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Serotonin involvement in the blockade of bulbospinal and recurrent inhibition of the monosynaptic reflex Sastry, Bhagavatual Sree Rama

Abstract

The monoamine uptake blocking agents, imipramine HC1 (5 mg/kg i.v.) and desipramine HC1 (4.8 mg/kg i.v.), and the monoamine oxidase inhibitor, pargyline HC1 (30 mg/kg i.v.) antagonized bulbospinal inhibition (BSI) of the monosynaptic reflex (MSR) in unanaesthetized cats decerebrated at the mid-collicular level. The effect of imipramine was quantitatively more on BSI of the quadriceps (QUAD)-MSR compared to that on BSI of the posterior biceps-semitendinosus (PBST)-MSR. Imipramine's action on this inhibition was also quantitatively greater compared to that of the equimolar dose of desipramine. Pretreatment of the animals with the tryptophan hydroxylase inhibitor, DL-p-chlorophenylalanine (p-CPA) (300 mg/kg i.p. for 3 consecutive days) completely eliminated the blocking action of imipramine. However, pretreatment of the animals with the tyrosine hydroxylase inhibitor, DL-α-methyl-p-tyrosine methyl ester HC1 (α-MPT) (126 mg/kg i.p. given 16 and 4 hours before the recording ) had no effect on imipramine's action. These findings strongly suggest that a 5-hydroxytryptamine (5-HT, serotonin) system antagonizes BSI of the MSR. They do not support the proposal of Clineschmidt and Anderson (1970) that the bulbospinal inhibitory pathway involves a 5-HT interneurone in the spinal cord. Imipramine HC1 (5 mg/kg i.v.) and pargyline HC1 (30 mg/kg i.v.) blocked recurrent inhibition (RI) of the MSR evoked by stimulation of a dorsal root. Imipramine blocked RI of the QUAD-MSR but had no effect on RI of the PBST-MSR. Pretreatment of the animals with either p-CPA or α-MPT prevented the blocking action of imipramine on RI. Application of a 'cold block' which potentiated RI of the QUAD-MSR also eliminated the blocking action of imipramine on this inhibition. These observations suggest that a supraspinal monoaminergic system which involves 5-HT and noradrenaline links has a tonic inhibitory effect on RI of the QUAD-MSR.

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