Effect of delayed weaning on opioid receptor control of swim stress-induced antinociception in the developing rat

Risk-taking behavior in adolescent mice: psychobiological determinants and early epigenetic influence

Epidemiological research has emphasized that adolescence is associated with some temperamental and behavioral traits that are typical of this age and that might substantially contribute to both psychological and psychobiological vulnerability. The contribution of the important developmental rearrangements in neurobiological and neuroendocrinological processes has received surprisingly little investigation. The present review summarizes recent work in animal models, indicating that adolescent rodents exhibit marked peculiarities in their spontaneous behavioral repertoire. When compared to adults, adolescents show an unbalanced and 'extremes-oriented' behavior, consisting of an increased novelty seeking, together with decreased novelty-induced stress and anxiety, an increased risk-taking behavior in the plus-maze, as well as elevated levels of impulsivity and restlessness. Age-related discontinuities in the function of monoaminergic systems, which are a main target of abused drugs, can perhaps account for such a profile. In particular, a peculiar function within reward-related dopaminergic brain pathways actually seems to underlie the search for novel and rewarding sensations, as well as changes in the magnitude of psychostimulant effects. The role played by early epigenetic factors in the shaping of novelty-seeking behavior of adolescent and adult rodents are also reviewed. Two examples are considered, namely, subtle variations in the hormonal milieu as a function of intrauterine position and precocious or delayed maturation of nutritional independence as a function of changes in time of weaning. As for spontaneous drug consumption, a prominent vulnerability to the oral intake of nicotine during early adolescence is reported. In conclusion, adolescence in rodents may represent a suitable animal model with enough face- and construct-validity. Actually, this model is able to show behavioral features that resemble those found in human adolescents, including vulnerability to the consumption of psychoactive drugs.

delta-Opioid modulation of social interactions in juvenile mice weaned at different ages

The environmental stimulus of weaning has been shown to affect both the developmental expression of social behavior and the maturation of the opioid delta-receptors' subpopulation in altricial rodents. The aim of this study was to address both these issues by using the social interaction paradigm. Separate groups of male and female mice were randomly assigned to three different weaning ages -- early (Wean-15), regular (Wean-20), and delayed (Wean-25) -- and assessed when 30 days old under intraperitoneal administration of the selective delta-opioid agonist SNC80 (0, 0.1, or 0.3 mg/kg). Wean-15 male and female subjects were much more involved in investigating the partner as well as the cage environment compared to the regular Wean-20 group. An increased social investigation was also found as a consequence of delayed weaning in the female group. The neurobehavioral changes induced by the manipulation of weaning age were also reflected in an altered responsivity to the effects of SNC80 administration. The drug-induced increase in the expression of investigative and affiliative social interactions was further magnified by early weaning. A delayed weaning time was instead associated with reduced sensitivity to the drug, which suggests a delayed maturation of the system. As a whole, the present results indicate that the time of weaning is able to markedly affect the expression of social interactions of adolescent mice by possibly exerting a direct modulatory role on the development of the still plastic delta-opioid system.

Weaning-induced development of delta-opioid receptors in rat brain: differential effects of guanine nucleotides and sodium upon ligand-receptor recognition

1. We have previously shown that weaning at day 21 increases delta-opioid receptor binding in the brain at day 25, which might be due to stimulation of the development of a delta-opioid receptor subtype or activation of G-protein coupling processes. 2. We have addressed the possibility that weaning stimulates coupling of the delta-receptor by homogenate binding studies with four agonist and one antagonist radioligand in the presence of a GTP analogue and Na+ in brain tissue from weaned and non-weaned animals. 3. Saturation studies with three agonist ligands ([3H]-deltorphin I, [3H]-S-Atc-Ile(5,6)deltorphin I and [3H]-R-Atc-Ile(5,6)deltorphin II) showed higher levels of maximal binding in brains from 25-day weaned than in brains from non-weaned rats. The magnitude of the effects of GMPPNP and Na+ in decreasing this binding was ligand dependent and in each case was significantly more marked in brains from weaned animals. GMPPNP and Na+ were completely without effect on Bmax for, [3H]-S-Atc-Ile(5,6)deltorphin I and [3H]-R-Atc-Ile(5,6)deltorphin II in brains from non-weaned rats. 4. [3H]-Ile(5,6)deltorphin II and [3H]-naltrindole showed no differences in labelling between weaned and non-weaned groups and both groups responded similarly to the effects of GMPPNP and Na+ treatment. 5. GMPPNP and Na+ had small effects on binding affinity (K(D)) for some of the agonist radioligands which were similar in both weaned and non-weaned groups. 6. Weaning induced increases in binding of delta-receptors in 25-day rats can be explained in terms of the way delta-agonist radioligands recognize the receptor environment.

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.

Ontogeny of P2-purinoceptors in the longitudinal muscle and muscularis mucosae of the rat isolated duodenum

1. The ontogeny of P2-purinoceptors in the longitudinal muscle and the muscularis mucosae of the rat isolated duodenum was investigated by use of functional assays in tissues from neonatal animals. The degradation of purinoceptor agonists by the rat duodenum muscularis mucosae was also investigated. 2. In the rat duodenum muscularis mucosae adenosine 5'-(alpha, beta-methylene)triphosphonate (AMPCPP), adenosine 5'-triphosphate (ATP), uridine 5'-triphosphate (UTP) and 2-methylthioadenosine 5'-triphosphate (2-Me-S-ATP) all caused a contraction from day 10 to day 40, day 10 being the earliest age it could be tested. The potency order of agonists above day 25 was AMPCPP > ATP = UTP > 2-Me-S-ATP and this is similar to the potency order previously obtained for the adult tissue. However, in the neonatal tissues below day 20, 2-Me-S-ATP was the most potent agonist and at days 10 and 15 the order was 2-Me-S-ATP > AMPCPP > ATP = UTP. 3. In the rat duodenum muscularis mucosae desensitization was observed with AMPCPP at day 30 but not at day 15. At day 30, cross-desensitization was also observed between AMPCPP and 2-Me-S-ATP but not between AMPCPP and ATP or UTP, whereas no cross-desensitization was observed at day 15 with AMPCPP and any of the agonists. At day 15 and below AMPCPP and 2-Me-S-ATP may therefore both activate P2Y-receptors (2-Me-S-ATP > AMPCPP, no desensitization with AMPCPP) whereas above day 20 the agonists activate P2X-receptors (AMPCPP > 2-Me-S-ATP, desensitization with AMPCPP) which is similar to the adult tissue. Since ATP and UTP were equipotent in the muscularis mucosae and as no cross-desensitization was observed with AMPCPP and UTP or ATP at days 15 or 30, it is likely that ATP and UTP both activate P2U-receptors throughout the ages, as in the adult. 4. The potency of all the agonists in causing contraction in the rat duodenum muscularis mucosae decreased with age. The potency of AMPCPP and 2-Me-S-ATP in causing contractions was highest in the neonates before day 25, and reached values not significantly different from adult by day 30, and the potency of ATP and UTP causing contractions in this tissue was also highest in the neonates at days 10 and 15, and reached values not significantly different from adult by day 20. This suggests either that the receptor populations mediating contraction are highest in the neonates below day 20 or that the agonists are degraded by the muscularis mucosae to a greater extent after day 20. 5. In the rat duodenum muscularis mucosae the degradation of ATP, UTP, 2-Me-S-ATP and AMPCPP was followed by high pressure liquid chromatography at days 15 and 30. ATP was degraded to adenosine 5'-diphosphate (ADP), adenosine 5'-monophosphate (AMP) and inosine with no adenosine being detected, 2-Me-S-ATP was degraded to 2-methylthioadenosine 5'-diphosphate (2-Me-S-ADP), 2-methylthioadenosine 5'-monophosphate (2-Me-S-AMP) and 2-methylthioadenosine (2-Me-S-adenosine), and UTP was degraded to uridine 5'-diphosphate (UDP), uridine 5'-monophosphate (UMP) and uridine. The rate of degradation of these agonists was much faster at day 30 than at day 15, probably due to the increase in the size of the tissue. AMPCPP was also degraded with adenosine 5'-(alpha,beta-methylene)diphosphonate (AMPCP) being detected at both ages. However, at day 30 the rate of degradation of AMPCPP was much slower than for ATP, UTP or 2-Me-S-ATP. 6. In the rat duodenum longitudinal muscle 2-Me-S-ATP and AMPCPP both caused a relaxation with a potency order of 2-Me-S-ATP > AMPCPP, suggesting the activation of P2Y-receptors, as previously found for the adult tissue. Weak relaxations were observed to both the agonists at day 15 (the earliest age it could be studied), and the potency of the agonists reached values not significantly different from adult tissues by day 25. 7. Overall, these results suggest that in the neonatal rat duodenum longitudinal muscle there are P2Y-receptors mediating relaxation and that the receptor population i

Differential development of adenosine A1 and A2b receptors in the rat duodenum

1. The development of the adenosine A1 and A2b receptors inducing relaxation of the rat duodenum was studied by use of a combination of functional and radioligand binding assays on rats aged between 5 and 30 days and compared with results previously found in adult rat duodenum. 2. 1,3-[3H]-dipropyl-8-cyclopentylxanthine ([3H]-DPCPX) bound with high affinity to a single site in duodenum preparations from rats aged 20, 25 and 30 days. At 10 and 15 days there was no detectable specific binding of [3H]-DPCPX. 3. The affinity (KD) of the binding site for [3H]-DPCPX was similar in membrane preparations from 20, 25 and 30 day old animals (1.58-2.27 nM), but the density (Bmax) of binding sites was found to increase up to 25 days where peak levels (72.0 +/- 9.5 fmol mg-1 protein) were observed and then decline at 30 days (45.5 +/- 2.9 fmol mg-1 protein) to levels commensurate with those previously determined in the adult rat duodenum. 4. In duodenum from 10 day old rats no responses to N6-cyclopentyladenosine (CPA, 1 nM-10 microM) were observed, at 15 days the duodenum responded to the highest concentration of CPA (3 microM) only, and at 20-30 days concentration-related responses were observed, with the potency of CPA increasing with an increase in age. DPCPX (10 nM) abolished the responses to CPA except at the highest concentration of CPA (3 microM) where the response was markedly attenuated, suggesting the presence of an A1, receptor. 5. In rat duodenum from animals of all ages (5-30 days), concentration-related responses to 5'-N-ethylcarboxamidoadenosine (NECA) were observed. The potency of NECA remained constant with an increase in age, whereas the maximum relaxation response increased from 20% at 5 days to 110% at 25 and 30 days. In the presence of 1 microM DPCPX a right-ward shift in the concentration-response curve to NECA was observed at all ages. In the presence of 10 nM DPCPX, the response to NECA was unaffected in the duodenum from animals aged 10 and 15 days. However, in duodenum from animals aged 20-30 days the concentration-response curve to NECA was shifted to the right suggesting that there is an A1 component to the action of NECA at these ages. Schild analysis of the effects of increasing concentrations of DPCPX versus NECA on the duodenum from 25 day old animals generated a slope of 0.62 suggesting that NECA acts at A1 and A2b receptors as in the adult. 6. The A2b-selective analogue, 2-[p-(carboxyethyl)-phenylethylamino]-5'-N-ethylcarboxamidoadenosi ne (CGS 21680) (10 nM-10 microM) was without effect on the carbachol-contracted duodenum from 15 day old rats and the duodenum from 25 day old rats responded to the highest concentration of CGS 21680 only, suggesting that the A2 receptors here, as in the adult, are not of the A2a subtype. The adenosine antagonist, 8-phenyltheophylline (8-PT) (10 microM), abolished the inhibitory effects of NECA (100 nM-100 microM) on 10, 15 and 25 day old rat duodenum indicating that the responses to NECA were not mediated via an adenosine A3 receptor. 7. These results show that adenosine A1 receptors in rat duodenum are present and functionally viable from day 20 onwards and that the density of A1 receptors varies with age, increasing up to day 25 and then declining at day 30 to a density commensurate with that found in the adult. The responses to CPA, mediated via the A1 receptor, increase with age in a similar fashion. In contrast however, the response to NECA was evident from day 5, the earliest age studied, and from days 5-15 NECA acted via the A2b receptor subtype. However, from day 20 onwards NECA acted at a mixed population of A1 and A2b receptors. These results demonstrate the differential development of the A1 and the A2b receptors in the rat duodenum.

Effects of selective opioid receptor antagonists on morphine-induced changes in striatal and limbic dopamine metabolism

The effects of selective opioid receptor antagonists, beta-funaltrexamine (selective for mu receptor), naloxonazine (microliter) and naltrindole (delta) on morphine-induced changes in striatal and limbic dopamine (DA) metabolism were studied in rats. beta-Funaltrexamine (20 micrograms intracerebroventricularly) and naloxonazine (15 mg/kg intraperitoneally) were given 24 hr before morphine (15 mg/kg subcutaneously), and the rats were decapitated 60 min. after morphine. Naltrindole (1 mg/kg intraperitoneally) was given twice, 15 min. before and after morphine. Morphine significantly increased the concentrations of DA metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). This effect was significantly antagonized by pretreatment with beta-funaltrexamine but not by naloxonazine or naltrindole. However, naloxonazine attenuated the antinociceptive effect of morphine in the hot-plate test. The concentration of DA was not significantly altered by any of the drugs studied. These results show that selective blockade of mu-opioid receptors totally blocks the increase of striatal and limbic DA metabolism induced by morphine. It seems that mu 2-subtype of mu-opioid receptor predominantly mediates this effect. Blockade of delta-opioid receptor did not alter these effects of morphine.

Evidence that weaning stimulates the developmental expression of a delta-opioid receptor subtype in the rat

To study whether weaning stimulates the development of a subtype of the delta-opioid receptor we have measured antinociceptive activity of putative delta 1 (DELT I) and delta 2 (DSLET) agonists in weaned and non-weaned 25-day-old rats. In weaned rats, DSLET produced antinociception reversed by the delta 2-antagonist naltriben, but in non-weaned rats DSLET had no effect. Responses to DELT I were evident in both weaned and non-weaned rats and were not antagonised by naltriben. This is the first evidence that delta-receptor subtypes develop independently and suggests that weaning is the stimulus for the expression of the delta 2-subtype.

Endogenous opiates: 1993

This paper is the sixteenth installment of our annual review of research concerning the opiate system. It is restricted to papers published during 1993 that concern the behavioral effects of the endogenous opiate peptides, and does not include papers dealing only with their analgesic properties. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; development; immunological responses; and other behaviors.