Spike Frequency Adaptation and Signaling Properties of Identified Neurons in Rodent Deep Spinal Dorsal Horn

Abstract

Using whole cell recordings, I analyzed the intrinsic discharge properties for 285 neurons in Rexed's laminae III–V of isolated hamster spinal cord preparations. Neurons were characterized by their responses to step-wise and ramp-hold depolarizing current applied through the recording pipettes. Tonic cells (133/285; 47%) fired repetitively during step-wise current application. Firing decayed linearly (–0.14 to –4.3 imp · s–1 · s–1) or was bimodal, with an initial exponential phase (τ ≈ 450 ms) followed by a linear decline (–0.02 to –6.3 imp · s–1 · s–1); discharge frequency was unrelated to current trajectory. Phasic-firing cells (108/285; 38%) responded with a burst discharge having an initial rapid, exponential decrease (τ ≈ 30 ms) and subsequent linear decline (–1 to –78 imp · s–1 · s–1). Phasic cells were activated preferentially by fast current ramps (slope, 70 pA/s–2.2 nA/s) with the number and frequency of impulses increasing with current slope. Delayed-firing cells (44/285; 15%), responded to current steps with an accelerating firing following a substantial latent period (0.5–4 s) and discharged during current ramps with slopes less than ≈100 pA/s. Intracellular staining revealed a significant association between electrophysiological profile and neuronal morphology. A majority of presumed projection cells (22/30; 73%) exhibited tonic firing to step-wise activation. The preponderance of phasic and delayed firing cells, 93% (42/45) and 71% (12/17), respectively, were interneurons with local or intersegmental terminations. Differential sensitivity to static and time-varying components of membrane current suggest differences in neuronal signaling properties that may have important implications for integration of mechanosensory information in the deep spinal dorsal horn.