Antagonism of the actions of glutamate by pentobarbitone or midazolam in the frog optic tectum in vitro

Plasticity in the tectum of Xenopus laevis: binocular maps

Xenopus frogs exhibit dramatic changes in the binocular projections to the tectum during a critical period of development. Their eyes change position in the head, moving from lateral to dorsal and creating an increasing region of binocular overlap. There is a corresponding shift of binocular projections to the tectum that keeps the two eyes' maps in register with each other throughout this period. The ipsilateral input is relayed via the nucleus isthmi. Two factors bring the ipsilateral projection into register with the contralateral projection. First, chemoaffinity cues establish a crude topographic map beginning when the shift of eye position begins. Approximately 1 month later, visual cues bring the ipsilateral map into register with the contralateral map. The role of visual input is demonstrated by the ability of the axons that bring the ipsilateral eye's map to the tectum to reorganize in response to a surgical rotation of one eye and to come into register with the contralateral eye's map. This plasticity can be blocked by NMDA receptor antagonists during the critical period. In normal adults, reorganization is minimal. Eye rotation fails to induce reorganization of the ipsilateral map. However, plasticity persists indefinitely in animals that are reared in the dark, and plasticity can be restored in normally-reared animals by treatment with NMDA. The working model to explain this plasticity posits that correlated input from the two eyes triggers opening of NMDA receptor channels and initiates events that stabilize appropriately-located isthmotectal connections. Specific tests of this model are discussed.

Long-term GABA treatment elicits supersensitivity of quisqualate-preferring metabotropic glutamate receptor in cultured rat cerebellar neurons

In primary cultures of rat cerebellar granule neurons, GABA treatment (50 microM, 7 days) caused a withdrawal supersensitivity selective for the metabotropic glutamate receptors that mainly prefer L-glutamate, quisqualate and, to a lesser extent, kainate. The withdrawal supersensitivity was absent when 10 microM SR-95531 was coadministered with GABA during the treatment period, an event that suggests the GABAA receptors primarily produced the GABA treatment effect. This was supported further by the inability of baclofen treatment to mimic completely the treatment effect of GABA. Withdrawal from 7 days of baclofen treatment only produced a slight increase in the metabotropic effect of L-glutamate and carbachol. In addition, in untreated neurons, baclofen had no acute effect, whereas GABA inhibited the effect of L-glutamate and carbachol. The inhibitory effect of GABA was reversed by SR-95531 and was absent in neurons treated with GABA. These observations suggest the involvement of GABAA receptors and the apparent development of tolerance to GABA, respectively. Also, dependence on GABA may have occurred; the metabotropic effects of glutamate, kainate, and quisqualate were not altered in neurons maintained with GABA treatment.

An electrophysiological study of the action of N-methyl-D-aspartate on excitatory synaptic transmission in the optic tectum of the frog in vitro

Excitatory synaptic field potentials, induced by stimulating optic nerve fibers, were recorded from in vitro preparations of the optic tectum of the frog. Bath-applied N-methyl-D-aspartate (NMDA), glutamate or quisqualate elicited transient enhancement in these field potentials, followed by a sustained depression reversible on washout. Responses to glutamate or quisqualate and the amplitude of control synaptic potentials, were not affected by the NMDA receptor antagonists aminophosphonovalerate (APV), 3(2-carboxypiperazin-4-yl)propyl-1-phosphonate (CPP), ketamine, magnesium ions or dizocipiline (MK 801) which, on the other hand, blocked the effects of NMDA. The antagonist dinitroquinoxaline-2,3-dione (DNQX), which is preferential for non-NMDA receptors, blocked the action of glutamate and synaptic transmission. In the presence of strychnine, glycine reversed the block of NMDA-mediated responses caused by magnesium. It is suggested that in the optic tectum of the frog, glutamate is the excitatory transmitter of at least one class of optic nerve fibers and that it acts through non-NMDA receptors. Although this area of the brain contains a well-developed NMDA receptor system, its function in physiological synaptic transmission remains to be elucidated.