Quantitative analysis of naloxone antagonism of the discriminative stimulus properties of morphine in the pigeon

Pharmacological analysis of the rate-decreasing effects of mu and kappa opioids in pigeons

The present study investigated the rate-decreasing effects of several mu (morphine and l-methadone) and kappa (bremazocine, U69,593 and U50,488) opioid agonists in pigeons. Mu and kappa agonists were examined alone, in combination with naltrexone or the mu-selective opioid antagonist, beta-funaltrexamine (beta-FNA), and in pigeons treated chronically with U50,488. Naltrexone was equipotent in shifting the morphine, l-methadone and bremazocine dose-effect curves to the right, but was less potent in shifting the U69,593 dose-effect curve and did not shift the U50,488 dose-effect curve. Beta-FNA shifted the l-methadone dose-effect curve to the right but did not shift the bremazocine, U69,593 or U50,488 dose-effect curves. Pigeons that developed tolerance to U50,488 following daily administration were cross-tolerant to bremazocine but not to l-methadone. Taken together, these experiments indicate that the rate-decreasing effects of morphine and l-methadone are mediated by mu opioid receptors, whereas the rate-decreasing effects of bremazocine, U69,593 and U50,488 in pigeons differ depending on the pharmacological procedures used to assess their effects.

Discriminative stimulus and response rate-decreasing effects of kappa opioids: antagonism by naloxone

The present study examined the discriminative stimulus and response rate-decreasing effects of kappa opioids in pigeons trained to discriminate a 0.017 mg/kg dose of bremazocine from saline. Bremazocine, spiradoline, CI977, U69,593 and U50,488 substituted completely for the bremazocine stimulus in a dose-dependent and naloxone-reversible manner. Apparent pA2 values (range, 6.01-6.81) of naloxone against the discriminative stimulus effects of these kappa opioids were smaller than those reported previously in the pigeon for naloxone against the discriminative stimulus effects of various mu opioids. Bremazocine, CI977, spiradoline and U69,593 also decreased rate of responding in a dose-dependent and naloxone-reversible manner. The apparent pA2 values (range, 6.25-6.44) for naloxone against the rate-decreasing effects of bremazocine, CI977 and U69,593 were not different from the apparent pA2 values for naloxone against their discriminative stimulus effects. An apparent pA2 for naloxone against the rate-decreasing effects of spiradoline could not be determined due to the shallow slope of the Schild plot. Although the rate-decreasing effects of U50,488 were antagonized by naloxone, the degree of antagonism was small and not dose-dependent. These findings indicate that the discriminative stimulus and rate-decreasing effects of some kappa opioids are mediated by similar mechanisms and that a non-opioid mechanism may contribute to the rate-decreasing effects of spiradoline and U50,488.

Effects of nimodipine on the discriminative stimulus properties of d-amphetamine in rats

The discriminative stimulus (DS) properties of d-amphetamine (AMP) are thought to be mediated by enhanced release of catecholamines, which may involve neuronal calcium influx through voltage sensitive channels. The present study examined the influence of nimodipine, a calcium channel blocker, on the DS properties of AMP. Rats (N = 8) were trained to discriminate AMP (0.5 mg/kg, IP) from saline in a two-lever, food-reinforced, drug discrimination paradigm. Nimodipine alone (2.0-5.6 mg/kg, IP) did not substitute for AMP. When given in combination with AMP, 2.0 mg/kg nimodipine increased by less than 2-fold the AMP dose necessary to induce AMP-appropriate responses. Higher doses of nimodipine combined with AMP did not increase the magnitude of this effect. Nimodipine enhanced the effects of AMP on response rate. Haloperidol (0.125 mg/kg) increased by approximately 4-fold, whereas diazepam (0.5 or 1.0 mg/kg) and morphine (5.0 mg/kg) increased by approximately 2-fold the AMP dose necessary to induce AMP-appropriate responses. The interaction with AMP was associated with enhanced reduction of response rate in the tests with diazepam and morphine but not haloperidol. These results suggest that nimodipine attenuates the DS properties of AMP, probably in a non-specific way, due to the ability of nimodipine itself to induce a discriminable internal state.

Endogenous opiates: 1986

This is the ninth installment of our annual review of research involving the endogenous opiate peptides. It is restricted to the non-analgesic and behavioral studies of the opiate peptides published in 1986. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic processes; mental illness; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; activity; sex, pregnancy, and development; and some other behaviors.