THE ACTION OF OXYTOCIN ORIGINATING IN THE HYPOTHALAMIC PARAVENTRICULAR NUCLEUS ON MITRAL AND GRANULE CELLS IN THE RAT MAIN OLFACTORY-BULB

The effects of electrical stimulation of the hypothalamic paraventricu lar nucleus on the spontaneous firing of mitral and granule cells in t he main olfactory bulb were examined in ovariectomized female rats und er urethane anaesthesia. High-frequency stimulation (0.5-1.0 mA, 10-20 pulses at 100 Hz) of the paraventricular nucleus produced inhibitory responses in 80% of mitral cells tested and excitatory responses in 74 % of granule cells tested, with latencies ranging from 2 to 150 s. Bot h responses were blocked by infusions into the olfactory bulb of [d(CH 2)(5),Tyr(Me)(2)]ornithine-vasotocin (10 pmol), an oxytocin antagonist , and mimicked by intracerebroventricular infusions (0.2 or 0.4 nmol) or microiontophoretic applications of oxytocin but not by intracerebro ventricular infusions of vasopressin (1 or 2 nmol). Infusions of 0.5% lignocaine, a local anaesthetic, into either the medial olfactory trac t or the medial forebrain bundle failed to block the responses of mitr al and granule cells to the stimulation. Unilateral transections at va rious levels between the bulb and the paraventricular nucleus also Fai led to block the responses. There were cases in which significant resp onses of mitral and granule cells to the stimulation required 60 or mo re pulses after the lignocaine infusions or transections, however.Thes e results suggest that oxytocin originating in the hypothalamic parave ntricular nucleus reaches the olfactory bulb following its release par tly into the cerebrospinal fluid and acts to decrease olfactory proces sing.