Inhibitory neurones of the spinal substantia gelatinosa mediate interaction of signals from primary afferents.

The spinal substantia gelatinosa (SG; lamina II) is a major synaptic zone for unmyelinated (C) primary afferents. Whereas a substantial proportion of intrinsic SG neurones are GABAergic inhibitory, their relationship to afferent activity is unknown. In spinal cord slices from a transgenic mouse in which certain GABAergic lamina II neurones are labelled with green fluorescent protein (GFP), we compared primary afferent input with local efferent connections made by inhibitory SG neurones. Simultaneous whole-cell recordings from characterized neurones establish that inhibitory SG neurones receive monosynaptic input from a subset of unmyelinated primary afferents and connect to other lamina II cells that have input from a different set of afferents, permitting interactions between distinctive afferent messages. Certain lamina II inhibitory cells were found to connect to one another by reciprocal links. Inhibitory lamina II connections appear arranged to modulate activity from different sets of peripheral unmyelinated fibres through neural circuitry that includes disinhibition.