Intrathalamic sensory connections mediated by the thalamic reticular nucleus.

The thalamus and cerebral cortex are linked together to form a vast network of interconnections. Different modes of interactions among the cells in this network underlie different states of consciousness, such as wakefulness and sleep. Interposed between the dorsal thalamus and cortex are the GABAergic neurons of the thalamic reticular nucleus (TRN), which play a pivotal role not only in switching between the awake and sleep states but also in sensory processing during the awake state. The visual, somatosensory, and auditory sectors of TRN share many of the same organizational features. Each of these sectors contains maps, which are related to its inputs and outputs, and organizational components called 'slabs.' It is proposed that, during wakefulness, TRN is crucially involved in resetting the activity levels in sensory nuclei of the dorsal thalamus, which allows the cortex to actively and periodically compare its ongoing sensory processing with the available sensory information.

Organization in the auditory sector of the cat's thalamic reticular nucleus.

This study describes the organization of cells in the thalamic reticular nucleus (TRN) that project to the auditory part of the cat's dorsal thalamus. Injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and fluorescent dyes were made into the medial geniculate complex (MG). The resultant retrograde labelling in the TRN was analyzed. Injections of WGA-HRP into the ventral (MGv), dorsal (MGd), or medial (MGm) nuclei of the MG label zones of cells that are restricted to a caudoventral sector of the TRN. In reconstructions, these zones resemble "slabs" that are elongated in the dorsoventral and oblique rostrocaudal dimensions of the nucleus. In comparisons of the zones of labelling in the TRN following tracer injections into different nuclei of the MG, inner and caudal cells project to the pars lateralis of the MGv (MGvl) or to the MGd, and outer and rostral cells project to the pars ovoidea of the MGv (MGvo) or to the MGm. Thus, cells projecting to the MGvl or MGd or to the MGvo or MGm occupy overlapping territories. Double injections of different fluorescent dyes into selected pairs of MG nuclei result in reticular cells that are labelled from either both nuclei or only one or the other nucleus in each pair. These results indicate that the projections of cells in the auditory sector of the TRN to the MGvl or MGvo or to the MGd or MGm are topographically organized. Furthermore, projections to more than one MG nucleus can arise from single reticular cells.

A new intrathalamic pathway linking modality-related nuclei in the dorsal thalamus.

Transmission of sensory information through the dorsal thalamus involves two types of modality-related nuclei, first order and higher order, between which there are thought to be no intrathalamic interactions. We now show that within the somatosensory thalamus, cells in one nucleus, the ventrobasal complex, can influence activity in another nucleus, the medial division of the posterior complex. Stimulation of ventrobasal complex cells evoked inhibitory postsynaptic currents in cells of the medial division of the posterior complex. These currents exhibited the reversal potential and pharmacology of a GABAA receptor-mediated chloride conductance, indicating that they result from the activation of a disynaptic pathway involving the GABAergic cells of the thalamic reticular nucleus. These findings provide the first direct evidence for intrathalamic interactions between dorsal thalamic nuclei.

Organization in the somatosensory sector of the cat's thalamic reticular nucleus.

This study describes the organization of cells in the thalamic reticular nucleus (TRN) that project to the somatosensory part of the dorsal thalamus in the cat. Injections of wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) and fluorescent dyes were made into the ventrobasal complex (VB) and the medial division of the posterior complex (POm) of the thalamus. The resultant retrograde labelling in TRN was analyzed. Large injections of a tracer in VB label many reticular cells that are restricted to a centroventral, or somatosensory, sector of TRN. Small injections of a tracer in VB produce narrow zones of labelled cells in this sector. In reconstructions these zones resemble thin "slabs," which lie parallel to the plane of TRN along its oblique rostrocaudal dimension and occupy only a fraction of its thickness. In comparisons of the zones of labelled cells in TRN resulting from tracer injections in different nuclei of VB, inner cells, intermediate cells, and outer cells across the thickness of TRN project to the ventral posteromedial, the medial division of the ventral posterolateral, and the lateral division of the ventral posterolateral nuclei, respectively. Furthermore, shifts in injected areas along the dorsoventral dimension of VB produce similar shifts in zones of labelled cells in TRN. Thus, reticular cells form an accurate map on the basis of their connections with VB. Large injections of a tracer in the ventral subdivision of POm label many reticular cells that are also restricted to the centroventral sector of TRN. Small injections of a tracer in ventral POm produce broad zones of labelled cells in this sector. In comparisons of the zones of labelled cells in TRN resulting from tracer injections in different regions of ventral POm, cells that project to these regions are scattered across the thickness of TRN and occupy overlapping territories. Large injections of a tracer in either VB or ventral POm also label cells in a restricted centroventral region of the perireticular nucleus. Double injections of different tracers in VB and ventral POm produce many cells in TRN that are labelled from both of these dorsal thalamic structures and fewer cells that are labelled from only one or the other of these structures. These results indicate that there is a dual organization in the projections of cells in the somatosensory sector of TRN to dorsal thalamus: Projections to VB are topographically organized whereas those to ventral POm lack a topographical organization. Furthermore, both of these mapped and nonmapped projections can arise from single reticular cells in the somatosensory sector.

Monoclonal antibody Cat-301 selectively identifies a subset of nuclei in the cat's somatosensory thalamus.

Recently it has been demonstrated that the monoclonal antibody Cat-301 is capable of identifying functionally related neurons in the mammalian visual thalamus. We have examined the possibility that this antibody might display a similar capacity in nonvisual thalamic areas. We demonstrate that in the cat's somatosensory thalamus the distribution of Cat-301-positive cells and neuropil is restricted to a subset of nuclei. These include the ventroposterior medial, ventroposterior lateral, and ventroposterior inferior nuclei. Staining with Cat-301 provides a clear visualisation of the entire somatotopic map within these nuclei. The somatosensory sector of the thalamic reticular nucleus and the perireticular nucleus, which may have a somatosensory sector, are also Cat-301-positive. In contrast, cells that do not express the Cat-301 antigen are located in the ventroposterior oralis nucleus, the ventroposterior shell region, the medial and lateral divisions of the posterior nuclear group, and the inner small cell region adjacent to the thalamic reticular nucleus. In comparison with previous physiological studies, cells that express the Cat-301 antigen most likely represent subpopulations in only a few of the somatic submodality-specific groups. These include cells in the small-field and Pacinian cutaneous-responsive groups, excluding cells in the wide-field cutaneous-, muscle-, joint-, and noxious-responsive groups. Taken together these findings indicate that monoclonal antibody Cat-301 is capable of selectively identifying neurons with distinct functional properties in the mammalian somatosensory thalamus.

Prenatal development of retinogeniculate projections in the rabbit: an HRP study.

The prenatal development of the rabbit's retinal projections to the dorsal lateral geniculate nucleus (dLGN) was studied by using anterograde axonal transport of HRP injected intraocularly. Further, the ontogenesis of the dLGN's alpha and beta sectors was studied. Fetuses aged embryonic day 18 (E18) to E29 were examined. Gestation in the rabbit is 30-31 days. On E18 the future dorsal lateral and medial geniculate nuclei appear as a continuous strip of cells along the lateral margin of the dorsal thalamus. On E21 labelled retinal fibers are invading the lateral margin of the dLGN contralateral, but not ipsilateral, to an injected eye. At this age the dorsal lateral and medial geniculate nuclei are separating. By E23 contralateral fibers occupy the entire presumptive alpha sector, while ipsilateral fibers are invading the caudal half of the sector, overlapping the contralateral fibers. At this age the alpha and beta sectors begin to differentiate. On E25 contralateral fibers are more densely distributed throughout the alpha sector and the ipsilateral fibers are concentrated dorsally within the caudal three-quarters of the sector. By E27 contralateral fibers begin to withdraw from a medial zone of the alpha sector, while ipsilateral fibers remain densest in this zone and begin to withdraw from more lateral and caudal aspects of the sector; contralateral fibers, but not ipsilateral fibers, invade the beta sector. At this age the alpha and beta sectors acquire an adult-like appearance. By E29 the contralateral fibers vacate the beta sector and the medial zone of the dLGN and the ipsilateral fibers are restricted to this zone. Thus, 1 or 2 days before birth, the locations of the ipsilateral and contralateral retinal projections to the dLGN resemble those seen in the adult. The early overlapping projections of ipsilateral and contralateral retinal fibers within the dLGN and their eventual segregation in the fetal rabbit are consistent with the development of these projections in other mammalian orders. Further, the brief invasion of the beta sector by the contralateral fibers resembles the transient occupation of the carnivores' perigeniculate nucleus by developing retinal fibers. In addition, direct comparisons of temporal and spatial events during retinal innervation of the dLGN and the superior colliculus indicate several developmental differences between the two nuclei.

Prenatal development of retinocollicular projections in the rabbit: an HRP study.

The prenatal development of the rabbit's retinal projections to the superior colliculus (SC) was studied by using anterograde transport of horseradish peroxidase injected intraocularly. Fetuses aged embryonic day 21 (E21) to E29 and an adult rabbit were examined. Gestation in the rabbit is 30-31 days. On E21 contralaterally projecting retinal fibers invade across the entire SC. Their distribution is initially diffuse within the superficial laminae, but by E29 they have a distinct stratified appearance. Ipsilaterally projecting retinal fibers invade the rostral half of the SC on E21. By E23 they cover the entire SC and overlap the contralateral fibers both tangentially and radially. The ipsilateral fibers for the most part are sparsely distributed, but they form a dense focal distribution in the rostrolateral quarter of the SC. This focus straddles the stratum griseum superficiale/stratum opticum (SGS/SO) border. On E25 the ipsilateral fibers maintain their widespread distribution and focal rostrolateral concentration. By E27 they are excluded almost entirely from the caudal half of the SC and are reduced in density in the rostromedial quarter of the nucleus. On E29 the ipsilateral terminal field forms distinct patches and bands that are restricted to the rostrolateral quarter of the SC and are confined to the SGS/SO border. Thus, a few days before birth the pattern and location of the ipsilateral retinocollicular projection resemble those seen in the adult. The early widespread distribution of the ipsilaterally projecting retinal fibers to the SC and their eventual restriction in the fetal rabbit are consistent with the development of this projection in other mammalian orders.

The beta sector of the rabbit's dorsal lateral geniculate nucleus.

The beta sector of the rabbit's dorsal lateral geniculate nucleus is a small region of nerve cells scattered among the fibres of the geniculocortical pathway. In its topographical relations it resembles the perigeniculate nucleus of carnivores, which contains neurons driven by geniculate and visual cortical neurons and which sends inhibitory fibres back into the geniculate relay. We have traced retinogeniculate, geniculocortical and corticogeniculate pathways in rabbits by using horseradish peroxidase or radioactively labelled proline and have found that the beta sector resembles the perigeniculate nucleus in receiving no direct retinal afferents, sending no efferents to the visual cortex (V-I), and receiving afferents from the visual cortex. The corticogeniculate afferents are organized so that the visual field map in the beta sector and the main part of the lateral geniculate relays are aligned, as are the maps in the cat's perigeniculate nucleus and the main part of the geniculate relay of carnivores. Electron microscopical studies show similar types of axon terminals in the rabbit and the cat for the main part of the geniculate relay on the one hand and for the beta sector and the perigeniculate nucleus on the other. Earlier observations that the proportion of putative inhibitory terminals (F-type terminals) is lower in the rabbit's than the cat's geniculate region are confirmed. A major difference between the beta sector and the perigeniculate nucleus has been revealed by immunohistochemical staining for GABA. Whereas almost all of the cat's perigeniculate cells appear to be GABAergic, the proportion in the beta sector is much lower, and not significantly different from that found in the main part of the rabbit's geniculate relay. It is concluded that the beta sector shares many of the organizational features of the perigeniculate nucleus. A common developmental origin seems probable, but the functional differences remain to be explored.

Effects of antibodies to large retinal ganglion cells on developing retinogeniculate pathways in the cat.

We investigated the effects of polyclonal antibodies, produced against ox large retinal ganglion cells, on the developing retinogeniculate pathways of cats. Four-week-old kittens were given an intraocular injection of either a low or a high concentration of the antibodies and effects were assessed 35-69 weeks later. After a low concentration (110 micrograms/33 microliter volume) injection, the density of retinal alpha-cells (the morphological counterpart of Y-cells) was reduced 44% in area centralis and 37% in peripheral retina. After a high concentration (333 micrograms/33 microliter volume) injection, alpha-cell density was reduced 76% in area centralis and 91% in peripheral retina. The same concentration of antibodies had no consistent effect on the numbers of medium- or small-size retinal ganglion cells. Electrophysiological recordings from single neurons in layers A and A1 of the lateral geniculate nucleus (LGN) revealed a 53% decrease in the percentage of Y-cells after a low-concentration injection and an 82% decrease after a high-concentration injection. There was a concomitant increase in the percentage of LGN cells that were non-responsive to light or that responded too poorly to be classified. No change was observed in the percentages of LGN X-cells or cells with mixed response properties. The reduced encounter rate of LGN Y-cells was not accompanied by significant changes in LGN cell-body size. Together, the results indicate that the immunoablation technique produces a large and apparently selective reduction of the Y-cell retinogeniculate pathway in developing kittens.

Effects of reduced numbers of lateral geniculate Y-cells on development of ocular dominance in cat striate cortex.

In the companion study (Dev. Brain Res., 34 (1987) 223-233), we showed that a monocular injection of antibodies against ox large retinal ganglion cells produces a 53% (low concentration) to 82% (high concentration) loss of Y-cells in lateral geniculate nucleus (LGN) layers A and A1 that receive inputs from the antibody-injected eye. At the same time, the percentage of LGN X-cells is unaffected. In the present study, we investigated the effect of this monocular antibody-induced reduction of LGN Y-cells on the development of ocular dominance in striate cortex. Four-week-old kittens were given an intraocular injection of either a low (110 micrograms/33 microliter volume) or a high (333 micrograms/33 microliter volume) concentration of antibodies and single-cell recordings were carried out in striate cortex 33-65 weeks later. Following an injection of either antibody concentration, we found only slight abnormalities in striate cortex ocular dominance compared to normal adult cats. There was a small, but significant, decrease in the percentage of binocularly driven cells and a concomitant increase in the percentage of cells driven exclusively by the normal or control-injected eye. No ocular dominance abnormalities were found in kittens injected monocularly with control gamma-globulins, indicating that the changes are due to effects of the antibodies. The changes in cortical ocular dominance produced by early antibody treatment are very different from those produced by rearing with monocular deprivation (MD) despite a similar loss of LGN Y-cells in the two conditions.(ABSTRACT TRUNCATED AT 250 WORDS)

The ipsilateral retinocollicular projection in the rabbit: an autoradiographic study of postnatal development and effects of unilateral enucleation.

The postnatal development of the ipsilateral retinocollicular projection in the rabbit and the effects of unilateral enucleation (performed on the day of birth, day 0) on that development were studied by using the anterograde axonal transport of tritiated proline injected intraocularly. Material from 1-, 6-, and 10-day-old (i.e., at days 1, 6, and 10) and adult animals was examined. On day 1, autoradiographically labelled optic fibers from the ipsilateral eye formed distinct patches and bands within the superior colliculus (SC), which were restricted primarily to the lateral one-half and anterior one-third to one-half of the nucleus. At subsequent ages no major changes in the location of this projection were found for normal animals or animals enucleated on day 0 (0-DE animals). From dorsal-view reconstructions, the pattern of the ipsilateral projection appeared wedge-shaped with a broad base aligned with the lateral SC border for all normal and 0-DE animals at the various postnatal ages examined. In normal animals the surface area of this projection increased with age and maintained a constant proportion of the increasing surface area of the total SC. In 0-DE animals the surface area of the projection initially increased more rapidly than in normal animals. Thus, by day 6 the area was already within the normal adult range but did not exceed this range at later postnatal ages. The only obvious difference in the appearance of the ipsilateral retinocollicular projection between normal and 0-DE animals at corresponding ages was an enhanced radial distribution of the projection across laminae in the 0-DE animals. Taken together these findings suggest that, in the rabbit, once topographically appropriate connections are established between the SC and the ipsilateral retinal projection, they are maintained regardless of substantial postnatal growth of the SC and removal of the contralateral retinal projection to the SC.

Contributions of Y- and W-cell pathways to response properties of cat superior colliculus neurons: comparison of antibody- and deprivation-induced alterations.

The cat's superior colliculus (SC) receives direct inputs from retinal W-cells (a W-D input) and Y-cells (Y-D input) and an indirect Y-cell input via the lateral geniculate nucleus and visual cortex (Y-I input). In previous studies we have shown that intraocular injection of antibodies raised against large retinal ganglion cells produces a dose-dependent reduction in the Y retinogeniculate pathway. Furthermore, when a sufficiently high antibody concentration is used, there is a substantial loss of the Y pathway and no apparent loss of the W pathway. In the present study, we used the antibodies to investigate the contributions of the Y and W pathways to functional organization within the SC. Binocular injections of low (330 micrograms/100 microliters) or high (1,000 micrograms/100 microliters) antibody concentrations were made. The antibody-mediated effects on SC cells' response properties were compared directly with effects of early binocular deprivation, which have been attributed to a loss of Y-I input. Extracellular single-cell recordings were made from the SC, and cells were classified as receiving Y-D, Y-I, or W-D inputs on the basis of their response latencies to electrical stimulation of the optic chiasm and optic tract. Injections of the low antibody concentration produced no significant effects on inputs to the SC. However, injections of the high antibody concentration resulted in a 70% reduction in SC cells with a Y-D input and an 82% reduction in SC cells with a Y-I input. There was no effect on the percentage of cells with a W-D input. Binocular deprivation produced a 76% reduction in the percentage of cells with Y-I input. Visual response properties of SC cells also were assessed. Injections of the high antibody concentration produced a 55% reduction in cells that respond with a directional preference and a 51% reduction in cells that respond to high-velocity stimuli. Binocular deprivation produced a 78% reduction in the proportion of directional cells and a 25% reduction in cells that respond to the ipsilateral eye. Taken together, the results of this and previous studies using cortical lesions, visual deprivation, and immunoablation suggest that Y-D input is the primary basis for responses to high stimulus velocity, Y-I input is an important basis for directional responses and response through the ipsilateral eye, and W-D input is important for responses to low stimulus velocity.(ABSTRACT TRUNCATED AT 400 WORDS)

Dose-response analysis of effects of antibodies to large ganglion cells on the cat's retinogeniculate pathways.

Previous studies have shown that antibodies against large retinal ganglion cells (alpha-/Y-cells) reduce the Y-cell retinogeniculate pathway while having little or no effect on the X- or W-cell pathways. The present study investigated the dose-response relationship of these effects. We began by studying effects on the T1 (largely Y-cell-mediated) and T2 (largely X-cell-mediated) waves of the retinal field-potential. Different concentrations of the antibodies were injected intraocularly in adult cats and retinal field-potentials evoked by optic chiasm stimulation were examined. The lowest concentration of immune serum tested (330 micrograms/100 microliter volume) reduced both the T1 and T2 amplitudes. With increasing concentrations, the ratio of T1:T2 amplitudes progressively decreased from 0.71 to only 0.05. The highest concentration of immune serum tested (1000 micrograms/100 microliter volume) virtually eliminated the T1 wave while the T2 wave remained (albeit reduced). Next, we carried out single-cell physiological and morphological studies to verify the effects of the highest antibody concentration and compare them with previous results on effects of the lowest antibody concentration (Kornguth et al., 1982; Spear et al., 1982). In single-cell recordings from the retina, the encounter rates of Y- and X-cells were reduced by 85 and 53%, respectively, after injection of the highest antibody concentration. There was no effect on the encounter rate of retinal W-cells. After injection of the lowest antibody concentration, there was no change in the encounter rates of any of the retinal cell types. Morphological studies revealed an 88-99% loss of alpha-cells in retinae treated with the highest antibody concentration. There also was a substantial (24-57%) loss of medium-size ganglion cells but no loss of small ganglion cells. The loss of alpha-cells was much greater after high-concentration injections than after low-concentration injections. In recordings from the LGN, the proportion of Y-cells was reduced by 87% in laminae receiving input from an eye injected with the highest antibody concentration. Laminae receiving input from an eye injected with the lowest concentration had a 77% reduction in Y-cells. The encounter rate of LGN X-cells was not affected by either concentration. Morphological analysis indicated that the loss of Y-cells in the LGN was not due to changes in cell size. These findings indicate that antibody-mediated effects on retinogeniculate pathways are dose-dependent.(ABSTRACT TRUNCATED AT 400 WORDS)

Selectivity of antibody-mediated destruction of axons in the cat's optic nerve.

The purpose of this study was to determine the selectivity with which polyspecific antibodies directed against large retinal ganglion cells destroy axons in the cat's optic nerve. Immune serum prepared against large ganglion cells isolated from ox retinas was injected into 1 eye of each of 2 cats. After more than 1 month, the cats were perfused with mixed aldehydes and the optic nerves were prepared for transmission electron microscopy. On the basis of a large sample of micrographs of transverse thin sections, we estimated that each of the nerves that issued from treated eyes contained approximately 62,500 necrotic fibers, amounting to 42-46% of the total fiber population in 1 case and 39-42% in the other. The diameters of 2900-5000 intact axons from each of 4 nerves (2 from immune-treated eyes and 2 from untreated eyes) were measured. Comparisons of histograms of axon diameter for nerves from treated and untreated eyes revealed that 90-100% of large axons with diameters above 3.5-4.0 micron were eliminated by the antibodies. Between 65 and 70% of medium-sized fibers were also eliminated. The number of small axons--those with diameters less than 1.2-1.6 micron--did not differ appreciably from normal. These results suggest that the immune serum destroyed virtually all alpha cell axons and a substantial fraction of beta cell axons but did not reduce the number of small fibers that largely stem from the gamma class of retinal ganglion cells.

Effects of bicuculline-induced epileptiform activity on development of receptive field properties in striate cortex and lateral geniculate nucleus of the rabbit.

Studies have shown that normal development of receptive field properties in striate cortex and lateral geniculate nucleus (LGN) of the rabbit is severely altered in the presence of penicillin-induced disruption of cortical neuronal activity. We wished to replicate these studies using a different convulsant drug in order to rule out possible effects due to the penicillin drug itself. Aqueous bicuculline was injected twice daily into a cannula implanted over the monocular region of one striate cortex. Drug administration was initiated on postnatal day 8-9 and was discontinued either on postnatal day 19-24 or on postnatal day 24-30 for studies of the LGN and striate cortex, respectively. Coincidental with the bicuculline injections, control solutions were similarly applied to the monocular region of the contralateral striate cortex. Single-unit recordings made from LGN ipsilateral to bicuculline-treated cortex revealed normal percentages of receptive field types. However, in single-unit recordings made from bicuculline-treated striate cortex, an abnormal percentage distribution of receptive field types was found. In such cortex there was an unusually high proportion of no-response type cells and a substantially reduced proportion of oriented type cells. These developmental abnormalities are virtually the same as those found in the striate cortex of similarly reared animals treated with penicillin. Our present results lend support to our previous conclusion that in the rabbit, disruption of orderly neuronal activity in the geniculostriate system has a detrimental effect on the development of receptive fields in this system.

Characterization of penicillin- and bicuculline-induced epileptiform discharges during development of striate cortex in rabbits.

A comparison was made between penicillin- and bicuculline-induced epileptiform activity. Aqueous solutions of either penicillin or bicuculline were applied to striate cortex of rabbit pups, and the electroencephalogram was monitored. Applications were made twice daily for 14 consecutive days beginning on postnatal day 8-9. During this time period, interictal spikes generated by bicuculline and penicillin displayed similar properties in most respects. However, the morphology of spikes induced by each convulsant was different.

Functional influences of the visuocortical projection on superior colliculus neurons in the rabbit.

The effects of electrical stimulation of the visual cortex on superior colliculus neurons were investigated in adult Dutch-belted rabbits. Single units were recorded in the superior colliculus and classified as to receptive field type. Stimulation of the ipsilateral visual cortex activated 29% of the recorded superior colliculus units. No units were driven by stimulation of the contralateral visual cortex. Comparison of the relative proportional distributions of cortically driven and not driven cells having various receptive field types revealed an over-representation of driven motion type cells. The excitatory influence of the visuocortical projection to the superior colliculus in the rabbit shows a preference for neurons responsive to moving visual stimuli.

Effects of progressively longer durations of monocular deprivation on development of visuocortical receptive fields in the rabbit.

Three groups of rabbits were reared with monocular eyelid suture. Percentages of cells with various receptive field types were determined by recording in the deprived visual cortex of animals 30-35 days, 46-55 days, and 20-36 months old. These results, together with previously published data from two other groups of monocularly deprived rabbits, are described relative to normative data for 9-10-day-old and adult rabbits. During the first postnatal year cells in the deprived visual cortex acquire normal adult proportions of receptive field types, but do so over a longer course than that which is characteristic of cells in non-deprived visual cortex. With continued deprivation for 2-3 years there is a subsequent loss of oriented type cells and a corresponding increase in non-responsive cells.

Development of receptive field properties in the visual cortex of rabbits subjected to early epileptiform cortical discharges.

We examined the effects of early epileptiform activity on development of visuocortical receptive fields in the rabbit. Aqueous penicillin was injected twice a day into a cannula implanted over the monocular area of one visual cortex. Drug administration was begun on postnatal day 8-9 and continued until postnatal day 24-30. Concomitant with the penicillin injections a mixture of penicillin and penicillinase was similarly applied to the monocular area of the contralateral control cortex. Interictal discharges were routinely observed only from the penicillin-treated or epileptic cortex. Single-unit recordings made on postnatal day 25-31 revealed that in the neuronal population adjacent to the cortical penicillin focus percentages of receptive field types were severely altered relative to control cortex percentages. Epileptic cortex showed an abnormally high percentage of no response type cells together with an abnormally low percentage of complex and oriented-directional type cells. These abnormalities were greater closer to the penicillin focus than further from it. Epileptic visual cortex receptive field percentages are compared with those for the neonatal rabbit and the rabbit subjected to early monocular deprivation. One interpretation of our results is that development of complex and oriented-directional type cells is impeded by epileptogenic disruption of organized geniculostriate activity.

Anomalous uncrossed retinal projections fail to activate superior colliculus neurons in rabbits unilaterally enucleated by fetal surgery.

Previous studies have shown that unilateral enucleation of rabbit pups produces an aberrant uncrossed retinotectal projection. These fibers failed to drive collicular units when stimulated with either light or electric shock. The present study attempts to assess the possibility that enucleation at earlier stages of development would lead to a greater degree of morphological and physiological reorganization in the uncrossed retinotectal projection. Rabbit fetuses were unilaterally enucleated at day 20 or 25 of gestation. Birth is at day 31. After 3 months, the degree of reorganization of the uncrossed retinotectal projection was assessed using the following techniques: (1) autoradiographic demonstration of the projection from the remaining eye, (2) electrophysiological recording of collicular unit activity, and (3) a combination of these methods. Autoradiographic data indicated a much greater expansion of the anomalous uncrossed projection in fetally enucleated animals than in those enucleated at birth. Labelled terminals were found to occupy more than the anterior third of the ipsilateral colliculus and were distributed to the entire depth of the stratum griseum superficiale and stratum opticum. Electrode penetrations within the boundaries of the expanded projection failed to locate collicular units which could be driven by either light stimulation of the eye or electric shock of the optic nerve. Only a few cells encountered in the lateral border area, receiving the normal uncrossed retinal projection, could be driven by light stimulation. These negative findings are in contrast to the data reported for the rat and hamster where anomalous retinal projections are capable of forming functional connections. Further comparative studies of reorganization are needed.