On GABAergic mechanisms in the optokinetic nystagmus of the frog: effects of bicuculline, allylglycine and SR 95103, a new GABA antagonist

The commissural transfer of the horizontal optokinetic signal in the rat: a c-Fos study

We applied the Fos method in rats subjected to horizontal optokinetic stimulation (OKS) to study whether optokinetic information is transferred through the commissural pretectal fibres from one optic tract nucleus (NOT) to the opposite. In binocular as well as in monocular nasalward OKS, the highest Fos immunoreactivity was found in the NOT contralateral to the nasalward stimulation, as expression of the activation either of direction-selective cells and of commissural neurons. Even the opposite NOT showed many Fos-positive cells activated by the opposite nucleus throughout the commissural pretectal pathway. They might be the GABA positive cells, which are thought to allow the activation in one nucleus to be transformed into inhibition of the opposite side. In monocular temporalward OKS, the inhibition on direction-selective cells and the consequent silencing of commissural neurons caused the faint immunoreactivity in the NOT contralateral to eye stimulated. In the opposite nucleus the few Fos-positive cells emerged as a consequence of the lack of the normal tonic commissurally mediated inhibition.

Optokinetic deficits in albino ferrets (Mustela putorius furo): a behavioral and electrophysiological study

We compared the horizontal optokinetic reaction (OKR) and response properties of retinal slip neurons in the nucleus of the optic tract and dorsal terminal nucleus (NOT-DTN) of albino and wild-type ferrets (Mustela putorius furo). In contrast to pigmented ferrets, we were unable to observe OKR in albino ferrets during binocular and monocular viewing using random dot full field stimulation and electro-oculography (EOG). Observations during early postnatal life indicate that regular OKR is present in pigmented pups 3 d after eye opening but is absent at any stage during development in albino ferrets. Unilateral muscimol injections to inactivate all neurons in the NOT-DTN containing GABA(A) and GABA(C) receptors caused spontaneous horizontal nystagmus with slow phases away from the injected hemisphere in albino as well as in pigmented animals. Retinal slip neurons in the NOT-DTN of albino ferrets identified by antidromic activation from the inferior olive and orthodromic activation from the optic chiasm were well responding to intermittent bright light stimuli, but many showed a profound reduction of responsiveness to moving stimuli. The movement-sensitive neurons exhibited no clear direction selectivity for ipsiversive stimulus movement, a characteristic property of these neurons in pigmented ferrets and other mammals. Thus, the defect rendering albino ferrets optokinetically nonresponsive is located in the visual pathway subserving the OKR, namely in or before the NOT-DTN, and not in oculomotor centers.

Temporal contrast enhancement via GABAC feedback at bipolar terminals in the tiger salamander retina

Most retinal amacrine (ACs) and ganglion cells (GCs) express temporal contrast by generating action potentials at only the onset and offset of the light stimulus. This study investigated the neural mechanisms that underlie this temporal contrast enhancement. Whole cell patch recordings were made from bipolar cells (BCs), ACs, and GCs in the retinal slice preparation. The cells were identified by the locations of their somas in the inner nuclear layer and ganglion cell layers, their characteristic light responses, and morphology revealed by Lucifer yellow staining. Depolarizing a single BC with a brief voltage pulse elicited a Cl- tail current that was completely abolished when Ca2+ entry to bipolar terminals was prevented, by either removing Ca2+ from the Ringer solution or blocking Ca2+ channels with Co2+. This suggests that the Cl- current is Ca2+-dependent. In those bipolar cells whose axon terminals were cutoff during slicing no Cl- current was observed, indicating that this current is generated at the synaptic terminals. The Cl- current consists of a predominant synaptic component that can be blocked by the non-N-methyl--aspartate (NMDA) glutamate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or by the gamma-aminobutyric acid-C (GABAC) receptor antagonist picrotoxin. There also exists a relatively small nonsynaptic component. Thus both glutamatergic and GABAergic transmission were involved in the generation of this Cl- current, suggesting that it is mediated by a recurrent feedback to bipolar cells. Picrotoxin, which blocks both GABAC receptors at BC terminals and GABAA receptors on the dendrites of ACs and GCs, converted the light-elicited voltage response in most - ACs and GCs from transient to sustained. Bicuculline, which blocks only the GABAA receptors, did not prolong the transient response in - ACs and GCs. This suggests that a negative feedback mediated by the GABAC receptor on the bipolar terminals is responsible for making these responses transient. After the GABAergic feedback was blocked with picrotoxin the light-elicited voltage responses (recorded under current clamp) were more sustained than the current responses (recorded under voltage clamp) to the same light stimuli. This suggests that a voltage-dependent conductance converts the relatively transient current responses to more sustained voltage responses. Our results imply a synaptically driven local GABAergic feedback at bipolar terminals, mediated by GABAC receptors. This feedback appears to be a significant component of the mechanism underlying temporal contrast enhancement in - ACs and GCs.

GABAA receptors in neurons of the nerve fiber layer in rabbit retina

Two synapse-rich layers are well recognized in the mammalian retina, the inner and outer plexiform layers. However, synapses occur also in other layers, particularly in the innermost nerve fiber layer. These synapses form a tenuous layer at times referred to as the superficial plexiform layer. We have found that staining for GABAA receptors in whole-mounted rabbit retina demonstrates this layer. It is most well developed in the region of the visual streak 2-4 mm below the center of the myelinated streak and is very sparse in other parts. Most or all of the processes in the plexus originate from cells in the ganglion cell layer.

N-methyl-D-aspartate antagonists suppress the development of frog symmetric monocular optokynetic nystagmus observed after unilateral visual deprivation

In monocular vision, frogs display a unidirectional optokinetic nystagmus (OKN), reacting only to temporal-nasal (T-N) stimulation. The OKN N-T component is almost absent. However, prolonged monocular visual deprivation by unilateral eyelid suture provoked the appearance of the N-T component. The analysis of search coil recordings showed that the slow phase velocity gain of both T-N and N-T components became similar. Chronic administration of N-methyl-D-aspartate (NMDA) antagonists for the duration of deprivation prevented the appearance of a symmetrical monocular OKN in frogs: following repeated intraperitoneal injections of either MK 801, CGS 19755 or intrapretectal microinjections of 2-amino-5-phosphonovalerate (APV), the N-T component did not appear, and OKN remained asymmetrical. Thus NMDA receptors appear to be involved in the control of the plasticity process which allows monocular OKN of adult lower vertebrates to become symmetrical.

Pharmacological study of the chicken's monocular optokinetic nystagmus: effects of GABAergic agonist and antagonists

When injected into the chicken open eye, the GABA-agonist THIP and the GABA-antagonists bicuculline and picrotoxin induced spontaneous eye movements in nasal-temporal (N-T) and in temporal-nasal (T-N) direction, respectively. These spontaneous movements were scarcely modulated by optokinetic stimulation, irrespective of the direction of stimulation. It is suggested that they are due to the suppression of directional selectivity of retinal ganglion cells. When injected into the closed eye, GABAergic drugs did not produce spontaneous nystagmus. THIP provoked a reduction of the N-T component, without modifying the T-N one, while GABA antagonists induced a significant increase in OKN performance, especially for the N-T direction of stimulation. In these conditions, picrotoxin also provoked an increase in the duration of both components of optokinetic after nystagmus, indicating a direct effect of the drug upon the velocity-storage system.

Pharmacological study of the chicken's monocular optokinetic nystagmus: involvement of the ON retinal channel evidenced by the glutamatergic separation of ON and OFF pathways

The chicken's monocular optokinetic nystagmus (OKN), recorded by the magnetic search coil technique, displays a directional asymmetry, the temporal-nasal (T-N) stimulation being more efficient than the nasal-temporal (N-T) one to evoke the reflex. The intravitreal administration of APB, a glutamate agonist which selectively blocks the ON retinal channel strongly reduced the eye monocular OKN; it also induced spontaneous eye movements in the T-N direction. The intravitreal injection of PDA another glutamate analog, which reduces the OFF channel, while increasing the activity of the ON channel, induced a large increase in OKN velocity gain, especially for a N-T stimulation at the lowest drum speeds. These results indicate the main involvement of the ON retinal channel in the OKN genesis and the inhibitory effect of the OFF channel upon this oculomotor reflex.

Plasticity of the frog monocular OKN: involvement of pretectal GABAergic and cholinergic systems

The frog horizontal monocular optokinetic nystagmus (OKN) is asymmetrical, the temporal-nasal (T-N) stimulation being the sole stimulation efficient to evoke the reflex, the nasal-temporal (N-T) component being almost absent. Coil recordings showed that, in adult animals, prolonged monocular visual deprivation by unilateral eyelid suture provoked the appearance of the N-T component. The OKN became symmetrical, reacting for both directions of stimulation. Microinjection of either gamma-aminobutyric acid (GABAA) agonist 4,5,6,7-tetrahydroisoxazolo (5,4-C) Pyridin-3-ol (THIP) or muscarinic cholinergic antagonist atropine into the nucleus lentiformis mesencephali, the pretectal mesencephalic structure involved in OKN, transiently abolished the presence of N-T component. This result suggests that the phenomenon of visual plasticity, occurring after a week of monocular deprivation, can be due, at least partially, to reduction in pretectal GABAergic inhibition, and to concomitant activation of cholinergic muscarinic receptors.

Directional asymmetry of the frog monocular optokinetic nystagmus: cholinergic modulation

The frog monocular optokinetic gaze nystagmus (OKN) was studied by coil recordings after intravitreal administration of cholinergic drugs into the closed eye. Before injection, the frog displayed OKN for stimulations in the temporo-nasal (T-N) direction only. The injection of muscarinic agonists, as well as that of nicotinic antagonists, provoked the appearance of a naso-temporal (N-T) component, the slow phase velocity gain then being strongly and significantly increased. The abolition of the OKN directional asymmetry indicates that acetylcholine seems to act in opposite ways through muscarinic and nicotinic binding sites. The GABAergic and cholinergic systems may interact to generate and modulate OKN in the frog.

Distribution of GABA immunoreactive systems in the forebrain and midbrain of the chameleon

An immunocytochemical method, using glutaraldehyde fixation and an antiserum developed against a GABA-glutaraldehyde protein conjugate, permitted direct visualization of GABAergic structures in the brain of a reptile (chameleon). GABA immunoreactive cell bodies and nerve terminals were observed to be evenly distributed throughout the forebrain and midbrain. In the forebrain, GABA-positive perikarya were shown in all cortical areas, the septal area, the striatum, the dorsal ventricular ridge, and in the nucleus accumbens. In the midbrain, the optic tectum contained a dense and laminar distribution of GABA neurons. These neurons were also observed in the lateral geniculate nucleus, nucleus profundus mesencephali, nucleus opticus tegmenti and substantia nigra. Immunoreactive nerve fibers and terminals were observed in the same structures and, additionally, in the tractus septo-hypothalamicus, habenula complex, median eminence, intermediate lobe of the pituitary, basal part of the subcommissural organ, torus semicircularis and nucleus reticularis isthmi. These results provide a framework for a further electron microscopic analysis of the GABAergic innervation of some encephalic areas involved in physiological regulations particular to this species especially the visual system.

Abolition of monocular optokinetic nystagmus directional asymmetry after unilateral visual deprivation in adult vertebrates: involvement of the GABAergic mechanism

In lower vertebrates such as frogs and chickens, monocular optokinetic nystagmus (OKN) displays directional asymmetry, temporal-nasal (T-N) stimulation being more efficient in evoking this visuomotor reflex than N-T stimulation. The N-T component of monocular OKN is significantly weaker in chickens, while it is almost absent in frogs. Coil recordings showed that in adult frogs and chickens, prolonged monocular visual deprivation by unilateral eyelid suture provoked the appearance of the N-T component in frogs as well as its significant and progressive increase in both species. The administration of THIP, a GABAA agonist, abolished reversibly the increase of the N-T component in both species. This fact suggests that the GABAergic system could be involved in determining this plasticity process observed in adult lower vertebrates.

Directional asymmetry of the horizontal monocular head and eye optokinetic nystagmus: effects of picrotoxin

Frog monocular eye and head optokinetic nystagmus (OKN) were studied by coil recordings after intravitreal administration of picrotoxin into the closed eye. Before injection, the frog displayed an OKN only for stimulations in the temporo-nasal (T-N) direction. The injection of picrotoxin provoked the appearance of a N-T component of the head and eye OKN: the slow phase velocity gain and the resetting fast phase frequency were strongly and significantly increased. Thus, picrotoxin abolished the directional asymmetry of head and eye OKN, indicating the involvement of GABAergic mechanisms in the inhibition of the N-T component of the monocular eye and head OKN. Picrotoxin administration had an additional effect on the monocular head OKN only, the performances (measured by the velocity gain and the frequency of resetting fast phases) were markedly increased for both directions of stimulation, suggesting an effect of the drug upon the motor output of head movements.

GAD immunoreactivity in pretectal and accessory optic nuclei of the frog mesencephalon

By an immunocytochemical technique using anti-GAD antibodies, we found a high density of GAD-immunoreactive puncta in the 5 pretectal nuclei and in the nucleus of the basal optic root. These results back up the suggestion that GABAergic modulation of the output of the visual pretectal relay nuclei might underlie the directional asymmetry of the horizontal optokinetic nystagmus and the directional selectivity of pretectal neurons.

On cholinergic mechanisms in the optokinetic nystagmus of the frog: antagonistic effects of muscarinic and nicotinic systems

In a monocular situation, an intravitreal injection of acetylcholine (ACh) agonists (especially muscarinic agonists like muscarine or oxotremorine) provoked both the suppression of the optokinetic nystagmus (OKN) related to the injected eye, and the appearance of a nasal-temporal (N-T) component in the OKN triggered by the contralateral non-injected eye. These two effects were added in a binocular condition. Similar results were obtained with ACh nicotinic antagonists (D-tubocurarine, alpha-bungarotoxin, hexamethonium and gallamine). ACh muscarinic antagonists (atropine, scopolamine) had only moderate effects and failed to provoke an N-T component in the contralateral OKN. These data show that ACh mechanisms are involved in the control of the frog OKN, especially in directional asymmetry, nicotinic and muscarinic systems acting in an antagonistic way.

Anisotropic inhibition in the receptive field surround of the frog retinal ganglion cells, evidenced by bicuculline and SR 95103, a new GABA antagonist

When GABA antagonists (picrotoxin, bicuculline methiodide and SR 95103) were intravitreally injected in the frog, they increased the number of spikes of transient retinal ganglion cells, as well as the duration of the response. Thus, the transient pattern of the response became more sustained. GABA antagonists also provoked a marked increase in the size of the receptive field, which might be due to the abolition of the inhibition exerted by the surround upon the centre of the field. In fact, a stimulus applied to the surround of the field simultaneously with one applied to the centre no longer provoked the reduction of the field area nor that of the number of spikes. These are effects which were always observed before drug injection. After picrotoxin injection, the enlarged field was concentric with the initial one, both angular diameters doubled, whereas after bicuculline or SR 95103, the enlarged field was not concentric with the initial one and only one diameter increased. Thus, GABA inhibition appears to be distributed according to an anisotropic spatial pattern. Whether this anisotropy might be an input for direction selectivity in the frog visual system is a topic of discussion. With respect to SR 95103, this compound proved to act like a selective GABA antagonist with long lasting effects.