Thermal control of mother-young contact revisited: hyperthermic rats nurse normally

Morphine (MOR) is known to inhibit maternal behavior and induce hyperthermia; at appropriate doses, concurrent administration of naloxone (NAL) counteracts its disruption of maternal behavior but not the hyperthermia. We used these findings to evaluate the view that lactating rats terminate nursing due to intolerable hyperthermia. After a dam-litter separation of 4 h on Day 7 postpartum (PP), mother-litter interactions were observed continuously for 1 h. One hour before reunion, the dams received two injections (1 ml/kg ip each) of saline (SAL), MOR (20 mg/kg) and/or NAL (1 mg/kg) in the following combinations (n = 7 each): SAL + SAL, SAL + NAL, MOR + SAL or MOR + NAL. MOR profoundly disrupted maternal behavior, thereby preventing litter weight gains; these effects were completely counteracted by NAL, which alone had no discernible effects. In contrast, MOR-induced hyperthermia (approximately 0.7 degrees C increase in each hour, before and after reunion with pups) was not antagonized by NAL at the doses used. Thus, an additional 0.7-1.4 degrees C of body temperature (T) did not delay the onset or reduce the duration of nursing compared with SAL-treated controls. Further, there were no group differences in behaviors displayed both shortly before and after a nursing bout that included milk ejections or in the resumption of nursing. Together with earlier methodological and empirical criticisms of the thermal control theory, as well as knowledge about the somatosensory determinants of nursing, the present results suggest that nursing bouts in lactating rats are not limited by the mother's T.     

Low doses of risperidone and morphine interact to produce more analgesia and greater extrapyramidal effects in rats

The search for non-narcotic drugs that will enhance the analgesic effects of opiates without enhancing their side effects has included the investigation of psychoactive drugs already approved for other uses. Some research has supported an analgesic effect of risperidone (RIS), an atypical neuroleptic. However, the analysis of the analgesic efficacy of RIS alone or as an adjuvant to morphine (MOR) has not considered the production of adverse motor effects that would limit its usefulness as a treatment for pain. We tested whether low doses of RIS would enhance the analgesic action of opiates without inducing untoward motor effects. The analgesia induced by a range of RIS doses (0.1-1.0 mg/kg, SC) was assessed alone and in combination with MOR (5 mg/kg, IP) in male Long-Evans (hooded) rats using two different algesiometric assays: hotplate and tail-flick test. The presence or absence of ptosis, vacuous chewing, and abnormal stationary postures was recorded to evaluate dyskinetic effects. No dose of RIS alone altered pain threshold. However, the highest dose of RIS, 1.0 mg/kg SC, significantly increased the analgesic effects of MOR. Dyskinetic effects of RIS were dose-dependent and enhanced in RIS+MOR treatment. These results do not support the hypothesis that RIS, alone or in combination with MOR, elevates pain threshold without also inducing motor side effects. These findings suggest caution in the use of RIS as either a primary treatment or opiate adjuvant treatment for pain.     

Morphine treatment during pregnancy modulates behavioral selection in lactating rats

Previous studies have demonstrated that treatment of postpartum female rats with morphine inhibits maternal behavior and stimulates foraging. Exposure to drugs of abuse may result in a progressive enhancement of their reinforcing effects. Puerperal treatment with morphine leads to reverse tolerance to this drug. The present study investigated whether repeated morphine treatment during late pregnancy may influence the effects of different morphine dosages on behavioral selection in lactating rats. Females were simultaneously exposed to pups and insects, and the choice between taking care of the pups and hunting insects was observed. Female Wistar rats were treated with morphine (3.5 mg/kg/day, subcutaneous [s.c.]) or saline for 5 days beginning on pregnancy day 17. On day 5 of lactation, animals were acutely challenged with morphine (0.5, 1.0, or 1.5 mg/kg, s.c.; MM0.5, MM1.0, and MM1.5 groups, respectively) or saline (MS group) and tested for predatory hunting and maternal behavior. Control groups were pretreated with saline and challenged with morphine (SM0.5, SM1.0, and SM1.5 groups) or saline (SS group). Animals treated with morphine during late pregnancy and acutely challenged with 1.0 mg/kg morphine (MM1.0 group) exhibited significantly decreased maternal behavior and enhanced hunting. This effect was not evident with the 0.5 mg/kg dose. The 1.5 mg/kg morphine dose decreased maternal behavior and increased hunting in both the MM1.5 group and in animals challenged with morphine after previous saline treatment (SM1.5 group). These results provide evidence of plasticity of the opioidergic role in behavioral selection during lactation.     

The biphasic effect of morphine on odor conditioning in neonatal rats.

Three experiments examined the dose-dependent biphasic effect of morphine on odor conditioning in neonatal rats. In Experiment 1, a single pairing of an odor and a low dose of morphine (0.5 mg/kg) in 5-day-old rats produced an odor preference, relative to an unpaired control group. In Experiment 2, pairing an odor with a high dose of morphine (2.0 mg/kg) produced an odor aversion, relative to an unpaired control group. A third experiment compared performance of a group given odor and morphine (2.0 mg/kg) paired to that of two unpaired groups: one given morphine 24 hr prior to and the other 24 hr after odor exposure. The paired group showed an odor aversion relative to both of the unpaired groups, which did not differ. The latter finding suggests that even if morphine metabolism is incomplete after 24 hr, behavior is unaffected. These results are discussed in reference to the functional development of the opioid system in rats.     

Chronic exposure to morphine does not alter the neural tissues subserving its acute rewarding properties: apparent tolerance is overshadowing.

Drug-naive, but not morphine-dependent, rats preferred places paired with morphine (2 mg/kg) over unfamiliar neutral places. Both drug-naive and morphine-dependent rats preferred places paired with higher doses of morphine (20 mg/kg) over unfamiliar places. Lesions of the tegmental pedunculopontine nucleus (TPP) blocked the conditioned place preferences produced by both 2 and 20 mg/kg morphine in drug-naive rats but not the preferences produced by 20 mg/kg morphine in dependent rats. When morphine-dependent animals received withdrawal-alleviating doses of morphine (20 mg/kg) 3.5 hr before pairing one environment with 2 mg/kg morphine, they showed morphine-conditioned place preferences that were abolished by TPP lesions. The apparent behavioral tolerance to the TPP-mediated rewarding effects may have resulted from overshadowing by separate withdrawal-related motivational mechanisms.     

Dissociation of opioid and nonopioid analgesic responses following adult monosodium glutamate pretreatment

Neonatal administration of monosodium glutamate (MSG: 2-4 mg/g, SC) selectively destroys circumventricular organs, especially the arcuate nucleus and median eminence of the hypothalamus, and also attenuates both nonopioid (continuous cold-water swim: CCWS) and opioid (morphine) analgesia when rats are tested as adults. The present study evaluated whether administration of MSG (1-6 g/kg, SC) or its equiosmotic control (2.37 M NaCl) to adult rats altered either basal nociception on the tail-flick and jump tests or analgesia following morphine (5 mg/kg, SC) or CCWS (2 degrees C for 3.5 min). MSG treatment dose-dependently produced small but significant increases in basal nociceptive thresholds in adult rats. Morphine analgesia was significantly reduced on both tests following pretreatment with MSG (30-32%) and hypertonic NaCl (17-25%). In contrast, MSG (55-247%), but not NaCl pretreatment potentiated both nonopioid CCWS analgesia on both tests and CCWS hypothermia. These data are discussed in terms of differential neonatal and adult MSG effects, dissociations between opioid and nonopioid pain-inhibition, and the role of MSG in altering adaptive mechanisms to environmental stressors.     

纳洛酮对吗啡所致大鼠γ波改变的阻断作用

目的探讨纳洛酮对吗啡急性注射所致大鼠EEG变化的阻断作用。方法将大鼠随机分为3组,做植入式脑电手术,手术后1周监测EEG作为各自的前对照,分别腹腔注射(ip)生理盐水1ml,吗啡10mg/kg,同时腹腔注射吗啡(10mg/kg)与纳洛酮(4mg/kg),10min后记录EEG。结果1生理盐水没有引发EEG变化;2注射吗啡后30～100分钟γ明显减少(P<0.05)。3纳洛酮拮抗吗啡组在30～90分钟γ变化不明显。结论吗啡急性注射会引起大鼠γ波的变化,纳洛酮能够阻断该变化,并且两者都具有时间依赖性。     

Effects of maternal state on the responsiveness to nest odors of hooded rats

Systemic antagonism of H1 or H2 receptors for histamine attenuated drinking elicited by SC 20 mg/kg histamine in adult male Sprague-Dawley rats. The H1 antagonist dexbrompheniramine (DXB; 0.5-16 mg/kg) and the H2 antagonist cimetidine (C; 0.5-100 mg/kg) each inhibited drinking elicited by histamine when given IP 10 min prior to SC histamine: The lowest doses to produce a statistically significant inhibition of drinking were 2 mg/kg DXB and 32 mg/kg C. While 1 mg/kg DXB alone or 16 mg/kg DXB plus 16 mg/kg C virtually abolished drinking elicited by histamine (1.25-20 mg/kg) in a dose-response study. In addition, such combined antagonism of H1 and H2 receptors failed to elicit drinking in the absence of exogenous histamine and failed to inhibit drinking elicited by deprivation from water for 7 or 24 hr. Because combined systemic antagonism of H1 and H2 receptors can specifically and completely inhibit drinking elicited by exogenous histamine, these findings provide a probe for a histaminergic component of drinking in the rat.     

Indwelling catheter for infusions into subcutaneous tissue of freely-moving rats

A permanently implanted catheter, designed for drug administration to freely-moving rats and involving minimal construction and surgery, is described and tested. The catheter, placed subcutaneously (SC) along the length of the rat's back, consists of silicone tubing punched with tiny holes to allow the drug solution to pass. The drug is administered to an unrestrained rat through a light tubing connecting the catheter to a syringe. Analgesia produced in male Wistar rats by 2.5 and 25 mg/kg morphine sulfate delivered through the catheter was comparable to that produced by SC hypodermic injection, but more rapid in onset. Control groups indicated that there was no appreciable effect on the observed analgesia of the implantation carried out 24 hr earlier. Three days later these rats were used to demonstrate that 30 mg/kg of sodium pentobarbital was absorbed by the blood significantly more rapidly following administration through the catheter than following hypodermic administration.     

Catecholaminergic-serotonergic balance in the CNS and reproductive cycling in aging rats

Treatment with the serotonin (5-HT) reuptake inhibitor zimelidine, 20 mg/kg/24 hr, SC, for 14 days increased the duration of vaginal cycles in 3 month-old Long Evans hooded rats. It induced persistent vaginal estrus in 12 of 16 ten-month-old animals, and blocked reinitiation of vaginal cycles by L-dopa in 10 of 10 twenty-month-old rats. A single injection of zimelidine at 1400 hr did not alter the vaginal smear pattern of young or middle-aged cycling females or old constant estrus females. Also, a single dose of zimelidine at 1400 hr on the day of vaginal proestrus had no effect on serum LH values in young females. The serotonergic neurotoxin 5,7-dihydroxytryptamine, 4 micrograms, injected into the ventral and dorsal raphe areas (after desipramine, 25 mg/kg IP) reinitiated vaginal cycling in 8 of 13 twenty-month-old rats. These results suggest that age-dependent changes in serotonin metabolism may contribute to the age-dependent changes in luteinizing hormone secretion which eventually lead to the cessation of ovarian function in the rat and that alterations in serotonin function are an important component of the mechanism by which treatments with catecholamine precursors reinstate ovarian function in the old female rat.     

Seizure activity involved in the up-regulation of BDNF mRNA expression by activation of central Mu opioid receptors

Chemical-induced seizures up-regulated brain-derived neurotrophic factor (BDNF) mRNA expression. Intracerebroventricular (i.c.v.) administration of endogenous opioids preferentially activating μ opioid receptor (MOR) could also increase BDNF mRNA expression. The aim of this study was to determine to what extent i.c.v. administration of synthetic MOR-selective agonists in rats can modulate both seizure activity and up-regulation of BDNF mRNA expression. Effects and potencies of i.c.v. administration of morphine and [d-Ala², N-Me-Phe⁴, Gly⁵-ol]-enkephalin (DAMGO), were directly investigated by scoring behavioral seizures and measuring BDNF mRNA expression. In addition, effects of the opioid receptor antagonist naloxone and antiepileptic drugs, diazepam, phenobarbital, and valproate, on i.c.v. MOR agonist-induced behavioral seizures and up-regulation of BDNF mRNA expression were determined. A single i.c.v. administration of morphine (10–100 μg) or DAMGO (0.15–1.5 μg) dose-dependently elicited behavioral seizures and increased BDNF mRNA expression in the widespread brain regions. However, s.c. administration of MOR agonists neither produced behavioral seizures nor increased BDNF mRNA expression. Pretreatment with naloxone 1 mg/kg significantly reduced behavioral seizure scores and the up-regulation of BDNF mRNA expression elicited by i.c.v. morphine or DAMGO. Similarly, diazepam 10 mg/kg and phenobarbital 40 mg/kg significantly blocked i.c.v. MOR agonist-induced actions. Pretreatment with valproate 300 mg/kg only attenuated behavioral seizures, but it did not affect morphine-induced increase of BDNF mRNA expression. This study provides supporting evidence that seizure activity plays an important role in the up-regulation of BDNF mRNA expression elicited by central MOR activation and that decreased inhibitory action of GABAergic system through the modulation on GABA receptor synaptic function by central MOR activation is involved in its regulation of BDNF mRNA expression.     

Induction of micronuclei in murine lymphocytes by morphine

Although individuals who abuse drugs are prone to an increased risk of malignancy, the mutagenic and carcinogenic potential of these agents has received relatively little attention. We report here on the potential of morphine to induce micronuclei in murine lymphocytes. Following a single intraperitoneal injection of 20 mg/kg morphine, the frequency of micronucleated binuclear (cytochalasin-blocked) murine T- and B-splenocytes was elevated from 1 2–36 hr after treatment. The maximum frequencies seen 24 hr after injection were 6.3– and 4.9–fold greater than the respective controls. A dose-dependent induction of micronuclei was observed from 5–20 mg/kg morphine, with no further increases in frequency produced by higher doses. In contrast, incubation of mitogen-stimulated splenocytes with 10-⁷-10-⁴ M morphine in vitro produced no change in frequency of micronucleated cells relative to controls. Treatment with the narcotic antagonist naloxone (5 mg/kg) alone had no effect on the frequency of micronuclei, but reduced the clasto-genic response of a subsequently administered dose of morphine (20 mg/kg). Thus, in murine lymphocytes morphine indirectly produces genetic damage, which is at least in part opioid receptor-mediated. © 1995 Wiley-Liss, Inc.     

Latency and duration of estrogen induction of maternal behavior in hysterectomized-ovariectomized virgin rats: Effects of pup stimulation

The latency for the onset of maternal behavior was measured in virgin rats which received either 100 μg/kg estradiol benzoate (EB) or Oil immediately following ovariectomy and hysterectomy. Different groups of EB and Oil-treated females were presented with 3–8 day old test pups at 0, 24, and 48 hr after their injections. Only the females given EB exhibited short-latency maternal behavior; latencies as short as 24 hr were observed in 46% of the animals which were presented with pups at 0 hr and within 48 hr, 92% of the group were maternal. The effects of EB on maternal responsiveness could be detected between 48 to 72 hr after injection: these effects were equal to those occurring over the first and second 24-hr periods.     

Prior morphine facilitates the occurrence of immobility and anhedonia following stress

The role of the activation of the opiate system either induced by a 120-min restraint session or by a single morphine administration (10 mg/kg, i.p.) on the behaviors performed in a subsequent forced-swim test has been evaluated. In addition, animals were pretreated with naloxone (2 mg/kg, i.p.) prior to restraint or to morphine. Furthermore, in order to evaluate if this opioid mechanism could participate in the effect of stress on the response to a rewarding stimulus, rats were administered with morphine (10 mg/kg, i.p.)--whether associated or not with prior naloxone (2 mg/kg, i.p.) administration--and subsequently exposed to a 90-min restraint period. Following stress, all rats were submitted to a sucrose (1%) preference test. Both morphine and restraint enhanced the time spent in immobility in the forced-swim test. Both behavioral effects were attenuated by naloxone pretreatment thus suggesting that the increased immobility is probably modulated by the previous activation of an opiate mechanism. Furthermore, only animals with the associated treatment with morphine and restraint showed a clear reduction in sucrose preference. The fact that this effect was blocked by naloxone suggests the involvement of an opiate process in this decreased response to reward. These behavioral data suggest that the activation of an endogenous opiate mechanism facilitates the occurrence of enhanced immobility and anhedonia in response to a subsequent stress experience.     

Once is too much: conditioned changes in accumbens dopamine following a single saccharin-morphine pairing

The present study tested whether presentation of a taste cue would support conditioned suppression of dopamine in the nucleus accumbens (NAcc) following a single taste-drug pairing. Nondeprived male Sprague-Dawley rats were given 20-min access to a 0.15% saccharin conditioned stimulus (CS). Immediately thereafter, experimental rats were injected with morphine (15 mg/kg ip); standard controls were injected with saline; and explicitly unpaired controls were injected with morphine, but approximately 24 hr later. All rats were then given one 20-min CS-only test. Microdialysis samples from the NAcc were measured over 20-min intervals before, during, and after CS access on the conditioning and test trial. The results showed that a single saccharin-morphine pairing led to a marked reduction in CS intake, and the reduction in intake was accompanied by a conditioned blunting of the accumbens dopamine response to the saccharin reward cue. In turn, a single exposure to the saccharin cue also blunted the unconditioned dopamine response to morphine. Reward comparison effects, then, are cross-modal, bidirectional, and immediate, resulting in both unconditioned and conditioned changes in brain and behavior.     

Milk-induced, opioid-mediated antinociception in rats at the time of cesarean delivery.

Four experiments were conducted in rats within 2 hr of cesarean delivery to assess antinociception at birth and its possible opioid bases. Morphine antinociception was established in a dose-dependent fashion (0.0625-5.0 mg/kg bw). Analgesia was naloxone (1.0 mg/kg) reversible. In succeeding experiments, antinociception equivalent to that produced by 0.0625-0.125 mg/kg morphine injections was induced by a single 20-microliters bolus of milk (commercial half-and-half) that was delivered over 1-2 s to the middle of the tongue. This, too, was naloxone reversible. Milk-induced antinociception was maintained for at least 4 min. Finally, baseline latencies were progressively reduced during the first 2 hr after delivery to levels (8-10 s) that are typically obtained in older (10-day-old) rats. This decline was not opioid mediated because it was not affected by naloxone. Thus, at birth, delivering milk to the mouth in physiological volumes can exert opioid-mediated antinociceptive effects in rats born by cesarean delivery that had never suckled or experienced any other form of maternal contact.     

Naltrexone reverses a long term depressive effect of a toxic lithium injection on saccharin preference

In 2 experiments, using female rats, we demonstrated that an injection of 2% body weight 150 mM lithium chloride (LiCl) reduced saccharin preference, measured 48 hr later. Furthermore, Experiment 1 showed that this effect of lithium was reversed by 10 mg/kg naltrexone injected immediately before lithium administration. Experiment 2 demonstrated that one injection of 10 mg/kg morphine also depressed saccharin preference 48 hr post-injection. Paradoxically, morphine given before lithium reversed the effects of lithium on saccharin preference. The experiments suggest the involvement of endogenous opioid peptides in the long term toxic effects of lithium on saccharin preference.     

Serotonin as a facilitatory neurotransmitter in the anticonvulsant activity of methaqualone

The neuromodulatory role of serotonin in the anticonvulsant activity of methaqualone was investigated. A dose-dependent increase in the ability of methaqualone to provide protection against pentylenetetrazol (90 mg/kg SC)-induced convulsions in mice was observed. The ED50 value for the anticonvulsant activity of methaqualone was calculated and found to be 60 mg/kg, IP. Pretreatment of mice with 5-hydroxytryptophan (100 mg/kg, IP, 2 hr) and p-chlorophenylalanine (300 mg/kg, IP, 2 hr), causing an increase in brain serotonin levels, resulted in a 60% and 80% increase, respectively, in the anticonvulsant activity of methaqualone. Similar pretreatment with p-chlorophenylalanine (300 mg/kg, IP, 48 hr), causing a lowering of brain serotonin, and methysergide (10 mg/kg, IP, 0.5 hr), causing blockade of brain serotonin receptors, resulted in a 40% and 20% decrease, respectively, in the ability of methaqualone to provide protection against pentylenetetrazol-induced convulsions. These results suggest a facilitatory role of serotonin in the anticonvulsant activity of methaqualone.     

Cyclosporine and alpha-interferon do not attenuate morphine withdrawal in rats but do impair thermoregulation

Immunomodulating drugs as diverse as alpha-interferon and cyclosporine have been reported to attenuate physical signs of morphine withdrawal in rats. On the basis of these results, the immune system has been claimed to be involved in opiate addiction. To assess whether this is the case, the effects of alpha-interferon and cyclosporine were studied on objective signs of morphine withdrawal in morphine-dependent rats. Rats made dependent upon morphine by implantation of a 75-mg morphine pellet were challenged three days later by naloxone (1 mg/kg). Pretreatment with alpha-interferon (150 U/g) or cyclosporine (15 mg/kg) did not attenuate the reduction in body weight or the behavioral suppression induced by naloxone in morphine-dependent rats trained to press a lever for food reinforcement on a fixed-ratio 10 schedule. Alpha-interferon pretreatment blocked the capacity of naloxone to decrease body temperature in these rats and actually induced an hyperthermic response. In contrast, cyclosporine tended to enhance the drop in body temperature induced by naloxone. This last effect was more striking when the rats were placed in a cold room at 3.5 degrees C. Cyclosporine by itself induced a drop in body temperature in normal rats exposed to 3.5 degrees C. These results indicate that alpha-interferon and cyclosporine impair thermoregulation but do not directly interfere with morphine withdrawal signs.     

Separation of opioid from nonopioid mediation of affect in neonatal rats: nonopioid mechanisms mediate maternal contact influences

A causal distinction is established in infant Norway rats between opioid- and nonopioid-mediated determinants of behavior. Contact influences are shown to be mediated by nonopioid pathways, whereas gustatory influences are shown to be opioid mediated. Specifically, naltrexone (0.5 and 1.0 mg/kg) did not at all diminish quieting exerted by contact with an anesthetized dam but completely reversed the quieting effects of morphine in isolated rats. Naloxone (5 mg/kg) did not affect the latencies with which nondeprived or 8-hr deprived rats 9, 12, 15, and 18 days of age attached to the nipples of anesthetized dams, nor did naloxone (5 and 10 mg/kg) cause any systematic change in nipple attachment in 10- and 18-day-old rats that had been deprived of their dam for either 0, 8, or 24 hr. In a 3rd experiment, naloxone (5 mg/kg) did not significantly reduce milk intake by 9-, 12-, 15-, or 18-day-old rats from the nipple when milk letdown was induced by oxytocin. Moreover, naloxone (5 and 10 mg/kg) did not reduce milk intake in Day-10 rats that, while suckling, received milk via a cannula placed in the posterior portion of the tongue at the level of the intermolar eminence or in rats that obtained milk directly from their awake mother. In contrast, milk intake was significantly reduced by naltrexone (0.25-1.0 mg/kg) in Day-10 rats that obtained milk (a) by licking it off a saturated substrate or (b) through an indwelling cannula located in the anterior portion of the lower jaw. (Milk delivered at this placement is thought to engage feeding systems by its taste and texture.) In a final set of experiments in Day-10 rats, intake of milk delivered via anterior jaw cannulae was reduced by naloxone (5 and 10 mg/kg) in rats that were either isolated, in contact with an anesthetized dam, or attached to her nipples. On the basis of resistance to naloxone and naltrexone administration, these experiments demonstrate that behavioral influences of the tactile (and possibly olfactory) qualities of the mother are not mediated by opioid systems. Implications for understanding the means through which mothers can influence their young and the infantile mediators of these maternal influences are discussed.