Activity patterns and osmosensitivity of rat supraoptic neurones in perfused hypothalamic explants

Extracellular recordings were obtained in vitro from supraoptic neurones in an explant of rat hypothalamus maintained viable through intravascular perfusion with artificial media. The spontaneous activity observed from 64% of cells included continuously active neurones (mean frequency 3.2 +/- 0.8 Hz) located throughout the nucleus, and phasically active neurones (8.6 +/- 0.6 Hz) located predominantly in the caudal half of the nucleus. Supraoptic neurones displayed antidromic activation (latency range 6.5-18.2 ms) following stimulation of the pituitary stalk and orthodromic excitatory or inhibitory responses following stimulation in the anteroventral third ventricular (AV3V) region. Synaptic responses were reversibly abolished during perfusion with media containing 12 mM-Mg2+. A 15-40 mosmol/l increase in the osmotic pressure of the perfusion media, through addition of NaCl, sucrose or mannitol, prompted nineteen of twenty-three cells to increase their discharge frequency. The patterns of response included the induction or simple increase in continuous firing frequency, the induction or enhancement of phasic or bursting activity and a change from continuous to phasic activity. Similar responses to an osmotic stimulus were obtained from thirteen of twenty-one supraoptic neurones, including two phasic neurones, where synaptic activity had been abolished during perfusion with 12 mM-Mg2+. Osmosensitivity appeared to be selective for supraoptic cells; no significant change in firing frequency was observed from any of six cells recorded in the lateral hypothalamus or thirteen cells recorded in the medial hypothalamus during exposure to a change in osmotic pressure of +20 to +40 mosmol/l, using either control media (seven cells) or media containing 12 mM-Mg2+ (twelve cells). These observations indicate that supraoptic neurones maintained in vitro can display spontaneous, antidromie , orthodromic and osmotically induced activity patterns identical to those observed with in vivo recordings. The persistence of a reduced osmosensitivity among supraoptic neurones in the absence of synaptic transmission indicates that although these cells can function as osmoreceptors, their osmosensitivity may be enhanced through synaptic input from adjacent neurones, possibly located in the AV3V area. The presence of phasic activity among supraoptic neurones maintained in media where synaptic transmission has been abolished suggests that the mechanisms responsible for such activity patterns are endogenous membrane properties of a subpopulation of supraoptic neurones.