Behavioral correlates of serotonin depletion

Depletion of telencephalic serotonin (5-HT) content by medical forebrain bundle lesions, which interrupt the ascending serotonergic pathways or by DL-p-chlorophenylalanine produces an increased sensitivity to pain as measured by the flinch-jump, stabilimetric, or hot-plate methods. Examination of the effects of a number of other lesions and drugs indicated that dopamine, norepinephrine and acetylcholine are not involved in pain sensitivity. Dosages of 75 mg/kg DL-5-hydroxytryptophan(5-HTP), 37.5 mg/kg L-5-HTP or 50 mg/kg Ro 4-4602 (NI-(DL-seryl)-N2-(2,3,4-trihydroxybenzyl)hydrazine) plus 37.5 mg/kg L-5-HTP administered to medical forebrain bundle lesioned rats returned both the telencephalic content of 5-HT and the pain threshold to normal values. Injection of 37.5 mg/kg of D-5-HTP or an equimolar dose of L-dopa had no effect on pain threshold. Normal animals display increased sensitivity to pain and decreased 5-HT contents in frontal pole, hippocampus, and amygdala during dark as compared to light hours. All three of these telencephalic areas are innervated by the ascending serotonergic pathways, and cells in these areas show inhibition of firing following the iontophoretic application of 5-HT. Taken together these data suggest that the serotonergic system normally acts to inhibit the effects of painful stimuli. A review of a variety of behavioral effects of 5-HT depletion including an enhanced response to lysergic acid diethylamide and amphetamine suggests that the ascending serotonergic system may have a general role in the inhibition of arousal, rather than a specific role with respect to various categories of behavior.