Effects of the estrous cycle and ovarian hormones on cue-triggered motivation and intrinsic excitability of medium spiny neurons in the Nucleus Accumbens core of female rats

Abstract

Naturally occurring alterations in estradiol influence food intake in females. However, how motivational responses to food cues are affected by the estrous cycle or ovarian hormones is unknown. In addition, while individual susceptibility to obesity is accompanied by enhanced incentive motivational responses to food cues and increased NAc intrinsic excitability in males, studies in females are absent. Therefore, we examined basal differences in intrinsic NAc excitability of obesity-prone vs. obesity-resistant females and determined how conditioned approach (a measure of cue-triggered motivation), food intake, and motivation for food vary with the cycle in naturally cycling female obesity-prone, obesity-resistant, and outbred Sprague-Dawley rats. Finally, we used ovariectomy followed by hormone treatment to determine the role of ovarian hormones in cue-triggered motivation in selectively-bred and outbred female rats. We found that intrinsic excitability of NAc MSNs and conditioned approach are enhanced in female obesity-prone vs. obesity-resistant rats. These effects were driven by greater MSN excitability and conditioned approach behavior during metestrus/diestrus vs. proestrus/estrus in obesity-prone but not obesity-resistant rats, despite similar regulation of food intake and food motivation by the cycle in these groups. Furthermore, estradiol and progesterone treatment reduced conditioned approach behavior in obesity-prone and outbred Sprague-Dawley females. To our knowledge, these data are the first to demonstrate cycle- and hormone-dependent effects on the motivational response to a food cue, and the only studies to date to determine how individual susceptibility to obesity influences NAc excitability, cue-triggered food-seeking, and differences in the regulation of these neurobehavioral responses by the estrous cycle.

Fig. 1.

Verification of female obesity-prone and obesity-resistant phenotype. A-C) Weight and adiposity measures at baseline: Although body weight is similar between groups, obesity-prone rats (OP) have greater fat mass and lower lean mass vs. obesity-resistant (OR) rats. D-F) Weight and adiposity measures after 4 weeks of junk-food diet (JF) or chow (CH) consumption: Obesity-prone rats remain heavier than obesity-resistant rats, and have greater fat mass and lower lean mass vs. obesity-resistant rats. Junk-food increases fat mass and reduces lean mass in both groups, but the magnitude of this effect is stronger in obesity-prone rats. G) Home cage food intake during 4 weeks of junk-food or chow consumption: Junk-food consumption was greater than chow consumption in both groups, with obesity-prone rats consuming more junk-food than obesity-resistant rats. All data are shown as mean ± SEM unless otherwise noted. ^# = differences between obesity-prone and obesity-resistant rats, * = differences between chow and junk-food, p < 0.05.

Fig. 2.

Home cage food intake decreases during estrus in both obesity-prone and obesity-resistant female rats. A) Representative pictures from vaginal lavages at each phase of the estrous cycle in female rats. B) Home cage chow consumption across the cycle. Home-cage chow consumption was greater in obesity-prone (OP) vs. obesity-resistant (OR) rats, and was reduced during estrus in both groups. * = planned post-hoc comparisons, ^# = obesity-prone vs. obesity-resistant rats, p < 0.05.

Fig. 3.

Motivation to work for a sucrose pellet decreases during proestrus/estrus in both obesity-prone and obesity-resistant rats. A, B) Average number of active and inactive lever presses across training. Acquisition of instrumental responding for sucrose was similar between groups. All rats preferentially responded on the active vs. inactive lever during fixed ratio 1 (FR1) and fixed ratio 5 (FR5) sessions. C) Average break point reached during progressive ratio testing. Break point was significantly lower during proestrus/estrus (P + E) vs. metestrus/diestrus (M + D) in both groups. No differences between obesity-prone (OP) and obesity-resistant (OR) groups were observed; * = p < 0.05.

Fig. 4.

NAc core MSN intrinsic excitability is enhanced in obesity-prone vs. obesity-resistant rats during metestrus/diestrus. A) Example traces of current-clamp recordings from MSNs in slices from obesity-prone (OP) and obesity-resistant (OR) rats prepared during metestrus/diestrus. B) Example traces of current-clamp recordings from MSNs in slices from obesity-prone and obesity-resistant rats prepared during proestrus/estrus. C, D) Current/voltage (I/V) relationship and number of action potentials in obesity-prone and obesity-resistant from slices made during metestrus/diestrus. I/V relationships are shifted to the right in in obesity-prone vs. obesity-resistant rats and the same current injection intensity elicits more action potentials in MSNs from obesity-prone vs. obesity-resistant rats. E, F) Current/voltage (I/V) relationship and number of action potentials in obesity-prone and obesity-resistant from slices made during proestrus/estrus. I/V relationships and the number of action potentials fired are similar between obesity-prone and obesity-resistant rats in proestrus/estrus. G, H) Input resistance and rheobase: Obesity-prone rats have a higher input resistance and lower rheobase vs. obesity-resistant rats. I, J) Input resistance and rheobase during proestrus/estrus: No differences were observed in input resistance or rheobase between obesity-prone and obesity-resistant rats. K, L) Cell parameters from recordings conducted in slices prepared during metestrus/diestrus and proestrus/estrus. No differences in resting membrane potential (RMP), action potential threshold, action potential amplitude (AP), after-hyperpolarization amplitude (AHP), inter-spike interval (ISI) or rise time were observed between groups or cycle phase metestrus/diestrus or proestrus/estrus; * = p < 0.05. M) Map of NAc core MSN recording area.

Fig. 5.

Cue-triggered motivation is modulated by the estrous cycle in obesity-prone but not in obesity-resistant rats. A) Average number of food cup entries during the first 10 s of CS presentation in obesity-prone rats (OP). Obesity-prone rats discriminate between the CS+ and the CS− during initial Pavlovian conditioning. B) Average number of food cup entries during the first 10 s of CS presentation in obesity-resistant rats (OR). Obesity-resistant rats discriminate between the CS+ and the CS− during initial Pavlovian conditioning. C) Average difference in the number of food cup entries during the first 10 s of CS+ vs. CS− presentation. No difference in magnitude of conditioned approach between obesity-prone and obesity-resistant rats was observed. D) Extinction Test: Average number of food cup entries during the CS+, CS− and inter-trial interval (ITI) in obesity-prone and obesity-resistant rats tested in metestrus/diestrus (M + D) and proestrus/estrus (P + E). Conditioned approach is stronger during metestrus/diestrus vs proestrus/estrus in obesity-prone, but not obesity-resistant rats. In addition, the magnitude of conditioned approach is greater in obesity-prone vs. obesity-resistant rats tested during the metestrus/diestrus phase. * = p < 0.05, ** = p < 0.01.

Fig. 6.

Repeated treatment with estradiol and progesterone decreased conditioned approach in obesity-prone females. A) Schematic of experimental timeline. Rats received 8 sessions of Pavlovian conditioning, followed by ovariectomy surgery (OVX) and 10 days of recovery before being treated with estradiol (+E) and progesterone (+P) or vehicle. 4–6 h after the last progesterone injection rats were tested in extinction conditions. B) Average number of food cup entries during the first 10 s of CS presentation in obesity-prone rats (OP). Obesity-prone rats learned to discriminate between the CS+ and CS−. C) Average number of food cup entries during the CS+, CS− and inter-trial interval (ITI) in vehicle and hormone treated groups. A single cycle of hormone treatment is not sufficient to decrease conditioned approach. D) Average number of food cup entries during the CS+, CS− and inter-trial interval (ITI) in vehicle and hormone treated groups. Repeated estradiol and progesterone treatment decreased conditioned approach compared to vehicle treated controls, although repeated testing reduced the magnitude of conditioned approach in both treatment groups. * = p < 0.05.

Fig. 7.

Repeated treatment with estradiol and progesterone decreased conditioned approach in outbred females. A) Schematic of experimental timeline. Rats received 8 sessions of Pavlovian conditioning, followed by ovariectomy surgery (OVX) and 10 days of recovery before being treated with estradiol (E) and progesterone (P) or vehicle. 4–6 h after the last progesterone injection rats were tested in extinction conditions. B) Average number of food cup entries during the first 10 s of CS presentation. Outbred rats learned to discriminate between the CS + and the CS−. C) Average number of food cup entries during the CS+, CS− and inter-trial interval (ITI) in vehicle and hormone treated groups. Four cycles of estradiol and progesterone treatment decreased conditioned approach in outbred rats.* = p < 0.05.

Fig. 8.

Estradiol and progesterone act synergistically to decrease cue-triggered motivation in outbred rats. A) Schematic of experimental timeline. Outbred rats received 8 sessions of Pavlovian conditioning, followed by ovariectomy surgery (OVX) and 10 days of recovery. Rats were the counterbalanced by weight and divided into 4 groups: vehicle, estradiol and progesterone (E + P), estradiol alone (E) and progesterone alone (P). B) Average number of food cup entries during the first 10 s of CS presentation. Outbred rats learned to discriminate between the CS+ and the CS−. C, D) Average body weight and food intake across the treatment period. Estradiol treatment (E or E + P) reduced body weight and food intake compared to the vehicle (Veh) and progesterone (P) groups. E) Average number of food cup entries during the CS+, CS− and inter-trial interval (ITI) in vehicle and hormone treated groups. Estradiol or progesterone alone did not affect conditioned approach. However, repeated estradiol and progesterone treatment decreased conditioned approach in outbred rats. Thus, although estradiol is sufficient to reduce weight and food intake, both estradiol and progesterone are needed to reduce conditioned approach. * = p < 0.05.