THE BIPHASIC EFFECTS OF LOCUS-COERULEUS NORADRENERGIC ACTIVATION ON DENDRODENDRITIC INHIBITION IN THE RAT OLFACTORY-BULB

Some forms of olfactory learning require intact noradrenegic terminals in the olfactory bulb that originate from the locus coeruleus. To cla rify the action of noradrenergic inputs on the dendrodendritic interac tion between mitral and granule cells in the rat olfactory bulb, we an alyzed field potentials in the granule cell layer of the olfactory bul b evoked by paired-pulse stimulation of the lateral olfactory tract be fore and after the activation of the locus coeruleus. Locus coeruleus activation by glutamate injection in the vicinity of the nucleus chang ed only the test response without any effect on conditioning response. Paired-pulse inhibition measured from the ratio of test response ampl itude to conditioning response amplitude was significantly depressed i mmediately after locus coeruleus activation. Conversely, 2 min later, paired-pulse inhibition was significantly potentiated. The significant potentiation of inhibition lasted for several minutes. The depression -potentiation sequence of paired-pulse inhibition was blocked by infus ion of timolol, a beta-antagonist, into the olfactory bulb, in a dose- dependent manner, but not by infusion of phentolamine, an alpha-antago nist. Infusion of isoproterenol, a beta-agonist, into the bulb mimicke d the depression of paired-pulse inhibition by locus coeruleus activat ion. These results suggest that glutamate activation of the locus coer uleus produces a depression-potentiation sequence in granule cell-medi ated feedback inhibition onto mitral cells in the olfactory bulb throu gh beta-adrenergic receptors. (C) 1998 Elsevier Science B.V.