Glutamate receptor antagonist infused into the hypothalamic suprachiasmatic nuclei interferes with the diurnal fluctuations in plasma prolactin and corticosterone levels and injected into the mesencephalic dorsal raphe nucleus attenuates the suckling stimulus-induced release of prolactin of the rat.

The aim of the present investigations was to examine the involvement of the rich glutamatergic innervation of the hypothalamic suprachiasmatic nucleus, a key structure in the control of circadian rhythms, in the regulation of the diurnal fluctuations in plasma prolactin and corticosterone, and to test the involvement of the glutamatergic innervation of the mesencephalic dorsal raphe nucleus in the prolactin response induced by the suckling stimulus. By means of a mini-pump a non-NMDA receptor antagonist (6-cyano-7- nitroquinoxaline-2,3-dione disodium, CNQX) was microinfused between the two suprachiasmatic nuclei for 3 days and on the third day blood samples were taken at different time points of the day. CNQX inhibited the afternoon rise in plasma prolactin and corticosterone. In lactating rats CNQX, similarly to the NMDA antagonist MK-801, microinjected into the dorsal raphe nucleus significantly attenuated the prolactin response to the suckling stimulus. The findings indicate that the glutamatergic innervation of the suprachiasmatic nuclei is involved in the diurnal fluctuations in plasma prolactin and corticosterone levels, and the glutamatergic innervation of the dorsal raphe nucleus has a prominent role in the mediation of the suckling stimulus to the hypothalamus.