Cholinergic Overstimulation Attenuates Rule Selectivity in Macaque Prefrontal Cortex

Abstract

Acetylcholine is released in the prefrontal cortex (PFC) and is a key modulator of cognitive performance in primates. Cholinergic stimulation has been shown to have beneficial effects on performance of cognitive tasks, and cholinergic receptors are being actively explored as promising targets for ameliorating cognitive deficits in Alzheimer's disease. We hypothesized that cholinergic stimulation of PFC during performance of a cognitive task would augment neuronal activity and neuronal coding of task attributes. We iontophoretically applied the general cholinergic receptor agonist carbachol onto neurons in dorsolateral PFC (DLPFC) of male rhesus macaques performing rule-guided prosaccades and antisaccades, a well established oculomotor task for testing cognitive control. Carbachol application had heterogeneous effects on neuronal excitability, with both excitation and suppression observed in significant proportions. Contrary to our prediction, neurons with rule-selective activity exhibited a reduction in selectivity during carbachol application. Cholinergic stimulation disrupted rule selectivity regardless of whether it had suppressive or excitatory effects on these neurons. In addition, cholinergic stimulation excited putative pyramidal neurons, whereas the activity of putative interneurons remained unchanged. Moreover, cholinergic stimulation attenuated saccade direction selectivity in putative pyramidal neurons due to nonspecific increases in activity. Our results suggest excessive cholinergic stimulation has detrimental effects on DLPFC representations of task attributes. These findings delineate the complexity and heterogeneity of neuromodulation of cerebral cortex by cholinergic stimulation, an area of active exploration with respect to the development of cognitive enhancers. The neurotransmitter acetylcholine is known to be important for cognitive processes in the prefrontal cortex. Removal of acetylcholine from prefrontal cortex can disrupt short-term memory performance and is reminiscent of Alzheimer's disease, which is characterized by degeneration of acetylcholine-producing neurons. Stimulation of cholinergic receptors is being explored to create cognitive enhancers for the treatment of Alzheimer's disease and other psychiatric diseases. Here, we stimulated cholinergic receptors in prefrontal cortex and examined its effects on neurons that are engaged in cognitive behavior. Surprisingly, cholinergic stimulation decreased neurons' ability to discriminate between rules. This work suggests that overstimulation of acetylcholine receptors could disrupt neuronal processing during cognition and is relevant to the design of cognitive enhancers based on stimulating the cholinergic system.

Neuromodulation of Persistent Activity and Working Memory Circuitry in Primate Prefrontal Cortex by Muscarinic Receptors

“…It is noteworthy that carbachol is a general cholinergic agonist that has agonist activity at both muscarinic and nicotinic receptors. However, as discussed earlier, studies heretofore report that stimulation of the major nicotinic receptor subtypes in PFC appear to be generally excitatory ( Yang et al, 2013 ; Sun et al, 2017 ), suggesting that the physiological actions of carbachol in Major et al (2018) were mediated by muscarinic receptors.…”

“… Influence of cholinergic agonist carbachol on persistent activity. (A) adapted from Egorov et al (2002) ; (B) adapted from Major et al (2018) . (A) Example of persistent activity evoked in a rat entorhinal cortical neuron by a current pulse in the presence of carbachol.…”

“…To clarify whether carbachol could induce or augment persistent activity in PFC during WM task performance, our group conducted experiments where we microiontophoretically applied carbachol on PFC neurons in rhesus monkeys performing the rule-memory guided pro- and antisaccade task ( Major et al, 2018 ). Surprisingly, we found that carbachol had mixed effects on neuronal physiology and persistent activity ( Figure 4B ).…”

“…Given the pervasiveness of scopolamine-induced suppression of PFC neurons described above ( Major et al, 2015 ), and that some or all of the effects of PFC carbachol stimulation were mediated by muscarinic receptors ( Major et al, 2018 ), the question arises as to which muscarinic receptor subtypes contributed to the various physiological effects on persistent activity and task-selectivity changes caused by these cholinergic manipulations.…”

Evolution in Neuromodulation—The Differential Roles of Acetylcholine in Higher Order Association vs. Primary Visual Cortices

“…Iontophoresis of the muscarinic antagonist, scopolamine, markedly abolished neural representations of rules in a working memory task at a dose that only moderately reduced activity in V1 (Herrero et al, 2008 ; Major et al, 2015 ). However, the muscarinic response in dlPFC is complex, as nonspecific excitation of muscarinic receptors in dlPFC also eroded rule selectivity (Major et al, 2018 ), indicating a need for precise levels and patterns of muscarinic receptor engagement. The role of muscarinic receptor mechanisms in monkey dlPFC, including the contributions of M1R vs. M2R stimulation, is an area of ongoing research, where preliminary data indicate a major contribution of M1R to network function (Galvin, Wang, and Arnsten, unpublished).…”