Many peptides, when released as chemical messengers within the brain, have powerful influences on complex behaviours. Most strikingly, vasopressin and oxytocin, once thought of as circulating hormones whose actions were confined to peripheral organs, are now known to be released in the brain where they play fundamentally important roles in social behaviours1. In humans, disruptions of these peptide systems have been linked to several neurobehavioural disorders, including Prader-Willi syndrome, affective disorders, and obsessive-compulsive disorder, and polymorphisms of the vasopressin V1a receptor have been linked to autism2,3. Here we report that the rat olfactory bulb contains a large population of interneurones which express vasopressin, that blocking the actions of vasopressin in the olfactory bulb impairs the social recognition abilities of rats, and that vasopressin agonists and antagonists can modulate the processing of information by olfactory bulb neurones. The findings indicate that social information is processed in part by a vasopressin system intrinsic to the olfactory system.

The peptides alpha-melanocyte stimulating hormone (alpha-MSH) and oxytocin, when administered centrally, produce similar behavioral effects. alpha-MSH induces Fos expression in supraoptic oxytocin neurons, and alpha-MSH melanocortin-4 receptors (MC4Rs) are highly expressed in the supraoptic nucleus, suggesting that alpha-MSH and oxytocin actions are not independent. Here we investigated the effects of alpha-MSH on the activity of supraoptic neurons. We confirmed that alpha-MSH induces Fos expression in the supraoptic nucleus when injected centrally and demonstrated that alpha-MSH also stimulates Fos expression in the nucleus when applied locally by retrodialysis. Thus alpha-MSH-induced Fos expression is not associated with electrophysiological excitation of supraoptic neurons because central injection of alpha-MSH or selective MC4 receptor agonists inhibited the electrical activity of oxytocin neurons in the supraoptic nucleus recorded in vivo. Consistent with these observations, oxytocin secretion into the bloodstream decreased after central injection of alpha-MSH. However, MC4R ligands induced substantial release of oxytocin from dendrites in isolated supraoptic nuclei. Because dendritic oxytocin release can be triggered by changes in [Ca2+]i, we measured [Ca2+]i responses in isolated supraoptic neurons and found that MC4R ligands induce a transient [Ca2+]i increase in oxytocin neurons. This response was still observed in low extracellular Ca2+ concentration and probably reflects mobilization of [Ca2+]i from intracellular stores rather than entry via voltage-gated channels. Taken together, these results show for the first time that a peptide, here alpha-MSH, can induce differential regulation of dendritic release and systemic secretion of oxytocin, accompanied by dissociation of Fos expression and electrical activity.

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