The substantia gelatinosa (SG) is a vital integrative centre for sensory information arriving from peripheral nerves. Despite receiving both noxious and innocuous input, the identity of distinct cell types within the SG remains to be fully established (Gobel 1978; Light et al. 1979). In order to further the understanding of information processing within the adult rat SG, we have attempted to define neuronal cell types based upon their characteristic electrophysiological properties.

Intrinsic cell properties were examined using parasagittal spinal cord slices (150-300 µm) prepared from terminally anaesthetised male Wistar rats (240-400g). Slices were superfused (2.5 ml min^-1, 35 °C) with artificial cerebrospinal fluid containing (mM): 125 NaCl, 25 NaHCO₃, 10 glucose, 2.5 KCl, 1.25 NaH₂PO₄, 2 CaCl₂, 1 MgCl₂, and bubbled with 95 % O₂-5 % CO₂. Whole-cell patch-clamp recordings were made from visually identified SG neurons, using electrodes filled with (mM): 120 potassium gluconate, 10 NaCl, 2 MgCl₂, 0.5 K₂EGTA, 10 Hepes, 4 Na₂ATP, and 0.3 Na₂GTP, pH 7.2. Data are presented as mean ± S.E.M.; statistical significance was confirmed with Student’s unpaired t test.

Under current-clamp recording conditions, current-voltage relationships obtained by the injection of negative and positive current steps revealed differential expression of active conductances in SG neurons in the presence of 1 µM TTX. From these data we classified neurons into clusters based upon the expression patterns of these conductances. Cluster 1 was characterised by a hyperpolarisation-activated cation conductance (I_h, n = 69), sensitive to 1 mM Cs⁺ (n = 8). A further 49 Cluster 1 neurons expressed a T-type Ca²⁺ conductance, sensitive to 100 µM Ni⁺ (n = 4). Cluster 2 neurons expressed anomalous inwardly rectifying K⁺ conductances (I_an, n = 49), sensitive to 100 µM Ba²⁺ (n = 10); and Cluster 3 neurons expressed both I_h and I_an (n = 25). Cluster 4 neurons were characterised by the presence of an A-type K⁺ conductance (n = 5), sensitive to 2 mM 4-aminopyridine (n = 3). Cluster 5 neurons expressed a T-type conductance (n = 9) and Cluster 6 expressed both I_an and an A-type K⁺ conductance (n = 21). These conductances were absent in all Cluster 7 neurons (n = 48). Significant cluster-specific variations were observed throughout cell properties, including resting membrane potential (Cluster 3: -59 ± 1.2 mV, Cluster 4:-67 ± 5 mV (P < 0.01)), firing threshold (Cluster 5: -35 ± 2.8 mV, Cluster 6: -24 ± 1.9 mV (P < 0.01)) and input resistance (Cluster 1: 247 ± 17.7 MV, Cluster 2: 432 ± 26.6 MV (P < 0.01)).

In conclusion, we have demonstrated the presence of electrophysiological phenotypes within the SG. How they differentially integrate afferent information remains to be defined.