Bilateral microinjection of oxytocin (OXY) into the ventral tegmental area (VTA) of rat brain produced a significant increase in grooming behaviors at doses from 100 pg to 400 ng. Sites in the caudal region of the VTA were sensitive to lower doses of OXY than sites in the rostral region of the VTA. The time course of action of OXY in the grooming paradigm indicated onset beginning immediately after injection, and termination at 60-75 minutes after injection. Comparison of OXY-induced grooming in male, female, and ovariectomized, estrogen-treated female rats showed no differences in potency for OXY among these groups, suggesting that the grooming effects of OXY are not regulated by sex steroids. Analysis of locomotor activity in rats microinjected with OXY 200 ng bilaterally into the caudal VTA revealed that OXY had no effect on ambulatory locomotion, suggesting that this peptide may activate neurons within the VTA which mediate grooming but not locomotion. The OXY receptor antagonist, [Pen1, pMePhe2, Thr4, Orn8]-OT, blocked OXY-induced grooming when both were simultaneously microinjected into the VTA. The dopamine D-2 receptor antagonist, haloperidol, and the D-1 receptor antagonist, SCH 23390, when microinjected into the VTA five minutes before microinjection of OXY into the VTA, did not block OXY-induced grooming, suggesting that OXY is not working through a dopamine autoreceptor on the VTA neurons. Systemic pretreatment with haloperidol and SCH 23390 effectively blocked grooming induced by OXY in the VTA, suggesting that OXY is directly stimulating OXY receptors on VTA neurons to release dopamine at postsynaptic sites regulating grooming behaviors.
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