Release of classical transmitters and nitric oxide in the rat olfactory bulb, evoked by vaginocervical stimulation and potassium, varies with the oestrus cycle Rosalinda Guevara-Guzma Ân, 1 Baltazar Barrera-Mera, 1 Carlos de la Riva 2 and Keith M. Kendrick 2 1 Departamento de Fisiologõ Âa, Facultad de Medicina, UNAM, Me Âxico 04510 2 Laboratory of Cognitive and Developmental Neuroscience, The Babraham Institute, Babraham, Cambridge CB2 4AT, UK Keywords: dopamine, GABA, glutamate, neurotransmitters, nitric oxide, oestrus cycle, olfactory bulb Abstract In vivo microdialysis was used to investigate the effects of ovariectomy and the oestrus cycle on vaginocervical stimulation-evoked classicaltransmitterandnitricoxidereleaseintheolfactorybulbofanaesthetized(urethane)andconsciousrats.Duringpro-oestrus/ oestrus,vaginocervicalstimulation(1or10min)signi®cantlyincreasedconcentrationsofglutamate,aspartate,GABA,noradrenaline, dopamine and nitric oxide (citrulline) but failed to do so in met-oestrus/di-oestrus or following ovariectomy. Potassium chloride- evoked GABA, noradrenaline and nitric oxide release in the olfactory bulb was also signi®cantly enhanced during pro-oestrus/ oestrus. The effects of vaginocervical stimulation on olfactory bulb transmitter release during pro-oestrus/oestrus were signi®cantly reducedbypelvicorvagusnervesection.Basalconcentrationsofclassicaltransmittersandnitricoxideintheolfactorybulbdidnot vary across the oestrus cycle although noradrenaline and dopamine levels were reduced following ovariectomy. These results con®rm our previous electrophysiological data showing that the olfactory bulb mitral cells are only excited by vaginocervical stimulation during pro-oestrus/oestrus. They also suggest that sex hormones acting primarily at the level of the olfactory bulb dramatically enhance the ability of vaginocervical stimulation to evoke release of both classical transmitters and nitric oxide in this region.Suchalterationsinneurochemicalreleaseintheolfactorybulbmaybeimportantformediatingplasticitychangesunderlying olfactory recognition of mates or offspring. Introduction In the rat, mechanical stimulation of the vagina and cervix (VCS) produces morphological and electrophysiological changes in a number of brain regions controlling reproductive behaviours (Haskins&Moss,1983;Lee&Erskine,1996).Wehavepreviously found that VCS increases mitral cell activity in the main olfactory bulb(OB)ofoestrusfemaleratsanddecreasesitindi-oestrusanimals (Guevara-Guzma Ân etal., 1997). Such VCS-induced changes in the OBmaybepartlymediatedviathevagusnerveandthenucleusofthe solitary tract since the OB responds to electrical stimulation of the vagus (Garcõ Âa-Dõ Âaz etal.,1984)andthenucleusofthesolitarytract to VCS (Ortega-Villalobos etal., 1990). However, the pelvic and hypogastric nerves may be involved as well since they are also activated by VCS (Berkley etal., 1993). The morphological and electrophysiological responses of several brain regions associated with reproductive functions can alter as a function of the oestrus cycle (Gorski etal., 1975; Kendrick, 1981; Warren etal., 1995). Further, VCS-induced changes in glucose utilization (Allen etal., 1981) or c-fos expression (Erskine, 1993; Pfaus etal.,1993,1996;Tetel etal.,1993;Polston&Erskine,1995; Da Costa etal., 1997; Guevara-Guzma Ân etal., 1997; Pfaus & Heeb, 1997)occurinolfactory,hypothalamic,limbicandbrainstemregions in both rodents and sheep. In the OB itself, VCS-induced changes occurinbothrats(Guevara-Guzma Ân etal.,1997)andsheep(daCosta etal., 1997) and are signi®cantly greater during pro-oestrus than di- oestrus in rats (Guevara-Guzma Ân etal., 1997). In the OB, VCS-induced changes are important for olfactory memory formation underlying recognition of offspring in sheep (Kendrick etal.,1997b)andintheaccessoryOBoffemalemicefor therecognitionofmalepheromonesfollowingmating(Brennan etal., 1990; Kaba & Nakanishi, 1995). Memory formation in both cases involves plasticity changes in the dendrodendritic synapses between the mitral and granule cells (Kendrick etal., 1992; Brennan etal., 1995). These plasticity changes involve VCS-induced activation of centrifugal noradrenergic inputs to the OB and accessory OB from the brainstem (Rosser & Keverne, 1985; Le Âvy etal., 1993; Brennan etal., 1995) as well as glutamate release acting on NMDA and AMPA receptors intrinsically (Brennan, 1994; Kendrick etal., 1997a). In the OB, the release of nitric oxide (NO) modulating guanylylcyclaseandcyclicGMPreleaseinthemitralcellsynapsesis the®nalcriticalstepincausingplasticitychangestooccur(Kendrick etal., 1997a). Nitric oxide release may also be important for mediating plasticity changes in the accessory olfactory bulb (AOB) butthisismorecontroversialsinceeffectscanonlybedemonstrated with NO-donors (Okere etal., 1996). In the male rat OB, potassium-evoked release of noradrenaline (NA)isfacilitatedbytestosterone(Guan&Dluzen,1991),suggesting that sex steroids may directly in¯uence OB neurochemical release and functioning. However, little is known about the effects of the ovariectomy or the oestrus cycle on OB neurochemical release in Correspondence:DrK.M.Kendrick,asabove. Email: keith.kendrick@bbsrc.ac.uk Received24May1999,revised7September1999,accepted8September1999 European Journal of Neuroscience, Vol. 12, pp. 80±88, 2000 Ó European Neuroscience Association