Figure 7.

Repeated administration of morphine alters neuronal excitability through a gut mediated mechanism. (a) Number of action potentials produced at 1×, 2× and 3× rheobase (Ph) in isolated DRG neurons from a naïve animal. Neurons were treated with colonic conditioned media (CCM) which conveyed the corresponding phenotype seen in Fig. 3. The CCM from morphine treated mice significantly increases the number of action potentials seen in a 500 ms pulse period at 3× rheobase stimulation compared to saline (morphine 3.50 AP’s ± 0.5 vs saline 1.93 AP’s ± 0.2 at 2× rheobase P = 0.04 vs saline, and morphine 5.6 AP’ s ± 0.73 vs saline 2.47 AP’s ± 0.46 at 3 × rheobase P = 0.006 vs saline), this enhanced excitability was not observed in the naïve neurons treated with CCM from NaBut treated animals (2.91 AP’s ± 0.5 at 3× rheobase, P = 0.03 vs morphine). Panels (b–d) show the expected changes in threshold potential, and relevant kinetic changes observed in rheobase (52.7 pA ± 8.64. P < 0.001 vs saline) which was not significantly shifted by CCM isolated from morphine + NaBut treated animals (112 pA ± 11.6, P = 0.02 vs morphine). Indicating that the relevant phenotypes reported in Fig. 4 are likewise conveyed to the naïve neurons by morphine treated CCM but not in the presence of NaBut treated CCM. Panel (e) shows representative traces of Morphine, and Morphine + NaBut CCM treated naïve neurons, note the similarity in AP morphology to those representative traces in Fig. 2. All data were generated from naïve neurons recorded 24 h after adding CCM fractions to the cells. Data were data analyzed by Two-way ANOVA with Bonferroni’s post-test, F (4, 68) = 5.84 P < 0.001 (N = 4 n = 10–15 per group).