- Activational Effects of Testosterone...

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Activational Effects of Testosterone Hormones like testosterone can simultaneously adjust many mechanisms throughout the brain and body: Hormones released into general circulation and so can reach any cells that express receptors for them Unlike neurotransmitters that communicate with neighboring cells, hormones are analogous to a radio broadcast to the entire body, received by any cells that tune to the right frequency General Theory: Testosterone promotes physiological and behavioral processes related to mating effort , and directs energy away from competing processes related to survival effort Muscle cells express testosterone receptors, and skeletal muscle mass increases under testosterone exposure
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Manipulation of Testosterone and Muscle Mass Bhasin et al. (2001) studied 61 men, 18-35 years At baseline (before treatment), measured all men for testosterone, fat free mass (measure of muscle mass), and lifting strength All men received a drug that blocked GnRH production each month, eliminating natural testosterone production Men randomly assigned to receive weekly testosterone injections for 20 weeks at one of 5 doses from 25 mg (produces testosterone below normal) to 600 mg (produces testosterone over twice normal) Measured fat free mass and lifting strength again at week 20
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Muscle Mass/Body Size Bhasin et al., 2001 Gains in fat-free mass and leg strength shown at higher doses.
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Where does energy for muscle construction come from? Skeletal muscle mass is energetically expensive to both construct and maintain Energy is finite, so it must be taken from elsewhere Testosterone promotes fat catabolism (breakdown) as source of energy Immune system is very energetically expensive, both for surveillance (cells searching for foreign cells) and acute responses
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Testosterone Regulation of Immune Responses Many cell-types within the immune system express testosterone receptors In vitro, adding androgens to cell cultures including immune cells causes those cells to replicate less (e.g., B cells – antibodies) Yesilova et al. (2000) studied young men with hypogonadism (low LH, testosterone, delayed puberty) Measured various antibody counts before and after treatment with synthetic LH Testosterone increased and antibody counts dropped 20-40% after treatment (antibody counts high vs. controls before treatment) Some evidence in nonhuman species that greater energy availability allows for both high testosterone and robust immune response simultaneously Suggests trade-offs between mating and survival effort are in fact caused by finite energy availability
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Behavioral Effects of Testosterone If physiological effects of testosterone are designed to promote intrasexual competition (muscle construction)/mate-seeking, then behavioral strategies should be coordinated with physiological changes Experimental studies in nonhuman species show testosterone injections in paternally investing birds: (1) reduce offspring
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