Release of Vasopressin from the Septum and Hippocampus of Freely Behaving Male Rats: Effect of Bacitracin and Synthetic Vasopressin

Abstract

This paper describes the release characteristics of arginine vasopressin (AVP) from the dorsal septum and dorsal hippocampus of freely behaving male rats using miniaturized push-pull cannulae and a slow rate of perfusion to minimize tissue damage. The major findings were that: 1) The spontaneous release of immunoreactive AVP (I-AVP) from the dorsal hippocampus was episodic in nature and did not require the presence of bacitracin in the perfusion medium to reliably demonstrate its pattern of release; 2) despite the fact that the septum has close to seven times more I-AVP per/mg of tissue, the dorsal septum perfusates had undetectable levels of the neuropeptide; 3) under these conditions hypertonic saline intraperitoneally did not alter the spontaneous release of this peptide; and 4) only when the perfusion medium contained effective concentrations of bacitracin M) were reliable and detectable levels of I-AVP measured in perfusates from the dorsal septum and exogenous synthetic AVP had a robust positive feedback action on its own release. The physiological relevance and importance of this robust and novel effect of AVP remains to be elucidated.There is growing evidence indicating that arginine vasopressin ( .\VP), besides its classical peripheral hormonal effects (l), plays a major role as a neurotransmitter within the CNS (2). Immmunocytochemical studies have demonstrated the presence of numero u s extrahypothalamic AVP neurons particularly in the bed nucleus of the striae terminalis (BNST) as well as AVP nerve tcrminals in limbic regions such as the lateral septum (LS), hippocampus and the amygdala (3, 4). In at least one of these regions, the LS, there are reports showing that AVP is released from in vitro (5) as well as from in vivo (6-8) preparations through an active Ca'+-dependent neurosecretory process. These data, together with the demonstration of AVP receptors in the septum and hippocampus (9-1 I), strongly support the role of AVP as an agent of neuronal communication and reinforces the concept that this peptide may be normally involved under specific conditions of the organism in the modulation of a variety of CNS functions. For example, since the pioneering work of De Wied and co-workers it is known that AVP can facilitate consolidation of memory processes since it can enhance retention of active and passive avoidance behavioral tasks (12). More recently, it was shown that this nonapeptide facilitates recognition of juveniles by male adult rats in the so-called social memory paradigm (1 3). In addition, AVP seems to be involved in an array of apparently disparate behaviors such as barrel rotation (14), grooming and flank marking (15), sleep (16), lordosis behavior (1 7) as well as being involved in temperature regulation as an antipyretic agent (18,19) and in pain as a nociceptive substance (20). However, in most of these cases high doses of AVP in the order of ng or pg ranges have been injected either systematically or intracerebrally. Therefore, it is of crucial importance to demonstrate une...

Positive Feedback of Vasopressin on its Own Release in the Central Nervous System: in vitro Studies

“…First, it appears to be site-specific within the septum since perfusion of the dorsal region of the septum and not the ventral septum led to increases in I-AVP release as shown in the preceeding paper (8). Secondly, the positive feedback action of AVP is dose-dependent and it tends to saturate at high doses.…”

“…In the previous paper (8) it was shown that to measure detectable levels of immunoreactive AVP (I-AVP) from the septum of freely behaving male rats the superfusion medium had to contain effective concentrations of bacitracin, an aminopeptidase inhibitor. Therefore, in these in vitro experiments we first studied the spontaneous release of I-AVP from septal fragments in the Presence of different concentrations of bacitracin in the perfusion medium as well as the response of the tissue to a pulse of synthetic AVP infused for 5 min.…”

“…Fig. 1 shows individual examples of the pattern of I-AVP release from septal fragments superfused in vitro with KRP with (A) or without bacitracin ( 8 ) . Notice the clear pulsatile release of I-AVP from septal fragments superfused with bacitracin as well as a much higher overall mean release than septal fragments superfused without bacitracin.…”

“…The release of this neuropeptide from extrahypothalamic regions in the CNS such as the lateral dorsal septum (DS) and dorsal hippocam-PUS (DH) in freely behaving male rats (8) as well as from in vitro preparations (9) of these tissues has contributed to strengthen such a concept.…”

“…In the preceding paper (8) we demonstrated that synthetic AVP infused directly into the DS of freely behaving rats by way of a push-pull cannula (PPC) had a robust and reliable stimulatory action on its own release. In this paper, we study in vitro the charxteristics of this novel positive feedback action of AVP in the septum by means of an in vitro superfusion system (10).…”