Physiological, neurochemical and morphological properties of a subgroup of GABAergic spinal lamina II neurones identified by expression of green fluorescent protein in mice.

The processing of sensory, including nociceptive, information in spinal dorsal horn is critically modulated by spinal GABAergic neurones. For example, blockade of spinal GABA(A) receptors leads to pain evoked by normally innocuous tactile stimulation (tactile allodynia) in rats. GABAergic dorsal horn neurones have been classified neurochemically and morphologically, but little is known about their physiological properties. We used a transgenic mouse strain coexpressing enhanced green fluorescent protein (EGFP) and the GABA-synthesizing enzyme GAD67 to investigate the properties of a subgroup of GABAergic neurones. Immunohistochemistry showed that EGFP-expressing neurones accounted for about one-third of the GABAergic neurones in lamina II of the spinal dorsal horn. They constituted a neurochemically rather heterogeneous group where 27% of the neurones coexpressed glycine, 23% coexpressed parvalbumin and 14% coexpressed neuronal nitric oxide synthase (nNOS). We found almost no expression of protein kinase Cgamma (PKCgamma) in EGFP-labelled neurones but a high costaining with PKCbetaII (78%). The whole-cell patch-clamp technique was used to intracellularly label and physiologically characterize EGFP- and non-EGFP-expressing lamina II neurones in spinal cord slices. Sixty-two per cent of the EGFP-labelled neurones were islet cells while the morphology of non-EGFP-labelled neurones was more variable. When stimulated by rectangular current injections, EGFP-expressing neurones typically exhibited an initial bursting firing pattern while non-EGFP-expressing neurones were either of the gap or the delayed firing type. EGFP-expressing neurones received a greater proportion of monosynaptic input from the dorsal root, especially from primary afferent C-fibres. In conclusion, EGFP expression defined a substantial but, with respect to the measured parameters, rather inhomogeneous subgroup of GABAergic neurones in spinal lamina II. These results provide a base to elucidate the functional roles of this subgroup of GABAergic lamina II neurones, e.g. for nociception.