Mammal-Like Striatal Functions in Anolis

Abstract

Serotonin (5-HT) 5-HT_2A and 5-HT_2C receptors are thought to play important roles in the mammalian striatum. As basal ganglia functions in general are thought highly conserved among amniotes, we decided to use in situ autoradiographic methods to determine the occurrence and distribution of pharmacologically mammal-like 5-HT_2A and 5-HT_2C receptors in the lizard, Anolis carolinensis, with particular attention to the striatum. We also determined the distributions of 5-HT_1A, 5-HT_1B/D, 5 HT₃, and 5-HT_uptake receptors for comparison. All 5-HT receptors examined showed pharmacological binding specificity, and forebrain binding density distributions that resembled those reported for mammals. Anolis 5 HT_2A/C and 5-HT_1A site distributions were similar in both in vivo and ex vivo binding experiments. 5-HT_{2A & C} receptors occur in both high and low affinity states, the former having preferential affinity for ¹²⁵I-(±)-2,5-dimethoxy-4-iodo-amphetamine hydrochloride (¹²⁵I-DOI). In mammals ¹²⁵I-DOI binding shows a patchy density distribution in the striatum, being more dense in striosomes than in surrounding matrix. There was no evidence of any such patchy density of ¹²⁵I-DOI binding in the anole striatum, however. As a further indication that anoles do not possess a striosome and matrix striatal organization, neither ³H-naloxone binding nor histochemical staining for acetylcholinesterase activity (AChE) were patchy. AChE did show a band-like striatal distribution, however, similar to that seen in birds.