Developmental origins of sex differences in the neural circuitry of play

Together, the coordinated actions of both androgens and estrogens are necessary for complete masculinization of the brain and behavior in rodents.

“Together, the coordinated actions of both androgens and estrogens are necessary for complete masculinization of the brain and behavior in rodents (McCarthy, 2008; Zuloaga et al., 2008).”

• cites McCarthy, 2008 sourceDisputed

  In “The Aromatization Hypothesis in Brain Sex Differentiation” (page unavailable), the review says estradiol was more effective than testosterone at masculinizing the rat brain, while the nonaromatizable androgen DHT was largely ineffective. It also says estradiol treatment appears to fully mimic the effects of estradiol derived from testicular androgens. That conflicts with the claim that coordinated actions of both androgens and estrogens are necessary. (LLM, conf=0.84)

  Checked by a reasoning model — may be wrong.

  Experimental — accuracy not yet measured
• cites Zuloaga et al., 2008 sourceNot checked — no full text of the source

The masculinizing effects of steroids are region-specific and diverse, ranging from epigenetic modifications to cell genesis, cell death, synaptogenesis, synaptic pruning, axonal and dendritic growth and branching, myelination, and neurochemical phenotype.

“The masculinizing effects of steroids are region-specific and quite diverse, ranging from epigenetic modifications to cell genesis, cell death, synaptogenesis, synaptic pruning, axonal and dendritic growth and branching, myelination, and neurochemical phenotype (McCarthy & Arnold, 2011).”

• cites McCarthy & Arnold, 2011 sourcePartly supported

  Split into 2 atoms — 0 confirmed, 2 partially supported, 0 disputed, 0 misattributed, 0 unverifiable. Expand the claim to see per-atom verdicts.

  Experimental — accuracy not yet measured

The development of male-typical play is largely androgen-dependent.

“Initial studies investigating the sexual differentiation of play discovered that the development of male-typical play is actually largely androgen-dependent.”

• cites Meaney et al., 1983 sourceSupported

In rodents, sexual differentiation processes occur during a critical period beginning just days prior to birth and ending shortly after parturition.

“In rodents, these processes occur during a critical period of life beginning just days prior to birth and ending shortly after parturition (McCarthy et al., 2018).”

• cites McCarthy et al., 2018 sourcePartly supported

  The “Critical periods versus sensitive periods” section places the onset of the rodent critical window before birth, with fetal androgen production beginning several days prenatally. But it says the window’s closing varies by endpoint and that females remain sensitive longer than males; the “beginning and an end but also a middle” section gives an example extending to about postnatal day 10. Thus, a single end point shortly after birth overgeneralizes the paper’s account. (LLM, conf=0.86)

  Experimental — accuracy not yet measured

Treating female rat pups with testosterone or 5α-dihydrotestosterone during the sensitive period increases their play to male-typical levels.

“Treating female rat pups with testosterone or 5α-dihydrotestosterone (a testosterone analog that cannot be converted into estradiol) during the sensitive period in development increases their play to male-typical levels.”

• cites Meaney et al., 1983 sourceSupported

When play is highly motivated following social isolation and the play bout is short, there are no consistent sex differences in the structure of or time spent engaged in play.

“When play is highly motivated, as is the case following periods of social isolation, and the play bout is short, there are no consistent sex differences in the structure of or time spent engaged in play (Panksepp & Beatty, 1980; Panksepp, 1981; Thor & Holloway, 1984).”

• cites Panksepp & Beatty, 1980 sourceNot checked — no full text of the source
• cites Panksepp, 1981 sourceNot checked — no full text of the source
• cites Thor & Holloway, 1984 sourcePartly supported

  The Procedures section says social isolation increases play fighting and that sex differences in measures of play fighting appear generally nonrobust. It does not establish that, specifically in short play bouts after isolation, males and females show no consistent differences in play structure or time spent playing. (LLM, conf=0.83)

  Experimental — accuracy not yet measured

The neurochemical underpinnings of motivated play appear to be distinct in each sex.

“Moreover, the neurochemical underpinnings of motivated play appear to be distinct in each sex (Bredewold & Veenema, 2018), which is discussed further below.”

• cites Bredewold & Veenema, 2018 sourceSupported

In primates, androgens mediate most, if not all, of the masculinization of the brain.

“However, this does not appear to be true in primates, where androgens mediate most, if not all, of the masculinization of the brain (Thornton et al., 2009).”

• cites Thornton et al., 2009 sourcePartly supported

  The Conceptual issues and conclusions section says prenatal androgens organize neural mechanisms underlying sexually dimorphic behaviors and reports that many rhesus-monkey behaviors are affected. It also says the precise neural effects have not been directly tested in rhesus monkeys. The paper therefore supports a role for androgens, but not the claim that they mediate most or all brain masculinization across primates. (LLM, conf=0.86)

  Experimental — accuracy not yet measured

For females, the critical period represents a sensitive period during which masculinization can be induced by exposure to high levels of steroids, naturally or experimentally.

“For females, this time represents a “sensitive period” during which masculinization can be induced as a result of exposure to high levels of steroids, either naturally or experimentally (McCarthy, 2010).”

• cites McCarthy, 2010 sourceSupported

The LS receives vasopressinergic inputs directly from the MeA and the bed nucleus of the stria terminalis (BNST).

“It receives vasopressinergic inputs directly from the MeA (Caffe et al., 1987) and the bed nucleus of the stria terminalis (BNST) (De Vries & Buijs, 1983).”

• cites Caffe et al., 1987 sourceNot checked — no full text of the source
• cites De Vries & Buijs, 1983 sourcePartly supported

  The citation to De Vries and Buijs is attached to the BNST input, not the MeA input. Their summary (p. 2) reports that BNST lesions sharply reduced vasopressin fibers in the lateral septum, but calls BNST cells only the probable source. The Discussion (p. 9) says fibers originating elsewhere might instead pass through the BNST, so it does not establish the direct BNST input as stated. (LLM, conf=0.84)

  Experimental — accuracy not yet measured

The sex difference in newborn cells is driven by a sex difference in endocannabinoids.

“The sex difference in newborn cells is driven by a sex difference in a class of signaling molecules that are essential to brain development: endocannabinoids (Krebs-Kraft et al., 2010).”

• cites Krebs-Kraft et al., 2010 sourcePartly supported

  The Abstract reports more newly generated cells in neonatal female rat amygdala and shows that inhibiting endocannabinoid degradation eliminated the proliferation difference. The Discussion says the data support the hypothesis that endocannabinoid/CB2 signaling underlies that difference. Calling it definitively “driven by” endocannabinoids is stronger than the paper’s qualified conclusion; the copy also describes their influence on brain development but does not establish that they are essential to it. (LLM, conf=0.82)

  Experimental — accuracy not yet measured

Lower astrocyte density in the MeA is correlated with male-typical levels of play.

“Lower astrocyte density in the MeA is correlated for male-typical levels of play (VanRyzin et al., 2019).”

• cites VanRyzin et al., 2019 sourceSupported

Disruption of estrogen synthesis within the developing cerebellum during the second postnatal week reduces play by juvenile males.

“Lastly, we have found that disruption of estrogen synthesis within the developing cerebellum during a later sensitive period, the second postnatal week, reduces play by juvenile males (Dean et al., 2012).”

• cites Dean et al., 2012 sourcePartly supported

  The Results (page unavailable) report that systemic nimesulide treatment from PN7 to PN13 reduced play in juvenile male rats. But the treatment inhibited prostaglandin synthesis, not estrogen synthesis; the Discussion says the relationship between prostaglandins and estradiol in the cerebellum remains for future work. (LLM, conf=0.88)

  Experimental — accuracy not yet measured

If females are treated with CB1 and CB2 receptor agonists to mimic higher endocannabinoid levels, their newborn cell number and eventual play behavior is masculinized, even in the absence of testosterone.

“If females are treated with CB1 and CB2 receptor agonists to “mimic” the higher endocannabinoid levels, their newborn cell number and eventual play behavior is masculinized, even in the absence of testosterone (Argue et al., 2017; VanRyzin et al., 2019).”

• cites Argue et al., 2017 sourcePartly supported

  The Results (Fig. 2; page not provided) show that neonatal coadministration of CB1 and CB2 agonists increased females’ juvenile play to male levels. The paper does not report that this combined treatment masculinized newborn cell number; its Discussion instead describes a prior finding involving a nonspecific agonist. It also does not test whether the effect occurs independently of testosterone. (LLM, conf=0.88)

  Experimental — accuracy not yet measured
• cites VanRyzin et al., 2019 sourceSupported

Blocking testosterone by using an androgen receptor antagonist in females treated with testosterone completely prevents these effects.

“Similarly, blocking testosterone by using an androgen receptor antagonist in females treated with testosterone completely prevents these effects (VanRyzin et al., 2019).”

• cites VanRyzin et al., 2019 sourcePartly supported

  The Results section ('Testosterone Programs 2-AG Content…'; page not provided) reports that flutamide, an androgen receptor antagonist, prevented the testosterone-induced increase in amygdala 2-AG in female pups. It does not show that flutamide completely prevented the other reported effects on newborn cells, microglial phagocytosis, or social play; the play experiment instead used cannabinoid receptor antagonists. (LLM, conf=0.85)

  Experimental — accuracy not yet measured

Pharmacological manipulation of the V1a receptor elicits a sex-dependent effect on social play.

“and pharmacological manipulation of the V1a receptor elicits a sex-dependent effect on social play.”

• cites Veenema et al., 2013 sourceSupported

Male rats with nonfunctional androgen receptors (Tfm males) engage in some aspects of play at male-typical levels.

“Supporting this idea, male rats with nonfunctional androgen receptors, so-called “Tfm males”, engage in some aspects of play at male-typical levels (Meaney et al., 1983; Field et al., 2006).”

• cites Meaney et al., 1983 sourcePartly supported

  The Discussion reports that weanling-castrated males and intact males did not differ in play-fighting frequency. But neither the Discussion nor the Results addresses Tfm males or nonfunctional androgen receptors; the studied groups received testosterone propionate, corticosterone, or oil. The male-typical play comparison is supported for a different condition, not for Tfm males. (LLM, conf=0.84)

  Experimental — accuracy not yet measured
• cites Field et al., 2006 sourceSupported

The amygdala contributes to processing the rewarding aspects of play.

“The amygdala, in addition to regulating the sex difference in play, contributes to processing the rewarding aspects of play (Trezza et al., 2012).”

• cites Trezza et al., 2012 sourceSupported

Female rodents become addicted faster than males.

“Female rodents become addicted faster than males and this is believed to be due to a greater activation of the reward circuit (Becker & Koob, 2016).”

• cites Becker & Koob, 2016 sourcePartly supported

  The “2. Animal Models” section says female rats progress to cocaine-related addictive-like behavior more rapidly than males, supporting the claim for that drug and model. The sentence’s proposed explanation is not supported: “IV. Sex Differences in the Neurobiology of Addiction” reports a lower cocaine-induced dopamine increase in the nucleus accumbens in females, rather than greater activation of that reward-related response. No page numbers were provided in our copy. (LLM, conf=0.85)

  Experimental — accuracy not yet measured

The MeA and LS are fundamental to expression of adult social behaviors that also exhibit sex differences, such as parenting, sex behavior, and aggression.

“Notably, the two brain regions implicated in generating sex differences in play, the MeA and LS, are also fundamental to expression of adult social behaviors that also exhibit sex differences, such as parenting, sex behavior, and aggression (Chen et al., 2019; Gammie & Nelson, 1999; Gogate et al., 1995; Tsukahara et al., 2014; Wong et al., 2016).”

• cites Chen et al., 2019 sourcePartly supported

  The Discussion (page not provided) establishes a sexually dimorphic role for the MeA in parenting and infanticidal behavior. The cited passages do not establish a role for the LS or specifically support the claims about sexual behavior and aggression; those parts of the multi-citation sentence are not supported by this paper. (LLM, conf=0.88)

  Experimental — accuracy not yet measured
• cites Gammie & Nelson, 1999 sourcePartly supported

  The paper’s introductory passage describes sex-related differences in aggression and places maternal aggression within maternal behavior. Its discussion of brain regions focuses on the MPOA, SCN, and SPa, however; it does not establish that the MeA or LS is fundamental to parenting, sexual behavior, or aggression. No page number is available in our copy. (LLM, conf=0.82)

  Experimental — accuracy not yet measured
• cites Gogate et al., 1995Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

These sex differences are maintained in adulthood by the actions of circulating gonadal hormones.

“Treating female or castrated male rat pups with testosterone increases the density of vasopressinergic inputs to the LS (De Vries et al., 1983), and these sex differences are maintained in adulthood by the actions of circulating gonadal hormones (Van Leeuwen et al., 1985; Miller et al., 1992).”

• cites Van Leeuwen et al., 1985Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

• cites Miller et al., 1992 sourceSupported

The lower astrocyte density in the MeA of juvenile males is associated with higher IEG expression following a play bout than females.

“In support of this idea, the lower astrocyte density in the MeA of juvenile males is associated with higher IEG expression following a play bout than females.”

The process by which microglia control astrocyte number is crucial to later playfulness.

“and the process by which microglia control astrocyte number is crucial to later playfulness.”

In the developing amygdala, microglia engulf and kill other brain cells which would otherwise survive.

“We found that in the developing amygdala, microglia, the innate immune cells of the brain, engulf and kill other brain cells which would otherwise survive.”

Neurons in some regions of the brain express high levels of aromatase, which converts testosterone into estradiol.

“Neurons in some regions of the brain express high levels of the enzyme aromatase, which converts testosterone into estradiol (Naftolin et al., 1975; McEwen et al., 1977).”

• cites Naftolin et al., 1975 sourceNot checked — no full text of the source
• cites McEwen et al., 1977 sourceNot checked — no full text of the source

The isolation-induced changes in play are not observed in adult rats.

“This appears to be strictly a juvenile phenomenon, however, as the same isolation-induced changes in play are not observed in adult rats.”

In spontaneous play paradigms, the sex difference in play behavior is remarkably consistent across the juvenile period.

“In these paradigms, the sex difference in play behavior is remarkably consistent across the juvenile period.”

• cites Argue & McCarthy, 2015bNot matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Sex differences in play are most robustly detected in spontaneous play paradigms.

“It is worth noting that sex differences in play are most robustly detected in spontaneous play paradigms (see discussion below).”

• Also stated in: § Juvenile male rats are more playful · plain

  “Sex differences in play are reliably observed in experimental conditions that favor spontaneous play (VanRyzin et al., 2020).”

• cites VanRyzin et al., 2020 sourceNot checked — no full text of the source

The sex difference in play manifests during the stage of life when sexual differentiation of the brain is complete, but before the pubertal increase in circulating steroid hormones.

“Second, the sex difference in behavior manifests during the stage of life when sexual differentiation of the brain is complete, but before the pubertal increase in circulating steroid hormones.”

Play is most robustly expressed during the juvenile period, which in rats corresponds to postnatal days 26-40.

“Play is most robustly expressed during the juvenile period, which in rats corresponds to postnatal days 26–40 (Panksepp, 1981).”

• cites Panksepp, 1981 sourceNot checked — no full text of the source

When a male and female are partnered, the time spent playing is an intermediate of that of same-sex pairs.

“When a male and female are partnered, the time spent playing is an intermediate of that of same-sex pairs (Argue & McCarthy, 2015b).”

• cites Argue & McCarthy, 2015bNot matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Males engage in social play more frequently and with higher physical intensity than females across many mammalian species.

“Social play consists of reciprocal physical interactions between conspecifics with many features conserved across species, including the propensity for males to engage in play more frequently and with higher physical intensity.”

• Also stated in: § Introduction · boosted

  “One fascinating feature of rough and tumble play is the presence of a reliable sex difference: males play more frequently and more intensely than females.”

• cites see Table 1Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

The sex difference in play is highly evolutionarily conserved, being observable in many mammalian species ranging from rats, cats, and dogs to humans.

“First, the sex difference is observable in many mammalian species ranging from rats, cats, and dogs, to humans, indicating that this sex difference in play (and play itself) is highly evolutionarily conserved, speaking to its importance (see Table 1).”

• cites see Table 1Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Early in life, the brain undergoes sexual differentiation, a process by which the brain develops to match the animal's gonadal sex.

“Early in life, the brain undergoes the process of sexual differentiation, a process by which the brain develops to match the animal’s gonadal sex (McCarthy et al., 2017).”

• cites McCarthy et al., 2017Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Sex differences in play must have their origins in perinatal development.

“The timing of this behavior makes it unique compared to most other sex differences in behavior, which are observed only in the adult animal (e.g. mating, parental behavior, aggression, etc.), and suggests that sex differences in play must have their origins in perinatal development.”

Juvenile females increase to the same level of play as males if isolated long enough.

“This appears to be because juvenile females increase to the same level of play as males if isolated long enough (Pellis, Field, Smith, & Pellis, 1997).”

• cites Pellis, Field, Smith, & Pellis, 1997 sourceNot checked — no full text of the source

Sex differences in play cannot be attributed to differences in circulating gonadal hormones because play is most robustly observed during the juvenile period prior to puberty.

“As noted above, play is most robustly observed during the juvenile period (i.e. prior to the onset of puberty) which means that differences in behavior cannot be attributed to differences in circulating gonadal hormones (estrogens in females, androgens in males).”

Once the sensitive period closes, by the end of the first week of the rodent's life, females become impervious to the masculinizing impact of steroids.

“Once the sensitive period closes, by the end of the first week of the rodent’s life, females become impervious to the masculinizing impact of steroids.”

Masculinization, initially attributed entirely to androgens, actually occurs through many estrogen-dependent mechanisms.

“Early in the study of steroid-induced sexual differentiation, it was discovered that masculinization—initially attributed entirely to the effects of androgens—actually occurred through many estrogen-dependent mechanisms.”

• cites Naftolin et al., 1975 sourceNot checked — no full text of the source
• cites McEwen et al., 1977 sourceNot checked — no full text of the source

Many components of play are also components of other complex behaviors.

“Complicating matters, many components of play are also components of other complex behaviors.”

The sex difference in frequency and intensity of play provides a contrast agent for visualizing differences within the circuit that are specific to play.

“However, the sex difference in frequency and intensity of play provides a contrast agent for visualizing differences within the circuit that are specific to play, whether they are differences in brain activation, connectivity and/or neurochemical phenotype.”

Estrogen signaling might regulate a "ceiling" for male-typical levels of play.

“These data suggest that estrogen signaling might regulate a “ceiling” for male-typical levels of play.”

Attempting to distinguish neurons as "play" or "not-play" is confounded.

“Thus, attempting to distinguish neurons as “play” or “not-play” is confounded.”

The sex difference in play is not a distributed property of the play circuit but instead is determined by sex differences in specific nodes.

“Importantly, the sex difference in play is not a distributed property of the play circuit but instead is determined by sex differences in specific nodes.”

By "tuning" the sensitivity of the MeA to social information, a common circuitry in males and females may produce behavioral sex differences.

“Thus, by “tuning” the sensitivity of the MeA to social information, a common circuitry in males and females may produce behavioral sex differences.”

Play activates several distinct brain regions, including those involved in motivation, reward, sensory, and motor processing.

“Using this methodology, two leading play research groups demonstrated that play activates several distinct brain regions, including those involved in motivation, reward, sensory, and motor processing (Gordon et al., 2002; van Kerkhof et al., 2014).”

• cites Gordon et al., 2002 sourceNot checked — no full text of the source
• cites van Kerkhof et al., 2014Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Implanting androgens directly into the MeA of developing females increases the frequency of their play to that of males.

“Conversely, implanting androgens directly into the MeA of developing females increases the frequency of their play to that of males (Meaney & McEwen, 1986).”

• cites Meaney & McEwen, 1986 sourceNot checked — no full text of the source

The mPFC is changed in response to play experience.

“The mPFC is also changed in response to play experience.”

• cites Bell et al., 2010 sourceNot checked — no full text of the source

The dendritic complexity of cortical pyramidal neurons is reduced in animals given multiple play partners over an extended period of time compared to those either socially deprived or given only limited access to the same partner.

“The dendritic complexity of cortical pyramidal neurons is reduced in animals given multiple play partners over an extended period of time compared to those either socially deprived or given only limited access to the same partner (Bell et al., 2010).”

• cites Bell et al., 2010 sourceNot checked — no full text of the source

Play-induced activation patterns within many PFC regions correlate with activity in other connected regions within the striatum and amygdala.

“play-induced activation patterns within many PFC regions correlate with activity in other connected regions within the striatum and amygdala (van Kerkhof et al., 2014).”

• cites van Kerkhof et al., 2014Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Pharmacological inactivation of the PFC, achieved by infusing GABA receptor agonists at the time of play, reduces the frequency and duration of play behavior.

“Moreover, pharmacological inactivation of the PFC, achieved by infusing GABA receptor agonists at the time of play, reduces the frequency and duration of play behavior (van Kerkhof et al., 2013).”

• cites van Kerkhof et al., 2013Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

The ultimate function of the sex difference in play may be to enable appropriate expression of sex-typical adult social behaviors.

“Thus, while play may facilitate the development of adult cognitive and social behaviors similarly in both sexes, the ultimate function of the sex difference in play may be to enable appropriate expression of sex-typical adult social behaviors.”

• AssumptionRests on an assumption: The MeA and LS generate sex differences in play.

Dopamine depletion in the striatum produces alterations in play behavior sequence.

“dopamine depletion in the striatum produces alterations in play behavior sequence (Pellis et al., 1993).”

• cites Pellis et al., 1993 sourceNot checked — no full text of the source

Treating female or castrated male rat pups with testosterone increases the density of vasopressinergic inputs to the LS.

“Treating female or castrated male rat pups with testosterone increases the density of vasopressinergic inputs to the LS (De Vries et al., 1983), and these sex differences are maintained in adulthood by the actions of circulating gonadal hormones (Van Leeuwen et al., 1985; Miller et al., 1992).”

• cites De Vries et al., 1983Not matched

  Couldn’t be matched to a paper in Crossref, so it can’t be checked.

Play is rewarding to both participants.

“Play is clearly rewarding to both participants (Vanderschuren et al., 2016)”

• cites Vanderschuren et al., 2016 sourceNot checked — no full text of the source

Neonatal lesions of the mPFC impair play in a variety of ways, largely resulting in a disruption of the animal's play structure and ability to respond to play initiations.

“As noted previously, neonatal lesions of the mPFC impair play in a variety of ways, largely resulting in a disruption the animal’s play structure and ability to respond to play initiations (Panksepp et al., 1994; Bell et al., 2009).”

• cites Panksepp et al., 1994 sourceNot checked — no full text of the source
• cites Bell et al., 2009 sourceNot checked — no full text of the source