Inhibition of stress-induced adrenocorticotropin and prolactin secretion mediating hypophysiotropic factors by antagonist of AMPA type glutamate receptor

Glutamate is the dominant excitatory neurotransmitter in a large number of physiological processes including neuroendocrine regulation. Some pharmacological studies have shown that different subtypes of glutamate receptor, such as the N-methyl-D-aspartic acid (NMDA) and alpha-amino-3-hydroxy-5-methy-4-isoxazolepropionic acid (AMPA) receptors, are involved in stress-induced adrenocorticotropin (ACTH) and prolactin secretion. However, the roles of the respective glutamate receptors and the mechanism of ACTH and prolactin secretion during stress via these receptors have not been investigated in detail. In the present study, we evaluated the role of AMPA-type glutamate receptor in ACTH and prolactin regulation under restraint stress in adult male rats. Male rats pretreated with a selective AMPA receptor antagonist, 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo(f)quinoxaline (NBQX; 50 microg), through a lateral ventricle cannula were stressed by immobilization. Administration of NBQX inhibited ACTH and prolactin secretion in response to restraint stress. However, NBQX had no significant effects on the activity of tyrosine hydroxylase (TH), the rate-limiting enzyme in dopamine biosynthesis, as measured by the accumulation of 3, 4-dihydroxyphenylalanine (DOPA). In addition, administration of NBQX suppressed stress-induced prolactin secretion in the male rats pretreated with alpha-MT, an inhibitor of dopamine synthesis, and infused with dopamine solution (2.5 microg/200 microl/10 min). These results indicated that the effects of NBQX on prolactin secretion might be mediated by non-dopamine mechanisms. The contents of corticotropin-releasing hormone (CRH) and arginine vasopressin (AVP) in the median eminence (ME) of the male rats decreased during restraint stress; however, the fluctuations in CRH and AVP were eliminated by NBQX administration. These results suggest that stress-induced ACTH and prolactin release mediated by neurotransmission via AMPA receptors might be partly attributable to hypophysiotropic regulatory factors in the hypothalamus.