Chp 4: Hormonal Influences on Male Sex Behavior

1
Chp 4 Hormonal Influences on Male Sex Behavior

• Male sex behavior can be divided into two main
  components
• precopulatory behavior
• production of ultrasonic vocalizations
• chemoinvestigation (sniffing of the females
  anogenital region)
• copulatory behavior
• mounts male mounts the female from behind
• intromissions male, with an erected penis,
  inserts penis into the females vagina
• ejaculations explusion of a copulatory
  plug--composite of different secretions from
  prostate gland, seminal vesicle and coagulating
  gland plus sperm this composite will coagulate
  forming a plug that serves to keep the sperm
  within females reproductive system and increase
  likelihood of fertilization

2
Sequence of Precopulatory and Copulatory Events
copulation
chemoinvestigation
mount
intromission
ejaculation
(repeated X times)
postejaculatory interval (PEI)
satiety
postejaculatory interval (PEI) refractory
period following ejaculation in which male does
not engage in sex behavior (seconds to
minutes) satiety refractory period following a
series of ejaculations in which a male will not
engage further in sex behavior with a given female
3
Quantification of Precopulatory Copulatory
Behaviors

• How to analyze male sex behavior?
• amount of time engaged in sniffing odors
  (chemoinvestigation)
• latency to mount number of mounts
• latency to intromit number of intromissions
• interintromission interval--length of time
  between intromissions
• latency to ejaculate number of ejaculations
• postejaculatory interval length of time from an
  ejaculation to the next intromission

4
Sexual Motivation

• In addition to precopulatory and copulatory
  behaviors, we we can also consider the males
  desire to engage in sex behavior.
• Sexual motivation males inclination to seek
  out and approach a female for the purpose of
  mating.
• Tests of sexual motivation
• latency to mount
• mount quickly--gtreally motivated
• mount slowly--gtnot very motivated
• has problems--??
• Train a male to press a bar to gain access to a
  female--how many bar presses will a male make?
  (more bar pressesgreater motivation)
• Separate males and females across an electrified
  grid--how much electrical current will a male
  withstand to gain access to a female? (greater
  current--greater motivation)

5
Hormonal Influences on Male Sex Behavior

• In females, transient elevations of estrogen
  and/or progesterone during the estrous cycle play
  an important role in activating female sex
  behavior.
• In males, testosterone (and/or its metabolites)
  also play an important role in activating male
  sex behavior.
• Testosterone secretion increases at puberty
  development of secondary sex characteristics,
  increased muscle mass, production of sperm, and
  development of an interest in sexual activity.
• However, testosterone levels do not show similar
  cyclical changes in levels reminiscent of the
  female estrus cycle.

6
Hormonal Influences on Male Sex Behavior

• Testosterone is the major androgen secreted by
  the testes.
• aromatization of testosterone to estrogen
• important for activating male sex behavior in the
  rat (and in many other species, although not all)
• involve aromatase and ERs
• reduction of testosterone to dihydrotestosterone
• important for penile erections and intromissions
  in the rat
• involve 5?-reductase and ARs
• testosterone as testosterone
• important for aspects of sexual motivation
• involve ARs

7
Testosterone Secretion in Males

• GnRH neurons secrete GnRH into the median
  eminence
• GnRH stimulates release of LH and FSH from
  anterior pituitary
• In interstitial space (testis)
• LH acts at the Leydig cells to secrete the
  testosterone (steroidogenesis)
• LH--gtincrease levels of cAMP
• In seminiferous tubules
• at puberty, FSH and testosterone act at Sertoli
  cells to initiate the production of sperm
  (spermatogenesis)
• FSH --gtincrease levels of cAMP
• FSH also stimulates the production of inhibin
  from Sertoli cells which inhibits release of FSH
  from anterior pituitary

GnRH Neuron
-

HYPO
GnRH
ANT PIT
-
FSH LH
Inhibin
TESTES
Testosterone
GnRH gonadotropin-releasing hormone FSH
follicle stimulating hormone LH luteinizing
hormone
8
Hormonal Influences on Male Sex Behavior

• Testosterone is the major androgen secreted by
  the testes.
• aromatization of testosterone to estrogen
• important for activating male sex behavior in the
  rat (and in many other species, although not all)
• involve aromatase and ERs
• reduction of testosterone to dihydrotestosterone
• important for penile erections and intromissions
  in the rat
• involve 5?-reductase and ARs
• testosterone as testosterone
• important for aspects of sexual motivation
• involve ARs

9
Species Differences Male Sex Behavior

• In rats (and hamsters)
• estrogen is critical for activating
  copulation--mounts, intromissions and
  ejaculations )
• dihydrotestosterone is critical for penile
  reflexes
• In some species (lizards, mice guinea pigs)
• dihydrotestosterone can stimulate male sex
  behavior
• In ferrets
• testosterone plays selective role in sexual
  motivation
• In primates
• testosterone may also have a selective effect on
  sex behavior not mediated by estrogen or
  dihydrotestosterone

10
Peripheral Mechanisms

• Androgens are critical for penile erections and
  deposition of the copulatory plug
• androgens stimulate the size and number of
  sensory receptors on the surface of the glans
  penis
• dihydrotestosterone (DHT) activates penile
  erections when rats are restrained on their backs
• penile erections and deposition of the copulatory
  plug are controlled (in part) by motoneurons
  within spinal nucleus of the bulbocavernosus
• SNB innervates penile musculature
• DHT acts to increase survival of penile
  musculature followed by survival of SNB
  motoneurons (during development)
• in the adult, DHT also affects the morphology of
  SNB neurons--increasing size of cell bodies and
  increasing length of dendrities
• Note penile erections and ejaculation are also
  controlled by the autonomic nervous system.

11
Central Mechanisms

• In the male rat
• estrogen acts to stimulate copulation--mounts,
  intromissions and ejaculations
• aromatization is important for this process!
• MPOA plays an important role the effects of
  estrogen
• sexually dimorphic area of the preoptic area
  (SDA-POA) sexual learning
• dorsal MPOA critical for stimulation of mounts,
  intromissions and ejaculations
• dopaminergic inputs--activating mounts,
  intromissions and ejaculations
• amygdala plays an important role in facilitating
  sex behavior--the amygdala attributes salience
  to cues associated with sex behavior
• corticomedial amygdala--pheromones to activate
  behavior
• basolateral amygdala--learned responses (e.g.,
  bar pressing to gain access to female)
• dopaminergic inputs into the nucleus
  accumbens--sexual motivation
• dopaminergic inputs into the striatum--general
  locomotion (e.g., chasing female)

12
MPOA

• In the rat
• Aromatization of testosterone to estrogen is
  important for sexual differentiation of the brain
  and activation of male sex behavior.
• The MPOA is an integrative center for hormonal
  stimulation of male copulation.
• Large lesions of the MPOA eliminate mounting,
  intromissions and ejaculations.
• Electrical stimulation of the MPOA stimulates
  these responses.
• Castrate an adult male rat and he will stop
  engaging in copulation.
• If you administer testosterone to a castrated
  male, he will again engage in copulation.
• If you administer testosterone or estrogen within
  the MPOA (with DHT to act peripherally), normal
  male copulation will be seen--gtMPOA is critical
  site!!
• The MPOA sends projections to neurons within the
  midbrain and brainstem, which subsequently
  project to neurons in the spinal cord 2 effects
  take place
• activation of autonomic centers that control
  process of erection and ejaculation
• activation of motoneurons that control mounting
  and pelvic thrusts

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MPOA

• Anatomical Subdivisions
• Study by Heimer and Larsson (1966/67)
• Large bilateral lesions of the MPOA eliminated
  mating.
• Study by Arendash Gorski (1983)
• Is the SDN-POA the critical brain site for
  control of male copulation?
• No in sexually experienced males lesions of the
  SDN-POA had no significant effect on male
  copulation.
• Instead, bilateral lesions of the dorsal MPOA
  produced substantial, long-term decreases in
  number of mounts, intromissions, and ejaculations
  compared to sham-lesioned control males.
• Study by De Jonge et al. (1989)
• Revisited role of SDN-POA in male sex
  behavior--found an effect!
• In sexually naïve males, bilateral lesions of the
  SDN-POA resulted in an increased latency to first
  mount, intromission and ejaculation (took longer
  to initiate copulation)
• However, this decrement in sexual performance
  disappeared after repeated testing.

14
Amygdala

• Corticomedial Amygdala (CMA)
• CMA plays an important role in linking
  pheromones to activation of male sex behavior
• CMA receives olfactory stimulation from the
  vomeronasal organ and olfactory mucosa (sensory
  structures responsd to odors project to the
  olfactory bulbs).
• Pheromones are secreted by female that can
  stimulate chemoinvestigation and copulation.
• CMA projects to the MPOA.
• Lesions of the vomeronasal organ and olfactory
  mucosa or bilateral lesions of the CMA can
  eliminate male sex behavior--effect is similar to
  bilateral lesions of MPOA.

Olfactory Bulbs
CMA
MPOA
brainstem
spinal cord
Copulatory reflexes
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Amygdala

• Basolateral Amygdala (BLa)
• BLa implicated in associative learning that
  feeds into motivation circuits.
• You can train a rat to bar press to gain access
  to an estrus female.
• If you lesion the BLa, the male rat will not show
  bar pressing for the female.
• However, if you place the receptive female in the
  chamber with the male, he readily copulates with
  her.
• It is thought that the association between bar
  pressing to gain access to a female rat (for
  copulation) has been lost.
• The BLa, via connections within the nucleus
  accumbens, is thought to play a role in sexual
  motivation--searching out a female (e.g., bar
  pressing) in order to mate.

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Nucleus Accumbens

• The nucleus accumbens (NAcc) has been linked to
  motivational processes.
• Mogensen (1980) suggested that the NAcc serves as
  the limbic-motor interface
• a system concerned with translating emotional
  (salient) stimuli into motor output

Limbic System
Prefrontal Cortex
Ventral Pallidum
BLa
NAcc
MPOA
associative learning
Motor System
Basal ganglia
Motor Responses
Mesencephalic Locomotor Region
a) foraging for a female
b) bar pressing for a female
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Central Mechanisms

• In the male rat
• estrogen acts to stimulate copulation--mounts,
  intromissions and ejaculations
• aromatization is important for this process!
• MPOA plays an important role the effects of
  estrogen
• sexually dimorphic area of the preoptic area
  (SDA-POA) sexual learning
• dorsal MPOA critical for stimulation of mounts,
  intromissions and ejaculations
• dopaminergic inputs--activating mounts,
  intromissions and ejaculations
• amygdala plays an important role in facilitating
  sex behavior--the amygdala attributes salience
  to cues associated with sex behavior
• corticomedial amygdala--pheromones to activate
  behavior
• basolateral amygdala--learned responses (e.g.,
  bar pressing to gain access to female)
• dopaminergic inputs into the nucleus
  accumbens--sexual motivation
• dopaminergic inputs into the striatum--general
  locomotion (e.g., chasing female)

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Dopaminergic Cell Groups
Dopamine Cell Bodies
General Effect
incertohypothalamic DA cell group
MPOA
mounts, intromissions and ejaculations (copulatory
reflexes)
ventral tegmental area (VTA)
nucleus accumbens
display of motivated behavior
substantia nigra
dorsal striatum
sensorimotor coordination (locomotion)
21
Dopamine

• The levels of dopamine increase within these
  areas during mating
• Study by Pfaus et al. (1990)
• in vivo microdialysis was used to follow levels
  of dopamine (and its metabolites) within nucleus
  accumbens and dorsal striatum during copulation
• in vivo microdialysis involves measurement of
  molecules present in the extracelular space
  through diffusion across a semipermeable membrane
  at the base of a probe implanted into brain
• nucleus accumbens levels of dopamine increased
  significantly during placement of a sexually
  experienced male in a sex chamber (with odors of
  prior matings present), with additional increases
  observed with placement of a female behind a
  screen, and then when the male was allowed to
  mate with female
• dorsal striatum levels of dopamine increased
  significantly only during interaction between
  the male and female (copulation)
• concluded DA released in the nucleus accumbens
  (from VTA) is involved in motivation processes
  (prior to, and in preparation of, the actual act
  of copulation) DA released in the striatum is
  involved in actual physical act of copulation
  (sensorimotor coordination)

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Dopamine

• The levels of dopamine increase within these
  areas during mating
• Studies by Hull et al. (1995, 1997)
• in vivo microdialysis was used to follow levels
  of dopamine (and its metabolites) within MPOA
  during copulation
• MPOA levels of dopamine increased significantly
  during precopulatory period in which a female is
  placed behind a perforated barrier (male can see,
  hear and smell female but not mate with her), and
  then further increases in dopamine occur when the
  male can interact with the female
• concluded DA released within MPOA is involved
  in sexual motivation (DA is increased prior to,
  and in preparation of, the actual act of
  copulation), and copulatory responses (DA
  increases further during the actual act of
  copulation)

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Central Mechanisms--Dopamine

• Data from lesions and dopamine release during
  behavior provide insight into functional roles of
  various brain regions to sex behavior
• dorsal striatum
• lesions of dopamine neurons in substantia nigra
  produce Parkinsonism symptoms difficulty
  initiating movements, slowness of actions and
  tremor
• DA levels increase significantly during the act
  of copulation
• dorsal striatum is thought to play a role in
  sensorimotor coordination (pursuit of female)
• MPOA
• lesions of MPOA block display of mounts,
  intromissions and ejaculations
• administration of DA antagonists into the MPOA
  can inhibit display of copulation
• DA levels increase prior to, and during, the act
  of copulating with a receptive female
• MPOA is thought to play a role in both sexual
  motivation and copulation
• MPOA receives pheromone-associated information
  from CMA this information provides one source of
  excitatory information linked to both sexual
  motivation and performance

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Central Mechanisms--Dopamine

• Nucleus Accumbens (NAcc)
• lesions do not block the display of copulation
• however, blocking input into the NAcc by
  bilateral lesioning the BLa can block bar
  pressing in order to gain access to a receptive
  female (discussed in terms of sexual motivation)
• DA levels increase prior to, and during, the act
  of copulating with a receptive female
• however, DA levels also increase in NAcc in
  response to feeding, drinking, drug
  self-administration
• Nacc is thought to play a role motivational
  processes and reward

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Hormonal Regulation

• Testosterone, and/or its conversion to estrogen,
  is important not only for stimulating copulation
  but also for motivational processes.
• If you train a male to bar press to gain access
  to a female, you can study the effects of
  specific brain lesions and compare those effects
  to that produced by castration.
• Sexual performance
• bilateral lesions of the MPOA, but not BLa,
  eliminate copulation (low of rats engaged in
  copulation)
• castration also dramatically decreases rats
  copulating
• Motivation processes
• bilateral lesions of the BLa, but not the MPOA,
  greatly reduce bar pressing to gain access to a
  receptive female
• castration also dramatically decreases bar
  pressing

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Hormones and Dopamine Regulation

• Does castration alter dopaminergic
  neurotransmission to effect changes in male sex
  behavior? Answer--yes, testosterone facilitates
  dopamine release!
• Studies by Alderson Baum (1981) Mitchell
  Stewart (1989)
• castration decreases the level of dopamine within
  the nucleus accumbens
• administration of testosterone, or estrogen plus
  dihydrotestosterone, increases dopamine levels
  within this nucleus
• Study by Hull et al. (1997)
• castration decreases release of dopamine within
  the MPOA in response to a precopulatory period
  (female behind barrier) or interactions with an
  estrous female
• some castrates (1 week) showed dopamine release
  in MPOA and engaged in sex behavior (short-term
  response eventually all castrated males will
  stop mating)
• administration of testosterone proprionate
  restored release of dopamine within MPOA in
  response to precopulatory period and facilitated
  mating

27
Opioid and Male Sex Behavior

• Opioids have been shown to inhibit male sex
  behavior.
• Study by Hughes colleagues (1988)
• infusion of B-endorphin into the medial preoptic
  area causes a reduction in males mating
  performance
• infusion of naloxone, an opioid receptor
  antagonist, can facilitate sexual performance and
  block inhibitory effect of B-endorphin

MPOA
Arcuate Nucleus
male copulation
naloxone
B-endorphin neurons
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Opioid and Male Sex Behavior

• Opioids have been shown to stimulate male sex
  behavior.
• Mehrara and Baum (1990)
• Design
• conditioned place preference is a test of
  motivation (desire to spend more time on one side
  of a chamber than the other)
• established conditioned place preference in male
  rats given the opportunity to mate on only one
  side of the chamber
• how much time would the males spend on the side
  of the chamber in which they had mated
• several variables 1) sham-operated vs.
  castration, 2) given naloxone at varying doses or
  saline, and 3) testing the animals 7 days after
  surgery or 14 days

29
Opioid and Male Sex Behavior

• Opioids have been shown to stimulate male sex
  behavior.
• Mehrara and Baum (1990)
• Results
• administration of naloxone decreased time spent
  in the chamber associated with mating (e.g., loss
  in conditioned place preference)
• administration of naloxone also decreased time
  spent in the chamber associated with mating in
  males 7 days after castration, but not 14 days,
  after castration
• Conclusion
• opioids act to facilitate conditioned place
  preference (blocking opioid activity can
  blockthis response)
• the ability of opioids to facilitate conditioned
  place preference is lost at 14 days
  postcastration
• opioids act to stimulate dopamine release from
  neurons within the VTA dopamine neurons in VTA
  project to NAcc NAcc is involved in motivational
  processes

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Opioid and Male Sex Behavior

• Opioids have been shown to stimulate male sex
  behavior.

opioids
Nucleus Accumbens
VTA


Motivation (e.g., conditioned place preference)
dopamine neurons
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Neurons Behavior

• How to study role of specific groups of neurons
  in specific behavioral responses?
• compare the behavior of lesioned animals to
  sham-operated controls
• lesions can be made with electrical current
  (electrolytic) or with neurotoxins
  (neurochemical)
• compare the behavior of different groups of
  animals following the administration of drugs
  that either increase or decrease the level of a
  specific neurotransmitter
• lesion specific neurons 6-hydroxydopamine is
  used to lesion dopamine neurons
• use agonists to stimulate activity at specific
  receptors
• use antagonists to block activity at specific
  receptors
• follow the activation of groups of neurons at
  various endpoints of behavior
• release of neurotransmitters
• expression of molecules that serve to mark
  activated neurons
• expression of c-fos proto-oncogene

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c-fos Expression

• c-fos expression is used as a marker of neuronal
  activation
• levels of c-fos mRNA and Fos protein are low in
  control, unhandled animals
• if you stimulate an animal--handling, mating,
  stress, etc., you will see significant increases
  in the level of these molecules
• the rise and fall occurs quickly
• c-fos mRNA peaks by 30-60 minutes, and declines
  to basal levels within 2 hours
• Fos protein peaks by 1-2 hour,s and declines to
  basal levels within 4 hours
• the transient nature of c-fos expression as well
  as the low levels basally enables detection of
  these molecules to serve as a marker of
  neuronal activation
• c-fos expression serves to couple inputs
  (neuronal or hormonal) with gene transcription
• Fos protein forms a dimer with Jun
  protein--called AP-1 factor
• AP-1 factor binds to specific DNA sequences (AP-1
  site) to control transcription of genes

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Components of Mating

• Kollack-Walker Newman (1997)
• Experimental Design
• sexually experienced male hamsters were exposed
  to female hamster vaginal secretion (FHVS) for 10
  minutes, or allowed to mate to different
  endpoints of copulation 5 intromissions, 1
  ejaculation, 5 ejaculations, long intromissions
• long-intromissions characterized by a period of
  extended intravaginal thrusting during which no
  sperm transfer occurs the occurrence of long
  intromissions is believed to reflect a state of
  sexual satiety (turning off male sex behavior)
• following FHVS exposure or copulation, males were
  kept in the mating arena (up to one hour) until
  they were anesthetized, perfused with fixative,
  their brains removed from the skull and sectioned
  on a freezing microtome
• a series of sections from each animal were
  processed for Fos immunocytochemistry
• the number of Fos-immunoreactive neurons were
  determined for specific brain regions in each
  animal, averaged per group, and compared
  statistically

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Components of Mating

• Kollack-Walker Newman (1997)
• Results
• significant patterns of activation were observed
  within medial amygdala (MeA), bed nucleus of the
  stria terminalis (BNST) and medial preoptic area
  (MPOA) following FHVS exposure alone, mating to
  1 ejaculation, and mating to multiple
  ejaculations
• in particular, neuronal cell clusters in the
  caudal part of the posterior medial nucleus of
  the amygdala (cMePD) were seen in 67 of males
  mating to 5 ejaculations, and in 100 of males
  mating to long intromissions
• Conclusion
• different patterns of c-fos expression were
  observed in males that engaged in different
  levels of sexual activity (chemoinvestigation,
  copulation)
• Question Does the selective pattern of neuronal
  activation seen following multiple ejaculations
  reflect a state of satiety or simply a phenomonon
  of experiencing multiple ejaculations?

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Sexual Satiety

• Parfitt Newman (1997)
• Experiment 1 Do the number of ejaculations
  prior to the occurrence of long intromissions
  make a difference in the pattern of c-fos
  expression?
• Design
• compare pattern of Fos-immunostaining within
  sexually experienced male hamsters mated to long
  intromissions in 2 groups 1) rested male
  hamsters (rested 1 week), and 2) male hamsters
  mated to long intromissions on 4 consecutive days
  prior to experimental day
• rested male hamsters show more ejaculations (on
  average 9 ejaculations) while repeatedly mated
  males show fewer ejaculations (on average 4
  ejaculations)
• Results (focus on neuronal cell clusters in
  cMePD)
• males mated to long intromissions in both groups
  (rested male hamsters and repeatedly mated male
  hamsters) showed the presence of neuronal cell
  clusters in cMePD
• Conclusion
• the number of ejaculations does not alter
  presence of neuronal cellclusters in cMePD

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Sexual Satiety

• Parfitt Newman (1997)
• Experiment 2 Does mating to only 1 ejaculation
  produce different patterns of neuronal activation
  in males that differ in their proximity to sexual
  satiety?
• Design
• compare pattern of Fos-immunostaining within
  sexually experienced male hamsters mated to only
  one ejaculation in 2 groups 1) rested male
  hamsters (rested 1 week), and 2) male hamsters
  mated to long intromissions on 4 consecutive days
  prior to experimental day
• Results (focus on neuronal cell clusters in
  cMePD)
• following mating to only 1 ejaculation,
  repeatedly mated males showed the presence of
  neuronal cell clusters within cMePD, while the
  rested males did not
• Conclusion
• the number of ejaculations are not critical for
  the occurrence of neuronal cell clusters within
  cMePD, rather the proximity to satiety was the
  critical factor
• neuronal cell clusters in cMePD may reflect
  development of a state of satiety!

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The Coolidge Effect

• The Coolidge Effect
• males recently mated with a particular female
  appear uninterested in further copulation (sexual
  satiation)
• however, if a new female is introduced, then the
  male will continue to mate
• thus, the introduction of novel females can
  extend a males sexual motivation and performance

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Sexual Satiety

• Summary
• different patterns of c-fos expression are
  observed in male hamsters mated to different
  endpoints of copulation--female odors, 1
  ejaculation and multiple ejaculations
• neuronal cell clusters in cMePD their
  appearance is dependent upon how close a male is
  to sexual satiety
• clusters are present in all males mated to long
  intromissions (LI)
• clusters are present after only 1 ejaculation in
  repeatedly mated males (4 ejaculations away from
  LI) but not in rested males (9 ejaculations away
  from LI)
• thus, neuronal cell clusters in cMePD may
  reflect development of a state of satiety
• in the literature, there is evidence that
  lesioning this part of the medial amygdala in
  female hamsters can produce a significant
  increase in the duration of copulation Takahashi
  Gladstone (1988) Behav. Neurosci., 102
  268-275

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Seasonality in Reproduction

• Seasonal breeders engage in sexual activity
  during certain times of the year.
• Reproductive axis also varies with the breeding
  season.
• Seasonal breeders hamsters, sheep, deer, zebra
  finches, ferrets
• Nonseasonal breeders rats, humans, primates
• Similar events within the reproductive axis are
  observed in males and females of species that are
  seasonal breeders.

Out of Breeding Season
During Breeding Season
GnRH
GnRH
LH FSH
LH FSH
testes
testes
testosterone
testosterone
No display of sex behavior
Display of sex behavior
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Seasonality in Reproduction

• The breeding season for different species varies,
  and is dependent upon an interaction between
  environmental factors and length of gestation.
• Environmental factors food availability,
  temperature--signaled by changes in day length
  (photoperiod).
• Goal ensure that the offspring are born under
  conditions optimal for their survival.

Ex. Sheep
Ex. Ferrets
Breeding season in fall
Breeding season in spring
Daylength is decreasing
Daylength is increasing
Gestation period 5 months
Gestation period 41 days
Newborn lambs will be born in spring
Newborn ferrets will be born in spring
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Seasonality in Reproduction

• Seasonality can affect how the brain responds to
  gonadal steroids
• How well does administration of exogenous
  testosterone restore male sex behavior?
• Testosterone is most effective in stimulating
  male sex behavior in males housed under long day
  photoperiod compared to males housed under short
  day photoperiod.

house under short day photoperiod
male hamsters
replace testosterone
castrate
house under long day photoperiod