Actions of opiates, substance P, and serotonin on the excitability of primary afferent terminals and observations on interneuronal activity in the neonatal rat's dorsal horn in vitro

Approximately 5 segments of lumbo-thoracic spinal cord together with connected dorsal root ganglia were removed from 1-11-day-old rats and maintained in vitro. Dorsal root afferents, recorded from the ganglion and stimulated at the root entry zone, had conduction velocities typical of unmyelinated fibers (< 2 m/s). The spinal terminals of individual afferents showed increased excitability with bath application of substance P and serotonin and decreased excitability with morphine sulfate, [d-ala₂]methionine-enkephalinamide, manganese ions and magnesium ions. Naloxone by itself elicited no change in excitability, although it appeared to reduce the ongoing effect of opiates. Neurons recorded extracellularly in the dorsal horn responded to afferent volleys with one or more of 3 distinct phases; an excitation roughly coincident with the volley's arrival, a 50-300 ms period of inhibition, and a late excitation of 150-300 ms latency. The excitability results are accounted for by a model in which substance P, γ-aminobutyric acid and possibly other depolarizing agents are contained in interneurons which synapse on afferent terminals. These interneurons could receive inhibitory enkephalinergic input, and, in the neonate but not the adult, excitatory serotoninergic input. An alternate scheme would have enkephalin and serotonin acting directly on afferent terminals, although perhaps by non-synaptic diffusion since the appropriate synapses have not been seen in histochemical studies. Such an action for enkephalin might explain the existence of opiate receptor on afferent terminals. The interneuronal responses to afferent volleys are parallel in most aspects to those found in the dorsal horns of adult mammals in vivo.