The existence of bilateral neural connections between the two SON was suggested by electrophysiological and in vivo studies, thus supporting our results that both SON are involved in the mediation of the cardiovascular response to the microinjection of carbachol into the BST. Takano Adriamycin order et al. (1990) reported that one-third of the vasopressin-containing neurons tested in the SON were excited by electric stimulation

of the contralateral SON. In the same study, those authors reported that vasopressin neurons tested in the SON were not antidromically activated by a contralateral SON stimulation, thus suggesting that neural connections between the bilateral SON are mainly polysynaptic. It was also reported that antidiuretic effect associated with noradrenaline microinjection into the SON was inhibited either by a lesion of the contralateral SON or its pretreatment

with adrenoceptor antagonists (Tsushima et al., 1996), indicating the existence of bilateral adrenergic neural connections between selleck chemical supraoptic nuclei. Because the pressor response to the microinjection of carbachol into the BST was inhibited by the blockade of either the ipsilateral or the contralateral SON, it is possible that carbachol administration into the BST activates a pathway from the BST to the ipsilateral SON, in relation to BST microinjection site, which would stimulate neuron(s) that project to contralateral SON, thus suggesting that carbachol responses would depend on a bilateral SON cross-talking. Therefore, activation of vasopressinergic neurons in the contralateral SON in relation to BST stimulation site would mediate pressor response to carbachol administration into the BST. A schematic representation sketching the mechanism by which carbachol microinjection

into the BST evokes a vasopressin-mediated pressor response is presented next in Fig. 9. The pathway for the neural connection between bilateral SON is not totally understood. Moos and Richard (1989) concluded that the supraventricular gray commissura is important for interconnection of oxytocin-containing neurons in the SON, because synchronization of oxytocin-containing neurons in the bilateral SON disappeared after an inter hemisphere sectioning (including the supraventricular gray commissura and the corpus callosum), but persisted after a superficial interhemisphere sectioning that was limited to the corpus callosum. Therefore, the supraventricular gray commissura is a possible pathway for interconnections between bilateral SON vasopressin-containing neurons. Also, other connections between bilateral supraoptic nuclei, through the medulla oblongata and pons, have been suggested to exist (Tsushima et al., 1996), thus indicating alternative pathways for a bilateral SON cross-talking.