Neurokinin actions on substantia gelatinosa neurones in an adult longitudinal spinal cord preparation.

We have used an adult longitudinal spinal cord preparation to study the effects of a range of selective neurokinin analogues on single neurones located exclusively within the substantia gelatinosa. Since the preparation retained attached dorsal roots it was possible synaptically to activate the substantia gelatinosa neurones by electrical stimulation of their afferent fibres, thus providing a means of studying directly the role of neurokinins in mechanisms of primary afferent transmission. The actions of three agonists selective for the three NK receptor subtypes (NK1, GR73632; NK2, GR64349; NK3, senktide), and a highly selective antagonist at NK1 receptors (GR82334) were investigated. Experiments were performed on a total of 274 substantia gelatinosa neurones, estimates of conduction velocity for evoked responses suggested that the majority of these neurones were innervated by unmyelinated afferents. A large proportion responded to iontophoretically applied neurokinin agonists. The majority responded to NK1, fewer responded to NK2; some, although not all, of the neurones tested responded to both NK1 and NK2 agonists. In most cases the responses were excitatory, although inhibitory effects were observed in some neurones. None of the neurones tested responded to NK3 agonist. Excitatory and inhibitory actions could be demonstrated following abolition of synaptic transmission by removal of calcium, suggesting direct mechanisms for both effects. The antagonist alone failed to modify either spontaneous firing or firing in response to afferent stimulation in any of the neurones studied, even though the doses used were shown to be effective in selectively antagonising responses to the NK1 agonist, suggesting that neither relied on the endogenous release of neurokinins.