Michael L. Thompson

Rapid increase in brain benzodiazepine receptor binding following defeat stress in mice

Defeat stress in mice, a model of social stress, increases benzodiazepine receptor binding as measured by specific [3H]Ro15-1788 binding in vivo, but not by [3H]flunitrazepam binding in vitro. This increase occurs rapidly, by 20 min following exposure to stress, and resolves by 60 min. Increased benzodiazepine receptor binding is observed in the cerebral cortex, cerebellum and hypothalamus, and appears to be due to an increase in receptor number rather than apparent affinity. The stress-induced increase in central benzodiazepine receptors is decreased in a dose-dependent fashion by lorazepam, a benzodiazepine agonist, but not by the receptor antagonist Ro15-1788. The stress-induced increase in benzodiazepine receptors is also blocked by adrenalectomy and is restored by corticosterone replacement.

Cocaine-induced hepatic necrosis in mice—The role of cocaine metabolism

Abstract Liver damage following cocaine injection in mice is due to the action of a metabolite of cocaine rather than of cocaine itself. The bioactivation of cocaine to a toxic metabolite appears to be a multi-step process, and is carried out by the cytochrome P-450 microsomal mixed function oxidase system. Inhibitors and inducers of this system blocked or potentiated liver damage respectively. Norcocaine was found to be more potent than cocaine, but also required further metabolism for hepatotoxicity. Inhibition of esterase activity increased damage from both cocaine and norcocainee. Most metabolites and analogues of cocaine were not hepatotoxic, indicating fairly strict structure requirements for activation. N -Hydroxynorcocaine, a possible metabolite of norcocaine, was also hepatotoxic. However, it too required further metabolism in order to produce liver damage. Glutathione in the liver was depleted after cocaine or norcocaine by 25–30 per cent at 1 hr after injection. Cysteine pretreatment offered protection from cocaine. These results suggest that an active metabolite of N -hydroxynorcocaine may be responsible for the liver damage observed after injection of cocaine into mice.

Liver damage from cocaine in mice.

Abstract Four daily injections of 20 mg per kg cocaine hydrochloride into B6AF1/J mice produced focal necrosis of liver parenchymal cells in the midzonal region. Massive liver damage and marked elevation of serum glutamic-oxaloacetic transaminase was observed after a single injection of 50 mg per kg cocaine-HC1. This damage led to increased sleep time after pentobarbital, and a decreased rate of pentobarbital metabolism in vivo .

Effect of early exposure to δ-9-tetrahydrocannabinol on the levels of opioid peptides, gonadotropin-releasing hormone and substance P in the adult male rat brain

The effects of neonatal exposure to delta-9-tetrahydrocannabinol (THC) on the adult animal brain neurochemistry and pain perception were evaluated. Newborn rat pups were culled to a litter size of 8 (males and females) and treated either with THC (2 mg/kg) or oil (control) daily, during days 1-4 after birth. After weaning, the THC-treated males were housed 4 per cage. During the juvenile period (day 50), the THC-treated animals exhibited significantly lower baseline tail-flick values (a measure of pain perception) than the control. However, as adults, the THC-treated animals exhibited significantly higher sensitivity to pain following 5 mg/kg morphine challenge. Furthermore, the THC-treated animals had significantly elevated beta-endorphin and methionine-enkephalin levels in almost all the brain areas sampled for the study. In addition, the neonatally THC-treated rats exhibited significantly higher levels of substance P (SP) and significantly lower levels of gonadotropin releasing hormone (GnRH) in the anterior hypothalamus-preoptic area. The SP and GnRH levels did not differ among the THC-treated and control animals in the medial basal hypothalamus. The results of this study indicate that even a very low dose of THC administered during the neonatal period has a long-lasting effect on the brain neurochemistry. In particular, neonatal administration of THC appears to alter functioning of the endogenous opioid system.

Innate and adaptive immune responses in a social conflict paradigm

Social conflict stress was examined for its effects on in vitro and in vivo immunity in mice. Adaptive immunity, as measured by the generation of primary IgM antibody responses to the T-dependent antigen keyhold limpet hemocyanin, was suppressed following chronic (greater than 1 day), but not acute (less than 1 day), stress periods while the IgM response to the T-independent antigen polyvinylpyrrolidone was not affected. In vitro proliferative responses of splenocytes to the T cell mitogen concanavalin A and the B cell mitogen lipopolysaccharide were unaffected. Acute (less than 1 day) stress dramatically increased innate immunity as measured by a luminol-dependent chemiluminescence assay of phagocytic cell function. DBA/2J mice averaged a 269% increase in phagocytosis as compared to a 412% increase in C57BL/6J. This differential effect of stress on immune responsiveness indicates that alterations in innate immunity in addition to adaptive immunity should also be considered when evaluating neuroendocrine and immune interactions in response to stress.

Naloxone injections into the periaqueductal grey area and arcuate nucleus block analgesia in defeated mice.

In a situation of social conflict, mice that are defeated by an opponent exhibit a marked analgesia. Microinjections of naloxone (1 or 10 μg) into the periaqueductal grey area (PAG) or into the region of the arcuate nucleus prior to the defeat prevented the emergence of analgesia. Microinjections of morphine (5 μg) into these sites had previously been shown to produce profound analgesia. Mice whose adrenals were removed rapidly developed analgesia when attacked by a stimulus animal. Injection of naloxone into PAG also antagonized defeat-induced analgesia in adrenalectomized mice. These observations indicate that sites and processes in the brain rather than in the periphery are responsible for the development of analgesia in mice that are subjected to social defeat.

Analgesia Following Defeat in an Aggressive Encounter: Development of Tolerance and Changes in Opioid Receptors

Stress-induced analgesia may actually by a misnomer for an array of phenomena that are relevant to the perception of and the response to pain. Neither the physical properties of a stressor nor the acute response of the pituitary-adrenal axis to this stimulus predict clearly whether or not the organism is rendered analgesic. Instead, the biological significance of the stimulus situation and the adaptive reaction to it appear to be of paramount importance for endogenous pain modulation. One such situation is that of an aggressive confrontation between a resident animal and an intruder. Both opponents display an intricate sequence of behavioral and physiological adaptations to the conflict; the intruder ultimately is defeated and fails to respond to painful stimuli. We have studied analgesia in mice that have been defeated by an opponent, and have explored several behavioral and physiological processes that characterize this phenomenon. Our evidence suggests that the analgesia in defeated mice is mediated by brain opioid peptides. Of particular interest are the observations suggesting longterm adaptations to the analgesia-causing events, a process that is closely similar to opiate tolerance. Stress-induced analgesia or the antinociceptive effects of stress may be produced by exposure to various physical and chemical stimuli, conditioned or

Sex and strain differences in response to cocaine

After pretreatment with phenobarbital, female B6AF1 mice showed considerably higher serum glutamic oxaloacetic transaminase (SGOT) elevations and more periportal necrosis from a single injection of cocaine than males. This sex difference was androgen dependent. Castration or treatment with flutamide made males respond like females, while testosterone made females behave like males. There was no significant sex difference in enzymes of cocaine metabolism. When the mice were induced by exposure to pine bedding, males showed higher SGOT elevations and more centrilobular necrosis after cocaine than females. In this case, the sex difference could be attributed to increased levels of cytochrome P-450 and cocaine N-demethylase in liver microsomes. BALB/cBy mice on pine bedding showed much less liver damage from cocaine than B6AF1 mice, but they were more sensitive to norcocaine and N-hydroxynorcocaine. This difference was correlated with low levels of cocaine N-demethylase in the BALB/cBy mice. Liver microsomes from phenobarbital-induced BALB/cBy mice had less norcocaine N-hydroxylase activity than those from B6AF1 mice. These studies demonstrate that the pattern of sex and strain differences in liver damage from cocaine depends on the inducing agent and can be related to a large extent to the microsomal enzymes induced by that agent.

Modulation of benzodiazepine receptor binding in mouse brain by adrenalectomy and steroid replacement.

Adrenal steroids alter neuronal excitability in the central nervous system (CNS), and evidence from in vitro studies indicates that at least some of these effects are mediated by the GABAergic system. Benzodiazepine receptor binding, among other sites on the GABA complex, has been implicated in steroid-induced alterations in the CNS. To investigate the modulation of benzodiazepine receptor binding by adrenal steroids, we examined receptor binding determined by an in vivo technique in mice after adrenalectomy, hypophysectomy and after replacement with several naturally occurring and synthetic steroids. Benzodiazepine receptor binding was substantially augmented in cortex, hypothalamus, and hippocampus in mice 1 week after adrenalectomy, and these increases appeared to be due to increased receptor number rather than changes in apparent affinity. Similar results in cortex were found after hypophysectomy. Replacement with physiologic, but not lower doses, of corticosterone reversed the changes induced by adrenalectomy. Chronic treatment with deoxycorticosterone also returned binding to control values, but chronic administration with dexamethasone, aldosterone and dihydroprogesterone did not alter binding after adrenalectomy. Adrenalectomy did not alter non-specific binding or GABA concentrations in cortex, and delivery of radioligand did not appear to be affected. These results indicate that adrenal steroids modulate benzodiazepine receptor binding in vivo, perhaps via the CR subtype of corticosteroid receptors. The steroid-benzodiazepine interaction may be especially important in the stress response.

Liver damage from narcotics in mice

Abstract Single or multiple intraperitoneal injections of morphine, dihydromorphinone or methadone into mice produced fatty infiltration of the liver and increases of up to 10-fold in the level of serum glutamate-oxaloacetate transaminase (SGOT). Changes in SGOT were proportional to the dose of morphine, and reached a peak at 12–16 hr after injection. Hepatotoxicity was blocked by naloxone and was not seen after dextrorphan, or in animals that had previously been made tolerant to narcotics. Liver damage was more severe in mice kept on pine shavings than in animals kept on corncob bedding. Liver damage was partially blocked by chloramphenicol, but not by SKF-525A. Liver damage was also partially blocked by reserpine and propranolol. Sex and strain differences in susceptibility were observed. The injection of 50–100 μg of morphine sulfate or [ d -Ala2]-Met-enkephalinamide into the cerebral ventricles produced liver damage comparable to that produced by 50 mg/kg morphine sulfate ip. Both central and peripheral actions of narcotics may be involved in hepatotoxicity.