Influence of birth and maternal experience on olfactory bulb neurotransmitter release

Abstract

The ewe's ability to selectively recognize her lamb depends upon vaginocervical feedback to the brain stimulating an interest in lamb odours. This process is facilitated by previous maternal experience. We have used in vivo microdialysis to measure changes in the release of intrinsic transmitters in the olfactory bulb (glutamate, dopamine and GABA) at parturition to determine if their release profiles differ depending upon the ewe's past maternal olfactory experience. Glutamate and GABA release increased significantly at parturition in multiparous but not primiparous ewes. Dopamine release increased in both groups but mean basal levels of this transmitter were significantly higher in primiparous ewes during the pre-partum period and the first few hours postpartum. The changes in the underlying neural circuitry which determine these differences are established within 6 h of parturition, as revealed by artificial stimulation of the reproductive tract. This procedure renders the system plastic enabling adoption of strange lambs and, contingent on this, the release of intrinsic transmitters no longer differs between the two groups of ewes. Pharmacological challenges to the olfactory bulb using retrodialysis in nulliparous and multiparous (maternally inexperienced and experienced) ewes produced significant differences between the groups for induction of glutamate and GABA release, but not that of dopamine. K + challenges produced greater increases in glutamate and GABA release in multiparous than in nulliparous ewes, while dopamine release did not differ with experience. Glutamate receptor blockade produced increases in glutamate release without changing GABA release. Bicuculline had no significant effect on glutamate, GABA or dopamine, but muscimol produced decreases in glutamate and increases in GABA which were greater in experienced than with inexperienced ewes. Taken together these results suggest differential effectiveness in transmitter release in the olfactory bulbs dependent on maternal/olfactory experience with lambs.