Traditional descriptions of the cortical cholinergic input system focused on the diffuse organization of cholinergic projections and the hypothesis that slowly changing levels of extracellular acetylcholine (ACh) mediate different arousal states. The ability of ACh to reach the extrasynaptic space (volume neurotransmission), as opposed to remaining confined to the synaptic cleft (wired neurotransmission), has been considered an integral component of this conceptualization. Recent studies demonstrated that phasic release of ACh, at the scale of seconds, mediates precisely defined cognitive operations. This characteristic of cholinergic neurotransmission is proposed to be of primary importance for understanding cholinergic function and developing treatments for cognitive disorders that result from abnormal cholinergic neurotransmission.The entire cortical mantle is innervated by cholinergic neurons that originate in the nucleus basalis of Meynert, the substantia innominata and the horizontal limb of the diagonal bandall structures of the basal forebrain (BF) (FIG. 1). Traditionally, the cortical cholinergic input system has been categorized as the rostral component of the brain's ascending arousal systems, complementing the modulatory roles of, and interacting with, noradrenergic, serotonergic and other projection systems that broadly influence the readiness of the forebrain for input processing, wakefulness and somnolence 1 . However, more recent evidence has supported the more specific hypothesis that cortical cholinergic inputs mediate essential aspects of attentional information processing [2][3][4][5][6][7][8][9] . As a result, efforts to develop treatments for a wide range of cognitive disorders have focused on cholinomimetic approaches, particularly acetylcholinesterase (ACHE) inhibitors and agonists at muscarinic (m) and nicotinic (n) acetylcholine (ACh) receptors (AChRs) [10][11][12] .The anatomical organization of the cortical cholinergic input system seems to be largely consistent with the notion of a diffuse pathway (this article does not address the hippocampal cholinergic projection system or cholinergic projections to the amygdala). Tracing studies revealed a roughly ventrolateral, dorsomedial and rostrocaudal topographical organization of cholinergic BF projections but did not suggest a more precise topography that would indicate, for example, that adjacent neurons in the BF innervate adjacent regions in the cortex [13][14][15][16] (FIG. 1 b, (FIG. 2). This seemingly diffuse organization of the cortical cholinergic input system has supported descriptions that it exerts general, uniform effects across the cortical hemispheres 20 .In contrast to other diffusely organized ascending systems, such as the ascending reticular systems of the brainstem, the axons of corticopetal cholinergic neurons (subcortical afferents that project to both cerebral hemispheres) do not seem to be extensively collateralized: individual neurons innervate a relatively small cortical field [22][23][24] . Thus, separat...

The cortical cholinergic input system has been described as a neuromodulator system that influences broadly defined behavioral and brain states. The discovery of phasic, trial-based increases in extracellular choline (transients), resulting from the hydrolysis of newly released acetylcholine (ACh), in the cortex of animals reporting the presence of cues suggests that ACh may have a more specialized role in cognitive processes. Here we expressed channelrhodopsin or halorhodopsin in basal forebrain cholinergic neurons of mice with optic fibers directed into this region and prefrontal cortex. Cholinergic transients, evoked in accordance with photostimulation parameters determined in vivo, were generated in mice performing a task necessitating the reporting of cue and noncue events. Generating cholinergic transients in conjunction with cues enhanced cue detection rates. Moreover, generating transients in noncued trials, where cholinergic transients normally are not observed, increased the number of invalid claims for cues. Enhancing hits and generating false alarms both scaled with stimulation intensity. Suppression of endogenous cholinergic activity during cued trials reduced hit rates. Cholinergic transients may be essential for synchronizing cortical neuronal output driven by salient cues and executing cue-guided responses.V irtually all cortical regions and layers receive inputs from cholinergic neurons originating in the nucleus basalis of Meynert, the substantia innominata, and the diagonal band of the basal forebrain (BF). Reflecting the seemingly diffuse organization of this projection system, functional conceptualizations traditionally have described acetylcholine (ACh) as a neuromodulator that influences broadly defined behavioral and cognitive processes such as wakefulness, arousal, and gating of input processing (1, 2). However, anatomical studies have revealed a topographic organization of BF cholinergic cell bodies with highly segregated cortical projection patterns (3-7). Such an anatomical organization favors hypotheses describing the cholinergic mediation of discrete cognitive-behavioral processes. Studies assessing the behavioral effects of cholinergic lesions, recording from or stimulating BF neurons in behaving animals have supported such hypotheses, proposing that cholinergic activity enhances sensory coding and mediates the ability of reward-predicting stimuli to control behavior (8-17).In separate experiments using two different tasks, we reported the presence of phasic cholinergic release events (transients) in the medial prefrontal cortex (mPFC) of rodents trained to report the presence of cues (18,19). These studies used choline-sensitive microelectrodes to measure changes in extracellular choline concentrations that reflect the hydrolysis of newly released ACh by endogenous acetylcholinesterase (SI Results and Discussion). Importantly, such cholinergic transients were not observed in trials in which cues were missed and in which the absence of a cue was correctly reported and rewarded. Cho...

We previously reported involvement of right prefrontal cholinergic activity in veridical signal detection. Here, we first recorded real-time acetylcholine release in prefrontal cortex during specific trial sequences in rats performing a task requiring signal detection as well as rejection of non-signal events. Cholinergic release events recorded with sub-second resolution (“transients”) were observed only during signal-hit trials, not during signal-miss trials or non-signal events. Moreover, cholinergic transients were not observed for consecutive hits; instead they were limited to signal-hit trials that were preceded by factual or perceived non-signal events (“incongruent hits”). This finding suggests that these transients mediate shifts from a state of perceptual attention, or monitoring for cues, to cue-evoked activation of response rules and the generation of a cue-directed response. Next, to determine the translational significance of the cognitive operations supporting incongruent hits we employed a version of the task previously validated for use in research in humans and BOLD-fMRI. Incongruent hits activated a region in the right rostral prefrontal cortex (BA 10). Furthermore, greater prefrontal activation was correlated with faster response times for incongruent hits. Finally, we measured tissue oxygen in rats, as a proxy for BOLD, and found prefrontal increases in oxygen levels solely during incongruent hits. These cross-species studies link a cholinergic response to a prefrontal BOLD activation and indicate that these interrelated mechanisms mediate the integration of external cues with internal representations to initiate and guide behavior.

Striatal pavalbumin (PV) and cholinergic (CHI) interneurons are poised to play major roles in behavior by coordinating the networks of medium spiny cells that relay motor output. However, the small numbers and scattered distribution of these cells has made it difficult to directly assess their contribution to activity in networks of MSNs during behavior. Here, we build upon recent improvements in single cell calcium imaging combined with optogenetics to test the capacity of PVs and CHIs to affect MSN activity and behavior in mice engaged in voluntarily locomotion. We find that PVs and CHIs have unique effects on MSN activity and dissociable roles in supporting movement. PV cells facilitate movement by refining the activation of MSN networks responsible for movement execution. CHIs, in contrast, synchronize activity within MSN networks to signal the end of a movement bout. These results provide new insights into the striatal network activity that supports movement.