Morphine-induced conditioned taste aversions in the LEW/N and F344/N rat strains.

Previous reports have shown that the LEW/N and F344/N inbred rat strains display a differential sensitivity to cocaine in a number of preparations, with the LEW/N rats displaying an increased sensitivity to both the reinforcing and aversive effects of cocaine (relative to the F344/N rats). Given that the LEW/N rats are also more sensitive to the reinforcing effects of morphine than the F344/N strain, the present experiment examined the ability of morphine to condition taste aversions in the LEW/N and F344/N strains to determine if the general sensitivity to cocaine generalizes to another drug of abuse. Specifically, on four conditioning trials, 35 LEW/N and 33 F344/N female rats were allowed access to a novel saccharin solution and then injected with varying doses of morphine (0, 10, 32 and 56 mg/kg). On intervening recovery days, subjects were allowed 20-min access to water. Following the fourth trial, a final aversion test was administered. The F344/N rats, but not the LEW/N rats, rapidly acquired morphine-induced taste aversions at all doses of morphine. Pharmacokinetic differences between the strains were also assessed. Specifically, 10 mg/kg morphine (or vehicle) was administered to subjects of both strains and plasma morphine levels were analyzed at 0.5, 2 and 4 h postinjection. No differences in plasma levels between the strains were observed. Unlike with cocaine, the LEW/N rats do not seem generally sensitive to morphine (relative to the F344/N rats). Rather, the differential sensitivity of the two strains to these compounds seems to be preparation dependent. Possible mechanisms underlying the differential sensitivity evident in the strains were discussed.

Acute effects of morphine on blood lymphocyte proliferation and plasma IL-6 levels.

Administration of morphine (10 mg/kg) to rats was found to decrease the proliferative potential of blood lymphocytes by 60-80% and concurrently elevate circulating levels of the cytokine, interleukin-6 (IL-6), 2- to 4-fold. Both parameters were similarly altered upon the central administration of morphine and were blocked upon pretreatment of animals with the opioid receptor antagonist, naltrexone. These results suggest that the activation of central opioid receptors is involved in morphine-induced inhibition of lymphocyte proliferation as well as increases in circulating levels of IL-6. Studies addressing the potential peripheral mechanisms demonstrated that intact ganglionic transmission was required for both effects of morphine. Although the suppression by morphine of lymphocyte proliferation appeared to be largely independent of stimulation of the hypothalamic-pituitary-adrenal axis, the elevation of IL-6 was completely abolished in adrenalectomized animals. Collectively, these results suggest that central opioid receptor activation results in changes in different immune parameters that can be mediated through distinct peripheral mechanisms.

Nicotinic receptor binding sites in rat primary neuronal cells in culture: characterization and their regulation by chronic nicotine.

We have characterized high affinity neuronal nicotinic acetylcholine receptors labeled by [3H]cytisine in primary neuronal cell cultures from fetal rat brains. After 15 days in culture, the highest density of [3H]cytisine binding sites (Bmax approximately 57 fmol/mg protein) was found in cells from the brainstem, which includes the following subcortical brain areas: the septum, thalamus, hypothalamus, midbrain, pons and medulla. A lower density of sites was found in cells from the cerebral cortex, hippocampus, and caudate nucleus. [3H]Cytisine binds to receptors in primary cells from the brainstem and cerebral cortex with a Kd of approximately 0. 5 nM, and the binding is inhibited by the agonists nicotine, acetylcholine, and epibatidine with IC50 values of 1 to 20 nM, and by carbachol and the antagonist dihydro-beta-erythroidine with IC50 values of 0.5 to 1.5 microM. Chronic treatment of neuronal cultures with nicotine for 7 days differentially affected the number of nicotinic receptors in cells from different brain areas; it significantly increased the number of nicotinic binding sites in cells from the cerebral cortex, hippocampus, and caudate, but not in cells from the brainstem. The nicotine-induced increase of receptors in cerebral cortical cultures was not blocked by either mecamylamine or dihydro-beta-erythroidine. These results indicate that primary cultures of rat neuronal cells provide a good model system in which to study and compare the properties and regulation of native neuronal nicotinic acetylcholine receptors.

Characterization of (+/-)(-)[3H]epibatidine binding to nicotinic cholinergic receptors in rat and human brain.

Epibatidine is an alkaloid that was first isolated from the skin of the Ecuadoran frog Epipedobates tricolor by Daly et al. [J. Am. Chem. Soc. 102:803-836 (1980)] and was found to have very high affinity for neuronal nicotinic receptors, where it acts as a potent agonist. Here we have measured and characterized the binding of (+/-)(-)[3H]epibatidine to nicotinic receptors in rat brain. In rat forebrain homogenates, (+/-)(-)[3H]epibatidine binds to two sites, with apparent affinities of 15 pM and 360 pM. Both of these binding sites have pharmacological profiles consistent with neuronal nicotinic receptors and a similar brain regional distribution. (+/-)(-)[3H]Epibatidine also binds to sites in rat adrenal gland, suggesting that it can label a subtype of nicotinic receptor found in peripheral ganglia as well as the subtype that predominates in brain. In human cerebral cortex as well, (+/-)(-)[3H]epibatidine binds two sites, one of which appears to have an affinity of < 1 pM. We conclude that (+/-)(-)[3H]epibatidine should be a very useful new tool for characterizing the properties and regulation of neuronal nicotinic receptors, including those not easily measurable with other radioligands.