The nature of the excitatory transmitter mediating X and Y cell inputs to the cat dorsal lateral geniculate nucleus

A specific subgroup of non-length tuned relay cells in the feline dorsal lateral geniculate nucleus

Length tuning was first described for the "hypercomplex cell category" in the visual cortex. However it has subsequently become apparent that cells in the dorsal lateral geniculate nucleus (dLGN) also exhibit a high degree of length tuning and that for the majority of the population this matches or exceeds that associated with cortical hypercomplex cells (Cleland et al. 1983; Jones and Sillito 1987). In this paper we describe a distinct subpopulation of dLGN Y cells that lack length tuning. These cells were also characterised by poor centre-surround antagonism, and tended to be located close to laminar borders. They appeared to constitute 25% of the Y cell population. Following recent evidence showing relay cells to be powerfully excited by acetylcholine, and inhibitory interneurones to be inhibited, we have examined the responses of these non-length tuned cells to iontophoretic application of acetylcholine. Their brisk excitatory responses suggest that these cells are in fact relay cells. Their presence raises the possibility of a discrete non-length tuned component to the geniculate input to the cortex, and has potentially important implications for the way in which synaptic processes contributing to the length tuning profiles of visual cortical cells are modelled.

Do NMDA receptors mediate the effects of light on circadian behavior?

We report here the results of experiments designed to evaluate whether a specific NMDA receptor antagonist, (+)-5-methyl-10,11-dihydro-5H-dibenzo[a,b]cyclohepten-5,10-imine maleate (MK-801), blocks the phase shifting effects of light on the circadian rhythm of wheel-running activity in golden hamsters. Intraperitoneal administration of (+)-MK-801 produced a dose-dependent blockade of both light-induced phase advances and delays. The effect was stereoselective and treatment with related compounds, phenylcyclidine and ketamine, also blocked light-induced phase shifts. MK-801, by itself, did not cause any consistent effect on the phase of the rhythm. These data, coupled with previous findings, indicate that excitatory amino acid receptors play an important role in the transmission of light information from the retina to the circadian system.

Sleep-promoting action of excitatory amino acid antagonists: a different role for thalamic NMDA and non-NMDA receptors

Changes in the sleep-waking cycle of freely moving cats were studied during application of excitatory amino acid antagonists in the ventro-posterolateral thalamic nuclei by microdialysis. DL-2-Amino-5-phosphono-pentanoic acid (APV), a selective N-methyl-D-aspartate (NMDA) receptor antagonist, produced an increase in the deep stages of slow wave sleep and in paradoxical sleep and a decrease in the light stages of slow wave sleep (SWS1), while 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), at a concentration selective for the non-NMDA receptors, produced a marked increase in SWS1. These results indicate a strong sleep-promoting action of excitatory amino acid antagonists and suggest that thalamic NMDA and non-NMDA receptors may play different roles in sleep regulation. Thus, changes in the sleep-waking cycle should be carefully evaluated when assessing the potential clinical use of excitatory amino acid antagonists.

Selective release of N-acetylaspartylglutamate from rat optic nerve terminals in vivo

Glutamate (Glu) and aspartate (Asp) are considered to be the neurotransmitters of the optic pathway in submammalian species, but their roles in mammals is uncertain. Recently, N-acetylaspartylglutamate (NAAG) has been proposed as a neurotransmitter in mammalian optic pathway; however, the release of endogenous NAAG on stimulation of the optic pathway has not been demonstrated. Using an in vivo microdialysis technique, we now report that electrical stimulation of rat optic nerve markedly increased the extracellular concentration of NAAG but not Glu/Asp in superficial superior colliculus where retinal afferents terminate, whereas non-specific stimulation of neurotransmitter release by high potassium or veratridine increased both extracellular Glu/Asp and NAAG concentration in the perfusate. The release of NAAG was dependent on Ca2+ and the presence of optic terminals. We conclude that NAAG is a better candidate as a neurotransmitter of rat optic nerve terminals than Glu/Asp.

N-methyl-D-aspartate receptors contribute to excitatory postsynaptic potentials of cat lateral geniculate neurons recorded in thalamic slices

Neurons of the cat's dorsal lateral geniculate nucleus were recorded intracellularly to study the contribution of N-methyl-D-aspartate (NMDA) receptors to excitatory postsynaptic potentials (EPSPs) and low-threshold calcium spikes. EPSPs were evoked by stimulation of retinogeniculate axons in the optic tract and/or corticogeniculate axons in the optic radiations; EPSPs from both sources were similar. These EPSPs had one or two components, and the second component had several characteristics of NMDA receptor-mediated events. For example, EPSP amplitude decreased when neurons were hyperpolarized and increased when stimulus frequency was increased; these EPSPs could also be blocked reversibly by application of the selective NMDA receptor antagonist DL-2-amino-5-phosphonovaleric acid (APV). We also studied the influence of NMDA receptors on low-threshold calcium spikes, which are large, voltage- and calcium-dependent depolarizations that are often accompanied by high-frequency action potential discharge. APV blocked synaptically activated low-threshold calcium spikes, but APV had no effect on low-threshold calcium spikes that were elicited by current injection. Therefore, APV does not appear to have a direct effect on the T-type calcium channel that is involved in generation of low-threshold calcium spikes. The voltage and frequency dependence of the NMDA receptor-mediated component of the EPSPs, as well as its ability to trigger low-threshold calcium spikes, provide for complex signal processing in the lateral geniculate nucleus.

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.

Quantitative immunogold analysis reveals high glutamate levels in synaptic terminals of retino-geniculate, cortico-geniculate, and geniculo-cortical axons in the cat

A postembedding immunogold procedure was used to estimate quantitatively, at the electron-microscopical level, the intensity of glutamate (GLU) immunoreactivity in different identifiable profiles of the lateral geniculate nucleus (LGN) and perigeniculate nucleus (PGN) of the cat. Synaptic terminals of retinal and cortical origins in the LGN, and of axon collaterals of geniculo-cortical relay cells in the PGN, were identified by previously determined ultrastructural features. Processes of interneurons or relay cells were identified by being immunoreactive or non-immunoreactive, respectively, in serial thin section reacted with a GABA antibody. The results showed that synaptic terminals of geniculo-cortical relay cells in the PGN have significantly higher levels of GLU immunoreactivity than their parent somata or dendrites in the LGN; this suggests transmitter storage of this amino acid in these terminals. By contrast, synaptic terminals of interneurons did not show enrichment of GLU relative to their parent somata. This argues against the possibility that the relative enrichment of GLU in relay cells terminals is due to factors other than presynaptic storage. In addition, axon collateral terminals of relay cells in the pGN, as well as retinal and cortical terminals in the LGN, showed significantly higher GLU immunoreactivity than GABAergic terminals. These immunocytochemical results suggest that GLU is a neurotransmitter in the retino-geniculate, cortico-geniculate, and geniculo-cortical pathways in the cat.

Presumptive catecholaminergic ganglion cells in the pigeon retina

An antiserum directed against tyrosine hydroxylase (TH), the rate-limiting enzyme in the synthesis of dopamine, was used to study the pigeon retina. Labeled cells were observed in both the inner nuclear layer (INL) and ganglion cell layer (GCL). Two populations of TH-immunoreactive neurons were observed in the INL. Some of these cells were 7-10 microns in diameter and gave rise to processes that arborized in three layers of the inner plexiform layer (IPL). These cells appeared similar to the dopaminergic amacrine cells described previously (Marc, 1988). Other labeled cells in the INL were 12-20 microns in diameter and were recognizable as a previously described subpopulation of TH-immunoreactive displaced ganglion cells (Britto et al., 1988). A population of labeled cells was observed in the GCL. Counts of these cells in two retinae revealed 5000 and 7000 cells, respectively. They ranged in size from 8-15 microns in diameter in the central retina and from 8-20 microns in diameter in the peripheral retina. The density of labeled cells was highest in the central retina and red field and lowest in the retinal periphery. The difference in cell size and cell density as a function of eccentricity is characteristic of the total population of ganglion cells in the avian retina (Ehrlich, 1981; Hayes, 1982). Some of the TH-positive cells in the GCL could be classified as ganglion cells for two reasons: (1) The axons of many of the TH-positive cells in the GCL were TH-immunoreactive as well and could be followed to the optic nerve head. (2) The injection of rhodamine-labeled microspheres into the nucleus geniculatus lateralis, pars ventralis (GLv), resulted in the retrograde labeling of many of the TH-positive cells in the contralateral retina.

Postnatal changes in the density and distribution of neurotensin-like immunoreactive fibers in the mediodorsal nucleus of the thalamus in the rat

A previous report (Inagaki et al., Brain Res. 260:143-146, '83) suggested that the peptide neurotensin is contained in neurons of the piriform cortex that project to the mediodorsal thalamic nucleus (MD) in young rats. To confirm this, we have studied the distribution of neurotensin-like immunoreactive (NTIR) fibers in MD during development, using three antisera directed at different parts of the neurotensin molecule (Emson et al., J. Neurochem. 38:992-999, '82). In adult rats, NTIR fibers in MD are sparse. They are located mostly at the medial edge of MD and in the adjacent midline thalamic nuclei, with a few poorly stained NTIR fibers in the central part of MD. In contrast, during the first postnatal week, both the medial and central portions of MD stain heavily for neurotensin. The density of NTIR fibers in MD then progressively decreases until the density typical of adult rats is reached, at about 5 weeks. Changes in the distribution of NTIR fibers in MD also occur. In 7-day-old rats, the patches of NTIR fibers in the medial and central parts of MD are contiguous, but by 10 days a sparsely immunoreactive zone forms between them. With maturation, this zone enlarges as the density of neurotensin staining decreases, until the medial contingent of NTIR fibers reaches its adult position at the medial edge of MD. From a comparison of the distribution of NTIR cells with that of cells that can be retrogradely labeled from MD or the midline thalamus, the probable source of the NTIR fibers to the central part of MD is in the deep layer of the piriform cortex, while the NTIR fibers to the medial edge of MD and the midline nuclei may arise from the preoptic region and the medial amygdala. In neonatal rats, neurons are found in the piriform cortex, the preoptic region, and the medial amygdala, which can be double-labeled both for neurotensin and with a retrograde tracer injected into MD and the midline thalamus. Projections of the preoptic region to the thalamus have a distribution similar to that of the medial population of NTIR fibers, whereas the distribution of piriform cortical afferents in central MD matches the central patch of NTIR fibers.

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.

Glutamate-immunoreactivity in the retina and optic tectum of goldfish

Glutamate was immunohistochemically localized in the goldfish retina and tectum at the light and electron microscopic (E.M.) levels using double affinity purified antisera against glutaraldehyde conjugated L-glutamate. In retina, glutamate-immunoreactivity (Glu+) was observed in cone inner segments, cone pedicles, bipolar cells, a small number of amacrine cells and the majority of cells in the ganglion cell layer. The latter were shown to be ganglion cells by simultaneous retrograde labeling. Centrally, Glu+ was observed in axons in the optic nerve and tract, and in stratum opticum and stratum fibrosum et griseum superficialis (SFGS) of the tectum. The Glu+ in the optic pathway disappeared four days after optic denervation and was restored by regeneration without affecting the Glu+ of intrinsic tectal neurons. In tectum, Glu+ was also observed in torus longitudinalis granule cells, toral terminals in stratum marginale, some pyramidal neurons in the SFGS, multipolar and fusiform neurons in stratum griseum centrale, large multipolar and pyriform projection neurons in stratum album centrale, and many periventricular neurons. Glu+ was also localized within unidentified puncta throughout the tectum and within radially oriented dendrites of periventricular neurons. At the E.M. level, a variety of Glu+ terminals were observed. Glu+ toral terminals formed axospinous synapses with dendritic spines of pyramidal neurons. Ultrastructurally identifiable Glu+ putative optic terminals formed synapses with either Glu+ or Glu- dendritic profiles, and with Glu- vesicle-containing profiles, presumed to be GABAergic. These findings are consistent with the hypothesis that a number of intrinsic and projection neurons in the goldfish retinotectal system, including most ganglion cells, may use glutamate as a neurotransmitter.

Synaptic connections between corticogeniculate axons and interneurons in the dorsal lateral geniculate nucleus of the cat

Anatomical evidence is provided for direct synaptic connections by axons from visual cortex with interneurons in lamina A of the cat's dorsal lateral geniculate nucleus. Corticogeniculate axon terminals were labeled selectively with 3H-proline and identified by means of electron microscopic autoradiography. Interneurons in the lateral geniculate nucleus were stained with antibodies that had been raised against gamma aminobutyric acid (GABA). We found that corticogeniculate terminals synapsed with dendrites stained positively for GABA about three times as often as with unstained dendrites. Of the corticogeniculate terminals that contacted GABA-positive dendrites, 97% made synaptic connections with dendritic shafts. Only 3% synapsed with F profiles, the vesicle-filled dendritic appendages characteristic of lateral geniculate interneurons. These results suggest that the corticogeniculate pathway in the cat is directed primarily at interneurons and is organized synaptically to influence the integrated output of these cells, rather than the local interactions in which their dendritic specializations participate.

Organization of cholinergic synapses in the cat's dorsal lateral geniculate and perigeniculate nuclei

In the preceding article, we showed that cholinergic fibers originating from the brainstem reticular formation provide a dense innervation of the lateral geniculate nucleus. In this report we describe the ultrastructure of these fibers and their relations with other elements in the neuropil of the lateral geniculate nucleus. Cholinergic fibers were labeled with an antibody to choline acetyltransferase (ChAT), the synthesizing enzyme for acetylcholine (ACh). In the A-laminae of the lateral geniculate nucleus, ChAT + profiles are small and contain tightly packed, mostly round vesicles. Some end in encapsulated synaptic zones where they form asymmetrical synaptic contacts with processes of both projection cells and interneurons. Others form synapses upon the shafts of dendrites. Of the four classical types of vesicle-containing profiles identified by Guillery (Z. Zellforsch. Mikrosk. 96:1-38, '69; Vision Res. [Suppl.] 3:211-227, '71), ChAT + profiles most closely resemble RSD profiles (Round vesicles, Small profile, Dark mitochondria). However, as a population, ChAT + profiles can be distinguished from the unlabeled population of RSD profiles because they are larger in size, contain more mitochondria, and make synapses with smaller postsynaptic membrane specializations. Each of these differences is statistically significant and together they indicate that ChAT + profiles are a distinct morphological type of synaptic profile. ChAT + profiles in the perigeniculate nucleus resemble those found in the lateral geniculate nucleus; they also make synapses with obvious postsynaptic thickenings.

The binocular input to cells in the feline dorsal lateral geniculate nucleus (dLGN)

1. Cells in the A laminae of the dorsal lateral geniculate nucleus receive their primary innervation from either the contralateral (A) or ipsilateral (A1) eye. This paper provides evidence concerning the responses they give to visual stimulation of what is commonly regarded as the ineffective or non-dominant eye. It also examines the contribution of the corticofugal input to these responses. 2. Cells were identified and classified according to their responses to stimulation of the dominant eye receptive field. This was then occluded and the corresponding location in the non-dominant eye field stimulated by a moving bar. Out of fifty-seven cells examined forty-three (75%) gave a response to stimulation of the non-dominant eye. There was no obvious difference between the effects on X and Y cells in these experiments. 3. In most cases (thirty-seven) the response involved an inhibition of the resting discharge level, but three cells gave a mixed excitatory and inhibitory response and three a pure excitatory response. All the responses were weak and only revealed by prolonged periods of averaging (20-100 trials). 4. Ionophoretic application of the GABA antagonist N-methyl-bicuculline (NMB) blocked the visually elicited inhibitory effects and in most cases (twenty-seven out of thirty-two tested) revealed an excitatory response. Out of a further eight cells previously unresponsive to the non-dominant eye, NMB application revealed excitatory responses in three. 5. Increasing background discharge levels and cell excitability by ionophoretic application of either acetylcholine or the excitatory amino acid, quisqualate, did not eliminate inhibitory responses and did not reveal excitatory responses. We suggest that the visually driven non-dominant eye suppression of the background discharge involves a GABA-mediated inhibitory input which masks an underlying excitatory input. 6. An excitatory non-dominant eye response could potentially derive from the influence of the corticofugal projection. However, removal of the corticofugal input by aspiration of areas 17 and 18 did not reduce either the excitatory or the inhibitory components of the response. 7. In the absence of corticofugal input all cells tested (fourteen) exhibited a non-dominant eye response and all studied during NMB application (eleven) gave an excitatory response. The primary effect of removing the corticofugal input appeared to involve the loss of a 'damping' influence on the excitatory and inhibitory responses, such that they were more easily revealed. The significance of these findings is discussed.

Dependence of Retinogeniculate Transmission on Membrane Voltage in the Cat

The mammalian lateral geniculate nucleus seems organized to gate or control the gain of retinogeniculate transmission, the result of which is then relayed to the visual cortex. We have performed in vivo intracellular studies of retinogeniculate transmission along these retino-geniculo-cortical pathways in cats by recording the retinally evoked excitatory postsynaptic potential (EPSP) in geniculate neurons. In cats, these pathways are organized into two parallel and functionally distinct channels, the X and Y pathways. We found that nearly all geniculate X cells display a fairly conventional voltage dependency for their retinally evoked EPSPs, because the amplitudes of these EPSPs decrease fairly linearly with membrane depolarization as the EPSP reversal potential is approached. Rare X cells and all Y cells, however, show an unconventional response: over a wide range of membrane potentials, their EPSP amplitudes increase with membrane depolarization. This increase does not result from alterations in neuronal input resistance and instead seems due to changes in synaptic conductance. The underlying cause of this voltage dependency remains to be determined. None the less, it does afford an interesting means by which retinogeniculate transmission can be gated, since non-retinal inputs (e.g. corticogeniculate axons) that can control a relay Y cell's membrane potential can also modulate the cell's EPSP amplitude.

Effects of excitatory amino acid receptor antagonists and agonists on suprachiasmatic nucleus responses to retinohypothalamic tract volleys

A slice preparation of the mouse hypothalamus that includes the suprachiasmatic nuclei (SCN), the optic chiasm and the optic nerves was used for pharmacologic investigations of the nature of the receptors mediating the excitation of SCN neurons by input from the retinohypothalamic tract (RHT). Bath application of cis-2,3-piperidinedicarboxylic acid, a non-selective antagonist of excitatory amino acid receptors, reversibly blocked the postsynaptic component of the field potentials evoked in the dorsolateral SCN by stimulation of the optic nerve. The selective antagonist of N-methyl-D-aspartate receptors, 2-amino-5-phosphonovaleric acid, had no effect on SCN responses. Glutamic acid diethyl ester and 2-amino-4-phosphonobutyric acid also were without effect, but gamma-D-glutamylglycine caused a small decrease in the amplitude of the postsynaptic wave. Addition of the agonists, kainate and N-methyl-D,L-aspartate, to the superfusate also blocked the postsynaptic response. Kainate was the most potent agonist. L-Glutamate was without effect at up to 100 microM. These results indicate that postsynaptic responses in the SCN to retinohypothalamic tract volleys are mediated by a non-NMDA class of excitatory amino acid receptors.

Quantitative immunogold analysis reveals high glutamate levels in retinal and cortical synaptic terminals in the lateral geniculate nucleus of the macaque

An immunogold procedure has been used on ultrathin sections of the parvo- and magnocellular layers of the dorsal lateral geniculate of the rhesus monkey to estimate quantitatively at the electron microscopic level the intensity of immunoreactivity to an antibody against glutamate over profiles of retinal, cortical, GABAergic synaptic terminals and glial cells. GABAergic terminals were identified directly by immunogold reactivity to a GABA antibody or by ultrastructural features. The results showed that in both of the main subdivisions of the geniculate the densities of immunogold particles over cortical and retinal terminals were about two- to three-fold higher than those over GABAergic terminals or glial profiles. In addition, cortical and retinal terminals showed higher positive correlations of glutamate immunogold particle densities to synaptic vesicle densities than did GABAergic terminals. These differences suggest higher and lower concentrations of glutamate corresponding to transmitter and metabolic pools of this amino acid in axon terminals of retinal and cortical origins versus GABAergic terminals, respectively, in the dorsal lateral geniculate nucleus of the macaque.

Modulatory effects of serotonin on excitatory amino acid responses and sensory synaptic transmission in the ventrobasal thalamus

Excitatory amino acid receptors are thought to mediate sensory input to the ventrobasal thalamus. There is evidence for a brainstem serotonergic projection to the ventrobasal thalamus which may have a modulatory role. The possibility that serotonin may selectively modulate responses to excitatory amino acid receptor agonists, and its effects on sensory synaptic transmission has been examined in the rat ventrobasal thalamus in vivo. Iontophoretic ejection of serotonin at low currents produced a marked facilitation of responses to excitatory amino acids. In contrast, excitatory responses to cholinomimetic agonists were attenuated. Synaptic transmission was concomitantly enhanced or unchanged in these circumstances. Higher serotonin ejection currents reversed the facilitation, or inhibited excitatory amino acid responses and synaptic transmission. It is concluded that serotonin can modulate responses to excitatory amino acids relatively selectively and that synaptic transmission of somatosensory information through the ventrobasal thalamus may be susceptible to brainstem serotonergic modulation.

Calcium-dependent evoked release of N-[3H]acetylaspartylglutamate from the optic pathway

N-Acetylaspartylglutamate (NAAG) is a neuropeptide localized to several putative glutamatergic neuronal systems, including the rodent optic pathway. To determine whether the peptide is released by depolarization, the superior colliculus of the rat was perfused with 2 microCi of [3H]NAAG, then with Krebs-bicarbonate buffer for 1 h, using a microdialysis system. Subsequently, 10-min fractions were collected and analyzed by HPLC for [3H]NAAG. Addition of 100 microM veratridine resulted in a several-fold increase in the evoked release of [3H]NAAG that was virtually abolished by coperfusion with Ca2+-free Krebs buffer containing 1 mM EGTA. When [3H]glutamate was used as the precursor, veratridine depolarization resulted in only an 80% increase in the release of [3H]NAAG. Prior enucleation of the right eye reduced the spontaneous release of [3H]NAAG by 50%, and the veratridine-evoked release by greater than 85%, from the left superior colliculus. These results suggest that NAAG is released upon depolarization and may serve as a neurotransmitter/neuromodulator in the optic tract.

Substance P-immunoreactivity in the developing human retinogeniculate pathway

Substance P has been immunohistochemically localized in the human optic nerves and lateral geniculate nuclei during the prenatal period from 13-14 to 37 weeks of gestation. Substance P-immunoreactive fibres were present in the optic nerves and lateral geniculate nuclei in all these ages thereby providing direct evidence of this undecapeptide being associated with the retinogeniculate pathway. At 16-17 weeks, greater numbers of fibres were observed than in the later ages. It is likely that the reduction in number of optic nerve fibres seen quantitatively during prenatal life may partly be due to the loss of substance P fibres.

The effects of N-acetylaspartylglutamate and distribution of N-acetylaspartylglutamate-like immunoreactivity in the rat somatosensory thalamus

The ventrobasal thalamus and adjacent regions were stained for the presence of N-acetylaspartylglutamate-like immunoreactivity. Immunoreactive axonal terminals were observed in this area and also in certain non-specific thalamic nuclei, the reticular thalamic nucleus and the lateral geniculate nucleus. Stained somata were found in the habenula, centrolateral thalamic nucleus and reticular thalamic nucleus. Iontophoretically applied N-acetylaspartylglutamate had variable, although predominantly inhibitory, actions on ventrobasal thalamus neurons. These results indicate that N-acetylaspartylglutamate is unlikely to be the neurotransmitter of ascending somatosensory afferents, but do not rule out the possibility that it has some other neurotransmitter or neuromodulator role in the ventrobasal thalamus.

Cholinergic excitation and inhibition in the visual thalamus of the cat--influences of cortical inactivation and barbiturate anesthesia

Neuronal responses to microiontophoretically applied pulses of acetylcholine (ACh) were recorded in the perigeniculate nucleus (PGN) and dorsal lateral geniculate nucleus (dLGN) of the cat. In the PGN, ACh inhibited 95% of the cells under N2O/O2-halothane anesthesia. Systemic application of sodium pentobarbitone did not abolish ACh inhibition. In the dLGN with N2O-halothane, ACh increased firing rates of neurons before (88%) and after (78%) interruption of their direct retinal inputs by photocoagulation of the receptive field area on the retina. Functional inactivation of the corticogeniculate loop by cooling of the ipsilateral visual cortex did not selectively influence ACh sensitivity in the retinally deafferented dLGN. It is concluded that ACh exerts a direct, excitatory action on geniculate neurons. Infusion of sodium pentobarbitone resulted in reduction of ACh firing rates to about 30% of the control values in 35% of the normally innervated cells (65% of that sample not being excited by ACh), whereas excitatory ACh responses after retinal deafferentation were completely blocked by the barbiturate. It is concluded that pentobarbitone influences direct ACh excitation, but not cholinergic disinhibition in the dLGN or ACh inhibition in the PGN. Possible differential effects of pentobarbitone on nicotinic and muscarinic types of ACh responses are discussed.

The modulation of the retinal relay to the cortex in the dorsal lateral geniculate nucleus

The translation of the retinal input through the dorsal lateral geniculate nucleus (dLGN) to the visual cortex is highly dependent on a range of influences. This article reviews the available evidence. One of the influences, the corticofugal projection to the dLGN from layer VI of the visual cortex, provides a synaptic input which in magnitude exceeds that from the retina. This makes direct synaptic contact on relay cells and the intrinsic and perigeniculate inhibitory interneurones influencing their activity. The corticofugal system appears to be spatially organised in such a way that for any given region in the dLGN, there is a central zone comprising an overlying field with facilitatory effect, and a surrounding zone with inhibitory influence. The extent to which these overlap is open to question at present. The inhibitory effect of the corticofugal projection can be clearly seen in its contribution to the length tuning of dLGN cells when tested with drifting bars. On average dLGN cells exhibit a very high degree of length tuning, matching that of cortical hypercomplex cells. Removal of the corticofugal influence causes a radical reduction in this, shifting the mean reduction in peak response with increasing bar length from 71% to 43%. One consequence of this corticofugal effect is that the selectivity of the dLGN cell receptive field towards stimuli spatially restricted to the vicinity of the centre mechanism, is as good for moving bars as it is for stationary flashing spots. The retinal output to dLGN relay cells appears to be mediated by excitatory amino acid receptors, of both NMDA and non-NMDA categories. The non-NMDA receptors appear to provide an initial level of depolarisation which enables the operation of the voltage dependent NMDA receptor channels. The NMDA receptor however sits as a critical gate regulating the transmission of retinal information in the dLGN, when it is blocked visual responses are virtually eliminated. Its voltage dependency makes it crucially dependent on the complex pattern of excitatory and inhibitory influences from the cortex and the "non-specific" modulatory influence of the cholinergic system.

The action of the corticofugal pathway on sensory thalamic nuclei: a hypothesis

The N-methyl-D-aspartate receptor has recently attracted great interest due to its nonlinear current-voltage behavior. In order to evoke a large depolarizing postsynaptic current, the synaptic-induced conductance change must be paired with a postsynaptic depolarization. This temporally tuned AND gate could underlie a number of different operations throughout the nervous system. We propose that the synapses made by the optical nerve onto projection cells in the mammalian dorsal lateral geniculate nucleus are of the N-methyl-D-aspartate type. [In this Commentary, we have pooled data regarding sensory thalamic nuclei from a number of different mammalian species. Unless otherwise mentioned, we have referred to the dorsal division of the cat lateral geniculate nucleus.] About half of all synapses in these cells--located almost exclusively in the peripheral two-thirds of the dendritic tree--are associated with axons originating in layer VI of visual cortex. It then follows that the massive corticogeniculate pathway controls the gain of the retinogeniculate pathway via its action on the N-methyl-D-aspartate receptors. Thus, near-simultaneous activation of the retinal and the cortical input will transiently enhance the geniculate cell response. Generalizing to other thalamic sensory nuclei, afferent information will be routed through the thalamus and on to the cortex as long as cortical activity is congruent with sensory input to the thalamus. Experimental evidence argues for such a mechanism to control the gain of the somatosensory input to the ventrobasal thalamic nucleus.

Excitatory amino acid receptors and synaptic transmission in the rat ventrobasal thalamus

1. Extracellular single-neurone recordings were made in the ventrobasal thalamus (v.b.t.) of urethane-anaesthetized rats with multi-barrel ionophoretic electrodes in order to test the hypothesis that excitatory amino acid receptors are involved in the responses of these neurones to stimulation of sensory afferents. 2. Responses of neurones to either physiological stimulation of hair and vibrissa follicle sensory afferents and to ionophoretically applied excitatory amino acids were challenged with the antagonists D-2-amino-5-phosphonovalerate (APV), kynurenate and gamma-D-glutamylaminomethyl sulphonate (GAMS). 3. In agreement with previous findings in other brain areas, ionophoretically applied APV was found to selectively antagonize responses of v.b.t. neurones to N-methylaspartate (NMA), whereas GAMS was found to be moderately kainate selective. Kynurenate was found to be relatively non-selective. 4. Responses of neurones to short-duration (10-20 ms) physiological stimulation of afferents were resistant to APV when this antagonist was applied with NMA-selective ionophoretic currents. In contrast, these APV currents were adequate to antagonize responses to maintained physiological stimulation. 5. The broad spectrum excitatory amino acid antagonist kynurenate was found to block synaptic responses of v.b.t. neurones to both short-duration and maintained stimuli when it was applied with currents which were sufficient to reduce responses to ionophoretic quisqualate. 6. GAMS was found to selectively block kainate responses in a proportion of the neurones tested. In such cases, there was little effect of the antagonist on the responses evoked by either short-duration or maintained sensory stimuli. 7. It is concluded that excitatory amino acid receptors of both the NMDA and non-NMDA type are involved in the synaptic responses of v.b.t. neurones to sensory afferent stimulation, and that the apparent synaptic pharmacology depends on the mode of stimulation of the afferent pathway.

N-acetylaspartylglutamate immunoreactivity in neurons of the cat's visual system

The acidic dipeptide, N-acetylaspartylglutamate (NAAG) was identified immunohistochemically within neurons of the cat's visual system. In the retina, NAAG-like immunoreactivity was observed in some horizontal and amacrine cells at the inner and outer margins of the bipolar cell layer. NAAG-like immunoreactivity was also observed in many retinal ganglion cell bodies, their neurites, and the neuropil of their target areas, the lateral geniculate nucleus (LGN) and the superior colliculus. Additionally, peptide immunoreactivity was also seen in the projection neurons of the LGN, in cells of the pulvinar nucleus, and in the pyramidal cells of layers III and V in areas 17, 18 and 19 of the cerebral cortex. These data suggest that NAAG or a structurally related molecule may have a prominent role in the communication of visual signals at retinal, thalamic and cortical levels.

Contributions of inhibitory mechanisms to the shift responses of X and Y cells in the cat lateral geniculate nucleus

1. Adult cats were anaesthetized with a mixture of halothane, nitrous oxide and oxygen to record from single neurones of the dorsal lateral geniculate nucleus (d.l.g.n.) with five-barrel glass micro-electrodes. Periphery effects (shift effects) were elicited by large-field phase-reversing gratings presented in the visual field outside the conventional receptive field area. 2. A range of transient excitatory responses was found in X and Y cells. Y cells had phasic shift effects with significantly higher amplitudes and shorter durations (mean 52 impulses/s, 135 ms) than those observed in the tonic shift effects of X cells (mean 24 impulses/s, 169 ms). All Y cells and most X cells responded to stimulation of remote retinal regions. About 7% of the X cells displayed no shift effect. 3. Micro-ionophoresis of the gamma-aminobutyric acid (GABA) antagonist bicuculline, acetylcholine (ACh) and L-glutamate specifically influenced the shift effects of X and Y cells. 4. During continuous application of the GABA antagonist bicuculline the differences in maximal response rates and amplitudes of X and Y cells were eliminated. The maintained activity raised predominantly in X cells and the early peak rates increased more in X- than in Y-cell shift effects, leading to equal average peak rates of 100 and response amplitudes of about 85 impulses/s in both cell classes. The characteristic time courses of X- and Y-cell responses were not affected. 5. Micro-ionophoretic application of ACh caused a combination of excitatory and disinhibitory effects. Maintained activity as well as early parts of stimulus-evoked responses were similarly raised in X and Y cells. In addition, the Y-cell shift effects became less phasic by elevation of the late response part. Sodium pentobarbitone, used to block ACh excitation, suppressed the ACh-induced effects in the early phase of the X- and Y-cell shift effects and the increase of maintained activity in Y-cells, while the effect on the late part of Y-cell responses persisted. Elevation of background activity partially remained in X cells, and the X-cell responses became tonically prolonged at the same time. 6. L-Glutamate increased the activity of X and Y cells without changing the characteristic shift-effect properties of both cell classes. 7. It is concluded that different short- and long-lasting inhibitory mechanisms shape the responses of d.l.g.n. neurones to stimulation outside the conventional receptive field.(ABSTRACT TRUNCATED AT 400 WORDS)

N-acetylaspartylglutamate identified in the rat retinal ganglion cells and their projections in the brain

N-Acetylaspartylglutamate like immunoreactivity (NAAG-L) was identified in retinal ganglion cell bodies and their axons. The presence of the dipeptide in ganglion cell projection areas, the lateral geniculate nucleus (LGN) and superior colliculus (SC), was confirmed following NAAG purification from these tissues by a high-performance liquid chromatographic method. NAAG-L was identified in the optic tract as well as within fibers and puncta in the LGN and SC. The hypothesis that NAAG is present within ganglion cell axons in the brain was tested by unilateral enucleation which resulted in loss of NAAG and NAAG-L within the contralateral LGN and SC.

Kynurenic acid blocks suprachiasmatic nucleus responses to optic nerve stimulation

An in vitro slice preparation of the mouse hypothalamus was used to determine the effects of pharmacological agents on the field potentials that are evoked in the suprachiasmatic nucleus (SCN) by stimulation of the optic nerve. Postsynaptic components of these responses were identified by lowering the concentration of calcium in the superfusate. Bath application of kynurenate, an antagonist of excitatory amino acid neurotransmission, reversibly blocked postsynaptic responses in the SCN. The evoked responses in the SCN were not affected by the acetylcholinergic agents (+)-tubocurarine, scopolamine, physostigmine, or carbachol. These results suggest that excitatory amino acid receptors mediate responses of SCN neurons to retinal input, but do not support a role for acetylcholine.

On the excitatory post-synaptic potential evoked by stimulation of the optic tract in the rat lateral geniculate nucleus

1. The electrophysiological and pharmacological properties of the excitatory post-synaptic potentials (e.p.s.p.) evoked by electrical stimulation of the optic tract were studied in projection neurones of the ventral and dorsal lateral geniculate nucleus (l.g.n.) of the rat in vitro. 2. No difference was found in the rise time of e.p.s.p.s. recorded in the dorsal and ventral l.g.n. and in their threshold for action potentials. At membrane potentials more negative than -60 mV, e.p.s.p.s. in the dorsal l.g.n. were always followed by a Ca2+-dependent potential. Its amplitude could easily reach threshold for generating an action potential and thus evoke firing from an e.p.s.p. that was subthreshold at resting potential. No Ca2+ potential was observed to follow e.p.s.p.s. recorded in the ventral l.g.n. 3. At resting potential the excitability of dorsal and ventral cells was unaffected following an initial shock to the optic tract. However, in dorsal neurones, at potentials more negative than -60 mV, the presence of Ca2+ potentials evoked by the e.p.s.p.s. resulted in a period of decreased excitability. 4. Using intrasomatic injection of Cs+ the reversal potential (E) of the e.p.s.p. and of the depolarization produced by glutamate could be measured in the same l.g.n. neurone. They were: Eepsp, -0.9 mV; and Eglut, -3.9 mV. 5. gamma-D-glutamylglycine (DGG), an excitatory amino acid antagonist, reversibly inhibited the e.p.s.p. and depolarization produced by quisqualate and glutamate by a competitive action. The concentration of DGG that produced 50% inhibition (IC50) was 2.7 mM. 6. D-2-amino-5-phosphonovalerate (APV), the potent and selective N-methyl-D-aspartate (NMDA) antagonist, had no effect on the e.p.s.p. both in the presence and absence of Mg2+. The isomers of 2-amino-4-phosphonobutyrate (APB) were inactive or had a non-specific action on the e.p.s.p. 7. No difference could be detected in either the reversal potential or the action of the antagonists between neurones of the dorsal and the ventral l.g.n. 8. These results suggest that Ca2+-dependent potentials play an important role in modulating synaptic efficacy in principal neurones of the dorsal l.g.n. The quisqualate/kainate nature of the optic nerve receptors and the similarity of Eepsp and Eglut constitute strong support in favour of a glutamate-like substance as the transmitter of the optic nerve.

Localization of gamma-aminobutyric acid (GABA) in type 3 cells and demonstration of their source to F2 terminals in the cat lateral geniculate nucleus: a Golgi-electron-microscopic GABA-immunocytochemical study

Postembedding immunocytochemistry with a gamma-aminobutyric acid (GABA) antiserum was done on semithin sections of cat lateral geniculate nucleus (LGN) previously processed with the rapid-Golgi and gold-toning procedures, to determine which of the three main morphological types (1, 2,3) of neurons in the A-laminae show immunoreactivity and are, therefore, presumably GABAergic. Only type 3 cells were found to be GABA positive. These cells were characterized by small somata and few, scarcely branched dendrites bearing almost exclusively appendages with long slender stalks. Some of these cells have extensive filiform "axonlike" processes originating from different regions of dendrites and having appendages similar to those originating directly from dendrites. Many of these Golgi gold-toned impregnated dendritic appendages of type 3 cells were analyzed in the electron microscope and were identified as typical F2 terminals by their content of pleomorphic synaptic vesicles; by being postsynaptic to retinal (RLP), cortical (RSD), and perigeniculate (F1) terminals; and by being presynaptic to dendrites. In addition, since it was previously demonstrated that glutamic acid decarboxylase (GAD) and GABA-positive cells are not retrogradely labeled with horseradish peroxidase (HRP) from the visual cortex, the present results, by showing that GABA-positive cells have type 3 morphology, provide supporting evidence for the interneuronal nature of type 3 cells in cat LGN.

Localization of N-acetylaspartylglutamate-like immunoreactivity in selected areas of the rat brain

N-acetylaspartylglutamate (NAAG) was detected immunohistochemically in the rat brain using an antiserum which recognizes carbodiimide-fixed NAAG. NAAG-like immunoreactivity is described in 5 areas of the brain; olfactory bulb, septal nuclear area, lateral geniculate nucleus, superior colliculus and the entorhinal cortex/hippocampal formation. Mitral cells of the olfactory bulb and neurons concentrated in the medial septum were densely immunostained. A dense population of immunoreactive puncta was found in the superior colliculus and lateral geniculate nucleus (LGN). The LGN also contained immunoreactive neurons. The entorhinal cortex contained numerous immunoreactive cells in layers II-III while the hippocampus had few neurons that were NAAG-positive.

Anticholinesterase agents affect contrast gain of the cat cortical visual evoked potential

The effects of the anticholinesterase agents physostigmine, pyridostigmine, diisopropylfluorophosphate and soman on the cortical visual evoked potential (VEP) were examined using phase reversed sine wave gratings of varying contrasts. All 4 agents decreased the slope (gain) of the response vs log contrast function while the intercept with the noise level (threshold, an indicator of contrast sensitivity) was unaffected. These results suggest that anticholinesterases modify the VEP by changing a contrast gain control mechanism.

Mediation of thalamic sensory input by both NMDA receptors and non-NMDA receptors

Excitatory amino acids such as L-glutamate and L-aspartate are well established as neurotransmitter candidates in the mammalian central nervous system, and three types of receptor for these substances have been proposed, characterized by the agonists N-methyl-D-aspartate (NMDA), kainate and quisqualate. All these receptors have been suggested to have synaptic roles in excitatory transmission in the brain. Here I demonstrate that NMDA receptors play a crucial role in the observed response of ventrobasal thalamus (VB) neurones to natural stimulation of somatosensory afferents, but do not appear to be responsible for the short-latency excitation seen on electrical stimulation of the afferents which is apparently mediated by excitatory amino-acid receptors of the non-NMDA type. This result indicates an involvement of NMDA and non-NMDA receptors in the responses of VB neurones to stimulation of somatosensory somatosensory afferents, depending on the mode of stimulation of the pathway.

The control of retinogeniculate transmission in the mammalian lateral geniculate nucleus

In the mammalian visual system, the lateral geniculate nucleus is commonly thought to act merely as a relay for the transmission of visual information from the retina to the visual cortex, a relay without significant elaboration in receptive field properties or signal strength. However, many morphological and electrophysiological observations are at odds with this view. Only 10-20% of the synapses found on geniculate relay neurons are retinal in origin. Roughly half of all synapses derive from cells in layer VI of visual cortex; roughly one third are inhibitory and GABAergic, derived either from interneurons or from cells of the nearby perigeniculate nucleus. Most of the remaining synapses probably derive from cholinergic, noradrenergic, and serotonergic sites within the brainstem reticular formation. Moreover, recent biophysical studies have revealed several ionic currents present in virtually all thalamic neurons. One is a Ca2+-dependent K+ current underlying the afterhyperpolarization (or the IAHP), which may last up to 100-200 ms following an action potential. Activation of the IAHP leads to spike frequency adaptation in response to a sustained, suprathreshold input. Intracellular recordings from other neuronal preparations have shown that the IAHP can be blocked by noradrenaline or acetylcholine, leading to an increased cellular excitability. Another ionic current results from a voltage- and time-dependent Ca2+ conductance that produces a low threshold spike. Activation of this conductance transforms a geniculate neuron from a state of faithful relay of information to one of bursting behavior that bears little relationship to the activity of its retinal afferents. We propose that state-dependent gating of geniculate relay cells, which may represent part of the neuronal substrate involved in certain forms of selective visual attention, can be effected through at least three different mechanisms: conventional GABAergic inhibition, which is largely controlled via brainstem and cortical afferents through interneurons and perigeniculate cells; the IAHP, which is controlled via noradrenergic and cholinergic afferents from the brainstem reticular formation; and the low threshold spike, which may be controlled by GABAergic inputs, cholinergic inputs, and/or the corticogeniculate input, although other possibilities also exist. Furthermore, it seems likely that gating functions involving the corticogeniculate pathway are suited to attentional processes within the visual domain (e.g., saccadic suppression), whereas brainstem inputs seem more likely to have more global effects that switch attention between sensory systems.(ABSTRACT TRUNCATED AT 400 WORDS)

Influence of excitatory amino acid receptor antagonists and of baclofen on synaptic transmission in the optic nerve to the suprachiasmatic nucleus in slices of rat hypothalamus

Electrical stimulation of the optic nerve evoked two positive waves with short latency, followed by a large negative wave in the suprachiasmatic nucleus of slices of hypothalamus of the rat. The latency to peak of the two positive waves and the large negative wave were 2.7 +/- 0.1, 6.1 +/- 0.1 and 10.3 +/- 0.5 msec, respectively. Only the large negative wave disappeared in low calcium Ca2+-high magnesium (Mg2+) Krebs solution and with the addition of tetrodotoxin (1 microM) all the waves disappeared. Baclofen inhibited the large negative wave in a dose-dependent manner but not the two positive waves. Excitatory amino acid antagonists also inhibited only the large negative wave, i.e. it was reduced to about 70% by 1 mM glutamic acid diethyl ester and to about 50% by both 1 mM 2-amino-4-phosphonobutyric acid and 1 mM DL-2-amino adipic acid. All waves were unaffected by 0.1 mM atropine, hexamethonium and curare. These results indicate that two positive waves, induced by stimulation of the optic nerve are attributed to nerve conduction and the large negative wave to the neurons of the suprachiasmatic nucleus, and that the neuronal pathway from the optic nerve to the suprachiasmatic nucleus may include aspartate and/or glutamate as an excitatory neurotransmitter.

The proportion and size of GABA-immunoreactive neurons in the magnocellular and parvocellular layers of the lateral geniculate nucleus of the rhesus monkey

Neurons containing GABA-immunoreactivity in LGN of the macaque monkey were analyzed quantitatively in semithin (1 micron) sections. The percentage of GABA(+) cells per unit area of the sections was 26% in the magnocellular layers and 19% in the parvocellular layers. However, the percentage of GABA(+) cells in a unit volume of LGN, calculated by a stereological method that takes into account the observed difference in size of labeled and unlabeled somata, was 35% in the magnocellular layers and 25% in the parvocellular layers. GABA(+) somata in the magnocellular layers were significantly larger than those in the parvocellular layers. The possible role of GABAergic cells in inhibitory mechanisms of receptive fields of parvo- and magnocellular neurons are discussed in the light of current knowledge of the physiology and neural circuits of macaque LGN.

Binocular interactions in the lateral geniculate nucleus of the cat: GABAergic inhibition reduced by dominant afferent activity

Single cells were recorded in layers A and A1 of the cat dorsal lateral geniculate nucleus (dLGN) with multibarrel micropipettes. Interocular inhibition ("binocular inhibition") was elicited by phase-reversing gratings presented monocularly to the nondominant eye. This inhibition was blocked by microiontophoretic application of bicuculline and thus proven to be GABAergic. Cortical cooling enhanced the inhibition from the nondominant eye. After localized destruction of the receptive field area in the dominant eye, duration and strength of discharge depression were increased. It is concluded that afferent activity from the dominant eye reduces inhibition from the nondominant eye in the dLGN.

Excitatory and differential disinhibitory actions of acetylcholine in the lateral geniculate nucleus of the cat

Single neurones were recorded in the dorsal lateral geniculate nucleus (d.l.g.n.) of adult cats anaesthetized with a mixture of halothane, nitrous oxide and oxygen. The multibarrel-glass micro-electrodes were filled with sodium acetate, L-glutamate, acetylcholine (ACh), gamma-aminobutyric acid (GABA) and bicuculline. In normally innervated, spontaneously active d.l.g.n. cells, ACh and L-glutamate elicited increased firing rates. After elimination of the excitatory input from the retina by retinal photocoagulation, the effects of ACh and L-glutamate were similar. This proves that both drugs have direct excitatory effects on d.l.g.n. cells and that disinhibition is not the most prominent influence of ACh in the d.l.g.n. The excitatory action of ACh on relay cells in the d.l.g.n. was strongly influenced by barbiturates. Sub-narcotic levels of sodium pentobarbitone completely abolished the excitation by ACh while the response to L-glutamate remained unchanged. Excitation, centre-surround antagonism and periphery effects were elicited by spots of light and by large field phase-reversing gratings with and without central sparing of the receptive field area. Binocular inhibition was elicited with the phase-reversing grating presented to the non-dominant eye. After localized destruction of the retinal receptive field area, retinogeniculate excitation ceased and an isolated lateral inhibition was observed in the acutely deafferented d.l.g.n. cells. The time course and strength of this inhibition was disclosed by raising the background discharge with microiontophoretically applied L-glutamate. With increasing size of retinal lesions the strength of isolated lateral inhibition decreased exponentially. A maximal intrageniculate range of more than 1000 microns was derived from computations of the lateral extent of deafferentation in the d.l.g.n. The inhibition acted beyond the classic surround inhibition of d.l.g.n. cells and thus was named long-range lateral inhibition. Microiontophoretically applied GABA elicits a strong inhibitory effect at the d.l.g.n. cells which is antagonized by bicuculline. Centre-surround antagonism, binocular inhibition and long-range inhibition were blocked by bicuculline and thus proven to be GABAergic. Each class of inhibition was differentially influenced by microiontophoretically applied ACh. Long-range inhibition was disinhibited, centre-surround antagonism was enhanced, and binocular inhibition was not significantly changed. In contrast to ACh excitation, the disinhibitory action of ACh was not suppressed by pentobarbitone.(ABSTRACT TRUNCATED AT 400 WORDS)

A microiontophoretic study of the actions of the putative sleep factor, piperidine, in the rat brainstem

By means of microiontophoresis, we have compared the actions of a putative sleep substance, piperidine, with other neurotransmitters in the rat anaesthetized with urethane. In the pons and midbrain, piperidine mimicked the actions of acetylcholine on more than 200 neurones. Piperidine- and acetylcholine-induced excitations were equally effectively antagonized by hexamethonium or atropine. In 32 neurones piperidine showed no affinity for the receptors for the excitatory amino acid agonists, quisqualate and N-methyl-D-aspartate, piperidine-evoked excitations being unaffected by the antagonists glutamate diethylester or 2-amino-5-phosphonovalerate. Similarly, piperidine-evoked excitations in 23 neurones were unaffected by alpha-methylnoradrenaline, suggesting that piperidine does not act at receptors for noradrenaline. Twenty per cent of neurones responsive to piperidine were inhibited. These inhibitions in 12 neurones were insensitive to either strychnine or bicuculline indicating that piperidine does not act on receptors for glycine or gamma-aminobutyric acid. In a further 68 neurones, neither hexamethonium (4 out of 59 cells) nor atropine (0 out of 9 cells) was effective in antagonizing the inhibitions evoked by piperidine or by acetylcholine. It is suggested that piperidine may exert its central hypnogenic effects by an action at cholinoceptors in brainstem areas involved in sleep regulation.

The role of gamma-aminobutyric acid mediated inhibition in the response properties of cat lateral geniculate nucleus neurones

We studied the effect of local ionophoretic application of bicuculline on the response of cat lateral geniculate nucleus (laminae A) cells to stimulation by sinusoidal gratings and spots of light. Application of bicuculline produced an increase both of spontaneous and visually driven discharge of both X and Y cells. On stimulation by drifting sinusoidal gratings, the average discharge of both X and Y cells remained constant with increasing contrast under normal conditions. Application of bicuculline caused the average discharge to increase with contrast, indicating that the constancy of the average discharge was maintained by gamma-aminobutyric acid mediated inhibition. Under normal conditions, the amplitude of response modulation of both X and Y cells to sinusoidal grating stimulation increased monotonically with stimulus contrast. During bicuculline application, the slope of the contrast-response curve for X cells but not for Y cells increased, indicating that the inhibition which dampened the modulation of X cells (but not Y cells) was contrast dependent. Application of acetylcholine also increased the average discharge and the amplitude of modulation of the cell responses, but this increase did not depend on stimulus contrast. Under normal conditions, X but not Y cells showed an attenuation of response and an increase in contrast threshold to low spatial frequencies. This attenuation vanished during bicuculline application. The shape of Y-cell response curves was unaffected by bicuculline. Bicuculline had the same effect on the non-linear component of Y-cell response as on the linear component. Although bicuculline had a different effect on the response of X and Y cells to stimulation by gratings, it reduced the antagonistic surround of both X and Y cells to a similar extent (revealed by plotting the cell receptive fields with flashed spots of light).

Comparison of sigma- and kappa-opiate receptor ligands as excitatory amino acid antagonists

Using the technique of microelectrophoresis in pentobarbitone-anaesthetized cats and rats, the effects of benzomorphans, with known actions at sigma- and kappa- opioid receptors, were tested on responses of spinal neurones to amino acids and acetylcholine. The racemic mixture and both enantiomers of the sigma opiate receptor agonist, N-allylnormetazocine (SKF 10, 047), and the dissociative anaesthetic, ketamine, reduced or abolished excitation evoked by N-methyl-aspartate (NMA) with only small and variable effects on responses to quisqualate or kainate. (+)-SKF 10, 047 was 1.2 +/- 0.7 times more potent than the (-)-enantiomer in antagonizing NMA. On Renshaw cells, (+)-SKF 10, 047 enhanced responses to acetylcholine whereas the (-) enantiomer produced only a small reduction. The kappa- opiate receptor agonist, ethylketocyclazocine, had no selective effects on responses to amino acids or to acetylcholine. We conclude that actions at sigma- but not kappa-, opiate receptors are responsible for the NMA antagonism observed with benzomorphans.

Acidic amino acid binding sites in mammalian neuronal membranes: their characteristics and relationship to synaptic receptors

This review summarizes studies designed to label and characterize mammalian synaptic receptors for glutamate, aspartate and related acidic amino acids using in vitro ligand binding techniques. The binding properties of the 3 major ligands employed--L-[3H]glutamate, L-[3H]aspartate and [3H]kainate--are described in terms of their kinetics, the influence of ions, pharmacology, molecular nature, localization and physiological/pharmacological function. In addition, the binding characteristics are described of some new radioligands--[3H]AMPA, L-[3H]cysteine sulphinate, L-[35S]cysteate, D-[3H]aspartate, D,L-[3H]APB, D-[3H]APV and D,L-[3H]APH. Special emphasis is placed on recent findings which allow a unification of the existing binding data, and detailed comparisons are made between binding site characteristics and the known properties of the physiological/pharmacological receptors for acidic amino acids. Through these considerations, a binding site classification is suggested which differentiates 5 different sites. Four of the binding site subtypes are proposed to correspond to the individual receptor classes identified in electrophysiological experiments; thus, A1 = NMDA receptors; A2 = quisqualate receptors; A3 = kainate receptors; A4 = L-APB receptors; the fifth site is proposed to be the recognition site for a Na+-dependent acidic amino acid membrane transport process. An evaluation of investigations designed to elucidate regulatory mechanisms at acidic amino acid binding sites is made; hypotheses such as the Ca2+-activated protease hypothesis of long-term potentiation are assessed in terms of the new binding site/receptor classification scheme, and experiments are suggested which will clarify and expand this exciting area in the future.

The cholinergic influence on the function of the cat dorsal lateral geniculate nucleus (dLGN)

The functional influence of the cholinergic input to cat dLGN has been examined by assessing the action of iontophoretically applied acetylcholine (ACh) on the visual responses of cells in layers A and A1. Iontophoretically applied pulses of ACh exerted a strong excitatory action on all 113 cells studied within these layers. In the presence of a sustained application of ACh, the excitatory responses to an optimal stimulus such as a spot of light located within the receptive field centre were greatly facilitated, but at the same time stimulus-specific inhibitory influences were also enhanced. The action of ACh on the stimulus-specific inhibitory influences had the consequence that the responses to non-optimal stimuli were not facilitated to the same extent as those to optimal stimuli and in some cases even diminished. The stimulus-specific inhibitory effects seen in the presence of ACh were very powerful and frequently resulted in complete suppression of the elevated background discharge. We suggest that the ACh directly excites both the relay cells and the Golgi type II inhibitory interneurones within the dLGN. The facilitation of the stimulus-specific inhibition may follow from a direct action on the presynaptic dendrites of the Golgi type II cells which arborize within the dendritic field of the relay cell. Supplementary observations on cells in the perigeniculate nucleus confirm previous findings showing that ACh has an inhibitory effect on these cells. We suggest a tripartite action for the cholinergic influence on the dLGN, involving direct facilitation of relay cells, enhancement of stimulus-specific inhibition via the Golgi type II cells, and disinhibition of the non-specific inhibitory influence form the perigeniculate nucleus.

The influence of GABAergic inhibitory processes on the receptive field structure of X and Y cells in cat dorsal lateral geniculate nucleus (dLGN)

Visually elicited inhibitory processes, underlying the receptive field structure of cells in layers A and A1 of the cat dorsal lateral geniculate nucleus (dLGN), have been examined by a combination of visual neurophysiological and iontophoretic techniques. Discrete visual stimulation of both centre and surround mechanisms, produced a powerful suppression of the elevated background discharge levels induced by iontophoretic application of an excitatory amino acid. These observations are consistent with the activation of a postsynaptic inhibitory input, a view supported by the fact that the suppressive effects were blocked by iontophoretic application of bicuculline, an antagonist of GABA, a putative inhibitory transmitter in the dLGN. These inhibitory effects were always elicited by the opposite phase of a flashed stimulus to that eliciting responses associated with the receptive field region. That is 'on' inhibitory effects were elicited from 'off' excitatory regions and 'off' inhibitory effects from 'on' excitatory regions. Plotting the responses of dLGN cells to flashed stimuli of increasing diameter, before and during iontophoretic application of bicuculline, revealed a marked reduction of the surround antagonism of centre response in the presence of the drug. During bicuculline application, surround antagonism of centre responses was at a level associated with that seen in retinal ganglion cells. Annular stimuli of internal and external diameter selected to be just outside the centre-surround border, insofar as they gave pure surround responses, were observed in the presence of bicuculline to elicit strong centre responses, as well as surround responses. These observations indicate that GABAergic inhibitory processes generate the enhanced centre-surround antagonism associated with dLGN cells and serve to increase the contrast between the two sets of mechanisms at the centre-surround border. The present conclusions apply equally to 'X' and 'Y' cells.

Amino acids in the dorsal lateral geniculate nucleus of the cat--collection in vivo

A dialysis sampling probe was used to collect amino acids from the dorsal lateral geniculate nucleus (LGN) in vivo. The sampling probe was equipped with an electrode to allow local stimulation and recording of nerve activity. The amino acids in the dialysates were determined fluorimetrically by precolumn derivation and hplc-separation. Local electrical stimulation of the LGN caused a multifold increase in glutamate, aspartate and GABA levels. Smaller changes were observed for taurine, alanine and glycine. The results indicate that the dialysis sampling probe is rather atraumatic and can be used to detect stimulation-induced changes in extracellular amino acid concentrations.

Release of endogenous amino acids from superior colliculus of the rabbit: in vitro studies after retinal ablation

Tissue slices from the superior colliculi (SC) of the rabbit were superfused and investigated 1 week after unilateral eye removal. Amino acid levels were determined both in the tissue slices and in the medium after chemical depolarisation (56 mM K). The amino acid determinations were done fluorimetrically by precolumn derivation and HPLC separation. Colliculi contralateral to the enucleation exhibited a 16% reduction in glutamate compared with the ipsilateral colliculi. The Ca-dependent release of glutamate or other amino acids tested was not appreciably affected by enucleation. However, both the total and the Ca-independent release of glutamate was lower from contralateral SC slices compared with the ipsilateral slices. The results do not favour glutamate as the major optic nerve transmitter in the rabbit, but do not rule out glutamate as a transmitter in a minor population of retinal fibres.