Investigating the Origin and Role of Catecholamines in the Visual Cortex of the Macaque Monkey

Abstract

The catecholamines - norepinephrine and dopamine- are released into cortex from subcortical nuclei. It is classically assumed, for example, that dopamine receptors in cortex bind molecules released by dopaminergic axons. In the case of cortex, those axons would be the ascending fibers of the ventral tegmental area (VTA). Interestingly, dopamine receptors are found in all layers of primary visual cortex (V1), but the innervation from the VTA is mostly restricted to layers 1 and 6. This anatomical mismatch raises the question as to whether or not dopamine from the VTA is the primary ligand for dopamine receptors in V1. An alternative possibility is that the locus coeruleus (LC) co-releases dopamine and norepinephrine into V1. LC axons innervate all layers of V1 and are thus anatomically positioned to provide a ligand to activate dopamine receptors. Another possibility is that dopamine from VTA axons passively diffuses to the middle layers of V1 from layers 1 and 6, and a third possibility is that norepinephrine is binding to these middle layer dopamine receptors. I will present results from a series of studies that address these three possibilities in which I find that activation of the LC can elicit release of both catecholamines into V1 in a manner that depends on experimental parameters such as the intensity and frequency of electrical stimulation of the LC. The data suggests that the LC co-releases dopamine with norepinephrine in a state-dependent manner.