The development of the reaction to thermal nociceptive stimulation was studied in neonatal rats of two strains (albino Wistar and pigmented Long-Evans). Using the tail-flick method in neonates of both strains the effects of daily repeated thermal stimulation (sensitisation) were compared to those of acute stimulation applied on 1 day only. During the whole postnatal development period studied (3-28 days) the pigmented rat pups were more sensitive to a repeated thermal nociceptive stimulation than the albinos. The latency of the response did not change linearly during the developmental time studied, but had a pronounced peak (decreased sensitivity) in the period of eye-lid opening. In the pigmented animals the peak appeared 1 day earlier than in the albinos. At the time of the weaning period the latencies of the response were again shorter.

Department of Normal Pathological and Clinical Physiology Charles University 3rd Faculty of Medicine Ke Karlovu 4 Praha 2 120 00 Prague Czech Republic

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