Opiate agonist-induced changes in behavioral sensitivity to clonidine are observed in perinatally malnourished rats exposed to chronic stress

Nonopioidergic mechanism mediating morphine-induced antianalgesia in the mouse spinal cord

Intrathecal (i.t.) pretreatment with a low dose (0.3 nmol) of morphine causes an attenuation of i.t. morphine-produced analgesia; the phenomenon has been defined as morphine-induced antianalgesia. The opioid-produced analgesia was measured with the tail-flick (TF) test in male CD-1 mice. Intrathecal pretreatment with low dose (0.3 nmol) of morphine time dependently attenuated i.t. morphine-produced (3.0 nmol) TF inhibition and reached a maximal effect at 45 min. Intrathecal pretreatment with morphine (0.009-0.3 nmol) for 45 min also dose dependently attenuated morphine-produced TF inhibition. The i.t. morphine-induced antianalgesia was dose dependently blocked by the nonselective mu-opioid receptor antagonist (-)-naloxone and by its nonopioid enantiomer (+)-naloxone, but not by endomorphin-2-sensitive mu-opioid receptor antagonist 3-methoxynaltrexone. Blockade of delta-opioid receptors, kappa-opioid receptors, and N-methyl-D-aspartate (NMDA) receptors by i.t. pretreatment with naltrindole, nor-binaltorphimine, and (-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801), respectively, did not affect the i.t. morphine-induced antianalgesia. Intrathecal pretreatment with antiserum against dynorphin A(1-17), [Leu]-enkephalin, [Met]-enkephalin, beta-endorphin, cholecystokinin, or substance P also did not affect the i.t. morphine-induced antianalgesia. The i.t. morphine pretreatment also attenuated the TF inhibition produced by opioid muagonist [D-Ala2, N-Me-Phe4,Gly-ol5]-enkephalin, delta-agonist deltorphin II, and kappa-agonist U50,488H. It is concluded that low doses (0.009-0.3 nmol) of morphine given i.t. activate an antianalgesic system to attenuate opioid mu-, delta-, and kappa-agonist-produced analgesia. The morphine-induced antianalgesia is not mediated by the stimulation of opioid mu-, delta-, or kappa-receptors or NMDA receptors. Neuropeptides such as dynorphin A(1-17), [Leu]-enkephalin, [Met]-enkephalin, beta-endorphin, cholecystokinin, and substance P are not involved in this low-dose morphine-induced antianalgesia.

Acute effects of D-amphetamine on progressive-ratio performance maintained by cigarette smoking and money

RATIONALE

Psychomotor stimulants often increase cigarette smoking, although the mechanisms responsible for that effect remain unclear.

OBJECTIVE

The present study was conducted to extend our investigation of the effects of psychomotor stimulant use on the reinforcing effects of cigarette smoking.

METHODS

A progressive-ratio (PR) schedule was used to examine the acute effects of orally administered D-amphetamine (placebo, 5, 10, 15 mg/70 kg) on the reinforcing effects of smoking (2 puffs/ratio) and money (1.00 US dollar/ratio) in 18 adult smokers.

RESULTS

Among the overall sample of 18 smokers, D-amphetamine produced only a modest, non-significant increase in the amount of responding maintained by smoking (PR break point), but effects varied systematically between subjects. D-Amphetamine increased the smoking break point among ten subjects (responders), while leaving unchanged or decreasing the break point in the remaining eight individuals (non-responders). Responders also were more sensitive than non-responders to d-amphetamine's effects on visual-analog ratings of "high", "good effects", "drowsy", and "slurred speech", but not on other subject ratings (i.e., "bad effects", "jittery", "confused") or on any physiological measures (i.e., heart rate, blood pressure, skin temperature). D-Amphetamine did not significantly alter PR break point maintained by money.

CONCLUSIONS

These results provide further evidence that D-amphetamine can increase the reinforcing effects of cigarette smoking, at least among a subset of smokers.

Endogenous opiates: 1998

This paper is the twenty-first installment of our annual review of research concerning the opiate system. It summarizes papers published during 1998 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.

Preweanling naltrindole administration differentially affects clonidine induced antinociception and plasma adrenaline levels in male and female neonatal rats

1. The influence of a chronic treatment with the delta-selective opioid antagonist naltrindole (1 mg kg-1) during the preweanling period (daily injections from birth to postnatal day 19), on the antinociceptive and sympatholytic effects of the alpha2-adrenergic agonist clonidine in male and female rats of 20 and 25 days of age was investigated. 2. Nociception was assessed using the tail immersion test (water at 50 degrees C) and plasma levels of adrenaline were measured by high-performance liquid chromatography. 3. The dose of clonidine (1.5 mg kg-1) and the time point at which nociceptive responses were recorded (30 min after the administration of the drug) were chosen on the basis of dose-response (0.5, 1, 1.5 and 2 mg kg-1) and time-response (5, 10, 15, 30 and 60 min) curves which were previously carried out in naive control neonatal rats. 4. In females, the functional blockade of the delta-receptor by neonatal naltrindole treatment did not modify the sympatholytic effect of clonidine but prevented clonidine induced antinociception. Conversely, in males naltrindole treatment allowed the appearance of clonidine antinociception and the sympatholytic effect of clonidine. 5. The results indicate that the delta-receptor is involved in the modulation of antinociceptive and sympatholytic responses to clonidine in neonatal rats and suggest the existence of sex differences in the interactions between delta-opioid and alpha2-adrenergic receptors.

Effect of perinatal malnutrition on the inactive behavior induced by long-term shock and conditioned analgesia

Inactive behavior in response to long-term inescapable foot shock (IS) and conditioned analgesia were evaluated in adult rats perinatally undernourished. During the IS session, control rats exhibited first a period of vigorous activity and then inactive behavior, during which they accepted the shock more passively. Moreover, when these same animals were subsequently placed in the environment associated with shock application, they showed an increase in the paw lick latency. Conversely, in malnourished rats, this IS schedule induced lower behavioral inactivity and rats failed to develop conditioned analgesia. When undernourished rats were injected with morphine (MOR) before IS exposure, it was observed that both phenomena--inactivity during shock and analgesia conditioned by submission to the shock context--were normalized. A possible alteration in the activation of an opiate process implicated in the stress response in early undernourished rats is suggested.