Binocular competition in the control of geniculate cell size depends upon visual cortical N-methyl-D-aspartate receptor activation

The lateral geniculate nucleus relays visual information from the retina to cortex. One well-known anatomical consequence of monocular deprivation during early postnatal development is a shrinkage of neurons in the lamina of the lateral geniculate nucleus that receive input from the deprived eye. This is thought to reflect the competition of afferents subserving the two eyes, possibly at the level of the visual cortex. We find that blockade of N-methyl-D-aspartate receptors in kitten visual cortex disrupts this process of binocular competition. These data provide direct evidence that postsynaptic activation of cortical neurons is required for competitive changes in lateral geniculate cell size and suggest a role for N-methyl-D-aspartate receptors in anatomical as well as physiological plasticity in the mammalian visual system.

Different populations of dorsal lateral geniculate nucleus neurons have concentration-specific requirements for a cortically derived neuron survival factor

A macromolecular fraction of conditioned culture medium (CM) derived from explant cocultures of embryonic rat posterior cortex and caudal thalamus is able to support the survival of neurons in the dorsal lateral geniculate nucleus (dLGN) of newborn rats following ablation of dLGN cortical target areas. In the present study we tested whether the survival-promoting activity of this target-derived neurotrophic agent was concentration dependent and whether different subpopulations of dLGN neurons were equally responsive. With the starting concentration of the CM fraction designated X, increasing concentration results in a progressive falloff in trophic activity so that at 200X overall dLGN survival is similar to that seen in unconditioned medium (UM) controls. In contrast, diluting the fraction produces an increase in activity until maximal survival is achieved at 0.2X. Further dilutions result in a decline in trophic activity until control values are reached at 0.001X. Two populations of neurons within the dLGN, defined by their time of origin, respond in a specific manner to the different concentrations. Neurons generated during the early stages of neurogenesis (E14) have maximal survival (25.8%) at 0.05X, whereas those neurons generated later (E15/16) are maximally supported (30.7% survival) at 10X, a 200-fold difference in concentration. While it is possible that separate neurotrophic and neurotoxic molecules exist for each of these populations of dLGN neurons, the most parsimonious interpretation of the data is that a single cortically derived neurotrophic factor exists whose production is strictly controlled during development to achieve maximal effect on different populations of thalamic neurons that may be functionally distinct.

Facilitation of the acoustic startle reflex by ponto-geniculo-occipital waves: effects of PCPA

The relationship between ponto-geniculo-occipital (PGO) waves and motor activity during waking and non-rapid eye movement (non-REM) sleep stages was studied in cats treated with the serotonin synthesis inhibitor p-chlorophenylalanine (PCPA). PGO waves appeared in waking after daily treatment with PCPA. The magnitude of the acoustic startle elicited in the absence of prior PGO waves was increased (by a mean of 555%) by the PCPA treatment as compared to that of the pre-drug level. When startle-eliciting stimuli were presented shortly after the occurrence of the PGO wave, the response amplitude was further enhanced as compared to that of the baseline startle. The effect was maximal 50 ms following the peak of the PGO wave (average 192% of the baseline level), with return to the baseline startle level within 200 ms. A similar effect could also be seen with waking eye-movement potentials (EMPs) in drug-naive animals. Over half of the spontaneous PGO waves were found to be preceded or followed by discrete head-body movements. After PCPA, the amplitude of auditory-evoked LGN PGO waves increased during quiet waking (QW) while those in non-REM and REM sleep states did not change. It was concluded that serotonergic systems produce a tonic suppression of startle response and PGO amplitude in waking. PGO spikes in waking are associated with a phasic facilitation of the sensorimotor mechanisms involved in startle.

Effects of monoamine reuptake blockade on ponto-geniculo-occipital wave activity

Norepinephrine (NE) and serotonin (5HT) likely inhibit the generation of ponto-geniculo-occipital (PGO) waves. Either desipramine (DMI) or sertraline (SER:1S,4S-N-methyl-4-(3,4-dichlorophenyl)-1,2,3,4-tetrahydro-1-naphthyl amine) was administered in the cat for 2.5 weeks to probe noradrenergic and serotonergic mechanisms, respectively. Placebo days were compared with the first day of drug and with days that followed 2.5 weeks of drug (chronic). PGO rates during REM sleep and the preceding transition period were significantly decreased by either chronic DMI or SER. Cat PGO waves resemble waves that accompany alerting to intense or novel stimuli in wakefulness. Depressive disorders in humans have features of hyperarousal; PGO wave suppression by antidepressant drugs may relate to clinical antidepressant actions.

Magnocellular and parvocellular contributions to responses in the middle temporal visual area (MT) of the macaque monkey

Many lines of evidence suggest that the visual signals relayed through the magnocellular and parvocellular subdivisions of the primate dorsal LGN remain largely segregated through several levels of cortical processing. It has been suggested that this segregation persists through to the highest stages of the visual cortex, and that the pronounced differences between the neuronal response properties in the parietal cortex and inferotemporal cortex may be attributed to differential contributions from magnocellular and parvocellular signals. We have examined this hypothesis directly by recording the responses of cortical neurons while selectively blocking responses in the magnocellular or parvocellular layers of the LGN. Responses were recorded from single units or multiunit clusters in the middle temporal visual area (MT), which is part of the pathway leading to parietal cortex and thought to receive primarily magnocellular inputs. Responses in the MT were consistently reduced when the magnocellular subdivision of the LGN was inactivated. The reduction was almost always pronounced and often complete. In contrast, parvocellular block rarely produced striking changes in MT responses and typically had very little effect. Nevertheless, unequivocal parvocellular contributions could be demonstrated for a minority of MT responses. At a few MT sites, responses were recorded while magnocellular and parvocellular blocks were made simultaneously. Responses were essentially eliminated for all these paired blocks. These results provide direct evidence for segregation of magnocellular and parvocellular contributions in the extrastriate visual cortex and support the suggestion that these signals remain largely segregated through the highest levels of cortical processing.

Brainstem genesis of reserpine-induced ponto-geniculo-occipital waves: an electrophysiological and morphological investigation

Several experimental results indicate that the peribrachial (PB) cholinergic area of the pedunculopontine nucleus is the final relay for the transfer of brainstem-generated pontogeniculo-occipital (PGO) waves to the thalamus. However, the mechanisms underlying the PGO-related activity of PB neurons remain unknown. In order to study these mechanisms, single unit recordings in the PB area were performed in reserpinized cats. Because PGO waves are closely related to rapid eye movements, our microelectrode explorations were also aimed to some structures of the preoculomotor network, namely, the superior colliculus (SC) and parts of the central tegmental field (FTC). We have found several classes of PGO-on cells in the PB area, most of them descharging 80 ms or less before the peak of PGO waves. These cell-classes comprised high-frequency bursting cells, slow-frequency bursting cells, and neurons discharging single spikes or doublets. Intracellular recordings showed that PGO-on single spikes arise from conventional excitatory postsynaptic potentials. Among PGO-related cells in structures outside the PB limits, it was found that most SC cells discharge during or after the PGO, whereas FTC cells increase their discharge rate several hundreds of ms before PGO waves, thus indicating that PGO waves are elaborated long before the activation of PB neurons. Massive retrograde labeling was found in FTC following horseradish peroxidase injections into the PB area. We suggest that long-lead FTC neurons provide an excitatory input to PGO-on PB neurons.

Iontophoresis of norepinephrine onto neurons of the pigeon's lateral geniculate nucleus: characterization of an inhibitory response

A group of neurons in the pigeon's lateral geniculate equivalent nucleus (LGNe) shows associative enhancement of their response to light during visually conditioned heart rate change. The source of the relevant unconditioned stimulus input to LGNe for this enhancement has been identified as the locus coeruleus (LC). Thus, we have begun to examine neurotransmitters synthesized in LC for possible involvement in associative modification of neuronal discharge in LGNe. As a first step, we have examined the responses of LGNe neurons to iontophoretic application of norepinephrine (NE) and identified the receptor mediating one response class. The majority of neurons in LGNe show inhibition of maintained activity in response to iontophoretic application of NE or its agonists. The potency of the NE agonists is alpha-methyl NE greater than epinephrine greater than NE greater than phenylephrine greater than isoproterenol. This profile is characteristic of an alpha 2-adrenoceptor. The alpha 2-agonist clonidine also induces inhibition of maintained activity. The alpha 2-antagonists WB-4101 and yohimbine block the NE-inhibition while the alpha 1-antagonist prazosin and beta-antagonist sotalol do not. It is thus suggested that the receptor mediating the NE-inhibition of maintained activity has the characteristics of an alpha 2-adrenoceptor.

Pacemaker-like and other types of spontaneous membrane potential oscillations of thalamocortical cells

During EEG-synchronized sleep, thalamic activity is characterized by rhythmic oscillations that till recently have been suggested to require the contribution of intra- and extra-thalamic inputs. The present experiments show that thalamocortical (TC) cells, mechanically and pharmacologically isolated from their intra-thalamic, cortical and brainstem inputs, are capable of different types of spontaneous membrane potential oscillations some of which resemble those observed in TC cells of the living animal during EEG-synchronization.

Dependence of retinogeniculate transmission in cat on NMDA receptors

1. We have examined the possibility that N-methyl-D-aspartate (NMDA) receptors may be involved in the visual response of relay cells in the cat dorsal lateral geniculate nucleus (dLGN). The selective NMDA receptor antagonists D-2-amino-5-phosphonovalerate (APV) and 3-[(+/-)-2-carboxypiperazin-4-yl]-propyl-1-phosphonic acid (CPP) have been iontophoretically applied to X and Y cells in the dLGN and their effects on the visual response to a light spot flashed within the receptive field center determined. 2. The antagonist effects were assessed at ejection current levels producing a selective blockade of the responses to iontophoretically applied NMDA with respect to those elicited by the non-NMDA receptor agonists quisqualate and kainate. These selective effects were determined in an experimental paradigm where the visual response and responses to NMDA and the non-NMDA receptor agonists were compared in the same test run. The data refer to a total population of 52 cells (28 X, 24 Y). 3. Application of APV abolished or greatly reduced the visual responses of both X and Y cells. The mean percentage reduction in the visual response for the X cells studied was 59 +/- 10% (SE; n = 7) and for the Y cells 66 +/- 8% (SE; n = 11). Both the early onset transient and the sustained component of the visual response to the flashed stimulus were equally affected. 4. The antagonist CPP produced a similar pattern of effect to APV, substantially reducing or abolishing the visual response in both X and Y cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Scopolamine and physostigmine do not alter visual detection of change: relationships to a model of lateral geniculate operations

The effects of physostigmine and scopolamine were tested on 5 male subjects using a task based on a model of cross-inhibition among lateral geniculate neurons. The task consisted of detecting and locating a change (appearance or disappearance) of one point of light in an array of points. Earlier research suggested that the task was sensitive to drugs and pathology. In the present study, neither drug showed significant effects on any of three task variables used, although differences between subjects were significant. These findings show that the task is sensitive to individual differences but insensitive to changes in cholinergic activity produced by physostigmine and scopolamine, even though such changes should affect neuronal functioning at the lateral geniculate.

The contribution of the non-N-methyl-D-aspartate group of excitatory amino acid receptors to retinogeniculate transmission in the cat

The N-methyl-D-aspartate receptor antagonist 3-((+/-)-2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid and the non-N-methyl-D-aspartate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione have been iontophoretically applied to cells in the cat dorsal lateral geniculate nucleus and their effects on the visual response compared. The objective was to examine the possibility of both N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors being involved in the transfer of the retinal input to X and Y cells in the dorsal lateral geniculate nucleus. The results show that selective blockade of either N-methyl-D-aspartate receptors or non-N-methyl-D-aspartate receptors can block the visual response of both X and Y cells. Overall, the most potent reductions of visual responses across the population of cells studied were obtained with the N-methyl-D-aspartate receptor antagonist with X cells showing a slightly greater reduction on average (80%) than Y cells (66%). The relatively smaller overall reductions in visual responses obtained with the non-N-methyl-D-aspartate receptor blockade reflected the lower levels of blockade that were compatible with selectivity using iontophoretic applications of 6-cyano-7-nitroquinoxaline-2,3-dione. It is concluded that N-methyl-D-aspartate and non-N-methyl-D-aspartate receptors are critically involved in the visual response of both "on" and "off" centre X and Y cells.

6-Hydroxydopamine treatment blocks the effects of chronic monocular paralysis in the cat's lateral geniculate nucleus

The abnormal patterns of binocular stimulation produced by unilateral eye immobilization (monocular paralysis) alter the physiology of the lateral geniculate nucleus (LGN), shifting the LGN X/Y ratio in such a way that X cells are encountered far less frequently than Y cells. These changes are not observed in cats treated with intraventricular injections of the neurotoxin, 6-hydroxydopamine (6-OHDA) during the period of chronic monocular paralysis. Additional experiments indicated that the blockade was not due to any non-specific effects associated with the injection procedures, nor to any direct effects the drug itself may have had on LGN cell recording. These results suggest that the neuronal mechanism mediating the shift in the X/Y ratio produced by monocular paralysis contains elements that are sensitive to 6-OHDA.

The cholinergic influence upon rat dorsal lateral geniculate nucleus is dependent on state of arousal

Several lines of evidence suggest a role for acetylcholine (ACh) in mediating the effects of state of arousal on transfer of visual information through the lateral geniculate nucleus (LGN). Local application of cholinergic agonists to geniculate relay cells in anesthetized cats and rats produces predominantly facilitatory effects. This indicates that presynaptic release of ACh may be responsible for the increased excitability of LGN relay cells that is observed during waking and REM sleep. In this study in rats we have examined the influence of cholinergic agonists applied during the 3 natural states of arousal: waking, slow-wave (SW) sleep and rapid eye movement (REM) sleep. Pharmacological agents were iontophoretically administered to identified, single cells in head-restrained, unanesthetized rats free to sleep and wake. Application of cholinergic agonist produced state-dependent differences in response in all geniculate relay-cells studied. During both waking and REM sleep, a facilitatory response was always observed, whereas in SW sleep responses were of three types: no effect (62%), inhibition (24%), and biphasic inhibition followed by facilitation (14%). All response types were antagonized by scopolamine. In contrast to the qualitatively different state-dependent effects of cholinergic agonists, response to application of glutamate, with quantitative variations, was uniformly facilitatory in all states, though responses in SW sleep tended to be lower in magnitude. The effects of gamma-aminobutyric acid (GABA), glycine, and serotonin were inhibitory in all states. These data are consistent with the suggested role of ACh in mediation of increased relay-cell excitability during REM sleep and waking. Our findings, however, also indicate that in the transition from SW sleep to REM or waking, local release of ACh is not solely responsible for alterations in cell excitability.

Ultrastructural and functional evidence for the survival of corticogeniculate neurons in kainic acid-lesioned lateral geniculate nucleus

After a kainic acid lesion in the dorsal lateral geniculate nucleus of rat, retrograde axonal transport of fluorescent dyes is blocked in corticogeniculate but not in retinogeniculate neurons. This inhibition, however, can be reversed by electrical stimulation in the subcortical white matter (Woodward and Coull, Brain Research 454 (1988) 106-115). These observations suggest that retrograde axonal transport in corticogeniculate neurons is impulse-dependent and that neuronal activity in this pathway is reduced as a consequence of the lesions. To test this we examined retrograde transport of horseradish peroxidase (HRP) and cytochrome oxidase activity in the cortex of lesioned animals. Unilateral kainic acid lesions in the geniculate inhibit the retrograde transport of HRP, but this inhibition is reversed by electrical stimulation of white matter. Moreover, histochemical staining for cytochrome oxidase activity is less intense over visual cortex on the lesioned side, implying that cortical activity in intrinsic and efferent pathways is reduced as a consequence of removal of geniculate afferents. Inasmuch as the retrograde transport of HRP is dependent upon impulse activity in neurons and is thought to be mediated by synaptic vesicle recycling, these results suggest that terminals of corticogeniculate fibers survive the kainic acid lesions in the geniculate and are capable of releasing synaptic vesicles. Ultrastructural examination of lesioned geniculates strongly supports this conclusion and reveals the presence of axon terminal profiles which are filled with small round synaptic vesicles and have membrane specializations reminiscent of synaptic contacts. These terminal profiles are presumed to be of retinal and cortical origin.

Effects of chronic developmental lead exposure on monkey neuroanatomy: visual system

The effects of lead on specific areas of the visual system were examined in two groups of monkeys (Macaca fascicularis). The first group (N = 3) received 2000 micrograms Pb/kg/day from infancy onward, while the second group (N = 4) received 25 micrograms Pb/kg/day from birth onward. Monkeys were killed at approximately 6 years of age. Areas of the visual system, including optic nerve, lateral geniculate nucleus, and primary area V1 and one visual projection area V2, were examined by a combination of light and electron microscopy and Golgi impregnation. No effect of lead on optic nerve was identified, nor were there consistent differences in the lateral geniculate nucleus. Within areas V1 and V2, the neuronal volume density was significantly reduced in the high dose compared to the low dose group. Moreover, analysis of the dendritic arborization by Golgi analysis revealed a relative decrease in the number of arborizations among pyramidal neurons in both areas V1 and V2. These data suggest that lead exposure beginning during the early postnatal period may result in changes in cytoarchitecture in visual areas V1 and V2.

Action of urethane on dorsal lateral geniculate neurons

Light evoked neuronal responses of urethane anaesthetized rats were compared with those recorded in freely moving animals. The results demonstrate that urethane reduces the firing frequency, the response probability as well as the number of excitatory light evoked response phases. These changes are combined with a prolongation of the postexcitatory inhibitory phases. The latencies to diffuse flashes are increased in geniculate neuronal responses as well as in evoked potentials recorded from the visual cortex. A similar relation exists between diminution of late geniculate excitations and suppression of cortical photic afterdischarges.

Effects of bicuculline on receptive field center sensitivity of relay cells in the lateral geniculate nucleus

The lateral geniculate nucleus (LGN) receives input from the retina that is spatially organized into a receptive-field center and surround. It maintains this organization in the signal that it sends to the visual cortex. Previous studies have focused on changes in the receptive-field 'surround' that are generated at the LGN, possibly as a local contrast enhancement mechanism. The present study suggests a role for the LGN in regulating the receptive-field center sensitivity under the control of GABAergic circuitry. Local microiontophoresis of the GABAA receptor antagonist bicuculline increased the contrast sensitivity of LGN relay cells to many spatial frequencies. Difference of Gaussians analysis showed that the increased was due to an increased sensitivity of the receptive-field center. Similar increases in receptive-field center sensitivity may be produced during behavioral arousal by the action of pontine and mesencephalic pathways upon the activity of the LGN GABAergic circuitry.

Effects of bicuculline on signal detectability in lateral geniculate nucleus relay cells

The lateral geniculate nucleus conveys the center-surround organized retinal receptive fields to the cortex in a way that does not significantly alter their spatial structure. However, non-retinal influences may change the 'strength' (detectability) of the signal under conditions of anesthesia, arousal and attention. A previous analysis of receiver operating characteristic curves in cat suggests that a reduction in signal detectability occurs in lateral geniculate nucleus (LGN) relay cells in anesthetized animals in comparison to the retinal afferents. In the present study, it was found that antagonism of GABAA receptors with bicuculline (BIC) increased signal detectability in LGN relay cells in the tree shrew (Tupaia belangeri). This change is consistent with the hypothesis that feedforward and/or feedback GABAergic circuits in the LGN differentially affect the retinogeniculate transfer ratio for visually driven activity versus maintained (spontaneous) activity. Under conditions of arousal or attention, signal detectability may be increased by brainstem activation, thus increasing the flow of information in the visual system.

Induction of audiogenic seizures in normal and genetically epilepsy-prone rats following focal microinjection of an excitant amino acid into reticular formation and auditory nuclei

An excitant amino acid (EAA), N-methyl-D-aspartate (NMDA), induces susceptibility to seizures when bilaterally microinjected into subcortical auditory nuclei of normal rats. Thirty-five percent of animals exhibit only audiogenic seizures (AGS) after infusions of NMDA into inferior colliculus (IC). Infusions into cochlear nucleus and medial geniculate body never produce susceptibility to AGS without non-audiogenic seizures (N-AGS). The overall seizure incidence (AGS and N-AGS) with IC infusions is 100%, but the incidence is less than 50% with infusions into cochlear nucleus or medial geniculate body. Although AGS susceptibility is induced by NMDA infusions in normal animals, the seizures are submaximal in severity and lack tonic components. Bilateral infusions of NMDA into IC or reticular formation of the substrain of genetically epilepsy-prone rats (GEPRs) that exhibits submaximal AGS (GEPR-3s) do not increase seizure severity. These data along with studies showing increased EAA levels and excitotoxic-like damage in the IC of the GEPR and blockade of AGS with an EAA receptor antagonist or synthesis inhibitor suggest that an EAA in the IC is involved in initiation of AGS in the GEPR. However, EAA action in the GEPR IC is not sufficient to induce the complete spectrum of seizure behaviors, and additional mechanisms may be required for induction of maximal severity audiogenic seizures.

Effects of kainic acid lesions in rat ventral lateral geniculate nucleus upon field potentials of the superior colliculus: correlation between morphological and physiological observations

Morphological and physiological effects of kainic acid (KA) lesions in rat ventral lateral geniculate nucleus (LGV) were studied 1.5 and 6 h after KA injection. Morphological changes were examined mainly by electron micrographs. At 1.5 h after KA injection dendrites were dilated and some vacuolations occurred in both dendrites and perikarya including geniculotectal relay neurons while axons were completely intact and cell organelles almost remained intact. Six h after KA injection dendrites and cell bodies were massively dilated with degeneration of cell organelles accompanied by sparse cytoplasm and deformed chromatin in the nucleus. However, almost all presynaptic axons, mainly retinogeniculate fibers, still remained intact. The electron micrographs demonstrate that destruction occurred first in dendrites, next in cell bodies and finally axons were likely to be affected. These morphological changes induced by KA are compatible with physiological effects which were assessed by the field response of the superior colliculus (SC) evoked by stimulation of the optic chiasm. During 1.5-2 h after KA injection all components of the SC response, the presynaptic and postsynaptic negative-positive waves were enhanced. The enhancement of the SC response may be correlated with morphological changes in terms of excitatory action of KA resulting in facilitation of geniculotectal transmission. Six h after KA injection postsynaptic negative-positive waves gradually declined in amplitude while the presynaptic wave returned to control level. The late suppression of postsynaptic components of the SC response may be attributable to a marked loss of geniculotectal transmission resulting from destruction of geniculotectal relay neurons by KA.

Optic tract stimulation evokes GABAA but not GABAB IPSPs in the rat ventral lateral geniculate nucleus

The inhibitory postsynaptic potentials (IPSPs) evoked in neurons of the rat ventral geniculate nucleus (vLGN) by electrical stimulation of the optic tract and the action of GABA and baclofen on the same cells were studied using intracellular recording technique in an in vitro slice preparation. A short latency short duration IPSP always followed the monosynaptic excitatory postsynaptic potential (EPSP). This IPSP reversed in polarity at about -65 mV and was reversibly blocked by bicuculline (50 microM) thus indicating that it represents a GABAA receptor-mediated IPSP. No long-lasting IPSP was evoked in vLGN cells by stimulation of the optic tract, while in the same slice, long-lasting GABAB IPSPs were routinely recorded in the dorsal lateral geniculate nucleus. GABA applied by ionophoresis evoked a hyperpolarization that had a reversal potential close to -70 mV and was antagonized by bicuculline. Baclofen hyperpolarized vLGN neurons and its action was reversibly blocked by the selective GABAB antagonist phaclofen (1 mM). In the presence of bicuculline GABA also produced a hyperpolarization that had properties similar to that evoked by baclofen. These results indicate that, although functional GABAA and GABAB receptors are present on vLGN neurons, stimulation of the optic tract evokes only GABAA but not GABAB mediated IPSPs. The lack of long-lasting GABAB IPSPs could explain the absence of long-lasting inhibition observed in vLGN neurons in vivo following stimulation of the optic tract.

Sleep state-specific neuronal activity in rat dorsal lateral geniculate nucleus is not altered by local serotonin and norepinephrine depletion

The relay cells in dorsal lateral geniculate nucleus (dLGN) represent one among many populations of neurons throughout the neuraxis that display systematic alteration of spontaneous rate and pattern of discharge concurrent with change in state of arousal. Both noradrenergic (NE) and serotonergic (5-HT) systems innervate dLGN and are implicated in sleep-wake control mechanisms. Our study was designed to test the influence of these systems upon sleep state-related multiple unit activity in the dLGN. Two monoamine neurotoxins, 5,7-dihydroxytryptamine and 6-hydroxydopamine, were injected locally into dLGN to destroy NE and 5HT afferents individually and in combination. In three separate treatment groups, mean monoamine concentrations in dLGN were reduced in relation to the contralateral, vehicle-injected, control dLGN to: 1) NE-17%, 5HT-14%; 2) NE-46%, 5HT-28%, and 3) NE-6%, 5HT-77%. In no case was chronic sleep state-related cell activity in dLGN significantly altered. We conclude that afferent monoaminergic inputs are not critically related to the mechanisms underlying normative shifts in sleep state-related neuronal activity in dLGN.

Amphetamine-induced recovery of visual cliff performance after bilateral visual cortex ablation in cats: Measurements of depth perception thresholds

After bilateral visual cortex ablation, cats exhibit a loss of depth perception as measured on a visual cliff, which recovers following administration of d-amphetamine. In this Study, 3 amphetamine-treated cats with visual cortex ablations showed a rapid and enduring recovery, with 2 of these animals obtaining levels of performance seen only with binocular vision, suggesting a restoration of binocular depth perception. Cats with asymmetrical lesions showed only a transient improvement during amphetamine treatment, and some animals not displaying autonomic signs of amphetamine intoxication did not improve. Saline-treated cats showed no signs of improvement, and the effect of amphetamine was blocked by the catecholaminergic antagonist haloperidol. These results indicate that amphetamine can induce an enduring recovery from a behavioral deficit after brain injury, which if left untreated would not spontaneously recover.

On the properties and origin of the GABAB inhibitory postsynaptic potential recorded in morphologically identified projection cells of the cat dorsal lateral geniculate nucleus

Intracellular recordings were performed from projection cells of the cat dorsal lateral geniculate nucleus in vitro to investigate the properties and origin of optic tract evoked inhibitory postsynaptic potentials mediated by GABAB receptors and their relationship to the physiologically different cell classes present in this nucleus. In all three main laminae of the dorsal lateral geniculate nucleus, stimulation of the optic tract evoked an excitatory postsynaptic potential followed by two inhibitory postsynaptic potentials. The first is a GABAA receptor mediated inhibitory postsynaptic potential since it was blocked by bicuculline, reversed in polarity following intracellular Cl- injection and had a reversal potential similar to the bicuculline sensitive hyperpolarizing effect of GABA. The second is a GABAB receptor mediated inhibitory postsynaptic potential. Its amplitude was not linearly related to membrane potential (maximal amplitude at -60 mV), it decreased when using frequencies of stimulation higher than 0.05 Hz and it was reversibly increased by addition of bicuculline to the perfusion medium. The reversal potential of GABAB inhibitory postsynaptic potentials was dependent on the extracellular K+ concentration but did not change in the presence of bicuculline or when recording with Cl- filled microelectrodes. While GABAA inhibitory postsynaptic potentials always abolished repetitive firing of projection cells, GABAB inhibitory postsynaptic potentials were able to block weak firing but unable to decrease strong activation of projection cells evoked by direct current injection. Optic tract evoked GABAB (as well as GABAA) inhibitory postsynaptic potentials could be recorded in slices which did not include the perigeniculate nucleus, thus indicating that they are generated by the interneurons of the dorsal lateral geniculate nucleus. Using intracellular injection of horseradish peroxidase, we have found that the GABAB inhibitory postsynaptic potentials are present in projection cells showing many different types of neuronal morphologies. In conclusion, GABA released from interneurons in the dorsal lateral geniculate nucleus is capable of evoking an early, short-lasting GABAA and a late, long-lasting GABAB inhibitory postsynaptic potential in projection cells with diverse morphology, indicating that the late inhibition in the dorsal lateral geniculate nucleus can no longer be associated exclusively with the recurrent inhibitory pathway through the perigeniculate nucleus.

Modification of retinal ganglion cell axon morphology by prenatal infusion of tetrodotoxin

The cellular mechanisms by which the axons of individual neurons achieve their precise terminal branching patterns are poorly understood. In the visual system of adult cats, retinal ganglion cell axons from each eye form narrow cylindrical terminal arborizations restricted to alternate non-overlapping layers within the lateral geniculate nucleus (LGN). During prenatal development, axon arborizations from the two eyes are initially simple in shape and are intermixed with each other; they then gradually segregate to form complex adult-like arborizations in separate eye-specific layers by birth. Here we report that ganglion cell axons exposed to tetrodotoxin (TTX) to block neuronal activity during fetal life fail to form the normal pattern of terminal arborization. Individual TTX-treated axon arborizations are not stunted in their growth, but instead produce abnormally widespread terminal arborizations which extend across the equivalent of approximately two eye-specific layers. These observations suggest that during fetal development of the central nervous system, the formation of morphologically appropriate and correctly located axon terminal arborizations within targets is brought about by an activity-dependent process.

The blockage of ponto-geniculo-occipital waves in the cat lateral geniculate nucleus by nicotinic antagonists

The cholinergic mechanism underlying the genesis of ponto-geniculo-occipital (PGO) waves recorded from the lateral geniculate (LG) nucleus was studied in reserpinized cats under urethane anesthesia. PGO field potentials and their related unit activities occurred spontaneously or were triggered by stimulation of the brainstem peribrachial (PB) area. It was found that PGO-related unit activities were strongly depressed by systemic or iontophoretic applications of nicotinic antagonists such as mecamylamine or hexamethonium but remained intact after similar applications of scopolamine. These results suggest that the genesis of thalamic PGO waves involves a nicotinic activation of relay neurons by PB cholinergic afferents.

Altered neuronal sensitivity of lateral geniculate neurones to noradrenaline and 5-HT following exposure to continuous lighting

Rats were housed in either continuous lighting (LL), an extended photoperiod of 18 h light-6 h dark (LD 18:6) or a 12 h light-12 h dark (LD 12:12) lighting cycle. The effects of these various lighting regimens on the sensitivity of dorsal lateral geniculate neurones to iontophoresed noradrenaline and serotonin (5-HT) was examined. Exposure to either continuous lighting or to an extended photoperiod (LD 18:6) resulted in the development of an enhanced responsiveness to iontophoresed noradrenaline (alpha 1-adrenoceptor) and 5-HT. The development of light-induced noradrenaline and 5-HT supersensitivity resembles the changes obtained with antidepressant treatments.

Lateral geniculate lesions block circadian phase-shift responses to a benzodiazepine

Several pharmacological treatments, including application of an excitatory neurotoxin to the lateral geniculate nucleus (LGN) and systemic administration of triazolam, a clinically effective benzodiazepine, can elicit large phase shifts in a circadian rhythm according to the time of administration. The hypothesis that the LGN might mediate the effect of triazolam on circadian clock function was tested. Bilateral lesions of the LGN, which destroyed the connection from the intergeniculate leaflet to the suprachiasmatic nucleus, blocked phase-shift responses to triazolam. The requirement of an intact LGN for triazolam to shift circadian phase suggests that the LGN may be a site through which stimuli gain access to the circadian clock to modulate rhythm phase and entrainment.

Effects of kainic acid lesions in lateral geniculate nucleus: activity dependence of retrograde axonal transport of fluorescent dyes

Kainic acid lesions in the dorsal lateral geniculate nucleus of rats block the retrograde axonal transport of fluorescent dyes in corticogeniculate neurons without affecting the retrograde transport of D-aspartate or the orthograde transport of radiolabelled proteins in these neurons. This blocking of dye transport does not appear to be a consequence of kainic acid-induced damage to axon terminals in the geniculate since retinal ganglion cells are still able to transport dyes retrograde. A more likely explanation for these results is that fluorescent dye transport requires electrical activity in neurons, and elimination of the geniculate afferents to visual cortex reduces impulse traffic in cortical output fibers to a level below that required to support detectable dye transport. This interpretation is supported by the observation that kainic acid lesions also reduce retrograde transport of dyes in cortical neurons which project to the superior colliculus. Electrical stimulation in the subcortical white matter restores the transport of dye compounds in corticogeniculate neurons: evidence consistent with an activity-dependent mechanism of retrograde transport for these substances. These results provide evidence that axon terminals of retinal ganglion cells and corticogeniculate neurons survive in kainate-lesioned geniculates and are capable of normal neuronal function.

Acute effects of sodium valproate on epileptic photosensitivity in the lateral geniculate-kindled cat

Epileptic photosensitivity was acquired as a result of kindling in the lateral geniculate body (GL), and a GL-kindled cat pretreated with DL-allylglycine was found to be a useful model of epilepsy for assessing the efficacy of anticonvulsant drugs. In the present study the acute anticonvulsant effects of sodium valproate (VPA) were examined in the GL-kindled cat under DL-allylglycine. An intravenous injection of VPA at 50 mg/kg induced a protective action against photically induced seizures and paroxysmal EEG activities. This protection persisted for up to 4 hours of the observation period and corresponded to plasma concentrations (61 to 123 micrograms/ml) similar to those considered therapeutic in humans. The present findings are consistent with those obtained in other animal models of photosensitive epilepsy, and lend experimental support to the idea that VPA possesses antiepileptic activity in human photosensitive epilepsy.

The GABAB antagonist phaclofen inhibits the late K+-dependent IPSP in cat and rat thalamic and hippocampal neurones

Phaclofen (0.5-1 mM) reversibly inhibited the late, bicuculline resistant, K+ dependent IPSP recorded in projection cells of the cat and rat dorsal lateral geniculate nucleus and in rat hippocampal CA1 pyramidal neurones. At the same concentrations, phaclofen reversibly blocked the K+ dependent, bicuculline insensitive hyperpolarization evoked by GABA and baclofen but had no effect on the GABAA IPSP. These results represent conclusive evidence that GABAB receptors mediate the late K+ dependent IPSP in cortical and subcortical neurones.

Origin of butyrylcholinesterase in the lateral geniculate nucleus of tree shrew

We examined the distribution and possible origins of pseudocholinesterase activity within the lateral geniculate nucleus (LGN) of the tree shrew. Butyrylcholinesterase (BuChE) activity was spread diffusely throughout the LGN and not localized to neuronal perikaryon. Lesions of the LGN eliminated this BuChE activity while not affecting acetylcholinesterase (AChE) activity; however, removal of retinal input by unilateral ocular enucleations failed to affect the BuChE activity within the denervated layers of the LGN. This lack of effect suggests that, unlike the macaque monkey, retinal terminals within the LGN of tree shrew are not the source of BuChE. Thus, in the tree shrew LGN it appears that BuChE is not metabolically related to or dependent upon AChE nor does it originate from retinal sources, but rather BuChE appears to represent an enzyme that is endogenous to the LGN.

Cl- - and K+-dependent inhibitory postsynaptic potentials evoked by interneurones of the rat lateral geniculate nucleus

1. Hyperpolarizing potentials evoked by electrical stimulation of the optic tract were studied in projection cells of the rat dorsal lateral geniculate nucleus (LGN) in vitro. In the same cells the effects of gamma-amino butyric acid (GABA), baclofen and acetylcholine (ACh) were also investigated. 2. In the majority of cells a short- (SHP) (34 ms) and a long-lasting (LHP) (240 ms) hyperpolarizing potential could be recorded in the presence and in the absence of a preceding EPSP. They were blocked by tetrodotoxin (1 microM) and were more sensitive than the monosynaptic EPSP to a low-Ca2+-high-Mg2+ solution. 3. The SHP was associated with a marked decrease (75%) in input resistance, was blocked by bicuculline (1-100 microM) and its reversal potential (-67 mV) was dependent on the extracellular Cl- concentration. 4. The LHP was associated with a smaller decrease (45%) in input resistance and its reversal potential (-76 mV) was dependent on the extracellular K+ concentration. It was increased by bicuculline (100% at 50 microM) and nipecotic acid (30% at 10 microM), blocked by Ba2+ (1 mM), and unaffected by eserine (1-10 microM), neostigmine (1-10 microM) or by recording with EGTA-filled electrodes. In the presence of bicuculline, a single LHP was able to evoke, as a rebound response, a low-threshold Ca2+ spike that was, however, not followed by another LHP (or any other long-lasting hyperpolarization). 5. Ionophoretic applications of GABA evoked in the same cell a Cl- -dependent hyperpolarization (reversal potential: -65 mV) and/or depolarization, both of which were associated with a marked decrease (91%) in input resistance and abolished by bicuculline. GABA was also able to evoke a bicuculline-insensitive, K+-dependent hyperpolarization that had a reversal potential of -75 mV and was associated with a smaller decrease (43%) in input resistance. 6. Baclofen, applied by ionophoresis, pressure ejection or in the perfusion medium (1-100 microM), produced a hyperpolarization that had a reversal potential of -79 mV and was associated with a decrease (45%) in input resistance. 7. In the majority of cells (thirty-seven out of forty) ACh evoked a slow depolarization and only in three cells a hyperpolarization which had a reversal potential of -80 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Carbon disulfide effects on the visual system. II. Retinogeniculate degeneration

We examined the morphological effects of carbon disulfide exposure on neurons and vasculature of the visual system of macaque monkeys. Five monkeys were exposed to 256 ppm carbon disulfide (CS2) by inhalation for 6 hr a day, 5 days a week. One monkey, sacrificed immediately after exposure, had numerous axonal swellings in the distal optic tract. Four other monkeys survived the exposure period for at least 1 year and were found to have suffered marked degeneration of central retinal ganglion cells, with little or no effect on other neurons in the retina. No evidence was found for arteriosclerotic or aneurysmal changes, suggesting that visual system injury in primates induced by carbon disulfide exposure is not dependent on the occurrence of structural changes in retinal blood vessels.

Nicotine enhances cerebral glucose utilization in central components of the rat visual system

The effect of nicotine in visual system components of the rat brain was examined using the 2-deoxy-D-[1-14C]-glucose ([14C]DG) technique. Nicotine was administered subcutaneously (SC) at doses of 0.1, 0.3, and 1.0 mg/kg 2 min before the infusion of the radiotracer. Local cerebral glucose utilization (LCGU) was stimulated by nicotine in a dose-dependent manner in many brain regions associated with the visual system. Increases of over 100% were seen in the superior colliculus, nucleus of the optic tract, and portions of the accessory optic system (medial and dorsal terminal nuclei, and the inferior fasciculus). Statistically significant increases were also observed in the lateral geniculate nucleus, the lateral terminal nucleus and the cerebellum. The effects were blocked by pretreatment with mecamylamine and by enucleation. The findings lend support to the involvement of the nicotinic cholinergic system in the processing of visual information or visual-motor function.

Excitant amino acids and audiogenic seizures in the genetically epilepsy-prone rat. I. Afferent seizure initiation pathway

The afferent pathway involved in initiation of audiogenic seizures in the genetically epilepsy-prone rat was investigated by bilateral microinfusion of the excitant amino acid antagonist 2-amino-7-phosphonoheptanoate into the major brain stem and subcortical nuclei of the auditory system. This antagonist has been shown to possess anticonvulsant properties in other seizure models, and an excitant amino acid has been implicated as a putative neurotransmitter in several of these nuclei. Seizure severity was significantly reduced following infusion of this agent into the cochlear nucleus, superior olivary complex, inferior colliculus, and medial geniculate body. Many of these animals exhibited a complete blockade of seizures. The smallest effective dose in the cochlear nucleus and the medial geniculate body was 5 nmol per side. The smallest effective dose in the olive was 1 nmol, and in the inferior colliculus 0.1 nmol per side was protective. The onset of anticonvulsant effectiveness was earliest in the inferior colliculus. These findings showed that the inferior colliculus was the most sensitive auditory center to the anticonvulsant action of 2-amino-7-phosphonoheptanoate and that imbalance between inhibitory and excitatory transmission within this brain structure may be crucial in the initiation of audiogenic seizures in the genetically epilepsy-prone rat.

The effect of altered neuronal activity on the development of layers in the lateral geniculate nucleus

The main objective of this study was to examine the role of neural activity in the development of cell layers in the dorsal lateral geniculate nucleus (LGN). We studied this relationship in postnatal tree shrews either by completely blocking retinal ganglion cell activity with TTX or by selectively blocking activity to the developing ON-center LGN layers (1 and 2) with 2-amino-4-phosphonobutyric acid (APB), using unilateral and bilateral eye injections. All manipulations were carried out from birth (P0), when no LGN cell layers are evident, to or past the point when layers are recognizable (i.e., 1-2 weeks). Nissl-stained and cytochrome oxidase (CO)-reacted material was examined for all cases. Our results show that in the absence of activity produced by bilateral TTX injections, interlaminar spaces between cell layers do begin to develop. Retinal afferents, which are segregated at birth, remain segregated, and differential CO staining between matched sets of LGN layers is evident. The normal pace of LGN development, however, is slowed significantly: LGN cells are smaller and interlaminar spaces are narrower than are seen in age-matched controls. Unilateral TTX injections produce similar, but more dramatic and asymmetric, effects on LGN cells, perhaps because cells are at a competitive disadvantage relative to their normally innervated counterparts in cortex. Combining unilateral eye enucleation at P0 with subsequent TTX treatment of the other eye clearly demonstrates that axons from the remaining eye are capable of producing their normal complement of LGN layers. The development of the LGN ON-center layers, 1 and 2, and the interlaminar space between them are more affected by TTX treatment than are the other layers. By contrast, APB eye injections do not selectively affect the development of the ON-center layers, but do result in some slowing of overall LGN development. Taken together, these results suggest that activity of retinal afferents is not essential for initiating interlaminar space formation, but is important for the normal pace of maturation of LGN cell layers.

Responsiveness of suprachiasmatic and ventral lateral geniculate neurons to serotonin and imipramine: a microiontophoretic study in normal and imipramine-treated rats

The suprachiasmatic nuclei (SCN) are a major pacemaker of circadian rhythms in mammals. The SCN receive a direct retinal projection and a second optic input via the ventral lateral geniculate nucleus (vLGN). Both visual pathways mediate the entrainment of circadian rhythms, whereas both the SCN and the vLGN receive serotonergic afferents from the raphe nuclei. We investigated the effects of microiontophoretically applied serotonin (5HT) on SCN and vLGN cells in normal rats and rats chronically treated with the 5HT reuptake blocker imipramine (IMI). In the SCN of both groups over 40% of all recorded cells (N = 80) responded to 5HT with a dose-dependent suppression of their spontaneous or glutamate-evoked discharge, while twenty percent were tonically light-responsive. Except for one cell with an inconsistent 5HT response, none of the visual SCN neurons were 5HT-sensitive. In the vLGN of normal and IMI-treated rats about 60% of the cells recorded (N = 42) were inhibited by 5HT. In IMI-treated rats a few cases of excitation by 5HT were encountered in the vLGN. Visual as well as non-visual vLGN cells were responsive to 5HT. Microiontophoretic application of IMI resulted in suppression of electrical activity in both brain regions and enhanced the response induced by 5HT. Chronic IMI-treatment produced a significant increase in the sensitivity of cells in the SCN and vLGN to iontophoresed 5HT, without affecting the relative magnitude of the inhibition. The recovery from 5HT-induced inhibition was slow in these animals. Interestingly, the spontaneous discharge rate of both 5HT-sensitive and 5HT-insensitive SCN and vLGN cells was significantly lower in the imipramine-treated group.(ABSTRACT TRUNCATED AT 250 WORDS)

Tonotopic organization in the medial geniculate body (MGB) of lightly anesthetized cats

In the medial geniculate body (MGB) of nitrous oxide anesthetized cats, the pars lateralis (PL) was the only nucleus to show a clear topographic arrangement of its neurons according to their characteristic frequency (CF). When compared to barbiturate anesthetized cats (Imig and Morel 1985a), the tonotopic organization in PL appeared less strict and was characterized by a significant local CF disparity. Furthermore, the degree of tonotopic organization varied along the rostrocaudal axis of the nucleus: it was lower in its caudal than in its rostral half. In the pars ovoidea, the rostral half of the pars magnocellularis (PM) and the suprageniculate nucleus, CF sequences and quantitative evaluations of the tonotopicity indicated the presence of some degree of tonotopic organization which was lower than in PL. No such organization was observed in the caudal part of PM nor in the ventrolateral nucleus, while in the dorsal nucleus, the proportion of tone-responding units was too low for a significant analysis.

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)

A new experimental model for drug studies: effects of phenobarbital and phenytoin on photosensitivity in the lateral geniculate-kindled cat

Photosensitivity was acquired as a result of kindling in the lateral geniculate body (GL), and the GL-kindled cat pretreated with DL-allylglycine showed a stable level of photosensitivity. To test the usefulness as a model for the evaluation of anticonvulsant drugs, the effects of phenobarbital (PB) and phenytoin (PHT) on photosensitivity were studied in the GL-kindled cat under DL-allylglycine. PB (5 and 10 mg/kg intravenously, i.v.) completely suppressed photically induced seizures in most subjects at plasma concentrations of 7-16 micrograms/ml, and this anticonvulsant action persisted for at least 4 h after the injection. PHT (15 mg/kg, i.v.) at plasma concentrations of 9-15 micrograms/ml produced toxic signs, e.g., pupil dilatation, hypersalivation, and tachypnea. At this dose, PHT was inactive against photically induced myoclonus but prevented the elicitation of a generalized tonic-clonic convulsion. From these results showing that the effects of anticonvulsant drugs on photically induced seizures can be assessed in relation to plasma concentration and acute neurologic toxicity, we suggest that the GL-kindled cat is a potentially useful animal model of epilepsy for testing the efficacy of anticonvulsant drugs.

Direct and indirect effects on the lateral geniculate nucleus neurons of prenatal exposure to methylazoxymethanol acetate

In this study the morphology of the lateral geniculate nucleus and occipital cortex in rats with methylazoxymethanol acetate (MAM Ac)-induced micrencephaly was examined. The aim was to examine the relative contributions of (a) the direct cytotoxic action of the drug on precursors of dorsal lateral geniculate nucleus (dLGN) neurons in the fetal brain and, (b) the postnatal degeneration of the dLGN following prenatal destruction of target neurons in the occipital cortex, to the final extent of damage to the dLGN. Exposure to MAM Ac on E13 produced severe necrosis in the fetal thalamus and caused a 77% deficit in neuronal numbers in the mature dLGN. Exposure to MAM Ac on E15 did not cause necrosis in the fetal thalamus but when animals exposed at this time were examined at 5 weeks postnatal age there was an 87% deficit in neuronal numbers in the dLGN. The hypothesis that this deficit was the result of postnatal death of the dLGN neurons following the destruction by MAM Ac of their normal target population in laminae iii and iv of the occipital cortex was supported by the observation of severe postnatal degeneration in the dLGN of animals exposed to MAM Ac on E15. The significance of these direct and indirect effects of the cytotoxic teratogen, MAM Ac, for understanding the mechanisms by which brain abnormalities in human micrencephaly are produced is discussed.

Altered morphology of dorsal lateral geniculate nucleus neurons in methylazoxymethanol acetate induced micrencephaly

The morphology of neurons in the dorsal lateral geniculate nucleus (dLGN) of rats made micrencephalic by prenatal exposure (E13 or E15) to the cytotoxic agent, methylazoxymethanol acetate, has been examined in Golgi impregnations. Type B neurons were unaffected by exposure on either day while type A neurons showed significant reductions in both soma diameter and dendritic field area following exposure on E15, but not E13. These results indicate that target deprivation of type A neurons (by destruction of neurons in the granular and supragranular layers of the occipital cortex with exposure on E15) has a more significant effect on development of type A neurons than the direct cytotoxic action of the agent on precursors of dorsal lateral geniculate nucleus neurons in the fetal thalamus (seen with exposure on E13 in a previous study). The findings are significant because they indicate that the indirect effects of cytotoxic teratogens on the developing brain (acting via the target dependence of some neuronal classes) may cause structural und functional alterations in the brain which differ from those predicted on the basis of the direct action above. This study also shows that the percentage of relay neurons in the dorsal lateral geniculate nucleus is unaffected in animals exposed to the agent on E15, despite pronounced reductions in neuronal numbers and changes in relay neuron morphology.

Serotonergic inhibition of the dorsal lateral geniculate nucleus

Electrophysiological studies were conducted on chloral hydrate-anesthetized rats to determine if the dorsal raphe nucleus (DR) exerts an inhibitory influence upon the dorsal lateral geniculate nucleus (dLGN), and if this inhibition is mediated by the release of serotonin (5-HT). Conditioning stimuli presented to the DR 100-400 ms before an optic tract (OT) shock significantly lowered the amplitude of OT shock-elicited, postsynaptic, field potentials of less than 3 ms latency. Rare, long-latency, field potentials (greater than 5 ms) were diminished in amplitude when preconditioning intervals were less than 15 ms. Six days after intracerebral injection of the 5-HT neurotoxin, 5,7-dihydroxytryptamine (8 micrograms), into the dLGN, significant reductions were observed in 5-HT and 5-hydroxyindole acetic acid in the dLGN. Field potentials recorded on the sixth day in indoleamine-depleted dLGN were significantly less inhibited by DR preconditioning. Intracerebral injections of a control solution neither altered monoamine levels nor the degree of inhibition by DR preconditioning. These data provide further evidence that inhibition of dLGN by DR is mediated by release of 5-HT.

Kainic acid-induced terminal degeneration in the dorsal lateral geniculate of tree shrew

The dorsal lateral geniculate nucleus of tree shrews is very susceptible to the neurotoxic effects of kainic acid. In addition to neuronal loss, there is a profound loss of retinal terminals that is manifested through a disruption of anterograde transport of WGA/HRP from the retina to the kainic acid-lesioned area of the geniculate nucleus. The actions of kainic acid upon both the presynaptic terminals and geniculate neurons may be mediated by a glutamatergic pathway and questions the hypothesis that kainic acid is solely neuron-specific in its toxic action.

Effects of intraocular tetrodotoxin on the postnatal development of the dorsal lateral geniculate nucleus of the rat: a Golgi analysis

The postnatal development of the rat dorsal lateral geniculate nucleus (dLGn) during tetrodotoxin (TTX)-induced monocular impulse blockade was investigated by quantitative Golgi techniques. Beyond 14 days postnatal (dpn), the effectiveness of TTX was monitored by loss of the pupillary light reflex. By 21 dpn, Golgi analysis indicated that TTX had no effect on the pattern of dendritic branching of class A or class B neurons, although the number of dendritic spinous protrusions was reduced. No evidence of any TTX-induced loss of optic axons or neuronal degeneration in the dLGn was found, despite a 16% decrease in the size of the nucleus, suggesting a reduction in the growth of neuropil. These data indicate that optic impulses are important in mediating the proper growth of postsynaptic specializations in the dLGn during ontogenesis, but that the postnatal development of the dendritic arbor of neurons in the dLGn appears to be independent of retinal impulse activity.

Effects of allylglycine on photosensitivity in the lateral geniculate-kindled cat

The effects of DL-allylglycine, an inhibitor of GABA synthesis, on the responses to photic stimulation were studied in the cat kindled in the lateral geniculate body (GL). For 3 to 8 h after the injection of DL-allylglycine at a subconvulsant dose (30 or 40 mg/kg, i.v.), the kindled cat showed a stable level of photosensitivity without any toxic effects and responded with various degrees of myoclonus or a generalized tonic-clonic convulsion when photic stimulation was repeated at hourly intervals. The incidence of photically induced myoclonus reached its plateau during this period. Our results suggest that photosensitivity of the lateral geniculate-kindled cat is related to the modification of GABAergic mechanisms, and that when the GL-kindled cat is pretreated with DL-allylglycine it is a reliable model of photosensitive epilepsy.