Response of brain and pituitary pools of dynorphin as compared to vasopressin to acute stress in the rat

Nicotinic Cholinergic System in the Hypothalamus Modulates the Activity of the Hypothalamic Neuropeptides During the Stress Response

BACKGROUND

The hypothalamus harbors high levels of cholinergic neurons and axon terminals. Nicotinic acetylcholine receptors, which play an important role in cholinergic neurotransmission, are expressed abundantly in the hypothalamus. Accumulating evidence reveals a regulatory role for nicotine in the regulation of the stress responses. The present review will discuss the hypothalamic neuropeptides and their interaction with the nicotinic cholinergic system. The anatomical distribution of the cholinergic neurons, axon terminals and nicotinic receptors in discrete hypothalamic nuclei will be described. The effect of nicotinic cholinergic neurotransmission and nicotine exposure on hypothalamic-pituitaryadrenal (HPA) axis regulation at the hypothalamic level will be analyzed in view of the different neuropeptides involved.

METHODS

Published research related to nicotinic cholinergic regulation of the HPA axis activity at the hypothalamic level is reviewed.

RESULTS

The nicotinic cholinergic system is one of the major modulators of the HPA axis activity. There is substantial evidence supporting the regulation of hypothalamic neuropeptides by nicotinic acetylcholine receptors. However, most of the studies showing the nicotinic regulation of hypothalamic neuropeptides have employed systemic administration of nicotine. Additionally, we know little about the nicotinic receptor distribution on neuropeptide-synthesizing neurons in the hypothalamus and the physiological responses they trigger in these neurons.

CONCLUSION

Disturbed functioning of the HPA axis and hypothalamic neuropeptides results in pathologies such as depression, anxiety disorders and obesity, which are common and significant health problems. A better understanding of the nicotinic regulation of hypothalamic neuropeptides will aid in drug development and provide means to cope with these diseases. Considering that nicotine is also an abused substance, a better understanding of the role of the nicotinic cholinergic system on the HPA axis will aid in developing improved therapeutic strategies for smoking cessation.

Differential effects of emotional and physical stress on the central and peripheral secretion of neurohypophysial hormones in male rats

The effects of various stressful conditions on the levels of oxytocin (OT) and vasopressin (VP) in plasma and cisternal cerebrospinal fluid (CSF) of male rats were investigated. Three experimental models were used: exposure to a novel environment for 5 min, immobilization for 15 min, and ether inhalation for 10 min resulting in anaesthesia. Novelty and immobilization induced a slight but significant increase in OT levels in the CSF immediately after the stress. The effect of ether was considerably more pronounced. The concentration of VP in the CSF was elevated only by ether stress. In plasma, the level of OT was increased immediately following immobilization and ether stress but not after novelty stress, whereas VP only showed a delayed response 20 min after immobilization. These results indicate a rapid preferential release of OT in the periphery in response to physical and pharmacological stress. In addition, they provide evidence that release of OT into the CSF is triggered by physical, pharmacological as well as emotional stress, while the central release of VP is rather resistant to emotional stress. The data suggest that OT is a stress hormone in the central nervous system.

Presence of arginine-vasopressin in bovine pituitary intraglandular colloid of intermediate lobe origin

In order to determine whether pituitary intraglandular colloid might contain enough hormonal material to be considered a transport medium for intermediate lobe peptides, arginine-vasopressin was assayed in three pools of colloid collected from nearly 100 steers. Colloid samples taken from the pituitary glands of freshly slaughtered cattle were pooled, freed of extraneous tissue, and lyophilized. Three such pools were further extracted on octadecylsilyl-silica and assayed. The radioimmunoassay showed that arginine-vasopressin was indeed present in the bovine intraglandular colloid at levels ranging from 0.18 to 6.95 ng/mg dry weight. These results indicate that the intraglandular colloid contains a sufficient amount of arginine-vasopressin to be considered as a transport medium for this and other intermediate lobe peptides.

Electric footshocks differentially affect plasma and spinal cord vasopressin and oxytocin levels

Rats exposed for three minutes to repeated electric footshocks showed an approximate 10-fold increase of basal plasma vasopressin (AVP) and oxytocin (OXT) levels. In contrast, spinal AVP and OXT contents measured in the same rats remained unchanged when compared to undisturbed controls. This observation suggests that spinal AVP and OXT do not play a major role in the short-term adaptation of the organism to stress.

Chronic arthritis in the rat: differential changes in discrete brain pools of vasopressin as compared to oxytocin

Rats inoculated in the tail-base with killed Mycobacterium butyricum developed an arthritic swelling and inflammation of the limbs accompanied by a hyperalgesia to noxious pressure applied thereto. These changes were maximal at 3 weeks and had subsided by 10 weeks post-inoculation. At 3 weeks, arthritic rats manifested an elevation in levels of immunoreactive (ir)-vasopressin (VP) but not ir-oxytocin (OT) in the midbrain. In contrast, ir-OT was increased in the medulla-pons while ir-VP was unaltered therein. These changes had disappeared by 10 weeks. No other brain region displayed changes. Thus, chronic arthritis is associated with selective and reversible effects upon discrete brain pools of ir-VP and ir-OT. The data clearly demonstrate that pools of ir-VP and ir-OT can be modulated independently of each other in particular brain tissues. Whether the changes are produced by, or reflect a functional response to, the pain rather than other characteristic of the arthritis, remains to be determined.

Chronic pain in the rat: selective alterations in CNS and pituitary pools of dynorphin as compared to vasopressin

Inoculation of rats at the tail-base with Mycobacterium led to arthritic swelling and inflammation of all four limbs. Immunoreactive (ir)-dynorphin (DYN) increased in anterior but not neurointermediate pituitary. In the brain, only thalamus showed a rise and, in spinal cord, a large elevation was seen. Ir-vasopressin (VP) was not affected in these tissues but increased in midbrain. These effects might reflect a role of DYN in the control of chronic pain. In addition, they support a differential modulation of DYN as compared to VP extrinsic to the hypothalamic-neurohypophyseal axis.

Kappa-receptor antagonist reverse 'non-opioid' stress-induced analgesia

We have evaluated the effects of antagonists to the mu, kappa and opiate receptors on 'opioid' and 'non-opioid mediated' analgesia. Our findings indicate that mu receptors might mediate 'opioid' and kappa receptors 'non-opioid' analgesia. It is suggested that endogenous opiates that act on mu or kappa receptors could be responsible for the different types of stress-induced analgesia.