Exploring ketamine's reinforcement, cue-induced reinstatement, and nucleus accumbens cFos activation in male and female long evans rats

Abstract

Ketamine (KET), a non-competitive N-methyl-d-aspartate (NMDA) receptor antagonist, has rapid onset of antidepressant effects in Treatment-Resistant Depression patients and repeated infusions are required to sustain its antidepressant properties. However, KET is an addictive drug, and so more preclinical and clinical research is needed to assess the safety of recurring treatments in both sexes. Thus, the aim of this study was to investigate the reinforcing properties of various doses of KET (0-, 0.125-, 0.25-, 0.5 mg/kg/infusion) and assess KET's cue-induced reinstatement and neuronal activation in both sexes of Long Evans rats. Neuronal activation was assessed using the protein expression of the immediate early gene cFos in the nucleus accumbens (Nac), an important brain area implicated in reward, reinforcement and reinstatement to most drug-related cues. Our findings show that KET has reinforcing effects in both male and female rats, albeit exclusively at the highest two doses (0.25 and 0.5 mg/kg/infusion). Furthermore, we noted sex differences, particularly at the highest dose of ketamine, with female rats displaying a higher rate of self-administration. Interestingly, all groups that self-administered KET reinstated to drug-cues. Following drug cue-induced reinstatement test in rats exposed to KET (0.25 mg/kg/infusion) or saline, there was higher cFos protein expression in KET-treated animals compared to saline controls, and higher cFos expression in the core compared to the shell subregions of the Nac. As for reinstatement, there were no notable sex differences reported for cFos expression in the Nac. These findings reveal some sex and dose dependent effects in KET's reinforcing properties and that KET at all doses induced similar reinstatement in both sexes. This study also demonstrated that cues associated with ketamine induce comparable neuronal activation in the Nac of both male and female rats. This work warrants further research into the potential addictive properties of KET, especially when administered at lower doses which are now being used in the clinic for treating various psychopathologies.

Keywords: Addiction; Dose-response; Relapse; Sex differences; cFos.

Fig. 1.

Mean number (±SEM) of infusions for independent groups of Long Evans male and female rats self-administering saline (A) and ketamine (0.125 (B), 0.25 (C), 0.5 (D) mg/kg) under various fixed ratios (FR) FR1, FR3 and FR5. The FR was progressively increased from 1 (sessions 1–10) to 3 (sessions 11–15) to 5 (sessions 16–20). (E).

Fig. 2.

AUC values for infusions received across FR1 (A), FR3 (B), and FR5 (C) sessions. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001 vs. saline or ketamine.

Fig. 3.

Mean number (±SEM) of active and inactive nose pokes for independent groups of Long Evans male and female rats self-administering saline (A) and ketamine (0.125 (B), 0.25 (C), 0.5 (D) mg/kg) under various fixed ratios (FR) FR1, FR3 and FR5. The FR was progressively increased from 1 (sessions 1–10) to 3 (sessions 11–15) to 5 (sessions 16–20).

Fig. 4.

AUC values for active and inactive responses made across FR1 (A,D), FR3 (B, E), and FR5 (C, F) sessions. *p < 0.05, **p < 0.01, ***p < 0.001, ****p < 0.0001 vs. saline or ketamine.

Fig. 5.

Mean number (±SEM) of active and inactive nose pokes for independent groups of Long Evans male and female rats during extinction from saline (A) and ketamine (0.125 (B), 0.25 (C), 0.5 (D) mg/kg) and their associated cues. Session 0 corresponds to the last day of acquisition, and sessions 1–10 are the extinction days.

Fig. 6.

Mean number (±SEM) of active (A) and inactive (B) nose pokes for independent groups of Long Evans male and female rats during reinstatement of cues previously associated with saline and ketamine (0.125, 0.25, 0.5 mg/kg) self-administration. EXT D10 = active responses during the last day of extinction in the absence of saline or ketamine and their associated cues; Reinstate = active responses during the one session of reinstatement in the presence of cues. *p < 0.05 Reinstate vs. EXT D10.

Fig. 7.

cFos protein expression in the nucleus accumbens core and shell subregions. (A) Comparative images of cFos protein expression in the NAc following cue-induced reinstatement to saline (SAL) and ketamine (KET, 0.25 mg/kg) (B) Quantification of cFos protein expression in the core and shell subregions of the NAc following cue-induced reinstatement to saline and ketamine (0.25 mg/kg). **p < 0.01 vs. SAL, ****p < 0.0001 Core vs. Shell.