NITRIC-OXIDE INHIBITS THE RELEASE OF NOREPINEPHRINE AND DOPAMINE FROMTHE MEDIAL BASAL HYPOTHALAMUS OF THE RAT

Previous research indicates that norepinephrine and dopamine stimulate release of luteinizing hormone (LH)-releasing hormone (LHRH), which t hen reaches the adenohypophysis via the hypophyseal portal vessels to release LH. Norepinephrine exerts its effect via alpha(1)-adrenergic r eceptors, which stimulate the release of nitric oxide (NO) from nitric oxidergic (NOergic) neurons in the medial basal hypothalamus (MBH). Th e NO activates guanylate cyclase and cyclooxygenase, thereby inducing release of LHRH into the hypophyseal portal vessels. We tested the hyp othesis that these two catecholamines modulate NO release by local fee dback. MBH explants were incubated in the presence of sodium nitroprus side (NP), a releaser of NO, and the effect on release of catecholamin es was determined. NP inhibited release of norepinephrine. Basal relea se was increased by incubation of the tissue with the NO scavenger hem oglobin (20 mu g/ml). Hemoglobin also blocked the inhibitory effect of NP. In the presence of high-potassium (40 mM) medium to depolarize ce ll membranes, norepinephrine release was increased by a factor of 3, a nd this was significantly inhibited by NP. Hemoglobin again produced a further increase in norepinephrine release and also blocked the actio n of NP. When constitutive NO synthase was inhibited by the competitiv e inhibitor N-G-monomethyl-L-arginine (NMMA) at 300 mu M, basal releas e of norepinephrine was increased, as was potassium-evoked release, an d this was associated in the latter instance with a decrease in tissue concentration, presumably because synthesis did not keep up with the increased release in the presence of NMMA. The results were very simil ar with dopamine, except that reduction of potassium-evoked dopamine r elease by NP was not significant, However, the increase following incu bation with hemoglobin was significant, and hemoglobin, when incubated with NP, caused a significant elevation in dopamine release above tha t with NP alone. In this case, NP increased tissue concentration of do pamine along with inhibiting release, suggesting that synthesis contin ued, thereby raising the tissue concentration in the face of diminishe d release. When the tissue was incubated with NP plus hemoglobin, whic h caused an increase in release above that obtained with NP alone, the tissue concentration decreased significantly compared with that in th e absence of hemoglobin, indicating that, with increased release, rele ase exceeded synthesis, causing a fall in tissue concentration. When N O synthase was blocked by NMMA, the release of dopamine, under either basal or potassium-evoked conditions, was increased. Again, in the lat ter instance the tissue concentration declined significantly, presumab ly because synthesis did not match release. Therefore, the results wer e very similar with both catecholamines and indicate that NO acts to s uppress release of both amines. Since both catecholamines activate the release of LHRH, the inhibition of their release by NO serves as an u ltra-short-loop negative feedback by which NO inhibits the release of the catecholamines, thereby reducing the activation of the NOergic neu rons and decreasing the release of LHRH. This may be an important mean s for terminating the pulses of release of LHRH, which generate the pu lsatile release of LH that stimulates gonadal function in both male an d female mammals.