GABAergic projection from the arcuate nucleus to the supraoptic nucleus in the rat.

Electrical stimulation of the neurones in the hypothalamic arcuate nucleus results in a transient inhibition followed by a marked post-stimulus excitation of magnocellular neurones of the supraoptic nucleus. Microdialysis administration of the gamma-aminobutyric acid agonist (GABA(A)), muscimol, directly into the supraoptic nucleus inhibited both oxytocin and vasopressin neurones and these actions were fully reversed by the GABA(A) antagonist bicuculline. In addition, bicuculline administration blocked the inhibition induced by arcuate stimulation, but had no effect on the post-stimulus excitation. Thus, part of the inhibitory pathway arising from or passing through the arcuate nucleus to the supraoptic nucleus is mediated by the neurotransmitter GABA. However, the post-inhibitory excitation induced by arcuate stimulation is not a rebound response, but appears to involve an independent excitatory pathway.