Lack of effect of microinjection of noradrenaline or medetomidine on stimulus-evoked release of substance P in the spinal cord of the cat: a study with antibody microprobes.

1. Experiments were performed on barbiturate anaesthetized, spinalized cats to investigate the effect of microinjected noradrenaline or medetomidine on the release of immunoreactive substance P in the dorsal spinal cord following peripheral nerve stimulation. The presence of immunoreactive substance P was assessed with microprobes bearing C-terminus-directed antibodies to substance P. 2. Noradrenaline or medetomidine were microinjected into the grey matter of the spinal cord, near microprobe insertion sites, at depths of 2.5, 2.0, 1.5 and 1.0 mm below the spinal cord surface with volumes of approximately 0.125 microliters and a concentration of 10(-3) M. 3. In the untreated spinal cord, electrical stimulation of the ipsilateral tibial nerve (suprathreshold for C-fibres) elicited release of immunoreactive substance P which was centred in and around lamina II. Neither noradrenaline nor medetomidine administration in the manner described produced significant alterations in this pattern of nerve stimulus-evoked release. 4. In agreement with recent ultrastructural studies these results do not support a control of substance P release by catecholamines released from sites near to the central terminals of small diameter primary afferent fibres.