A study of the effects of noradrenaline in the rat olfactory bulb using evoked field potential response

In the rat, the main olfactory bulb receives a strong noradrenergic (NA) input from the locus coeruleus which is critical for different types of olfactory learning. However, the resulting effect of NA modulation on on the olfactory bulb electrical activity and its pharmacology are not well understood. In this study, we investigated the action of NA on the bulbar neuronal population using evoked field potentials (EFP) elicited antidromically in the olfactory bulb of anesthetized rats, by stimulation of the lateral olfactory tract (LOT). EFPs in response to single and paired-pulse stimulation of the LOT were collected before, during and until 2 h after a 10 min perfusion of pharmacological agents through a push-pull cannula. Four concentrations of NA were tested ranging from 10(-5) M to 10(-2) M. NA induced a reversible dose-dependent effect. The major effect was observed at 10(-3) M. It consisted of an increase in Component 2 amplitude (depolarization of granules cell dendrites) and a decrease in Component 3 amplitude (depolarization of granule cell bodies). In parallel, paired-pulse inhibition of mitral cells by granule cells was increased. The alpha 1 agonist phenylephrine (10(-3) M) mimicked most of the effects of NA whereas the alpha 1 antagonist prazosin (10(-3) M) blocked its main action. Isoproterenol (beta agonist, 10(-3) M) and clonidine (alpha 2 agonist, 10(-3) M) could not reproduce the effects of NA. Thus mainly through the activation of alpha 1 receptors, NA enhances synaptic activation of granule cells and increases feed-back inhibition of mitral cells. Consequences of such effects in the context of learning and memory are discussed.