Impairment in Pain Perception in Adult Rats Lesioned as Neonates with 5.7-Dihydroxytryptamine.

BACKGROUND: Whereas some studies have demonstrated the essential role of 5-hydroxytryptamine (5-HT) in tramadol and acetaminophen analgesia, other research has presented conflicting results. To dispel doubts, some aspects of the involvement of 5-HT in the antinociceptive properties of these drugs remain to be clarified. OBJECTIVES: The aim of this study was to determine whether the serotoninergic system dysfunction produced by neonatal 5-HT lesion in rats may affect the antinociceptive effects of tramadol and acetaminophen administered in adulthood. MATERIAL AND METHODS: Three days after birth, the control rats were pretreated with desipramine HCl (20 mg/kg i.p.) 30 min before intraventricular saline--vehicle injection. A separate group received 5.7-DHT; 2×35 µg in each lateral ventricle. At the age of 8 weeks, 5-HT and 5-hydroxyidoleaceticacid (5-HIAA) concentrations were determined in the thalamus and spinal cord by an HPLC/ED method. The antinociceptive effects of tramadol (20 mg/kg i.p.) or acetaminophen (100 mg/kg i.p.) were evaluated by a battery of tests. RESULTS: 5.7-DHT lesioning was associated with a reduction in 5-HT and 5-HIAA content of the thalamus (>85% and >90%) and spinal cord (>58% and 70%). Neonatal 5.7-DHT treatment produced a significant reduction in the antinociceptive effect of tramadol in the hot plate, tail-immersion, paw withdrawal and writhing tests. In the formalin hind paw test, the results were ambiguous. 5-HT lesion was also associated with a decrease in the analgesic effect of acetaminophen in the hot plate and writhing tests. A similar relationship wasn't found in the other assessments conducted with the use of acetaminophen. CONCLUSIONS: The present study provides evidence that (1) an intact serotoninergic system is required for the adequate antinociceptive action of tramadol, and (2) the serotoninergic system exerts a negligible influence on acetaminophen-induced analgesia in rats. We hypothesize that similar abnormalities in nociception may occur in patients with 5-HT dysfunction (e.g. depression), so these results should be complied in analgesic dosage adjustment.

Neonatal DSP-4 treatment impairs 5-HT(1B) receptor reactivity in adult rats. Behavioral and biochemical studies.

To examine the effect of a central noradrenergic lesion on the reactivity of the 5-HT(1B) receptor we compared intact male rats with rats in which noradrenergic nerve terminals were largely destroyed with the neurotoxin DSP-4 (50 mg/kg x 2, on the 1st and 3rd days of postnatal life). When rats attained 10 weeks of age, control and DSP-4 rats were divided into two subgroups receiving either saline or the serotonin (5-HT) synthesis inhibitor (p-chlorophenylalanine; p-CPA; 100 mg/kg). Employing an elevated plus maze test, we demonstrated that CP 94,253 (5-propoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-pyrrolo[3,2-b]pyridine hydrochloride) (4.0 mg/kg; 5-HT(1B) agonist) induced an anxiogenic-like action in control rats; however, it failed to elicit this effect in the DSP-4 group. Surprisingly, in p-CPA pretreated rats anxiogenic-like activity was observed both in control and DSP-4 treated rats. CP 94,253 significantly attenuated 5-HT synthesis in the medial prefrontal cortex (mPFC) of control rats, and SB 216641 (N-{3-[3-(dimethylamino)ethoxy]-4-methoxyphenyl}-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-[1,1'-biphenyl]-4-carboxamide hydrochloride) (4.0 mg/kg; 5-HT(1B) antagonist) was able to antagonize this effect. Conversely, CP 94,253 failed to significantly inhibit the 5-HT synthesis rate in DSP-4-treated rats. In the microdialysis study CP 94,253 induced long-lasting attenuation of 5-HT release in the mPFC of control rats but had no effect in DSP-4 rats. These data lead to the proposal that presynaptic 5-HT(1B) autoreceptors underwent desensitization in DSP-4 treated rats.