Neurons of the dorsal lateral geniculate nucleus of the albino rat

Temporal and spatial tuning of dorsal lateral geniculate nucleus neurons in unanesthetized rats

Visual response properties of neurons in the dorsolateral geniculate nucleus (dLGN) have been well described in several species, but not in rats. Analysis of responses from the unanesthetized rat dLGN will be needed to develop quantitative models that account for visual behavior of rats. We recorded visual responses from 130 single units in the dLGN of 7 unanesthetized rats. We report the response amplitudes, temporal frequency, and spatial frequency sensitivities in this population of cells. In response to 2-Hz visual stimulation, dLGN cells fired 15.9 ± 11.4 spikes/s (mean ± SD) modulated by 10.7 ± 8.4 spikes/s about the mean. The optimal temporal frequency for full-field stimulation ranged from 5.8 to 19.6 Hz across cells. The temporal high-frequency cutoff ranged from 11.7 to 33.6 Hz. Some cells responded best to low temporal frequency stimulation (low pass), and others were strictly bandpass; most cells fell between these extremes. At 2- to 4-Hz temporal modulation, the spatial frequency of drifting grating that drove cells best ranged from 0.008 to 0.18 cycles per degree (cpd) across cells. The high-frequency cutoff ranged from 0.01 to 1.07 cpd across cells. The majority of cells were driven best by the lowest spatial frequency tested, but many were partially or strictly bandpass. We conclude that single units in the rat dLGN can respond vigorously to temporal modulation up to at least 30 Hz and spatial detail up to 1 cpd. Tuning properties were heterogeneous, but each fell along a continuum; we found no obvious clustering into discrete cell types along these dimensions.

Dendritic changes in rat visual pathway associated with experimental ocular hypertension

PURPOSE

Increasing evidence shows that structural changes in dendrites play an important role in neuronal degenerative processes. The aims of this study were to characterize and delineate morphological changes of dendrites in retinal ganglion cells (RGCs) and their central target neurons in the superior colliculus (SC) and lateral geniculate nucleus (LGN) in experimental rat glaucoma.

METHODS

Chronic ocular hypertension (OHT) was surgically induced in rats and animals were sacrificed at 1, 4, 8, 16 and 32 weeks following IOP elevation. Animals without IOP elevation served as normal control. Dendritic morphology of neurons was visualized by ex vivo DiI labelling using confocal microscopy and dendritic length and number was quantified using Image J.

RESULTS

We found significant dendritic shrinkage (p < 0.001) and loss (p < 0.001) in RGCs and neurons in the SC and LGN in OHT animals compared to age-matched controls. Analysis of the temporal morphological profiles among them revealed the RGCs to have the earliest changes compared to the SC and LGN although the most prominent changes occurred in the SC.

CONCLUSION

Our study has demonstrated that OHT results in dendritic changes of the neurons throughout the visual pathways, from RGCs to SC cells and LGN cells, suggesting that both the retina and the brain should be targeted when considering diagnosis and therapeutic strategies for glaucoma.

Resolving the detailed structure of cortical and thalamic neurons in the adult rat brain with refined biotinylated dextran amine labeling

Biotinylated dextran amine (BDA) has been used frequently for both anterograde and retrograde pathway tracing in the central nervous system. Typically, BDA labels axons and cell somas in sufficient detail to identify their topographical location accurately. However, BDA labeling often has proved to be inadequate to resolve the fine structural details of axon arbors or the dendrites of neurons at a distance from the site of BDA injection. To overcome this limitation, we varied several experimental parameters associated with the BDA labeling of neurons in the adult rat brain in order to improve the sensitivity of the method. Specifically, we compared the effect on labeling sensitivity of: (a) using 3,000 or 10,000 MW BDA; (b) injecting different volumes of BDA; (c) co-injecting BDA with NMDA; and (d) employing various post-injection survival times. Following the extracellular injection of BDA into the visual cortex, labeled cells and axons were observed in both cortical and thalamic areas of all animals studied. However, the detailed morphology of axon arbors and distal dendrites was evident only under optimal conditions for BDA labeling that take into account the: molecular weight of the BDA used, concentration and volume of BDA injected, post-injection survival time, and toning of the resolved BDA with gold and silver. In these instances, anterogradely labeled axons and retrogradely labeled dendrites were resolved in fine detail, approximating that which can be achieved with intracellularly injected compounds such as biocytin or fluorescent dyes.

Morphology, classification, and distribution of the projection neurons in the dorsal lateral geniculate nucleus of the rat

The morphology of confirmed projection neurons in the dorsal lateral geniculate nucleus (dLGN) of the rat was examined by filling these cells retrogradely with biotinylated dextran amine (BDA) injected into the visual cortex. BDA-labeled projection neurons varied widely in the shape and size of their cell somas, with mean cross-sectional areas ranging from 60–340 µm². Labeled projection neurons supported 7–55 dendrites that spanned up to 300 µm in length and formed dendritic arbors with cross-sectional areas of up to 7.0×10⁴ µm². Primary dendrites emerged from cell somas in three broad patterns. In some dLGN projection neurons, primary dendrites arise from the cell soma at two poles spaced approximately 180° apart. In other projection neurons, dendrites emerge principally from one side of the cell soma, while in a third group of projection neurons primary dendrites emerge from the entire perimeter of the cell soma. Based on these three distinct patterns in the distribution of primary dendrites from cell somas, we have grouped dLGN projection neurons into three classes: bipolar cells, basket cells and radial cells, respectively. The appendages seen on dendrites also can be grouped into three classes according to differences in their structure. Short “tufted” appendages arise mainly from the distal branches of dendrites; “spine-like” appendages, fine stalks with ovoid heads, typically are seen along the middle segments of dendrites; and “grape-like” appendages, short stalks that terminate in a cluster of ovoid bulbs, appear most often along the proximal segments of secondary dendrites of neurons with medium or large cell somas. While morphologically diverse dLGN projection neurons are intermingled uniformly throughout the nucleus, the caudal pole of the dLGN contains more small projection neurons of all classes than the rostral pole.

Neuronal pentraxins mediate silent synapse conversion in the developing visual system

Neuronal pentraxins (NPs) are hypothesized to play important roles in the recruitment of AMPA receptors (AMPARs) to immature synapses, yet a physiological role for NPs at nascent synapses in vivo has remained elusive. Here we report that the loss of NP1 and NP2 (NP1/2) leads to a dramatic and specific reduction in AMPAR-mediated transmission at developing visual system synapses. In thalamic slices taken from early postnatal mice (<P10) NP1/2 knock-out (KO) neurons displayed severely reduced AMPAR-mediated retinogeniculate transmission. The reduced currents reflected an increased number of silent synapses with no change in quantal amplitude or presynaptic release. These are the first data to demonstrate that NP1/2 are required in vivo for the normal development of AMPAR-mediated transmission. In addition, they suggest a novel role for NP1/2 in silent synapse conversion during a discrete developmental period when visual circuit connections are undergoing eye-specific refinement. After this period, retinogeniculate transmission not only recovered in the knock-outs but became excessive. The enhanced currents were attributable, at least in part, to a deficit in the characteristic elimination of functional inputs that occurs in the developing dLGN. These data indicate that the loss of NP1/2 disrupts several aspects of retinogeniculate development including the initial establishment of AMPAR transmission and the subsequent elimination of inappropriate circuit connections.

GABAergic pathways in the rat subcortical visual system: a comparative study in vivo and in vitro

Inhibitory pathways project from the pretectal nuclear complex to the ipsilateral superior colliculus (SC) and dorsal lateral geniculate nucleus (dLGN). Both pathways arise from GABAergic neurones that are located in the dorsal pretectal nuclear complex. In the present experiments, we compared the anatomy and physiology of these two pathways with the objective of determining whether they have similar functions. First, we injected retrograde axonal tracers that fluoresce at different wavelengths in the dLGN and SC of single animals to determine if individual GABAergic neurones in the pretectum project to both structures. The results showed that the dLGN and SC receive input from different cell groups. Next, morphological reconstructions of cells labelled after in-vitro whole-cell patch-clamp recordings demonstrated that the pretectal-recipient cells in the dLGN are GABAergic interneurones, whereas those in the SC are projection cells. Finally, with in-vitro whole-cell patch-clamp recordings we showed that inhibitory currents generated by both pathways are mediated by GABA(A) receptors. Taken together, these results suggest that these inhibitory projections may function to facilitate the relay of information from the dLGN to the visual cortex by suppressing the activity of dLGN interneurones, and to reduce the level of activity leaving the SC by inhibiting the projection neurones. These hypotheses will be discussed in the context of the known functions of the pretectal complex.

GABAC receptor mediated inhibition in acutely isolated neurons of the rat dorsal lateral geniculate nucleus

In the dorsal lateral geniculate nucleus (dLGN), GABA(C) receptors seems to be specifically expressed by local GABAergic interneurons. Although the presence of GABA(C) receptors has been demonstrated, a quantitative estimation of their contribution to inhibition in dLGN is lacking. Because the amount of inhibition mediated by these receptors might reflect their functional importance we performed whole-cell patch clamp recordings from dLGN cells acutely dissociated from brain slices. We focally applied the GABA receptor agonist muscimol and quantified effects mediated through either GABA(C) or GABA(A) receptors. Because their basic dendritic morphology was preserved, we tried to morphologically differentiate between thalamocortical cells and local interneurons. In the majority of multipolar cells, representing thalamocortical projection neurons, the specific GABA(A) receptor antagonist bicuculline completely blocked muscimol induced currents. In contrast, in most of the bipolar cells, representing interneurons, bicuculline blocked only 70-80% of the muscimol induced currents. The remaining currents were blocked by co-application of TPMPA, a specific GABA(C) receptor antagonist, or picrotoxin, an unspecific GABA(A) and GABA(C) receptor blocker. The latter neurons were also sensitive to the selective GABA(C) receptor agonist cis-aminocrotonic acid. These results indicate that in those dLGN neurons that express GABA(C) receptors, these receptors contribute considerably to GABAergic inhibitory inputs.

Contribution of transient receptor potential channels to the control of GABA release from dendrites

Neuronal dendrites have been shown to actively contribute to synaptic information transfer through the Ca2+-dependent release of neurotransmitter, although the underlying mechanisms remain elusive. This study shows that the increase in dendritic gamma-aminobutyric acid (GABA) release from thalamic interneurons mediated by the activation of 5-hydroxytryptamine type 2 receptors requires Ca2+ entry that does not involve Ca2+ release nor voltage-gated Ca2+ channels in the plasma membrane but that is critically dependent on the transient receptor potential (TRP) protein TRPC4. These data ascribe a functional role of agonist-activated TRP channels to the release of transmitters from dendrites, thereby indicating a principle underlying synaptic interactions in the brain.

Dendritic organization of neurons of the superior colliculus in animals with different visual capability

The aim of the study was to compare several morphological characteristics of neurons in the superficial layers of the superior colliculus in diurnal and nocturnal mammals with different visual specialization. Thus, we investigated the rat (Rattus norvegicus), a nocturnal animal; the tree shrew (Tupaia glis), a diurnal animal, and the Mongolian rodents, Microtus brandti (nocturnal) and Alticola barakshin (diurnal). The investigation was focused on the study of the organization and extent of dendrites of Golgi-impregnated projection neurons, which were divided in two classes: narrow-field and wide-field cells. We determined that the ratios between the volumes of dendritic fields of the investigated neuronal types and the total volume of the superior colliculus differed to a great extent between the different species. The tree shrew had the largest superior colliculus and the smallest wide-field neurons, while the rat had the largest wide-field neurons. As for the Mongolian rodents, we provided the first description of superior colliculus neurons. The day-active animal Alticola barakshin was found to have a 50% larger volume of the superior colliculus than that of the night-active animal Microtus brandti, and the size of the dendritic field of both wide-field neurons and narrow-field neurons was smaller than that of Microtus brandti. Electron microscopic investigation of wide-field neurons performed in the rat revealed only a few symmetric synaptic contacts on the arborizations of distal and terminal dendrites and numerous asymmetric synapses on the dendritic stem. Our findings support the hypothesis that whereas the narrow-field neurons are relay neurons in the retino-tecto-thalamic pathway of the visual system, the wide-field neurons may play additional roles in the retino-tecto-reticulo-spinal system.

Neonatal monocular enucleation and the geniculo-cortical system in the golden hamster: shrinkage in dorsal lateral geniculate nucleus and area 17 and the effects on relay cell size and number

We have examined the effects of neonatal monocular enucleation on the volume of the dorsal lateral geniculate nucleus (dLGN), the area of area 17, and the size and numbers of geniculate relay neurons identified by retrograde transport of HRP from cortex. Compared to values for normal animals, the only significant change contralateral to the remaining eye was an increase in relay cell radius. The effects ipsilateral to the remaining eye were more widespread: we found significant reductions in the volume of the dLGN (27% reduction), the area of striate cortex (22%), and the number (16%) and average soma radius (6%) of geniculate relay neurons. The relay neurons were also more densely packed, suggesting that other geniculate cell types were affected similarly, although this was not explicitly examined. These changes were not uniform throughout the nucleus, and as such, reflected the changes in retinal input. The greatest reduction in cell size occurred in the region of the ipsilateral dLGN receiving the most sparse retinal input subsequent to enucleation. Nor was the shrinkage of the dLGN uniform, being most apparent in the coronal plane especially along the axis orthogonal to the pia; there appeared to be little change in the anteroposterior extent. Shrinkage in area 17 ipsilateral to the remaining eye was the same (about 22%) whether it was defined by myelin staining or transneuronal transport of WGA-HRP. These results show that the transneuronal changes seen in the organization of visual cortex after early monocular enucleation in rodents are associated with only a moderate loss of geniculate relay cells.

Two types of interneuron in the dorsal lateral geniculate nucleus of the rat: a combined NADPH diaphorase histochemical and GABA immunocytochemical study

The rationale for this study was to provide a comprehensive light microscopical description of the morphology of diaphorase-reactive neurons and neuropil elements in the dorsal lateral geniculate nucleus (dLGN) of the rat. An additional objective was to quantitatively assess whether a subpopulation of the diaphorase-reactive neurons, previously shown to be GABA-immunoreactive, constitute a distinct type of local-circuit neuron in the rat dLGN. Diaphorase activity was localised in a population of predominantly bipolar fusiform neurons. These cells were weak to moderately stained and possessed the morphological features of intrinsic inhibitory neurons, previously called class B neurons in the rat dLGN. Quantitative estimates indicated that the diaphorase-reactive neurons constituted approximately 10% of the total neuron composition of the dLGN. The majority (about 83%) of the diaphorase-reactive cells were located in the lateral half of the nucleus. In addition, a dense plexus of diaphorase-reactive varicose fibres was found throughout the dLGN lying between the oriented fibre bundles coursing dorsoventrally through the LGN. Diaphorase-reactive punctae were found to be closely associated with the somata and proximal dendritic segments of nonreactive neurons and also with the stained proximal dendritic segments of diaphorase-reactive dLGN neurons. The source of the diaphorase-reactive fibres in the dLGN was unknown. Evidence suggests, however, that they are of extrinsic origin. The GABA-immunoreactive nature of the diaphorase neurons in the dLGN was demonstrated by colocalising GABA immunoreactivity within the somata of diaphorase-reactive cells. The majority (> 90%) of diaphorase-reactive dLGN neurons were GABA-immunopositive. Also present was a distinct population of GABA-immunopositive neurons that were not diaphorase-reactive. In this study, cells that were solely GABA-immunopositive have been called class B1 neurons, while cells that were both diaphorase-reactive and GABA-immunoreactive have been called class B2 neurons. Size-frequency distributions of somatic profile areas established that the two populations of GABA-immunoreactive neuron were significantly different. Class B1 neurons constituted 57%, with class B2 cells representing 43% of all GABA-immunostained neurons in the rat dLGN. The characteristic morphological features, neurochemical identity and frequency of the diaphorase-reactive neurons in the rat dLGN indicate that they represent a subpopulation of inhibitory interneurons with the ability to affect intrinsic dLGN operations and thalamocortical interactions using the neuromodulator nitric oxide.

Immunohistochemical changes of neuronal calcium-binding proteins parvalbumin and calbindin-D-28k following unilateral deafferentation in the rat visual system

The neuron-specific calcium-binding proteins, parvalbumin and calbindin-D-28k, were studied in the subcortical visual system of normal and unilaterally deafferented albino rats. Immunohistochemistry with monoclonal antibodies was used on vibratome sections through optic tract (OT), dorsal lateral geniculate nucleus (dLGN), olivary pretectal nucleus (OPN), and superior colliculus (SC). In controls, OT stained strongly for parvalbumin and weakly for calbindin-D-28k. The dLGN contained a plexus of parvalbumin-positive fibers. In dLGN, calbindin-D-28k-antibodies showed strong labeling of some neurons with long dendrites and weak staining of the cytoplasm in other neurons. In OPN, parvalbumin stained a ring of neurons and terminals in the shell region, whereas calbindin-D-28k was contained in medial cell populations. In SC, parvalbumin was contained in fibers, terminals, and neurons throughout the visual layer. Calbindin-D-28k showed a laminar distribution of neurons with a predominance in deep portions of superficial grey matter and in ventral portions of stratum opticum. Following unilateral deafferentation induced by optic nerve section, retinal axons showed immunohistochemical changes related to Wallerian degeneration and target neurons reacted by changes of calcium-binding proteins. Parvalbumin and calbindin-D-28k immunostaining decreased during Wallerian degeneration of OT. In the deafferented dLGN, immunohistochemical labeling for calbindin-D-28k declined in strongly stained neurons from 4 to 21 days after lesion. Measurement of dendritic length per number of cells or per area of dLGN showed a significant decline for the contralateral side at 4, 8, and 21 days (ANOVA, P less than 0.05). In deafferented OPN, terminal-like staining for parvalbumin decreased and neuronal labeling was enhanced. In deafferented SC, the neuronal and dendritic staining for parvalbumin increased beginning from Day 1 on and persisting at Day 21, whereas fibers and terminal-like elements decreased in staining. Measurement of parvalbumin-positive neurons per area of SC showed a significant increase of labeling in the contralateral side from Day 1 to Day 21 (ANOVA, P less than 0.05). These studies show that cellular responses to deafferentation of visual neurons involve a regulation of calcium-binding proteins. The decline in staining for calbindin-D-28k in dLGN may relate to reduced retinal afferent activity. The progressive cellular changes in parvalbumin staining may be related to unmasking of intrinsic neurons after removal of parvalbumin-containing, afferent fibers and terminals. Additionally, the changes of parvalbumin labeling in SC neurons may reflect a plastic reorganization of local circuits known to occur in rat SC in response to deafferentation.

Morphometry and frequency of afferent synaptic terminals in the rabbit dorsal-lateral geniculate nucleus

Morphological and morphometric features of the retinal synaptic terminals (RLP) and cortical synaptic terminals (RSD) were analyzed in the alpha E sector of the rabbit dorsal-lateral geniculate nucleus (dLGN). A methodological approach was selected which allowed us to determine volume of the neuropil and elsewhere record variations in the size and distribution of the two types of terminals found in the three zones (superior, middle, and inferior) from up to down into which the alpha E sector of the dLGN was divided. After obtaining an isotropic, uniform, and pseudorandom (IUR) sample, the terminals were examined on the basis of a set of morphometric parameters. An analysis of these data showed the retinal terminals (RLP) to be more numerous and to occupy a greater total area of the neuropil in the dorsal (superior) zone of the nucleus, whereas the number and total area occupied by cortical terminals (RSD) did not vary in the superior, middle, and inferior zones. Upon comparing the two types of terminals, the RLP were larger and more widely distributed, the greatest differences between the two appearing in the dorsal (superior) zone of the dLGN.

Morphological organization of thalamocortical relay cells in the dorsal lateral geniculate nucleus of the North American opossum

The morphology of relay cells in the dorsal lateral geniculate nucleus of the North American opossum was studied by using both Golgi-Cox material and cells stained from retrograde transport of horseradish peroxidase. In general, soma sizes were largest in the part of the nucleus representing the central retina and decreased from the middle third of the nucleus to the anterior to posterior poles. Relay cells labeled with horseradish peroxidase were found to constitute approximately 90% of the dorsal lateral geniculate nucleus cells and have larger soma diameters than most unlabeled cells. From morphometric analysis of several structural characteristics, three classes of relay cells were identified in both Golgi-Cox and horseradish peroxidase material. Type 1 cells, the predominant class, exhibited radially arranged primary dendritic fields, symmetrically organized relative to projection lines. Type 2 cells had relatively few primary dendrites, and complex dendritic fields that were oriented parallel to projection lines. Least numerous were Type 3 cells, which were characterized by relatively sparse dendritic fields oriented perpendicular to projection lines. An additional class of neuron, Type 4 cells, with small somata and sparse dendritic branching, was found only in Golgi-Cox material. Cells with Type 4 dendritic morphology were not found with retrograde horseradish peroxidase labeling and may represent interneurons. The classification of morphologically characterized cells in the opossum dorsal lateral geniculate nucleus was evaluated quantitatively with multivariate discriminant analysis. The classes are compared to physiologically identified Y-, X-, and W-like relay cells in the opossum and to relay cell classes in other species.

T-type calcium channels mediate the transition between tonic and phasic firing in thalamic neurons

Thalamic neurons undergo a shift from tonic to phasic (burst) firing upon hyperpolarization. This state transition results from deinactivation of a regenerative depolarizing event referred to as the low-threshold spike. Isolated adult guinea pig thalamic (dorsal lateral geniculate) neurons exhibited low-threshold spikes that could be blocked by low concentrations of nickel but were unaffected by the dihydropyridine nimodipine. Whole-cell voltage-clamp recordings from these cells demonstrated a low-threshold, rapidly inactivating (T) Ca2+ current that manifested similar voltage dependency and time course as the low-threshold spike. Like low-threshold spikes, the T-type Ca2+ current was eliminated by nickel but was unaffected by nimodipine. In thalamic neurons, T-type Ca2+ channels underlie the low-threshold spike and, therefore, play a critical role in regulating the firing pattern of these cells.

Synaptogenesis in the dorsal lateral geniculate nucleus of the rat

Synapse formation and maturation were examined in the rat dorsal lateral geniculate nucleus (dLGN) from birth to adulthood. Examination of animals, whose ages were closely spaced in time, showed that the maturation of the synaptic organization of the nucleus takes place chiefly during the first 3 weeks of postnatal life. This period of maturation may be divided into 3 broad stages. During the first stage, which spans the first 4 days of life, there are only a few immature synapses scattered throughout the nucleus; occasionally aggregates of 3 or 4 synapses are encountered. Dendrodendritic synapses first appear at the end of this stage. The second stage, which lasts from the end of the first stage through day 8, is characterized by intensive synaptogenesis as well as extensive growth and degeneration. For the first time, large boutons resembling retinal terminals form multiple synaptic contacts with dendrites and dendritic protrusions; these synaptic arrangements are partially covered by glial processes. A feature characteristic of the developing dLGN during the first 2 postnatal weeks, and particularly during the second stage, is the presence of membrane specializations that resemble vacant postsynaptic densities. These specializations, which may be unapposed or opposite another neuronal process, decrease in frequency as the number of synapses increases. It is not known whether these densities are converted to synapses or whether they result from loss of presynaptic elements. The third stage in the process of synaptogenesis, which spans a period between days 10 and 20, is characterized by myelination and by the diminution of growth cones, degenerating profiles and vacant postsynaptic densities. There is also a very significant increase in the number and maturation of synapses including synaptic glomeruli. However, it is not until the end of this stage that synapses appear qualitatively indistinguishable from synaptic arrangements identified in adult animals.

Development of the rat thalamus: VI. The posterior lobule of the thalamic neuroepithelium and the time and site of origin and settling pattern of neurons of the lateral geniculate and lateral posterior nuclei

Short-survival, sequential, and long-survival thymidine radiograms of rat embryos, fetuses, and young pups were analyzed in order to determine the time of origin, site of origin, migratory route, and settling pattern of neurons of the dorsal lateral geniculate (LGD), ventral lateral geniculate (LGV), and lateral posterior (LP) nuclei of the thalamus. Quantitative examination of long-survival radiograms established that the neurons of the LGD are produced on days E14 and E15. Within the LGD there is an external-to-internal neurogenetic gradient; the majority (77%) of neurons of the external half are generated on day E14, while in the internal half the majority (64%) of neurons originate on day E15. The late-generated LGD neurons are located in the termination field of the uncrossed fibers of the optic tract. Examination of short-survival radiograms indicated that the neurons of the LGD originate in a discrete neuroepithelial eversion situated ventral to the pineal rudiment and dorsal to the putative neuroepithelium of the ventral nuclear complex. In sequential radiograms from rats injected with 3H-thymidine on day E15 and killed on days E16 and E17, the migration of young LGD neurons was followed in a posterolateral direction to the formative lateral geniculate body. By day E17, the day when the optic tract fibers begin to disperse over the lateral surface of the posterior diencephalon, the distribution of early and late-generated neurons of the LGD resembles that seen in young pups. As a whole, the neurons of the LGV are produced earlier than the neurons of the LGD. The bulk of LGV neurons are generated on days E14 and E15 in a caudal-to-rostral intranuclear neurogenetic gradient. Caudal LGV neurons are generated mainly on day E14 (82%), while a substantial proportion of rostral neurons (32%) are generated on day E15. Examination of short-survival and sequential radiograms suggest that the LGV neurons originate in an inverted sublobule situated beneath the putative neuroepithelium of the LGD. At anterior levels the putative inverted sublobule of the LGV merges imperceptibly with the neuroepithelium that produces the neurons of the lateral habenular nucleus. Like the neurons of the LGD and LGV, so also those of the LP are generated on days E14 and E15, but the neurogenetic gradients are different. There is a lateral-to-medial gradient within the LP as a whole. Peak production of neurons is on day E14 laterally (58%) and on day E15 medially (59%).(ABSTRACT TRUNCATED AT 400 WORDS)

The orientation of interneurones in the dorsal lateral geniculate nucleus of the rat: a quantitative study

The orientation of the processes of 60 Golgi-impregnated interneurones in the rat dorsal lateral geniculate nucleus (dLGN) was studied quantitatively. A statistical analysis of the orientation of 238 interneuronal processes (dendrites and axon-like processes) showed that they were aligned preferentially along a dorsoventral axis through the dLGN and were predominantly oriented parallel with the outlying optic tract. Computer reconstructions of two of the Golgi-impregnated dLGN interneurones and their subsequent 3-dimensional computer rotations showed that their processes ramified in long columnar-shaped territories aligned dorsoventrally. There was little extension of these processes along the rostrocaudal axis of the nucleus. The data of this investigation provide evidence that the processes of 'inhibitory' interneurones in the rat dLGN are predominantly aligned along the dorsoventral axis, parallel with both the optic tract and with the afferent and efferent fibre tracts coursing through the nucleus.

A synaptically evoked late hyperpolarization in the rat dorsolateral geniculate neurons in vitro

Intracellular potentials were recorded from presumed relay neurons in the rat dorsolateral geniculate nucleus maintained in vitro preparations. In this material, the neuronal circuit includes the excitatory optic tract which innervates monosynaptically both relay and intrinsic neurons, the latter providing a feed-forward GABAergic inhibition on the former. Electrical stimulation of the optic tract evokes in the dorsolateral geniculate neurons an early excitatory postsynaptic potential followed by an inhibitory postsynaptic potential which precedes a so far unreported long-lasting late hyperpolarization. The properties of the inhibitory postsynaptic potential are consistent with the notion that they are of disynaptic (feed-forward) origin and that they are the consequence of GABAA receptor activation. In contrast, the late hyperpolarization, which was found in almost every neuron, was enhanced by GABAA blockers, without accompanying changes in the resting membrane potential or the input resistance of the recorded cells. The late hyperpolarization had a lower threshold than the excitatory postsynaptic potential, a long latency (m = 38 +/- 4 ms, n = 10) and was of long duration (m = 308 +/- 57 ms, n = 10). The occurrence and threshold for producing these two potentials were uncorrelated, and paired stimulations of the optic tract showed a marked difference of their recovery time-courses. The late hyperpolarization could be elicited only by afferent stimulations; it never followed intracellularly induced depolarizations and/or anodal break calcium spikes. It was associated with a small conductance increase, sufficient, however, to inhibit high-frequency discharges induced by intracellular injection of depolarizing currents. The late hyperpolarization decreased in amplitude with membrane hyperpolarization and ultimately reversed polarity. The apparent reversal potential followed shifts in extracellular potassium concentration in an almost Nernstian relation (47 mV for a tenfold increase in [K]0). Involvement of GABAB receptors in the generation of this potential may be postulated since baclofen readily hyperpolarized the neurons and decreased their input resistance in the presence of GABAA blockers. We conclude that the late hyperpolarization is a postsynaptic potential mediated by an increased conductance to K ions. Our results further suggest that a minimal disynaptic feed-forward circuit impinging on the relay neurons of the dorsolateral geniculate nucleus is sufficient to subserve this late hyperpolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological study of synaptic connections between a transplanted lateral geniculate nucleus and the visual cortex of the host rat

The lateral geniculate nucleus (LGN) of a fetal rat was transplanted to the visual cortex (VC) of a neonatal rat. A current source-density analysis of field potentials and an intracellular study of neuronal responses were conducted in slice preparations by electrical stimulation of transplanted LGN and host VC. The results indicated that synaptic connections were established reciprocally between the transplanted LGN and the host VC.

The ventral and dorsal lateral geniculate nucleus of the rat: intracellular recordings in vitro

1. The membrane properties and the electrotonic structure of neurones in the ventral and dorsal lateral geniculate nucleus (l.g.n.) of the rat were studied using an in vitro slice preparation. 2. Following electrophysiological characterization, horseradish peroxidase (HRP) was injected intrasomatically and the morphological features of impaled cells were characteristic of principal neurones of the rat ventral and dorsal l.g.n. 3. Neurones in the ventral l.g.n. had a higher input resistance but similar membrane time constants (tau o) and resting potentials than cells in the dorsal l.g.n. 4. Using a simple neuronal model, the electrotonic length (L) and the dendritic to somatic conductance ratio (rho) were calculated and found to be similar for cells in both divisions of the l.g.n. The mean value of L (0.7) and rho (1.5) suggest that both groups of neurones are electrotonically compact. 5. The width and after-hyperpolarization of directly evoked action potentials, but not their threshold or their amplitude, were different between cells of the ventral and dorsal l.g.n. 6. At potentials more negative than -55 mV, a slow rising and falling potential could be evoked in each neurone (n = 310) of the dorsal l.g.n. but only in three cells of the ventral l.g.n. (n = 94). The electrophysiological and pharmacological properties of this potential were identical with those of the low-threshold Ca2+-dependent potential observed in other thalamic nuclei. 7. These results indicate that some of the passive and active membrane properties of ventral and dorsal l.g.n. neurones are different. The implications of these findings for the control of the integrative capability and the response of l.g.n. neurones to visual stimulation are discussed.

Complex convolutions in neurons of the dorsal lateral geniculate nucleus of the normal albino rat

The postnatal development of complex convolutions (CCs) of the dorsal lateral geniculate nucleus (LGNd) in normal rats has been studied quantitatively with light microscopy. We report that immature neurons do not contain these scarcely understood organelles, since they can be seen for the first time in very few, mature neurons of the 30 day rat; their number constantly increases during the following 4 months. These cytoplasmic inclusions can be equally seen in the aged rat. CCs are present in neurons of all sizes, except the smallest, which correspond to the interneuron population. Although, morphologically, CCs of the LGNd of the rat are similar, but not identical, to the cytoplasmic multilaminated bodies of the cat, intermediate forms are described.

A quantitative investigation of the neuronal composition of the rat dorsal lateral geniculate nucleus using GABA-immunocytochemistry

The proportion of neurons immunoreactive for gamma-aminobutyric acid (GABA), and their rostrocaudal distribution in the dorsal lateral geniculate nucleus of the rat, were determined quantitatively using post-embedding GABA-immunochemistry on semithin resin embedded coronal sections followed by stereological analysis. The mean total volume numerical density of neurons (total number of neurons per mm3) in the dLGN was 67,077 +/- 4412 mm-3 (mean +/- SEM; n = 5), comprising a mean volume numerical density for GABA-immunopositive neurons of 14,584 +/- 1324 mm-3, and a mean volume numerical density of GABA-immunonegative neurons of 52,493 +/- 3419 mm-3, GABA-immunopositive neurons constituted 21.7 +/- 0.5% of the total neuronal composition of the rat dorsal lateral geniculate nucleus. Although no rostrocaudal variation was detected in the total volume numerical density of neurons, the relative proportion of GABA-immunopositive neurons was significantly lower in the caudal segment (18.1 +/- 0.6%) compared with the middle (24.9 +/- 0.9%) and the rostral segments (22.1%). Furthermore, on the basis of somatic size distributions, GABA-immunonegative neurons were seen to be significantly smaller in the caudal segment than in the more anterior two segments. The somatic size of GABA-immunopositive neurons showed no rostrocaudal variation through the dorsal lateral geniculate nucleus. These data provide a morphological correlate for the structural and functional subdivision of the dorsal lateral geniculate nucleus described previously in electrophysiological and morphological studies.

Neurons of the dorsal lateral geniculate nucleus of the hereditary microphthalmic rat: a Golgi study

Neurons of the dorsal lateral geniculate nucleus (LGNd) in the microphthalmic rat were examined by the Golgi-Cox method. LGNd neurons in the microphthalmic rat were classified into the multipolar (I) and bipolar (II) types as in the normal rat. The multipolar type was further divided into two subclasses (Ia and Ib) on the basis of their dendritic patterns. The proximal portion of their primary dendrites was thinner than in normal LGNd neurons. The Ia cells had 6-7 primary dendrites extending radially, while the Ib cells had 3-4 primary dendrites spreading primarily parallel to the optic tract. Type II cells had two or three primary dendrites emerging from the cell bodies. In both types, primary dendrites were shorter in length or less branched than usual. These results suggested that LGNd neurons in the microphthalmic rat had smaller dendritic fields than those in the normal rat.

GABA-immunoreactive neurons in the rat dorsal lateral geniculate nucleus: light microscopical observations

The dorsal lateral geniculate nucleus (dLGN) of the rat was investigated immunocytochemically using an antiserum against the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). The appearance of GABA-immunopositive dendrites, dendritic appendages, and the size and shape of neuronal somata closely resembled the putative intrinsic neurons described previously in Golgi-impregnation studies of the rat dLGN.

The morphology of relay neurons in the dorsal lateral geniculate nucleus of the marsupial brush-tailed possum (Trichosurus vulpecula)

The retinal terminal zones and the morphology of relay neurons within the dorsal lateral geniculate nucleus (LGNd) of the brush-tailed possum (Trichosurus vulpecula) have been investigated with horseradish peroxidase (HRP) tracing techniques. Anterograde transport of HRP from the retina confirmed previous descriptions of the laminar distribution of retinal afferents in this nucleus. In addition, it was found that lamina III consists of two adjacent bands (IIIa and IIIb) of contralateral retinal input, separated by a terminal-free zone 20-40 micron wide. This zone is not apparent with Nissl or fibre stains. Relay neurons in the LGNd were retrogradely filled following cortical injections of HRP, and two classes (A and B) were distinguished. Class A neurons are found in the alpha portion of the LGNd (laminae I, II, III, and IV) and class B neurons in the beta portion (laminae V, VI, and VII). Class A cells are more densely packed and have shorter and more numerous dendrites, less-extensive dendritic arbors, and thicker axons than class B cells. No significant differences were found between the two classes in perikaryal size or thickness of proximal dendrites. Neurons in each lamina of the nucleus have dendritic arbors which ramify extensively within adjacent laminae, except cells in lamina IIIb, which have relatively few dendrites that cross into the cell-free zone and lamina IV.

Development of dendritic patterns in the lateral geniculate nucleus of monkey: a quantitative Golgi study

Quantitative analysis of dendritic branching patterns was performed in rapid Golgi sections of the lateral geniculate nucleus of Old World monkeys at several ages, using a computer-microscope and a tree-analysing program. In parvocellular and magnocellular multipolar neurons the dendrites were analysed in centrifugal order and according to whether they were intermediate or terminal segments. Between late gestation and birth there is an increase in the mean length of dendrites, and in the total length of dendrites per neuron, more pronounced in magnocellular layers; there follows a progressive decrease in their length until adulthood. However, only terminal dendritic segments are involved in these changes in length. Intermediate segments are shorter and show a more or less constant length throughout life. The final length of a segment seems more determined by it being terminal or intermediate than by its order of branching and there is greater plasticity in the terminal part of the dendritic tree. Magnocellular neurons are characterized by a greater number of both intermediate and terminal segments than parvocellular neurons but not by a greater length of individual segments. This accounts for the greater dendritic length in magnocellular neurons and may offer them a larger surface for synaptic connectivity.

Morphology of identified relay cells and interneurons in the dorsal lateral geniculate nucleus of the rat

The morphology of neurons in the lateral geniculate nucleus of the rat has been examined in both Golgi impregnated and in horseradish peroxidase (HRP) filled material. Two major classes of neurons are seen in Golgi material which encompass the variety of cells described in previous reports. Cells of one group (class A) are routinely labelled by injections of HRP into the visual cortex or optic radiations. This group also displays some morphological variation which may be related to the presence of parallel information channels in the retino-fugal pathway, but clear subgroups cannot be identified on the basis of morphological criteria alone. Cells of the other group (class B) are not labelled by HRP injections into visual cortex or the optic radiations, and are probably local circuit interneurons.

Neuronal types in the lateral geniculate nucleus of man. A Golgi-pigment study

Nerve cell types of the lateral geniculate body of man were investigated with the use of a transparent Golgi technique that allows study of not only the cell processes but also the pigment deposits. Three types of neurons have been distinguished: Type-I neurons are medium- to large-sized multipolar nerve cells with radiating dendrites. Dendritic excrescences can often be encountered close to the main branching points. Type-I neurons comprise a variety of forms and have a wide range of dendritic features. Since all intermediate forms can be encountered as well, it appears inadequate to subdivide this neuronal type. One pole of the cell body contains numerous large vacuolated lipofuscin granules, which stain weakly with aldehyde fuchsin. Type-II and type-III neurons are small cells with few, sparsely branching and extended dendrites devoid of spines. In Golgi preparations they cannot be distinguished from each other. Pigment preparations reveal that the majority of these cells contains small and intensely stained lipofuscin granules within their cell bodies (type II), whereas a small number of them remains devoid of any pigment (type III). Intermediate forms do not occur.

Bilateral changes in soma size of geniculate relay cells and corticogeniculate cells after neonatal monocular enucleation in rats

Soma areas of relay cells of the lateral geniculate nucleus and of corticogeniculate cells of normal rats (n = 4) were compared with those of neonatally unilaterally eye-enucleated adult rats (n = 13). These cells were labeled by retrogradely transported HRP. Monocular enucleation was performed on postnatal days 1 (PND 1) (n = 4), 3 (PND 3) (n = 5) and 6 (PND 6) (n = 4). The results are summarized as follows. In PND 1 rats soma areas of relay cells were 12-16% smaller than those of normal rats, but only for the geniculate nucleus ipsilateral to the remaining eye. In PND 3 and 6 rats the areal shrinkage of relay cells was 27-39% of the normal control for both hemispheres, though it was less marked in the hemisphere contralateral to the remaining eye. The corticogeniculate cells were distributed in layers V and VI in eye-enucleated rats as well as in normal rats. Soma areas of both layer V and VI cells increased in PND 1 rats for both hemispheres by about 15-47% of the normal control. In PND 3 rats increase in soma size tended to occur for layer VI cells, although the data varied from animal to animal. In summary, it was established that unilateral eye-enucleation in rats at birth induced soma size changes of the geniculate relay cells and of the corticogeniculate cells in the non-deafferented as well as in the deafferented hemisphere. Possible mechanisms for the bilateral changes in soma area of central visual cells after neonatal monocular enucleation are discussed.

A Golgi study of the opossum ventral basal complex

The morphology of neurons in the ventral basal complex (VBC) of the adult opossum (Didelphis virginiana) is described from thick coronal brain sections, using Golgi-, horseradish peroxidase (HRP)-, and Nissl-staining methods. Soma cross-sectional area, dendritic field shape, and the number of appendages (spines) in a defined major branch zone (MBZ) are quantified and statistically analyzed. Results indicate that neurons in opossum VBC have relatively large cell bodies, dendrites which branch in a tufted pattern, and numerous dendritic appendages. These neurons are designated as relay cells because of (1) their tufted dendritic branch patterns, considered characteristic of thalamic relay cells (Ramon-Moliner, '62), and (2) the similarity of their soma sizes with HRP-labeled somata after somatosensory cortical injections. Neurons with traditionally described interneuron morphology do not appear to be present in the VBC of this animal, and, in this respect, the neuronal morphology of opossum VBC is similar to that in rat (McAllister and Wells, '81). Based on statistical analysis of the structural features observed, the presumed relay cells in opossum VBC do not show significant differences in morphology, and consequently are not subdivided into classes. Opossum VBC neurons are recognized as forming a single category in which broad and continuous variations in morphology are indicated. Recognition of a singular class of relay cell is consistent with descriptions for rat and cat VBC (Scheibel and Scheibel, '66), but at variance with a previous report for the primate Galago VBC (Pearson and Haines, '80) subdividing thalamic relay cells into Types I, II, and intermediate categories.

The localization of AChE in the dorsal lateral geniculate nucleus of different mammals--a light and electron microscopical study

AchE has been histochemically demonstrated in the dLGN of different mammals. In the rat dLGN the enzyme is light microscopically localized in fibres only. This reaction is somewhat higher in the superficial than in the innermost region of the dLGN. The same distribution has been observed in other rodents like hamster, mouse, and guinea pig. Reaction products have been found electron microscopically in afferent fibres of the rat's dLGN with characteristic vesicle types and synaptic contacts to dendrites. We concluded that these fibres originate from the formatio reticularis of the brain stem. Nerve cells show a very weak reaction in the nuclear envelope and in the endoplasmatic reticulum. In the rabbit dLGN cell bodies with strong AchE-reaction are demonstrable. The AchE-activity in the laminae of the dLGN of tree shrew, cat and monkey is compared with the morphologically characterized cell types. It is discussed that X-cells in the mammalian dLGN are more influenced by AchE-containing afferent axons than other types of geniculo-cortical relay cells. Light microscopic results of AchE-localization in the visual cortex of rodents, tree shrew, and cats are described.

The distribution of ganglion cells in the retina of the North American opossum (Didelphis virginiana)

The distribution of ganglion cells in the retina of the opossum was determined from whole-mounted retinae stained with cresyl violet. Isodensity lines were approximately circular with a peak density of 2,000 to 2,700 cells/mm2 in superior temporal retina (area centralis). The total number of retinal ganglion cells was estimated to be 72,000 to 135,000 (mean 101,026) in retinae ranging from 125 to 187 mm2 in total area. Three groups of ganglion cells were distinguished on the basis of soma size and retinal topography. Large cells (24 to 32 micrometer diameter) were fairly evenly distributed across the retina. Medium cells (12 to 23 micrometer diameter) were more numerous in the superior temporal quadrant than in other regions of the retina. Small cells (7 to 11 micrometer diameter) were prominent in all retinal regions, but particularly in nasal and inferior retina. An analysis of topographical differences in soma size distribution suggests that the medium size cells can be further subdivided into small-medium and large-medium groups.

Spatial contrast sensitivity of cells in the lateral geniculate nucleus of the rat

1. The responses to visual stimuli of cells in the dorsal lateral geniculate nucleus of the rat were recorded with micro-electrodes. 2. Maps made with small spots of light showed that most units had concentrically organized receptive fields. Some units gave 'on-off' responses to spots flashed anywhere within the receptive field. These units were not directionally selective. 3. By the use of grating patterns as stimuli, units with concentrically organized receptive fields could be divided into groups that showed linear or non-linear spatial summation. Those unit showing linear spatial summation behaved like the 'X' cells of the cat, those showing non-linear summation like 'Y' cells. 4. 'On-off' units showed non-linear spatial summation of a kind that readily distinguished them from Y cells. 5. Measurements of spatial contrast sensitivity made with moving gratings showed, for both X and Y cells, peak sensitivities for spatial frequencies between 0.05 and 0.09 c/deg X and Y cells were not distinguished by their preferred spatial frequencies at any eccentricity.

Somatosensory thalamus of a prosimian primate (Galago senegalensis). II. An HRP and Golgi study of the ventral posterolateral nucleus (VPL)

The topographic arrangement and cytoarchitecture of cells in the ventral posterolateral nucleus (VPL) of a prosimian primate (Galago senegalensis) were studied using horseradish peroxidase (HRP) and Golgi impregnation techniques. Following cortical implants of HRP, reactive neurons in VPL are organized into medially concave lamellae which extend through the dorsoventral and rostrocaudal dimensions of the nucleus. After implant in forelimb and hindlimb areas of motor-sensory cortex, labeled cells are confined to the medial (VPLm) and lateral (VPLl) portions of VPL, respectively. HRP-positive cells in the ventral part of each lamella are organized into clusters which correspond to the clusters of cells and "parcellated-bursts" of preterminal debris previously described in bushbaby VPL (Pearson and Haines, this volume). HRP-reactive cells in the ventral intermediate nucleus (Vim) are evenly distributed as contrasted to the tightly clustered groups of somata in the adjacent VPL. This evidence argues in favor of the presence of Vim in dorsal thalamus of this prosimian. Golgi impregnations reveal two main types of relay cells in Galago VPL. Type I cells have multiangular somata, straight distal dendrites, and primary dendritic branch points which are free of appendages. Tye II cells have rounded somata, sinuous distal dendrites, and clusters of appendages located at primary branch points. Intermediate cells (i.e., cells with morphological features in between types I and II) are also present in VPL. Comparison of tufted Golgi impregnated cells with neurons labeled with HRP shows definite similarities in somata size and shape, and in the orientation of proximal dendrites. This evidence corroborates the relay nature of the tufted neurons in VPL. Relay cells in Galago VPL have morphological features which are similar to those of relay cells in lateral and medial geniculate nuclei of cat and primate. Type III cells have small round somata, radiate dendrites with elaborate appendages, and axons which appear to be intrinsic to VPL. Consequently, these cells are considered to be interneurons in the VPL of Galago. Glial-like neurons (type IV cells) were also observed. These have beaded dendritic processes similar to those which presumably represent presynaptic boutons in other species. Consequently, these cells are also assumed to function intrinsically within VPL.

Development of the diencephalon in the rat. V. Thymidine-radiographic observations on internuclear and intranuclear gradients in the thalamus

Groups of pregnant rats were injected with two successive daily doses of 3H-thymidine from gestational days 13 and 14 (E13 + 14) until the day before birth (E21 + 22). Internuclear and intranuclear cytogenetic gradients were examined in radiograms of the thalamus sectioned in the coronal, sagittal and horizontal planes. There was a precise and segregated lateral-to-medial gradient between and within the habenular nuclei. In the ventral thalamus the reticular nucleus had a lateral-to-medial gradient, the subthalamic nucleus a laterodorsal-to-medioventral gradient. There was a caudal-to-rostral gradient between the medial geniculate and dorsal lateral geniculate nuclei, and between the pars posterior and pars anterior of the lateral nucleus. A clear intranuclear gradient could not be detected in the sensory relay nuclei with the exception of the medial geniculate nucleus. A lateral-to-medial internuclear gradient was seen between the relay nuclei and the intralaminar nuclei, and between the latter and some of the midline nuclei. On the basis of a consideration of the time of origin and time span of production of neurons of various thalamic nuclei, and taking into account some of the recognizable internuclear and intranuclear gradients, the thalamus was divided into five principal cytogenetic components; the epithelamus, the ventral thalamus, the dorsal thalamus, the medial thalamus, and the posterior thalamus. The epithalamic nuclei form over a protracted period resembling the nuclei of the hypothalamus. The nuclei of the ventral thalamus are generated early and over a relatively long period. The dorsal thalamus consists of the relay nuclei and the intralaminar nuclei; they form rapidly and ahead of the medial thalamus. The medial thalamus was subdivided into the earlier-forming anteromedial nuclei and the latest-forming midline nuclei. The posterior thalamus was not examined in detail.

Anatomical and neurobehavioral investigations concerning the thalamo-cortical organization of the rat's visual system

The organization of thalamic afferents to the rat's visual cortex was investigated autoradiographically and through the retrograde transport of horseradish peroxidase (HRP) following infections into striate and peristriate cortex. The results revealed that Nucleus lateralis posterior (NLP) projects to a large peristriate cortical field that includes areas 18A, 7, and the anterior portion of area 18, and to a circumscribed temporal area corresponding to Krieg's ('46a,b) area 20. The dorsal lateral geniculate nucleus (LGNd) was shown to project to two spatially discontinuous cortical areas. The largest geniculate receiving area is partially coextensive with Krieg's area 17, but an extension of this projection posterior and medial to the striate cortex was found. In addition, a geniculate projection to a restricted field located in the lateral peristriate cortex was identified. Concurrent investigations were designed to assess the pattern discrimination abilities of rats prepared with striate cortical ablations, lesions in NLP and combined striate-cortical and thalamic ablations. Comparison of these animals with normal control subjects revealed that the striate cortex in the rat (as in the cat [Doty, '71; Sprague et al., '77] and the tree shrew [Killackey and Diamond, '71; Ware et al., '74]) is not necessary for successful pattern discrimination, and that the geniculo-striate and NLP-extra-striate projection systems are both involved in mediating the visual discriminative abilities of the rat. The results add species generality to the concept that the central connections to the visual cortex are characterized by parallel-conducting thalamic channels and contribute to the growing number of demonstrations that the extra-striate cortex and associated thalamic cell groups contribute significantly to the process of visual-pattern recognition.

The postnatal development of neurons in the dorsal lateral geniculate nucleus of the rat: a Golgi study

The postnatal differentiation of neurons in the dorsal lateral geniculate nucleus of the albino rat was studied using the Golgi-Cox technique. At least four animals were used at postnatal ages 12 hours, 2, 4, 6, 8, 10, 11, 12, 13, 14, 15, 16, 18, 20, 24, 28, 35 days and adult. Presumptive thalamo-cortical projection cells (Class A cells of Grossman et al., '73) and non-projection, intrinsic neurons (Class B cells) are distinguishable at 12 hours after birth. At this stage both types of neuron are immature, with prominent growth cones at their dendritic extremities. Dendritic growth and differentiation appear to be complete by 18 days. Relay cells display two apparent "growth spurts" characterized by noticeable enlargement of the perikaryon and a marked increase in dendritic length and complexity. One occurs between days 4 and 6 and coincides with a period of enhanced synaptogenesis and gliogenesis: the second occurs between days 14 and 15, around the time of eye opening (day 14) and may be related to a further phase of increased synaptogenesis and gliogenesis (Karlsson, '67; Biesold et al., '76). Class B cells appear to "lag" behind class A cells in their initial postnatal differentiation, but also reach their mature condition by day 18.

Response properties of cells in the dorsal lateral geniculate nucleus of the albino rat

The response patterns of cells in the dorsal lateral geniculate nucleus of the albino rat were studied in order to examine the functional organization of the lateral geniculate nucleus. Both photic stimulation and electrical stimulation of the optic tract were used to activate single units in the lateral geniculate nuclei. Three different types of response patterns were found for principal cells, while interneurons all had similar response patterns. The first class of principal cells, E-S cells, responded to stimulation with a period of excitation, followed by a period when activity was suppressed. A second class of cells, S cells, responded to photic stimulation with an initial period when activity was suppressed. The final class of cells, E cells, responded with a period of excitation followed by a return to spontaneous rates of firing. The response patterns of E cells suggest that this type of principal neuron does not receive feedback inhibition of the type proposed in previous models of the lateral geniculate nuclei. Based on these and other observations, a new model of the functional organization of the lateral geniculate nuclei is proposed.

Diazepam modification of evoked and spontaneous lateral geniculate activity

The effects of diazepam on evoked and spontaneous activity of dorsal lateral geniculate nucleus (dLGN) principal (P) and inhibtory (I) cells were examined in rats. In the majority of P cells tested both spontaneous and evoked activity were suppressed following diazepam treatment with these effects being altered little by a pentylenetetrazol (Metrazol) challenge. In two P cells, which were offresponding to photic pulse stimulation, evoked and spontaneous activity was enhanced by diazepam but again these effects were altered little by the Metrazol challenge. I cell spontaneous activity was suppressed by diazepam and augmented by the Metrazol challenge; however, post-stimulus activity was relatively unaffected by either treatment. These results were discussed in terms of support for a functional re-evaluation of the rat dLGN.

The ventral lateral geniculate nucleus, pars lateralis of the rat. Synaptic organization and conditions for axonal sprouting

The synaptic organization of the pars lateralis portion of the ventral lateral geniculate nucleus is similar to that of other thalamic nuclei. There are four types of synaptic knobs (RL, RS, F1, F2). RL knobs are large and irregularly shaped, contain round synaptic vesicles and make multiple asymmetrical junctions. They are found primarily in "synaptic islands" making contact with gemmules, spines, small dendrites, and other synaptic profiles containing pleiomorphic synaptic vesicles (F2). Smaller RS knobs contain round vesicles and make asymmetrical junctions with the same type of elements as RL knobs, with the exception of the F2 profiles, but are seldom found in synaptic islands. F1 knobs contain flattened synaptic vesicles and form symmetrical junctions with F2 knobs, gemmules, spines, and small-medium dendrites in synaptic islands, throughout the neuropil, and on the proximal dendrites and soma of the largest type of neuron. F2 knobs are irregularly shaped, contain pleiomorphic synaptic vesicles and make symmetrical junctions primarily with gemmules and spines in synaptic islands. They are postsynaptic to RL and F1 knobs. Occipital decortication indicates that cortical terminals are of the RS type. Bilateral enucleation indicates that retinal terminals are of both the FL and RS type. The large amount of geographic overlap of retinal and cortical terminals on gemmules, spines, and small dendrites found in the neuropil outside of synaptic islands logically would maximize axonal sprouting between these two sources.

The neurons in the centromedian-parafascicular complex of the monkey (Macaca mulatta): a Golgi study

The neurons of the nucleus centrum medianum and the neurons of the nucleus parafascicularis were studied in Golgi preparations of the adult monkey (Macaca mulatta). The cell bodies of the prinicipal neurons in the nucleus centrum medianum have a few somatic spines and vary in shape: some are cubical with protruding angles; some are egg-shaped; some are elongated and sausage-shaped. Four to six slightly branched dendrites of unequal thickness radiate from the cell body. Some dendrites extend for nearly 500 microns; all have dendritic spines. In the nucleus parafascicularis there are two varieties of principal neurons: (1) neurons with somatic spines and (2) neurons without somatic spines. The neurons with somatic spines are most numerous. They have polygonal-shaped cell bodies, prominent somatic spines and processes, larger than spines but considerably smaller than dendrites. These processes bear spines and are designated here "microdendrites." Spines and occasionally a "microdendrite" are found on the axon-hillocks. Five to six dendrites of unequal thickness emerge from the cell bodies. Some extend for more than 500 microns; all have prominent dendritic spines. The neurons without somatic spines are relatively few. Usually three exceptionally long, slightly branched dendrites, one apical and two basal, emerge from their elongated, slim cell bodies. Some dendrites extend for more than 800 microns; all have few scattered spines. The Golgi type II neurons found in both of these intralaminar nuclei have small cell bodies and a few, relatively long, undulating dendrites, which bear bulbous dendritic appendages and beaded axon-like processes. Distally on these dendrites, where the appendages and processes are more numerous, the dendritic appendages and axon-like processes form complex entanglements. Distally on these dendrites, where the appendages and processes are more numerous, the dendritic appendages and axon-like processes form complex entanglements. Beaded axons are found on some but not all of the cell bodies. Morphologically these neurons resemble the local interneurons that have been described in various thalamic nuclei.

The neurons in the primate subthalamic nucleus: a Golgi and electron microscopic study

In Golgi preparations of the adult monkey (Macaca mulatta) local interneurons and two varieties of principal neurons, radiating and elongated fusiform, are found in the subthalamic nucleus. The cell bodies of the radiating neurons have a few delicate, somatic spines some of which are occasionally bilobed and trilobed. Five to eight dendritic trunks give rise to branching, tapering dendrites, which may extend for over 400 microns. These dendrites are much thinner than the dendrites in the globus pallidus and the substantia nigra. Some neurons have many and some neurons have few dendritic spines. When numerous the dendritic spines are concentrated on the dendritic trunks and proximal dendrites. The relatively few elongated fusiform neurons are found not only in the capsule but also in the center of the nucleus. Most dendrites emerge from the opposite poles of their smooth surfaced cell bodies. They have a few dendritic spines. Some of these dendrites extend for more than 750 microsn. In 1-micron thick plastic sections lipofuscin granules are present in some but not all principal neuron cell bodies of the monkey (Macaca mulatta); but these granules are present in all principal neuron cell bodies of the pig-tail monkey (Macaca nemestrina) and of the squirrel monkey (Saimiri sciureus). The local interneurons have small cell bodies and a few relatively long undulating dendrites. The dendrites have bulbous dendritic appendages of varying complexity and beaded axon-like processes. The dendritic appendages and axon-like processes are more numerous distally and on the distal ends of the dendrites they form complex entanglements. Axons coming from the cell body have not been observed. The cell bodies of the local interneurons are identified in cresyl violet stained sections of the monkey (Macaca mulatta), in 1-micron thick plastic sections and electron micrographs of the squirrel monkey (Saimiri sciureus). They have relatively large nuclei surrounded by a thin rim of cytoplasm rich in polyribosomes.