Exp Neurobiol 4:127-134, 1995

Effects of capsaicin to the serotonergic neurons in the raphe nuclei of the rat midbrain.

Jong Eun Lee, Ki Sook Park, Won Taek Lee, Kyung Ah Park

Department of Anatomy, Yonsei University Colleage of Medicine, Seoul 120-752, Korea

Capsaicin, the pungent algesic substance of the red pepper, is known to be a neurotoxic substance that interrupts the pain conducting pathway. To investigate its effects on the serotonergic neurons in the rat raphe nuclei, capsaicin was administered to the neonatal and adult rats and morphometric, immunohistochemical, and electron microscopical studies were done. Capsaicin (50 mg/kg) was injected subcutaneously to neonatal rats on the 2nd and 4th day after birth. The same amount of capsaicin was injected once subcutaneously in the adult rats. The neonatal rats were sacrificed 10 weeks after capsaicin treatment and the adult capsaicin treated rats were sacrificed 1 week, 1 month, and 2 months after injection. Brain stem specimens were processed for histological and immunohistochemical investigation of the dorsal and median raphe nuclei. Serotonin immunoreactive (5-HT-I) neurons count and area of the 5-HT-I cell body were measured. The immunostained raphe nuclei were observed with electron microscope.

The 5-HT-I neurons in dorsal and median raphe nuclei showed no change in its counts after capsaicin treatment. The area of the neuronal cell bodies were decreased from 1 week to 1 month after capsaicin treatment but it recovered slightly after 2 months in adult capsaicin treated rats. The area of the neuronal cell bodies also decreased in neonatal capsaicin treated rats. In the immunoelectron-microscopic study, serotonin-immunoreactive profiles similar to the control group were well observed.

The 5-HT-I neurons did not change significantly after capsaicin treatment except for slight changes in neuronal cell body areas.

Key words : Capsaicin, Serotonin, Immunohistochemistry, Rat Raphe Nuclei