Biphasic effects of morphine on locomotor activity in hamsters

Mu opioid receptor modulation in the nucleus accumbens lowers voluntary wheel running in rats bred for high running motivation

The exact role of opioid receptor signaling in mediating voluntary wheel running is unclear. To provide additional understanding, female rats selectively bred for motivation of low (LVR) versus high voluntary running (HVR) behaviors were used. Aims of this study were 1) to identify intrinsic differences in nucleus accumbens (NAc) mRNA expression of opioid-related transcripts and 2) to determine if nightly wheel running is differently influenced by bilateral NAc injections of either the mu-opioid receptor agonist D-Ala2, NMe-Phe4, Glyo5-enkephalin (DAMGO) (0.25, 2.5 μg/side), or its antagonist, naltrexone (5, 10, 20 μg/side). In Experiment 1, intrinsic expression of Oprm1 and Pdyn mRNAs were higher in HVR compared to LVR. Thus, the data imply that line differences in opioidergic mRNA in the NAc could partially contribute to differences in wheel running behavior. In Experiment 2, a significant decrease in running distance was present in HVR rats treated with 2.5 μg DAMGO, or with 10 μg and 20 μg naltrexone between hours 0-1 of the dark cycle. Neither DAMGO nor naltrexone had a significant effect on running distance in LVR rats. Taken together, the data suggest that the high nightly voluntary running distance expressed by HVR rats is mediated by increased endogenous mu-opioid receptor signaling in the NAc, that is disturbed by either agonism or antagonism. In summary, our findings on NAc opioidergic mRNA expression and mu-opioid receptor modulations suggest HVR rats, compared to LVR rats, express higher running levels mediated by an increase in motivation driven, in part, by elevated NAc opioidergic signaling.

Wheel running reduces high-fat diet intake, preference and mu-opioid agonist stimulated intake

The ranges of mechanisms by which exercise affects energy balance remain unclear. One potential mechanism may be that exercise reduces intake and preference for highly palatable, energy dense fatty foods. The current study used a rodent wheel running model to determine whether and how physical activity affects HF diet intake/preference and reward signaling. Experiment 1 examined whether wheel running affected the ability of intracerebroventricular (ICV) μ opioid receptor agonist D-Ala2, NMe-Phe4, Glyol5-enkephalin (DAMGO) to increase HF diet intake. Experiment 2 examined the effects of wheel running on the intake of and preference for a previously preferred HF diet. We also assessed the effects of wheel running and diet choice on mesolimbic dopaminergic and opioidergic gene expression. Experiment 1 revealed that wheel running decreased the ability of ICV DAMGO administration to stimulate HF diet intake. Experiment 2 showed that wheel running suppressed weight gain and reduced intake and preference for a previously preferred HF diet. Furthermore, the mesolimbic gene expression profile of wheel running rats was different from that of their sedentary paired-fed controls but similar to that of sedentary rats with large HF diet consumption. These data suggest that alterations in preference for palatable, energy dense foods play a role in the effects of exercise on energy homeostasis. The gene expression results also suggest that the hedonic effects of exercise may substitute for food reward to limit food intake and suppress weight gain.

Different time schedules affect conditioned place preference after morphine and morphine-6-glucuronide administration

A number of studies have investigated the reward potential of morphine, using the Conditioned Place Preference (CPP) procedure. The morphine-metabolite morphine-6-glucuronide (M6G) is known to have analgesic activity comparable to morphine, but its reward properties are unclear. An unbiased two compartment counterbalanced procedure was used to investigate the induction of CPP by morphine or M6G in C57BL/6J-Bom mice using different conditioning schedules. The conditioning sessions took place either immediately after the injections and lasted either 20 or 40 min, or were delayed until 15 min after the injections and lasted for 20 min. Locomotor activity was recorded during the conditioning sessions. Morphine induced CPP when the 20-minute conditioning sessions were conducted directly after the injections, but not when they were delayed. M6G induced CPP when the 20-minute conditioning sessions were delayed, but not when the animals were conditioned directly after the injections. Neither morphine nor M6G induced CPP after 40-minute direct conditioning sessions. M6G had a biphasic effect on locomotor activity, with an initial decrease followed by excitation. This study indicates that M6G has rewarding effects, and might contribute to the development of addiction after heroin or morphine administration. However, in any attempts to explore the reward properties of M6G, the choice of time schedule should be carefully considered.

Effects of morphine on the plus-maze discriminative avoidance task: role of state-dependent learning

RATIONALE

The amnesic effects of morphine may be related to its action on nociception, anxiety, or locomotion. This effect is also suggested to be related to state dependency.

OBJECTIVES

The aims of this study were to verify the effects of morphine on mice tested in the plus-maze discriminative avoidance task (DAT) that uses light and noise as aversive stimuli and allows the concomitant evaluation of learning, memory, anxiety, and locomotion and also to verify the possible role of state-dependent learning in the effects of morphine.

METHODS AND RESULTS

The DAT was conducted in a modified elevated plus-maze. In the training, the aversive stimuli were applied when mice entered in one of the enclosed arms, whereas in the test, no stimuli were applied. The main results showed that (1) pretraining morphine (5-20 mg/kg i.p.) induced retrieval deficits (evaluated by the time spent in the aversive arm in the test) but not acquisition deficits (evaluated by the decrease in aversive arm exploration along the training); (2) pretest morphine (5-10 but not 20 mg/kg) counteracted this deficit; (3) morphine induced hypolocomotion (decreased number of entries in the arms), irrespective of memory alterations; and (4) morphine did not alter anxiety-like behavior (evaluated by the time spent in the open arms) during the training.

CONCLUSIONS

Morphine given before training induces retrieval deficits in mice tested in the DAT, and these deficits could be related to morphine-induced state-dependent learning. Neither the memory deficit induced by pretraining morphine nor the reversal of this deficit by pretest morphine seems to be related to anxiety levels or locomotor alterations.

Delta opioid inhibition of light-induced phase advances in hamster circadian activity rhythms

A master neuronal pacemaker located within the suprachiasmatic nucleus in the ventral hypothalamus generates circadian activity rhythms in hamsters. The circadian pacemaker receives afferent input from many brain regions, one of which is the intergeniculate leaflet of the thalamus. This thalamic input to the suprachiasmatic nucleus in hamsters contains enkephalins, neuropeptide Y, neurotensin, and GABA. The role of enkephalins in modulating light-induced phase shifts of hamster activity rhythms has not been reported. Therefore, in this study, we examined the ability of enkephalin-mimetic and other opioid compounds to modulate light-induced phase advances in hamster circadian activity rhythms. The delta opioid agonists SNC 80 and BW373U86 both inhibited light-induced phase advances of hamster circadian activity rhythms. Neither the mu opioid agonist morphine, nor the kappa opioid agonist U50488H had any effect on light-induced phase shifts. The antagonists naltrindole, naltrexone, and nor-binaltorphimine, selective for delta, mu, and kappa opioids respectively, were also without effect on light-induced phase advances. Therefore, we found that only delta opioid agonists modulate light-induced phase advances in hamster circadian activity rhythms. These results imply that enkephalins released from the intergeniculate leaflet onto components of the suprachiasmatic pacemaker may be capable of inhibiting the responsiveness of the pacemaker to photic input arriving from the retina. The inability of antagonists to modulate light-induced phase advances suggests that endogenous opiate systems are not tonically active in generating circadian activity rhythms, but rather that enkephalins are probably used by the circadian system to modulate responses only under certain conditions or time of day.

Naloxone suppression and morphine enhancement of voluntary wheel-running activity in rats

This study examines the effects of the opioid receptor antagonist, naloxone, and the agonist, morphine, on voluntary wheel-running activity (WR) in rats. Male Sprague-Dawley rats were given 1-h access to a running wheel under non-deprived conditions. Naloxone injections (1.0, 0.5, or 0.25 mg/kg, ip), administered immediately before access to running wheels, dose-dependently suppressed WR. In another experiment, subjects were given 6-h access to running wheels under nondeprived conditions for 5 consecutive days. Morphine injections (2.0 mg/kg, sc) were found to increase WR after an initial suppression. These data demonstrate that naloxone inhibits WR, while morphine both suppresses and enhances WR depending on time and dose. These are in agreement with data on other behaviors that indicate that endogenous opioid systems play a major role in the mediation of motivational behaviors.

Opioid induced non-photic phase shifts of hamster circadian activity rhythms

The phase of the circadian pacemaker in hamsters can be shifted by the application of certain non-photic stimuli late in the subjective day. A projection from the intergeniculate leaflet of the thalamus to the circadian pacemaker in the suprachiasmatic nucleus is believed to mediate some types of non-photic phase-shifting stimuli. In hamsters, this projection is immunoreactive to both Neuropeptide Y and enkephalin. Previous work in other laboratories has shown that Neuropeptide Y administration is capable of phase shifting circadian rhythms without the application of light. The present study was undertaken to determine if enkephalinergic compounds likewise have the ability to non-photically phase shift hamster activity rhythms. Hamsters were maintained under conditions of constant darkness and circadian wheel running activity was recorded. Agonists and antagonists selective for kappa, mu, and delta opioid receptors were systemically applied without light to hamsters at circadian times 8 and 10 to determine if they were able to elicit phase shifts in wheel running activity rhythms. Of the compounds tested, only the delta opioid agonist BW373U86 significantly affected circadian phase. BW373U86 phase advanced hamster wheel running activity rhythms by approximately 45 min, although total activity levels following drug application were not significantly affected. Changes in the amount of wheel running activity were detected after administration of some mu and kappa opioids, although the circadian phase was not altered. These results indicate that enkephalin-mimetic delta opioid agonists are capable of producing non-photic phase shifts in hamster activity rhythms, and that opioids can independently affect circadian phase and activity levels in hamsters.

Effects of diurnal phase and pimozide on cholecystokinin-elicited hypoactivity in the hamster

Locomotor activity in golden Syrian hamsters was measured following IP injections of cholecystokinin (CCK; 25 micrograms/kg) and pimozide (0.5 mg/kg), the dopamine receptor antagonist. In addition, animals were tested during either the dark or light phase of the diurnal cycle in either dark or light running wheel environments. Results indicated that CCK-elicited hypoactivity was blocked by pimozide and that the effect of CCK was evident only among animals tested during the light phase of the daily cycle. Ambient lighting conditions in the test environment did not modify the drug effects. Independently of any drug effect, locomotor activity was affected by diurnal phase and ambient lighting in the test environment. Animals were more active when tested during the dark phase than during the light phase and locomotor activity was higher under dark than under light ambient conditions. It is concluded that diurnal phase modulates CCK's effect on hamster locomotion and that CCK's effect on locomotion is mediated, in part, by dopaminergic mechanisms.

Morphine, D-Pen2, D-Pen5 enkephalin and U50,488H differentially affect the locomotor activity and behaviours induced by quinpirole in guinea-pigs

The effects of morphine D-Pen2, D-Pen5 enkephalin (DPDPE) and U50,488H on the behavioural syndrome elicited by the dopamine (DA) D-2 agonist quinpirole, were investigated. Morphine (1, 5 and 15 mg/kg SC) and morphine administered intracerebroventricularly (ICV) (2 x 5 microliters, 10(-3) M; total dose = 10 nmol) produced piloerection and sedation. DPDPE-ICV (2 x 5 microliters and 2 x 10 microliters, 10(-3) M; total doses = 10 and 20 nmol) produced piloerection and sedation similar to morphine. U50,488H (1 mg/kg SC) induced locomotor activity and some stereotyped behaviour, whereas U50,488H (5 and 10 mg/kg SC) induced muscle rigidity and dystonic-like movements. The locomotor and behavioural response elicited by quinpirole (3 mg/kg IP) was attenuated in guinea-pigs pretreated with morphine (1, 5 and 15 mg/kg SC), morphine-ICV (2 x 5 microliters, 10(-3) M), and DPDPE-ICV (2 x 5 microliters and 2 x 10 microliters, 10(-3) M). These effects were reversed by naloxone (15 mg/kg SC). U50,488H (1 mg/kg SC) increased the quinpirole-induced locomotor activity, whereas U50,488H (5 and 10 mg/kg SC) decreased the locomotor activity and stereotyped behaviours produced by quinpirole. These results indicate that the gross behavioural effects of mu, delta and kappa opioids differ in guinea-pigs compared to other rodent species, and suggest differential involvement of these opioid receptor subtypes with DA D-2 receptor-mediated activity.

Inhibitory effect of opiates on male rat sexual behavior may be mediated by opiate receptors outside the central nervous system

The importance of opiate receptors outside the central nervous system for the inhibitory actions of morphine on male rat sexual behavior was evaluated. Morphine (10 mg/kg) produced an almost complete inhibition of sexual behavior. This inhibition was antagonized by naloxone at a dose of 1 mg/kg but not at a dose of 0.25 mg/kg. The quaternary opioid antagonist methylnaloxone effectively blocked the inhibitory actions of morphine at a dose of 20 mg/kg but not at a dose of 5 mg/kg. Since the affinity of methylnaloxone for opiate receptors is about 5% of that of naloxone, it may be concluded that both antagonists were about equally effective in inhibiting the effects of morphine. Furthermore, the opiate-like drug loperamide was found to inhibit sexual behavior. This drug acts mainly outside the central nervous system. Its effect was blocked by both naloxone and methylnaloxone, suggesting that opiate receptors are involved. It was also shown that methylnaloxone is unable to block the reinforcing effects of morphine in the conditioned place preference procedure. Because the reinforcing effects of opiates seem to be localized to the central nervous system, it may be proposed that methylnaloxone does not antagonize morphine's central effects. Moreover, loperamide had no effect in the place preference procedure, suggesting that this drug does not act at central opioid receptors. Taken together, these data show that peripheral opioid receptors are responsible for at least some of the inhibitory actions of morphine on male sexual behavior. After treatment with morphine + methylnaloxone, ejaculatory mechanisms were facilitated, reflected in a reduced number of preejaculatory intromissions and a shortened ejaculation latency.(ABSTRACT TRUNCATED AT 250 WORDS)

Acute morphine dependence in the hamster

Two experiments investigated naloxone-precipitated withdrawal following a brief course of morphine administration in hamsters. In Experiment 1, observable withdrawal symptoms (e.g., wet-dog shakes) were elicited by two doses of naloxone (0.4 and 1.0 mg/kg) following four and eight daily injections of morphine (15 mg/kg), a regimen that replicated previous studies in our laboratory using a locomotor activity paradigm. At the lower dose of naloxone, the frequency of withdrawal signs was greater after eight than after four morphine injections. In Experiment 2, observable withdrawal symptoms were elicited by the same two doses of naloxone, 70 min after a single morphine injection. These results suggest that acute dependence in the hamster, as in other species, beings to develop with the first morphine exposure.

Morphine conditioned place preference in the hamster

Two experiments investigated the ability of morphine to produce a conditioned place preference in the hamster. In Experiment 1, it was found that a 15 mg/kg dose of morphine produced a conditioned place preference after eight conditioning trials. In addition, naloxone (0.4 mg/kg blocked the development of the morphine-conditioned place preference and itself produced a conditioned place aversion after four conditioning trials. In Experiment 2, the effects of four doses of morphine (0, 2.5, 5 and 15 mg/kg) on the acquisition of a conditioned place preference were studied. Only the 15 mg/kg dose produced a significant place preference. Compared to similar findings in the rat, the present results indicate that a relatively high dose of morphine is required to produce a conditioned place preference in the hamster.

Effects of U50,488H on locomotor activity in the hamster

Two experiments investigated the effects of the specific kappa opiate agonist, U50,488 on locomotor activity in the golden Syrian hamster. In Experiment 1, the effects of U50,488 were found to be dose-related, with a 1 mg/kg dose eliciting hyperactivity and a 10 mg/kg dose eliciting hypoactivity. In Experiment 2, the dual effects of U50,488 on locomotor activity were shown to be naloxone (1 mg/kg) reversible. It is suggested that the effects of U50,488 on activity are consistent with the reported dual opposing influences of kappa agonists in the substantia nigra pars compacta and pars reticulata.

Inhibitory and stimulatory effects of morphine on locomotor activity in mice: biochemical and behavioral studies

A possible interaction between the opposite effect (inhibition and stimulation) of morphine on locomotor activity in mice and monoaminergic systems in the striatum was studied. Ten minutes after systemic administration, morphine at 1.25 mg/kg decreased locomotor activity and the levels of 3-methoxytyramine (3-MT), whereas at 20 mg/kg locomotor activity and 3-MT levels increased. At the same time, no change in the other monoamine metabolite (DOPAC, HVA, MHPG, and 5-HIAA) levels was observed. Sixty minutes after administration, morphine at 1.25 mg/kg did not induce any change in locomotor activity or in all the monoamine metabolite levels measured. On the other hand, morphine at 20 mg/kg maintained an initial increase in locomotor activity and increased not only 3-MT levels, but also other metabolite (DOPAC, HVA, MHPG, and 5-HIAA) levels. These results suggest that, at low dosages, the inhibitory effect of morphine on locomotor activity in mice may be related to a decrease of the presynaptic dopamine release in the striatum and that the stimulatory effect of morphine, at high dosages, may be related to an increase of the presynaptic dopamine release in the striatum.

Affiliative behavior in voles: effects of morphine, naloxone, and cross-fostering

Species differences in affiliative behavior were examined in prairie and montane voles. Unfamiliar male-female pairs were placed in a test-cage for 2 hr and side-by-side huddling was recorded during the third hour. Prairie vole pairs spent a mean of 31.2 minutes in contact whereas montane voles were in contact only 1.3 minutes. In order to examine the effects of experience on affiliative differences, pups of each species were cross-fostered. Fostered prairie vole parents did not survive longer than 7 days, whereas fostered montane voles were successfully weaned; cross-fostering had no effect on their huddling behavior when tested as adults. The effects of morphine (5 and 10 mg/kg) and naloxone (5 and 10 mg/kg) on side-by-side contact were evaluated in both species. Morphine (10 mg/kg) reduced huddling duration and activity levels in prairie voles. There were no other drug effects in either species.

Independent effects of morphine and apomorphine on stereotyped gnawing in the hamster

Eight experiments were conducted to investigate the effects of apomorphine, a dopaminergic agonist, and of morphine, an opiate, on stereotyped behavior in the hamster. Animals were observed at two minute intervals for one hour and incidents of stereotyped gnawing, licking and sniffing were recorded using a time-sampling method. Both morphine and apomorphine produced dose-related increases in stereotyped gnawing. A low dose of the opiate antagonist, naloxone (0.4 mg/kg), blocked morphine-induced gnawing but neither that dose nor higher doses of naloxone (1, 4 and 10 mg/kg) blocked apomorphine-induced gnawing. A low dose of the dopaminergic antagonist, haloperidol (0.05 mg/kg), blocked apomorphine-induced gnawing but did not block morphine-induced gnawing. Further experiments indicated that morphine administration did not sensitize, or influence in any way, subsequent apomorphine-induced stereotyped behavior.

Opioids and sexual behavior in the male rat

Naloxone in the doses of 4 or 16 mg/kg failed to affect copulatory behavior of testosterone-treated castrated male rats. Morphine 10 mg/kg, administered 60 min before behavioral observation, reduced the proportion of animals displaying sexual behavior. Doses of 2.5 or 5 mg/kg reduced the latency to the second ejaculation, whereas the few animals still copulating after morphine 10 mg/kg showed a reduced latency to the first ejaculation. The same doses of morphine administered 5 min before behavioral observation produced a dose-dependent reduction of mount, intromission and ejaculation percentages. However, those animals that did copulate showed a normal copulatory behavior. D-Ala2-Met5 enkephalinamide (DALA) infused into the left cerebral ventricle in a dose of 5 micrograms 5 or 60 min before tests had no effect. When the peptide was infused 30 sec after the first intromission, the number of intromissions as well as the latency to ejaculation were reduced. Opioids may facilitate ejaculatory mechanisms, perhaps as a consequence of their rewarding properties. Moreover, in animals treated with DALA after the first intromission, the number of intromissions and the latency to ejaculation were similar for the first and second copulatory series, while these parameters were much reduced upon the second ejaculation for control animals. It is possible that liberation of endogenous opioids is the cause of ejaculation-induced facilitation of subsequent sexual behavior.

Naloxone reversal of morphine elicited hyperactivity

When naloxone is administered during morphine elicited hyperactivity, hyperactivity is reversed and hypoactivity occurs in its place. The present experiment tested the hypothesis that this effect is the result of morphine induced supersensitivity to naloxone. Two groups of hamsters received equivalent pretreatment with 15 mg/kg morphine (Groups M/M and M/S) for three days while a third group received saline (Group S/S). During subsequent testing one group received a morphine injection (Group M/M) while the others received saline (Groups M/S and S/S) before being placed in running wheels for a three hour session. Two hours later half the animals in each group received an injection of 0.4 mg/kg naloxone and half received saline. Naloxone produced hypoactivity in animals running under the influence of morphine (Group M/M), but neither in those with an equivalent history of morphine pre-treatment (Group M/S), nor in saline controls (Group S/S). These results are inconsistent with the hypothesis under test, but congruent with a modified dual-action hypothesis.

Cholecystokinin antagonizes morphine induced hypoactivity and hyperactivity in hamsters

Three experimental replications were used to test the effects of three doses (25, 50 or 75 micrograms/kg) of cholecystokinin octapeptide (CCK-8) on morphine induced changes in activity. For each dose of CCK-8, running wheel activity of golden Syrian hamsters was monitored for three hours following a series of two injections. The first injection consisted of either saline or CCK-8, the second of either saline or morphine sulfate (15 mg/kg). Thus, in each replication four groups were created: Group SAL/SAL (n = 8) received two saline injections, Group CCK/SAL (n = 8) an injection of CCK-8 followed by an injection of saline, Group SAL/MS (n = 8) an injection of saline followed by an injection of morphine and Group CCK/MS (n = 8) an injection of CCK-8 followed by an injection of morphine. Results indicated that a 25 micrograms/kg dose of CCK-8 blocked the hypoactivity elicited by morphine 40-60 min after opiate injection, whereas a 75 micrograms/kg dose of CCK-8 blocked the hyperactivity elicited by morphine 80-100 min after opiate injection. These findings are consistent with previous reports that CCK-8 antagonizes the effects of opiate agonists on a variety of behaviors and is supportive of the hypothesis that endogenous CCK-8 may antagonize endogenous opioid peptides in the control of behavior.

Evidence for an underlying opponent process during morphine elicited hyperactivity in the hamster

Two experiments investigated the effects of naloxone on morphine elicited hyperactivity in the hamster. In Experiment 1, naloxone (0.4 mg/kg) administered two hours after morphine (15 mg/kg) produced sedation in animals running at high rates under the influence of morphine. Saline control animals running at comparable rates were unaffected by naloxone. In Experiment 2, naloxone administered two hours after morphine converted morphine elicited hyperactivity into sedation. These results are discussed in terms of a modified dual-action hypothesis which holds that morphine elicited hyperactivity masks an underlying opponent process.

Effects of naloxone on morphine induced sedation and hyperactivity in the hamster

Three experiments investigated the effects of naloxone on morphine elicited changes in hamster locomotor activity. In Experiment 1, a prior subcutaneous injection of naloxone (0.4 mg/kg) converted morphine (15 mg/kg) elicited hypoactivity into hyperactivity: Compared with saline controls, naloxone pretreated animals were hyperactive following a subcutaneous injection of morphine. Experiment 2 investigated the effects of four doses of naloxone (0, 0.04, 0.1, 0.4 mg/kg) on morphine elicited hyperactivity. Results indicated that naloxone reversal of morphine elicited hyperactivity is directly related to dose of naloxone. In Experiment 3, naloxone (0.4 mg/kg) was administered one and two hours after a morphine injection. Compared with saline controls, morphine treated animals were hypoactive for approximately 40 minutes after each of the naloxone injections. Results are discussed in terms of a modified dual-action hypothesis.

Morphine tolerance and sensitization in the hamster

The effects of morphine on hamster locomotor activity were studied. Repeated administration of morphine in doses from 5 to 40 mg/kg produced systematic changes in morphine's biphasic time effect pattern: morphine's sedative effects decreased (tolerance) while morphine's excitatory effects increased (sensitization). These results extend findings of behavioral tolerance and sensitization in the hamster to a range of higher doses than those used previously.

Evidence for the activation of the endogenous opiate system in hamsters infected with human blood flukes, Schistosoma mansoni

Nociceptive thresholds were investigated in golden hamsters infected with the human blood fluke, Schistosoma mansoni. Increases in thermal thresholds suggestive of analgesia were evident by 20-25 days of infection. These increased further during a 40-42 day period. The altered responses were suppressed by the opioid antagonist naloxone. Non-invasive inhibition of the activity of the pineal gland by exposure to light also reduced nocturnal analgesia in schistosome infected animals. Naloxone antagonism and pineal inhibition of morphine- induced analgesia was obtained similarly in control, uninfected animals. Taken together, these findings suggest strongly that infection with S. mansoni results in a chronic activation of the endogenous opiate system.

Locomotor activity and opiate effects in male and female hamsters

Locomotor activity of golden Syrian hamsters was investigated in three experiments. In Experiment 1, running wheel activity of male and female hamsters was compared under the following conditions: no injection, saline injection, morphine injection (15 mg/kg) and naltrexone injection (1 mg/kg). During two hour test sessions, females maintained high levels of activity, whereas males slowed down considerably during the second hour. The difference between males and females was evident under all conditions except following morphine which produced a biphasic time-effect pattern in both sexes. Naltrexone, however, had no detectable effects on either males or females. Experiment 2 investigated the effects of four doses of naltrexone (0.3, 1, 3, 10 mg/kg) on hamster locomotion. Results indicated that none of the doses tested had an effect. Experiment 3 demonstrated that a 1 mg/kg dose of naltrexone antagonizes two of morphine's (15 mg/kg) effects on activity. First, naltrexone partially blocked morphine elicited sedation. Second, naltrexone blocked the increase in activity that characterizes recovery from morphine.

A biphasic effect of chlordiazepoxide on animal locomotor activity

Centrally acting drugs often act 'biphasically', especially on animal locomotor activities: the same drug stimulates and depresses, depending either on dose or on time since administration. We propose a third, hitherto neglected, type of biphasic effect, well illustrated by chlordiazepoxide, which depends, instead, on the length of time an animal is exposed to a test environment: in an unfamiliar Y-maze, rats' spontaneous locomotion (number of entries into the arms of the Y) was stimulated to begin with, but switched to depression in a few minutes. The stimulant phase appeared to be independent of dose, while the depressant phase showed log linear dose dependence.

Tolerance and sensitization to the biphasic effects of low doses of morphine in the hamster

Two experiments investigated the dose and time related effects of morphine sulfate on wheel running behavior in golden Syrian hamsters. In Experiment 1, within-subject comparisons were made of the acute effects of 5, 20, and 40 mg/kg doses of morphine sulfate on running wheel activity. Compared with saline, morphine produced a dose related decrease in activity followed by a dose related recovery. At the lowest dose, the time effect curve was biphasic, with sustained hyperactivity following the recovery. In Experiment 2, the effects of low doses of morphine sulfate (0.5, 1.0, 2.5, 5.0 mg/kg) on running wheel activity were monitored for three days. Biphasic time effect patterns were evident at each dose: An initial period of hypoactivity was followed by recovery and subsequent hyperactivity. Moreover, repeated administration produced both tolerance and sensitization to morphine's effects on activity. Implications for mechanisms underlying the biphasic response pattern are discussed.