Involvement of serotonin in mediation of inhibition of substantia nigra neurons by noxious stimuli.
Neuronal activities of the substantia nigra pars compacta (SNC) are known to be inhibited by noxious stimuli. Extracellular recording of spontaneous firing of SNC neurons were made to investigate how such inhibition would be influenced by treatment with 5,7-dihydroxytryptamine (5,7-DHT) or p-chlorophenylalanine (PCPA) or by IV injection of the serotonin antagonist, cyproheptadine. Noxious stimuli were produced by tail pinching (TP) and tail heating (HW). In 5,7-DHT-treated experiment, the inhibition indices were 62.8 +/- 4.4 (TP) and 63.4 +/- 4.6 (HW)% for vehicle-treated control, then the inhibition indices were 26.3 +/- 2.6 (TP) and 27.8 +/- 2.9 (HW)% for 5,7-DHT-treated animals. As to the PCPA-treated experiment, the inhibition indices were 63.1 +/- 2.6 (TP) and 64.8 +/- 2.5 (HW)% for saline control rats, while the inhibition indices were 30.3 +/- 1.6 (TP) and 30.1 +/- 1.6 (HW)% for the PCPA-treated ones. Furthermore, in another observation, the firing rates of SNC neurons were reduced from the saline (control) level by 53.2 +/- 2.12 and 52.1 +/- 2.07% during the application of stimulation of TP and HW, respectively. Following the intravenous injection of cyproheptadine, the firing rates of those were reduced by only 6.71 +/- 1.38 and 4.38 +/- 1.79% for TP and HW, respectively. That is, the TP- and HW-induced inhibition were attenuated about 47% by the injection of cyproheptadine. The results strongly suggested that the serotonergic mechanism would be involved in the mediation of the inhibition of SNC neurons by noxious stimuli.
Tsai
C T
CT
Department of Biology, National Taiwan College of Education, Changhua.
D015116:5,7-Dihydroxytryptamine / Q000494:pharmacology; D000818:Animals; D004558:Electric Stimulation; D010134:Fenclonine / Q000494:pharmacology; D007276:Injections, Intraventricular; D008297:Male; D009433:Neural Inhibition / Q000187:drug effects*; D010146:Pain / Q000378:metabolism* / Q000503:physiopathology; D051381:Rats; D011919:Rats, Inbred Strains; D012701:Serotonin / Q000378:metabolism / Q000502:physiology*; D013378:Substantia Nigra / Q000187:drug effects / Q000378:metabolism* / Q000503:physiopathology
D015116:5,7-Dihydroxytryptamine; D012701:Serotonin; D010134:Fenclonine