Male Reproductive Behaviors pt 2

Spinal Nucleus of the Bulbocavernosus Muscle (SNB)

Responsible for penile erection and ejaculation

Sexually dimorphic in rats and humans

Maintained by prenatal androgens stimulating bulbocavernosus muscle in males

Muscle atrophy in females causes neurons in the SNB to die off due to lack of stimulation (smaller, only for sphincter control)

Preoptic Area

Integrates information from social signals and internal state (mPOA is most studied)

POA lesions result in disappearance of male copulatory behaviors but not motivation

POA sends projections to dopaminergic neurons in the PAG and VTA (necessary for copulatory behaviors)

Receptors in Preoptic Area

µ-opioid receptors in mPOA which are activated and internalized after ejaculation (receptors associated with pleasure)

Ejaculations induce secretion of endogenous opioids (rewarding effect of sexual behavior)

Age and environment matters (Prepubertal lesions in mates raised with other rats, there is no effect on behavior)

Dopamine and Sexual Behaviors

Part of the reward system

Reward: Wanting (seeking/motivations) Liking (pleasure/satiation)

Interacts with age and environment to facilitate a social reward and increase dopamine

Dopamine is involved in the wanting part

Chemosensory System

More important in non-human mammals

Combination of olfaction and vomeronasal organ (likely vestigial in humans)

These areas project to the olfactory bulb

Olfaction and Sexual Behaviors

Disruption in olfactory ability of a male rat greatly impair sexual behavior

Anosmic males fail to achieve ejaculation and lose interest

Appetitive behavior or anogenital investigation is important and can be lost

Vomeronasal Organ

VNO input is absent in most primates but important for rodents

Signals travel from the VNO to the accessory olfactory bulb

Activated on pheromones

BNST

Bed nucleus of the Stria Terminalis part of the amygdala

Chemosensory system projects here

Activated by vaginal secretions

Lesions on corticomedial amygdala impairs male copulatory behavior

Urine and VNO activity facilitate mate recognition in M/F (facilitates sexual copulatory posture)

Cumulates in hypothalamus

Ventromedial Hypothalamus

Activation of VMH induces aggression in males against other males

When presented with a female VMH neurons reduce activity

If stimulated experimentally males will become aggressive towards females but usually neurons in VMH are inhibited by presence of female

Distribution of Sex Steroids

Rich in receptors: POA, BNST, corticomedial AMY (effect on male copulatory behavior)

VMH and arcuate nucleus lesions do not change copulatory behaviors

More widespread distribution of E2 receptors than androgen receptors (estradiol plays important role)

DHT regulates aromatase transcription (castrated males have less aromatase and this is restored by androgen not estrogen)

Brain Activation During Male Sexual Behaviors

Social behavior neural network

Concerted activity of the network rather than a single region

Many regions activated but some are more important than others

Neural Circuts

Two different circuits

Spinal: Controls penile erection and ejaculation, flows from mPOA to spinal cord (usually happens after brain but can be on its own)

Brain: Chemosensory signals will activate the BNST and then go to medial amygdala and relay other places like the mPOA (these give rise to the motivation, appetite)

Dopamine and Sexual Behavior

Can influence 3 different tracts

1. Nigrostriatal tract: Works with basal ganglia for motor control and inhibition

2. Hypothalamic mPOA which facilitates male sexual behaviors

3. Mesolimbic tract involved in the reward system controls learning, motivation, appetitive behaviors

Serotinin and Sexual Behaviors

Reduces sexual behavior

Secretion in mPOA reduces dopamine activity downstream

SSRIs inhibit sexual behavior

Social Influences in Mating Behavior

Physical Environment: Encountering mates

Social Environment: Growing in isolation, social groups, novel females

Sexual Experience: Can change brain chemistry and physiology (mPOA, NO, DA, androgen receptors)

Individual Differences in Sex Drive

When guinea pigs were tested there are differences in sexual behaviors

After castration they equalize

When testosterone is restored, the individual differences are restored independent of hormone concentrations

Mating postures

Many different types of mating postions especially in primates

Some use mating as a social aspect

Licking and Mating

If female licks the genitals of male, they spend more time mating as opposed to when the do not

Variation in Sexual Activity in Men

Kinsey reported human behavior in both men and women

Described it at a spectrum

Most men have sex 1-3 times a week and very little have more than 5 times a week

Testosterone Throughout Life

There is a trend where testosterone peaks between 20-70 however there is no normal range of testosterone

Testosterone Treatment in Hypogonadal Men

Copulatory behaviors are more dependent on testosterone than orgasm

Visual Stimulation and Sexual Behavior

Testosterone does not increase much when just observing

When participating in sexual activity testosterone has ~75% increase

When heterosexual males watch gay porn their levels of arousal are low and their levels of unpleasantness are high

Pseudocopulation

Present in parthenogenetic whiptail lizards

All females and do not need sexual reproduction

However, their estrogen levels do change and there is high estrogen and ovulation female will exhibit female like sexual behaviors and another female not in ovulation will exhibit male like