Physiology &Behavior, Vol. 54, pp. 403--405, 1993 0031-9384/93 $6.00 + .00 Printed in the USA. Copyright© 1993PergamonPressLtd. BRIEF COMMUNICATION Functional Development of Central Nervous System in the Rat: Ontogeny of Nociceptive Thresholds ABDOULAYE B~, ~ AND BIALLI V. SERI D@artement de Biologie et Physiologie animale, Facult( des Sciences et Techniques, Universit~ Nationale, 22 BP 582 Abidjan 22, C6te d'Ivoire Received 19 March 1992 B~,, A. AND B, V. SERI. Functional development of central nervous system in the rat: Ontogeny of nociceptive thresholds. PHYSIOL BEHAV 54(2) 403-405, 1993.--The functional development of central nervous system (CNS) in the rat has been studied from the 10th to the 45th postnatal day, through a sensory function: nociception. Baseline pain responsiveness has been assessed with the tail-flick procedure. The mean tail-flick latency clearly decreases from the 10th to the 25th day, and remains stationary from this age to 45 days. The maturational processes underlying these reductions of tail-flick latencies in developing rats are discussed. Nociceptive thresholds Developing rat Tail flick VARIOUS mechanisms which mediate normal nociception in adult animals are reported in recent years. However, information about nociception in immature organisms is scanty. The few published reports aimed at understanding the maturational changes in the behavioral responses of rats suggested that pain sensitivity may change with age. For example, 5- and 10-day- old rats have a more limited repertoire of responses to presen- tations of electroshock, compared with 15- or 20-day-old animals (11). Some pharmacologic studies also indicate differences in pain sensitivity in the developing rat. Thus, morphine is 40-fold less effective in producing analgesia in 2-day-old than in 14-day- old rats (19), and B-endorphin is not effective in producing an- algesia until 14 days of age (22). Other studies have also reported that the analgesic efficacy of morphine increases during the first 2 weeks of life in the rat (2,4). Recently, Bronstein et al. (9) reported that visceral nociceptive responses of developing rats change as a function of age. Their study revealed that hypertonic NaCl-induced abdominal con- striction responses were absent in 5-day-old rats, but were clearly discernible in 20-day-old animals, while adult-like patterns of responses were observed at the time of weaning. As many aspects of peripheral and central neural structure and function are immature in the neonate rat, it seems plausible that some of the ontogenetic differences in nociception may be related to the relative immaturity of neuroanatomical and/or neurochemical mechanisms that mediate this response (9). Thus, evaluation of nociceptive threshold in the developing rat would provide an index of maturation of the CNS. The present experiments were undertaken to determine changes of the nociceptive responses in developing rats, by mea- suring tail-flick latency to radiant heat in terms of age. METHOD Animals Wistar rat pups from rats mated in our colony were used. Approximately I week prior to parturition, the dams were housed individually in plastic cages (27 × 37 × 18 cm) with the floor covered by wooddust. The dams were checked daily in the morning for pups. The colony was bred in an aerated noiseless vivarium room subjected to diurnal daylight/night cycles and ambient temperature (31 _ 2°C). Litter sizes were adjusted within 24 h following birth so that every mother nursed 8 to l0 pups. The date of parturition being designated as postnatal day l (P 1). After birth, offsprings were left undisturbed until l0 days of age. Testing sessions were per- formed at 10, 15, 20, 25, 30, and 45 days of age. Mothers re- mained with the pups at all times, except during testing sessions. At weaning, rats pups were housed in some sex groups of three l Requests for reprints should be addressed to A. Bfi. 403