Glutamate enhances the adenylyl cyclase-cAMP system-induced beta-endorphin secretion and POMC mRNA expression in rat hypothalamic neurons in culture: NMDA receptor-mediated modulation

Abstract l -Glutamate, a major excitatory amino acid of the central nervous system, plays important roles as neurotransmitter and neuromodulator in the brain. Increasing evidence suggests that glutamate may also involve in the regulation of the neuroendocrine system at the hypothalamus. Employing long term monolayer hypothalamic cell cultures prepared from neonatal rats, we reported here that whereas glutamate significantly enhanced forskolin-, or N 6 ,2′-O-dibutyrrladenosine-3′5′-cyclic monophosphate [(Bu) 2 cAMP]-stimulated immunoreactive (ir)-β EP release form cultures treated daily for 4 consecutive days, the excitatory amino acid alone produced little effect. This potentiation of glutamate was time-related and dose-dependent with an E max value of the amino acid being approximately 50 μM; at this concentration glutamate augmented ir-β EP secretion about 1.8 times ( P ) that induced by 2 μM forskolin alone. Similar effects were also observed for POMC and mRNA levels in cultures subjected to 6 h of the above treatment regime. This potentiating effect of glutamate appears to be mediated specifically through NMDA receptor as it can be mimicked by NMDA but not by kainic acid or quisqualic acid, and blocked by the NMDA receptor antagonist 2-amino-5-phosphonovalerate (APV), but not by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a non-NMDA glutamate receptor antagonist. Interestingly, glutamate was found not to enhance high doses of forskolin (10 μM) or (Bu) 2 cAMP (100 μM) stimulated β EP release and POMC mRNA levels in hypothalamic cell cultures. These novel findings suggests that (i) hypothalamic β EP neurons are subjected to direct glutamatergic modulation, and (ii) the modulating effect of glutamate is mediated, at least in part, through the adenylyl cyclases-cAMP system. Our novel findings have provided a direct evidence supporting the assertion that cortical glutamatergical system may directly involve in the functional modulation of hypothalamic β EP producing neurons.