ACTION OF VASOPRESSIN ON HYPOGLOSSAL MOTONEURONS OF THE RAT - PRESYNAPTIC AND POSTSYNAPTIC EFFECTS

The distribution of vasopressin binding sites in the hypoglossal nucle us of newborn rats was determined using autoradiography on film and a radioiodinated vasopressor antagonist. These sites predominated in the ventromedial and dorsal divisions of the nucleus. The effect of vasop ressin on hypoglossal neurones was studied in brainstem slices of newb orn animals, using the single-electrode voltage-clamp technique. Vasop ressin, at 0.1-0.5 mu M, generated a sustained inward current in a maj ority of neurones, an action which was mediated by V-1-type receptors. Antidromic activation or morphological characterization of biocytin-l abelled neurones indicate that part of the vasopressin-sensitive cells were motoneurones. When synaptic transmission was blocked by perfusin g the preparation with a low-calcium/high-magnesium solution, the aver age vasopressin current decreased by 65%; and following TTX treatment, the peptide current decreased by 55%. In contrast, in a low-calcium s olution, i.e., under conditions of reduced synaptic transmission but o f increased neuronal excitability, the vasopressin current was not sig nificantly altered. These results may be interpreted by assuming that the action of vasopressin is in part postsynaptic and in part presynap tic, the latter effect probably depending upon action potential propag ation. Current-voltage relations suggest that the postsynaptic effect of vasopressin was due to the induction of a non-inactivating inward c urrent, reversing in polarity at around -15 mV. The data raise the pos sibility that, in young animals, endogenous vasopressin may modulate t he activity of hypoglossal motoneurones.