Clorgyline-induced switch from locomotion to mouthing in sensitization to the dopamine D2/D3 agonist quinpirole in rats: role of sigma and imidazoline I2 receptors

Rationale. The monoamine oxidase inhibitor (MAOI) clorgyline, blocks locomotor sensitization to the D₂/D₃ dopamine agonist quinpirole and sensitizes self-directed mouthing behavior in rats by a mechanism independent of MAO inhibition. Clorgyline has a high affinity for imidazoline I₂ and sigma receptors, which could account for its effects on quinpirole sensitization. Objectives. To examine whether the effect of clorgyline on quinpirole sensitization is attributed to stimulation of either I₂ or sigma receptors. Methods. In one experiment, rats received injections of the I₂ receptor agonist 2-BFI (0.2 mg/kg, IP) or vehicle, 90 min prior to each injection of quinpirole (0.5 mg/kg, SC, × 8, twice weekly) or saline. A similar protocol was used to examine the effects of the MAOI Ro 41-1049 (10 mg/kg, SC) on quinpirole sensitization. Unlike clorgyline, Ro 41-1049 has no affinity for sigma or I₂ sites. An initial experiment demonstrated that intermittent injections of clorgyline (1 mg/kg, SC) are as effective as a continuous clorgyline administration (1 mg/kg per day via osmotic mini-pump) on quinpirole sensitization. Results. Like clorgyline, Ro 41-1049, but not 2-BFI, blocked the development of quinpirole-induced locomotor sensitization and induced instead sensitization of self-directed mouthing. Conclusions. Because Ro 41-1049 produced the same effects as clorgyline, and 2-BFI had no effects on quinpirole sensitization, it is unlikely that clorgyline exerts its effects via an action at sigma or I₂ receptors. Our results are consistent with the suggestion that clorgyline and Ro 41-1049 affect the behavioral response to quinpirole via the MAOI-displaceable quinpirole binding (MQB) site, and the hypothesis that the MQB site selects what motor output becomes sensitized to repeated injections of quinpirole. Electronic Publication