dendrites of PV-IR neurons (mean = 1.4 mum diameter) were larger than those of CB-IR neurons (mean = 1.1 mum), indicating more proximal synapses. Distinct input patterns of intrinsic axons to the two populations of neurons suggest unique roles in cortical processing.     

Distinct origin of GABA-ergic neurons in forebrain of man, nonhuman primates and lower mammals

In this mini-review we present recent data about origin of GABA-ergic (gama-aminobutyric acid) neurons in the mammalian forebrain, including the diencephalon and telencephalon. The interest in GABA-ergic neurons, which in cerebral cortex mostly correspond to local circuit neurons (interneurons), has increased in the past decade. Many studies have shown that in lower mammals all hippocampal and almost all neo-cortical GABA-ergic neurons are born in the specific region named ganglionic eminence, and not locally in proliferative layers all around telencephalic vesicle. The ganglionic eminence, that represents a region with thick proliferative-subventricular layer in the ventral (basal) part of telencephalon, was classically thought to give neurons to basal ganglia and septal nuclei, whereas proliferative layers of dorsal telencephalon give neurons to cerebral cortex including hippocampus. It was thought that neurons migrate from proliferative layer to their target region following a radial orientation. However, data in lower mammals showed that this is the case only for glutamatergic principal cells, i.e. projection neurons. GABA-ergic neurons use long distance tangentional migration, parallel to pial surface to reach, from ganglionic eminence, their targeting layer in the cerebral cortex. Especially intriguing, but frequently neglecting, several studies suggest that mammalian evolution might use different developmental rules to provide GABA-ergic neurons to an expending brain. In this review we focus on specific events underlying GABA-ergic neuron development in human and non-human primates. Disturbances of the GABAergic network are found in many neurological and psychiatric disorders, some of them might result from altered production or migration of these neurons during development. Therefore, it is crucial to understand human-specific mechanisms that regulate the development of GABA-ergic neurons.     

Principalis- or parabrachial-projecting spinal trigeminal neurons do not stain for GABA or GAD

Retrograde transport and immunohistochemical double-labeling methods (Weinberg et al., 1985) were used to assess the distribution and projection status of spinal trigeminal (SpV) neurons that stain positively for glutamic acid decarboxylase (GAD) or gamma-aminobutyric acid (GABA). Large bilateral injections of diamidino yellow into the rostral and lateral pons, inclusive of V nucleus principalis and the parabrachial nucleus, retrogradely labeled large numbers of cells in each SpV subnucleus. Many cells in SpV subnuclei caudalis, interpolaris, and oralis also exhibited GABA immunoreactivity; the largest numbers were in caudalis and the smallest numbers were in oralis. However, none of the GABA- or GAD-immunoreactive SpV cells were double-labeled with diamidino yellow, though some reticular neurons displayed both GABA and the retrograde tracer. This negative result refutes a previously offered hypothesis that SpV local-circuit neurons with principalis collaterals are GABA-ergic (Jacquin et al., 1989b). These data also indicate that parabrachial-projecting SpV neurons are not GABA-ergic.     

Intrinsic properties of and thalamocortical inputs onto identified corticothalamic-VPM neurons

Corticothalamic (CT) feedback plays an important role in regulating the sensory information that the cortex receives. Within the somatosensory cortex layer VI originates the feedback to the ventral posterior medial (VPM) nucleus of the thalamus, which in turn receives sensory information from the contralateral whiskers. We examined the physiology and morphology of CT neurons in rat somatosensory cortex, focusing on the physiological characteristics of the monosynaptic inputs that they receive from the thalamus. To identify CT neurons, rhodamine microspheres were injected into VPM and allowed to retrogradely transport to the soma of CT neurons. Thalamocortical slices were prepared at least 3 days post injection. Whole-cell recordings from labeled CT cells in layer VI demonstrated that they are regular spiking neurons and exhibit little spike frequency adaption. Two anatomical classes were identified based on their apical dendrites that either terminated by layer V (compact cells) or layer IV (elaborate cells). Thalamic inputs onto identified CT-VPM neurons demonstrated paired pulse depression over a wide frequency range (2–20?Hz). Stimulus trains also resulted in significant synaptic depression above 10?Hz. Our results suggest that thalamic inputs differentially impact CT-VPM neurons in layer VI. This characteristic may allow them to differentiate a wide range of stimulation frequencies which in turn further tune the feedback signals to the thalamus.     

Activation of a wide-spread network of inhibitory neurons in barrel cortex

Abstract The one-to-one correspondence of whiskers to barrels in layer IV of rodent somatosensory cortex can be demonstrated by a precise match between columns of heavy 2-deoxyglucose (2DG) label in layer IV barrels and other layers which correspond to stimulated whiskers. While there is specificity of peripheral-to-central mapping, the extent to which integration and/or modulation are generated by circuitry within or interactions between the barrel-defined whisker columns is not clear. Following stimulation of selected whiskers, large cells at the layer IV-V boundary throughout the barrel field are heavily labeled by 2-deoxyglucose (2DG) at high resolution. Many of these cells are outside the barrel columns of the stimulated whiskers. Further, the number of cells labeled is not directly related to the number of activated barrel columns. These neurons are not labeled in animals anesthetized before 2DG injection and are not as heavily labeled in barrel fields of somnolent animals. Most of the heavily labeled neurons immunolabel for glutamate decarboxylase (GAD) and are presumed to be inhibitory, while a smaller number of labeled neurons, presumed to be excitatory, immunolabel for glutamate (Glu). Similar populations of large, heavily 2DG-labeled neurons are found in other cortical areas. These relatively few neurons are exceptionally active and may modulate integrative functions of cerebral cortex.     

In vivo transduction of central neurons using recombinant Sindbis virus: Golgi-like labeling of dendrites and axons with membrane-targeted fluorescent proteins.

A new recombinant virus which labeled the infected neurons in a Golgi stain-like fashion was developed. The virus was based on a replication-defective Sindbis virus and was designed to express green fluorescent protein with a palmitoylation signal (palGFP). When the virus was injected into the ventrobasal thalamic nuclei, many neurons were visualized with the fluorescence of palGFP in the injection site. The labeling was enhanced by immunocytochemical staining with an antibody to green fluorescent protein to show the entire configuration of the dendrites. Thalamocortical axons of the infected neurons were also intensely immunostained in the somatosensory cortex. In contrast to palGFP, when DsRed with the same palmitoylation signal (palDsRed) was introduced into neurons with the Sindbis virus, palDsRed neither visualized the infected neurons in a Golgi stain-like manner nor stained projecting axons in the cerebral cortex. The palDsRed appeared to be aggregated or accumulated in some organelles in the infected neurons. Anterograde labeling with palGFP Sindbis virus was very intense, not only in thalamocortical neurons but also in callosal, striatonigral, and nigrostriatal neurons. Occasionally there were retrogradely labeled neurons that showed Golgi stain-like images. These results indicate that palGFP Sindbis virus can be used as an excellent anterograde tracer in the central nervous system.     

Effect of rabies virus infection on the expression of parvalbumin, calbindin and calretinin in mouse cerebral cortex

Some clinical features of rabies and experimental evidence from cell culture and laboratory animals suggest impairment of gabaergic neurotransmission. Several types of gabaergic neurons occur in the cerebral cortex. They can be identified by three neuronal markers: the calcium binding proteins (CaBPs) parvalbumin (PV), calbindin (CB) and calretinin (CR). Rabies virus spreads throughout the cerebral cortex; however, rabies cytopathic effects on gabaergic neurons are unknown. The expression of calcium-binding proteins (CaBPs) parvalbumin (PV), calbindin (CB) and calretinin (CR) was studied in the frontal cortex of mice. The effect of gabaergic neurons was evaluated immunohistochemically. The distribution patterns of CaBPs in normal mice and in mice infected with 'fixed' or 'street' rabies virus were compared. PV was found in multipolar neurons located in all cortical layers except layer I, and in pericellular clusters of terminal knobs surrounding the soma of pyramidal neurons. CB-immunoreactivity was distributed in two cortical bands. One was composed of round neurons enclosed by a heavily labeled neuropil; this band corresponds to supragranular layers II and III. The other was a weakly stained band of neuropil which contained scattered multipolar CB-ir neurons; this corresponds to infragranular layers V and VI. The CR-ir neurons were bipolar fusiform cells located in all layers of cortex, but concentrated in layers II and III. A feature common to samples infected with both types of viruses was a more intense immunoreactivity to PV in contrast to normal samples. The infection with 'street' virus did not cause additional changes in the expression of CaBPs. However, the infection with 'fixed' virus produced a remarkable reduction of CB-immunoreactivity demonstrated by the loss of CB-ir neurons and low neuropil stain in the frontal cortex. In addition, the size of CR-ir neurons in the cingulate cortex was decreased.