Separation-induced body weight loss, impairment in alternation behavior, and autonomic tone: effects of tyrosine

We have investigated the effects of tyrosine on alternation behavior acid hippocampal adrenergic and cholinergic tone in a model of self-induced weight loss caused by separation stress. Separation decreased body weight in mice(P<.001) and spontaneous alternations in the T-maze (P<.001). This impairment was associated with depletion of both norepinephrine (NE, P<.001) and dopamine (P<.01) while increasing MHPG (P<.05) and the ratio of MHPG/NE (P<.05). Increasing tyrosine availability restored performance to control levels (P<.001) and repleted dopamine (P<.05) and presumably also NE (indicated by increases in both MHPG, P<.001, and MHPG/NE, P<.05). Stress increased adrenergic cut-receptor density (P<.001) without changing its K-d and the B-max and K-d of -receptors, suggesting that it decreased NE transmission through action on alpha (2)-receptors. The balance between beta- and alpha (2)-receptors appeared to be related to alternation behavior as shown by the decrease (P<.01) and increase (P<.05) in their ratios induced by stress and tyrosine, respectively. With regard to cholinergic tone, separation stress increased M1 receptor density (P<.05) and its mRNA signal (P<.001). Tyrosine further increased M1 receptor density of stressed mice (P<.05). Tyrosine might be a potential therapy for cognitive and mood problems associated with the maintenance of a reduced body weight in the treatment of obesity and in the extreme case of anorexia nervosa. <(c)> 2001 Elsevier Science Inc. All rights reserved.