CONVERGENCE OF NEURONAL INPUT FROM EXCITATORY AND INHIBITORY POPULATIONS OF ENKEPHALINERG CELLS IN THE SPINAL DORSAL HORN

The modulatory system of the spinal dorsal horn (SDH) that includes local enkephalinergic (ENK) neurons is characterized by a complex network where excitatory and inhibitory neurons often converge on to the same postsynaptic cell. The concurrent activity may therefore result in signal masking, and makes it difficult to assess the net effect of the modulatory input. While the descending control and local targets of dorsal horn enkephalinergic neurons in the SDH is relatively well characterized, we have limited information of their interactions within the SDH. Our aim in the present work was to describe the interconnection of enkephalinergic neurons and to determine how putative convergent input from the inhibitory and excitatory ENK neuron populations influence the target SDH neurons.
In order to manipulate ENK neurons selectively we used 4-10 week old proenkephalin (PENK) / channelrhodopsin2 hybrid mice. Intact spinal lumbar enlargements with attached dorsal roots (L4/L5) were removed and dorsal horn neurons visualized by the oblique IR-LED illumination technique. Single and double whole-cell patch-clamp recordings were performed. Neurons were characterized based on their firing pattern, primary afferent input and post-hoc morphological identification.
Upon repeated short light-pulse stimulation recorded SDH neurons with ENK input showed either IPSCs (58,3%) or EPSCs (41,7%). Interestingly pharmacological blockade of the dominant PSC unmasked PSCs with the opposite effect in all of the neurons with ENK input.
We conclude that neurons in the SDH circuitry are characterized by a high degree of convergence of excitatory and inhibitory ENK neurons.