Sex differences in nicotine-enhanced Pavlovian conditioned approach in rats

Abstract

Background: Nicotine exposure enhances Pavlovian conditioned approach (PCA), or the learned approach to reward-predictive cues. While females show elevated approach to conditioned stimuli compared to males, potentially indicating heightened addiction vulnerability, it is unknown how sex may interact with nicotine to influence approach behavior. Additionally, brain-derived neurotrophic factor (BDNF) levels can be altered significantly after repeated nicotine exposure, suggesting a potential mechanism contributing to nicotine-induced behavioral phenotypes. The present study investigated the role of sex on nicotine-induced changes to stimulus-response behavior and associated BDNF protein levels.

Methods: Male and female rats were exposed to nicotine (0.4 mg/kg, subcutaneously) or saline 15 min prior to each PCA session. PCA training consisted of 29 sessions of 15 trials, in which a 30-s cue presentation ended concurrently with a sucrose reward (20% w/v in water, 100 μL), and a 120-s variable intertrial interval occurred between trials. Approach behavior to the cue and reward receptacle was recorded. Preference toward the reward receptacle indicated a goal-tracking phenotype, and preference toward the cue indicated a sign-tracking phenotype, demonstrating that the cue had gained incentive salience. Twenty-four hours after the last PCA session, brain tissue was collected and BDNF levels were measured in the basolateral amygdala, orbitofrontal cortex, and nucleus accumbens using Western blot analysis.

Results: Nicotine exposure enhanced both sign- and goal-tracking conditioned approach, and females showed elevated sign-tracking compared to males. There were no sex-by-drug interactions on conditioned approach. Day-to-day variability in conditioned approach was similar between sexes. In contrast to prior studies, neither repeated exposure to nicotine nor sex significantly affected BDNF expression.

Conclusions: Drug-naïve females exhibited heightened sign-tracking compared to males, and nicotine enhanced conditioned approach similarly in males and females. Further, non-significant changes to BDNF expression in brain regions highly associated with PCA indicate that BDNF is unlikely to drive nicotine-enhanced conditioned behavior.

Keywords: BDNF; Goal tracking; Nicotine; Pavlovian conditioning; Sex differences; Sign tracking.

Fig. 1

Nicotine and female sex enhance sign-tracking in rats. Expression of sign-tracking behaviors over 29 days of training (left) and averaged across the last 10 days of training (right) in male and female rats that received nicotine injections prior to each session, compared to saline-injection control groups. Data are expressed as mean ± SEM, and reflect separate measures of conditioned approach to the conditioned stimulus: Top latency to press the lever; Middle lever presses per trial; Bottom probability of pressing the lever during a trial. * Main effect of nicotine exposure or sex, p < 0.05

Fig. 2

Nicotine enhances goal-tracking in male and female rats. Expression of goal-tracking behaviors over 29 days of training (left) and averaged across the last 10 days of training (right) in male and female rats exposed to nicotine or to saline. Data are expressed as mean ± SEM, and reflect separate measures of conditioned approach to the reward receptacle: Top latency to enter the receptacle; Middle receptacle elevation score per trial; Bottom probability of entering the receptacle during a trial. * Main effect of nicotine exposure, p < 0.05

Fig. 3

BDNF protein levels in the OFC, NAc, or BLA after nicotine exposure. BDNF protein was normalized to GAPDH loading control and expressed as a proportion of female saline controls. Protein was measured in the a OFC, b NAc, and c BLA. Representative bands of GAPDH and BDNF protein are presented for each region, aligned with their respective groups in the above bar graphs