Social isolation is a stress factor increasing animal anxiety and impairing food-reinforced instrumental learning. Social isolation modulates sensitivity to psychoactive substances: it potentiated the depressive and analgesic effects of morphine, but attenuated the activating and anxiogenic effects of caffeine. These behavioral changes and changes in sensitivity to psychoactive agents can be explained by a well-known phenomenon of activation of the endogenous opioid system during stress.
Effect of one-day and chronic (3 week) drinking of 0.1% caffeine as a single source of fluid on anxiety in rats with different anxiety level (either genetically determined or individually acquired) was studied. When housed in groups, Fisher-344 rats had a significantly higher anxiety level than WAG/G rats. When the rats were housed individually, their anxiety gradually increased. The effect of short-term and chronic intake of caffeine in high doses depended on both genetically determined sensitivity to this agent and environmental factors (social isolation). It was concluded that chronic caffeine drinking does not induce persistent emotional changes similar to drug abuse, because caffeine withdrawal produced little effect on animal anxiety.
The effect of circulation arrest on the development of stress-induced injuries was studied in Krushinsky-Molodkina rats genetically predisposed to audiogenic seizure. Resuscitated rats were subjected to acoustic stress 1.5 month after circulation arrest. The severity of neurological disorders and the frequency and severity of intracranial hemorrhages increased, while excitability of the central nervous system remained unchanged during stress. Thus, the resistance to stress considerably decreased in rats survived a short-term circulatory arrest due to dysfunction of the autonomic nervous regulation of hemodynamics rather than enhanced excitability of the central nervous system.
We studied the effects of substance P on intravenous self-administration of morphine in WAG/G and Fischer-344 rats. By the end of week 2 the daily amount of self-administered morphine in WAG/G rats was higher than in Fischer-344 rats. Treatment with substance P markedly suppressed self-injection of morphine, particularly in low doses. The most pronounced effects were observed in Fischer-344 rats. Substance P did not change food-procuring behavior of animals in the same experimental chambers. Since the content of substance P in the hippocampus, hypothalamus, and midbrain of Fischer-344 rats is much lower than in WAG/G rats, and morphine addiction in Fischer-344 rats is less pronounced than in WAG/G rats, the degree of opiate addiction is not determined by the content of substance P in rat brain. However, in our experiments treatment with substance P abolished morphine addiction, particularly in animals with low content of this compound in the brain.
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