Blockade of central delta-opioid receptors inhibits salt appetite in sodium-depleted rats

Opioid antagonism mitigates antipsychotic-associated weight gain: focus on olanzapine

BACKGROUND

The endogenous opioid system affects metabolism, including weight regulation. Evidence from preclinical and clinical studies provides a rationale for targeting this system to mitigate weight-related side effects of antipsychotics. This review describes the role of the opioid system in regulating weight and metabolism, examines the effects of opioid receptor antagonism on those functions, and explores the use of opioid antagonists to mitigate antipsychotic-associated weight gain and/or metabolic effects.

METHODS

A PubMed literature search was conducted to identify representative opioid antagonists and associated preclinical and clinical studies examining their potential for the regulation of weight and metabolism.

RESULTS

The mu opioid receptor (MOR), delta opioid receptor (DOR), and kappa opioid receptor (KOR) types have overlapping but distinct patterns of central and peripheral expression, and each contributes to the regulation of body weight and metabolism. Three representative opioid antagonists (eg, naltrexone, samidorphan, and LY255582) were identified for illustration. These opioid antagonists differed in their receptor binding and pharmacokinetic profiles, including oral bioavailability, systemic clearance, and half-life, and were associated with varying effects on food intake, energy utilization, and metabolic dysregulation.

CONCLUSIONS

Preclinical and clinical data suggest that antagonism of the endogenous opioid system is a mechanism to address antipsychotic-associated weight gain and metabolic dysregulation. However, evidence suggests that the differing roles of MOR, DOR, and KOR in metabolism, together with the differences in receptor binding, pharmacokinetic, and functional activity profiles of the opioid receptor antagonists discussed in this review, likely contribute to their differential pharmacodynamic effects and clinical outcomes observed regarding antipsychotic-associated weight gain.

Inhibition of salt appetite in sodium-depleted rats by carvacrol: Involvement of noradrenergic and serotonergic pathways

Carvacrol, a monoterpene phenol present in the essential oil of oregano, possesses several biological properties, such as antioxidant, anti-inflammatory, anxiolytic, anticonvulsive and antinociceptive. In vitro studies have shown that carvacrol inhibits serotonin, noradrenaline and dopamine transporters and the enzymes monoamine oxidase-A and B. Different brain functions are controlled by monoamines, including cardiovascular control, thirst and sodium appetite. In the present study we investigated the effects of intracerebroventricular (i.c.v.) injection of carvacrol on sodium appetite, and the participation of brain serotonergic and noradrenergic pathways on carvacrol effects. Neuronal activation in homeostasis-related brain areas induced by i.c.v. injection of carvacrol was also evaluated. Carvacrol dose-dependently inhibited hypertonic saline intake (1.5%) in sodium-depleted rats, and this antinatriorexigenic effect was reduced by brain serotonergic depletion and by alpha-adrenergic blockade. Furthermore, i.c.v. injections of carvacrol significantly increased the neuronal activation in brain areas involved in the control of salt appetite, such as MnPO, OVLT, PVN, SON, CeA and MeA. Taken together, our data show that carvacrol presents antinatriorexigenic activity through serotonin and noradrenaline pathways within brain circuits involved in the modulation of the body fluid homeostasis.

[µ-opioid receptors in the central nucleus of the amygdala mediate sodium intake in rats]

OBJECTIVE

To investigate the opioidergic mechanism of the central nucleus of the amygdala (CeA) for regulating sodium appetite in rats.

METHDOS

Using the elaborate invasive cerebral cannulation and brain microinjection method, we observed the effects of bilateral intra-CeA injections of DAMGO (a selective µ-opioid receptor agonist) and CTAP (a highly selective µ-opioid receptor antagonist), either alone or in combination, on NaCl solution (0.3 mol/L) and water intake by rats in different models of Na⁺ ingestion.

RESULTS

In the two-bottle tests, bilateral injections of DAMGO at 1, 2, and 4 nmol into the CeA induced a dose-related increase of NaCl and water intake in rats treated with water deprivation with partial rehydration (WD-PR), and pretreatment with 0.5, 1, and 2 nmol CTAP injected into the CeA significantly suppressed DAMGO-induced NaCl and water intake in a dose-dependent manner: in the one-bottle tests, bilateral injections of DAMGO (2 noml) into the CeA had no effect on water intake of the rats. In rats with subcutaneous injection of furosemide (FURO) combined with captopril (CAP) (FURO+CAP), bilateral intra-CeA injections of DAMGO (2 nmol) caused increased NaCl and water intake in the two-bottle tests, but such effects were suppressed by pretreatment with CTAP injection into the CeA; in the one-bottle tests, bilateral intra-CeA injections of DAMGO had no effect on water intake of the rats.

CONCLUSION

µ-opioid receptors in the CeA are involved in the excitatory regulation of sodium appetite to mediate sodium intake. µ-opioid receptor antagonists are expected to be targets for developing inhibitors of sodium appetite.

Endogenous central amygdala mu-opioid receptor signaling promotes sodium appetite in mice

Due to the importance of dietary sodium and its paucity within many inland environments, terrestrial animals have evolved an instinctive sodium appetite that is commensurate with sodium deficiency. Despite a well-established role for central opioid signaling in sodium appetite, the endogenous influence of specific opioid receptor subtypes within distinct brain regions remains to be elucidated. Using selective pharmacological antagonists of opioid receptor subtypes, we reveal that endogenous mu-opioid receptor (MOR) signaling strongly drives sodium appetite in sodium-depleted mice, whereas a role for kappa (KOR) and delta (DOR) opioid receptor signaling was not detected, at least in sodium-depleted mice. Fos immunohistochemistry revealed discrete regions of the mouse brain displaying an increased number of activated neurons during sodium gratification: the rostral portion of the nucleus of the solitary tract (rNTS), the lateral parabrachial nucleus (LPB), and the central amygdala (CeA). The CeA was subsequently targeted with bilateral infusions of the MOR antagonist naloxonazine, which significantly reduced sodium appetite in mice. The CeA is therefore identified as a key node in the circuit that contributes to sodium appetite. Moreover, endogenous opioids, acting via MOR, within the CeA promote this form of appetitive behavior.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).