The involvement of noradrenergic mechanisms in the suppressive effects of diazepam on the hypothalamic-pituitary-adrenal axis activity in female rats

Aim

To elucidate the involvement of noradrenergic system in the mechanism by which diazepam suppresses basal hypothalamic-pituitary-adrenal (HPA) axis activity.

Methods

Plasma corticosterone and adrenocorticotropic hormone (ACTH) levels were determined in female rats treated with diazepam alone, as well as with diazepam in combination with clonidine (α₂-adrenoreceptor agonist), yohimbine (α₂-adrenoreceptor antagonist), alpha-methyl-p-tyrosine (α-MPT, an inhibitor of catecholamine synthesis), or reserpine (a catecholamine depleting drug) and yohimbine.

Results

Diazepam administered in a dose of 2.0 mg/kg suppressed basal HPA axis activity, ie, decreased plasma corticosterone and ACTH levels. Pretreatment with clonidine or yohimbine failed to affect basal plasma corticosterone and ACTH concentrations, but abolished diazepam-induced inhibition of the HPA axis activity. Pretreatment with α-MPT, or with a combination of reserpine and yohimbine, increased plasma corticosterone and ACTH levels and prevented diazepam-induced inhibition of the HPA axis activity.

Conclusion

The results suggest that α₂-adrenoreceptors activity, as well as intact presynaptic noradrenergic function, are required for the suppressive effect of diazepam on the HPA axis activity.Benzodiazepines are used for their anxiolytic, sedative-hypnotic, muscle relaxant, and anticonvulsant properties in the treatment of a variety of neuropsychiatric disorders (1,2), including anxiety and depression, which are often related to disturbances in the activity of hypothalamic-pituitary-adrenal (HPA) axis (3,4). Although these drugs exert most of their pharmacological effects via γ-aminobutyric acid_A (GABA_A) receptors (5,6), benzodiazepine administration has been associated with alterations in neuroendocrine function both in experimental animals and humans (7-9). However, even after years of extensive studies, the complex mechanisms by which these widely used drugs produce their effects on the HPA axis are still not known.Although most of the previous studies have demonstrated that classical benzodiazepines such as diazepam decrease the HPA axis activity in stressful contexts (10-14), under basal conditions they have been shown to stimulate (9,11,15-18), inhibit (15,19-22), and not affect (17,23-25) the HPA axis activity. Such diverse results might be related to several factors such as the dose and gender (15,16,20,21,26-28), or may also be a consequence of the net effect of non-selective benzodiazepines on the various GABA_A receptor isoforms (9).Our previous studies demonstrated that while diazepam (1 mg/kg) produced no change in plasma corticosterone levels in male rats (15,20), it decreased basal levels of corticosterone in female rats (15,26). However, although diazepam inhibited the HPA axis activity of female rats following administration of lower doses (1 or 2 mg/kg) (15,20,21,26), it stimulated the HPA axis activity following administration of high doses (10 mg/kg) (15,16,26). Moreover, whereas the suppressive effect of the lower doses of diazepam (2.0 mg/kg) on the HPA axis activity in female rats involves the GABA_A receptor complex (21), increases in corticosterone levels by a higher dose of diazepam (10 mg/kg) do not involve the stimulation of GABA_A receptors (16). In addition, stimulatory effect of 10 mg/kg diazepam on the HPA axis activity in rats seems not to be mediated by the benzodiazepine/GABA/channel chloride complex or by peripheral benzodiazepine receptors, but rather by a cyclic adenosine monophosphate (AMP)-dependent mechanism (18).Since our previous results suggested that the effect of a high dose of diazepam on the activity of the HPA axis in female rats might be due to a blockade of α₂-adrenergic receptors (16), the aim of this study was to elucidate whether noradrenergic system also has a modulatory role in the inhibitory effect of 2.0 mg/kg diazepam on basal plasma adrenocorticotropic hormone (ACTH) and corticosterone levels in female rats.