Basic Research on Cholinergic Systems and Cognition

Cholinergic transients. Our discovery of fast cholinergic release events (“transients”) in attention task-performing rodents has had considerable impact on theories about cholinergic regulation and function. These transients were discovered using cutting-edge electrochemical methods that allow us to monitor, at real-time, to synaptic release of acetylcholine as well as glutamate (see Fig. 1 for a schematic of the electrodes and the neurochemical measurement schemes).

Fig. 1. Biosensors and measurement scheme for monitoring real-time release of acetylcholine and glutamate

We demonstrated that cholinergic transients mediate the detection of cues in certain trials. In other words,  cholinergic transients assure “hit” responses in such trials. Importantly, however, these cholinergic transients are only needed if such hits also require a shift in the attentional mode – away from monitoring for cues and to cue-directed behavior. This conclusion is based on the finding that cholinergic transients are only seen in cued trials (always ending with a hit) that part of trial sequences involving such an attention mode shift.

This conclusion implies that relatively complicated cognitive operations depend on these transients. As it is typical for research in the lab, we therefore also investigated humans while they are performing the type of hits that evoke cholinergic transients. Functional imaging (BOLD-fMRI) indicated an activation in the right frontal region in humans specifically when scoring this class of hits.

We published the findings form animals and humans in the same paper; this may have been the very first paper ever that presented converging evidence from electrochemical experiments in rats and imaging studies from humans – presented side-by side (see here).

More recently, we have used optogenetic methods to  show that cholinergic transients are indeed functional (Howard Gritton et al., in preparation). As illustrated in Fig. 2, these experiments involve complicated experimental designs as they involve mice performing our translational attention task while undergoing optogenetic manipulations (watch a mouse performing this task).

Opto timeline

Fig. 2. Design of studies using optogenetic methods for generating cholinergic transients.

The perhaps most fascinating finding from these studies comes from trials that do not involve a cue, and that normally result in a response indicating the animals’ decision that there was indeed no cue. Generating a cholinergic transient during these non-cue events caused false alarms, that is, false claims that a cue was present. It is difficult to envision a stronger evidence supporting the hypothesis that (right) prefrontal cholinergic transients cause the decision that a cue was present.

In addition to transients, cholinergic neurons have neuromodulatory influences on cortical information processing. In recent years, we ave begun to understand how different populations of cholinergic neurons generate transients and neuromodulate their targets, respectively. Furthermore, we developed hypotheses about the mechanisms responsible for interactions between these two components of cholinergic activity. We have recently published a paper that integrates this evidence (here), and Fig. 3 illustrates one major component of the cortical neurocircuitry involved in the interactions between neuromodulatory and transient cholinergic activity. Transients are generated by a separate population of cortical cholinergic inputs (upper red triangle illustrating a cholinergic terminal). Cue-evoked glutamatergic activity from thalamic projections (blue triangle illustrating a glutamatergic terminal) is necessary, but not sufficient, for generating cholinergic transients. Cholinergic modulatory activity influences the probability and amplitude of cholinergic transients by stimulating thalamic glutamatergic terminals, primarily via a4b2* nicotinic acetylcholine receptors (nAChRs) expressed by these terminals.

Fig4a inset

Fig. 3. Cortical circuitry required for generating cholinergic transients and effects of cholinergic neuromodulation.

This is also the primary reason why agonists at these nAChRs can benefit attentional performance. One more word about the pharmacology derived from the model illustrated in Fig. 3: It is rare that we understand the circuitry and neuronal mechanics underlying drug effects. For agonists at a4b2*nAChRs, their effects can be fairly precisely mapped on this circuit model and on the attentional performance mediated via this circuit. You can read more about this circuit-compliant neuropsychopharmacology here.

Genetic variations of cholinergic synaptic capacity. Given our primary interest in how cholinergic neurons work, we are obviously interested in finding opportunities to study species and situations that are associated with, or cause variations in, the capacity of cholinergic neurons to support attention. In recent years, we focused on mice that are heterozygous for the choline transporter (CHT). The CHT is essential for bringing choline into the synapse for the synthesis and release of acetylcholine (ACh). Mice lacking about half of these transporters have trouble to move enough CHTs into the synaptic plasma membrane when the going gets tough, and they exhibit attentional impairments that parallel the limited CHT-based capacity for cholinergic activity. This research, conducted in collaboration with Randy Blakely’s lab at Vanderbilt), is described in detail here and here.

As usual in our lab, we’d like to have a parallel and converging line of research in humans. To develop such a strategy in support of our research on CHT regulation and function, my colleague Cindy Lustig and our students have been searching for humans who are heterozygous for a CHT gene variant that reduces the capacity of the CHT by about 50% (see here). Many years later, we have found around 50 of such people. Amazingly, these folks self-report greater distractibility, Indeed, they are way more distractible than control subjects and their brains work quite differently when paying attention (see here). As we are learning more about the specific cognitive operations that differ between these folks and control subjects, we are translating this information back to our research in mice.

We now also have mice that over-express the CHT. We still have little data on these critters (see here) but there are indications to support the idea that these are indeed super-CHT mice. Are there humans with a CHT gene variant that makes them super-cholinergic? Stay tuned…

Translational Research on Cholinergic Systems and Attention

Over the years, we have extensively studied the role of abnormally-regulated cholinergic systems for the attentional problems that characterize a range of neuropsychiatric and neurodegenerative disorders, using animal models and also conducting research in patients. Furthermore, we have been eager bringing pharmacological strategies, derived form our basic research, to patient groups. Currently, our research concerns two main translational research interests.

Cognitive control of complex movements in Parkinson’s disease (PD). Cholinergic systems decline early in PD and are hypothesized to contribute to cognitive impairments as well as to impairments in performing complex movements. The main objectives of this project are to determine the impact of interactions between decline in cholinergic and dopaminergic brain systems on complex movement control, and the usefulness of new cholinergic treatments to improve cognitive functions as well as complex movement control in PD. We have produced a model of the greater propensity for falls in PD and also developed a new behavioral test system for these symptoms. As an example, Fig. 4 shows a rat traversing a rotating rod. Animals with partial losses of the cortical cholinergic input system and striatal dopamine – modeling the main neuronal losses that are present early in PD – fall very frequently when traversing such a task (see publications here and here ).

Fig. 4 .Control rat (a-e) and a rat with cortical cholinergic as well as striatal dopamine loss (f-j) traversing a rotating rod (10 rpm) at 0° incline. Both slip (b,g,i), but the rat with neuronal losses fails to evoke correcting steps and compensatory postural control after the second slip and falls (into a net).

We are currently assessing pharmacological strategies to benefit complex movement control in the model. The not-so-distant goal of this work is to form the basis for a clinical trial on treatments that will reduce the number of falls in PD patients.  This project is part of  our newly funded UDALL Center of Excellence for Parkinson’s Disease Research.

Attentional control of addictive cues. We think of addiction, specifically relapse, as a failure to constrain the attentional resources that are deployed to process drug-associated cues. As illustrated in Fig. 5, we conceptualize a vicious circle of attentional control loss that begins with poor attentional control as a trait (right figure). Such a trait contributes to impaired attentional control of drug cues (letting drug cues control behavior). Finally, extensive addictive drug use damages the integrity of brain regions mediating attentional control. As a result, attentional control deficits are mounting and the addict’s ability to shift cognitive activity to non-drug cues becomes more and more impaired.

Fig. 5. The vicious cycle of attentional control loss in addiction.

Our research focuses on the attentional capacities of animals that more readily acquire addiction-like behaviors than others. These are the sign-tracking rats (STs) that have been discovered by my colleague Terry Robinson. We have found that these rats have very poor attentional control and that this is associated with diminished capacity of the cholinergic system to modulate cortical networks (see here).

A main goal of this research is to develop treatments that limit the power of drug cues to consume attentional resources and to instigate drug-seeking behavior. Toward this goal, we are developing new experimental paradigms to assess the ability of rats to make such decisions, particularly if there are costs to the decision to go for the addictive drug. A promising treatment would be expected to increase the probability for task cues – over drug cues – to control behavior.

Meanwhile, we are also eager to find out whether we can identify human STs and address the question whether a propensity for sign-tracking predicts vulnerability for a range of addiction-like behaviors in humans…