Continuous opioid administration in adult rats causes differential tolerance to develop in measures of respiratory, behavioral, and analgesic effects
Authors: Twilling Christopher L. , Bell Harold J. 
Published: April 2020

Opioid drugs cause potent suppression of augmented breaths (ABs). Since many physiological effects of opioids demonstrate tolerance, in this study we sought to determine the temporal pattern and degree of any such tolerance in the suppression of ABs during 6 days of chronic opioid administration. Adult male rats (average 439.8 ± 19 g) received a continuous infusion of either saline (SAL, 0.01 ml/hr, n=5) or methadone (MET, 6 mg/kg/day, n=5) over 6 days via subcutaneously implanted osmotic pumps (Alzet, model 2ML1). Another control group received the cannabinoid receptor agonist WIN 55,212‐2 (WIN, 40 mg/kg/day, n=5), as a means of examining the potential confounding effect of altered behavioral activity on variables of interest. Animals were monitored before surgical implantation (D0), and again on days 2, 4 and 6 post‐implant (D2, D4, D6, respectively). Respiratory variables were measured using whole‐body plethysmography, pain tolerance assessed via tail flick latency (TF), and rearing behavior recorded as an index of behavioral alteration. All outcome variables of interest were analyzed using a mixed model ANOVA and appropriate post‐hoc comparisons, with monitoring day being the within‐animal factor and experimental group being the between‐animal factor considered. We found no differences in baseline measurements between experimental groups. During treatment, MET animals experienced a powerful suppression of ABs (D0 = 6.2 ± 0.383 ABs/15 min, vs. D2 = 1.4 ± 0.447, p<0.001), with tolerance developing across D4 and D6 (3.0 ± 0.523, p<0.001 and 5.0 ± 0.469, p=0.083 vs D0, respectively). No suppression of ABs was observed in either SAL or WIN animals across D0‐D6. Rearing behavior was suppressed in MET (P<0.001 for D2‐D6 vs. D0) and WIN animals across D2‐D6 (P<0.001 for D2‐D6 vs. D0), but not in SAL. However, MET animals demonstrated tolerance in rearing behavior over the 6 days of treatment, increasing back towards baseline on D6 vs. D2, P<0.001). In contrast, WIN animal rearing behavior continued to decrease across D2‐D6 vs baseline. TF latency was prolonged in MET (D0 = 2.812± 0.246 s vs 11.520± 0.389 s, 10.796± 0.152 s, & 10.739± 0.281 s on D2, D4, D6 respectively, all p<0.001), and no significant tolerance was observed across 6 days of treatment in MET animals. TF latency was prolonged in WIN only on D2 vs D0 (6.162± 0.389 s vs 3.826± 0.246, P= 0.002) to a lesser degree than in the MET animals, with no effect observed in SAL animals (p<0.001). We conclude that continuous opioid administration causes profound suppression of ABs with significant tolerance developing towards 6 days of treatment. This tolerance has a temporal pattern dissociated from behavioral effects, and markedly dissimilar to that of the analgesic effect. WIN does not cause any significant alteration of AB production despite a progressively profound suppression of behavioral activity. We propose that the mechanisms of suppression of ABs during opioid treatment follows an opioid‐receptor‐mediated pathway that is distinct from those mediating either analgesic or behavioral effects. It is therefore promising that targeted adjunct therapies to prevent the suppression of ABs during opioid treatment may be developed, that spare therapeutically desirable analgesic effects.