Striatal projections from the cat visual thalamus

Anterograde transport methods reveal an extensive thalamostriate projection from the extrageniculate visual thalamus. These projections distribute to approximately the same regions of the striatum innervated by the corticostriate projections from over a dozen higher visual cortical areas [visual-recipient sector; Updyke, B.V. (1993) J. Comp. Neurol., 327, 159-193.]. Like their cortical counterparts, the thalamostriate projections to the caudate distribute in a patchy manner that suggests potential overlap or intermingling spatial relationships between the two major afferents. All of the visually related thalamic nuclei projecting to the striatum receive ascending signals from the superior colliculus, suggesting that the constraints placed upon tectal processing by striatonigral control have important consequences for central perceptuomotor processing at the striatal and cortical levels.     

Salivation mediated by central M-2 muscarinic receptors in the cat

In these experiments the effect on the cat's salivation of the choline ester, carbachol, the anticholinesterase, eserine, and the muscarinic ganglionic stimulants, McN-A-343 and AHR-602, injected into the cerebral ventricles was investigated and compared. Carbachol and eserine injected intracerebroventricularly (ICV) produced dose-related salivation. On the other hand, ICV McN-A-343 and AHR-602 evoked inconsistent salivation when injected in large doses. The antimuscarinic drug, atropine, injected into the cerebral ventricles abolished the salivation caused by carbachol and eserine similarly injected. However, salivation induced by ICV carbachol and eserine was not significantly altered by the ganglionic blocking agent, mecamylamine, injected into the cerebral ventricles. It is concluded that the ICV injection of carbachol and eserine evokes salivation in the cat by way of an action on central M-2 muscarinic receptors.     

Interneurons and triadic circuitry of the thalamus

The thalamus is strategically placed to control the flow of information to cortex and thus conscious perception. A key player in this control is a local GABAergic interneuron that inhibits relay cells. This interneuron is especially interesting because, in addition to a conventional axonal output, most of its output is via distal dendrites. The latter seem to be electrotonically and thus functionally isolated from the soma and axon, and they enter into complex synaptic arrangements. It is proposed that, because of special synaptic properties of its dendritic outputs, this local GABAergic interneuron of the thalamus provides gain control for the relay cell and thereby keeps relay of information to cortex within a fairly linear regime.     

Evidence for spatiotemporal firing patterns within the auditory thalamus of the cat

Multiple spike trains were recorded in the auditory thalamus of cats. Each unit was studied before, during and after cooling of the ipsilateral primary auditory cortex, during spontaneous activity and acoustically evoked activity. The search for spatiotemporal firing patterns provided evidence that excess of patterns does exist and that the acoustical stimulation increased their number. Cortical cooling did not affect the probability of finding the firing pattern.     

Differential effects of quinolinic acid lesions on muscarinic acetylcholine receptors in cat visual cortex during postnatal development

Quinolinic acid (QA) lesions of neurons in cat visual cortex were combined with conventional in vitro autoradiographic methods in order to define the cellular locus of the muscarinic acetylcholine receptor (mAChR). Animals of various postnatal ages had QA unilaterally injected into the visual cortex. Four to fourteen days later they were sacrificed and processed for electron microscopy (EM) or in vitro autoradiography. QA lesions at the various postnatal ages were found to eliminate intrinsic cortical neurons and their processes while leaving intact glia, fibers of passage and axon terminals from outside the lesion zone. Autoradiograms of visual cortex labelled with [3H]QNB (which labels M1 and M2 subtypes) showed an age-dependent loss of binding sites, with the greatest decreases occurring after 65 days postnatal. Examined separately, only the M1 mAChRs labelled with [3H]pirenzepine exhibited these age-dependent alterations. The results indicate a differential distribution of the M1 mAChRs during postnatal development. The loss of receptors late in postnatal life following QA suggests a dominantly neuronal locus; the relatively small loss early in postnatal life suggests a locus on other cellular elements.     

Neuronal and synaptic arrangement in the lateralis posterior-pulvinar complex of the thalamus in the cat

Golgi and electron microscope studies on the cat's LP-pulvinar formation of the thalamus reveal the presence of two characteristically different cell populations: principal (or relay) cells and Golgi type II interneurons. These two cell types show little if any difference, both at the light and electron microscope level, from the characteristics of the similar cell types and the ultrastructural arrangement found in the sensory relay nuclei. Characteristic synaptic glomeruli (or encapsulated zones) are found in abundance in the LP-pulvianr complex. Four different types of terminal profiles containing synaptic vesicles can be identified in these glomeruli:RL = large axonal terminals with spheroid vesicles;RL = small axonal terminals with similar vesicles (relatively rare in the glomeruli and abundant in the general extraglomerular neuropil);F₁ = probably axonal profiles with flattened vesicles (present both in the glomeruli and in the general neuropil); andF₂ = profiles interpreted as of dendritic nature and belonging to Golgi type II interneurons. The numbers, percentages of different types of terminal profiles and of various combinations of interprofile contacts have been determined.     

Local circuitry of identified projection neurons in cat visual cortex brain slices

The relationship between pyramidal cell morphology and efferent target was investigated in layer 6 of cat primary visual cortex (area 17). Layer 6 has 2 projections, one to the lateral geniculate nucleus (LGN) and another to the visual claustrum. The cells of origin of each projection were identified by retrograde transport of fluorescent latex microspheres. The labeled cells were visualized in brain slices prepared from area 17, using an epifluorescence compound microscope modified for intracellular recording. Individual retrogradely labeled cells were penetrated and intracellularly stained with Lucifer yellow to visualize the patterns of axons and dendrites associated with each projection. The neurons that give rise to the 2 projections had very different patterns of dendrites and local axonal collaterals, but the patterns within each group were highly stereotyped. The differences between their axonal collaterals were particularly dramatic. Claustrum projecting cells had fine, horizontally directed collaterals that arborized exclusively in layer 6 and lower layer 5. Most LGN projecting cells had virtually no horizontal arborization in layer 6. Instead, they sent widespread collaterals vertically, which arborized extensively in layer 4. The apical dendrites of the 2 groups also differed markedly. Claustrum projecting cells had apical dendrites reaching to layer 1, with branches in layer 5 only, while LGN projecting cells never had an apical dendrite reaching higher than layer 3, with side branches in layers 5 and 4. Therefore, each efferent target must receive inputs from neurons whose synaptic connections within area 17 are significantly different from those of neurons projecting to other targets. This further suggests that distinct visual response properties should be associated with each projection. In addition to the claustrum and LGN projecting cells, about 20% of layer 6 pyramidal neurons lacked an efferent axon. Morphologically, most resembled LGN projecting neurons, but a few had characteristics of claustrum projecting cells. These neurons may represent cells that either failed to make an efferent connection or cells that lost an efferent axon during development. Their frequency suggests that such intrinsic, presumably excitatory, neurons may play a significant role in cortical processing.     

Cellular and subcellular localisation of muscarinic acetylcholine receptors during postnatal development of cat visual cortex using immunocytochemical procedures

A monoclonal antibody against the muscarinic acetylcholine receptor was used to study the distribution of this receptor within the cat visual cortex at the light and electron microscopic level. Immunolabelling was found to be distributed mainly in cell bodies and dendrites in both young and adult cats. The laminar distribution, however, changed during development from cortical layers IV and to a lesser extent II in kittens of 28-42 days of age into one favouring the upper (L I-III) and lower layers (L V and VI) in kittens and cats more then 60 days of age. Electronmicroscopy revealed staining of membranes of cell bodies and dendrites and a great amount of intra-cellular staining of vesicles and internal membranes. Since the receptors are found on cell bodies and dendrites in both kittens and adults this indicates that different populations of cortical cells express these muscarinic receptors at different postnatal ages.     

Histochemical localization of synaptic zinc in the developing cat visual cortex

The terminal boutons of many neurons in the telencephalon are known to contain a vesicle-bound, chelatable pool of zinc (Zn²⁺) that can be selectively visualized with histochemical procedures. In this paper, the normal laminar, areal, and ultrastructural distribution of histochemically reactive zinc in the visual cortex of the adult cat as well as its development from birth are described. In the adult cat visual cortex, intense zinc staining was found in layers I, II, III, and V, with layer VI staining only lightly. The primary geniculostriate input zone, layer IV, was conspicuously distinguished by the relative absence of zinc. This distinct pattern was restricted only to areas 17 and 18 and differentiated them from adjacent cortical area 19 laterally and the subadjacent cingulate cortex. The earliest zinc-positive staining in visual cortical areas 17 and 18 was first apparent by postnatal day 2 (P2) and was characterized by staining of a thin layer at the bottom of the cortical plate. By P10, and continuing through P20, synaptic zinc formed a trilaminar pattern of dense staining in areas 17 and 18, which included the top of layer I, and layers III and V. The laminar pattern of synaptic zinc in visual cortex appeared mature by P30, except that the distribution of zinc in layer IV was not uniform. This was most apparent around P50 in tangential sections through layer IV from opened and flattened cortex, where columnar patches of increased zinc staining were apparent in area 17. These columns were approximately 400 μm in diameter, with a centre-to-centre spacing of approximately 900 μm. The distribution of synaptic zinc apparently reflects the process of synaptic maturity of the cat visual cortex and appears to demarcate a particular form of columnar organization in visual cortex. © 1993 Wiley-Liss, Inc.     

Multiple pathways from the superior colliculus to the extrageniculate visual thalamus of the cat

The projection from the cat's superior colliculus to the extrageniculate visual thalamus were examined by the anterograde and retrograde transport of WGA-HRP. An acetylthiocholinesterase (ATChE) stain was employed to facilitate the differentiation of regions within the posterior thalamus. On the basis of the distribution of terminal label as well as the laminar origin of projection neurons, four pathways were delineated. Cells in the stratum griseum superficiale (primarily sublaminae II and III) innervate two regions within the nucleus lateralis posterior (LP): the medial zone, which stains darkly for ATChE, and a restricted portion of the lateral zone, adjacent to the pulvinar. Both of these pathways were found to be topographically organized. By using the fluorescent retrograde tracers, fast blue and rhodamine labeled microspheres, it was determined that the inputs to the medial and lateral zones of LP originate primarily from separate cell populations since very few neurons were found to be double-labeled. A third pathway originates principally from cells in the stratum opticum and terminates in an area just below the cholinesterase-rich region of the LP, designated as the ventral division of the LP. The fourth projection is primarily from the stratum griseum intermedium to the suprageniculate complex. Each of these four pathways arises from a population of neurons with heterogeneous morphological characteristics, and for the most part, each pathway comprises morphologically similar cells. These results suggest that visual information from the superior colliculus is conveyed to the extrageniculate thalamus via multiple pathways that may subserve diverse functions.     

Role of muscarinic receptors, G-proteins, and intracellular messengers in muscarinic modulation of NMDA receptor-mediated synaptic transmission.

Previously, we reported that activation of muscarinic receptors modulates N-methyl-D-aspartate (NMDA) receptor-mediated synaptic transmission in auditory neocortex [Aramakis et al. (1997a) Exp Brain Res 113:484-496]. Here, we describe the muscarinic subtypes responsible for these modulatory effects, and a role for G-proteins and intracellular messengers. The muscarinic agonist oxotremorine-M (oxo-M), at 25-100 microM, produced a long-lasting enhancement of NMDA-induced membrane depolarizations. We examined the postsynaptic G-protein dependence of the modulatory effects of oxo-M with the use of the G-protein activator GTP gamma S and the nonhydrolyzable GDP analog GDP beta S. Intracellular infusion of GTP gamma S mimicked the facilitating actions of oxo-M. After obtaining the whole-cell recording configuration, there was a gradual, time-dependent increase of the NMDA receptor-mediated slow-EPSP, and of iontophoretic NMDA-induced membrane depolarizations. In contrast, intracellular infusion of either GDP beta S or the IP3 receptor antagonist heparin prevented oxo-M mediated enhancement of NMDA depolarizations. The muscarinic receptor involved in enhancement of NMDA iontophoretic responses is likely the M1 receptor, because the increase was prevented by pirenzepine, but not the M2 antagonists methoctramine or AF-DX 116. Oxo-M also reduced the amplitude of the pharmacologically isolated slow-EPSP, and this effect was blocked by M2 antagonists. Thus, muscarinic-mediated enhancement of NMDA responses involves activation of M1 receptors, leading to the engagement of a postsynaptic G-protein and subsequent IP3 receptor activity.     

Synaptic targets of thalamic reticular nucleus terminals in the visual thalamus of the cat

A major inhibitory input to the dorsal thalamus arises from neurons in the thalamic reticular nucleus (TRN), which use gamma‐aminobutyric acid (GABA) as a neurotransmitter. We examined the synaptic targets of TRN terminals in the visual thalamus, including the A lamina of the dorsal lateral geniculate nucleus (LGN), the medial interlaminar nucleus (MIN), the lateral posterior nucleus (LP), and the pulvinar nucleus (PUL). To identify TRN terminals, we injected biocytin into the visual sector of the TRN to label terminals by anterograde transport. We then used postembedding immunocytochemical staining for GABA to distinguish TRN terminals as biocytin‐labeled GABA‐positive terminals and to distinguish the postsynaptic targets of TRN terminals as GABA‐negative thalamocortical cells or GABA‐positive interneurons. We found that, in all nuclei, the TRN provides GABAergic input primarily to thalamocortical relay cells (93–100%). Most of this input seems targeted to peripheral dendrites outside of glomeruli. The TRN does not appear to be a significant source of GABAergic input to interneurons in the visual thalamus. We also examined the synaptic targets of the overall population of GABAergic axon terminals (F1 profiles) within these same regions of the visual thalamus and found that the TRN contacts cannot account for all F1 profiles. In addition to F1 contacts on the dendrites of thalamocortical cells, which presumably include TRN terminals, another population of F1 profiles, most likely interneuron axons, provides input to GABAergic interneuron dendrites. Our results suggest that the TRN terminals are ideally situated to modulate thalamocortical transmission by controlling the response mode of thalamocortical cells. J. Comp. Neurol. 440:321–341, 2001. © 2001 Wiley‐Liss, Inc.     

Intracellular HRP study of nociceptive neurons within the ventrobasal complex of the cat thalamus

Locations and morphological characteristics of nociceptive specific (NS) and wide dynamic range (WDR) neurons of the ventrobasal (VB) thalamic complex were studied using an intracellular HRP injection technique in cats anesthetized with urethane and chloralose. Both NS and WDR neurons were found within the marginal zone of the VB complex. Their somata were confined to the VB complex, but dendrites were extended into the structures of the thalamus surrounding the VB complex. NS neurons were found in the caudal part of the VB complex, whereas WDR neurons were found more rostrally. Examinations of their locations ithin the marginal zone of the VB complex confirmed the somatotopic organization, as found previously. We also confirmed the previous reports that there are 3 different types of low-threshold mechanoreceptive (LTM) neurons within the VB complex. They are type I, type II and type III neurons. Both NS and WDR neurons were either type I or type II neurons. Obvious morphological differences were not found between LTM neurons and two classes of nociceptive VB neurons investigated.     

A golgi study of the class V cell in the visual thalamus of the cat

Golgi methods were used to study class V cells within the cat visual thalamus. Counterstaining was combined with Golgi staining to assess the distribution of dendrites relative to cytoarchitectural boundaries. Class V cells were encountered within all laminae of the lateral geniculate nucleus, the medial interlaminar nucleus, and the lateral posterior complex. The cells possess medium-sized perikarya and smooth and varicose or moniliform dendrites. Dendritic appendages are sparse and occur as single or serial swellings on thin processes. Many class V cells exhibit large, sparse dendritic arbors which span laminar or nuclear borders; dendrites were seen to lie within and to cross the interlaminar zones of the lateral geniculate nucleus, and extend beyond this nucleus into the perigeniculate nucleus and medial interlaminar nucleus. Class V cells of the lateral posterior complex send dendrites into the external medullary lamina. Indirect evidence favors the interpretation that the class V cells are thalamo-cortical relay cells.     

Topography and cortical projections of morphologically identified neurons in the visual thalamus of the cat

Visual thalamic neurons were stained in a Golgilike fashion following cortical injections of horseradish peroxidase (HRP). Such staining allows neurons to be classified on the basis of their dendritic morphology, and it enables the direct relation of morphologically identified neurons with their respective cortical projection target to be visualized. All classes of neurons categorized on the basis of Golgi material (Guillery, '66) were identified in the dorsal part of the lateral geniculate body (LGBd): class 1 and class 2 neurons in the laminae A, A₁, and C; class 3 neurons in laminae A and A₁; class 4 cells in the C-laminae. Three subtypes of class 4 neurons were differentiated: round cells, spindle cells, and fusiform cells. The round subtype is similar in dendritic morphology to the class 3 neurons, but its soma is larger (mean soma diameter 17 μm compared with 13 μm). Other types of neurons were identified in the laminae A, A₁, and C, having morphological characteristics intermediate between class 1 and class 2, and between class 2 and class 3. Class 1 neurons, class 2 neurons, and flat neurons were identified in the medial interlaminar nucleus (MIN). The flat type is similar in morphology to the fusiform type in the C-laminae; however, its soma is larger (mean soma diameter 23 μm compared with 19 μm), and its dendrites are free of appendages. In the extrageniculate nuclei three types of neurons were found: class 2 neurons in the pulvinar, medium-sized cells—probably class 5—in the pulvinar and in the nucleus lateralis posterior, and neurons with an isodendritic branching pattern in the intralaminar nuclei. The cortical projections of the various thalamic cell classes were analyzed by injection of HRP into acallosal parts, or callosal parts of area 17 or area 18. Area 17 receives input from a large number of class 2 neurons and from a moderate number of class 1 neurons in the LGBd. In contrast, area 18 receives a dominant input from class 1 neurons and only weak input from class 2 neurons (up to 10% of the population of projection cells in the individual cases). Class 3 neurons in the A-laminae and class 4 neurons in the C-laminae project to either cortical area. Class 2 neurons in MIN project only to callosal parts of area 18, whereas class 2 neurons in pulvinar project in addition to acallosal parts of area 18. The significant change in the relative number of class 1 and class 2 neurons in the LGBd occurs gradually across the callosal zone from area 17 to area 18; it is not strictly related to the 17/18 border as defined by cytoarchitectonic or physiological criteria. The class 1/class 2 relation in the projection to area 18 is independent of eccentricity in the A-laminae. An increase in the frequency of occurrence with eccentricity is noted, however, for class 4 neurons in the C-laminae and for flat neurons in MIN. The correlation between neuronal function and morphology is discussed. It is concluded that, in addition to strong input with y-characteristics, area 18 receives significant inputs with non-y properties. latter inputs arise mainly from the laminae of the LGBd and from extra-geniculate nuclei, but also may be seen to originate from MIN and the A-laminae of the LGBd.     

Immunocytochemical study of GABAA receptors in the cat visual cortex

The laminar distribution and morphological structures associated with GABA_A receptor immunoreactivity in the cat visual cortex were studied by using two different polyclonal antibodies directed either against the purified GABA_A receptor protein (antibody “967”) or against a specific domain of the β₁-subunit of the GABA_A receptor (antibody “Q”). Immunoblots of cat visual cortex tissue with these antibodies revealted that antibody “Q” recognizes only one subunit, namely the β₁-subunit of the GABA_A receptor, and that antibody “967” recognizes three subunits. Both antibodies produced very similar staining patterns, indicating that the β₁-subunit may be an essential component of the GABA_A receptor in the cat visual cortex. The typical staining pattern showed a clear membrane structure around neuronal somata. Using cell body shape criteria, immunopositive neurons included both pyramidal cells in cortical layers II, III, and V, and nonpyramidal cells in all cortical layers. Immunopositive neurons were uniformly distributed in layers II to VI, whereas the density of immunopositive cells in layer I was lower. Some immunopositive neurons were also found in the white matter underlying the visual cotex. In gray matter, immunopositive structures also included dendrites, especially the proximal dendrites, and axon initial segments of pyramidal neurons. The immunopositive processes usually ran vertically toward the pial surface. Some astrocytes were also immunostained. They were localized in layer I and in the white matter. The overall pattern of immunostaining was similar in areas 17, 18, and 19. © 1993 Wiley-Liss, Inc.     

The distribution of M1 and M2 muscarinic acetylcholine receptor subtypes in the developing cat visual cortex

The binding site characteristics and ontogenesis of [3H]pirenzepine ([3H]PZ) (M1 receptor) and [3H]oxotremorine-M ([3H]OXO-M) (M2 receptor) binding sites were investigated in the cat visual cortex. Scatchard analysis of [3H]PZ binding in adult cat visual cortex revealed a single site with a Kd of 17.3 nm and a Bmax of 352.45 fmol/mg protein. [3H]OXO-M also bound to a single site with a Kd of 7.1 nM and a Bmax of 256.39 fmol/mg protein. Receptor autoradiography revealed that [3H]PZ binding sites were present only in telencephalic structures while [3H]OXO-M sites were distributed heterogeneously throughout the brain. [3H]PZ binding sites in adult visual cortex were present in the superficial and deep cortical layers with the densest labeling in layer I and a distinct band in layer V. [3H]OXO-M sites also avoided the middle cortical layers, but were most prominent in layers V and VI with less pronounced binding in layers I and II. Deafferentation of extrinsic inputs to the visual cortex did not reduce [3H]PZ nor [3H]OZO-M binding, but neuron-specific excitotoxic lesions of visual cortex abolished both populations of binding sites. This indicates that both populations of binding sites are located on cells intrinsic to the cortex. In early postnatal life, both [3H]PZ and [3H]OXO-M binding sites were localized to intermediate cortical layers. Following this, the laminar distribution of both populations redistributed; each with its own idiosyncratic profile. By postnatal day 49, [3H]PZ binding sites redistributed into the superficial and deep layers, the pattern of adult animals, while [3H]OXO-M sites maintained a pattern similar to younger animals, with substantial binding persisting in layer IV. As late as postnatal day 70, well after [3H]PZ binding sites had achieved their mature laminar pattern, [3H]OXO-M binding sites in visual cortex had not achieved their characteristic adult pattern. In addition, the normal laminar redistribution of both [3H]PZ and [3H]OXO-M binding sites during postnatal development of the cat visual cortex was prevented by eliminating cortical afferents in early postnatal life. This indicates that muscarinic receptor rearrangement in development is dependent upon cortical input or output.