Cholinergic modulation of a decrement in social investigation following repeated contacts between mice.

Social recognition has been inferred from a decline in olfactory investigation of conspecific intruders during repeated or protracted confrontation with a resident rat. A stimulus-response relationship defined by lack of response remains somewhat ambiguous. Since it is likely that behavior continues to be emitted by the resident animal, how behavior reorganizes as the resident becomes familiar with an intruder represents an important issue in the characterization of recognition. We examined the decline in olfactory investigation of ovariectomized females by adult male mice. The duration and frequency of olfactory investigation was measured during four 1 minute confrontations with 10-min intertrial intervals (Training trials). If the same female was presented in each trial, investigation declined to less than 50% of initial levels. Aggressive behavior gradually increased with repeated trials. No decline in investigation or increased aggression was measured when females were changed in each trial. Administration of doses of scopolamine (0.16-1.0 mg/kg, IP) blocked decrements in olfactory investigation in repeated confrontations and significantly reduced aggression. Co-administration of heptylphysostigmine (0.32-5.0 mg/kg, IP) reversed scopolamine's effects on olfactory investigation but not aggression. Acetylcholinesterase inhibitors heptylphysostigmine, galanthamine (0.63-2.5 mg/kg, IP) and tacrine (0.63-10.0 mg/kg, IP) all enhanced the rate of decrement of olfactory investigation when administered alone, but had differential effects on aggression. The decline in investigation corresponds to criteria for habituation. Increased responsivity expressed as aggression indicates recognition may also be characterized as a change in behavioral strategy dependent on the sexual and social status of the stimulus animal. Pharmacological data support a role for acetylcholine release in the development of social recognition as an olfactory memory, or through modulation of olfactory perception.