Methadone in brain and its effects on locomotor activity of young and aged mice

Unique pharmacodynamic properties and low abuse liability of the µ-opioid receptor ligand (S)-methadone

(R,S)-methadone ((R,S)-MTD) is a µ-opioid receptor (MOR) agonist comprised of (R)-MTD and (S)-MTD enantiomers. (S)-MTD is being developed as an antidepressant and is considered an N-methyl-D-aspartate receptor (NMDAR) antagonist. We compared the pharmacology of (R)-MTD and (S)-MTD and found they bind to MORs, but not NMDARs, and induce full analgesia. Unlike (R)-MTD, (S)-MTD was a weak reinforcer that failed to affect extracellular dopamine or induce locomotor stimulation. Furthermore, (S)-MTD antagonized motor and dopamine releasing effects of (R)-MTD. (S)-MTD acted as a partial agonist at MOR, with complete loss of efficacy at the MOR-galanin Gal1 receptor (Gal1R) heteromer, a key mediator of the dopaminergic effects of opioids. In sum, we report novel and unique pharmacodynamic properties of (S)-MTD that are relevant to its potential mechanism of action and therapeutic use. One-sentence summary: (S)-MTD, like (R)-MTD, binds to and activates MORs in vitro, but (S)-MTD antagonizes the MOR-Gal1R heteromer, decreasing its abuse liability.

Involvement of hypothalamic neuropeptide Y in pentazocine induced suppression of food intake in rats

The potent orexigenic peptide neuropeptide Y (NPY) has been considered as a possible endogenous ligand for a subpopulation of sigma receptors (SigR). However, their mutual interaction with reference to feeding behavior remains poorly understood. In the present study, we explored the possible interaction between sigma1 receptors (Sig1R) agonist, pentazocine, and NPY on food intake in satiated rats. While pentazocine dose-dependently reduced the food intake, NPY significantly increased it at 2, 4 and 6h post injection time points. In combination studies, pretreatment with NPY (0.1 nmol/rat, intra-PVN) normalized the inhibitory effect of pentazocine (60 μg/rat, intra-PVN) on food intake. Similarly, pre-treatment with pentazocine (30 μg/rat, intra-PVN) significantly antagonized the orexigenic effect of NPY (0.5 and 1.0 nmol/rat, intra-PVN). Moreover, pentazocine treatment decreased NPY immunoreactivity in arcuate (ARC), paraventricular (PVN), dorsomedial (DMH) and ventromedial (VMH) nuclei of hypothalamus. However, no change was observed in lateral hypothalamus (LH). Study implicates the reduced NPY immunoreactivity for the anorectic effect observed following pentazocine injections. Therefore, the concomitant activation of the NPYergic system along with the Sig1R agonist treatment may serve a useful purpose in the management of the unwanted side effects related to energy homeostasis.

Differential effects of methadone and buprenorphine on the response of D2/D3 dopamine receptors in adolescent mice

BACKGROUND

There is a lack of studies that examine the effects of opioid maintenance drugs on the developing adolescent brain, limiting the ability of physicians to conduct a science-based risk assessment on the appropriateness of these treatments for that age group. Our recent observations indicate higher potential risks in repeated exposure to morphine during adolescence, specifically to the D2/D3 dopamine receptors' signaling. Disturbances in dopaminergic signaling could have broader implications for long-term mental health. Thus, this study examined whether buprenorphine and methadone differentially alter the responses of the D2/D3 dopamine receptors in adolescents.

METHODS

Adolescent mice were orally administered buprenorphine (0.1-0.4 mg/kg), methadone (25-100 mg/kg), or saline once daily for 6 days. Two hours or three days later, the mice were tested for their locomotor response to 10 mg/kg quinpirole, a D2/D3 dopamine receptor agonist.

RESULTS

Buprenorphine-treated adolescent mice did not significantly differ from control drug-naïve animals in their response to quinpirole. However, an enhanced response was observed in methadone-treated adolescent animals. This enhanced locomotion was significantly higher two hours following the final dose of methadone, as compared to three days afterwards.

CONCLUSIONS

This study suggests that exposure to various opioids carries differential probabilities of altering the highly sensitive neurochemistry of adolescent brains. Methadone exposure disturbs the D2-like receptor's response, indicating a potential risk in administering methadone to adolescents (either for the treatment of opioid dependency/abuse or for pain management). In contrast, buprenorphine appears to have a significantly lower effect on the behavioral sensitivity of D2/D3 dopamine receptors in adolescents.

Alterations in delta opioid receptor levels in discrete areas of the neocortex and in the globus pallidus of the aging guinea pig: a quantitative autoradiographic study

The effect of aging on delta opioid receptors was examined in the brains of guinea pigs aged 1, 6, 24 and 36 months. Quantitative autoradiography was used to monitor the concentrations of delta receptors in various anatomical regions at five rostro-caudal levels. delta opioid receptor populations were found to be remarkably stable throughout the life span of this species. We have, however, discovered anatomical areas which offer striking exceptions. In the globus pallidus, progressive age-related losses of delta receptors reached 50% in the senescent animal. In contrast, laminae I, II of the lateral agranular frontal cortex and laminae I, II and III, IV of the primary somatosensory cortex demonstrated age-related increases in the concentrations of delta receptors ranging from 30 to 45%. These changes are discussed with the view to their being functionally related components of motor circuitry involving pyramidal and extrapyramidal elements.

Age-related changes in kappa opioid receptors in the guinea-pig brain: a quantitative autoradiographic study

Investigation into the effect of aging on kappa opioid receptors in the brains of guinea-pigs was carried out in animals aged one, six, 24 and 36 months. Quantitative autoradiography was used to monitor the concentration of kappa receptors in various anatomic regions at five rostrocaudal levels in each age-group. Areas of high binding were found in the deep layers (laminae V, VI) of the neocortex and in the internal band of the periallocortical, dorsal agranular insular cortex. Among non-cortical areas examined, the nucleus accumbens and the substantia nigra possessed kappa binding levels equal to those seen in the deep neocortical layers. In all cases where an age-related change in the level of kappa receptors was detected, the direction of the change was one of decreased binding with advancing age. Statistical analysis of the binding data revealed that the one-month-old animal possessed the highest levels of kappa binding among all age groups examined. The vast majority of age-related changes in kappa binding levels occurred in laminae V and VI of neocortical regions. The per cent decreases (18-42%), as well as their age of onset (six to 36 months) varied in different anatomical regions. Possible mechanisms to explain the age-related decreases in kappa opioid binding are presented. The majority of the age-related decreases in kappa opioid binding are found in areas of the neocortex which are characterized by their motor, sensory and associative functions. It is within these three areas of function that diminutions in performance are most apparent in senescence.

Aging and day-night rhythms in feeding in mice: effects of the putative sigma opiate agonist, N-allylnormetazocine (SKF-10,047)

Day-night rhythms in feeding behavior and response to the putative sigma opiate agonist, N-allylnormetazocine (+/- SKF-10,047, 0.10-10 mg/kg), were measured in young (1-2 months), mature (8-12 months) and old (24-30 months) male CF-1 mice. The mice consumed more food at night than in the day-time, though this nocturnal peak was markedly reduced in the mature and old animals. The young mice also displayed a significant nocturnal enhancement in SKF-10,047 (0.10-1.0 mg/kg) stimulated feeding, that could, in part, be suppressed by the opiate antagonist naloxone (1.0 mg/kg). The day-night rhythm in ingestive responses to SKF-10,047 (0.10-1.0 mg/kg) was reduced in the mature animals and absent in the old animals. The old mice failed to show any significant increase in ingestive response following opiate administration. A higher dose of SKF-10,047 (10 mg/kg) had no significant ingestive effects in any of the age groups of mice; the excitatory, psychotomimetic-related effects, being also reduced in the old animals.

The influence of opiate agonists on day-night feeding rhythms in young and old mice

Daily rhythms of feeding behavior and responses to ketocyclazocine, morphine and naloxone were measured in young (1-2 months) and old (24-30 months) male CF-1 mice. All of the mice consumed more food at night than in the day-time, though this nocturnal peak was markedly reduced in the old animals, who consumed more in the day. The young mice also displayed a significant nocturnal enhancement in ketocyclazocine- and morphine-stimulated feeding. This day-night rhythm in ingestive responses was absent in the old mice. In comparison to the young mice, the opiate-stimulated food consumptions of the old animals were reduced at all times. Additionally, the old animals failed to show any day-night variations in the suppressive effects of naloxone on deprivation-induced food intake that were displayed by the young animals.

The effects of aging on day-night rhythms of kappa opiate-mediated feeding in the mouse

Day-night rhythms in feeding behavior and response to the specific kappa opioid agonist U-50,488H (0.10-10. mg/kg) were measured in young (1-2 months), mature (8-12 months) and old (24-30 months) male CF-1 mice. All the mice consumed more food at night than in the day-time, though this nocturnal peak was markedly reduced in old and mature animals. Young mice also displayed a significant, dose-related, nocturnal enhancement in U-50,488H-stimulated feeding. This day-night rhythm was reduced in mature animals and absent in old mice. In old mice, U-50,488H significantly stimulated feeding only after the high dose of 10 mg/kg. Additionally, old animals did not show the dose-dependent latency to initiation of feeding after administration which was observed in young mice and to a lesser extent in mature animals.