Resultsand Discussion

In sham-operated rats (Figure 1 A), immobilization stress decreased the content of AVP in the NIL both in vehicle- and MLT-treated animals. Plasma AVP concentration was higher in MLT-treated stressed rats (Figure 1B). In sham-operated rats, MLT decreased the NIL AVP content in animals of all experimental subgroups (Figure 1 A), while in pinealectomized animals MLT treatment had an opposite effect (Figure 2A). After the pineal removal, plasma AVP concentrations were increased in MLT-treated rats of subgroups 2 and 3 (Figure 2B). The efficiency of the experimental immobilization procedure as an stressor was confirmed by an increased ACTH concentration in blood plasma (the ACTH plasma levels in these animals are here not shown [see: 2]).

In the present study we demonstrated that the pineal and MLT are involved in the functional mechanisms related to the neurohypophysial response to immobilization stress. Moreover, we have confirmed that the NIL AVP content decreased under immobilization stress as it was earlier reported (7). Also, the finding that MLT injection decreased the NIL AVP content was consistent with results described previously (8, also for references). However, the mechanisms of the MLT-induced enhancement of AVP release (as noted during this study) remain to be investigated. In this regard, the interactions of MLT with its brain receptors (9) and/or neurotransmitters that

Figure 1. The effect of melatonin on the NIL (A) and blood plasma (B) AVP levels in sham-operated male rats under immobilization stress (mean ± S.E.M.; n = 8-10).
Figure 2. The effect of melatonin on the NIL (A) and blood plasma (B) AVP levels in pinealectomized male rats under immobilization stress (mean ± S.E.M.; n = 7-8).

regulate AVP release (10) are conceivable as to both sham-operated and pinealec-tomized rats. It is concluded that under stress conditions MLT modifies both AVP (this study) as well as OT, PRL and ACTH (2) release. The fact that MLT attenuated the stress-induced ACTH secretion implied possible anti-stress activity of the pineal.

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