Introduction:
Central serotoninergic system is involved in numerous functions in the human body including mood, emotional and cognitive processes, the control of appetite, temperature regulation and nociception. In the latter case we have to consider interaction between serotonin and opioid system which consists of endogenic peptides, i.e. endorphins, enkephalin and dynorphins together with specific receptors synthetized in various brain’s parts. The main goal of the present work was to examine the effect of neonatal central serotoninergic system lesion on antinociceptive effect of morphine assessed in adult rats.

Materials and methods:
On the 3 rd day of postnatal life male rats were administered with 5.7-dihydroxytryptamine (5.7-DHT) (70 µg/10µl). Rats continued to be housed until 8-10 weeks, for further experimentation. The antinociceptive effects of morphine (2.5 mg/kg) was assessed by hot plate, tail immersion, paw withdrawal, formalin and writhing tests. Furthermore cerebral frontal cortex serotonin level and its metabolism was assayed.

Results:
It was demonstrated that central serotoninergic lesion attenuated antinociceptive effects evoked by morphine injection assessed in hot plate, writhing and paw withdrawal tests being without effect in tail immersion and formalin tests. Morphine accelerated serotonin metabolism in the cerebral frontal cortex in control and lesioned rats.

Conclusions:
Dysfunction of the central serotoninergic system may lead to modified biological reaction to analgesic drugs (morphine).