Behavioral and thalamic nociceptive responses in rats following noxious ischaemia of the tail

In rats anaesthetized with urethane, we have investigated the response of neurones in the ventrobasal complex of the thalamus to noxious ischaemia of the tail, and to graded noxious thermal stimulation of the tail before and after ischaemia. In behavioural experiments conscious rats were exposed to the same experimental procedure. After ischaemia the threshold tail temperature required to elicit both a neuronal response and aversive behaviour in conscious rats, to thermal stimulation, was decreased significantly (P less than 0.01 paired t tests). Threshold temperatures for the neuronal response and the behavioural response were not significantly different, either before or after ischaemia. The time course of recovery to pre-ischaemic threshold temperatures was the same for both the behavioural and neuronal responses. Most thalamic neurones responding to noxious thermal stimulation of the tail also increased firing rate during ischaemia. The latency of response of the thalamic neurones to ischaemia was 12.1 +/- 1.8 min and the latency of the behavioural response to the same stimulus was 11.9 +/- 2.1 min. Ventrobasal thalamic neurones, therefore, which responded to noxious thermal stimulation of the tail also responded to noxious ischaemia, and exhibited a neuronal correlate of post-ischaemic hyperalgesia which paralleled behavioural responses closely.