Neural and Neurochemical Control of Olfactory Recognition of Offspring in Sheep

Abstract

In sheep, rapid onset of maternal behaviour occurs at parturition and an exclusive bond between a mother and her lambs is established within a few hours after birth. Parturient ewes discriminate between their own lambs and alien lambs: they allow the former to access and suckle the udder, while they reject suckling attempts from the latter, often violently. Olfactory signals from the lambs are the most important cues in this context. Microdialysis measurement of neurochemical substances in the sheep olfactory bulb (OB) reveals increased release of intrinsic (glutamate (GLU), Y-aminobutyric acid (GABA) and dopamine) and centrifugal (noradrenaline (NA) and acetylcholine) neurotransmitters at parturition. Increased release of these transmitters also occurs after artificial vaginocervical stimulation (VCS), which induces ewes to accept alien lambs. Therefore VCS which takes place at parturition plays a key role in triggering the release of these transmitters in the OB and establishing a selective maternal bond. Rapid changes in effectiveness of VCS on release of neurotransmitters occur after the first experience of parturition in primiparous ewes, suggesting that maternal experience with lambs enhances the establishment of a lamb recognition memory at the time of subsequent parturitions. The formation of maternal olfactory memory depends in part on release of nitric oxide (NO) and NA, because inhibition of NO synthase activity or blockade of β-adrenergic receptors or NA release during labour prevents selective bonding and allows ewes t o accept alien lambs. Following selective bonding, the release of GLU and GABA in the OB increases in response to own lamb odours, but not to alien ones, indicating that the importance of these transmitters for mediating the recall of the olfactory memory. Moreover, blockade of GABAA receptors in the OB overcomes a selective bond with own lambs and induces acceptance of alien lambs.