Responses to Osmotic Stimulation of Presumed Vasopressin and OxytocinNeurons in the Supraoptic Nucleus of Ovariectomized Rats

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This study re-evaluated the osmosensitivity of the supraoptic (SON) vasopressin (AVP) and oxytocin (OXT) neurons in rats in vitro. Brain slice preparations made from ovariectomized (OVX) rats were incubated in 300 mOsm/kg H_2O artificial cerebrospinal fluid (ACSF) with low-Ca^<2+> (0.2 mM), and high-Mg^<2+> (5.0 mM), to block neuronal activation by way of synaptic inputs. Identified in this study by their discharge patterns according to Poulain and Wakerely were AVP neurons which had a phasic pattern, and OXT neurons, which showed a tonic pattern. In the phasic SON neurons (n=29,1.74±0.34 Hz, mean±SE), a significant (P<0.0005) increase in the mean discharge rate (5.59±0.77 Hz) was detected in the hyperosmolar condition (340mOsm/kg H_2O). In contrast to the phasic neurons, the SON neurons (n=14) with a continuous discharge pattern (7.25±1.11 Hz), did not show any significant change in the mean discharge rate in the same hyperosmolar condition (7.27±1.07 Hz). These results suggest that the OXT neuron but not the AVP neuron is much less sensitive to increased osmolality in OVX rats.
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