Age-related differences in the effect of chronic administration of naloxone on opiate binding in rat brain

Developmental consequences of fetal exposure to drugs: what we know and what we still must learn

Most drugs of abuse easily cross the placenta and can affect fetal brain development. In utero exposures to drugs thus can have long-lasting implications for brain structure and function. These effects on the developing nervous system, before homeostatic regulatory mechanisms are properly calibrated, often differ from their effects on mature systems. In this review, we describe current knowledge on how alcohol, nicotine, cocaine, amphetamine, Ecstasy, and opiates (among other drugs) produce alterations in neurodevelopmental trajectory. We focus both on animal models and available clinical and imaging data from cross-sectional and longitudinal human studies. Early studies of fetal exposures focused on classic teratological methods that are insufficient for revealing more subtle effects that are nevertheless very behaviorally relevant. Modern mechanistic approaches have informed us greatly as to how to potentially ameliorate the induced deficits in brain formation and function, but conclude that better delineation of sensitive periods, dose-response relationships, and long-term longitudinal studies assessing future risk of offspring to exhibit learning disabilities, mental health disorders, and limited neural adaptations are crucial to limit the societal impact of these exposures.

Naloxone treatment alters gene expression in the mesolimbic reward system in 'junk food' exposed offspring in a sex-specific manner but does not affect food preferences in adulthood

We have previously reported that the opioid receptor blocker, naloxone, is less effective in reducing palatable food intake in offspring exposed to a maternal cafeteria diet during the perinatal period, implicating a desensitization of the central opioid pathway in the programming of food preferences. The present study aimed to investigate the effect of a maternal cafeteria diet and naloxone treatment on the development of the mesolimbic reward pathway and food choices in adulthood. We measured mRNA expression of key components of the reward pathway (mu-opioid receptor, proenkephalin, tyrosine hydroxylase, D1 and D2 receptors and the dopamine active transporter (DAT)) in the nucleus accumbens (NAc) and ventral tegmental area (VTA) of the offspring of control and cafeteria fed (JF) dams at weaning and after a 10-day naloxone treatment post-weaning and determined food preferences in adulthood in the remaining offspring. Naloxone treatment decreased the expression of DAT by 8.2 fold in female control offspring but increased it by 4.3 fold in female offspring of JF dams relative to the saline-injected reference groups. Proenkephalin mRNA expression was higher in the NAc of female JF offspring compared to controls, independent of naloxone treatment (P<0.05). There was no effect of naloxone treatment on food preferences in adulthood in either control or JF offspring. These data indicate that prenatal exposure to a cafeteria diet alters the impact of opioid signaling blockade in the early post-weaning period on gene expression in the central reward pathway in a sex specific manner, but that these changes in gene expression do not appear to have any persistent impact on food preferences in adulthood.

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

The effects of a daily injection of the delta selective opioid antagonist naltrindole (1 mg/kg), from birth to postnatal day 19, on antinociceptive and behavioral responses to the mu selective agonist alfentanil (65 microg/kg) and the kappa selective agonist CI-977 (50 microg/kg) in 20-day-old male rats were investigated. Antinociception was assessed using the tail immersion test and behavioral testing was performed by employing an open field. The functional blockade of the delta receptor by naltrindole blocked the antinociceptive response to alfentanil but did not affect the antinociception induced by CI-977. The effects of alfentanil (increased exploration) and CI-977 (a marked hypoactivity) in the open field were not modified by neonatal naltrindole treatment. The results suggest a functional interaction between delta and mu receptors in the postnatal period but not between delta and kappa receptors. The data also suggest differences in the delta and mu receptors interacting in the modulation of antinociception and those involved in behavioral responses in the open field.

Effects of preweanling chronic naltrindole administration on stress-induced antinociceptive responses in rats

The effect of a daily injection of the delta-selective opioid antagonist naltrindole (1 mg/kg), from birth to postnatal day 19, on the development of stress-induced-antinociception (SIA) and on the antinociceptive response to the mu-selective agonist alfentanil (65 microg/kg) in female rats was investigated. Functional blockade of the delta-receptor during the preweanling period markedly reduced the antinociceptive response to swim-stress in 25-day-old rats, and SIA was only mediated by delta-receptors at this age. In 20-day-old rats and in adults, SIA was predominantly mu-receptor mediated and unaffected by delta-receptor blockade. The lack of interference with mu-receptor function was confirmed as alfentanil responses were unaffected by preweanling naltrindole treatment. The data show independence of mu- and delta-receptors in the control of SIA during development and an impairment of delta- but not mu-mediated SIA after chronic delta-antagonist treatment.

Naltrexone administration effects on regional brain monoamines in developing rats

The effects of the opioid antagonist naltrexone (NALTX) daily administration (1 mg/kg SC) from birth on the levels of dopamine (DA), serotonin (5-HT), and their respective major metabolites, in the striatum, midbrain, and hypothalamus of 7-, 14-, and 22-day-old rats were investigated. Naltrexone treatment increased the striatal HVA/DA ratio on postnatal day 7. At day 14, two subpopulations (A and B) were found among the treated animals. The subpopulation A showed decreased HVA/DA and increased DOPAC/DA ratios, whereas the subpopulation B presented a higher DA concentration. No significant effect appeared on the striatal dopaminergic system in 22-day-old pups. The serotonergic system was affected by exposure to naltrexone only from day 14. The subpopulation A showed a reduction in all the parameters measured in the three regions studied, although in the subpopulation B, lower 5-HIAA/5-HT ratios appeared in the midbrain and hypothalamus. At 22 days of age NALTX treatment elevated striatal 5-HT and 5-HIAA and the ratio of 5-HIAA/5-HT in the midbrain and hypothalamus. These data suggest an endogenous opioid modulation on the central aminergic systems during the neonatal period and point out the consequences of opioid plasticity on related neurotransmitter systems.

The potentiating effects of restraint stress and continuous naloxone infusion on the analgesic potency or morphine are additive

The analgesic potency of morphine in the rat can be increased by either continuous administration of an opiate antagonist, which also increases the number of mu-opioid receptors in the brain, or by immobilization stress. To examine further the plasticity of brain opioid mechanisms, this study assessed the combined effects of continuous naloxone treatment and physical restraint on the analgesic potency of morphine. Osmotic pumps releasing 0.3 mg/kg/h of naloxone were implanted subcutaneous (SC) in two groups of rats for 7 days and empty (SHAM) pumps were implanted in two other groups. Twenty-four hours after the pumps were removed, all animals were injected with morphine in cumulative doses and tested in the tail-flick procedure. One group of naloxone-treated rats and one group of SHAM-treated rats were tested while restrained; the other two groups were tested while unrestrained. Naloxone infusion alone and restraint alone each increased significantly the analgesic potency of morphine by 1.4-fold whereas the combination of these two treatments increased analgesic potency by almost 2-fold, significantly more than either treatment alone. All animals were retested with morphine 6 days later; the analgesic potency of morphine in the SHAM groups was still potentiated by stress but the potentiating effect of the earlier naloxone infusion was no longer evident. It appears that the opioid receptors mediating stress-induced potentiation of morphine-induced analgesia can be upregulated transiently by 7-day infusion of naloxone.

Effects of beta-funaltrexamine treatment and sexual isolation in the perinatal period on the development of mu-opioid receptors and nociception

Male and female rats were segregated from birth (sexually isolated animals). Additional litters containing both male and female pups were kept as controls (mixed-housed animals). beta-FNA (5 mg/kg) or water was administered SC at day 0 or day 7 to isolated and mixed-housed animals. The development of mu-opioid receptors and nociceptive responses in the two groups was assessed at day 7 or 14, respectively. Mu-receptor binding was measured in whole brain using (3H) DAGO as a binding ligand and nociception assessed using the tail immersion test. beta-FNA treatment depressed mu-receptors when measured 1 but no 7 days later. However, male and female rats treated at day 0 with beta-FNA had lower brain protein content. Sexual isolation had little effect on mu-receptor number and did not augment the beta-FNA effect. However, isolation increased pain sensitivity in 7-day-old animals and in 14-day-old females. beta-FNA treatment had little effect on nociceptive threshold but reversed the effects of sexual isolation.

Involvement of mu-opioid receptors in the antitussive effects of pentazocine

The effect of pentazocine on the capsaicin-induced cough reflex in rats was investigated. Intraperitoneal injection of pentazocine, in doses from 1 to 10 mg/kg, significantly decreased the number of coughs in a dose-dependent manner. The antitussive effect of pentazocine (10 mg/kg, i.p.) was significantly reduced by prior injection of naloxone (0.3 mg/kg, i.p.), but it was unaffected by Mr-2266 BS (5 mg/kg, i.p.), an antagonist of kappa-opioid receptors. The antinociceptive potency of pentazocine (30 mg/kg, i.p.), as determined by the formalin test, was significantly reduced by pretreatment with Mr-2266 BS (5 mg/kg, i.p.), whereas naloxone (0.3 mg/kg, i.p.) had no significant effect on the antinociceptive effect of pentazocine. The antitussive effects of pentazocine (3 mg/kg) and morphine (0.1 mg/kg) were significantly enhanced in rats treated chronically with naloxone (5 mg/kg/day, 5 days), whereas the antitussive effect of U-50,488 H (1 mg/kg, i.p.), a selective kappa-opioid agonist, was not enhanced in these rats. By contrast, the antinociceptive effect of morphine (0.01 mg/kg, i.p.) was significantly enhanced in rats that had been pretreated chronically with naloxone. However, the antinociceptive effects induced by pentazocine (3 mg/kg, i.p.) and U-50,488 H (1 mg/kg, i.p.) were unchanged. These results suggest that pentazocine-induced antitussive effects in rats are mediated via stimulation of mu-opioid receptors.

Increased sensitivity to rate-altering and discriminative stimulus effects of morphine following continuous exposure to naltrexone

Two experiments evaluated whether termination of a continuous infusion of naltrexone altered sensitivity to the rate-suppressing or discriminative stimulus effects of morphine in rats. An 8-day infusion of saline or doses of 3, 10, or 18 mg/kg/day naltrexone did not alter rates of lever pressing maintained under fixed-ratio 30 schedules of food delivery. A dose of 10 mg/kg day naltrexone produced insurmountable antagonism of the rate-suppressing and analgesic effects of morphine. The ED50 of morphine for rate suppression decreased by 2-fold 1 day after termination of the 8-day infusion of 10 or 18 mg/kg/day naltrexone. The ED50 of morphine returned to initial values within 8 days. Termination of infusion of either saline or 3 mg/kg/day naltrexone did not alter the ED50 of morphine. Changes in morphine stimulus control were evaluated in rats trained to discriminate saline and 3.2 mg/kg morphine under fixed-ratio 15 schedules of food delivery. The ED50 of morphine for stimulus control or rate suppression decreased by 2-fold 1 day after termination of an 8-day infusion of 18 mg/kg/day naltrexone. The ED50 of morphine for rate suppression returned to initial values within 3 days; that for stimulus control, within 5 days. Thus, termination of exposure to high doses of naltrexone produced brief changes in sensitivity to the rate-altering and discriminative stimulus effects of morphine that parallel reported changes in sensitivity to the analgesic and lethal effects of morphine.

Conditioning of morphine-induced taste aversion and analgesia

The process of selective associations is evident in the aversive conditioning literature, where it has been shown that external cues are readily associated with peripheral pain, whereas taste cues are more easily associated with effects of drug administration. Within this framework, it is of interest that the failures to obtain a conditioned analgesic response to a morphine-associated CS have used external cues as conditioned stimuli. In Experiment 1, subjects re-exposed to a morphine-associated CS not only expressed the anticipated taste aversion, but also exhibited a decrease in pain sensitivity that was evident 15 or 30 min following CS re-exposure. Experiment 2 suggested that the conditioned analgesic response was opioid mediated, as pre-test administration of naloxone blocked expression of the analgesic CR. In Experiment 3, an increase in opiate receptor sensitivity produced by chronic naltrexone treatment did not affect the strength of the taste aversion, but resulted in an increase in the magnitude of the conditioned analgesic response. Collectively, these data suggest a neuropharmacological dissociation in systems mediating the two responses.

Long-lasting effects of naltrexone, an opioid receptor antagonist, on cell proliferation in developing rat forebrain

Several studies have demonstrated in the past that endogenous opioid peptides and opioid receptors may be involved as mediators of brain tissue growth and function in the neonate. Applying histological and autoradiographic methods, we have examined the effect of the mu-receptor-specific antagonist, naltrexone, on the proliferation of the 4-12-week-old rat forebrain subependymal layer. We found that naltrexone, when given daily throughout the weaning period, evoked a long-lasting increase of the mitotic rate and the [3H]thymidine labelling index. This effect was most significant about 8-10 weeks after ending the naltrexone treatment. Although a direct influence of naltrexone on long-term subependymal cell proliferation cannot be excluded, we are discussing evidence of an indirect effect via suppression of noradrenergic activity in the forebrain.

Delayed suppression of serum luteinizing hormone after naloxone treatment in neonatal female rats

In female neonatal rats, opiate receptor blockade markedly raises serum luteinizing hormone (LH) levels. The LH effect of acute treatment with opiate antagonists is apparently brief in older rats; however, age-related differences in antagonist pharmacokinetics may result in different LH response patterns. The duration of LH response to naloxone (NAL) and naltrexone (NTX) was examined in 5 day-old (d.o.) female rats and compared to the duration of analgesia blockade. The rise in serum LH following opiate receptor blockade in 5 d.o. rats was of similar duration to that previously observed in older animals and much briefer than blockade of analgesia. Furthermore, neonatal rats exhibited a delayed suppression of LH 6 hr following NAL, but not NTX, treatment. Stimulation and later suppression of LH were still observed after five repetitive NAL treatments at 6 hr intervals.

Chronic treatment with naltrexone enhances morphine-stimulated dopamine neurotransmission: neurochemical and behavioral evidence

Rats were implanted for 10 days with a slow-release pellet of naltrexone or were given sham surgery. At one of various different intervals during or after implantation of the pellet, the synthesis of dopamine (DA) was assessed in the nigrostriatal and mesolimbic systems. The results indicated that naltrexone alone was without effect on the synthesis of DA. However, one day after removal of the pellet, naltrexone-treated animals displayed an enhanced response to the DA-stimulatory action of morphine (15 mg/kg) in both the nigrostriatal and mesolimbic systems. This change was accompanied by an increase in specific binding of the mu-specific radioligand [3H]DAGO in whole brain and by an increase in the depressant action of morphine on locomotor activity. In contrast, at 10 days after removal of the pellet, naltrexone was without effect on morphine-induced changes in the synthesis of DA and locomotor activity, thus indicating that the supersensitivity to morphine was transient. These results support the idea that opioids modulate DAergic neurotransmission in the nigrostriatal and mesolimbic pathways and that this modulatory role may underlie opiate-induced changes in locomotor behavior.

Chronic naltrexone supersensitizes the reinforcing and locomotor-activating effects of morphine

Rats were implanted for 10 days with a slow-release naltrexone pellet and then the pellet was removed. Sham-control animals were treated similarly, except no pellet was implanted. One day after pellet removal or sham treatment, animals were assessed for morphine-induced conditioned place preference (CPP) or locomotor activity. CPP was evident in sham animals following two conditioning trials using 5 mg/kg subcutaneous morphine (Experiment 1) and following one conditioning trial using 8 mg/kg intravenous morphine (Experiment 2). Animals conditioned while implanted with a naltrexone pellet showed no morphine-induced CPP. More important, one day after pellet removal, naltrexone-pretreated animals given one conditioning trial with 5 mg/kg intravenous morphine displayed a greater preference for morphine-associated cues relative to sham animals given morphine (Experiment 3 and 4). This single IV morphine dose was insufficient to produce CPP in sham animals, suggesting that naltrexone-induced supersensitization may only be evident at a morphine dose below the reinforcing threshold in control animals. Further, chronic naltrexone potentiated the locomotor-activating effect of 2 mg/kg subcutaneous morphine but not of either 1 or 5 mg/kg morphine (Experiment 5). Behavioral supersensitization assessed by morphine-induced locomotor activation was transient, as it was evident one day, but not either three or 10 days following pellet removal (Experiment 6). These results confirm the functional significance of opiate receptor up-regulation following chronic opioid blockade.

Endogenous opioid systems regulate cell proliferation in the developing rat brain

The role of endogenous opioid systems in modulating the proliferation of developing cerebellar cells was examined autoradiographically in 6-day-old rats. The blockade of endogenous opioid-opioid receptor interaction by naltrexone, a potent opioid antagonist, was accompanied within 1-2 h by an increased proportion of cells incorporating [3H]thymidine. When high doses of naltrexone (50 mg/kg) were administered this index was still elevated 12 h later; however, when low doses of naltrexone (1 mg/kg) were administered the index of labeled cells was decreased markedly. Injection of methionine-enkephalin, an endogenous opioid peptide, also resulted in a decrease in the proportion of cells incorporating [3H]thymidine. Concomitant injection of 1 mg/kg naloxone, however, blocked the inhibitory effects of methionine-enkephalin on cell division but did not itself affect cell generation. These studies demonstrate that endogenous opioid systems can regulate the proliferation of cell populations in the developing nervous system and do so through an inhibitory mechanism.

Cross-tolerance between analgesic low doses of morphine and naloxone in arthritic rats

The effects of acute injections of naloxone (3-3000 micrograms/kg i.v.) and morphine (100-1000 micrograms/kg i.v.) on the vocalization threshold induced by pressure on the paw were analyzed in adjuvant-induced arthritic rats pretreated either with naloxone or with morphine administered at low doses (9 micrograms/kg s.c. and 3000 micrograms/kg s.c., respectively) over 4 consecutive days. In naloxone-pretreated arthritic rats, the paradoxical analgesic effect of low doses of naloxone was almost abolished, and the potent analgesic effects of low doses of morphine were also strongly and dose-dependently reduced. In morphine-pretreated arthritic animals, the analgesic effect of low doses of naloxone was significantly attenuated. These results attest that a cross-tolerance with low analgesic doses of morphine and naloxone can be demonstrated in these chronic suffering animals. By contrast, in rats pretreated either with naloxone or morphine, the hyperalgesic effect of naloxone produced by higher doses persisted and even was unmasked for doses which were analgesic before the pretreatment. These data emphasize the involvement of opiate receptors different in their sensitivity and/or their functions in the two opposite effects of naloxone. They also suggest that opiate receptors and endorphinergic systems differ in normal animals and animals which experience persistent pain.

Effect of withdrawal from chronic naltrexone on regional cerebral oxygen consumption in the cat

This investigation determined the effects of withdrawal from chronic naltrexone administration on average and regional cerebral blood flow, oxygen extraction and oxygen consumption. The relationship between the effects of withdrawal from chronic administration of this opiate receptor antagonist, which may increase the numbers of postsynaptic opiate receptors, and these parameters was investigated. Fourteen adult mongrel cats were administered subcutaneous injections of 1 mg/kg naltrexone HCl or 1 ml 0.9% saline twice daily for 21 days. Two days later, regional cerebral blood flow was monitored using radioactively tagged microspheres. The animals were sacrificed and prepared for microspectrophotometric analysis of regional cerebral venous and arterial oxygen saturation. Regional cerebral oxygen consumption was calculated as the product of cerebral blood flow and oxygen extraction for each area examined. After 2 days of withdrawal from chronic naltrexone treatment, the blood pressure, heart rate and blood gas parameters did not change significantly when compared to saline-treated animals. Average cerebral blood flow was significantly increased from 47.9 +/- 3.4 ml/min/100 g (mean +/- S.E.M.) in the control group to 80.3 +/- 6.5 (ml/min/100 g) in the chronic naltrexone-treated group. Flow was significantly increased in the cortex, lenticulate nuclei, thalamus and pons. Neither average cerebral oxygen consumption, which increased slightly, nor cerebral oxygen extraction, which decreased slightly, were significantly altered by treatment. The distribution of flow among the examined regions was, however, significantly altered in the animals 2 days after receiving chronic naltrexone injections. These changes were not restricted to brain regions dense in neuronal opiate receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioid antagonist-induced modulation of cerebral and hippocampal development: histological and morphometric studies

The role of endogenous opioid systems in preweaning cerebral and hippocampal development was explored in rats utilizing naltrexone, a potent opioid antagonist. Sprague-Dawley rats were given daily injections (s.c.) of either 1 or 50 mg/kg naltrexone to invoke a temporary or complete blockade, respectively, of opioid receptors throughout the first 3 weeks of postnatal life; animals injected with sterile water served as controls. At weaning (Day 21), macroscopic, morphometric, and histological assessments were undertaken. In general, 50 mg/kg naltrexone had a stimulatory action on brain development, whereas 1 mg/kg naltrexone had an inhibitory influence. In most cases, both males and females were affected comparably. Opioid antagonist action was especially directed at cellular and tissue differentiation, with marked changes in macroscopic and areal dimensions and histotypic organization observed in the cerebrum. A prominent effect on the cerebrum of the 1 mg/kg naltrexone group was a substantial increase in packing density of the neural cells, reflecting a reduced area for accommodating neural elements. Changes in the hippocampus were largely restricted to the 1 mg/kg group. However, the number of granule cells was increased in the dentate gyrus of the 50 mg/kg group, suggesting that opioid receptor blockade affects cell types undergoing postnatal proliferation. Cellular elements derived prior to naltrexone treatment (e.g., pyramidal neurons) were capable of being influenced in only differentiative capacity. Our results show that endogenous opioids are natural trophic factors in brain development and provide evidence for the crucial role of endogenous opioid-opioid receptor interaction in neuro-ontogeny.

Unexpected effects of nalmefene, a new opiate antagonist, on the hypothalamic-pituitary-gonadal axis in the male rat

In order to gain additional information on the role of brain opioid peptides in the regulation of the hypothalamic-pituitary-gonadal axis, we studied the effects of nalmefene, a new opiate antagonist, on gonadotropin and testosterone secretion in male rats. The results were compared with those obtained with naloxone, a well-studied antagonist. Acute injections of either nalmefene or naloxone (2 mg/kg) produced 4-fold increases in LH and testosterone secretion. In castrated male rats treated with testosterone propionate (TP), nalmefene (10 mg/kg) reversed the androgen negative feedback on LH secretion; surprisingly, when higher doses (25 and 50 mg/kg) were injected, the compound lost its ability to antagonize the testosterone-induced inhibition of LH levels. In contrast, naloxone was able to increase LH levels in TP-treated castrated rats even at the highest dose tested (50 mg/kg). Chronic administration of these antagonists resulted in suppression of the acute release of LH and T secretion in nalmefene-treated but not in naloxone-injected animals. These data are consistent with previous observations suggesting that opioid peptides a) exert a tonic inhibitory effect on LH and testosterone production and b) participate in the negative androgen-induced feedback control of LH secretion. Our results also show that the antagonistic action of nalmefene, but not naloxone, is reversed when higher doses are used or following chronic administration.

Increased analgesic potency of morphine and increased brain opioid binding sites in the rat following chronic naltrexone treatment

Implantation of rats with prolonged-release naltrexone pellets increased both morphine's analgesic potency in the tailflick assay and radiolabeled opioid binding in the brain. The increases in both radiolabeled opioid binding and morphine potency were time-dependent. Implantation for 24 hours did not increase binding, whereas increases of approximately 45% were seen following 8 days of implantation. Similarly, morphine's analgesic potency, measured as ED50 values, was increased by 50% following 8 days of exposure to naltrexone while a 24 hour exposure had no significant effect.

Developmental, behavioral, and opiate receptor changes after prenatal or postnatal beta-endorphin, CRF, or Tyr-MIF-1

Developmental and long-term behavioral effects of perinatal injection of beta-endorphin (BE), CRF and Tyr-Pro-Leu-Gly-NH2 (Tyr-MIF-1) in male rats were investigated along with the possibility that opiate receptors may be altered by the injection of BE during this critical time. Daily injections of peptide were given to pregnant females (100 micrograms/rat) in the week before birth or to the offspring (50 micrograms/rat) of untreated mothers during the first week of life. Prenatal BE and CRF reduced body weight on day 1, in contrast to Tyr-MIF-1 which produced a significant increase over controls by day 7 as well as a slight but significant acceleration of eye opening. Among the postnatal treatments, CRF-treated animals showed the most dramatic changes. These included decreased body weight, accelerated eye opening, and, in adulthood, increased open field rearing behavior and a tendency for a monotonic body temperature response to low doses of morphine, in contrast to the biphasic response shown by controls. BE, when given to pregnant mothers, increased the number (Bmax) of [3H]naloxone-labeled (mu) receptors in whole brains of offspring assayed on day 14, but it did not significantly alter [3H]D-Ala-D-Leu-enkephalin-labeled (delta) receptors. In contrast, a significant decrease in both mu and delta receptors was observed on day 14 in rats given BE postnatally. These differences in receptors were no longer apparent in adulthood, and no significant differences in tail-flick response were detectable at this time. Nevertheless, some of the effects of these three peptides endured well beyond their presence, and for BE included changes in the number of opiate receptors.

Chronic naloxone increases opiate binding in SHR and WKY rats

We have previously reported that chronic administration of naloxone to SHR and WKY rats results in a significant increase in their systolic blood pressure relative to control animals. In the present study we show that chronic naloxone is also accompanied by a marked increase in the number of brain opiate receptors. Although the relative difference in blood pressure diminishes with increasing maturity, the elevation in brain opiate receptors remains in the treated animals. Mechanisms for these differential effects are discussed.

Opiate receptor supersensitivity produced by chronic naloxone treatment: dissociation of morphine-induced antinociception and conditioned taste aversion

In three separate experiments, rats were used to assess the effects of chronic administration of naloxone on specific binding of 3H-naloxone in various regions of the central nervous system (CNS) and on the efficacy of morphine to produce antinociception and a conditioned taste aversion. Chronic naloxone treatment increased opiate binding in medulla-pons, midbrain, hypothalamus, hippocampus, striatum, and prefrontal cortex, but not in either spinal cord or cerebellum. In those CNS regions exhibiting increased opiate binding, the duration of increased binding following termination of the naloxone treatment differed between regions. In conjunction with the increase in opiate binding, the efficacy of morphine to produce antinociception was potentiated, while the efficacy to produce a conditioned taste aversion was unchanged. Moreover, the administration of naloxone during behavioral testing blocked completely the antinociceptive effect, but not the aversive effect, of morphine. These results indicate that morphine-induced antinociception and conditioned taste aversion may be dissociated neuropharmacologically.

Opiates, endorphins, and the developing organism: a comprehensive bibliography, 1982-1983

A comprehensive bibliography of the literature concerned with opiates, endorphins, and the developing organism for 1982 and 1983 is presented. Utilized with a companion paper (Neurosci Biobehav Rev 6: 439-479, 1982) these articles cover clinical and laboratory references beginning in 1875. For the years 1982 and 1983, a total of 385 citations was recorded. A series of indexes accompanies the citations in order to make the literature more accessible. These indexes are divided into clinical and laboratory topics. The clinical section is subdivided into: age of subject examined, maternal aspects, the fetus, and the offspring. The laboratory section is subdivided into: type of opiate/endorphin studied, species utilized, and major subject areas explored.

Developmental changes in environmentally induced analgesia

Previous studies have shown that exposure to inescapable, front-paw shock produces an opioid-mediated analgesia. Additionally, research has revealed that the majority of opioid receptors in the central nervous system are formed between birth and adulthood. The purpose of the present experiment was to examine the relationship between the development of the opioid receptors and the function of the endogenous opioid pain-inhibitory system activated by shock. Ten-day-old, 28-day-old and 5-7-month-old rats were exposed to 90 s of front-paw shock (1.6 mA). Results revealed that 10-day-old rats displayed lower levels of shock-induced analgesia than 28-day-old and 5-7-month-old rats. This age-related difference in shock-induced analgesia confirms a parallel in the development of opioid receptors and the function of an endogenous pain-inhibitory system. In addition, injection of naloxone produced an increased analgesia in the 10-day-old rats. In the 28-day-old rats naloxone completely blocked the shock-induced analgesia while in the 5-7-month-old rats naloxone only partially attenuated analgesia. This age-related difference (28-day-old vs 5-7-month-old) in the effectiveness of naloxone in blocking shock-induced analgesia suggests the involvement of a non-opioid analgesia system in the 5-7-month-old rats that is not present in the 28-day-old rats. This last difference led to the speculation that the non-opioid analgesia system develops more slowly than the opioid system.

Naloxone administration following brief exposure to novelty reduces activity and rearing in mice upon 24-h retest: a conditioned aversion?

It has recently been reported that naloxone treatment, prior to initial exposure to a novel arena, results in significant behavioural change when animals are retested 24 h later. In an attempt to clarify the nature of this delayed action of the opiate antagonist, three further studies have been performed. In the first experiment, male mice were injected with naloxone hydrochloride (0-10 mg/kg, IP) immediately after their initial experience of the test arena. When retested 24 h later, all groups that had previously received naloxone exhibited greatly reduced activity and rearing, with no evidence of a dose-response relationship. In the second experiment, naloxone (0-10 mg/kg) failed to induce a conditioned place aversion when administered according to the above regimen. In the final experiment, no evidence for a naloxone-induced taste aversion to saccharin was observed. It is concluded that the behavioural changes observed in the open-field study may reflect either (a) subtle aversive properties of naloxone which are insensitive to traditional one-trial paradigms or (b) opioid modulation of memory for non-painful experiences.

Chronic naltrexone increases opiate binding in brain and produces supersensitivity to morphine in the locus coeruleus of the rat

Rats were implanted subcutaneously for 2-4 weeks with slow-release pellets of naltrexone (10 mg) or placebo and then the pellets were removed. One day after removal of the pellet, animals were either (1) sacrificed and various CNS regions examined for specific binding of [3H]naloxone, [3H]etorphine or [3H]rauwolscine or (2) they were anesthetized and prepared acutely for assessing morphine-induced changes in the spontaneous activity of neurons in the locus coeruleus (LC). Naltrexone treatment significantly increased the number of specific binding sites for opiates, but not for alpha 2-adrenergic antagonists, in spinal cord, hypothalamus, striatum and cortex. Specific binding of [3H]naloxone was also increased in the LC. The spontaneous activity of neurons in the LC was reduced by the chronic naltrexone treatment, suggesting that these neurons became supersensitive to the tonic inhibitory effect of endogenous opioid peptides. Moreover, neurons in the LC of chronic naltrexone-treated rats exhibited an enhanced response to the inhibitory effects of morphine administered systemically. These results demonstrate that chronic opiate receptor blockade increases the number of receptor sites for morphine and that this increase in receptors is accompanied by a neuronal supersensitivity in the LC to morphine which can be assessed electrophysiologically.