FLUOXETINE ALTERS MU OPIOID RECEPTOR EXPRESSION IN OBESE ZUCKER RAT EXTRAHYPOTHALAMIC REGIONS

Fluoxetine reverses the behavioral despair induced by neurogenic stress in mice: role of N-methyl-d-aspartate and opioid receptors

Opioid and N-methyl-d-aspartate (NMDA) receptors mediate different effects of fluoxetine. We investigated whether opioid and NMDA receptors are involved in the protective effect of fluoxetine against the behavioral despair induced by acute physical stress in male mice. We used the forced swimming test (FST), tail suspension test (TST), and open-field test (OFT) for behavioral evaluation. We used fluoxetine, naltrexone (opioid receptor antagonist), MK-801 (NMDA receptor antagonist), morphine (opioid receptor agonist), and NMDA (NMDA receptor agonist). Acute foot-shock stress (FSS) significantly induced behavioral despair (depressive-like) and anxiety-like behaviors in tests. Fluoxetine (5 mg/kg) reversed the depressant-like effect of FSS, but it did not alter the locomotion and anxiety-like behavior in animals. Acute administration of subeffective doses of naltrexone (0.3 mg/kg) or MK-801 (0.01 mg/kg) potentiated the antidepressant-like effect of fluoxetine, while subeffective doses of morphine (1 mg/kg) and NMDA (75 mg/kg) abolished this effect of fluoxetine. Also, co-administration of subeffective doses of naltrexone (0.05 mg/kg) and MK-801 (0.003 mg/kg) with fluoxetine (1 mg/kg) induced a significant decrease in the immobility time in FST and TST. Our results showed that opioid and NMDA receptors (alone or in combination) are involved in the antidepressant-like effect of fluoxetine against physical stress.

Effects of chronic treatment with citalopram on cannabinoid and opioid receptor-mediated G-protein coupling in discrete rat brain regions

RATIONALE

There is growing interest in investigating the mechanisms of action of selective serotonin reuptake inhibitors (SSRIs), beyond their association with the serotonergic system, due to their wide therapeutic potential for disorders including depression, pain and addiction.

OBJECTIVE

The aim of this study was to investigate whether chronic treatment with the SSRI, citalopram, alters the functional coupling of G(i/o)-associated cannabinoid type 1 (CB(1)) and mu-opioid receptors in selected areas of rat brain implicated in psychiatric disorders and pain.

METHODS

Using an autoradiographic approach, the effects of the cannabinoid receptor agonist, HU210 (in the presence or absence of the CB(1) receptor antagonist AM251), or the mu-opioid receptor agonist, [D: -Ala(2),N-Me-Phe4,Gly(5)-ol]-enkephalin (DAMGO; in the presence or absence of the mu-opioid receptor antagonist D: -Phe-Cys-Tyr-D: -Trp-Orn-Thr-Pen-Thr-NH(2)), on [(35)S]GTPgammaS binding in discrete brain regions of citalopram-treated (10 mg kg(-1) day(-1) for 14 days by subcutaneous minipump) and control rats were investigated.

RESULTS

The HU210-induced increase in [(35)S]GTPgammaS binding observed in the hypothalamic paraventricular nucleus of control rats was abolished after chronic treatment with citalopram. Reduced response to HU210 in rats receiving chronic treatment with citalopram was also observed in the hippocampus and medial geniculate nucleus. Citalopram had no significant effect on DAMGO-induced [(35)S]GTPgammaS binding in the brain regions investigated, with the exception of the medial geniculate nucleus where a modest impairment was observed.

CONCLUSIONS

These data provide evidence for reduced cannabinoid receptor-mediated G-protein coupling in the hypothalamus, hippocampus and medial geniculate nucleus of rats chronically treated with citalopram, effects which may, in part, underlie the mechanism of action of SSRIs.

Endogenous opiates and behavior: 2006

This paper is the 29th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning 30 years of research. It summarizes papers published during 2006 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurological disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Effects of fluoxetine on hippocampal-dependent and hippocampal-independent learning tasks

It is generally assumed that fluoxetine does not produce cognitive impairments, based on observations that fluoxetine-treated animals do not show impairment in learning the spatial water-maze task. As fluoxetine has different effects on different brain regions and as learning is not a unitary phenomenon, it may be the case that fluoxetine has different effects on different types of learning and memory paradigms. In this study, 15 male Sprague-Dawley rats were given chronic injections of either fluoxetine or saline and received training in two hippocampal-independent tasks in addition to a spatial water-maze task. The two hippocampal-independent tasks were a short-delay appetitive Pavlovian-conditioning task and an object-recognition task. The results showed that the fluoxetine-injected rats did not show any impairment relative to the saline controls in either the acquisition or the retention phases of the water-maze task, but were significantly impaired in both of the hippocampal-independent tasks. Fluoxetine-injected rats spent significantly less time exploring the novel object in the object-recognition task and took longer to learn the association between the conditional stimulus and the appetitive unconditional stimulus in the appetitive Pavlovian-conditioning task.