Here we will examine an additional rewarding behavior. Rodents, such as rats, are used as a model for studying sexual behavior and reward.

Rodent Sexual Behavior

When in the presence of a sexually receptive female rat, a male rat will engage in male sexual behavior, called mounting behavior. Mounting behavior is controlled by the medial preoptic area (mPOA) of the hypothalamus in males.

Female rats display their own sexual behavior called lordosis. Lordosis behavior is displayed as an arched-back posture that is controlled by the ventromedial hypothalamus (VMH). Female rats also have the ability to pace their sexual interactions by controlling the timing of the sexual interaction. The female rat will run around the cage and choose when they will stop and allow the male to engage in mounting behavior.

Steroid hormones such as testosterone, estrogen, and progesterone regulate activity of the mPOA and VMH.

When considering sexual behavior as a model for motivated behaviors, it is important that we understand why an animal engages in sexual behavior.

A modified operant chamber was constructed by Everitt and colleagues (1990) that helped to answer this question. A male rat was trained to press a lever within the chamber that when pressed would open a door and allow for access to a sexually-receptive female rat, with whom the male could engage in a sexual bout. Males easily learned this paradigm and consistently pressed the lever to gain access to the female (Everitt, 1990).

When the mPOA was lesioned in the male rats and they were tested in this same operant chamber, the male rats still pressed the lever to gain access to the female rat, even though they no longer had the ability to perform sexual mounting behavior. However, when the nucleus accumbens was lesioned and the mPOA remained intact, the male rats no longer pressed the lever. They had lost the motivation associated with the prospect of sexual experience, thus demonstrating that the nucleus accumbens, and the reward pathway drive the motivation for sexual behavior in rodents, not the brain structure that controls sexual behavior. It follows that in female rats, the motivation to engage in sexual behavior is dependent on activity of the nucleus accumbens, and not on activity within the VMH.

Nucleus Accumbens Dopamine in Sexual Behavior

Measuring extracellular dopamine concentrations outside the nucleus accumbens through microdialysis techniques has demonstrated that extracellular dopamine increases during many different behaviors considered to be rewarding, including male and female sexual behavior.

Measuring Reward: Conditioned Place Preference

Conditioned place preference (CPP) is a behavioral test that can be used as way of measuring the reward associated with different stimuli. CPP is commonly used as a way to measure reward associated with different drugs of abuse, but the paradigm can also be used to measure the rewarding aspects of natural rewards, such as sexual behavior. In a CPP paradigm, a CPP appartus is used that typically has two distinct testing chambers that have different colors or patterns on the walls, and different tactile surfaces on the floor of the chamber. Additionally, there is a neutral compartment between the two testing chambers. As an example, one testing chamber might have white walls and hard bedding on the floor, whereas the other testing chamber has gray walls and soft bedding.

An experimental male animal can be placed in the neutral central compartment and then given 10 minutes to explore the entirety of the apparatus. The amount of time that the animal spends in each chamber is recorded as a pre-test measurement. Next, the chambers are sectioned off from each other and the experimental male animal is placed with a sexually-receptive female animal in the gray chamber with soft bedding.  The experimental male then has a 10-minute sexual experience within the gray chamber. After the sexual experience, they are also placed alone in the white chamber with hard bedding. This is repeated for several weeks, such that the animals learn to associate sexual experience with the gray chamber environment and learn to associate nothing with the white chamber environment. Lastly, the experimental animal is placed alone in the central neutral compartment and again permitted to access the entire apparatus. The amount of time spent in each of the chambers is recorded as a post-test measurement.

By comparing the time spending in the conditioning chamber during the pre-test to the time spent in the conditioning chamber during the post-test, it can be determined if the animal spends significantly more time in the chamber that was paired with the sexual experience. Researchers interpret a significantly increased time spent in the conditioning compartment as a measurement of the reward that the animal feels being in that environment. If no significant difference is observed between the pre-test and post-test times in the conditioning compartment, then the testing substance or behavior is not considered to be rewarding to the animal.

Using this paradigm, administration of a dopamine antagonist to animals prior to the conditioning sessions prevents the formation of a conditioned place preference, indicating that dopamine within the reward pathway underlies the reward associated with sexual behavior.

References

“Sexual Motivation: A Neural and Behavioral Analysis of the Mechanisms Underlying Appetitive and Copulatory Responses of Male Rats” 1990. Everitt BJ. Neuroscience and Biobehavioral Reviews, Vol 14 pp 217-232.

Attributions

This chapter is adapted from “Motivated Behavior: Sexual Behavior” in Introduction to Neuroscience by Valerie Hedges which is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License.

Media Attributions

• Rat sex behavior © Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license
• CPP apparatus © Valerie Hedges is licensed under a CC BY-NC-SA (Attribution NonCommercial ShareAlike) license