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Tyrosine hydroxylase expression is affected by sexual vigor and social environment in male Cnemidophorus inornatus

Although the distribution of catecholamine‐synthesizing cells has been described for a variety of taxa, less is known about the functional significance of particular populations in nonmammalian species, especially reptiles. To understand the role of these populations in the display of social behaviors in lizards, we studied the interactive effects of sexual vigor (sexually vigorous vs. sluggish) and social condition (housing in isolation vs. with females) on the number and somal areas of cells expressing tyrosine hydroxylase (TH), a rate‐limiting enzyme in catecholamine synthesis, in male whiptail lizards, Cnemidophorus inornatus. We found that, regardless of social condition, sexually vigorous males had more TH‐immunoreactive (TH‐ir) cells in the dorsal hypothalamus (DH) relative to sluggish males. Sexually vigorous males also had more TH‐ir cells in the substantia nigra pars compacta (SNpc), but this difference was significant only among males housed with females. Sexually vigorous males that had been housed with females had smaller TH‐ir cells in the preoptic area (POA) than vigorous males housed in isolation. On the other hand, no significant differences were found in the anterior hypothalamus. These results highlight the regional heterogeneity in the plasticity of TH expression and suggest that, just as in other species, the DH, SNpc, and POA might be involved in the expression of social behaviors and in behavioral plasticity following social experiences in lizards. J. Comp. Neurol. 476:429–439, 2004. © 2004 Wiley‐Liss, Inc.