Enhancement of morphine analgesia and prevention of morphine tolerance in the rat by the cholecystokinin antagonist L-364,718.

The potent and selective non-peptide cholecystokinin (CCK) antagonist L-364,718 (0.5-2.0 mg/kg s.c.) enhanced the analgesia induced by acute morphine treatment in the rat tail flick test. Chronic treatment with L-364,718 (1.0 mg/kg) prevented the development of tolerance to morphine analgesia (after a 6 day period of morphine treatment) but did not influence the onset of opioid dependence. Since L-364,718 is considerably more potent in inhibiting CCK binding to peripheral tissues than to brain membranes its interaction with morphine is surprising. The exact locus of this interaction, or whether it involves 'peripheral-type' (CCK-A) or 'central-type' (CCK-B) receptors is not known.

Influence of chemical and environmental stressors on acute cadmium toxicity.

Previous investigations have demonstrated that the cytosolic protein metallothionein (MT) is induced not only by exposure to certain heavy metals but also by a variety of other factors, including environmental stress. While MT synthesis has been observed with exposure to cold temperatures, there is a paucity of data concerning the influence of cold on heavy-metal toxicity. The present investigation focused on the influence of metal and cold pretreatments on the acute toxicity of cadmium. Mortalities of 80% and 100% were observed for mice orally administered challenge doses of 100 mg Cd/kg and 150 mg Cd/kg, respectively. To determine a protective cadmium pretreatment dose, animals were administered 2.5, 5, 10, 20, 25, and 50 mg Cd/kg 24 h prior to cadmium challenge. In animals pretreated with 10 mg Cd/kg, mortalities of 20% and 70% were observed with the respective challenge doses. Immediately following cold stress (4 degrees C, 12 h), mortalities of 30% and 90% were observed with cadmium challenge doses of 100 and 150 mg Cd/kg, respectively. Significant correlations were demonstrated between induced hepatic MT concentrations and cadmium pretreatment (r = 0.99), as well as cold pretreatment (r = 0.87). Results of this investigation indicate that stressors, such as cold, influence the acute toxicity of cadmium to the same magnitude as metal pretreatment. This induced tolerance to cadmium was attributed, in part, to the induction of MT synthesis. Furthermore, the induced levels of MT resulting from cold stress may confound the simplistic approach of using MT as a biological monitor of occupational exposure to cadmium.

The effect of alpha-interferon, cyclosporine A, and radiation-induced immune suppression on morphine-induced hypothermia and tolerance.

An interconnection between the immune and the central nervous systems has been suggested by investigators studying the actions of several types of immune modifying agents and procedures upon opiate related phenomena. These studies have included the effects of altering immune system function by administration of either alpha-interferon, cyclosporine or radiation exposure upon naloxone-precipitated opiate withdrawal and upon opioid antinociceptive effects. The present study extends these earlier investigations by examining the effect of immune modulation upon opiate induced hypothermia. The results demonstrate that interferon and cyclosporine have no effects on baseline temperature or morphine induced hypothermia, while irradiation exposure elicits hyperthermia without affecting morphine-induced hypothermia. Finally, neither cyclosporine nor irradiation affect the development of tolerance to morphine induced hypothermia, while a single injection of the immune system modifier interferon was able to prevent the development of such tolerance. These observations suggest that yet another opiate-related phenomenon may be regulated at least in part by the immune system. These results together with our previous findings are further evidence of a link between the immune system and the CNS mediated through the opioid system. In addition, these studies further support our earlier hypothesis that "Interferon" is one of the endogenous substances which serves to prevent the development of tolerance and dependence to endogenous opioids.

D-pipecolic acid inhibits ethanol tolerance in mice.

The effects of graded doses of D-pipecolic acid (0.005-5 micrograms/animals s.c.) on tolerance to the hypothermic effect of ethanol (4 g/kg i.p.) were investigated in mice. D-Pipecolic acid itself did not change the core temperature or the acute hypothermic response to a single dose of ethanol. Repeated D-pipecolic acid administration, however, blocked the development of tolerance to the hypothermic effect of ethanol. The development of tolerance could be observed in the control group. It is assumed that D-pipecolic acid is capable of counteracting the tolerance effect of ethanol.

Characteristics of tolerance, recovery from tolerance and cross-tolerance for cocaine used as a discriminative stimulus.

Rats were trained to discriminate an injection of cocaine (10 mg/kg) from saline using a two-lever choice procedure with food as a reinforcer. Subsequently, training was stopped, and cocaine was injected chronically. In one experiment, 10.0 or 20.0 mg/kg/8 hr 8 hr of cocaine for 7 days produced a 2-fold shift to the right of the discriminative stimulus dose-effect curve. A dose of 5.0 mg/kg/8 hr for up to 14 days did not shift the dose-effect curve. In a second experiment, 20 mg/kg/8 hr for 7 or 14 days produced comparable degrees of tolerance. In a third experiment, after termination of 12 days of chronic cocaine injection (20 mg/kg/8 hr), base-line sensitivity to the training stimulus recovered progressively across 18 days. Additional experiments tested the effects of chronic administration of drugs that were substituted (d-amphetamine) or were not substituted (morphine) for the training stimulus. d-Amphetamine (2.5 mg/kg/8 hr for 7 days) produced a 4-fold shift to the right of the dose-effect curve for the detection of both d-amphetamine and cocaine; in contrast, 7 days of escalating morphine doses produced signs of physical dependence but did not alter the sensitivity of rats to the cocaine training stimulus. These results provide evidence that tolerance for cocaine used as a discriminative stimulus occurs as a function of chronic dose, dosing regimen and class of drug administered.

Behaviorally active oxytocin fragments simultaneously attenuate heroin self-administration and tolerance in rats.

Maintenance of intravenous heroin self-administration and the degree of tolerance to the analgesic effect of self-injected heroin were simultaneously measured in heroin-tolerant rats. Subcutaneous injection of oxytocin (OXT-(1-9)) and of its behaviorally active fragments desglycinamide9-oxytocin (OXT-(1-8)) and [pGlu4,Cyt6]-oxytocin-(4-8) (OXT-(4-8)) decreased the amount of heroin self-injected. The C-terminal tripeptide of oxytocin (prolyl-leucyl-glycinamide, PLG, OXT-(7-9)) and desglycinamide9-[Arg]8-vasopressin (AVP-(1-8] were ineffective in this respect. In spite of the lower amount of self-injected heroin after pretreatment with oxytocin fragments, no differences in the antinociceptive effect of self-injected heroin, as assessed by the lick response using a hot plate device, were observed after pretreatment with placebo and oxytocin fragments. These findings suggest that oxytocin and some of its behaviorally active fragments attenuate heroin tolerance and that this effect may result in a diminished heroin intake in tolerant animals self-injecting heroin.

The effect of oxytocin and fragment (MIF-I) on the development of tolerance to hypothermic and hypnotic action of ethanol in the rat.

The daily pretreatment of rats with oxytocin (OXY) or MIF-I prior to ethanol (Et-OH) administration markedly altered the alcohol tolerance when tested on the fifth day of treatment. OXY (800 and 2400 nmole/kg SC) and MIF (800 nmole/kg SC) inhibited the development of tolerance to the hypnotic effect of Et-OH. MIF at this dose also inhibited the tolerance to the hypothermic effect. Only OXY in the dose of 800 nmole/kg suppressed hypothermia in an acute experiment with Et-OH and produced by itself hypothermia after acute administration (2400 nmole/kg). The tolerance to this last effect developed after four days of peptide treatment. The results indicate that OXY and MIF-I can influence the processes of development of tolerance to some central depressive effects of Et-OH in rats.

Differing effects of short-chain alcohols on body temperature and coordinated muscular activity in mice.

Administration of single doses of ethanol, 1-propanol, 1-butanol or 1-pentanol to mice caused hypothermia and impairment of rotarod performance. Repetitive doses, at 24-72 hr intervals led to development of tolerance to the hypothermic effects of ethanol but not of the other alcohols. No tolerance was seen in the impairment of rotarod performance with repeated doses of any of the alcohols. Ethanol did show an intersession tolerance on rotarod performance; at 20 and 80 min after injection, blood levels were similar, while performance was impaired at 20 but not at 80 min.

Effects of oxytocin and a derivative (Z-prolyl-D-leucine) on morphine tolerance/withdrawal are mediated by the limbic system.

Recent data indicate that the neurohypophyseal hormone oxytocin (OXT) and Z-prolyl-D-leucine (Z-Pro-D-Leu), a synthetic dipeptide derived from the C-terminal part of OXT, attenuate the development of tolerance to and dependence on morphine in the mouse. Biochemical and behavioral data raise the possibility that these effects of the peptides might be associated with their effects on the central nervous system and in particular on limbic brain structures. The present results confirm this hypothesis, since intracerebroventricular (i.c.v., 50 ng) and local (0.5 ng) injections of OXT and Z-Pro-D-Leu into the dorsal hippocampus and the mesolimbic nucleus accumbens attenuate morphine tolerance/dependence, similarly to systemic injections of these peptides in higher amounts (5-50 micrograms). Local injections of these peptides into other brain sites (e.g. the nucleus caudatus, ventral tegmental area and the external cortical surface) are without effect. Lesion of the nucleus accumbens by the catecholaminergic neurotoxin 6-hydroxydopamine (6-OHDA) completely prevents the effects of Z-Pro-D-Leu and partially those of OXT on morphine tolerance/dependence. The data point to the role of limbic structures as mediators of the effects of neuropeptides on morphine addiction.

Potentiation of opiate analgesia and apparent reversal of morphine tolerance by proglumide.

Exogenous cholecystokinin selectively antagonizes opiate analgesia, which suggests that endogenous cholecystokinin may act physiologically as an opiate antagonist and may play a role in opiate tolerance. The use of the selective cholecystokinin antagonist proglumide provided a test of these hypotheses in rats that were either inexperienced with or tolerant to opiates. Proglumide potentiated analgesia produced by morphine and endogenous opiates and seemed to reverse tolerance. These results suggest that endogenous cholecystokinin systems oppose the action of opiates.

The contribution of the active metabolites to the tolerance developing to diazepam in man: relationship to bioassayed serum benzodiazepine levels.

Seven healthy students cooperated in this study. They underwent four experimental periods at one-month intervals, each period comprising a single-dose test with diazepam (D) 15 mg, oxazepam (OX) 45 mg, nordiazepam (ND) 15 mg or placebo (given double-blind in randomized order) on day 1, followed by maintenance with respective drug for 7 days, and a retest with D 15 mg on day 8. Thus, psychomotor responses to D 15 mg were measured after one-week treatment with D 5 mg, OX 15 mg, ND 5 mg, or placebo, all taken twice daily. Serum samples were taken at each session day before and 2.5 hr after the drug intake for bioassay of serum benzodiazepine (BZ) levels against commensurable diazepam standard. On day 1, a single dose of OX 45 mg resulted in fourfold concentrations of serum BZ levels in comparison with the results of the doses D 15 mg and ND 15 mg. Psychomotor skills were also most impaired by oxazepam. ND 15 mg was less effective than D 15 mg, irrespective of the same serum BZ levels. One-week of treatment with D and OX seemed to diminish most responses to D, despite increased BZ effects on these functions. Pretreatment with ND reduced the subjective sedative effects of D. Our results support the view that the rate of development of tolerance to BZs is task-dependent. Tolerance developed mostly to the complex tests subject to learning while Maddox wing test (reflecting the degree of muscle relaxation) was resistant. ND, the main metabolite of D, plays little part in the tolerance that developed to D.

Reversal of morphine tolerance after medial thalamic lesions in the rat.

Tolerance, manifested by a diminished electroencephalographic response at cortical and subcortical recording sites, was found in rats subjected to repeated systemic injections of morphine sulfate. Reversal of tolerance to morphine resulted from destruction of the medial thalamus.