Table 4.

Proposed mechanisms of the behavioral effects of AAS and other enhancers.

Type of effect	Behavior and effector	Mechanism and mediator
Positive	Enhanced mood (5-HT↑)	(1) Increased free Trp availability to the brain by  (a) Trp displacement by T and AAS  (b) TDO inhibition by E2 (classes I and II)
		(2) Enhanced 5-HT synthesis and function by E2 (classes I and II)
		(3) Effects of insulin, GH, and IGF-1
	Anxiolysis (5-HT, IPA)	(1) Enhanced serotonin synthesis and function
		(2) Trp transamination to IPA
Negative	Lowered mood (5-HT↓)	(1) TPH inhibition by excess Trp
		(2) PLP depletion by Trp transamination
		(3) PLP depletion by estrogens (classes I and II)
		(4) Cytokine induction of cerebral IDO by GH
	Anxiety and psychosis	(1) Increased production of KA and QA by  (a) Excessive Trp availability to the brain  (b) Increased cerebral uptake of kynurenine (class III)  (c) Induction of cerebral IDO  (d) Increased brain entry of QA by exercise and large AAS doses

Abbreviations: 5-HT, 5-hydroxytryptamine or serotonin; AAS, androgenic-anabolic steroids, E2, estradiol; GH, growth hormone; IDO, indoleamine 2,3-dioxygenase; IPA, indolepyruvic acid; KA, kynurenic acid; PLP, pyridoxal phosphate; QA, quinolinic acid; T, testosterone; TDO, tryptophan 2,3-dioxygenase; TPH, tryptophan hydroxylase; Trp, l-tryptophan.