Introduction:
Activation of the central histaminergic system leads to the reversal of experimental haemorrhagic shock, whereas neuropeptide Y (NPY) administered intracerebroventricularly (icv) induces a depressor effect in haemorrhagic hypotension. Since histaminergic neurons of the tuberomammillary nucleus receive input from neurons producing NPY localized in the caudal magnocellular nucleus of the hypothalamus, the aim of the study was to examine (1) the cardiovascular effects of the NPY type 1 (Y1) receptor antagonist in haemorrhagic shock and (2) the possible involvement of the histaminergic system in this action.

Material and methods:
Experiments were performed in ketamine/xylazine-anaesthetised male Wistar rats subjected to irreversible haemorrhagic hypotension, with mean arterial pressure (MAP) 20–25 mmHg and 0% survival at 2 h. Immediately after terminating bleeding, the animals were pre-treated icv with histamine H1 and H2 receptor antagonists chlorpheniramine (50 nmol) and ranitidine (50 nmol) as well as the H3/4 receptor antagonist/inverse agonist thioperamide (50 nmol), respectively, or saline. Five minutes later, the rats were injected icv with the Y1 receptor antagonist BIBP 3226 (64 nmol/kg).

Results:
BIBP 3226 evoked rises in MAP, pulse pressure and renal blood flow (RBF) with an increase in survival to 100% at 2 h. Chlorpheniramine inhibited cardiovascular changes evoked by BIBP 3226 and decreased to 0% survival at 2 h. In contrast, ranitidine and thioperamide had no effect.

Conclusions:
We demonstrate for the first time (1) the pressor effect resulting from the blockage of central Y1 receptors in haemorrhage-shocked rats and (2) the involvement of the histaminergic system in this action.