Role of vasopressin in the control of systemic hemodynamics--lessons learned from the Brattleboro rat

Chronic activation of central alpha2-adrenoceptors prevents hypertension in DOCA-salt rats

The effects of chronic intracerebroventricular (i.c.v.) injections of the alpha2-adrenoceptor agonist, xylazine, on blood pressure were examined in DOCA-salt rats. Acute studies also examined the renal sympathetic nerve activity (RSNA) and renal excretory responses produced by i.c.v. xylazine in rats with established DOCA-salt hypertension. Rats implanted with a chronic i.c.v. cannula for drug injection were used. In chronic studies, four groups were investigated: control rats treated with s.c. soybean oil and i.c.v. saline; DOCA-salt rats (s.c. deoxycorticosterone acetate) receiving i.c.v. saline, xylazine or the alpha2-adrenoceptor antagonist, yohimbine. During vehicle or DOCA-salt treatment, xylazine (0.2 ng/microg) or yohimbine (10O microg/kg) was injected i.c.v. daily (three times). In DOCA-salt rats receiving i.c.v. saline, resting mean arterial pressure (MAP) was elevated on days 15 and 30 (135 +/- 5 and 160 +/- 6 mmHg, respectively). Chronic i.c.v. xylazine significantly attenuated the rise in MAP produced by DOCA-salt (day 15, 118 +/- 5 mmHg; day 30, 121 +/- 4 mmHg). Alternatively, chronic i.c.v. yohimbine shortened the onset (day 15, 152 +/- 7 mmHg) and augmented the hypertension in DOCA-salt rats (0 survival by day 30). In acute studies, i.c.v. xylazine elicited a profound natriuresis and diuresis as well as a reduction in RSNA without altering MAP. This study demonstrates that the ongoing (tonic) activity of central alpha2-adrenoceptor mechanisms are critically involved in regulating blood pressure in the DOCA-salt treated rat. In this manner, an enhanced activity of central alpha2-adrenoceptor systems acts to protect against a rise in blood pressure. In contrast, the attenuation of central alpha2-adrenoceptor stimulation evokes hypertension. The central action of xylazine to prevent hypertension may be associated with the inhibition of sympathetic outflow to the kidneys and evokes an enhanced natriuresis. By inhibiting the avid sodium retention elicited by DOCA-salt treatment, the central activation of alpha2-adrenoceptors delays the onset and the severity of hypertension in this pathological model.

Vasopressin analgesia: specificity of action and non-opioid effects

Recent neuroanatomical and behavioral evidence has indicated that vasopressin (VP) increases pain thresholds. In the present study intracerebroventricular (ICV) administration of both arginine VP (AVP: 75-500 ng) and 1-deamino-8-D-arginine vasopressin (DDAVP: 150-500 ng) elevated tail flick latencies. Oxytocin (OXY, ICV), also elevated tail-flick latencies (150-1000 ng); however this increase was accompanied by "barrel-roll" seizure activity. VP analgesia was eliminated by pretreatment with 1-deamino-penicillamine-2(O-methyl)tyrosine-AVP (dPTyr(me)AVP: 500 ng, ICV), a VP antagonist, but not naloxone (1 or 10 micrograms, ICV), suggesting that VP modulates nonciceptive thresholds through its own binding sites. Conversely, pretreatment with naloxone (1 micrograms, ICV) but not dPTyr(me)AVP (1 microgram, ICV) attenuated the analgesic efficacy of systemic morphine (10 mg/kg), further dissociating VP and central opiate analgesic processes. Finally, systemic pretreatment with dexamethasone potentiated VP analgesia. These data support the notion that VP is a specific non-opioid pain inhibitor.

Altered baroreceptor inputs to the supraoptic nucleus of the Brattleboro rat

In Brattleboro rats, many more supraoptic neurones are baroresponsive than in normal rats. This may reflect the more widespread noradrenergic innervation of the supraoptic nucleus in Brattleboro rats. If so, the present data suggest that baroreceptor influences on vasopressin secretion are mediated by a noradrenergic pathway and that the altered responsiveness reflects the altered innervation.