Unit activity in the supraoptic and paraventricular nuclei — the effects of anaesthetics

Abstract

Summary Recordings were made from neurons in the supraoptic (SO) and paraventricular (PV) nuclei of the hypothalamus, which could be antidromically activated by stimulation of the neural lobe, and from a group of other hypothalamic units under anaesthesia with urethane, pentobarbitone, tribromoethanol, chloral hydrate or ethanol. No significant differences were seen between the firing rates of any of the groups of neurones under urethane, tribromoethanol or chloral hydrate. The firing rate of SO neurones but not PV neurones or other hypothalamic units was slower under ethanol anaesthesia than under urethane, tribromoethanol or chloral hydrate. This may have been due to the water-load given together with the ethanol. The axonal conduction velocity was faster under ethanol anaesthesia than chloral hydrate. The firing rates of each group of neurones were significantly lower under pentobarbitone anaesthesia than under the other anaesthetics, and the axonal conduction rate the antidromically activated units in the PV nucleus was also slower under pentobarbitone. Pentobarbitone thus appears to be a less suitable anaesthetic for studying SO and PV neurones than the other anaesthetics. The firing rates of all groups of neurones were similar under urethane anaesthesia which releases substantial quantities of vasopressin and tribromoethanol which does not. This implies that the effect of urethane on vasopressin release is at the neurohypophysial rather than the hypothalamic level and does not render it unsuitable for unit recording from SO and PV neurones.