G. J. Kant

Diurnal Variation in Neuroendocrine Response to Stress in Rats: Plasma ACTH, β-Endorphin, β-LPH, Corticosterone, Prolactin and Pituitary Cyclic AMP Responses

The effects of restraint stress applied at different times of the day on levels of five stress-responsive plasma hormones (ACTH, β-endorphin, β-LPH, corticosterone and prolactin) and pituitary cyclic AMP levels were assessed. Different groups of rats were subjected to 15 min of restraint stress at 2-hour intervals over a 24-hour period. Rats were sacrificed immediately upon removal from their home cage (controls) or immediately following restraint (stressed). The time of day of stress exposure markedly affected the stress responses measured. Generally, responses to stress applied at the beginning of the dark cycle (18.00) were less than those seen following stress applied at the beginning of the light cycle (06.00). Stress at 06.00 increased levels of pituitary cyclic AMP 10-fold, while stress applied at 18.00 did not significantly increase pituitary cyclic AMP levels. In stressed rats, high correlations were seen among levels of hormones derived from the common precursor, proopiomelanocortin (ACTH, β-endorphin, β-LPH) and between these hormones and levels of pituitary cyclic AMP. These findings support the hypothesis that pituitary cyclic AMP is involved in the stress-induced release or synthesis of the pituitary hormones ACTH, β-endorphin, and β-LPH.

Effects of repeated stress on pituitary cyclic AMP, and plasma prolactin, corticosterone and growth hormone in male rats

The effects of five putative stressors (saline injection, cold exposure, forced running, immobilization, and footshock) on levels of pituitary cyclic AMP, plasma prolactin, corticosterone and growth hormone were examined. In naive rats exposed to 15 min of these stressors for the first time, running, immobilization and footshock increased levels of pituitary cyclic AMP, plasma corticosterone and prolactin and decreased growth hormone, typical of stress response in the rat. Cold exposure only increased corticosterone and saline injection did not affect any measured parameter. In rats chronically exposed to the same stressor (once a day for 15 min) for 10 days immediately prior to the experiment, an attenuated pituitary cyclic AMP and plasma prolactin response was seen upon application of 15 min of that stressor on the day of the experiment, compared to the responses observed in the naive rats.

Specific Hormonal and Neurochemical Responses to Different Stressors

The neuroendocrine and neurochemical responses of rats to 5 min of cold exposure versus 5 min of forced immobilization were determined and compared. We found that plasma hormones and brain neurochemical systems responded differently to the two different stressors. Plasma prolactin levels were elevated over 10-fold in the immolilized group, while rising only 2-fold in the cold stress group. Levels of corticosterone were significantly increased and growth hormone levels were decreased in both stressed groups as compared to controls. Levels of cyclic GMP were markedly elevated in 11 brain regions following cold exposure. Surprisingly, no elevation of cyclic GMP was found after forced immobilization. Cyclic AMP, norepinephrine, and dopamine levels throughout the 17 regions of brain examined showed no significant response to 5 min of either stressor. Lesions of the ventral medial tegmental area did not affect the cyclic GMP or neuroendocrine responses to cold stress. Lesion of the nucleus locus ceruleus did not affect the cyclic GMP response but significantly reduced growth hormone levels in the cold-stressed rats.

Paraventricular Lesions Abolish the Stress-Induced Rise in Pituitary Cyclic Adenosine Monophosphate and Attenuate the Increases in Plasma Levels of Proopiomelanocortin-Derived Peptides and Prolactin

Acute exposure to footshock stress increased the pituitary level of cyclic adenosine monophosphate (AMP) in vivo and sharply increased plasma levels of adrenocorticotropic hormone, beta-endorphin and beta-lipotrophic hormone, as well as prolactin. Seven days after bilateral lesions of the paraventricular nucleus of the hypothalamus, the pituitary cyclic AMP response to stress was totally eliminated and the increases in plasma levels of these pituitary hormones were blunted. We conclude that while the pituitary hormonal responses to stress might be mediated by several neurohumoral factors, the stress-induced increases in pituitary levels of cyclic AMP in vivo are mediated largely via corticotropin-releasing factor, released from neurons which project from the paraventricular nucleus to the median eminence.

ACTH, prolactin, corticosterone and pituitary cyclic AMP responses to repeated stress

The present experiment was conducted to determine whether the plasma hormonal and pituitary cyclic AMP responses observed following a single exposure to an acute stressor would diminish following reexposures to the same stressor. Fifteen-min stress exposures (forced running) were separated by 45-min recovery periods. Separate groups of control and stressed animals were sacrificed before and after each of four 15-min stress periods and after each recovery period. The first exposure to 15 min of forced running raised plasma ACTH, corticosterone and pituitary cyclic AMP levels approximately 6-fold and more than tripled levels of plasma prolactin. Plasma ACTH and pituitary cyclic AMP responses to the second, third and fourth stress exposures were very similar to the responses to the first stress exposure, and levels of these substances returned to prestress levels during each 45-min recovery period. Plasma prolactin responses to the four stress sessions were somewhat variable but no significant trend among the responses was seen. Plasma prolactin levels also returned to prestress levels between stress exposures. Corticosterone levels were similar following each of the four stress sessions but levels remained elevated compared to prestress levels between stress exposures. These data suggest that pituitary responses to acute stress are rapid, that return to prestress levels is also rapid, with the exception of corticosterone, and that repeated responses of the same magnitude may be evoked when stressors are separated by short recovery periods.

Regulation of Pituitary Cyclic AMP, Plasma Prolactin and POMC-Derived Peptide Responses to Stressful Conditions

We have been investigating the role of pituitary cyclic AMP in regulating the stress-induced release of pituitary hormones in vivo. This paper will review a series of experiments describing the effects of both physical and psychological Stressors on these responses and exploring possible mechanisms of regulation. Exposure to stressful conditions elevates plasma levels of corticosterone (CS) (1–3), and increases plasma levels of several hormones secreted from the anterior pituitary, including adrenocorticotrophic hormone (ACTH), beta-endorphin (β-EP), beta-lipotropin (β-LPH), and prolactin (PRL) (3–8). ACTH, β-LPH and β-EP are derived from a common precursor, a 31,000 dalton glycoprotein currently referred to as proopiomelanocortin (POMC) (9,10). β-LPH is a 91 amino acid peptide (11) which contains β-EP as the 61–91 segment (12). In the rat, the three POMC-derived peptides are co-localized in the same secretory granules in anterior pituitary corticotrophs (13,14), while the intermediate lobe contains and releases predominantly β-EP (15–18). Plasma levels of β-EP are high relative to β-LPH (8,19), but exposure to stressors markedly increases plasma levels of all three POMC-derived peptides (5,8,19).

Dexamethasone suppresses ACTH release without attenuating pituitary cyclic AMP response to stress in vivo

Dexamethasone, a synthetic glucocorticoid, has been shown to decrease basal and stress-elevated levels of the pituitary hormone ACTH. Glucocorticoids are known to bind to multiple sites within the brain and pituitary and it is not known which site(s) is most important in mediating the observed inhibition of ACTH release. At the level of the corticotroph, there is contradictory data from in vitro studies regarding whether dexamethasone acts proximal or distal to the formation of the cyclic AMP second messenger that has been shown to be involved in CRF-stimulated ACTH release. In the present report, we have examined the effects of dexamethasone pretreatment on stress-induced elevations in pituitary cyclic AMP and the release of ACTH in vivo. Acute stress (15 min of intermittent footshock) elevated levels of pituitary cyclic AMP and plasma ACTH consistent with previous studies. Dexamethasone administration (0.4 mg/kg 24 hr prior to sacrifice plus 0.2 mg/kg 2 hr prior to sacrifice) inhibited stress-induced elevations in plasma ACTH but did not affect pituitary cyclic AMP response to acute stress. These findings suggest that dexamethasone inhibits the release of ACTH via an action distal to the generation of cyclic AMP.

Effect of Cholinergic Drugs, Stress and Locomotor Activity on Cyclic Nucleotides in Brain and Pituitary

ABSTRACT It has been suggested that the cyclic nucleotides, cyclic adenosine 3′,5′-monophosphate (cyclic AMP) and cyclic guanosine 3′,5′-monophosphate (cyclic GMP) serve as second messengers, biochemically mediating the effects of neurotransmitters and hormones in a number of tissues, including brain and pituitary. Using high-intensity microwave irradiation to achieve simultaneous sacrifice and inactivation of brain enzymes in the rat, we have studied the effect of cholinergic agonists, stress, and locomotor activity on levels of cyclic AMP and cyclic GMP in brain regions and pituitary in vivo . Cholinergic agonists have previously been reported to increase cyclic GMP in brain tissue without increasing cyclic AMP. Upon careful examination of individual brain regions, however, we found that cyclic AMP was also increased in certain specific areas. Oxotremorine and nicotine each produced marked increases in cyclic AMP in the pituitary and the interpeduncular region, with oxotremorine producing increases in the hypothalamus and substantia nigra as well. These effects of oxotremorine and nicotine were attenuated with atropine or mecamylamine, respectively. Locomotor activity increased cyclic GMP in several hindbrain regions-especially the cerebellum. Oxotremorine produced a tremor and increases in cyclic GMP in the cerebellum and several other regions in a pattern not unlike that produced by locomotor activity alone. Nicotine decreased both locomotor activity and cerebellar cyclic GMP. The effects of oxotremorine and nicotine on motor activity and cyclic GMP were attenuated by pretreatment with atropine or mecamylamine, respectively. We suggest that many drug effects on brain cyclic GMP may be indirectly mediated via a secondary effect on motor activity. Oxotremorine consistently increases cyclic GMP in the septal region, however, and this effect is not seen following locomotor activity alone. Recently we have shown that stress markedly increases levels of cyclic AMP in the pituitary, without affecting levels of cyclic AMP in brain. Thus, stress could contribute to apparent drug-induced increases in pituitary cyclic AMP. Stress does not, however, increase cyclic AMP in the hypothalamus, interpeduncular region, or substantia nigra; hence, the effects of cholinergic agonists on cyclic AMP levels in these regions may be considered to be independent of any stress effects. Taken together, these data emphasize the importance of interpreting pharmacological and neurochemical data in a physiological context.

Contribution of Locomotor Activity to Change in Cerebellar Cyclic GMP Following Administration of Drugs

ABSTRACT Numerous drugs and stressors have been reported to affect cerebellar cyclic GMP. We have previously demonstrated that locomotor activity independent of stress increases cyclic GMP in cerebellum and other brain regions. In the present study, we examined the effects of immobilization on the increases in cerebellar cyclic GMP induced by cold exposure or apomorphine. We found that immobilization attenuated the elevations in cerebellar cyclic GMP after either cold exposure or injection of apomorphine. We suggest that locomotor activity may be an intervening variable in producing changes in cerebellar cyclic GMP attributed to drugs or other environmental stimuli.