Blandine Jardon

Glutamatergic separation of ON and OFF retinal channels: possible modulation by glycine and acetylcholine

When intravitreally injected into the frog in vivo, 2-amino-4-phosphonobutyrate (APB) and cis-2,3-piperidine dicarboxylic acid (PDA) showed opposite effects on ON and OFF retinal channels: APB abolished the ON responses in the electroretinogram and in ganglion cell activity, and increased OFF responses. At the same time the receptive field area was enlarged, and the inhibition exerted by the surround was suppressed. A cholinergic/glycinergic loop involving amacrine cells was suggested to be the pathway of the inhibitory ON input upon the OFF channel. PDA abolished the OFF responses in the ERG and in ganglion cell activity, while increasing the ON response in the ERG and decreasing the ganglion cell sensitivity at ON. The receptive field area was not modified, but the inhibition exerted by the surround was suppressed, probably by a blockade of horizontal cell glutamate receptors.

On gabaergic mechanisms in the optokinetic nystagmus of the frog: Effects of bicuculline, allyglycine and SR 95103, a new GABA antagonist

In a monocular situation, an intravitreal injection of the GABA antagonists, bicuculline or SR 95103 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 (this N-T component being absent in control OKN). These two effects were added in a binocular condition. Similar results were obtained with L-C allylglycine which reduces the endogenous GABA level, but these effects were delayed when compared to those of GABA antagonists. All these data are roughly analogous to those previously obtained with picrotoxin (a non-competitive GABA antagonist) and thus confirm that GABA mechanisms are involved in the control of the frog OKN. Furthermore, SR 95103 acted in this model as a potent selective GABA antagonist, as has been demonstrated in another system.

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.

Unilateral pretectal microinjections of SR 95 531,a GABA A antagonist: effects on directional asymmetry of frog monocular OKN

SummaryMonocular eye movements have been studied in frogs using the search coil technique before and after unilateral microinjection of SR 95 531, a GABA A antagonist, into the pretectal nuclei contralateral to the open eye. Before injection, monocular, horizontal optokinetic nystagmus (OKN) in frogs, as in other lower vertebrates, displays a directional asymmetry: the stimulation in the T-N (temporo-nasal) direction is more efficient in evoking OKN than is stimulation in the N-T (naso-temporal) direction The N-T component is almost absent and displays only slow phases of very low speed. Unilateral SR 95 531 microinjection into the pretectum reversibly decreased the directional asymmetry of monocular horizontal OKN, by strongly increasing the N-T component slow phase velocity while the T-N slow phase velocity remained unchanged. These data show that SR 95 531 injected into the pretectum contralateral to the open eye reversibly decreased the inhibition upon the N-T component of monocular horizontal OKN, which suggests that a pretectal GABAergic system is involved in the directional asymmetry of monocular horizontal OKN in frogs.

Possible involvement of cholinergic and glycinergic amacrine cells in the inhibition exerted by the ON retinal channel on the OFF retinal channel

In the frog retina, the inhibition exerted by the ON channel on the OFF channel was evidenced by the increase in transient ganglion cell OFF responses, when the ON channel was blocked by 2-amino-4-phosphonobutyrate (APB). Intraocular administration of the neurotoxic choline analog ethylcholine mustard arizidinium ion (ECMA) also provoked an increase in the number of spikes of transient ganglion cell OFF responses, without suppressing the ON responses. APB, when administrated after ECMA, abolished the ON responses, but did not modify the OFF responses already increased by ECMA. Neurons located in the inner part of the inner nuclear layer were histologically altered by the toxin, and choline acetyltransferase activity was significantly depressed in ECMA-treated retinas. A double immunostaining experiment showed that amacrine cells containing glycine bear muscarinic binding sites. These results confirm the participation of cholinergic neurons in the inhibition exerted by the ON retinal channel on the OFF retinal channel, and suggest the involvement of a cholinergic/glycinergic loop of amacrine cells in this mechanism.

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.

Involvement of ON and OFF retinal channels in the eye and head horizontal optokinetic nystagmus of the frog

The specific role of ON and OFF retinal information channels in the generation of the horizontal optokinetic nystagmus (OKN) of the frog was studied. Coil recordings of monocular eye and head OKN were obtained before and after intravitreal injection of two drugs that block either ON or OFF channels. The intravitreal injection of 2-amino-4-phosphonobutyrate (APB), a glutamate analog that selectively blocks the ON retinal channel, strongly reduced or even cancelled the monocular OKN of the head and of the eye. The intravitreal injection of another glutamate analog, the cis-2,3-piperidine dicarboxylic acid (PDA) that especially blocks the OFF retinal channel, did not affect the gain velocity of the slow phase of both the horizontal monocular head and eye OKN, for low stimulus velocities. Our results suggest that the retinal ON information channel, but not the OFF channel, is involved in the generation of the slow phase of the OKN of the frog, at least at low drum velocities.

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 chickens 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.

Effects on the chicken monocular OKN of unilateral microinjections of GABAA antagonist into the mesencephalic structures responsible for OKN.

SummaryThe SR 95531, a GABAA antagonist was microinjected into either the pretectum nuclei, (nucleus Superficialis Synencephali nSS) or the nBOR (nucleus Ectomammillaris nEM) of chickens. Monocular optokinetic nystamus (OKN) of each eye was recorded by the search coil technique before and after unilateral intracerebral drug administration. Before injection, monocular horizontal OKN in chickens, as in other lower vertebrates, displays a directional asymmetry: the stimulation in the Temporo-Nasal (T-N) direction is more efficient in evoking OKN than is stimulation in the Naso-Temporal (N-T) direction. Unilateral microinjections of SR 95531 into either the nSS or nEM induce a reversible increase of gain in OKN directed by the contralateral eye for both directions of stimulation. However administration into the nSS increased directional asymmetry by increasing the T-N component slow phase velocity more strongly than the N-T component slow phase velocity. On the other hand, the unilateral administration of the drug into the nEM suppressed the directional OKN asymmetry by increasing the N-T component slow phase velocity more strongly than the T-N component slow phase velocity. These results indicate that the drug suppresses GABAergic inhibition at the mesencephalic level. Moreover the nSS seems especially involved in monocular OKN in response to a T-N stimulation, while the nEM seems more involved in the OKN response to N-T stimulation. The increase in gain of OKN directed by the ipsilateral eye to microinjected nuclei could account for the strong interactions existing between these mesencephalic structures responsible for horizontal OKN.

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.