Vasopressin as a Neurotransmitter in the Central Nervous System: Some Evidence from the Supraoptic Neurosecretory System

Goldfish hypothalamic neuroendocrine cells have been investigated with intracellular recordings. The cells showed resting potentials of 50 mv and action potentials up to 117 mv followed by a long lasting and prominent diphasic hyperpolarizing afterpotential. The action potential occurred in two steps indicating sequential invasion. "Total" neuron (input) resistance was measured to be 3.3 x 107 Ω and total neuron time constant was 42 msec. Orthodromic volleys, produced by olfactory tract stimulation, generated graded excitatory postsynaptic potentials. These neuroendocrine cells seem, therefore, to have electrical membrane properties that are similar to those of other central neurons. Antidromic volleys (pituitary stimulation) produced inhibitory post-synaptic potentials whose latency was only slightly longer than that of the antidromic spike indicating the presence of recurrent collaterals. This finding suggests that the concept of the neuroendocrine cell as a neuron whose axon forms contacts only on blood vessels and not on other neurons or effector cells is too restrictive. Perfusion of the gills with dilute (0.3 per cent) sea water produced an inhibition of spontaneous activity. This inhibition is discussed in relation to recent work which demonstrates that goldfish hypothalamic hormones facilitate Na+ influx across the gill membrane.