Effects of neonatal naltrindole treatment on antinociceptive and behavioral responses to mu and kappa agonists in rats

Role of kappa and delta opioid receptors in mediating morphine-induced antinociception in morphine-tolerant infant rats

We have previously noted that the antinociceptive efficacy of morphine was significantly decreased in rat pups chronically infused with morphine from implanted osmotic minipumps. In this study, morphine was fully efficacious (i.e., 100% maximum possible effect, %MPE) in the 52 degrees C tail-immersion test after a 72-h infusion from implanted saline-filled osmotic minipumps. However, administration of up to 1000 mg/kg, s.c. morphine failed to elicit greater than a 27% MPE in rats infused with morphine at 2 mg/kg/h. Morphine was more efficacious when the water bath temperature was decreased to 49 degrees C. Experiments were conducted to determine the mechanisms whereby chronic morphine administration leads to a decrease in antinociceptive efficacy. The kappa-opioid antagonist nor-binalorphimine completely blocked the antinociceptive effects of morphine in morphine-infused rat pups. The kappa agonist U50,488 elicited antinociception; however, the requirement to use higher doses in morphine- than saline-infused rats indicates that kappa cross-tolerance was present. Thus, in tolerant rats the antinociceptive effects of high doses of morphine appear to be mediated through kappa-opioid receptors. The delta-opioid antagonist naltrindole was inactive in both treatment groups. DAMGO-stimulated [(35)S]GTPgammaS and [(3)H]naloxone binding reveals that the anatomical distribution of the mu-opioid receptor was consistent with that of the adult rat brain. In adult rats, the mu-opioid receptor is desensitized during morphine tolerance. However, desensitization was not evident in P17 rats based on the lack of significant decreases in [(35)S]GTPgammaS binding. Furthermore, [(3)H]naloxone binding indicated a lack of mu receptor downregulation in morphine-tolerant rat pups.

Functional responses to the cannabinoid agonist WIN 55,212-2 in neonatal rats of both genders: influence of weaning

We have studied behavioural, biochemical and endocrine responses to the cannabinoid agonist WIN 55,212-2 (WIN) in neonatal rats, as well as the effects of weaning on such responses. We used preweanling rats (20 days of age), 25-day-old weaned rats (weaning at Day 22) and 25-day-old nonweaned rats of both sexes. The behavioural effects of WIN were assessed in the nociceptive tail immersion test and in the open field. We also analysed the effect of weaning on corticosterone responses to WIN (radioimmunoassay) as well as on WIN-stimulated [35S] GTPgammaS binding in periaqueductal grey (PAG) and striatum. The cannabinoid agonist induced a modest increase in pain thresholds, whereas the effect of the drug on open-field activity, particularly on vertical activity, was much more marked. The weaning process appeared to reduce the baseline nociceptive latencies of the female rats. No significant effect of weaning on the behavioural responses to WIN was found. However, the group of weaned females (but not males) showed a significantly reduced WIN-stimulated [35S] GTPgammaS binding in the striatum. The cannabinoid agonist significantly increased the corticosterone levels of 25-day-old rats with the effect being more marked in weaned than in nonweaned animals. The results suggest that the weaning process might produce some sexually dimorphic developmental changes in CB1 receptor function.

Antinociceptive, behavioural and neuroendocrine effects of CP 55,940 in young rats

The peripubertal period appears to be critical in relation to the abuse of cannabinoids and opioids in humans. However there is little information about the acute effects of cannabinoids and their interactions with opioids in young experimental animals. We have studied the effects of the cannabinoid agonist CP 55,940 (0.1, 0.2, 0.4 and 0.6 mg/kg) on the nociceptive responses (tail immersion test) and holeboard activity of 40-day-old rats, and the involvement of the CB(1) receptor (antagonism by SR 141716A, 3 mg/kg). The implication of the opioid system was evaluated using the opioid antagonist naloxone (1 mg/kg) and a combined treatment with subeffective doses of CP 55,940 (0.1 mg/kg) and morphine (1 mg/kg). The effects of CP 55,940 on the serum corticosterone levels (radioimmunoassay) and on the dopamine and DOPAC contents of discrete brain regions (high-performance liquid chromatography) were also assessed. The antinociceptive effect of CP 55,940 was of a similar magnitude at all the doses used. The results show the involvement of the CB(1) receptor. The cannabinoid agonist significantly depressed the holeboard activity in a dose-dependent manner. The results indicate that the CB(1) receptor is involved in the effects on motor activity but not in the effects on the exploratory activity. The behavioural effects of CP 55,940 were modulated by morphine. The cannabinoid agonist (0.6 mg/kg) induced a CB(1)-mediated increase in the serum corticosterone levels, but no effect on the dopaminergic systems of either the striatum or the limbic forebrain was found.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 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; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.

Postnatal naltrindole treatments induce behavioural modifications in preweanling rats

To investigate the physiological role of the delta-opioid receptor during the preweanling period, we have studied the effects of chronic (daily injections from birth to postnatal day 19) and acute treatments with the selective delta-antagonist naltrindole (1 mg/kg), on behavioural and nociceptive responses in 20-day old male rats. Behavioural testing was performed using an open field paradigm. Acute naltrindole induced significant decreases in external and total ambulation (horizontal activity) and rearing behaviour (vertical activity), as well as a significant increase in grooming frequency. In animals chronically treated with naltrindole there was an increase in total ambulation one day after the discontinuation of the treatment. In a test of nociception (tail immersion) no significant effect of chronic naltrindole treatment on baseline latencies or of acute naltrindole on latency quotients (post-treatment latency/pre-treatment latency) were found. However, chronic naltrindole administration significantly decreased the latency quotients. The results show that the delta-opioid receptor participates in the tonic regulation of motor activity during the preweanling period and might be involved in certain aspects of stress responsiveness.

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.