Subpopulations of GABAergic and non‐GABAergic rat dorsal horn neurons express Ca 2+ ‐permeable AMPA receptors

Subpopulations of α‐amino‐3‐hydroxy‐5‐methylisoxazole‐4‐propionic acid (AMPA) receptors that are either permeable or impermeable to Ca 2+ are expressed on dorsal horn neurons in culture. While both mediate synaptic transmission, the Ca 2+ ‐permeable AMPA receptors provide a Ca 2+ signal that may result in a transient change in synaptic strength [Gu, J.G., Albuquerque, C., Lee, C.J. & MacDermott, A.B. (1996) Nature, 381, 793]. To appreciate the relevance of these receptors to dorsal horn physiology, we have investigated whether they show selective expression in identified subpopulations of dorsal horn neurons. Expression of Ca 2+ ‐permeable AMPA receptors was assayed using the kainate‐induced cobalt loading technique first developed by Pruss et al . [Pruss, R.M., Akeson, R.L., Racke, M.M. & Wilburn, J.L. (1991) Neuron, 7, 509]. Subpopulations of dorsal horn neurons were identified using immunocytochemistry for γ‐aminobutyric acid (GABA), glycine, substance P receptor (NK1 receptor) and the Ca 2+ ‐binding proteins, calretinin and calbindin D28K. We demonstrate that, in dorsal horn neurons in culture, kainate‐induced cobalt uptake is selectively mediated by Ca 2+ ‐permeable AMPA receptors, and that a majority of GABA and NK1 receptor‐expressing neurons express Ca 2+ ‐permeable AMPA receptors. GABAergic dorsal horn neurons are important in local inhibition as well as in the regulation of transmitter release from primary afferent terminals. NK1 receptor‐expressing dorsal horn neurons include many of the projection neurons in the nociceptive spino‐thalamic pathway. Thus, we have identified two populations of dorsal horn neurons representing important components of dorsal horn function that express Ca 2+ ‐permeable AMPA receptors. Furthermore, we show that several subpopulations of putative excitatory interneurons defined by calretinin and calbindin expression do not express Ca 2+ ‐permeable AMPA receptors.