Changes in acetylcholine content, release and muscarinic receptors in rat hippocampus under cold stress

The effects of stress on muscarinic receptors. Heterologous receptor regulation: yes or no?

1 Stress is usually comprehended as an event affecting mainly the catecholaminergic system, the hypothalamo-pituitary-adrenocortical (HPA) axis and the receptor systems connected to these neurotransmitters/hormones. Other neurotransmitter/hormone systems can be affected too. Here we review the available data on the effects of different stressful stimuli (physical, chemical, psychological/social, cardiovascular, affecting multiple system) on muscarinic receptors (MR). 2 The data suppose the existence of specific mechanisms that regulate the signalization through MR during different type of stress. 3 Physical stressors (cold vs. heat) reveal opposite type of changes on peripheral-tissue MRs. Chemical stressors (oxidative stress) are tightly connected with MR and it is especially interesting that the sensitivity of MR to oxidative stress is subtype-specific. It is also suggested that heterologous regulation can occur with psychological/social stressors on the organism. Cardiovascular system-disturbing stressors cause imbalance between autonomic receptors or down-regulate MR in the peripheral tissue. Immobilization caused opposite effects on MR in the central nervous system and periphery, where the changes are supposed to be due to heterologous regulation between receptor systems. 4 In conclusion, some data indicate that in specific conditions MR are regulated as a consequence of other changes rather than as a primary effect of stress. On the contrary, in some situations, MR are the first targets to respond to the stress. 5 These findings on stress-induced activity of the cholinergic system and changes in muscarinic receptors support the view that stress is a specific response of the organism.

Exposure to an elevated platform increases plasma corticosterone and hippocampal acetylcholine in the rat: reversal by chlordiazepoxide

There is evidence that the septohippocampal cholinergic system is activated in response to stressful stimuli. In addition, prior studies indicate that stimulating the hippocampal cholinergic neurotransmission increases open arm exploration in the elevated plus-maze. This raises the possibility that exposing the rat to an elevated platform, which would be similar to confining the animal to the open arms of the plus-maze, would alter hippocampal acetylcholine levels. Results from the present study suggest that an elevated platform can be used as an animal model of stress in that exposure to the platform significantly increased plasma corticosterone levels. Importantly, exposure to a platform significantly increased hippocampal acetylcholine efflux. Interestingly, the increase in plasma corticosterone and hippocampal acetylcholine levels upon exposure to an elevated platform could be prevented by chlordiazepoxide at a dose that had no effect on basal hippocampal acetylcholine or plasma corticosterone levels. However, the elevated platform-induced increase in hippocampal acetylcholine could not be blocked by prior administration of buspirone. These results provide direct evidence for the importance of the hippocampal cholinergic system in stress and provide validation for the elevated platform as a model of stress.

In Vivo Hippocampal Acetylcholine Release During Exposure to Acute Stress

Hippocampal extracellular acetylcholine (ACh) and choline levels were evaluated using in vivo microdialysis in male Fischer 344 rats before, during, and following an 80-min exposure to two different stress conditions. Measurements were taken in rats restrained and immersed in a water bath containing either 37 degreesC (normothermic-restraint) or 20 degreesC (cold-restraint) water. Results were compared to normothermic-freely-moving rats. Cold-restrained rats displayed decreased ACh levels during cold exposure relative to both normothermic-restrained and normothermic-freely-moving rats. By the end of the cold exposure period and following removal from cold, ACh levels had returned to near-baseline values. Normothermic-restrained rats had levels similar to those of normothermic-freely-moving rats, except for a marked increase in ACh following removal from restraint. Cold-restrained rats displayed a gradual elevation in choline levels during cold stress, followed by a gradual decline after stress termination, whereas both normothermic-restrained and normothermic-freely-moving rats displayed gradual decreases during the microdialysis session. These findings demonstrate that central cholinergic neurotransmission can be altered by the application of, and removal from, acute stressors. In addition, the results suggest a possible relationship between the magnitudes of both the stressor and its cholinergic consequences.

Interferon-alpha and transforming growth factor-beta 1 regulate corticotropin-releasing factor release from the amygdala: comparison with the hypothalamic response

Interferon-alpha (IFN-alpha) and transforming growth factor-beta 1 (TGF-beta 1) have been reported in different brain regions. The amygdala contains high levels of corticotropin releasing factor (CRF) and has been implicated as a central site for its stress-related autonomic and behavioral response. IFN-alpha will release arginine vasopressin (AVP) from both amygdala and hypothalamus, which further supports a role for the amygdala in neuroimmune interactions. In the present study, we compared the effects of these cytokines on the in vitro release of CRF from the amygdala and hypothalamus. In addition, we evaluated the possible involvement of guanylate cyclase-mediated signaling in CRF release. IFN-alpha stimulates CRF release from both amygdala and hypothalamus. The CRF release by IFN-alpha, Interleukin-2 (IL-2) and acetylcholine is blocked by guanylate cyclase inhibitors, indicating a role for cGMP accumulation in this CRF release. TGF-beta 1 had no effect on basal release of CRF, nor on the CRF-release induced by IL-2, but selectively blocked the acetylcholine-induced release in both amygdala and hypothalamus. Taken with a previous report that TGF-beta 1 specifically inhibits AVP release by acetylcholine, these results suggest that TGF-beta 1 may modulate HPA axis activation, by antagonizing (acetylcholine-evoked) CRF and AVP release. These data further support a role for the amygdala in the bidirectional communication between neuroendocrine and immune system.

Inescapable stress enhances extracellular acetylcholine in the rat hippocampus and prefrontal cortex but not the nucleus accumbens or amygdala

A number of experimental results has pointed to a cholinergic involvement in the stress response. Recently, analytical techniques have become available to measure acetylcholine release in vivo during exposure to various stressors. In these experiments, microdialysis was used to monitor acetylcholine output every 15 min in the dorsal hippocampus, amygdala, nucleus accumbens and prefrontal cortex before, during and after 1 h of restraint, including a 15-min session of intermittent tail-shock (1/min, 1 mA, 1-s duration) in rats. In response to the stressful event, acetylcholine release was significantly increased in the prefrontal cortex (186%; p < 0.01) and hippocampus (168%; P < 0.01) but not in the amygdala or nucleus accumbens. The sole effects observed in the amygdala and nucleus accumbens occurred upon release from the restrainer, at which point acetylcholine levels were significantly elevated in both areas (amygdala: 150%; P < 0.05; nucleus accumbens: 13%; P < 0.05). An enhanced acetylcholine release was also evident during this sample period in the hippocampus and prefrontal cortex. These data demonstrate an enhancement of cholinergic activity in response to stress in two acetylcholine projection systems (hippocampus and prefrontal cortex) but not in the intrinsic acetylcholine system of the nucleus accumbens or the extrinsic innervation of the amygdala. Moreover, the data showed that relief from stress was accompanied by a more ubiquitous acetylcholine response that extended to each site tested.

Medial septal injection of naloxone elevates acetylcholine release in the hippocampus and induces behavioral seizures in rats

The effects of injections of naloxone, a universal opioid receptor antagonist, into the medial septal nucleus on hippocampal acetylcholine (ACh) release and behavior were investigated in freely moving rats by means of the microdialysis method. The injection of naloxone (2, 10 and 20 micrograms) produced a marked increase in hippocampal ACh release in a dose-dependent manner. These effects of naloxone were reversed by the post-injection of [D-Ala2, N-Me-Phe4, Gly-ol]-enkephalin (DAGO; 10 micrograms), an opioid mu receptor agonist. Furthermore, basal release of hippocampal ACh was significantly reduced by the injection of DAGO alone. It was also found that rats given an injection of naloxone showed an increase in motor activity and occasionally exhibited behavioral seizures. These effects of naloxone were also reversed by the post-injection of DAGO. The present results suggest that endogenous opioids ionically inhibit the activity of septo-hippocampal cholinergic neurons via mediation of mu opioid receptors in the medial septal nucleus. They also suggest that endogenous opioids modulate the incidence of seizures, at least in part, through opioid mu receptors in the medial septal nucleus.

Effects of chronic mild stress on serum complement activity, saccharin preference, and corticosterone levels in Flinders lines of rats

Complement proteins and fragments participate in the induction and modulation of specific and nonspecific immune reactions. We have examined the effect of 4 weeks of chronic mild stress (CMS) on complement sheep red blood cell hemolytic activity measured in CH50 units in two selectively bred lines of rats, the Flinders resistant line (FRL) and the Flinders sensitive line (FSL), that differ in cholinergic sensitivity and behavioral characteristics. Additionally, CMS-induced hedonic deficit (decreased preference for 0.02% saccharin over water) and serum corticosterone levels were compared in FRL and FSL rats. CMS caused a significantly (p < 0.01) greater decline in CH50 responses in FSL (-15%) than in FRL (-7%) rats. This was accompanied by a significant (p < 0.01) suppression of saccharin preference over a 24 h period in both FRL and FSL rats. Both lines showed a similar, more than 2-fold (p < 0.01) increase in corticosterone levels following CMS. These results further confirm that CMS induces a depressive-like state in rats as well as the validity of the FSL rat as a genetic model of depression. They also indicate that the effect of stress on the immune system can be monitored by measuring the complement CH50 response.

Chronic corticosterone administration modulates nicotine sensitivity and brain nicotinic receptor binding in C3H mice

Glucocorticoid regulation of nicotine sensitivity was investigated in adrenalectomized (ADX) and sham-operated C3H mice administered chronic corticosterone (CCS) replacement therapy. Hormone pellets (60% CCS or pure cholesterol) were implanted at the time of surgery and animals were tested for nicotine sensitivity in a battery of behavioral and physiological tests. ADX-induced increases in nicotine sensitivity were reversed by chronic CCS replacement. Sham-operated animals that received CCS supplementation were subsensitive to the effects of nicotine. In both ADX and sham-operated animals, chronic CCS administration induced a decrease in the number of CNS nicotinic cholinergic receptors labeled by alpha-[125I]-bungarotoxin. Binding was decreased by 30-60% depending on brain region; no changes in affinity (KD) were detected. The number of brain nicotinic sites labeled by [3H]-nicotine was unaltered following 1 week of chronic CCS administration. These data support the hypothesis that glucocorticoids modulate nicotine sensitivity in the C3H mouse. In animals chronically treated with CCS, nicotine tolerance may be due to CCS-induced changes in nicotinic cholinergic receptor binding or the presence of high CCS titers at the time of testing.