Diverse immunocytochemical expression of opioid receptors in electrophysiologically defined cells of rat dorsal root ganglia

The expression of opioid receptors in the DRG cell types was diverse. Type 1 (25-30 μm cell diameter) and type 9 (35-45 μm) expressed MOR-IR, but were negative for DOR-IR and KOR-IR. Type 2 (25-30 μm) co-expressed DOR-IR and MOR-IR, but did not express KOR-IR. Type 3 (15-20 μm), the non-nociceptive cell type, was not immunoreactive. Type 4 (35-45 μm), type 6 (35-45 μm), and type 7 (15-20 μm) expressed all three opioid receptors. Type 5 (35-45 μm) and type 8 (35-45 μm), co-expressed KOR-IR and MOR-IR, but did not express DOR-IR. The co-expression of opioid receptors in some of the cell types suggests that these sensory afferents might contain heteromeric opioid receptors. Additionally, the diverse expression patterns of opioid receptors between cell types and the consistency of these patterns maintained within each cell type provides further evidence of distinct functional properties of DRG nociceptors.