INTRINSIC-PROPERTIES OF DEEP DORSAL HORN NEURONS IN THE L(6)-S-1 SPINAL-CORD OF THE INTACT RAT

1. Stable intracellular recordings were obtained from neurons (n = 62) in the L(6)-S-1 deep dorsal horn of the spinal cord in pentobarbital- sodium-anesthetized, intact rats (n = 26). All neurons responded to na tural mechanical stimuli and/or electrical stimulation of peripheral a fferents. 2. Intracellular penetrations were maintained for 30 min-2 h . Action potentials occurred spontaneously in most neurons (n = 50) an d could be evoked in the remainder (n = 12) by depolarizing current pa ssage. Mean resting membrane potential was -60.9 mV, mean action poten tial height amplitude was 75.2 mV, mean half-width of the action poten tials was 0.33 ms, mean input resistance was 38 M Ohm, and mean time c onstant was 9.1 ms. 3. Action potentials were followed by afterpotenti als made up of at least three components: a fast afterhyperpolarizatio n (fAHP), a slow afterhyperpolarization (sAHP), and an afterdepolariza tion (ADP). Most neurons (n = 40) exhibited all three afterpotentials, although some displayed only a fAHP and an ADP (n = 10) or a fAHP and a sAHP (n = 12). The durations and magnitudes of the afterpotentials varied widely among neurons. 4. Steady-state current-voltage relations were investigated in 14 neurons with depolarizing and hyperpolarizing current pulses. Of these 14 neurons, 5 exhibited inward rectification , 3 had outward rectification, and the remaining 6 showed a predominan tly linear change of membrane potential to current injection. In addit ion, several neurons (n = 9) exhibited a postinhibitory rebound that w as sometimes (n = 4) accompanied by a ''sag'' in voltage during the pr eceding hyperpolarizing current step. 5. Four patterns of spike freque ncy adaptation occurred during step depolarizing current passage. The firing of most neurons gradually decreased with a simple, approximatel y exponential time course (n = 21), in some neurons it decreased with both a fast and a slow time course (n = 8), in several it incremented in rate (n = 3), and one neuron showed a complex combination of multi pie decrementing and incrementing adaptations. Time constants, magnitu de of adaptation, and the slopes of the steady-state current-voltage r elation varied widely. 6. Oscillations in membrane potential and firin g rate occurred in three neurons. The oscillations arose from endogeno us mechanisms in at least one neuron because manipulation of membrane potentials altered the frequency of oscillation; a depolarizing curren t increased the period of oscillation and eventually produced tonic fi ring, and a hyperpolarizing current increased the frequency of oscilla tion and eventually terminated firing. 7. The results demonstrate that neurons in the L(6)-S-1 region of the dorsal horn exhibit a diversity of cellular mechanisms that may significantly modulate normal somatos ensory and visceral input.