Sexually Dimorphic Behaviour lecture 3

Sexually Dimorphic Behaviour

Learning Objectives

  • Illustrate the relevance of sexual dimorphism in mammalian brains and behaviour using specific pathways and stimuli.
  • Identify the role of hormone signalling in sex determination and behaviour, including:
    • Hormones during the setup of male versus female brains.
    • Hormones acting on the elicitation of sex-specific behaviours later in life.
  • Discuss the neural pathways underlying sexual and parental behaviour, which are both sexually dimorphic.
  • Provide examples of how neuropeptides regulate social behaviour.

Rodent Hypothalamus (AVP)

  • The anteroventral periventricular nucleus (AVP) shows sexually dimorphic differences in the number of dopaminergic neurons.
  • Using an antibody for tyrosine hydroxylase (required for dopamine biosynthesis), a different number of fluorescently labelled nuclei are observed in the female AVP compared to the male AVP.
  • Males with a mutated estrogen receptor revert to the female brain state.
  • Testosterone in the fetal rodent acts on the estrogen receptor because it enters the cell without being blocked by alpha-fetoprotein. Aromatase converts testosterone to 17-estradiol, which then acts on the estrogen receptor, determining the brain for its male phase.

Sizes of Nuclei

  • The sexually dimorphic nucleus of the preoptic area in the hypothalamus is larger in males than in females.
  • Quantification shows a much larger volume in the male brain.
  • Castration of males or treatment of females with testosterone leads to an intermediate state.

Human Anterior Hypothalamus & Spotted Hyena

  • Sexual dimorphism exists in similar nuclei in the human anterior hypothalamus.
  • Female hyenas are larger and more aggressive than males and are socially dominant.
  • Their clitoris is as large as the male penis and is used for urination, receiving semen, and giving birth.
  • There is a deficiency in placental aromatase in the small hyena, and the mother makes androstenedione, exposing the fetuses to it.
  • Hyena pups are aggressive and fight their siblings from birth.
  • Despite androstenedione exposure, sexual dimorphism is still observed in:
    • Spinal cord (SNB)
    • Hypothalamus (sexually dimorphic area of the optic area)
Androstenedione Suppression
  • Limited feminization of the male fetus occurs with androstenedione suppression:
    • The fetal penis becomes a clitoris.
    • No prostate develops.
    • Female-like cavernous muscle develops.
    • Reduced size in the nucleus.
  • Limited hyper-feminization of the female fetus:
    • Increased size and elasticity of the uterus.
    • Decreased urogenital meatus or opening at the clitoris length.
    • Increased glans diameter.

Sexual Dimorphism Across the Rodent Brain

  • Using the aromatase promoter, markers are expressed where the aromatase gene is expressed.
  • This indicates where testosterone would enter the cell and get converted to act on the estrogen receptor before and after birth.
  • The amygdala has more aromatase-positive neurons in the male than in the female.
  • The ventromedial hypothalamus has more staining of the reporter gene than in the female.

Summary of Areas in the Rodent Brain with Sexual Dimorphism

  • AVP (Anteroventral Periventricular Nucleus):
    • Increased volume in females.
    • Increased number of neurons in females.
    • Neurons produce dopamine and glutamate (important in the control of the ovarian cycle).
  • Preoptic Area:
    • Sexually dimorphic nucleus increased in males.
    • More cells and more volume in males.
    • Keltner neuropeptide is expressed in these cells.
  • Bed Nucleus of the Stria Terminalis (BNST):
    • Ventral part is larger with more cells in the female.
    • Another part has more volume and cells in the male.
    • Enlarged male neurons are Keltner positive, unlike the ventral part.

Sexual Behaviour in Rodents

  • Rodents are olfactory-oriented animals.
  • They use their noses more than their eyes to assess their environments.
  • They have two ways of sensing sexual signals:
    • Primary nasal organ and olfactory bulb.
    • Accessory olfactory bulb (relay for the vomeronasal organ inputs).
  • The olfactory bulb links to part of the brain itself, and there are neural input pathways that track inputs from both these areas, although there is some taste input as well.
  • Areas involved include:
    • Nucleus of the stria terminalis.
    • Medial preoptic area of the hypothalamus.
    • Ventromedial hypothalamus.
    • Bed nucleus of the stria terminalis.
    • All asterisked areas are sexually dimorphic in size (larger in males, except for one part of the nucleus of the stria terminalis).
Olfactory Epithelium
  • Cyclic nucleotide-gated channels are excited upon suction, triggering depolarization and neural signals.
  • This input is required for mating behaviour, including:
    • Sniffing
    • Mounting
    • Intermission/penetration
    • Territorial aggression (depending on the intruder)
Maternal Behaviour
  • The same pathway is used for maternal behaviour (retrieval).
  • Mother mice collect their pups if they are scattered around the home cage.
  • Wild-type mice versus knockout mice lacking the gated channel that acts in the olfactory epithelium display suppressed mating behaviours in the knockout mice.
Drip C2
  • The Cat on a channel acting from the nasal organ is required for sex-specific courting.
  • Male mice have a courtship song (high-pitched vocalizations).
  • Drip C2 is required to trigger the courtship song and to suppress the courting of other males in males.
  • Triple C2 mutation leads to courting of other males almost as high as that of females (distinction depends on vomernasal organ input via drip C2).
  • Pheromone perception happens in sexually dimorphic nuclei (nucleus of the stria terminalis in the medial amygdala).

Neuronal Activity

  • We can record neuronal activity in the nucleus of the stria terminalis (pheromone sensation), triggered by the presence of other mice.
  • Male resident mice with territory and aromatase-expressing bed nucleus of the stria terminalis are studied.
  • Inert objects do not excite those neurons, but female or male stimuli do, especially females.
  • Removing the drip C2 sensor in the vomeronasal organ eliminates the distinction between males and females in downstream neuron excitation.
  • Neuronal activity correlates with changes in behaviour dependent on input from the vomeronasal organ via A52.

Optogenetic Stimulation

  • Channelrhodopsin-induced excitation (blue light) in aromatase-positive neurons of the nucleus of the stria terminalis.
  • Normally, those neurons don't get excited as much by males as with females.
  • Artificially increase the excitement of those neurons to female levels.
  • The male is interpreted as a female and is mounted rather than attacked.
  • Neurons trigger a female-like behavioural response if given female-like stimulation.

Female Sexual Behaviour

  • Females have a lordosis reflex, triggered by touches on the flank and perineal regions.
  • Electrical stimulation of the ventromedial hypothalamus can trigger lordosis reflexes.
  • Ventromedial hypothalamus is associated with female sexual behaviour in rodents.
  • Lesioning that area results in loss of the lordosis reflex (partial recovery).
Progesterone Receptor-Positive Neurons
  • Ventromedial hypothalamus has progesterone receptor-positive neurons.
  • Manipulation of those cells can influence the behavioural choice between rejection of a partner and mating (lordosis behaviour).
  • In the absence of these cells, the behaviour turns into rejection.
  • These neurons in the hypothalamus trigger female sexual behaviour.
  • Killing the same progesterone receptor-positive cells in the male ventromedial hypothalamus changes male sexual behaviour.
  • Feminized brain: trigger of mating when presented with a female and an attack when presented with a male.
  • There is a hint that it's similar cells that do that, however, males have more cells in the ventromedial hypothalamus.
Recording of Cells
  • Recording of cells in the ventromedial hypothalamus is possible.
  • Look for cells that respond to females, males, or both.
  • Individual cells in the ventromedial hypothalamus specialize into male response versus female response over time.
  • Mapping development in the ventromedial hypothalamus of males where certain parts of that nucleus will start responding to males and then trigger aggression, while other parts of the ventromedial hypothalamus will be specialized in responding to females and trigger sexual behaviour.

Summary of Circuits

  • The vomeronasal organ receives pheromones from females in the male.
  • It projects to the medial preoptic area and the medial preoptic area of the hypothalamus (both sexually dimorphic).
  • Testosterone circulates and acts on these aromatase-positive sites.
  • Responses go to the ventral midbrain and then the brainstem and spinal cord, leading to sexual behaviour, including mounting, etc.
  • Androgens also act on these spinal neurons to augment the reflexes.
  • Testosterone acts downstream on the pathway (feedback from the ventral midbrain).
  • Females: ventromedial hypothalamus plays an important role (signals to the midbrain).
  • Locations are responsive to estrogens. Estrogens act on receptors and respond.
  • The reticular formation in the brainstem leads to actions in the spinal cord and motor neurons that lead to lordosis.

Conserved Pathways

  • Both cases have pathways that link sensory input through the vomeronasal organ to sexually dimorphic nuclei in the brain to pathways that go and trigger motor reflexes in the spinal cord.
  • Modulation of those behaviours by steroid hormones

Maternal Behaviour

  • Lactating mothers keep pups together (retrieval).
  • Experienced virgins collect the pups they are given.
  • Fathers can also do pup retrieval.
Galin-Expressing Neurons
  • The medial preoptic area of the hypothalamus has neurons associated with parenting in fathers.
  • Ablating neurons that express Galin neuropeptide in fathers increases the chance of ignoring pups (controls retrieve more often).
  • Optogenetic stimulation (channelrhodopsin) in Galin-expressing neurons in the medial preoptic area of virgin males reduces attacks on pups.
  • The Galin-expressing neurons in the sexually dimorphic nucleus in mothers elicit parental behaviour.
  • Ablating these neurons manipulates parental behaviour.
  • Virgin females can be induced to attack pups in the absence of these neurons.
  • These neurons elicit parental/socially positive behaviour towards pups.
Outputs
  • Galin-expressing neurons in the medial preoptic area have outputs to:
    • Peri-aqueductal grey.
    • Ventral tegmental area.
  • Leads to:
    • Motor control of retrieval.
    • Motivation for pup retrieval.
    • Inhibition of interactions with other adults.

Summary

  • Olfactory epithelium pathway (cyclic nucleotide gated channels) projects to the olfactory bulb and sexually dimorphic hypothalamus areas.
  • This pathway triggers mating behaviours in males and impacts retrieval in both males and females.
  • Vomeronasal organ pathway involves the drip C2 channel and projects to sexual dimorphic areas through the accessory olfactory bulb (nucleus of the stria terminalis and medial amygdala).
  • That pathway triggers sex-specific male courting and ultrasonic vocalization in males. Additionally, the ventromedial hypothalamus is involved in lordosis in females.
Specific Roles
  • The nucleus of the stria terminalis helps in sex determination of intruding mice for males.
  • Males can tell whether they should act upon the intruder as a male or a female.
  • Ventromedial hypothalamus (progesterone receptor-positive cells) controls female receptiveness and male sexual versus aggressive behaviour choices.
  • Medial preoptic area: parental behaviour encoded in Galin-positive neurons.

Neuropeptides

  • Primitive animals (C. elegans) use madison peptide for mating.
  • Oxytocin and vasopressin play a role in sexually dimorphic behaviours in voles.
  • Monogamous versus promiscuous voles have different patterns of receptors for these neuropeptides.
  • generalalanine↔leucinegeneral alanine \leftrightarrow leucine and leucine↔arginineleucine \leftrightarrow arginine
  • Key differences for the two neuropeptides.
Experiment
  • Experimental manipulation shows a change in contact in males when there's a partner versus a stranger. Monogamous males are normally much more interested in their partner than the stranger.
  • Antagonizing vasopressin signalling reverts this
  • A dopamine component to it in females. the preference for the partner is also reduced with an antagonist to dopamine signalling
  • Dopaminergic signalling and vasopressin signalling are involved in pair bonding in monogamous prairie voles.
  • Monogamous voles can partner males too, over your actualized females that normally would not have the same hormonal signals without needing if we infuse them with vasopressin or oxytocin placed together with female partners without mating. Given a choice between a stranger, an empty cage, or the partner:
    • Control
    • Vasopressin
    • Oxytocin
    • The preference for the partner was really encoded by vasopressin.
  • Vasopressin is the stimulant for partnering, not so much oxytocin.
Findings
  • Research cements vasopressin as the key signal for creating and maintaining a partner bond that can last over a lifetime.
  • Human fMRI experiment exposed individuals to a picture of their loved one versus a picture of a friend or acquaintance.
  • Certain areas in the brain stimulated:
    • Ventral tegmental area
    • Caudate nucleus
  • These areas were stimulated by the presence of a picture of one's loved one.
  • Certain areas in the brain are deactivated. Certain areas in the brain respond to images that are associated with an emotional bond. Also worked for pictures of their children

Motivational Encoding

  • The ventral tegmental area is involved in motivation for retrieval in rodents.
  • In humans, that same area seems to be associated with emotional bonding.
  • Sign of conservation of some of these pathways.
Human Differences
  • Sexual dimorphism in the brain translates in humans when, of course, we see differences in sex in humans
  • Hard to know which of these are truly biologically encoded and which are culturally encoded in parts or a combination of both.
  • Identify structures in the brain that are larger in a healthy female brain compared to cerebrum size or larger in the male brain compared to cerebrum size.
  • Some areas appear to have some bias to being enlarged in the male versus female brain.
  • For humans, there's still a lot of work to be done.