Enhanced Cocaine-Associated Contextual Learning in Female H/Rouen Mice Selectively Bred for Depressive-Like Behaviors: Molecular and Neuronal Correlates

Abstract

Background: Major depression has multiple comorbidities, in particular drug use disorders, which often lead to more severe and difficult-to-treat illnesses. However, the mechanisms linking these comorbidities remain largely unknown.

Methods: We investigated how a depressive-like phenotype modulates cocaine-related behaviors using a genetic model of depression: the Helpless H/Rouen (H) mouse. We selected the H mouse line for its long immobility duration in the tail suspension test when compared to non-helpless (NH) and intermediate (I) mice. Since numerous studies revealed important sex differences in drug addiction and depression, we conducted behavioral experiments in both sexes.

Results: All mice, regardless of phenotype or sex, developed a similar behavioral sensitization after 5 daily cocaine injections (10 mg/kg). Male H and NH mice exhibited similar cocaine-induced conditioned place preference scores that were only slightly higher than in I mice, whereas female H mice strikingly accrued much higher preferences for the cocaine-associated context than those of I and NH mice. Moreover, female H mice acquired cocaine-associated context learning much faster than I and NH mice, a facilitating effect that was associated to a rapid increase in striatal and accumbal brain-derived neurotrophic factor levels (BDNF; up to 35% 24 h after cocaine conditioning). Finally, when re-exposed to the previously cocaine-associated context, female H mice displayed greater Fos activation in the cingulate cortex, nucleus accumbens, and basolateral amygdala.

Conclusions: Our data indicate that neurobiological mechanisms such as alterations in associative learning, striato-accumbal BDNF expression, and limbic-cortico-striatal circuit reactivity could mediate enhanced cocaine vulnerability in female depressive-like mice.

Keywords: BDNF; H/Rouen mice; cocaine; depression; sex differences.

Figure 1.

Psychomotor stimulant effects of acute and repeated cocaine injections in male and female mice of the different mouse lines. Total locomotion over 45 min after an acute injection of saline or cocaine at different doses (1, 3, 10, 20, or 30 mg/kg i.p.) are plotted in (A) male and (B) female mice of the three mouse lines: non-helpless (NH), intermediate (I), and helpless (H; n = 8–12). Expression of cocaine behavioral sensitization is depicted as the mean distance traveled over 15 min after a challenge injection of cocaine (1 mg/kg i.p.) in mice that have been saline- or cocaine-pretreated; measurement was performed after a drug-free period of 7 days in (C) male and (D) female mice of the three mouse lines (n = 8–9). (E) Percentage changes in locomotor activity after the cocaine injection challenge (1 mg/kg i.p.) in cocaine-treated mice compared to respective saline control mice (n = 8–9). Data are expressed as mean + standard error of the mean (SEM). *Significant differences from respective saline control (p<0.05). #Significant differences from I mice (p<0.05).

Figure 2.

Cocaine-induced conditioned place preference (CPP) in the different mouse lines. (A) Four-cocaine and four-saline conditioning sessions were performed and mice were tested for CPP 1 day after the last conditioning session (day 8), while drug free. (B) Expression of cocaine-induced CPP, as measured by preference scores, was investigated during a 15-min CPP session (in s + standard error of the mean [SEM]) both in male and female mice of the 3 mouse lines (n = 7–14). (C) Eight cocaine and eight saline conditioning sessions were performed and mice were tested for CPP 1 day after the last conditioning session while in a drug-free state (drug-free day 12), and 2 days later under cocaine-primed condition (cocaine-priming day 13; a 10 mg/kg cocaine injection was given immediately before the CPP test). (D) Preference scores observed during a 15-min CPP session (in s + SEM) in females of the three mouse lines under drug-free and cocaine-primed conditions (n = 4–9). Data are expressed as mean + SEM. *Significant differences from respective saline control mice (p<0.05). #Significant differences from I mice (p<0.05). Δ Significant differences between male and female mice within the same mouse line (p<0.05). H, helpless; I, intermediate; NH, non-helpless.

Figure 3.

Effect of cocaine conditioning on striatal and accumbal brain-derived neurotrophic factor (BDNF) levels. (A) Female mice were subjected to four-cocaine and four-saline conditioning sessions but did not perform the conditioned place preference test. These mice were killed 24 h after the last conditioning session (day 8) for BDNF Western blotting (WB). (B) Schematic representation of the location of punches for BDNF protein level quantification (Bregma +1.18/+0.98, adapted from the Paxinos and Franklin [2001] mouse brain atlas). (C) Representative immunoblots showing BDNF and β-actin levels in saline- and cocaine-conditioned depressive-like H mice. (D) BDNF protein levels (normalized to β-actin for sample loading control) in cocaine- and saline-conditioned mice of the three mouse lines (n = 8–12). Data are expressed as mean + standard error of the mean. *Significant differences from respective saline control (p<0.001). #Significant differences from I mice (p<0.001). Acb, nucleus accumbens; dStr, dorsal striatum; H, helpless; I, intermediate; NH, non-helpless.

Figure 4.

Fos-expressing cells in the nucleus accumbens of female mice during retrieval of cocaine-associated contextual memory. (A) Four-cocaine and four-saline conditioning sessions were performed and female mice were tested for conditioned place preference (CPP) 5 days later (day 12) while in a drug-free state. Mice were perfused 2 h after the CPP test and the brain used for Fos immunohistochemistry. (B) Expression of cocaine-induced CPP, as measured by preference score, was investigated during a 15-min CPP session (in s + standard error of the mean) in female mice of the three mouse lines (n = 7–14). (C) Representative photomicrographs of Fos-positive nuclei on neutral red-counterstained coronal sections in the shell of the nucleus accumbens (AcbSh) of saline- and cocaine-conditioned H and I mice perfused 2 h after the CPP test. *Significant differences from respective saline control (p<0.05). #Significant differences from I mice (p<0.05). Acb, nucleus accumbens; NaCl, (saline); COC, (cocaine); H, helpless; I, intermediate; NH, non-helpless.

Figure 5.

Neuronal activation profile elicited by retrieval of cocaine-associated contextual memory in female mice. (A) The diagrams adapted from the Paxinos and Franklin (2001) mouse brain atlas indicate the placement of grids for counting. Bregma +1.98/+1.70: Cg1, cingulate cortex 1; PrL, prelimbic cortex; IL, infralimbic cortex. Bregma +1.18/+0.98: AcbC, core of the nucleus accumbens; AcbSh, shell of the nucleus accumbens; LSV, ventral part of the lateral septal nucleus. Bregma −1.34/-1.94; DG, dentate gyrus; CA1, CA1 region of the dorsal hippocampus; CA3, CA3 region of the dorsal hippocampus; BLA, basolateral amygdala. (B) Effect of 4 days of cocaine conditioning on Fos expression 2 h after a 15-min conditioned place preference test performed 5 days after the last cocaine conditioning session (Figure 4A; n = 6–9). Percentage of Fos-expressing cells/mm² in cocaine-conditioned female of the three mouse lines compared to corresponding saline-conditioned mice. The dotted lines on the graphs represent the mean for each saline mouse line. Data are expressed as mean + standard error of the mean. Significant differences from respective saline control: *p<0.05, **p<0.01 and ***p<0.001. H, helpless; I, intermediate; NH, non-helpless.