Lamina-specific membrane and discharge properties of rat spinal dorsal horn neurones in vitro

Membrane and discharge properties determine the input-output relationship of neurones and are therefore of paramount importance for the functions of neural circuits. Here, we have tested the hypothesis that neurones in different laminae of the spinal dorsal horn differ in their electro-physiological properties. Whole-cell patch-clamp recordings from dorsal horn neurones in a rat transverse spinal cord slice preparation were used to record active and passive membrane properties. Neurones from superficial dorsal horn laminae had higher membrane resistances and broader action potentials than deep dorsal horn neurones. Action potential thresholds were highest in lamina II neurones, representing low membrane excitability. Five types of firing patterns were identified in response to depolarising current injections. Tonic-firing neurones discharged action potentials at regular intervals throughout the current pulse. Delayed-firing neurones showed a delayed onset of firing in response to current injections that was due to activation of a transient voltage-dependent outward current, presumably an A-current. Another group of neurones fired a short initial burst of action potentials. Single-spiking neurones discharged only one action potential at the onset of a depolarising pulse. Phasic-bursting neurones showed irregular bursts of action potentials. Firing patterns were unequally distributed among laminae. Tonic-firing neurones were numerous in lamina I and deeper laminae but were not found in lamina II Delayed-firing neurones were encountered in laminae I and II but not in deeper laminae. Most of the neurones showing an initial burst were found in lamina II. These differences in membrane and discharge properties probably contribute to lamina-specific processing of sensory, including nociceptive, information.