POSTNATAL DEVELOPMENTAL-CHANGES OF RECEPTOR RESPONSIVENESS IN RAT MESENTERIC VASCULAR BED

1 The response to perivascular nervous stimulation (PNS) and the respo nsiveness to receptor agonists, in different stages of neurogenesis, o n rat mesenteric vascular bed (MVB), was investigated. Rats of differe nt age groups (5-7, 9-11, 14-16, 20-22 days) were tested, using 60-day -old rats as controls. 2 In the 5-7 days age group, the response to PN S was resistant to TTX treatment (1x10(-6) M). The TTX inhibition incr eased with age and became almost complete in 60-day-old rats. 3 In the 1st week of postnatal life (pre-innervation period), noradrenaline (N A) and 5-hydroxytryptamine (5-HT) produced contraction, whereas isopre naline (ISO) and dopamine (DA) caused relaxation. During the 1st and 2 nd week, pD(2) values of NA and ISO were significantly higher than in adult control rats. No significant difference in pD(2) values of 5-HT and DA was observed during postnatal development. 4 At 5-7 days, the r elaxation by acetylcholine (ACh), typical of adult age, was absent and ACh evoked only contractile responses. The relaxant effect by ACh app eared at 9-11 days, increased with age and, by the end of the 2nd week , did not differ from that of the adult group. 5 These results provide evidence that responsiveness of all tested receptors in the MVB is al ready present in the pre-innervation period (1st week). Adrenergic rec eptor responsiveness is higher at birth and decreases by the end of th e 3rd week of postnatal life, when connections between the central ner vous system and effector organs are established. Only muscular muscari nic receptors, responsible for ACh-induced contraction, are functional in the Ist week, while endothelial muscarinic receptors, responsible for ACh-induced relaxation, become gradually responsive later in postn atal life.