Coordinated release of multiple neurotransmitters in monkey primary visual cortex (V1) following acetylcholine application

Acetylcholine (ACh) can enhance the amplitude of synaptic potentials and thereby exert an influence on various cognitive functions including attention and memory encoding. The primary visual cortex is among the brain regions receiving cholinergic innervation from neurons originating in subcortical areas such as the basal forebrain. Here we study the effects of local application of ACh in V1 simultaneously on the concentration of several neurotransmitters in the extracellular brain fluid. Chemical analysis was performed by hydrophilic interaction chromatography (HILIC) coupled to tandem mass spectrometry (MS/MS). Extracellular fluid was withdrawn at 30nl/min by push-pull sampling method and collected in vials for sequence analyses. In addition to Ach, we monitored serotonin, dopamine, GABA, glutamate and aspartate. In addition however, we observed increases in other neurotransmitters lasting tens of minutes. Among the five neurotransmitters, GABA presented the most significant changes, reaching on average 1200 nM from the basal level at 50 nM. As expected, measured concentrations of ACh increased robustly following the ACh application, but it remained at low nM range given injection of 1 mM solution, due to the fast cholinergic metabolism in the brain. In control experiments we verified that increases in the concentrations of these neuro transmitters were not observed after the local injection of lidocaine. Our results indicate that ACh injection leads to an increase not only of ACh itself but also of other neurotransmitters and modulators, highlighting the closely coupled nature of cortical neurotransmitter systems