Characterisation of neurotensin-expressing interneurons in the mouse spinal dorsal horn.

The spinal dorsal horn contains numerous excitatory interneurons, which can be assigned to functional classes based on morphological, electrophysiological and neurochemical criteria. One population consists of neurons that express neurotensin, and these belong to a larger group defined by the presence of protein kinase Cγ (PKCγ). It has been proposed that PKCγ neurons form part of a circuit that can convey low-threshold mechanoreceptive information to nociceptive projection neurons in lamina I, forming a pathway that could underlie mechanical allodynia in pathological pain states. However, despite their potential importance, relatively little is known about the properties of the neurotensin-expressing cells. Here we have used a neurotensin-Cre line, together with intraspinal injection of AAVs coding for Cre-dependent constructs, to characterise the morphological and electrophysiological properties of these cells. Reconstruction of their dendritic trees revealed that they were morphologically diverse, although many could be assigned to a class known as central cells. All cells examined received synaptic contacts from putative A- and C-low-threshold mechanoreceptors (identified by expression of VGLUT1 and VGLUT3, respectively). However, these only accounted for a minority of their excitatory synapses. Around 40% of their synapses were from VGLUT2-immunoreactive boutons, which are likely to have originated mainly from local excitatory interneurons. Electrophysiological analysis revealed similarities to, and differences, from other neurochemically-defined excitatory interneuron populations. Our findings are compatible with the proposed role of neurotensin cells in mechanical allodynia, but suggest additional functions for these cells.