British journal of pharmacology
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1. Population synaptic responses of motoneurones were recorded from a ventral root following electrical stimulation of the corresponding lumbar dorsal root in neonatal rat hemisected spinal cord preparations in vitro. Two levels of electrical stimulation were used to elicit dorsal root compound action potentials that contained either an A fibre component alone or both A and C fibre components. ⋯ CGP40116 (3 microM) depressed the high intensity e.p.s.p. to 62 +/- 8%, the low intensity e.p.s.p. to 22 +/- 4% and the train e.p.s.p. to 16 +/- 2% of control values. 5. The depressant actions of morphine were fully reversed by naloxone (1 microM) and those of clonidine were fully reversed by atipamezole (1 microM). 6. These results show that, in contrast to previous findings, activation of primary afferent C fibres in dorsal roots is not required for generation of morphine- or clonidine-sensitive synaptic responses in ventral roots of this in vitro preparation.