Inoculation with herpes simplex virus that encodes μ-opioid receptor (hsvMOR), herpes simplex virus that encodes preproenkephalin (hsvPPE), or hsvMOR + preproenkephalin (PPE) increases the number of enkephalin immunoreactivity (ENK-ir) and colabeled μ-opioid receptor immunoreactivity (MOR-ir) and green fluorescent protein–positive dorsal root ganglion cells. (A) Quantification of Enk-ir–positive dorsal root ganglia (DRGs) on day 16 after viral inoculation. There is a significant increase in Enk-ir in hsvPPE and hsvMOR + PPE treatment groups compared to sham and control virus (hsvCON, two-way ANOVA, factors: group × neuron size, F[12, 60] = 7.5, P < 0.001). (Bottom) Histology images of enkephalin-positive dorsal root ganglion with control virus (left) and hsvPPE (right). (B) Quantification of MOR-ir–positive DRGs on day 16 after viral inoculation. (Bottom) Histology images of MOR–positive dorsal root ganglion with control virus (left) and hsvMOR (right). There is a significant increase in MOR-ir in large and medium sized DRGs of hsvMOR, hsvPPE, and hsvMOR + PPE treatment groups compared to sham and control virus (hsvCON, two-way ANOVA, neuron size, F[12, 60] = 10.5, P < 0.001). Bonferroni post hoc test was used for multiple comparisons. #significant at P < 0.05 versus sham; ##significant at P < 0.01 versus sham; ###significant at P < 0.001 versus sham; *significant at P < 0.05 versus control virus-treated (hsvCON); **significant at P < 0.01 versus control virus-treated (hsvCON); ***significant at P < 0.001 versus control virus-treated (hsvCON). CON = control; Enk = enkephalin; hsv = herpes simplex virus; ir = immunoreactivity; MOR = µ-opioid receptor.