Supersensitivity to apomorphine and stress two years after chronic methadone treatment

Methadone treatment, bruxism, and temporomandibular disorders among male prisoners

There is little information on bruxism related to illicit drug use. Prolonged drug use may damage the stomatognathic system via oral motor overactivity. The aim of the present study was to compare the rates of bruxism and temporomandibular disorders (TMDs) between prisoners with and without drug-use disorders, to evaluate the association between methadone treatment and bruxism and to assess the possible relationship between bruxism and pain. The sample included 152 male prisoners, 69 of whom were drug users maintained on methadone. All prisoners were examined by an experienced dentist and completed a questionnaire on their oral habits, with the aim of detecting signs or symptoms of TMD and/or bruxism. Additional data were collected from medical files. The prevalence of sleep bruxism and awake bruxism, but not of TMDs, was significantly higher among drug-user than non-drug user prisoners (52.2% vs. 34.9% for sleep bruxism, 59.7% vs. 30.1% for awake bruxism, and 46.3% vs. 25.6% for TMDs, respectively). Participants with awake bruxism were statistically more sensitive to muscle palpation compared with participants with sleep bruxism [rating scores (mean ± SD): 0.32 ± 0.21 vs. 0.19 ± 0.28, respectively]. An association was found between sleep bruxism and awake bruxism. It seems that there is a direct or an indirect association between methadone maintenance treatment and sleep bruxism or awake bruxism in male prisoners.

Enhanced immune sensitivity to stress following chronic morphine exposure

Chronic administration of escalating doses ofmorphine leads to neuroadaptive changes precipitating development of tolerance to many of the acute effects of morphine, such as analgesia, activation of the hypothalamic-pituitary-adrenal (HPA) axis and suppression of immune cell activities. Interestingly, morphine tolerance has also been shown to be accompanied by heightened immunosuppressive effects of restraint stress using a rodent model. These observations have led to the hypothesis that the altered neuronal state accompanying opioid tolerance may contribute to this enhanced immune sensitivity to stress. To further test this hypothesis using different stressors, Sprague-Dawley rats were treated chronically with morphine for at least 8 days and then challenged with either psychological (water stress) or systemic stressors [morphine withdrawal, lipopolysaccharide (10 mug/kg i.p. challenge)]. It was found that, independent of the type of stress employed, morphine-tolerant animals displayed significantly lower mitogen-stimulated blood lymphocyte responses when compared to the responses of similarly treated saline controls. To determine whether direct activation of central stress pathways may also lead to enhanced immune sensitivity, morphine-tolerant animals were centrally injected with IL-1beta (1 ng/mul i.c.v.), a cytokine that activates the HPA axis by central mechanisms. Similar to the other types of stress, this direct central challenge was also found to be more immunosuppressive in morphine-tolerant animals compared to controls. Collectively, these studies demonstrate that morphine-tolerant animals have an enhanced susceptibility to the debilitating effects of a variety of stressors on immune cell function, an effect that is likely due to the neuroadaptive changes that develop during chronic morphine exposure.

Antagonism of N-methyl-D-aspartate receptors reduces the vulnerability of the immune system to stress after chronic morphine

It has been shown that morphine-tolerant animals have an altered immunological sensitivity to stress. Although the glutamatergic system has been implicated in the neuroadaptive process underlying this tolerant state, its potential role in development of the altered immunological sensitivity consequent to chronic morphine treatment is not known. To determine this, a morphine-tolerant state was induced by 10-day administration of an escalating dose of morphine from 10 to 40 mg/kg (s.c., b.i.d.), and lymphocyte proliferative response to a T-cell mitogen was measured. Morphine challenge (10 mg/kg s.c.) after days of treatment was gradually less immunosuppressive, and this tolerance progression was delayed by concurrent administration of the N-methyl-D-aspartate (NMDA) receptor antagonist (-)-5-methyl-10,11-dihydro-5H-dibenzo[a,d]cyclohepten-5,10-imine maleate (MK-801) (0.1 mg/kg s.c.) with chronic morphine. The effect was independent of glucocorticoid level changes and was not a result of an acute interaction of the drugs or the prolonged presence of the antagonist alone. Subsequent to chronic treatment, animals were subjected to opioid withdrawal and water stress. Both stressors induced 50% immunosuppression in morphine-tolerant animals compared with saline-treated controls. Increased immunological sensitivity to these stressors was attenuated when MK-801 was administered with chronic morphine as demonstrated by an accelerated recovery rate and lack of immunosuppression from opioid withdrawal and water stress, respectively. Together, these findings provide the first evidence that the neuroadapted state of the immune response after chronic morphine can be modified by NMDA receptor antagonism, as illustrated by a temporal deceleration of the development of immunological tolerance during chronic treatment that is associated with an attenuation of the immunological vulnerability of morphine-tolerant animals to stress.

Antagonism of footshock stress-induced inhibition of intracranial self-stimulation by naloxone or methamphetamine

Rats were trained to lever-press for intracranial self-stimulation (ICSS) with electrodes implanted in the ventral tegmental area (VTA). The effect of inescapable footshock on response rates to ICSS was examined in the present study. Markedly decreased response rates to ICSS were observed 15 min to 24 h following inescapable footshock. Naloxone (10.0 mg/kg) itself was without effect on response rates to ICSS, but completely antagonized the decreased response rates by the stressor treatment. A relatively low dose of methamphetamine (0.5 mg/kg), which showed no effect on ICSS rates in naive rats, also antagonized the decreased response rates to ICSS. The present results suggest that inescapable footshock may release endorphin in the mesolimbic or mesocortical area; the released endorphin may act on dopaminergic nerve endings and interrupt dopaminergic transmission. The decreased activity of dopaminergic neurons may cause the decreased response rates to ICSS.

The persistence of hyperresponsiveness to apomorphine in rats following REM sleep deprivation and the influence of housing conditions

The hyperresponsiveness to apomorphine disappears after REM sleep deprivation of rats; the time course of the disappearance was investigated. When REM sleep-deprived (REMd) rats were caged individually during the recovery period the increase in ambulation and in stereotyped behaviour elicited by apomorphine subsided within 8-24 h; on the other hand, the exaggerated aggressiveness persisted up to 64 days. However, when the deprived rats were allowed to recovery in groups of 3 per cage the increase in aggressiveness subsided after 4 days. Since individual housing of non-deprived rats for 4--11 days increased the apomorphine-aggressiveness steadily, 'isolation' probably summed with REM deprivation in facilitating the effects of apomorphine on aggressive behaviour. The results also suggest that the dopaminergic systems implicated in aggressive behaviour are sensitized by REM deprivation for a longer period than those subjacent to ambulation and stereotype.

Morphine differentially affects ventral tegmental and substantia nigra brain reward thresholds

In order to differentiate the roles of the nigrostriatal and mesolimbic-mesocortical dopamine systems in the action of opiates on dopaminergically mediated intracranial self-stimulation (ICSS), the effects of chronic morphine administration and acute naloxone administration on ICSS were tested in rats with electrode placements in the substantia nigra pars compacta (A-9) and the ventral tegmentum (A-10). Acute morphine (5.0 mg/kg SC) did not affect ICSS thresholds of rats with electrodes in the A-9 nucleus when tested 1, 3, 5, and 23 hours after administration. With repeated daily administration, though, these animals showed increases in thresholds which grew progressively larger with each day of morphine treatment. This threshold elevation was not reversed by naloxone given 0.5 hour after the final morphine treatment on the fifth day. In contrast, acute morphine significantly lowered self-stimulation thresholds in rats with A-10 placements. Tolerance to this facilitatory effect was evident with chronic administration. Naloxone attentuated the lowering of threshold caused by opiate administration in these A-10 animals. The present data suggest a specificity of action of opiates on different brain systems subserving reward and reinforcement. These findings also suggest that the mesolimbic-mesocortical system may play an important role in mediating the rewarding properties of morphine.

Time course of dopaminergic hypersensitivity following chronic narcotic treatment

Guinea pigs were injected SC for 3 weeks with 3 different dosage schedules of morphine or methadone, or with saline. For 8 weeks thereafter they were challenged weekly with the dopamine agonist apomorphine. Hypersensitivity was manifested in more intense stereotypies, as compared to the saline group, by all morphine and methadone groups. Hypersensitivity persisted longer after the termination of methadone treatment (maximum of 8 weeks) than after morphine administration (maximum of 3 weeks). The degree of hypersensitivity, and its duration after treatment, was positively related to methadone dosage. In some groups a period of hyposensitivity was seen following hypersensitivity. These data are interpreted with reference to the hypothesized mechanism underlying the development of hypersensitivity, the different durations of action of morphine and methadone, and the retention of methadone in brain following treatment.

Responses of substantia nigra neurones to noxious stimulation

Experiments were performed to determine whether substantia nigra neurones respond to noxious stimulation. Some nigral neurones were found to respond to both noxious radiant heat and intra-arterially administered bradykinin. Most nociceptive neurones were excited by noxious stimulation although some exhibited a reduction in firing rate. Following the injection of bradykinin, responses to noxious heat were reduced in magnitude for periods of up to an hour. These results show that some nigral neurones are nociceptive and provide further support for the suggestion that dopamine is involved in nociceptive mechanisms. In addition it is suggested that bradykinin may represent a rather gross of non-specific form of noxious stimulation.