SYNAPTICALLY RELEASED HISTAMINE INCREASES DYE-COUPLING AMONG VASOPRESSINERGIC NEURONS OF THE SUPRAOPTIC NUCLEUS - MEDIATION BY H-1 RECEPTORS AND CYCLIC-NUCLEOTIDES

Activating direct olfactory (glutamatergic) inputs to supraoptic nucle us (SON) neurons increases interneuronal coupling in slices from lacta ting but from not virgin or male rats. Studied here were influences on coupling of another monosynaptic input to SON, the histaminergic tube romammillary nucleus (TM) projection, activation of which selectively excites phasically firing (putative vasopressin) cells. Effects of TM stimulation and its possible downstream consequences on Lucifer yellow (LY) dye coupling among putative vasopressin cells were determined in male rat SONs. in unstimulated slices, 12 LY injections (1 cell/SON) yielded eight single and four pairs of coupled neurons. In slices in w hich TM was stimulated for 10 min at 10 Hz, 13 injections yielded 4 si ngle and 28 coupled cells, with groups of 2 to 4 cells coupled to the injected neuron, a threefold increase in the number of coupled cells p er injection (p < 0.02). Bathing slices in medium containing 10 mu M p yrilamine (H-1 antagonist) blocked this stimulation-induced coupling i ncrease, suggesting mediation by activation of guanylate cyclase-cGMP to which H-1 receptors often are linked. Bathing slices in medium cont aining 0.5-1 mM 8-bromo-cGMP yielded results similar to those of TM st imulation, a 2.5-fold increase over control in the number of coupled c ells per injection. Effects of TM stimulation on coupling also were bl ocked by bathing slices in a guanylate cyclase inhibitor (10 mu M LY83 583). in contrast to cGMP, 1 mM 8-bromo-cAMP significantly reduced cou pling. We conclude that synaptically released histamine increases coup ling via cGMP-dependent mechanisms.