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Effects of monoamines on interneurons in four spinal reflex pathways from group I and/or group II muscle afferents

Effects of locally applied serotonin (5‐HT) and noradrenaline (NA) were tested on extracellularly recorded responses of single spinal interneurons in deeply anaesthetized cats. These effects were tested on: (i) interneurons mediating reciprocal inhibition from group Ia afferents; (ii) interneurons mediating non‐reciprocal inhibition from group Ia and Ib afferents; (iii) intermediate zone interneurons co‐excited by group I and II afferents; and (iv) dorsal horn interneurons excited by group II afferents. Effects of monoamines were tested on responses evoked at latencies compatible with monosynaptic coupling. Responses evoked by group Ia and/or Ib muscle afferents were facilitated in all of the tested interneurons both by NA and 5‐HT. Responses evoked by group II muscle afferents were depressed in the majority of the interneurons but were facilitated in some of them. 5‐HT depressed these responses in all dorsal horn interneurons and in one subpopulation of intermediate zone interneurons, while it facilitated them in another subpopulation of intermediate zone interneurons. NA depressed them in all intermediate zone interneurons and in one subpopulation of dorsal horn interneurons, while it facilitated them in another subpopulation of dorsal horn interneurons. The results of this study lead to the conclusions that: (i) modulation of synaptic actions of muscle spindle and tendon organ afferents on spinal interneurons by 5‐HT and NA is related to both the type of the afferent and the functional type of the interneuron; and that (ii) 5‐HT and NA counteract each others' actions on some interneuronal types but mutually enhance them on the others.