Postnatal development of the distribution of nitric oxide-producing neurons in the rat corpus callosum

The postnatal development of nitric oxide (NO)-producing intracallosal neurons was studied in rats by nicotinamide adenine dinucleotide phosphate-diaphorase (NADPH-d) histochemistry from postnatal day 0 (P0) to P30. NADPH-d-positive neurons (NADPH-d+_Ns) were detected already at P0, mainly in the rostral region of the corpus callosum (cc). Their location and the intensity of staining allowed them to be classified as type I NO-producing neurons. At P0, tufts of intensely labeled fibers, probably corresponding to the callosal septa described in the monkey and human cc, entered the ventral cc region and reached its dorsal portion. From P5, cell bodies and dendrites were often associated to blood vessels. The number of intracallosal NADPH-d+_Ns rose in the first postnatal days to peak at P5, it declined until P10, and then remained almost constant until P30. Their size increased from P0 to P30, dramatically so (>65%) from P0 to P15. From P10 onward their distribution was adult-like, i.e. NADPH-d+_Ns were more numerous in the lateral and intermediate portions of the cc and diminished close to the midline. In conjunction with previous data, these findings indicate that intracallosal NADPH-d+_Ns could have a role in callosal axon guidance, myelination, refinement processes, and callosal blood flow regulation.

Substance P NK1 receptor in the rat corpus callosum during postnatal development

INTRODUCTION

The expression of substance P (SP) receptor (neurokinin 1, NK1) was studied in the rat corpus callosum (cc) from postnatal day 0 (the first 24 hr from birth, P0) to P30.

METHODS

We used immunocytochemistry to study the presence of intracallosal NK1-immunopositive neurons (NK1_IP-n) during cc development.

RESULTS

NK1_IP-n first appeared on P5. Their number increased significantly between P5 and P10, it remained almost constant between P10 and P15, then declined slightly until P30. The size of intracallosal NK1_IP-n increased constantly from P5 (102.3 μm²) to P30 (262.07 μm²). From P5 onward, their distribution pattern was adult-like, that is, they were more numerous in the lateral and intermediate parts of the cc, and declined to few or none approaching the midline. At P5, intracallosal NK1_IP-n had a predominantly round cell bodies with primary dendrites of different thickness from which originated thinner secondary branches. Between P10 and P15, dendrites were longer and more thickly branched, and displayed several varicosities as well as short, thin appendages. Between P20 and P30, NK1_IP-n were qualitatively indistinguishable from those of adult animals and could be classified as bipolar (fusiform and rectangular), round-polygonal, and pyramidal (triangular-pyriform).

CONCLUSIONS

Number of NK1_IP-n increase between P5 and P10, then declines, but unlike other intracallosal neurons, NK1_IP-n make up a significant population in the adult cc. These findings suggest that NK1_IP-n may be involved in the myelination of callosal axons, could play an important role in their pathfinding. Since they are also found in adult rat cc, it is likely that their role changes during lifetime.

Connections from the rat dorsal column nuclei (DCN) to the periaqueductal gray matter (PAG)

Electrical stimulation of the dorsal columns (DCs; spinal cord stimulation; SCS) has been proposed to treat chronic neuropathic pain. SCS may activate a dual mechanism that would affect both the spinal cord and supraspinal levels. Stimulation of DCs or DC nuclei (DCN) in animals where neuropathic pain has been induced causes activation of brainstem centers including the periaqueductal gray (PAG), which is involved in the endogenous pain suppression system. Biotinylated dextran-amine (BDA) was iontophoretically injected into the DCN to analyze the ascending projection directed to the PAG. Separate injections into the gracile nucleus (GrN) and the cuneate nucleus (CunN) showed BDA-positive fibers terminating in different regions of the contralateral PAG. GrN-PAG afferents terminated in the caudal and middle portions of PAG-l, whereas CunN-PAG fibers terminated in the middle and rostral portions of PAG-l. Based on the DCN somatotopic map, the GrN sends information to the PAG from the contralateral hindlimb and the tail and the CunN from the contralateral forelimb, shoulder, neck and ear. This somatotopic organization is consistent with earlier electrophysiological and PAG stimulation studies. These fibers could form part of the DCs-brainstem-spinal cord loop, which may be involved in the inhibitory effects of SCS on neuropathic pain.

Functional MRI cortical activations from unilateral tactile-taste stimulations of the tongue

Functional magnetic resonance imaging (fMRI) was used for revealing activations in the human brain by lateralized tactile-gustatory stimulations of the tongue. Salt, a basic taste stimulus, and water, now recognized as an independent taste modality, were applied to either hemitongues with pads similar to the taste strips test for the clinical psychophysical evaluation of taste. With both stimuli, the observed cortical patterns of activations could be attributed to a combined somatosensory and gustatory stimulation of the tongue, with no significant differences between salt and water. Stimulation of each hemitongue evoked a bilateral activation of the anterior insula-frontal operculum, ascribable to the gustatory component of the stimulation, and a bilateral activation of the inferior part of the postcentral gyrus, ascribable to the tactile component of the stimulation. The results are in line with the notion that the representation of the tongue in the cerebral hemispheres in both the touch and the taste modalities is bilateral. Clinical and brain stimulation findings indicate that this bilaterality depends primarily on a partial crossing of the afferent pathways, perhaps with a predominance of the crossed pathway in the touch modality and the uncrossed pathway in the taste modality. Previous evidence suggests that the corpus callosum is not indispensible for this bilateral representation, but can contribute to it by interhemispheric transfer of information in both modalities.

Functional topography of the corpus callosum investigated by DTI and fMRI

This short review examines the most recent functional studies of the topographic organization of the human corpus callosum, the main interhemispheric commissure. After a brief description of its anatomy, development, microstructure, and function, it examines and discusses the latest findings obtained using diffusion tensor imaging (DTI) and tractography (DTT) and functional magnetic resonance imaging (fMRI), three recently developed imaging techniques that have significantly expanded and refined our knowledge of the commissure. While DTI and DTT have been providing insights into its microstructure, integrity and level of myelination, fMRI has been the key technique in documenting the activation of white matter fibers, particularly in the corpus callosum. By combining DTT and fMRI it has been possible to describe the trajectory of the callosal fibers interconnecting the primary olfactory, gustatory, motor, somatic sensory, auditory and visual cortices at sites where the activation elicited by peripheral stimulation was detected by fMRI. These studies have demonstrated the presence of callosal fiber tracts that cross the commissure at the level of the genu, body, and splenium, at sites showing fMRI activation. Altogether such findings lend further support to the notion that the corpus callosum displays a functional topographic organization that can be explored with fMRI.

Intracallosal neuronal nitric oxide synthase neurons colocalize with neurokinin 1 substance P receptor in the rat

The corpus callosum (cc) contains nitric oxide (NO)-producing neurons. Because NO is a potent vasodilator, these neurons could translate neuronal signals into vascular responses that can be detected by functional brain imaging. Substance P (SP), one of the most widely expressed peptides in the CNS, also produces vasomotor responses by inducing calcium release from intracellular stores through its preferred neurokinin 1 (NK1) receptor, thus inducing NO production via activation of neuronal NO synthase (nNOS). Single- and double-labeling experiments were performed to establish whether NK1-immunopositive neurons (NK1IP -n) are found in the rat cc and the extent of NK1 colocalization with nNOS. NK1IP -n were seen to constitute a large neuronal population in the cc and had a distribution similar to that of nNOSIP neurons (nNOSIP -n). NK1IP -n were numerous in the lateral cc and gradually decreased in the more medial portions, where they were few or absent. Intracallosal NK1IP -n and their dendritic trees were intensely labeled, allowing classification into four morphological types: bipolar, round, polygonal, and pyramidal. Confocal microscopic examination demonstrated that nearly all NK1IP -n contained nNOS (96.43%) and that 84.59% of nNOSIP -n co-expressed NK1. These data suggest that the majority of intracallosal neurons can release NO as a result of the action of SP. A small proportion of nNOSIP -n does not contain NK1 and is not activated by SP; these neurons may release NO via alternative mechanisms. The possible mechanisms by which intracallosal neurons release NO are also reviewed.

Further evidence for the topography and connectivity of the corpus callosum: an FMRI study of patients with partial callosal resection

BACKGROUND AND PURPOSE

This functional MRI study was designed to describe activated fiber topography and trajectories in the corpus callosum (CC) of six patients carrying different degree of partial callosal resection.

METHODS

Patients receiving gustatory, tactile, and visual stimulation according to a block-design protocol were scanned in a 1.5 Tesla magnet. Diffusion tensor imaging (DTI) data were also acquired to visualize spared interhemispheric fibers.

RESULTS

Taste stimuli evoked bilateral activation of the primary gustatory area in all patients and foci in the anterior CC, when spared. Tactile stimuli to the hand evoked bilateral foci in the primary somatosensory area in patients with an intact posterior callosal body and only contralateral in the other patients. Callosal foci occurred in the CC body, if spared. In patients with an intact splenium central visual stimulation induced bilateral activation of the primary visual area as well as foci in the splenium itself.

CONCLUSION

Present data show that interhemispheric fibers linking sensory areas crossed through the CC at the sites where the different sensory stimuli evoked activation foci, and that topography of callosal foci evoked by sensory stimulation in spared CC portions is consistent with that previously observed in subjects with intact CC.

Characterization of NO-producing neurons in the rat corpus callosum

INTRODUCTION

The aim of this study was to determine the presence and distribution of nitric oxide (NO)-producing neurons in the rat corpus callosum (cc).

MATERIAL AND METHODS

To investigate this aspect of cc organization we used nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry and neuronal NO synthase (nNOS) immunocytochemistry.

RESULTS

Intense NADPH-d-positive (NADPH-d+) neurons were found along the rostrocaudal extension of the cc (sagittal sections). They were more numerous in the lateral cc and gradually decreased in the more medial regions, where they were very few or absent. The Golgi-like appearance of NADPH-d+ intracallosal neurons allowed dividing them into five morphological types: (1) bipolar; (2) fusiform; (3) round; (4) polygonal; and (5) pyramidal. The number of NADPH-d+ neurons (both hemispheres) was counted in two brains using 50-μm thick sections. In the first brain, counts involved 145 sections and neurons were 2959; in the second, 2227 neurons were counted in 130 sections. The distribution and morphology of nNOS-immunopositive (nNOSIP) neurons was identical to that of NADPH-d+neurons. Some of these neurons were observed in the cc ependymal region, where they might be in contact with cerebrospinal fluid (CSF), monitoring its composition, pH, and osmolality changes, or playing a role in regulating the synthesis and release of several peptides. The somatic, dendritic, and axonal processes of many NADPH-d+/nNOSIP neurons were closely associated with intracallosal blood vessels.

CONCLUSIONS

Such close relationship raises the possibility that these neurons are a major source of NO during neural activity. As NO is a potent vasodilator, these findings strongly suggest that NO-positive neurons transduce neuronal signals into vascular responses in selected cc regions, thus giving rise to hemodynamic changes detectable by neuroimaging.

Differential distribution of parvalbumin- and calbindin-D28K-immunoreactive neurons in the rat periaqueductal gray matter and their colocalization with enzymes producing nitric oxide

The distribution, colocalization with enzymes producing nitric oxide (NO), and the synaptic organization of neurons containing two calcium-binding proteins (CaBPs) - parvalbumin (Parv) and calbindin-D28K (Calb) - were investigated in the rat periaqueductal gray matter (PAG). Parv-immunopositive (ParvIP) neurons were detected in the mesencephalic nucleus and rarely in the PAG. CalbIP neurons were found both in the dorsolateral (PAG-dl) and ventrolateral PAG (PAG-vl); their size ranged from 112.96 μm(2) (PAG-dl) to 125.13 μm(2) (PAG-vl). Ultrastructurally Parv and Calb immunoreactivity was mostly found in dendritic profiles. Axon terminals containing each of the two CaBPs formed symmetric synapses. Moreover both Parv and Calb were used to label a subpopulation of NO-producing neurons. Colocalization was investigated using two protocols: (i) a combination of Calb and Parv immunocytochemistry (Icc) with nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d) histochemistry (Hi) and (ii) neuronal NO synthase-Icc (nNOS) (immunofluorescence). Both techniques demonstrated a complete lack of colocalization of Parv and NADPH-d/nNOS in PAG neurons. Double-labeled (DL) neurons (Calb-NADPH-d; Calb-nNOS) were detected in PAG-dl. NADPH-d-Hi/Calb-Icc indicated that 41-47% of NADPH-d-positive neurons contained Calb, whereas 17-23% of CalbIP cells contained NADPH-d. Two-color immunofluorescence revealed that 53-66% of nNOSIP cells colocalized with Calb and 24-34% of CalbIP neurons contained nNOS. DL neuron size was 104.44 μm(2); neurons labeled only with NADPH-d or Calb measured 89.793 μm(2) and 113.48 μm(2), respectively. Together with previous findings (Barbaresi et al. [2012]) these data suggest that: Therefore the important aspect of the PAG intrinsic organization emerging from this and previous double-labeling studies is the chemical diversity of NO-synthesizing neurons, which is likely related to the different functions in which these neurons are involved.

Heterogeneity of glutamatergic and GABAergic release machinery in cerebral cortex

We investigated whether cortical glutamatergic and GABAergic release machineries can be differentiated on the basis of the proteins they express, by studying the degree of co-localization of synapsin (SYN) I and II, synaptophysin (SYP) I and II, synaptosomal-associated protein (SNAP)-25 and SNAP-23 in vesicular glutamate transporter (VGLUT) 1-, VGLUT2- and vesicular GABA transporter (VGAT)-positive (+) puncta in the rat cerebral cortex. Co-localization studies showed that SYNI and II were expressed in approximately 90% of VGLUT1+, approximately 30% of VGLUT2+ and 30-50% of VGAT+ puncta; SYPI was expressed in approximately 95% of VGLUT1+, 30% of VGLUT2+, and 45% of VGAT+ puncta; SYPII in approximately 7% of VGLUT1+, 3% of VGLUT2+, and 20% of VGAT+ puncta; SNAP-25 in approximately 94% of VGLUT1+, 5% of VGLUT2+, and 1% of VGAT+ puncta, and SNAP-23 in approximately 3% of VGLUT1+, 86% of VGLUT2+, and 22% of VGAT+ puncta. Since SYPI, which is considered ubiquitous, was expressed in about half of GABAergic axon terminals, we studied its localization electron microscopically and in immunoisolated synaptic vesicles: these studies showed that approximately 30% of axon terminals forming symmetric synapses were SYPI-negative, and that immunoisolated VGAT-positive synaptic vesicles were relatively depleted of SYPI as compared with VGLUT1+ vesicles. Overall, the present investigation shows that in the cerebral cortex of rats distinct presynaptic proteins involved in neurotransmitter release are differentially expressed in GABAergic and in the two major types of glutamatergic axon terminals in the cerebral cortex of rats.

GABA-immunoreactive neurons and terminals in the cat periaqueductal gray matter: a light and electron microscopic study

Immunocytochemical and electron microscopic methods were used to study the GABAergic innervation in adult cat periaqueductal gray matter (PAG). A mouse monoclonal antibody against gamma -aminobutyric acid (GABA) was used to visualize the inhibitory neuronal system of PAG. At light microscopy, GABA-immunopositive (GABA(IP)) neurons formed two longitudinally oriented columns in the dorsolateral and ventrolateral PAG that accounted for 36% of the neuronal population of both PAG columns; their perikaryal cross-sectional area was smaller than that of unlabeled (UNL) neurons found in the same PAG subdivisions. At electron microscopic level, patches of GABA immunoreactivity were readily detected in neuronal cell bodies, proximal and distal dendrites, axons and axon terminals. Approximately 35-36% of all terminals were GABA(IP); they established symmetric synapses with dendrites (84.72% of the sample in the dorsolateral PAG and 86.09% of the sample in the ventrolateral PAG) or with cell bodies (7-10% of the sample). Moreover, 49.15% of GABA(IP) axon terminals in the dorsolateral and 52.16% in the ventrolateral PAG established symmetric synapses with GABA(IP) dendrites. Immunopositive axon terminals and unlabeled terminals were also involved in the formation of a complex synaptic arrangment, i.e. clusters of synaptic terminals in close contact between them that were often observed in the PAG neuropil. Moreover, a fair number of axo-axonic synapses between GABA(IP) and/or UNL axon terminals were present in both PAG subdivisions. Several dendro-dendritic synapses between labeled and unlabeled dendrites were also observed in both PAG subdivisions. These results suggest that in the cat PAG there exist at least two classes of GABArgic neurons. The first class could exert a tonic control on PAG projecting neurons, the second could act on those GABAergic neurons that in turn keep PAG projecting neurons under tonic inhibition. The functional implications of this type of GABAergic synapse organization are discussed in relation to the dishinibitory processes that take place in the PAG.

Neuronal localization of the GABA transporter GAT-3 in human cerebral cortex: A procedural artifact?

Gamma-amino butyric acid (GABA) plasma membrane transporters (GATs) contribute to the modulation of GABA's actions and are implicated in neuropsychiatric diseases. In this study, the localization of GAT-3, the major glial GAT, was investigated in human cortex using immunocytochemical techniques. In prefrontal and temporal cortices, GAT-3 immunoreactivity (ir) was present throughout the depth of the cortex, both in puncta and in neurons. GAT-3-positive puncta were dispersed in the neuropil or closely related to cell bodies; neuronal staining was in perikarya, especially of pyramidal cells, and proximal dendrites. Electron microscopic studies showed that GAT-3 ir was in astrocytic processes as well as in neuronal elements. All GAT-3-positive neurons co-expressed heat shock protein 70. To test the possibility that the collection procedure of human samples induced the expression of GAT-3 in neurons which normally do not express it, we analyzed rat cortical tissue resected using the same procedure and found that numerous neurons are GAT-3-positive and that they co-express heat shock protein 70. Results show that in human cortex GAT-3 is expressed in astrocytic processes and in neurons and suggest that neuronal expression is related to the procedure used for collecting human samples.

Localization of the glutamine transporter SNAT1 in rat cerebral cortex and neighboring structures, with a note on its localization in human cortex

SNAT1 mediates glutamine (Gln) influx into neurons and is believed to replenish the transmitters pools of glutamate (Glu) and gamma-aminobutyric acid (GABA). We investigated its distribution and cellular localization in the cerebral cortex and neighboring regions of rats and humans using light and electron microscopic immunocytochemical methods with specific antibodies. In the first somatic sensory cortex of rats and in areas 9, 10, 21 and 46 of the human cortex, numerous SNAT1-positive (+) cells were present in the cortical parenchyma and in the white matter; >95% of SNAT1+ cells were neurons, but some were astrocytes. Most SNAT1+ cells were pyramidal neurons, but numerous non-pyramidal neurons were also observed: SNAT1/GABA double-labeling studies showed that SNAT1 is expressed in all GABA+ neurons. SNAT1/synaptophysin studies showed that <0.1% of all synaptophysin+ puncta coexpressed SNAT1. SNAT1 immunoreactivity (ir) was also in leptomeninges, ependymal cells and choroid plexus. Electron microscopic studies showed that neuronal SNAT1 ir was almost exclusively observed in perikarya and dendritic profiles. SNAT1 ir was also in distal astrocytic processes, including end feet profiles, and in leptomeninges. These findings suggest that the major function of SNAT1 is not to replenish the transmitter pools of Glu and GABA.

Postnatal development of high-affinity plasma membrane GABA transporters GAT-2 and GAT-3 in the rat cerebral cortex

We investigated the developmental profile of plasma membrane gamma-aminobutyric acid (GABA) transporters (GATs) GAT-2 and GAT-3 expression by immunocytochemistry with affinity-purified polyclonal antibodies in the rat neocortex. At all developmental ages investigated, GAT-2 ir was prominent in the arachnoid and in the trabeculae of the subarachnoid space, whereas it was weak within the cortical parenchyma; the adult pattern was reached during the third week of postnatal life. GAT-3 ir was present at birth and increased rapidly in the first week, when numerous positive cells were present throughout the cortical layers; at P10, GAT-3-positive cells became less numerous and GAT-3 ir switched to the adult pattern, which was expressed at P20. Confocal and electron microscopic investigations showed that GAT-3 positive cells were both neurons and astrocytes. The present evidence indicates that early in development GAT-3 is abundantly expressed in the cerebral cortex, where its expression appears to correlate with developmental variations in GABA levels, and suggests that it accounts for the largest fraction of GABA transport observed in the neonatal cerebral cortex.

Direct synaptic connections between corticothalamic axon terminals and interneurons in the cat ventrobasal complex

Lesion-induced degeneration was combined with immunocytochemistry to study, with electron microscopy, the synaptic connectivity between corticothalamic axon terminals from the first and second somatosensory areas and local circuit neurons of the ipsilateral ventrobasal complex (VB), selectively labelled with an antibody raised against gamma-aminobutyric acid (GABA). Four days from the cortical ablation many degenerating axon terminals, forming asymmetric synapses, were found on dendritic trees of both labelled and unlabelled neurons of VB and occasionally on presynaptic dendrites. The main finding of the present paper is that 64.01% of degenerating axon terminals synapsed with GABA-immunopositive dendrites, suggesting that the principal target of the cortical projection to VB are local circuit neurons.

The glial glutamate transporter GLT-1 is localized both in the vicinity of and at distance from axon terminals in the rat cerebral cortex

Glutamate transporter-1 (GLT-1) is responsible for the largest proportion of glutamate transport in the brain and the density of GLT-1 molecules inserted in the plasma membrane is highest in regions of high demand. Previous electron microscopic studies in the hippocampus and cerebellum have shown that GLT-1 is concentrated both in the vicinity of and at considerable distance from the synaptic cleft [Chaudry et al., Neuron 15 (1995) 711-721], but little is known about its distribution in the neocortex. We therefore studied the spatial relationships between elements expressing the presynaptic marker synaptophysin and those containing GLT-1 in the rat cerebral cortex using confocal microscopy. Preliminary studies confirmed that GLT-1 positive puncta were exclusively astrocytic processes; moreover, they showed that in most cases GLT-1 positive processes either completely surrounded asymmetric synapses or had no apparent relationship with synapses; occasionally, they were apposed to terminals containing pleomorphic vesicles. In sections double-labeled for GLT-1 and synaptophysin, codistribution analysis revealed that 61.2% of pixels detecting fluorescent emission for GLT-1 immunoreactivity overlapped with pixels detecting synaptophysin. The percentages of GLT-1/synaptophysin codistribution were significantly different from controls. In sections double-labeled for GLT-1 and the vesicular GABA transporter, codistribution analysis revealed that 27% of pixels detecting GLT-1 overlapped with those revealing the vesicular GABA transporter.The remarkable 'synaptic' localization of GLT-1 provides anatomical support for the hypothesis that in the cerebral cortex GLT-1 contributes to shaping fast, point-to-point, excitatory synaptic transmission. Moreover, the considerable fraction of GLT-1 immunoreactivity localized at sites distant from axon terminals supports the notion that glutamate spillout occurs also in the intact brain and suggests that 'extrasynaptic' GLT-1 regulates the diffusion of glutamate escaped from the cleft.

Glutamate transporter EAAC1 in the cat periaqueductal gray matter

The neuron-specific glutamate (Glu) transporter, excitatory amino acid carrier 1 (EAAC1), plays an important role in regulating Glu levels in the synaptic cleft. Using a specific EAAC1 monoclonal antibody, we investigated its regional distribution and ultrastructural localization in cat periaqueductal gray matter. In light microscopy EAAC1 immunoreactivity was randomly distributed to neurons and punctate structures. In electron microscopy, it was observed in the soma of many neurons, dendrites, in a discrete number of axon terminals, in ependymal cells and in few distal astrocytic processes. EAAC1 is thus not restricted to neurons, but could play an important role in glial cells. Moreover, EAAC1 protein could participate both in regulating the action of the Glu released at synaptic level and in other aspects of Glu metabolism.

gamma-Aminobutyric acid transporters in the cat periaqueductal gray: a light and electron microscopic immunocytochemical study

The gamma-aminobutyric acid (GABA) plasma membrane transporters (GATs) mediate GABA uptake into presynaptic axon terminals and glial processes, thus contributing to the regulation of the magnitude and duration of the action of GABA at the synaptic cleft. The aim of the present study was to investigate the expression of three high-affinity GABA transporters (GAT-1, GAT-2, and GAT-3) in the periaqueductal gray matter (PAG) of adult cats by using immunocytochemistry with affinity-purified antibodies. Light microscopic observations revealed GAT-1 immunoreactivity in punctate structures, particularly dense in the lateral portion of the dorsolateral PAG column. Weak GAT-2-immunopositive puncta were homogeneously distributed in the PAG. GAT-3 immunoreactivity was detected in each column of the PAG but was more intense in the dorsolateral PAG column and around the aqueduct. Electron microscopic studies showed GAT-1 immunoreactivity in distal astroglial processes, in unmyelinated and small myelinated axons, and in axon terminals making symmetric synapses on both PAG neurons and dendrites. GAT-2 immunoreactivity was present mostly in the form of patches of different sizes in the cytoplasm of neuronal elements like the perikarya and dendrites of PAG neurons, in myelinated and unmyelinated axons, and in the axon terminals forming both symmetric and asymmetric synapses. Labeling was also observed in nonneuronal elements. Astrocytic cell bodies and their distal processes as well as the ependymal cells lining the wall of the aqueduct showed patches of GAT-2 immunoreactivity. Electron microscopic observation revealed GAT-3 immunoreactivity exclusively in distal astrocytic processes adjacent to the somata of PAG neurons and in axon terminals making both symmetric and asymmetric synapses. The present results suggest that three types of termination systems of GABAergic transmission are present in the cat periaqueductal gray matter.

Neuronal, glial, and epithelial localization of gamma-aminobutyric acid transporter 2, a high-affinity gamma-aminobutyric acid plasma membrane transporter, in the cerebral cortex and neighboring structures

Neuronal and glial high-affinity Na+/Cl(-)-dependent plasma membrane gamma-aminobutyric acid (GABA) transporters (GATs) contribute to regulating neuronal function. We investigated in the cerebral cortex and neighboring regions of adult rats the distribution and cellular localization of the GABA transporter GAT-2 by immunocytochemistry with affinity-purified polyclonal antibodies that react monospecifically with a protein of 82 kDa. Conventional and confocal laser-scanning light microscopic studies revealed intense GAT-2 immunoreactivity (ir) in the leptomeninges, choroid plexus, and ependyma. Weak GAT-2 immunoreactivity also was observed in the cortical parenchyma, where it was localized to puncta of different sizes scattered throughout the radial extension of the neocortex and to few cell bodies. In sections double-labeled with GAT-2 and glial fibrillary acidic protein (GFAP) antibodies, some GAT-2-positive profiles also were GFAP positive. Ultrastructural studies showed GAT-2 immunoreactivity mostly in patches of varying sizes scattered in the cytoplasm of neuronal and nonneuronal elements: GAT-2-positive neuronal elements included perikarya, dendrites, and axon terminals forming both symmetric and asymmetric synapses; nonneuronal elements expressing GAT-2 were cells forming the pia and arachnoid mater; astrocytic processes, including glia limitans and perivascular end feet; ependymal cells; and epithelial cells of the choroid plexuses. The widespread cellular expression of GAT-2 suggests that it may have several functional roles in the overall regulation of GABA levels in the brain.

Neuronal and glial localization of NR1 and NR2A/B subunits of the NMDA receptor in the human cerebral cortex

N-Methyl-D-aspartate (NMDA) receptors play a critical role in many cortical functions and are implicated in several neuropsychiatric diseases. In this study, the cellular expression of the NMDAR1 (NR1) and NMDAR2A and B (NR2A and B) subunits was investigated in the human cerebral cortex by immunocytochemistry with antibodies that recognize the NR1 or the NR2A and B subunits of the NMDA receptor. In frontal (areas 10 and 46) and temporal (area 21) association cortices and the cingulofrontal transition cortex (area 32), NR1 and NR2A/B immunoreactivity (ir) were similar and were localized to numerous neurons in all cortical layers. NR1- and NR2A/B-positive neurons were mostly pyramidal cells, but some nonpyramidal neurons were also labeled. Electron-microscopic observations showed that NR1 and NR2A/B ir were similar. In all cases, labeling of dendrites and dendritic spines was intense. In addition, both NR1 and NR2A/B were consistently found in the axoplasm of some axon terminals and in distal astrocytic processes. This investigation revealed that numerous NMDA receptors are localized to dendritic spines, and that they are also localized to axon terminals and astrocytic processes. These findings suggest that the effects of cortical NMDA activation in the human cortex do not depend exclusively on the opening of NMDA channels located at postsynaptic sites, and that the localization of NMDA receptors is similar in a variety of mammalian species.

Immunocytochemical localization of substance P receptor in rat periaqueductal gray matter: a light and electron microscopic study

The distribution of substance P receptor (SPR) protein in the rat periaqueductal gray matter (PAG) was investigated with a polyclonal antibody in the four subdivisions obtained by cytochrome-oxidase histochemistry (Co-hi). At light microscopic analysis, immunoreactivity appeared particularly dense in the dorsal subdivision of the PAG, was less intense in the other subdivisions, and formed several longitudinally organized columns. SPR-like immunoreactivity (SP(R-i)) was localized mostly to cell bodies and dendrites of small and medium-sized neurons, which constituted about 6% of the total neuronal population of the PAG. At the electron microscopic level, SP(R-i) could be observed on postsynaptic as well as on nonsynaptic regions of both cell bodies and dendrites. A small proportion of axons (4.2%) and axon terminals (5.3%) showed SP(R-i), the majority of labeled axon terminals, amounting to about 70% of synapsing elements, formed asymmetric synapses with dendrites. Rare astroglial processes displaying SP(R-i) were also observed scattered throughout the neuropil of all PAG subdivisions. Our observations suggest that 1) also in the PAG, SP may act in a diffuse, nonsynaptic manner, probably on targets that are distant from its sites of release; and 2) SP may modulate excitatory neurotransmission acting presynaptically on those labeled axons that form asymmetric synapses.

GABA transporter-1 (GAT-1) immunoreactivity in the cat periaqueductal gray matter

Using light- and electron-microscopic immunocytochemistry, we investigated the regional distribution and the ultrastructural localization of GAT-1, a prominent GABA transporter, in the cat PAG. Light microscopic observations indicate that GAT-1-immunoreactive elements are particularly dense in PAG-DL and form a pair of longitudinal columns extending in the intermediate region of this structure. At electron-microscopic level, GAT-1 immunoreactivity was present in axon terminals forming symmetric synapses and in the distal processes of astroglial cells. These data further confirm the existence of longitudinal columns within PAG. They also indicate that GAT-1 could influence the action of GABA on its receptors, probably regulating the magnitude and duration of GABA's synaptic action on PAG neurons, and suggest that astrocytes may play an important role in this process.

Glutamate-positive neurons and terminals in the cat periaqueductal gray matter (PAG): a light and electron microscopic immunocytochemical study

The morphology, distribution, proportion, size, and synaptic organization of periaqueductal gray matter neurons labeled with immunocytochemical techniques by an anti-glutamate (Glu) polyclonal serum were investigated in six adult cats (PAG-GLU 1-6). At the light microscopic level, numerous Glu-positive neurons were found throughout each subdivision of the periaqueductal gray matter. Their proportion and size, calculated in semi-thin sections (1-microm-thick), varied slightly among the subdivisions of the periaqueductal gray matter. The morphology of Glu-positive neurons was similar to that of the multipolar, triangular, and fusiform cells described in previous Golgi studies. Numerous puncta, interpreted as dendrites, axons, and axon terminals were also present in all subdivisions without preferential distribution. At the electron microscopic level, all synaptic contacts made by Glu-positive axon terminals were of the asymmetric type, but not all presynaptic elements making asymmetric synapses were labeled. The vast majority of postsynaptic elements contacted by Glu-positive axon terminals were labeled and unlabeled dendrites. The present results describe for the first time the presence of both Glu-positive neurons and terminals in the feline periaqueductal gray matter and provide further evidence that Glu is the probable neurotransmitter of numerous excitatory neurons of this structure.

Laminar pattern of termination of the ipsilateral cortical projection from SII to SI in cats

The present light and electron microscopic experiments were carried out on the first somatic sensory area (SI) of cats to determine the laminar distribution of axon terminals from the ipsilateral second somatic sensory area (SII) and to identify the types of synapses between these terminals and the neuronal elements of SI. Phaseolus vulgaris-leucoagglutinin (PHA-L) was iontophoretically injected into multiple sites and at different cortical depths of the forepaw representation zone of SII. Fixed brain blocks containing the injected SII and ipsilateral SI were cut into slices and processed immunocytochemically to stain PHA-L-filled fibers and terminals. Light microscopic examination of SI revealed patches of anterograde labeling in the forepaw representation zone, concentrated mainly in supragranular layers. In these layers, thin immunolabeled fibers branched extensively and formed a dense plexus that was more prominent in layers II and I. Conversely, the infragranular layers contained fragments of vertically oriented thick fibers that rarely emitted axon collaterals. PHA-L-labeled axons had numerous swellings along their course, interpreted as boutons en passant, and stalked boutons. Of 19,661 labeled terminals (17,833 beads and 1,828 stalked boutons), 84.74% were observed in supragranular layers, with the highest concentration in layer II (33.15%) and lower in layers I (26.27%) and III (25.30%). The proportion of terminals was lower in layers IV (6.49%) and V (5.45%) and lowest in layer VI (3.32%). These counts also showed that boutons en passant were the majority (90.70%) and stalked boutons, the minority (9.30%). The ratio of these two types of presynaptic specializations was similar (9:1) in all six layers. Electron microscopic examination of the labeled regions of SI showed that both axon swellings and stalked boutons formed synapses of the asymmetric type with SI neuronal elements. The majority (85.37%) of a sample of 130 labeled terminals synapsed on SI neurons in layers I-III. The identified postsynaptic profiles were dendritic spines (61.11%) or medium-sized and small dendrites (38.89%). These results are discussed in relation to those of a companion study on the laminar pattern of the projection from SI to SII of cats (P. Barbaresi, A. Minelli, and T. Manzoni, 1994, J. Comp. Neurol. 343:582-596). Based on the anatomical organization of these reciprocal connections, there seems to be no clear hierarchicalal relationship between SI and SII in cats.

Commissural connections of the cat periaqueductal gray matter studied with anterograde and retrograde tract-tracing techniques

The commissural connections of the periaqueductal gray matter were investigated by light and electron microscopy by using the anterograde tracer Phaseolus vulgaris leucoagglutinin and the retrograde tracer horseradish peroxidase. In the first group of seven animals (1-7), single injections of Phaseolus vulgaris leucoagglutinin were performed iontophoretically (4.5 microA for 30 min) into various subdivisions of the periaqueductal gray matter. On light microscopic examination, injection sites were characterized by several immunolabeled neurons of different sizes and morphology, with the cytoplasm, nucleus and neuronal processes intensely stained. Many labeled fibers turned from injection sites toward all contralateral periaqueductal gray matter subdivisions, but anterograde labeling was densest in the regions homotopic to those injected. Commissural fibers bore along their course many en passant boutons of different sizes and morphology, and gave off spine-like processes, at the end of which one terminal bouton was observed. Labeled fibers branched into numerous collaterals which ended in a terminal array of 10-20 en passant and en grappe boutons. At the electron microscopic level, commissural axons were observed in close proximity to the cytoplasmic membranes of cells. Axon terminals formed symmetric or asymmetric synapses mainly on dendritic shafts of neurons and rarely on vesicle-containing profiles. Horseradish peroxidase experiments were carried out in four cats (1-4). The tracer was injected iontophoretically into different regions of the periaqueductal gray matter of three cats (1-3). Retrogradely labeled neurons giving rise to commissural connections had a morphology similar to that of polygonal, triangular and fusiform cells described in previous Golgi studies. The perikaryal cross-sectional area of commissural neurons was smaller than that of neurons projecting outside the periaqueductal gray matter (mean value of commissural neurons 149.77 microns 2 vs 261.19 microns 2 for projecting neurons), which were retrogradely labeled by pressure-injecting horseradish peroxidase into several targets of periaqueductal gray matter (4). Moreover, since the distribution of sizes of the two populations of the periaqueductal gray matter overlapped in the range of 90-300 microns 2, a considerable number of projecting neurons were as small as commissural neurons. The present results suggest that commissural fibers could reciprocally connect zones of the periaqueductal gray matter with similar functions, and originate from small and medium-sized neurons, some of which are also projecting neurons.

Topographical relations between ipsilateral cortical afferents and callosal neurons in the second somatic sensory area of cats

Experiments were carried out on the second somatic sensory area (SII) of cats to study 1) the laminar distribution of axon terminals from the ipsilateral first somatic sensory cortex (SI); and 2) the topographical relations between their terminal field and the callosal neurons projecting to the contralateral homotopic cortex. To label simultaneously in SII both ipsilateral cortical afferents and callosal cells, cats were given iontophoretic injections of Phaseolus vulgaris-leucoagglutinin (PHA-L) in the forepaw zone of ipsilateral SI, and pressure injections of horseradish peroxidase (HRP) in the same zone of contralateral SII. The possibility that ipsilateral cortical axon terminals synapse callosal neurons was investigated with the electron microscope by combining lesion-induced degeneration with retrograde HRP labelling. Fibers and terminations immunolabelled with PHA-L from ipsilateral SI were distributed in SII in a typical patchy pattern and were mostly concentrated in supragranular layers. Labelled fibers formed a very dense plexus in layer III and ramified densely also in layers I and II. Labelled axon terminals were both en passant and single-stalked boutons. Counts of 8,303 PHA-L-labelled terminals of either type showed that 82.40% were in supragranular layers. The highest concentration was in layer III (43.99%), followed by layers II (30.32%) and I (8.09%). The remaining terminals were distributed among layers IV (6.96%), V (4.93%), and VI (5.68%). The same region of SII containing anterogradely labelled axons and terminals also contained numerous neurons retrogradely labelled with HRP from contralateral SII. Callosal projection neurons were pyramidal, dwelt mainly in layer III, and were distributed tangentially in periodic patches. Patches of anterograde and retrograde labelling either interdigitated or overlapped both areally and laminarly. In the zones of overlap, numerous PHA-L-labelled axon terminals were seen in close apposition to HRP-labelled pyramidal cell dendrites. Combined HRP-electron microscopic degeneration experiments showed that in SII axon terminals from ipsilateral SI form asymmetric synapses with HRP-labelled dendrites and dendritic spines pertaining to callosal projection neurons. These results are discussed in relation to the layering and function of the SI to SII projection, and to the evidence that SII neurons projecting to the homotopic area of the contralateral hemisphere have direct access to the sensory information transmitted from ipsilateral SI.

Thalamic connections of the second somatic sensory area in cats studied with anterograde and retrograde tract-tracing techniques

The thalamic connections of the second somatosensory area in the anterior ectosylvian gyrus of cats have been investigated using the retrograde tracer horseradish peroxidase and the anterograde tracer Phaseolus vulgaris leucoagglutinin. Horseradish peroxidase was injected iontophoretically in several somatotopic zones of the second somatosensory area map of six cats. Sites of horseradish peroxidase delivery were identified preliminarily by recording with microelectrodes the responses of neurons to skin stimulation. Phaseolus vulgaris leucoagglutinin was iontophoretically injected within the ventrobasal complex (one cat) or in the posterior complex (one cat). Horseradish peroxidase injections into cytoarchitectonic area SII retrogradely labeled neurons in the ipsilateral ventrobasal complex and in the posterior complex. Counts of labeled neurons from the ipsilateral thalamus showed that the overwhelming majority of horseradish peroxidase-labeled neurons were in the ventrobasal complex (96.3-96.9%) and few were in the posterior complex (3.1-3.7%). Neurons labeled in the ventrobasal complex were observed throughout the anteroposterior extent of the nucleus, while their mediolateral distribution varied with the site of horseradish peroxidase delivery in the body map of the second somatosensory area, which indicates that the projections from the ventrobasal complex to the second somatosensory area are somatotopically organized. In the cat in which the horseradish peroxidase injection involved both the second somatosensory area proper and the second somatosensory area medial, which lies in the lower bank of suprasylvian sulcus, labeled neurons were almost as numerous in the ventrobasal complex as in the posterior complex. Phaseolus vulgaris leucoagglutinin injected in the ventrobasal complex anterogradely labeled thalamocortical fibers in the ipsilateral anterior ectosylvian gyrus. In this case, patches of labeled fibers and terminals were distributed exclusively within the cytoarchitectonic borders of the second somatosensory area proper. Labeled terminals were numerous in layer IV and lower layer III, but terminal boutons and fibers with axonal swellings, probably forming synapses en passant, were frequently observed also in layers VI and I. Injection of Phaseolus vulgaris leucoagglutinin in the posterior complex labeled thalamocortical fibers in two distinct regions in the ipsilateral anterior ectosylvian gyrus, one lying laterally and the other medially, which correspond, respectively, to the fourth somatosensory area and the second somatosensory area medial. In both areas the densest plexus of labeled fibers and axon terminals was in layer IV and lower layer III, but numerous labeled fibers and terminals were also observed in layer I. In this case, only rare fragments of labeled fibers were present in second somatosensory area proper, but no labeled terminals could be observed.

Organization of claustro-cortical projections to the primary somatosensory area of primates

Different fluorescent dyes were injected in the face (S1fa) and hand (S1hn) representations of the primary somatosensory cortex, involving both areas 3b and 1. Claustral neurons labeled by either S1fa or S1hn were divided in two populations. One population was located in the dorsal part of the nucleus: neurons labeled from S1fa were placed laterally to those labeled from S1hn. The second population was located more ventrally, with a rostro-caudal distribution of S1fa vs S1hn neurons. These findings demonstrate the existence of ordered and possibly multiple somatosensory representations in the monkey claustrum.

Matching of receptive fields in the association projections from SI to SII of cats

Anatomical and electrophysiological experiments were performed on cats to investigate the pattern of divergence and convergence in the association projections from the first (SI) to the second (SII) somatic sensory cortex and to ascertain whether diverging and converging fibre components from SI have receptive fields (RFs) matching those of target neurons in SII. In the first group of six cats, a single deposit of horseradish peroxidase (HRP) was iontophoretically placed (2-4 microA for 20 minutes) into an electrophysiologically identified site of the SII map: the digit (3 cats), forepaw (2 cats), and arm (1 cat) zones. The forelimb representation in ipsilateral SI was subsequently explored with microelectrodes and RFs from small clusters of neurons systematically mapped. Planar maps of this area were reconstructed with the aid of a computer from serial sections, to correlate on the tangential plane the topographical distribution of retrogradely labelled association neurons with the physiological map of the forelimb. Since diverging projections were observed from a zone of SI to multiple zones of SII, double-labelling experiments were carried out in a second group of three cats, in which two retrograde fluorescent dyes (diamidino yellow and fast blue) were injected by pressure into two different sites of the SII map, to ascertain whether SI sends diverging projections by branching axons. HRP injections in SII retrogradely labelled a discrete number of association neurons in SI. Their distribution area was several tens of times wider than that covered by the injection site. This suggests that a remarkable amount of divergence and convergence exists in the association projections from SI to SII. Despite the substantial difference in the extent of the injected and labelled areas, RFs of afferent and target neurons corresponded closely. Injections covering a small region within a single digit zone of SII labelled neurons throughout the entire representation of the same digit in SI, while neurons labelled in somatotopically inappropriate zones were rare. RFs mapped from several sites of the labelled region in SI were individually smaller than the RF mapped from the injection site in SII, but the overall size of afferent RFs encompassed that of target neurons. Divergence and convergence in the SI projections to SII zones representing more proximal portions of the forelimb may be even greater since HRP injections in the forepaw and arm zones of SII labelled a number of neurons also in the digit zone of SI, providing the RFs mapped from the injection sites were sufficiently wide to include the digits.(ABSTRACT TRUNCATED AT 400 WORDS)

Callosal connections of the somatic sensory areas II and IV in the cat

The homotopic and heterotopic callosal connections in the forelimb representations of the second (SII) and fourth (SIV) somatic sensory areas of cats were investigated by means of the axonal transport of horseradish peroxidase (HRP) in conjunction with microelectrode recording. The tracer was injected in the electrophysiologically identified hand and/or digit zone of SII (six cats) or SIV (four cats). The homotopic area in the contralateral hemisphere was explored with microelectrodes in five animals (three injected in SII and two in SIV) to map neuronal receptive fields. The aim was to correlate in the same experimental case the topography of labelled callosal neurons with the physiological map of the forelimb. Labelled cells and recording sites were plotted on planar maps reconstructed with the aid of a computer from serial coronal sections from the anterior ectosylvian gyrus. After SII injections, labelled callosal neurons were observed throughout the forelimb representation in the contralateral area, but in the tangential plane their distribution was uneven. Each somatotopic zone composing the forelimb map, that is, the arm, hand, and digit zones, contained several subzones in which callosal neurons were either dense or rare. Microelectrode explorations showed that receptive fields mapped from callosal and relatively acallosal subzones representing the same body part were similar in extent and location. After SIV injections, labelled callosal neurons were observed throughout the forelimb and proximal body representation of the contralateral area. Although slight regional variations in the density of labelled cells were apparent, no subzones bare of callosal labelling were observed in SIV. In both SII and SIV, callosal neurons were concentrated mainly in layer III, but a significant number was also evident in the infragranular layers. After HRP injections in the digit zone of SII or SIV, labelled cell bodies were also observed in heterotopic areas of the contralateral hemisphere. Most of these neurons were clustered in the medial bank of the coronal sulcus and in two other heterotopic cortical regions lying, respectively, in the anterior suprasylvian sulcus and in the lateral branch of the ansate sulcus. Some callosal cells interconnecting SII and SIV were also labelled. The results show that the distal forelimb zones in SII and SIV are callosally connected with the respective homotopic zones and with several somatosensory fields located heterotopically in the contralateral hemisphere.

Glutamate decarboxylase-immunoreactive neurons and terminals in the periaqueductal gray of the rat

The periaqueductal gray of 5 rats was processed for immunocytochemistry using an antiserum to glutamate decarboxylase. In both colchicine-pretreated (4 rats) and untreated (1 rat) animals, glutamate decarboxylase-positive cell bodies were present in all periaqueductal gray subdivisions, especially in the dorsal and ventrolateral subdivision. The perikaryal cross-sectional area of labelled neurons was smaller than that of periaqueductal gray projecting neurons retrogradely labelled with horseradish peroxidase in separate experiments. The morphology of glutamate decarboxylase-containing neurons resembled that of small polygonal, triangular and fusiform cells described in previous Golgi studies. Glutamate decarboxylase immunoreactivity was also observed in a large number of terminal-like structures, most of which were distributed close to the somata and dendrites of both glutamate decarboxylate-positive and -negative neurons. At all rostrocaudal levels the highest concentration of these elements was observed around the aqueduct. These results suggest that two sub-populations of neurons are present in the periaqueductal gray of rats, one consisting of small-sized glutamate decarboxylase-positive neurons (intrinsic neurons) and the other of large-sized glutamate decarboxylase-negative neurons (projecting neurons). Intrinsic circuits could be present between glutamate decarboxylase-positive and -negative neurons and between glutamate decarboxylase-positive neurons.

Cytochrome oxidase histochemistry reveals regional subdivisions in the rat periaqueductal gray matter

The identification of different anatomical regions of the periaqueductal gray matter of rats was addressed in the present study by using the histochemical staining for the mitochondrial enzyme cytochrome oxidase. At caudal and middle levels, cytochrome oxidase histochemistry clearly demonstrates the existence of four subdivisions: dorsal, dorsolateral, ventrolateral and medial, whereas in sections from the rostral periaqueductal gray matter only two concentric bands are identifiable on the basis of the degree of cytochrome oxidase activity.

D-[3H]aspartate retrograde labelling of callosal and association neurones of somatosensory areas I and II of cats

Experiments were carried out on cats to ascertain whether corticocortical neurones of somatosensory areas I (SI) and II (SII) could be labelled by retrograde axonal transport of D-[3H]aspartate (D-[3H]Asp). This tritiated enantiomer of the amino acid aspartate is (1) taken up selectively by axon terminals of neurones releasing aspartate and/or glutamate as excitatory neurotransmitter, (2) retrogradely transported and accumulated in perikarya, (3) not metabolized, and (4) visualized by autoradiography. A solution of D-[3H]Asp was injected in eight cats in the trunk and forelimb zones of SI (two cats) or in the forelimb zone of SII (six cats). In order to compare the labelling patterns obtained with D-[3H]Asp with those resulting after injection of a nonselective neuronal tracer, horseradish peroxidase (HRP) was delivered mixed with the radioactive tracer in seven of the eight cats. Furthermore, six additional animals received HRP injections in SI (three cats; trunk and forelimb zones) or SII (three cats; forelimb zone). D-[3H]Asp retrograde labelling of perikarya was absent from the ipsilateral thalamus of all cats injected with the radioactive tracer but a dense terminal plexus of anterogradely labelled corticothalamic fibres from SI and SII was observed, overlapping the distribution area of thalamocortical neurones retrogradely labelled with HRP from the same areas. D-[3H]Asp-labelled neurones were present in ipsilateral SII (SII-SI association neurones) in cats injected in SI. In these animals a bundle of radioactive fibres was observed in the rostral portion of the corpus callosum entering the contralateral hemisphere. There, neurones retrogradely labelled with silver grains were present in SI (SI-SI callosal neurones). Association and callosal neurones labelled from SI showed a topographical distribution similar to that of neurones retrogradely labelled with HRP. The laminar patterns of corticocortical neurones labelled with D-[3H]Asp or with HRP were also similar, with one exception. In the inner half of layer II, SII-SI association neurones and SI-SI callosal neurones labelled with the radioactive marker were much less numerous than those labelled with HRP. In cats injected in SII, D-[3H]Asp retrogradely labelled cells were present in ipsilateral SI (SI-SII association neurones). Their topographical and laminar distribution overlapped that of neurones labelled with HRP but, as in cats injected in SI, association neurones labelled with silver grains were unusually rare in the inner layer III.(ABSTRACT TRUNCATED AT 400 WORDS)

SII-projecting neurons in the rat thalamus: a single- and double-retrograde-tracing study

Experiments were performed on adult albino rats, using single-labeling (free horseradish peroxidase [HRP] or wheatgerm agglutinin conjugated to HRP [WGA:HRP]) and double-labeling (fluorescent dyes) techniques to investigate the thalamic projections to the secondary somatosensory cortex (SII) and to demonstrate the presence and location of thalamic neurons projecting to both the primary somatosensory cortex (SI) and SII by way of branching axons. In single-labeling experiments, the tracer was injected in SI or SII with or without electrophysiological control; in double-labeling experiments, fast blue and diamidino yellow were injected into the electrophysiologically identified forelimb areas of SI and SII. Single-tracer experiments showed that after injections in SI, focused in the forelimb representation area, retrogradely labeled neurons were present mainly in the ventral third of the nucleus ventralis posterolateralis (VPL) and in the anterior part of the posterior nuclear complex (PO); labeled neurons were also present consistently in the caudal portion of PO. Injection of tracers in the forelimb or forelimb and hindlimb representation areas of SII resulted in labeling of neurons in the posterior part of PO and in the caudal part of VPL. Double-labeling experiments confirmed the distribution of neurons projecting to SI or to SII, as observed in single-labeling experiments. Some neurons labeled with both tracers were also present. These neurons are interpreted as projecting to both SI and SII by means of axon collaterals and were observed in areas of overlap of the two single-labeled population of neurons--that is, at the border between PO and the ventroposterior complex, and in the medial part of caudal PO. Comparison of these data with those obtained after injections of tracers in SI and SII of cats (Spreafico et al., 1981b) suggests that in both species thalamic neurons projecting to these two areas are largely segregated, though partially overlapping; and that thalamic neurons projecting simultaneously to SI and SII, modest in number in cats, are even sparser in rats.

GABAergic neurons are present in the dorsal column nuclei but not in the ventroposterior complex of rats

Neurons containing glutamatic acid decarboxylase (GAD) are known to exist in the spinal dorsal horn, dorsal column nuclei (DCN), n. ventralis posterior (VP), and somatosensory cortex of cats. Recent work suggested that species differences exist concerning the presence and/or density of GAD-positive neurons in VP. The present experiments demonstrate that, in contrast with carnivores and primates, the rat's VP contains virtually no GAD-positive neurons and that virtually all neurons in it project to the cortex. This conclusion is supported by the failure to find, in Golgi-impregnated material, neurons with characteristics commonly attributed to Golgi type II neurons in VP of cats. The lack of GAD-positive neurons in VP of rats contrasts also with the presence of such neurons in the DCN in the same species. As in cats, about one third of the neurons in the cuneate n. are GAD-positive; these have mostly small perikarya and they are present throughout the nucleus. It is likely that these are intrinsic neurons, i.e. non-projecting beyond the limits of the DCN since a comparable percentage of neurons are unlabeled by simultaneous injections of horseradish peroxidase in multiple targets of the DCN. Like GAD-positive neurons, neurons unlabeled by the retrograde transport of HRP have, for the most part, small perikarya. It is possible that inhibitory mechanisms necessary for basic transfer functions in VP of rats are sustained through projections to this nucleus from the n. reticularis thalami. Extrinsic source of GABAergic input to the DCN seem to be absent or very weak. From this and previous evidence it may be proposed that intrinsic inhibitory interneurons have gradually developed in VP of rabbits, carnivores, and primates in parallel with more elaborate levels of thalamic integration of somatosensation.

D-[3H]aspartate retrograde labelling of association neurones in area SI of the cat

Injection of D-[3H]aspartate into area SII of cats retrogradely labelled association cells in area SI. Numerous intensely labelled cells were found in layer II and in the upper layer III but labelling was scanty in other layers. In contrast, association neurones labelled by horseradish peroxidase injected in area SII mixed with the radioactive marker were also numerous in the other sublaminae of layer III and in infragranular layers of SI. Association neurones of the outer laminae of this area are likely to use aspartate and/or glutamate as neurotransmitter(s).

Retrograde labeling of dorsal root ganglion neurons after injection of tritiated amino acids in the spinal cord of rats and cats

The present experiments are based upon evidence that neurons may selectively take up at their terminals, and retrogradely transport, the same chemical they use as a neurotransmitter or its analogues. In an attempt to identify dorsal root ganglion (DRG) neurons that use glutamic acid as a neurotransmitter, [3H]D-aspartate ([3H]D-Asp) was chosen as a marker, since it is a metabolically inert amino acid known to be taken up by the same affinity mechanism as L-aspartate and L-glutamate. Adult rats and cats received injections of 50 nl to 1.5 microliter of [3H]D-Asp (500 microCi/microliter) in the dorsal horn of cervical segments (C3 to C6). At 9 to 48 hr after injection, all animals were perfused with 5% glutaraldehyde. After sections were processed for autoradiography, the DRG neurons situated most closely to the injection site were chosen from representative cases, and the number and cross-sectional area of labeled and unlabeled perikarya with a nucleolus in the plane of the section were calculated. In rats, about 4% of the sampled DRG neurons were autoradiographically labeled, and the mean perikaryal area of these neurons was about twice that of unlabeled perikarya. In cats, the percentage of labeled perikarya ranged between 6.5% and 13.27% of the sampled population. The ratio of the mean perikaryal area of labeled neurons to that of unlabeled neurons ranged between 1.6 and 2.5. In a control cat injected with [3H]proline at C7, all perikarya in the C7 DRG were autoradiographically labeled. However, with injection of [3H]gamma-aminobutyric acid ([3H]GABA) selective retrograde labeling was observed. Quantitative data in rat showed that perikarya labeled at the C6 level after injection of this amino acid constituted about 8% of the sample population in C6 DRG. The ratio of the size of labeled to unlabeled perikarya was 2.02. In one cat injected with [3H]GABA at caudal C3, the largest number of labeled perikarya were in C4 DRG and comprised up to 5.32% of the sampled population. The ratio of the size of labeled to unlabeled perikarya was 1.57. The results in cases of injection with [3H]D-Asp may be interpreted as consistent with the idea that a fraction of DRG neurons use glutamate and/or aspartate as neurotransmitter(s).(ABSTRACT TRUNCATED AT 400 WORDS)

Changes in the structure of somato-sensitive neurons in cat following retrograde uptake of HRP

Neurons of cat cerebral cortex, area S1, labelled by retrograde transport of HRP, have been studied by electron microscopy; the animals were sacrificed 48 h after HRP injection. The cells studied are those in which the enzyme is found both in granular and in diffused form in the cytoplasm (diffuse-granular HRP neurons, DGN), or in diffused form in the cytoplasm (Golgi-like labelled neurons, GLN). The observations have shown that the certain recognition of DGN through electron microscopy, requires a comparison with a semithin section adjacent to the ultrathin section since the labelling granules are light-scattering in dark field. Instead the GLN are not light-scattering in dark field but are directly and unmistakably identifiable even by electron microscopy. The DGN are well preserved on the whole in all their organelles; the GLN show a highly damaged aspect in relation to both the Golgi apparatus and the smooth reticulum, to the cytoplasmic organelles and to the nucleus. Our hypothesis is that the latter neurons take on this appearance because of the combined action of: a) the peripheral mechanical lesions caused by the needle and b) the enormous amount of HRP that the cell is forced to take up (through these pathways) because of its lost integrity and because of the large quantity of marker around the axons. In conclusion intracellular HRP is identifiable by electron microscopy only when it is found in diffused form in the cytoplasm.

[Topography of the thalamo-cortical projections on trunk representation demonstrated by fluorescent neuro-tracers]

With the aim to study the detailed topography of the thalamo-cortical neurones projecting to the trunk representation zone of the first somatosensory area (SI), punctate injections of three different fluorescent tracers (Evans Blue, Nuclear Yellow and Fast Blue) were performed in the three physiologically defined subareas forming the trunk region of SI. These injections resulted in the labelling of three different cell aggregates, narrow in dorsoventral and mediolateral extent but elongated rostrocaudally, located in topographically distinct regions of the nucleus ventralis posterio-lateralis. The results suggest that the highly organized topography of the trunk representation of area SI is imposed by the thalamo-cortical input from VPL.

[Callosal connections of the somatosensory area in the primate: anatomical and electrophysiological studies]

In order to study the callosal connections of the hand sensory field of the second somatosensory area of the monkey, experiments were carried out by combining the method of retrograde neuronal tracing with microelectrode recording. In six monkeys, Macaca Irus, single or multiple (5-8) injections of horseradish peroxidase (HRP) were performed into the cortex of the parietal operculum of one side. Neurones retrogradely labelled with HRP (callosal neurones) were found in the post-central gyrus and in the parietal operculum of the contralateral hemisphere. Microelectrode recording from this hemisphere showed that the cortical zones of both the first and the second somatosensory area containing neurones excited by sensory stimulation of the contralateral hand also contained HRP-positive neurones.

Topography and receptive field organization of the body midline representation in the ventrobasal complex of the cat

The topography and receptive field (RF) organization of neurones in the trunk zone of the thalamic ventrobasal complex (VB) projecting to the homologous zone of the ipsilateral first somatosensory area (SI) were studied in the cat by performing experiments of retrograde neuronal tracing and microelectrode recording. Punctate cortical injections of small amounts of either horseradish peroxidase or fluorescent tracers (Evans Blue, Nuclear Yellow and Fast Blue) retrogradely labelled cell aggregates lying in the dorsal half of a VB region interposed between subnucleus VPL1 and VPLm. Aggregates of labelled cells were narrow in dorsoventral and mediolateral extent and elongated rostrocaudally. The distribution of VB cells projecting to the cortical subareas representing the dorsal midline, lateral trunk and ventral midline of the body in area SI, was established by injecting a different fluorescent marker into a physiologically defined site in each subarea. These injections resulted in labelling of three different cell aggregates located in topographically distinct regions of the VB trunk zone. Each aggregate of labelled cells only projected to one cortical subarea. Microelectrode analysis of cell populations of the VB trunk zone showed that neurones lying in regions projecting to dorsal and ventral midline zones of area SI had bilateral RFs, straddling the dorsal and the ventral midline of the body respectively. Neurones lying in the region projecting to the lateral trunk representation of area SI had contralateral RFs located on the lateral surface of the trunk. The results suggest that the detailed topography of the trunk map in the area SI and the bilaterality of the cortical representation of the body midlines, described in previous experiments, is imposed by the thalamocortical input from the VB.

Callosal mechanism for the interhemispheric transfer of hand somatosensory information in the monkey

The retrograde transport of horseradish peroxidase (HRP) was combined with extracellular microelectrode recording from single and multiple-neurones to study the anatomical and functional organization of the callosal connections of the hand sensory projection field in the parietal operculum of monkeys (Macaca Irus). In 3 animals anaesthetized with ketamine, a single injection of HRP (0.5 microliter) was delivered into the cortex forming the upper bank of the sylvian sulcus at a site where neuronal responses to somatic sensory stimulation of the hand were recorded. In the ipsilateral hemisphere, retrogradely HRP-labelled cells were found in the cortex of the post-central gyrus and in the thalamic nuclei ventralis posteroinferior and pulvinar oralis. In the contralateral hemisphere HRP-labelled neurones were present in the opercular cortex lying dorsal, and slightly caudal, to the posterior pole of the insula. Few scattered callosal neurones were also found in the post-central gyrus. In 3 other animals, multiple injections (5-8; 0.5 microliter each) of HRP were performed in the parietal operculum. In the ipsilateral hemisphere, retrogradely labelled cells were present in the post-central gyrus and in the following thalamic nuclei: ventralis posteroinferior, pulvinar oralis and medialis, ventralis posteromedialis and posterior complex. Few labelled cells were also present in the ventral part of the nucleus ventralis posterolateralis. In the contralateral hemisphere, numerous callosal cells were labelled with HRP. These cells were found, with regional variations in density, in wide regions of the buried and exposed cortex of the parietal operculum and in the post-central gyrus. These 3 monkeys were subjected to microelectrode mapping experiments (N2O and halothane anaesthesia) to explore the peripheral receptive fields of neurones in the parietal operculum and post-central gyrus contralateral to the injected side. HRP labelled callosal neurones were found in regions of the second and first somatosensory cortical areas which also contained units driven from the contralateral hand.

Axonal branching in the periaqueductal gray projections to the thalamus: a fluorescent retrograde double-labeling study in the cat

The double-labeling technique based on the retrograde axonal transport of fluorescent tracers (Evans blue, EB; Fast blue, FB; Nuclear yellow, NY) was used in the cat in order to investigate the occurrence of axonal branching in the periaqueductal gray (PAG) projections to some thalamic nuclei (n. ventralis postero-lateralis, VPL; n. ventralis postero-medialis, VPM; n. parafascicularis, Pf). In a first group of cats, FB and EB were injected, respectively, within the right and left VPM. In another two groups of cats, FB injections into Pf were combined with either EB or NY injections within VPL or VPM. Double-labeled neurons were found within the PAG only in the animals of the first group. The present results show that some PAG neurons project bilaterally to VPM by means of axons collaterals.

Periaqueductal grey projection to the ventrobasal complex in the cat: an horseradish peroxidase study

Horseradish peroxidase (HRP) was injected within the thalamic ventrobasal complex of 14 cats. The aim was to ascertain whether the periaqueductal grey matter (PAG) sends fibres to this complex. Retrogradely labelled cells were found within the PAG following HRP delivery either in the nucleus ventralis posterolateralis (VPL) or ventralis posteromedialis (VPM). PAG-VPL projection is only ipsilateral and arises mainly from lateral PAG, PAG-VPM projection is bilateral and originates from latero-ventral regions of the central grey. The hypothesis that PAG might control the activity of ventrobasal nociceptive neurones is proposed.