Activation of neurosecretory cells by osmotic stimulation of anteroventral third ventricle.

Extracellular action potentials of paraventricular neurosecretory cells were obtained from urethan-anesthetized rats during microinjection of a hyper- or hypotonic solution into the anteroventral third ventricle (AV3V), third ventricle, or the vicinity of the recording site. The injection of a 0.2-microliter NaCl solution with a concentration range from 0.16 to 0.8 M into the AV3V excited most of the neurons tested. However, the magnitude of the response was dose related. In most cases, arterial blood pressure recorded simultaneously with the excitatory response of the neuron did not change after the injection. Distilled water (0.2 microliter) was injected into the AV3V after the intraperitoneal injection of a 1.5 M NaCl solution (1 ml), which decreased the firing rate in 67% of the neurons tested. The small volume (0.2 microliter) of hypertonic saline, which was effective when injected into the AV3V, excited none of the neurons tested when injected into the third ventricle or the vicinity of the recording site. These results strongly suggest that the AV3V is sensitive to the osmotic stimulus within a physiological range and that the region plays an important role as an osmoreceptor site for the osmoregulation of paraventricular neurosecretory cells.