Effects of adrenal steroids on brain function and behavior

Habituation of the prolactin response in rats to psychological stress

It is well known that stress is a stimulant for prolactin release. However, relatively few studies have investigated the role of psychological factors in prolactin secretion, and investigators have typically used one-time exposure and a single collection period in their studies. In our studies, attempts were made to carefully characterize the prolactin response to different psychological stressors by serially sampling blood from an indwelling cannula and to determine if repeated exposure to the stressor leads to habituation of the prolactin response. Exposure of the male rats to different novel situations such as being placed in a new cage, being placed on a platform in water, or being handled resulted in increased prolactin levels. As the rats habituated behaviorally to repeated exposure to similar situations, the prolactin response also attenuated. These findings show that psychological factors do play a role in influencing prolactin secretion and are consistent with the idea that as the psychological stress imposed by a stimulus becomes habituated, the prolactin response to that stimulus also becomes habituated.

Effect of corticosteroids in the hippocampus on passive avoidance behavior in the rat

The site of action of corticosteroids in avoidance learning was investigated in 110 rats. Injection of cycloheximide, 30 min before one-trial training on a passive avoidance task suppressed corticosteroid secretion in response to footshock, and produced an avoidance deficit in a test 6 days later. However, an additional injection of hydrocortisone, either subcutaneously or intra-hippocampally within 5 min of training, restored the avoidance response in the test. Septal and hypothalamic injections of the hormone were ineffective in reversing the cycloheximide effect, whereas the effect of hormone injection into the amygdala was equivocal because of an increased level of activity. Corticosteroids secreted following an aversive experience appear to act upon the steroid-sensitive neurons in the hippocampus to influence the animal's later performance of passive avoidance response.

Effect of thyrotropin-releasing hormone (TRH) and antidepressant agents on brain stem and hypothalamic multiple unit activity in the cat

The EEG and MUA (multiple unit activity) of mesencephalic reticular formation (MRF), area hypothalami posterior (PH), and area hypothalami anterior (AH) were studied in chronically implanted freely moving cats. The effects of thyrotropin-releasing hormone (TRH) and some antidepressant agents were tested on neuronal activity. Desipramine and imipramine resulted in a dose-dependent decline of MUA of all structures with the most significant decrease of activity in PH. A single injection of TRH resulted in slight or moderate gross behavioral changes and vegetative excitation lasting for 30-50 min with variable MUA levels. In the course of repetitive TRH treatment on consecutive days the gross behavioral changes and the vegetative symptoms failed to develop by the 3rd or 4th day. By that time the MUA changes of PH and MRF showed similar characteristics in response to TRH administration which was observed following the injection of desipramine and imipramine. The drugs, except for TRH, induced a suppression of paradoxical sleep cycles.

Multiple unit activity study of brain stem and limbic structures during environmental habituation and circadian rhythm

Multiple unit activity (MUA) of brain stem, hypothalamic and limbic structures was studied during habituation to a novel environment and circadian rhythm in chronically implanted freely moving rats. MUA was analysed in the mesencephalic reticular formation (MRF), area hypothalami posterior (PH), basal nuclear group of amygdala (AMY), area septalis (SEPT), dorsal hippocampus (HIPP) and area hypothalami anterior (AH). It was found that in the novel environment MUA of all subcortical structures increased to a high level. During habituation MUA in each phase of wakefulness--sleep cycle decreased to stable low level both in brain stem and forebrain structures. Gradual decrease in MUA was characteristic to MRF, and a sharp decrease occured in AH and AMY. The environmental habituation proved to be a long lasting process in rat. During all phases of wakefulness--sleep cycle activity was significantly higher in the light period than in the dark, and MUA base level showed circadian variation both in brainstem and limbic structures. Close correlation was found between the actual MUA level and responsiveness to various sensory modalities both during habituation and circadian rhythm. The higher the MUA level, the higher the responsiveness, and a fall in activity was accompanied by decreased neuronal responsiveness.