Brain sites involved in mu-opioid receptor-mediated actions: a 2-deoxyglucose study

Addiction and the cerebellum with a focus on actions of opioid receptors

Increasing evidence suggests that the cerebellum could play a role in the higher cognitive processes involved in addiction as the cerebellum contains anatomical and functional pathways to circuitry controlling motivation and saliency. In addition, the cerebellum exhibits a widespread presence of receptors, including opioid receptors which are known to play a prominent role in synaptic and circuit mechanisms of plasticity associated with drug use and development of addiction to opioids and other drugs of abuse. Further, the presence of perineural nets (PNNs) in the cerebellum which contain proteins known to alter synaptic plasticity could contribute to addiction. The role the cerebellum plays in processes of addiction is likely complex, and could depend on the particular drug of abuse, the pattern of use, and the stage of the user within the addiction cycle. In this review, we discuss functional and structural modifications shown to be produced in the cerebellum by opioids that exhibit dependency-inducing properties which provide support for the conclusion that the cerebellum plays a role in addiction.

Application of in vitro [³⁵S]GTPγ-S autoradiography in studies of growth hormone effects on opioid receptors in the male rat brain

Chronic treatment with opiates may inhibit cell growth and trigger apoptosis. On the contrary, growth hormone (GH) has been demonstrated to stimulate neurogenesis and counteract apoptosis. We recently demonstrated that recombinant human GH (rhGH) may reverse opiate-induced apoptosis in cells derived from prenatal mouse hippocampus. Thus, GH might be able to prevent the impaired cognitive capabilities that may occur in both humans and other mammals in connection to chronic opiate treatment. In order to explore the mechanism by which GH exerts its beneficial effects we here examined the impact of GH treatment on the levels of delta and mu opioid peptide (DOP and MOP, respectively) receptors in the male rat brain. The rats were treated with rhGH (Genotropin®) at two different doses (0.07 and 0.7 IU/kg), twice daily, during 7 days. Following decapitation, the levels of DOP and MOP receptor functionality were determined using [³⁵S]GTPγS autoradiography. The results demonstrate that rhGH affects the levels of the MOP receptor functionality in certain areas of the brain. These alterations were seen in e.g. amygdala and thalamus, i.e. regions that recently have been implicated in learning and memory. The activity level of DOP receptors was not affected. Thus, the data support that the beneficial effect of GH on counteracting apoptosis might involve a direct or indirect effect on the MOP but not the DOP receptor.

Effects of the intravenous administration of [Lys7]dermorphin on local cerebral glucose utilization in the rat

The use of analgesic opioids in the clinical setting is hampered by the reinforcing and addictive properties of these drugs. Moreover, chronic administration of conventional opioids is accompanied by progressive reduction of the analgesic effects, that often forces clinicians to increase dosages, exposing a subject to serious side-effects. Thus, interest is growing in the development and characterization of synthetic opioid agonists with lower reinforcing properties than conventional opioids. [Lys7]dermorphin is a mu1 receptor agonist with 20-30 times stronger analgesic properties than morphine. Previous data indicate that the drug causes fewer side-effects than conventional opioids, and is less likely to produce physical dependence than morphine. In this study we investigated the effects of the intravenous administration of a range of doses of [Lys7]dermorphin (0.002, 0.01 and 0.05 mg/kg) on local cerebral glucose utilization in the rat, by means of the quantitative [14C]2-deoxyglucose method. The results of the study showed dose-related reductions of cerebral metabolic rates for glucose in limbic, sensory-motor and autonomic regions following the intravenous administration of [Lys7]dermorphin. Such pattern of changes is similar to those measured earlier following the administration of analgesic doses of drugs stimulating mu-opioid receptors. Within the nucleus accumbens, and the shell portion in particular, we did not measure any increase of glucose utilization, rather a significant decrease following the administration of the higher dose of [Lys7]dermorphin. These findings contribute to the definition of the functional consequences of the administration of [Lys7]dermorphin, and indirectly suggest the lack of effect of the drug on mesolimbic dopamine neurotransmission.

Testosterone Reverses Ethanol-Induced Deficit in Spatial Reference Memory in Castrated Rats

The present study was designed to evaluate the effects of ethanol, testosterone and combination of ethanol and testosterone, on spatial reference memory and beta-endorphin (beta-EN) levels in castrated rats. Male Sprague-Dawley rats (120-150 g) were used in this study, Animals were castrated and ethanol, testosterone or combination of the drugs were administered to rats at 09:00 h. The drugs were administered after a training period of 5 days and spatial reference memory was evaluated for 7 days using the Morris water maze. One hour after the last injection, animals were sacrificed, their brains removed and dissected into cortex, hypothalamus, hippocampus and midbrain. The beta-EN levels in these brain regions were determined by radioimmunoassay. The time to find the platform (latency period) was significantly increased in ethanol-treated rats, indicating that ethanol induces deficit in spatial reference memory. On the other hand, testosterone administration improved spatial reference memory by significantly decreasing the latency period. In addition, there was a significant decrease in latency period in the animals treated with combination of ethanol and testosterone. Results also indicate that administration of ethanol resulted in a significant increase in beta-EN levels in the hippocampus and in the cortex while concurrent administration with testosterone abolished this increase. These findings clearly indicate that administration of testosterone did not only improve memory but also abolished the spatial memory deficit induced by ethanol in castrated rats.

Fentanyl, a upsilon-opioid receptor agonist, phase shifts the hamster circadian pacemaker

The phase-shifting effects of the mu-opioid receptor agonist fentanyl on the circadian timing system were investigated in the hamster. Fentanyl injections during the mid-subjective day induced phase advances of the hamsters' wheel-running activity rhythm. The shifts were not accompanied by an increase in locomotor activity but instead a decrease of activity was often observed. A dose-response curve indicated that with increasing dosage, the response probability increased, while the magnitude of the induced shift remained stable. The present data suggest that there is some role for opioid regulation of the circadian system.

Naltrexone does not prevent the weight gain and hyperphagia induced by the antipsychotic drug sulpiride in rats

Few pharmacological tools are currently available to counteract the excessive body weight gain often observed during prolonged administration of antipsychotic drugs. Most antipsychotic drugs block dopamine receptors, and both the brain dopaminergic and opioid systems appear to be involved in initiation and maintenance of feeding behavior, respectively. We evaluated whether the opioid antagonist naltrexone (NAL, 0.5-16 mg/kg/ip for 21 days) (a) affects body weight and food intake in gonadally-intact and drug-free female rats, (b) prevents obesity, hyperphagia, hyperprolactinemia and vaginal cycle disruption induced by long-term administration of the antipsychotic drug sulpiride (SUL, 20 mg/kg/ip for 21 days), or (c) reverses the acute hyperphagia induced by SUL (15 microg bilaterally), when directly applied in the perifornical lateral hypothalamus (PFLH). In drug-free rats, only NAL doses above 4 mg/kg, significantly decreased weight gain and food intake. Even though NAL (1 and 8 mg/kg) significantly attenuated SUL-induced hyperphagia and hyperprolactinemia, it did not reverse at any dose the weight gain and permanent diestrous induced by SUL. In addition, local NAL did not prevent the hyperphagia and polidypsia observed after acute intrahypothalamic SUL. Unexpectedly, the cumulative and 24 h food intake in SUL-treated rats was significantly increased by NAL. Collectively, these results do not support a role for endogenous opiates in the neural and endocrine mechanisms involved in weight gain during prolonged antipsychotic drug administration in rats.

Ultrastructural localization of mu-1 opioid receptor in the dorsal raphe nucleus of the rat

A simple pre-embedding avidin-biotin-peroxidase complex technique was used to study the ultrastructural localization of mu-1 opioid receptor in the rat dorsal raphe nucleus. Using low concentrations of the first antiserum for incubation with a short reaction time to 3,3'-diaminobenzidine, the immunostaining was faint at the light microscopic level. However, at the electron microscopic level strong immunoreaction was observed. Mu-1 opioid receptors were found to be localized on the postsynaptic membrane of dendrites, extra-synaptic plasma membrane, and the surface of the small, clear vesicles in axon terminals. Of the total 407 immunopositive profiles observed, 76.4% (311/407) were dendrites and 18.9% (77/407) were axon terminals. The immunostained myelinated axons and perikarya were relatively rare, with frequencies of 1.0% (4/407) and 3.7% (15/407), respectively. About 50.8% of the immunopositive dendrites (158/311) were immunostained having their MOR-LI results beneath the postsynaptic membrane, although about 19.6% of them (31/158) also exhibited MOR-LI on other components, including the extrasynaptic plasma membrane. Other immunopositive dendrites showed staining in some other contents, including extrasynaptic plasma membrane (82/311, 26.4%) or not on the plasma membranes (71/311, 22.8%). Less than half of the immunopositive axon terminals (35/77, 45.5%) were found to make synapses with nonimmunoreactive dendrites (31/77, 40.3%) or immunopositive dendrites (4/77, 5.2%); none were found to make synapses with immunoreactive perikarya. The present study shows that mu-1 opioid receptor in the dorsal raphe nucleus plays a role at both synapse or not.

Similar involvement of several brain areas in the antinociception of endogenous and exogenous opioids

The complete inhibitor of the enkephalin degrading enzymes, RB 101, N-{(R,S)-2-benzyl-3[(S)-(2-amino-4-methylthio)butyldithio]-1- oxopropyl}-L-phenylalanine benzyl ester, which crosses the blood-brain barrier, induced antinociceptive effects similar to those of exogenous opiates. The almost complete absence of tolerance and dependence after chronic administration of RB 101 is therefore due to limited stimulation of opioid receptors by 'protected' endogenous enkephalins. In order to clarify the mechanisms involved in these response, we have investigated the participation of several brain structures in the antinociceptive effects induced by systemic administration of morphine or RB 101. Rats were implanted with bilateral cannulae into the ventro-basal thalamus, central amygdala and periaqueductal gray matter, or with a cannula into the raphe magnus nucleus. The antinociceptive responses induced by systemic morphine or RB 101 were measured by using the tail-electrical stimulation test, where three different thresholds were determined: motor response, vocalization and vocalization post-discharge. The ability of the opioid receptor antagonist methylnaloxonium to block these antinociceptive responses was evaluated after local injection into the different brain structures. The blockade of morphine- and RB 101-induced antinociception was similar, and was stronger when methylnaloxonium was injected into the periaqueductal gray matter and raphe magnus nucleus than when it was injected into the ventro-basal thalamus and amygdala. These results suggest that brain structures related to the control of pain seem to be the same for the antinociception induced by exogenous opiates and endogenous opioids.