Nitric oxide influences the release of histamine and glutamate in the rat hypothalamus

To investigate the influence of nitric oxide (NO) on the release of histamine and glutamate, the anterior hypothalamus of anaesthetized rats was superfused through a push-pull cannula either with artificial cerebrospinal fluid (CSF) or with various drugs dissolved in CSF. Hypothalamic superfusion with the NO-donating compounds linsidomine (200 mumol/l) or diethylamine-NO (DEANO, 100 mumol/l) led to a pronounced and sustained decrease in the histamine release rate, whereas the release rate of glutamate was enhanced. Superfusion with the inhibitor of NO synthase L-NG-nitro-L-arginine methyl ester (L-NAME, 200 mumol/l) increased the histamine release rate. The inhibitory effect of 200 mumol/l linsidomine was abolished by atropine (10 mumol/l). Superfusion with the glutamate receptor agonists glutamate (100 mumol/l) or N-methyl-D-aspartate (NMDA, 50 mumol/l) enhanced the histamine release rate. In the presence of linsidomine, the releasing effect of NMDA was not changed. These findings demonstrate that the release of histamine in the hypothalamus is diminished by endogenous NO. This effect of NO on histamine release seems to be due to enhanced release of acetylcholine from vicinal cholinergic neurons via stimulation of muscarinic acetylcholine receptors located presynaptically on histaminergic neurons. The NO-induced glutamate release seems to exert a subordinate stimulatory effect on histamine release. Finally, the inhibition of histamine release by NO is not due to blockade of NMDA receptors.