Long-term sequential determination of behavioral ontogeny of 5-HT1A and 5-HT2 receptor functions in the rat

Activation of 5-hydroxytryptamine(1A) (5-HT1A) receptors in rats produces h ypothermia and a number of behaviors [hindleg abduction (HLA), lateral head -weaving (LHW), forepaw treading (FPT), flat body posture (FBP), rollover ( RO), tremor (T), and straub tail (ST)] known collectively as the serotonin syndrome (SS). Stimulation of 5-HT2A receptors produces wet-dog shakes (WDS ), whereas 5-HT2C sites induce back muscle contraction (BMC). We investigat ed the functional ontogeny of the cited receptors in rat pups on postnatal days (PD) 7, 14, 18, 22, 28, 35, 60, and 120 by using (1) the 5-HT1A agonis t 8-hydroxy-2-dipropylaminotetralin (0, 1.25, and 5 mg/kg) to induce the SS and hypothermia and (2) the 5-HT2A/C agonist (+/-)-1-(2,5-dimethoxy-4-iodo phenyl)-2-aminopropane (0, 0.5, and 4 mg/kg) to produce both WDS and BMC. T he age of onset for most symptoms of SS [FBP, HLA, RO, and T] was the first week of life. They attained maximal intensities at ages 7 to 14 days, afte r which their maxima either reduced or dissipated to zero. Per contra, the onset of LHW and FPT required 14 to 18 days, and their maxima developed lat er. The onset of (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane-induce d WDS occurred after PD 14, and by PD 18, it reached its maximal intensity, which persisted up to PD 60, after which it declined. The onset of BMC was evident on PD 28 and attained its maximal frequency at ages 90 to 120 days . The results show that different components of SS appear within 14 days of birth, but they mature differentially, whereas the hypothermic effect of 5 -HT1A receptors remains relatively constant during aging. The times of onse t and maturation of WDS were intermediate (between the second and third wee ks of life), whereas BMC required 1 to 2 months for its appearance and matu ration.