Effects of l-DOPA on aggressive behavior and central monoaminergic activity in the lizard Anolis carolinensis, using a new method for drug delivery

The dopamine (DA) precursor, l-DOPA (500 μg), was injected into living crickets, which were ingested (one each) by adult male Anolis carolinensis. This method of delivery elevated plasma l-DOPA and DA concentrations by ∼1000-fold. In contrast, plasma epinephrine (Epi) and norepinephrine (NE) were not influenced by l-DOPA treatment, although they were elevated following the consumption of the cricket. Lizards that ingested l-DOPA treated crickets had elevated l-DOPA in all brain regions measured, with DA and/or DOPAC also increased significantly in most brain regions studied. Despite increased DA levels in the striatum and nucleus accumbens as a response to l-DOPA, the treatment had no influence on general motor activity. Central serotonin, NE, and Epi systems were not affected in any brain region by oral l-DOPA treatment. In addition, aggression was inhibited by this dose of l-DOPA, even though there was no effect on serotonergic systems. This is surprising because controlling aggressive behavior is usually considered the province of serotonergic activity. Aggression was measured before and after treatment, and while saline-treated lizards retained the full vigor of aggressive activity, those fed a cricket injected with l-DOPA were only one-third as aggressive after treatment. As l-DOPA treatment did not affect general motor activity, the effect appears to be directly associated with aggression. This is supported by the observation that l-DOPA treatment delayed latency to eyespot darkening, which predicts the latency to aggression.