P2 purinoreceptors are present in hypothalamic and brainstem nuclei that are involved in the regulation of body temperature (T_b) and development of fever (Kanjhan et al. 1999). Recently, using the intracerebroventricular (I.C.V.) injections of ATP analogues and P2 receptor antagonists we have shown that ATP acting on certain P2 receptors may play an important role in thermoregulation (Gourine et al. 2002). However, the site of ATP action in relation to regulation of T_b has not been investigated.

Experiments were performed in adult male Wistar rats (280-350 g) and were approved by the Institutional Animal Care and Use Committee (in Minsk). Rats were anaesthetised (ketamine 87.0 mg kg^-1 + xylazine 13.0 mg kg^-1), a steel guide cannula was implanted into the third cerebral ventricle or anterior hypothalamus (AH), and a telemetry transmitter was implanted into the abdomen for monitoring of T_b. After a 7 day recovery period, fever was induced by intraperitoneal injection of E. coli lipopolysaccharide (LPS; 50 µg kg^-1). Effects of I.C.V. and intrahypothalamic administration of the ATP analogue α,β-methyleneATP (α,β-meATP, 0.2 µmol) or artificial cerebrospinal fluid (ACSF) on T_b during fever were determined. The rat was humanely killed by overdose of anaesthetic at the end of the experiment. In addition, activity of the hypothalamic thermosensitive neurones was recorded in brain slices (400 µM) containing AH. Firstly, the thermosensitivity of the recorded neurones was determined by changing the bath temperature and relating the firing rate of the neurone to the slice temperature. Then, the effect of ATP on the activity of these cells was determined.

It was found that following I.P. injection of LPS, fever reached a maximal T_b (around 39 °C) 2.5 h after injection. α,β-MeATP injected I.C.V. or into the AH at the peak of fever caused a profound decrease in febrile T_b. Ten minutes after I.C.V. injection of α,β-meATP, T_b of febrile rats decreased to 37.84 ± 0.19 °C, some 0.9 °C lower than that of the T_b of febrile rats 10 min after I.C.V. treatment with ACSF (38.74 ± 0.19 °C, mean ± S.E.M., P < 0.05, Student’s unpaired t test). The decrease in T_b of febrile rats induced by intrahypothalamic injection of α,β-meATP developed more slowly and lasted longer compared with the response evoked by this ATP analogue injected into the third cerebral ventricle. Forty minutes after injection of α,β-meATP into the AH, T_b of febrile rats decreased to 37.43 ± 0.38 °C, mean ± S.E.M., some 1.0 °C lower than that of the T_b of febrile rats 40 min after intrahypothalamic administration of ACSF (38.42 ± 0.10 °C, mean ± S.E.M., P < 0.05, Student’s unpaired t test). It was also found that application of ATP increases the firing rate of 60 % of warm-sensitive neurones in the AH in vitro.

These data indicate that a population of warm-sensitive neurones in the AH is likely to be the site of action of ATP on T_b during fever, supporting our previous hypothesis that extracellular ATP acting on P2X receptors may be involved in the mechanisms of fever development, limiting the magnitude of the febrile response.

This work was supported by The Wellcome Trust.