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A model system consisting of two rat strains bred for nervous system excitability in response to electric shocks was used to study changes in the number density of neurons in hippocampal field CA3 at 24 h, two weeks, and two and six months after prolonged emotional-pain stress (PEPS). Neuron density in hippocampal field CA3 decreased after completion of PEPS. These changes arose at different time points in the different rat strains (one day for low-excitability rats, two months for high-excitability rats) and persisted to six months. Thus, this is the first demonstration that persistent differential effects of stress on the number density of neurons in hippocampal field CA3, which plays an important role in learning and memory processes, depend on genetically determined constitutive characteristics of the nervous system.

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An enhancement of stress reactivity of the hypophysial-adrenocortical system in response to emotional and physical influence was shown in rats with a low threshold of sensitivity to electrical current. This phenomenon was observed as a rise in the maximum level of blood corticosterone and acceleration of stressor hormonal response. In the high-threshold rats a decrease in sensitivity of the hypophysial adrenocortical system to the feedback signals was observed. Key Words: hypophysial adrenocortical system; stress reactivity; corticosterone; line ratsAnimal experiments confirm that basic properties of the nervous system are determined genetically [6]. Selection according to the excitability threshold of the nerve-muscle apparatus [2] show that not only behavioral characteristics, but also reactivity to stress and ability to adapt to the environment depend on the excitability of the nervous system [1,9,10]. These findings support the hypothesis that the divergence of these properties is determined predominantly by hypophysial-adrenocortical system (HACS). There are data indicating that neuron excitability depends on corticosteroids [13]. We studied functional state of HACS in rats, in which not only the peripheral subdivision of nervous system [2], but also its central structures, the midbrain reticular formation in particular [5], had different threshold of excitability [2]. MATERIALS AND METHODSThe study was carried out on 120 male Wistar rats weighing 350+_.25 g, which were genetieaUy selected according to the threshold of sensitivity of the tibial I.P. Pavlov Institute of Physiology, Russian Academy of Sciences, St. Petersburg nerve to electrical current. Selection was performed from two independent autobred populations: the high sensitivity LT-1 and LT-2 rats (low threshold: first selection program --34th generation and the second selection program --24th generation) and low sensitivity HTol and HT-2 rats (high threshold: the first and second selection programs of the same generations). The rats were grown and maintained on the standard diet at the Laboratory of Genetics of Highest Nervous System (I.P. Pavlov Institute of Physiology).Basal state of HACS was evaluated by plasma content of cortieosterone (CS) at rest. Blood samples were taken from the caudal vein at 4 p.m. Stress response of HACS was estimated by the maximum rise in blood CS. Several types of stressor stimulation were used. Emotional stress caused by new environment was estimated by measuring blood CS after a 30 rain vein (I series). In the 2nd series, the rats were stimulated transeutaneously with electrical current (0.5 rnA, 50 Hz, maximum duration 15 see) which was applied 15 times to the floor of a 20x20x 13 em chamber. Blood was taken from the caudal vein 1, 3, and 24 h after stimulation.The sensitivity of HACS to feedback signals was determined in a 2-day dexamethasone test [4].Two