Differential plasma corticosterone responses to hippocampal stimulation

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Abstract
The effect of limbic forebrain stimulation on pituitary-adrenal function was assessed by evaluating plasma corticosterone obtained prior to and following sham or electrical stimulation of urethane (1.20 g/kg) anesthetized female rats. Cortical electroencephalogram (EEG), electrocardiogram (ECG), heart rate (HR), mean arterial blood pressure (MAP), and respiration were routinely monitored. Timed blood samples (0.25 ml) were obtained from a catherized femoral artery. The HR (Bts/min), MAP (mm of Hg), and corticosterone levels (μg/dl) for 7 non-stimulated rats averaged over 6 sampling periods were 385±19, 95±6, and 70.3±5.8 respectively. In electrically or sham stimulated rats, blood samples were taken just prior to stimulation (biphasic square waves, 100 μA, 50 or 60 Hz, 1 ms, 1 s on/1 s off for 15 or 30 min) and 5, 10, 15, and 30 min after initiation of stimulation. Significant changes in plasma corticosterone levels were obtained following stimulation of hippocampal and amygdaloid areas. In contrast, no change in corticosterone concentration was observed following stimulation of cortex, corpus callosum, fornix and a variety of other CNS areas. Detailed analysis of hippocampal influence on urethane stimulated plasma corticosterone levels showed increased plasma corticosterone levels following stimulation of CA1. In contrast, stimulation of CA3, dentate (includes CA4) and the subiculum produced significant decreases in plasma corticosterone levels. No change in corticosterone levels was observed following sham stimulation. Collectively, these data indicate that consideration must be given to the possibility that differential neuroendocrine regulatory mechanisms reside within various limbic forebrain complexes and that electrical stimulation of limbic forebrain sites of urethane anesthetized rats may provide information regarding sites inhibitory to pituitary-adrenal activity.