Acetylcholine levels and choline acetyltransferase activity in turtle cortex

Histochemical localization of acetylcholine esterase (AChE) shows that the reaction product is in the outer half of the molecular layer of dorsal cortex of the turtle Pseudemys. Thalamic and noradrenergic locus coeruleus fibers are found in the same location. Two hypotheses could account for this apparent overlap of inputs. First, a cholinergic fiber system could exist in turtle cortex that occupies the same portion of the molecular layer. On the other hand, the AChE enzyme could be associated with a non-cholinergic fiber system, for example the adrenergic fibers. In the latter alternative no cholinergic fiber input would need to be present in turtle cortex at all. Our experiments analyzed the levels of acetylcholine (ACh) and the activity of its synthetic enzyme, choline acetyltransferase (ChAT) in adult turtle thalamic input to cortex as a first step toward distinguishing between these alternatives. The results show that turtle cortex contains ACh and exhibits ChAT activity. These biochemical results support the idea that the AChE staining pattern in the outer half of the molecular layer may reflect the laminar distribution of cholinergic fiber activity in this simple cortex.