Introduction:
Central cardiovascular regulation in haemorrhagic shock is influenced by many neuronal systems, including the melanocortinergic and histaminergic systems. The aim of the study was to examine the involvement of the histaminergic system in the resuscitating effect of melanocortin type 4 (MC4) receptor agonist Ro 27-3225.

Materials and methods:
Studies were carried out in male Wistar rats subjected for irreversible haemorrhagic shock with mean arterial pressure (MAP) 20-25 mmHg. At 5 min of critical hypotension the animals were injected intracerebroventricularly (icv) with Ro 27-3225 or 0,9% NaCl solution (control group).

Results:
Hypovolaemia was accompanied by a decrease in pulse pressure (PP), heart rate (HR) and renal blood flow (RBF). In the control group, there were no increases in measured parameters, and the survival rate of 2 h was 0%. Ro 27-3225 (10 nmol, icv) evoked long-lasting rises in MAP, PP and RBF as well as a 100% survival at 2 h. Pre-treatment with histamine H1 receptor antagonist chlorpheniramine (50 nmol, icv) inhibited Ro 27-3225-induced increases in MAP, PP and RBF, with no influence on HR and survival at 2 h (100%). In contrast, antagonists of H2 and H3/4 receptors – ranitidine (50 nmol, icv) and thioperamide (25 nmol, icv), respectively, had no influence on Ro 27-3225 action. In control groups, histamine receptor antagonists given alone did not affect measured parameters in comparison to the saline-treated group.

Conclusions:
The histaminergic system participates in the resuscitating effect induced by the activation of central MC4 receptors in haemorrhagic shock in rats, and histamine H1 receptors are involved.