Ionic Basis of Tonic Firing in Spinal Substantia Gelatinosa Neurons of Rat

Abstract

Ionic conductances underlying excitability in tonically firing neurons (TFNs) from substantia gelatinosa (SG) were studied by the patch-clamp method in rat spinal cord slices. Ca(2+)-dependent K(+) (K(CA)) conductance sensitive to apamin was found to prolong the interspike intervals and stabilize firing evoked by a sustained membrane depolarization. Suppression of Ca(2+) and K(CA) currents, however, did not abolish the basic pattern of tonic firing, indicating that it was generated by voltage-gated Na(+) and K(+) currents. Na(+) and K(+) channels were further analyzed in somatic nucleated patches. Na(+) channels exhibited fast activation and inactivation kinetics and followed two-exponential time course of recovery from inactivation. The major K(+) current was carried through tetraethylammonium (TEA)-sensitive rapidly activating delayed-rectifier (K(DR)) channels with a slow inactivation. The TEA-insensitive transient A-type K(+) (K(A)) current was very small in patches and was strongly inactivated at resting potential. Block of K(DR) rather than K(A) conductance by 1 mM TEA lowered the frequency and stability of firing. Intracellular staining with biocytin revealed at least three morphological groups of TFNs. Finally, on the basis of present data, we created a model of TFN and showed that Na(+) and K(DR) currents are sufficient to generate a basic pattern of tonic firing. It is concluded that the balanced contribution of all ionic conductances described here is important for generation and modulation of tonic firing in SG neurons.

Physiological types of substantia gelatinosa neurons in the rat spinal cord

“…All the studied SG neurons had passive electrical properties close to those reported earlier (membrane potential -67 ± 2.1 mV and input resistance 1.1 ± ± 0.2 GΩ, on average) [14][15][16]. These parameters were similar in neurons of different types (see below) and therefore were pooled.…”

“…Tonic neurons (n = = 26, 41.2%) generated repetitive APs throughout the entire duration of the stimulus; the frequency of APs increased with increasing stimulation intensity (A, 1). The maximum frequency reached >200 sec -1 , which was about two times greater than that in our previous reports [14,15]. This can be explained by the high temperature of recordings (32 o vs 22 o C).…”

“…Possible co-expression of T-type calcium currents in DFNs was tested in eight cells by replacing Cs + for K + in the pipette solution. Classification of a neuron was performed within the initial few seconds after rupturing of the membrane, as was reported earlier [14]. After 5-10 min of perfusion of the cell, the outward current decreased 15-20 times, but T-type calcium currents were not observed.…”

“…In practice, a physiological approach might be more useful. Thus, most authors have concluded that there are only three main types of SG neurons that differ from each other in their firing responses evoked by long membrane depolarization, namely tonic, adapting, and delayedfiring neurons (DFNs) [8,[10][11][12][13][14][15][16]. At the same time, the reported ratios of these neuronal groups varied significantly, which can be partially related to dissimilar experimental conditions and, in particular, to the way the electrical signals are recorded (i.e., a "classical" sharp microelectrode or a current-clamp mode of a patch-clamp recording).…”

“…First, a patch-clamp/current-clamp configuration was used, and second, a large part of the recordings was obtained using a physiological solution with low Ca 2+ [14][15][16]. In this study, we tried to record under more adequate conditions, and the existence of three neuronal types was confirmed.…”