This study investigates the possibility of glutamate and/or aspartate as an excitatory transmitter in the retinohypothalamic fibers' terminal within the anterior hypothalamus, including the suprachiasmatic nucleus (SCN). We studied the release of H-Glu and 3H-Asp from the SCN of rat hypothalamic slices produced by optic nerve stimulation. Stimulation of the optic nerve released 3H-Glu, 3H-Asp but not 3H-GABA, while stimulation of the SCN released all three. These releases were suppressed in both Ca2+ free Krebs and tetrodotoxin (TTX) 1 μM containing Krebs. The present result with previous electrophysiological observations strongly suggest that glutamate and/or aspartate are included in this retinohypothalamic termination, while GABA is included in intrinsic and/or extrinsic neurons of the SCN, excluding the terminal of the retinohypothalamic fibers.
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