GABA and Glycine Receptor-Mediated Inhibitory Synaptic Transmission onto Adult Rat Lamina II PKCγ-Interneurons: Pharmacological but Not Anatomical Specialization.

Mechanical allodynia (pain to normally innocuous tactile stimuli) is a widespread symptom of inflammatory and neuropathic pain. Spinal or medullary dorsal horn (SDH or MDH) circuits mediating tactile sensation and pain need to interact in order to evoke mechanical allodynia. PKCγ-expressing (PKCγ) interneurons and inhibitory controls within SDH/MDH inner lamina II (II) are pivotal in connecting touch and pain circuits. However, the relative contribution of GABA and glycine to PKCγ interneuron inhibition remains unknown. We characterized inhibitory inputs onto PKCγ interneurons by combining electrophysiology to record spontaneous and miniature IPSCs (sIPSCs, mIPSCs) and immunohistochemical detection of GABARα2 and GlyRα1 subunits in adult rat MDH. While GlyR-only- and GABAR-only-mediated mIPSCs/sIPSCs are predominantly recorded from PKCγ interneurons, immunohistochemistry reveals that ~80% of their inhibitory synapses possess both GABARα2 and GlyRα1. Moreover, nearly all inhibitory boutons at gephyrin-expressing synapses on these cells contain glutamate decarboxylase and are therefore GABAergic, with around half possessing the neuronal glycine transporter (GlyT2) and therefore being glycinergic. Thus, while GABA and glycine are presumably co-released and GABARs and GlyRs are present at most inhibitory synapses on PKCγ interneurons, these interneurons exhibit almost exclusively GABAR-only and GlyR-only quantal postsynaptic inhibitory currents, suggesting a pharmacological specialization of their inhibitory synapses.