Nociceptive integration in the rat spinal cord: role of non-linear membrane properties of deep dorsal horn neurons

Deep dorsal horn neurons (DHNs) involved in nociception can relay long-last ing inputs and generate prolonged afterdischarges believed to enhance the t ransfer of nociceptive responses to the brain. We addressed the role of neu ronal membrane properties in shaping these responses, by recording lamina V DHNs in a slice preparation of the rat cervical spinal cord. Of 256 neuron s, 102 produced accelerating discharges in response to depolarizing current pulses, whereas the other neurons showed spike frequency adaptation. Two m echanisms mediated the firing acceleration: a slow inactivation of a K+ cur rent expressed upon activation of the neuron from hyperpolarized holding po tentials, and the expression of a regenerative plateau potential activating around resting membrane potential. The increase in firing frequency was mu ch stronger when sustained by the plateau potential (71 DHNs, 28%). A few n eurons produced adaptation and both types of acceleration, in different mem brane potential domains, showing that the firing pattern of a deep DHN is n ot a rigid characteristic, Plateau potentials could be elicited by stimulat ion of nociceptive primary afferent fibres. The bistability associated with plateau potentials permitted afterdischarges. Because plateau potentials h ad slow activation kinetics and were voltage-dependent, the neurons had non -linear input-output relationships in both the amplitude and time domains. Nociceptive primary afferent stimulation elicited intense and prolonged res ponses in plateau-generating DHNs, while brief bursts of spikes were evoked otherwise. These results indicate that in a population of deep DHNs, inten se firing and prolonged afterdischarges in response to nociceptive stimulat ion depend on non-linear intrinsic membrane properties.