Placebo-induced analgesia in an operant pain model in rats

Abstract

Analgesia is particularly susceptible to placebo responses. Recent studies in humans have provided important insights into the neurobiology underlying placebo-induced analgesia. However, human studies provide incomplete mechanistic explanations of placebo analgesia because of limited capacity to use cellular, molecular, and genetic manipulations. To address this shortcoming, this article describes the development of a rat model of conditioned analgesia in an operant pain assay. Specifically, rats were conditioned to associate a placebo manipulation with the analgesic effect of 1mg/kg morphine (subcutaneously) on facial thermal pain. We found that conditioned (placebo) responding bore 3 of the hallmarks of placebo-induced analgesia: (1) strong interanimal variability in the response, (2) suppression by the opiate antagonist naloxone (5mg/kg subcutaneously), and (3) a positive predictive relationship between the unconditioned analgesic effect and the conditioned (placebo) effect. Because of the operant nature of the assay and the use of only a mild noxious thermal stimulus, we suggest that these results provide evidence of placebo-induced analgesia in a preclinical model that utilizes an affective behavioral end point. This finding may provide opportunities for invasive preclinical studies allowing greater understanding of placebo-induced analgesia, thus paving the way for avenues to harness its benefits.

Figure 1

(A) Schematic representation of the behavioral paradigm used. Boxes represent exposure to the operant training apparatus wherein rats had access to palatable reward contingent upon contacting their muzzles on a heated thermode stimulus. Variations of PBS vehicle, morphine (MOR, 1mg /kg), or naloxone (NLX, 5 mg/kg) were administered 30 mins before the final three sessions. (B) Summary of major groups in the current study.

Figure 2

(A) Mean operant licking responses for palatable reward under various drug treatment conditions as described in fig. 1B. Dashed line indicates responding in the PBS→PBS-treated group during COND1 as a point of reference. Data is expressed as responses normalized to mean responding during BASE48 as depicted in fig. 1A within each treatment groups. Data is expressed as mean ± S.E.M. Group sizes (n): PBS→PBS, 8; MOR→PBS, 19; MOR→NLX, 10. (B) p values for t-test, Mann Whitney-U and F-test comparisons for responses at all time points shown in A. p values (t-test or F-test) less than 0.05 are bolded.

Figure 3

Operant licking responses for palatable reward in individual rats during the TEST session as described in fig. 1A. Solid horizontal line indicates mean responding in PBS→PBS-treated rats. Dashed horizontal lines indicate increasing standard deviations (SD) away from the mean responding for the PBS→PBS-treated group.

Figure 4

Correlative relationships between operant lick responding during COND2 and TEST for the three main treatment groups. Each point represents an individual rat. Solid lines indicate linear regressions. Additional analysis of correlational relationships is shown in table 2. p values (Pearson's test) less than 0.05 are bolded.