Metformin in nucleus accumbens core reduces cue-induced cocaine seeking in male and female rats

Abstract

This study investigated the potential therapeutic effects of the FDA-approved drug metformin on cue-induced reinstatement of cocaine seeking. Metformin (dimethyl-biguanide) is a first-line treatment for type II diabetes that, among other mechanisms, is involved in the activation of adenosine monophosphate activated protein kinase (AMPK). Cocaine self-administration and extinction is associated with decreased levels of phosphorylated AMPK within the nucleus accumbens core (NAcore). Previously, it was shown that increasing AMPK activity in the NAcore decreased cue-induced reinstatement of cocaine seeking. Decreasing AMPK activity produced the opposite effect. The goal of the present study was to determine if metformin in the NAcore reduces cue-induced cocaine seeking in adult male and female Sprague Dawley rats. Rats were trained to self-administer cocaine followed by extinction prior to cue-induced reinstatement trials. Metformin microinjected in the NAcore attenuated cue-induced reinstatement in male and female rats. Importantly, metformin's effects on cocaine seeking were not due to a general depression of spontaneous locomotor activity. In female rats, metformin's effects did generalize to a reduction in cue-induced reinstatement of sucrose seeking. These data support a potential role for metformin as a pharmacotherapy for cocaine use disorder but warrant caution given the potential for metformin's effects to generalize to a natural reward in female rats.

Keywords: cocaine; metformin; nucleus accumbens; reinstatement; self-administration.

FIGURE 1

Cocaine self‐administration in male and female Sprague Dawley rats. (A) Experimental timeline for cocaine self‐administration, extinction, and reinstatement. (B) Active and inactive lever pressing in males. (C) Active and inactive lever pressing in females. (D) Daily cocaine infusions over the 10 days of self‐administration at criteria did not differ between male and female rats. E. The discrimination index remained higher in female rats compared to males during the extinction phase, *p < 0.05, n = 10–12 per sex. (F) Self‐administration active lever pressing did not vary by estrous cycle stage. (G) Cocaine infusions delivered during self‐administration did not vary by estrous cycle stage. In (F) and (G), each data point represents the rat mean for each cycle stage. (H) Active lever pressing on extinction day 1 was higher during the follicular phase (P/E = proestrus/estrus) compared to the luteal phase (M/D = metestrus/diestrus), *p < 0.05, n = 3–4 per phase. All data expressed as mean ± SEM

FIGURE 2

Effects of metformin on cue‐induced reinstatement of cocaine seeking. (A) Number of active and inactive lever presses during extinction (EXT) or induced by cocaine‐associated cues during reinstatement in male rats, n = 10. **p < 0.01 comparing saline (SAL) to extinction (MET = metformin). (B) Number of active and inactive lever presses during extinction or induced by cocaine‐associated cues during reinstatement in female rats, n = 12. ^**** p < 0.0001 comparing saline to extinction; **p < 0.01 comparing metformin to extinction or saline. (C) Cumulative active lever pressing in 5‐min bins during cue‐induced reinstatement in male rats. *p < 0.05 comparing saline to metformin. (D) Cumulative active lever pressing in 5‐min bins during cue‐induced reinstatement in female rats.*p < 0.01 comparing saline to metformin. (E) Cumulative inactive lever pressing in 5‐min bins during cue‐induced reinstatement in male rats. (F) Cumulative inactive lever pressing in 5‐min bins during cue‐induced reinstatement in female rats. All data expressed as mean ± SEM

FIGURE 3

Locomotor activity. (A) Locomotor activity in male rats immediately after microinjection of saline or metformin, n = 5 per treatment. (B) Locomotor activity in female rats immediately after microinjection of saline or metformin, n = 5–7 per treatment. (C) Locomotor activity in male rats following 1‐h pretreatment time after saline or metformin microinjection, n = 7 per treatment. All data expressed as mean ± SEM

FIGURE 4

Sucrose self‐administration in male and female Sprague Dawley rats. (A) Experimental timeline for sucrose self‐administration, extinction, and reinstatement. (B) Active and inactive lever pressing in males. (C) Active and inactive lever pressing in females (mean ± SEM). (D) Daily sucrose pellets earned over the 10 days of self‐administration was higher in female rats compared to male rats, **p < 0.05, n = 7–10. (E) The discrimination index was higher in female rats compared to males, ***p < 0.001, n = 7–10 per sex. (F) Self‐administration active lever pressing did not vary by estrous cycle stage. (G) Sucrose pellets delivered during self‐administration did not vary by estrous cycle stage. In (F) and (G), each data point represents the rat mean for each cycle stage. (H) Active lever pressing on extinction day 1 did not differ between groups, n = 5–7 per phase. P/E = proestrus/estrus and M/D = metestrus/diestrus. All data expressed as mean ± SEM

FIGURE 5

Effects of metformin on cue‐induced reinstatement of sucrose seeking. (A) Number of active and inactive lever presses during extinction or induced by sucrose‐associated cues during reinstatement in male rats, n = 10. *p < 0.05 comparing extinction (EXT) to saline (SAL) and metformin (MET). (B) Number of active and inactive lever presses during extinction or induced by sucrose‐associated cues during reinstatement in female rats, n = 7. *p < 0.05 comparing saline to extinction. (C) Cumulative active lever pressing in 5‐min bins during cue‐induced reinstatement in male rats. (D) Cumulative active lever pressing in 5‐min bins during cue‐induced reinstatement in female rats. *p < 0.05 comparing saline to metformin. (E) Cumulative inactive lever pressing in 5‐min bins during cue‐induced reinstatement in male rats. (F) Cumulative inactive lever pressing in 5‐min bins during cue‐induced reinstatement in female rats. All data expressed as mean ± SEM