Opioid receptor subtypes associated with ventral tegmental facilitation of lateral hypothalamic brain stimulation reward

Abstract

Rats were trained to lever-press for lateral hypothalamic electrical stimulation, and tested following ventral tegmental microinjections of morphine, delta ([ d-Pen 2, d-Pen 5]enkephalin: DPDPE), or kappa (U-50, 488H) receptor agonists or saline. Across the range of effective stimulation frequencies, morphine (8 nmol) and DPDPE (8 nmol) reduced the current necessary to sustain responding, while U-50, 488H (8 nmol) had no effect. Naloxone (1 mg/kg) blocked the effects of morphine and DPDPE. Since each of these opioids facilitates eating induced by stimulation of the lateral hypothalamus, but only the mu and delta agonists facilitate brain stimulation reward, it would appear that ventral tegmental kappa receptors are linked to circuit elements which play a role in eating but not lateral hypothalamic brain stimulation reward. Ventral tegmental mu and delta receptors, on the other hand, appear to be linked to circuit elements involved in both behaviors.