Stimulation of hypothalamic opioid peptide release by lithium is mediated by opioid autoreceptors: evidence from a combined in vitro, ex vivo study

Lithium Enhances the GABAergic Synaptic Activities on the Hypothalamic Preoptic Area (hPOA) Neurons

Lithium (Li+) salt is widely used as a therapeutic agent for treating neurological and psychiatric disorders. Despite its therapeutic effects on neurological and psychiatric disorders, it can also disturb the neuroendocrine axis in patients under lithium therapy. The hypothalamic area contains GABAergic and glutamatergic neurons and their receptors, which regulate various hypothalamic functions such as the release of neurohormones, control circadian activities. At the neuronal level, several neurotransmitter systems are modulated by lithium exposure. However, the effect of Li+ on hypothalamic neuron excitability and the precise action mechanism involved in such an effect have not been fully understood yet. Therefore, Li+ action on hypothalamic neurons was investigated using a whole-cell patch-clamp technique. In hypothalamic neurons, Li+ increased the GABAergic synaptic activities via action potential independent presynaptic mechanisms. Next, concentration-dependent replacement of Na+ by Li+ in artificial cerebrospinal fluid increased frequencies of GABAergic miniature inhibitory postsynaptic currents without altering their amplitudes. Li+ perfusion induced inward currents in the majority of hypothalamic neurons independent of amino-acids receptor activation. These results suggests that Li+ treatment can directly affect the hypothalamic region of the brain and regulate the release of various neurohormones involved in synchronizing the neuroendocrine axis.

Isolation stress impacts Met-enkephalin in the hypothalamo-pituitary-adrenocortical axis in growing Polish Mountain sheep: a possible role of the opioids in modulation of HPA axis

It was hypothesized that there is cross-talk between the classical constituents of the hypothalamo-pituitary-adrenocortical axis (HPA) and Met-enkephalin in the HPA axis. The study examined effects of isolation stress, sex, and age on concentrations of native Met-enkephalin and pro-enkephalin (PENK) gene expression in tissues of the HPA (hypothalamus, pituitary gland and adrenal cortex) in 3-, 6- and 9-month old female and male lambs. In addition, the effects of isolation stress on in vitro release Met-enkephalin from fragments of the hypothalamus or adrenal cortex were examined. Isolation stress was followed by decreases in the concentration of Met-enkephalin in both the pituitary gland and adrenal cortex. There were also increases in the hypothalamic concentration of Met-enkephalin together with increases in PENK gene expression in the HPA in 6- and 9-months old females and males. There were reductions in release of Met-enkephalin from hypothalamic and adrenocortical tissue in vitro after isolation stress. In the presence of naltrexone, there were increases in basal release in vitro of Met-enkephalin from hypothalamic tissue from control and stressed female lambs but a decrease in tissue from stressed male lambs. In a somewhat similar manner, the presence of naltrexone was associated with increases in the basal release of Met-enkephalin from adrenocortical tissue from control female lambs but a decrease with tissue from stressed female and both stressed and control male lambs. Lay summary   The present studies examine the impact of isolation stress on Met-enkephalin in growing female and male lambs. The results clearly showed the involvement of Met-enkephalin modulation of the psychological stress response in growing female and male lambs.

Lithium reverses mechanical allodynia through a mu opioid-dependent mechanism

Background

Lithium is widely used to treat bipolar disorders and displays mood stabilizing properties. In addition, lithium relieves painful cluster headaches and has a strong analgesic effect in neuropathic pain rat models.

Objectives

To investigate the analgesic effect of lithium on the cuff model of neuropathic pain.

Methods

We used behavioral and pharmacological approaches to study the analgesic effect of a single injection of lithium in wild-type and mu opioid receptor (MOR) null cuffed neuropathic mice. Mass spectrometry and enzyme-linked immunosorbent assay allowed to measure the levels of endogenous MOR agonist beta-endorphin as well as monoamines in brain and plasma samples 4 h after lithium administration.

Results

A single injection of lithium chloride (100 mg/kg, ip) alleviated mechanical allodynia for 24 h, and this effect was absent in MOR null neuropathic mice. Biochemical analyses highlight a significant increase in beta-endorphin levels by 30% in the brain of lithium-treated mice compared to controls. No variation of beta-endorphin was detected in the blood.

Conclusions

Together, our results provide evidence that lithium induces a long-lasting analgesia in neuropathic mice presumably through elevated brain levels of beta-endorphin and the activation of MORs.

Temporal Variation in Drug Interaction Between Lithium and Morphine‐Induced Analgesia

The administration-time-dependent aspects of the drug interaction between lithium and morphine-induced analgesia were studied using the mouse hot-plate test at six different times of day, each scheduled at 4 h intervals. Lithium treatment alone, in doses of 1 to 10 mmol/kg administered intraperitoneally (i.p.) did not significantly alter test latencies compared to the corresponding clock-time in saline-injected controls. Basal pain sensitivity and morphine-induced antinociceptive activity displayed significant circadian rhythms as assessed by the hot-plate response latencies, with higher values occurring during the nocturnal activity than during the daytime rest span. Acute administration of lithium, in a dose of 3 mmol/kg, 30 min prior to morphine dosing did not influence morphine-induced analgesia compared to all the clock-time test-matched morphine groups, except the 9 HALO (Hours After Lights On) one. There was a prominent potentiation of the morphine-induced antinociception at this biological time during combined drug treatment. The latter finding demonstrates that administration-time-dependent differences in drug-drug interactions need to be considered in both experimental designs and clinical settings.

Naloxone rapidly evokes endogenous kappa opioid receptor-mediated hyperalgesia in naïve mice pretreated briefly with GM1 ganglioside or in chronic morphine-dependent mice

Low-dose naloxone-precipitated withdrawal hyperalgesia is a reliable indicator of physical dependence after chronic morphine treatment. A remarkably similar long-lasting (>3-4 h) hyperalgesia is evoked by injection of a low dose of naloxone (10 microg/kg, s.c.) in naïve mice after acute pretreatment with the glycolipid, GM1 ganglioside (1 mg/kg) (measured by warm-water-immersion tail-flick assays). GM1 treatment markedly increases the efficacy of excitatory Gs-coupled opioid receptor signaling in nociceptive neurons. Co-treatment with an ultra-low-dose (0.1 ng/kg, s.c.) of the broad-spectrum opioid receptor antagonist, naltrexone or the selective kappa opioid receptor antagonist, nor-binaltorphimine, blocks naloxone-evoked hyperalgesia in GM1-pretreated naïve mice and unmasks prominent, long-lasting (>4 h) inhibitory opioid receptor-mediated analgesia. This unmasked analgesia can be rapidly blocked by injection after 1-2 h of a high dose of naltrexone (10 mg/kg) or nor-binaltorphimine (0.1 mg/kg). Because no exogenous opioid is administered to GM1-treated mice, we suggest that naloxone may evoke hyperalgesia by inducing release of endogenous bimodally acting opioid agonists from neurons in nociceptive networks by antagonizing putative presynaptic inhibitory opioid autoreceptors that "gate" the release of endogenous opioids. In the absence of exogenous opioids, the specific pharmacological manipulations utilized in our tail-flick assays on GM1-treated mice provide a novel bioassay to detect the release of endogenous bimodally acting (excitatory/inhibitory) opioid agonists. Because mu excitatory opioid receptor signaling is blocked by ultra-low doses of naloxone, the higher doses of naloxone that evoke hyperalgesia in GM1-treated mice cannot be mediated by activation of mu opioid receptors. Co-treatment with ultra-low-dose naltrexone or nor-binaltorphimine may selectively block signaling by endogenous GM1-sensitized excitatory kappa opioid receptors, unmasking inhibitory kappa opioid receptor signaling, and converting endogenous opioid receptor-mediated hyperalgesia to analgesia. Co-treatment with kelatorphan stabilizes putative endogenous opioid peptide agonists released by naloxone in GM1-treated mice, so that analgesia is evoked rather than hyperalgesia. Acute treatment of chronic morphine-dependent mice with ultra-low-dose naltrexone (0.1 ng/kg) results in remarkably similar rapid blocking of naloxone (10 microg/kg)-precipitated withdrawal hyperalgesia and unmasking of prominent opioid analgesia. These studies may clarify complex mechanisms underlying opioid physical dependence and opioid addiction.

Molecular targets of lithium action

Lithium is an effective drug for both the treatment and prophylaxis of bipolar disorder. However, the precise mechanism of lithium action is not yet well understood. Extensive research aiming to elucidate the molecular mechanisms underlying the therapeutic effects of lithium has revealed several possible targets. The behavioral and physiological manifestations of the illness are complex and are mediated by a network of interconnected neurotransmitter pathways. Thus, lithium's ability to modulate the release of serotonin at presynaptic sites and modulate receptor-mediated supersensitivity in the brain remains a relevant line of investigation. However, it is at the molecular level that some of the most exciting advances in the understanding of the long-term therapeutic action of lithium will continue in the coming years. The lithium cation possesses the selective ability, at clinically relevant concentrations, to alter the PI second-messenger system, potentially altering the activity and dynamic regulation of receptors that are coupled to this intracellular response. Subtypes of muscarinic receptors in the limbic system may represent particularly sensitive targets in this regard. Likewise, preclinical data have shown that lithium regulates arachidonic acid and the protein kinase C signaling cascades. It also indirectly regulates a number of factors involved in cell survival pathways, including cAMP response element binding protein, brain-derived neurotrophic factor, bcl-2 and mitogen-activated protein kinases, and may thus bring about delayed long-term beneficial effects via under-appreciated neurotrophic effects. Identification of the molecular targets for lithium in the brain could lead to the elucidation of the pathophysiology of bipolar disorder and the discovery of a new generation of mood stabilizers, which in turn may lead to improvements in the long-term outcome of this devastating illness (1).

Acute and conditioned sickness reduces morphine analgesia

Animals made ill by intraperitoneal injection with toxins, such as lithium chloride (LiCl) or lipopolysaccharides (LPS), or presented with cues associated with LiCl become hyperalgesic [Pain 56 (1994) 227]. The descending pronociceptive neurocircuitry and spinal pharmacology that underlie these effects bear the same features as those that mediate analgesic tolerance to morphine [Neurosci. Biobehav. Rev. 23 (1999) 1059]. Thus, we examined whether LiCl, LPS or cues paired with LiCl could reduce morphine analgesia. Morphine analgesia in the tail flick test was reduced 24 h but not 7 days following injection with LiCl, and 24 h following injection with LPS. In addition, morphine analgesia was reduced in the hot plate test 40 min and 24 h following LiCl. Furthermore, these effects occurred in the absence of detectable hyperalgesia indicating that illness-induced tolerance was not the result of an increase in pain sensitivity offsetting analgesia. Finally, rats tested in a context associated with LiCl demonstrated less morphine analgesia than rats tested in a context not associated with LiCl or rats naive to LiCl suggesting that illness activates descending mechanisms that antagonize analgesia rather than simply desensitizing opioid receptors. Thus, in addition to provoking hyperalgesia, illness-inducing agents also activate endogenous antianalgesic mechanisms.

Lithium alters mu-opioid receptor expression in the rat brain

Lithium can potentiate the effects of antidepressant drugs and alters morphine analgesia and phosphoinositide turnover. Analysis of mu-opioid receptor immunostaining after chronic lithium administration in rats revealed an increase in the density of cells expressing mu-opioid receptors in the caudatus-putamen, the dentate gyrus, the lateral septum and the frontal, parietal and piriform cortices. These data suggest that mu-opioid receptor expression in the rat forebrain is altered by in vivo chronic lithium treatment. This could be a compensatory mechanism, induced in part by the effects of lithium on mu-opioid receptor transduction mechanism.