ELECTROPHYSIOLOGICAL AND NEUROCHEMICAL STUDY OF THE RAT GENICULOCORTICAL PATHWAY - EVIDENCE FOR GLUTAMATERGIC NEUROTRANSMISSION

The projection from the dorsal lateral geniculate nucleus to the prima ry visual cortex of the rat was studied electrophysiologically. Electr ical stimulation of the dorsal lateral geniculate nucleus and the opti c tract produced three types of responses on neurons of area 17: excit ation followed by inhibition, excitation and inhibition. These results extend and confirm, in adult rats, previous studies done in rat genic ulate-visual cortex cocultures preparations in vitro. The role of glut amate in the neurotransmission of the rat geniculo-cortical pathway wa s also investigated. In a first set of experiments, the effects of kyn urenate, an antagonist of glutamate receptors, on visual cortex neuron s with a monosynaptic excitatory response to dorsal lateral geniculate nucleus stimulation were studied. Microiontophoresis of kynurenate in area 17 neurons selectively suppressed the excitatory response to dor sal lateral geniculate nucleus and optic tract stimulation. In a secon d set of experiments, the effects of electrical stimulation of the dor sal lateral geniculate nucleus and the optic tract on the release of a mino acids in the rat visual cortex in vivo were studied. Using the pu sh-pull method, we perfused a discrete region of the visual cortex wit h artificial cerebrospinal fluid (CSF), and the amino acid content of the perfusates was analysed by high performance liquid chromatography (HPLC). Stimulation of either the dorsal lateral geniculate nucleus or the optic tract significantly increased glutamate release in area 17. The rest of the amino acids studied did not show significant changes. The results provide evidence for the participation of glutamate in th e neurotransmission of the geniculo-cortical pathway in the rat.