Action of neurotensin on spinal cord neurons in the rat

Histochemical, biochemical and pharmacological data suggest that the tridecapeptide neurotensin (NT) may play a role as an intercellular messenger in the spinal cord of rats. In the present study the response of spinal cord neurons to NT was investigated employing conventional extra- and intracellular recording techniques in combination with iontophoresis or a microperfusion system which permits the control of the maximal concentration of NT reached near the neuron. Recordings were obtained from motoneurons, interneurons and from neurons in the dorsal horn receiving synaptic input from low and high threshold mechanoreceptors activated in their peripheral cutaneous receptive fields. The most commonly observed action of NT applied by either mode was an excitation after a dose-dependent delay. The response was dose-dependent, repeatable and reversible. The intracellular recordings revealed that these excitatory responses were due to a depolarizing action of NT associated with an increase in input resistance, not attributable to non-linearities in the current-voltage relationship. The present data are in agreement with most previous investigations reporting excitatory actions of NT on neurons in various neuronal structures in the peripheral and central nervous system. It seems unlikely that these excitatory effects are indirect actions mediated by an inhibition of other neurons in the vicinity.