The effects of naloxone on behavioural depression due to hypotensive haemorrhage in unanaesthetized spontaneously hypertensive rats

The purpose of the study was to examine the effects of naloxone on signs of cerebral ischaemia during hypotensive haemorrhage in unanaesthetized spontaneously hypertensive rats. Mean arterial blood pressure (MAP), heart rate (HR) and somatosensory evoked potentials (SEP) were recorded. Arousal tests were also performed and the behavioural responses quantified. The SEP alone were a poor indicator of cerebral function in these unanaesthetized rats, because they were markedly influenced by changes in activity and arousal of the animals. Hypotensive haemorrhage resulted in a biphasic tachycardia response, an attenuation of the first SEP component and a reduction of the behavioural response score. Naloxone, 5 mg kg-1 i.v., induced transient bradycardia and a dramatic improvement in arousal test responses, while SEP were not clearly altered. The MAP was kept constant after naloxone injection by adjustments of bleeding and transfusion. Injection of naloxone in unbled control SHR also induced bradycardia but without any changes in SEP and the behavioural responses. The results indicate that naloxone can have beneficial effects in cerebral ischaemia. Possible mechanisms are discussed.