VASOPRESSIN REGULARIZES THE PHASIC FIRING PATTERN OF RAT HYPOTHALAMICMAGNOCELLULAR VASOPRESSIN NEURONS

Vasopressin (AVP) magnocellular neurons of hypothalamic nuclei express specific phasic firing (successive periods of activity and silence), which conditions the mode of neurohypophyseal vasopression release, In situations favoring plasmatic secretion of AVP, the hormone is also r eleased at the somatodendritic level, at which it is believed to modul ate the activity of AVP neurons. We investigated the nature of this au tocontrol by testing the effects of juxtamembrane applications of AVP on the extracellular activity of presumed AVP neurons in paraventricul ar and supraoptic nuclei of anesthetized rats. AVP had three effects d epending on the initial firing pattern: (1) excitation of faintly acti ve neurons (periods of activity of <10 sec), which acquired or reinfor ced their phasic pattern; (2) inhibition of quasi-continuously active neurons (periods of silences of <10 sec), which became clearly phasic; and (3) no effect on neurons already showing an intermediate phasic p attern (active and silent periods of 10-30 sec). Consequently, AVP app lication resulted in a narrower range of activity patterns of the popu lation of AVP neurons, with a Gaussian distribution centered around a mode of 57% of time in activity, indicating a homogenization of the fi ring pattern. The resulting phasic pattern had characteristics close t o those established previously for optimal release of AVP from neurohy pophyseal endings. These results suggest a new role for AVP as an opti mizing factor that would foster the population of AVP neurons to disch arge with a phasic pattern known to be most efficient for hormone rele ase.