Browsing by Subject "acetylcholine"

Studies in the ankyrin-B+/- mouse reveal that ankyrin-B deficiency is associated with both the benefits of enhanced cardiac contractility and the costs of arrhythmia, early senescence, reduced lifespan, and impaired glucose tolerance. This constellation of traits is known as ankyrin-B syndrome, which may have important implications for humans possessing functional ankyrin-B mutations. We found that ankyrin-B variants are surprisingly common, ranging from 2 percent of European individuals to 8 percent in individuals from West Africa. Furthermore, by studying of the metabolic phenotype associated with ankyrin-B mouse, we have uncovered a major new dimension to ankyrin-B syndrome, a link between ankyrin-B and parasympathetic control of insulin secretion. Stimulation of pancreatic beta cells by acetylcholine augments glucose-stimulated insulin secretion by inducing inositol-trisphosphate receptor (InsP3R)-mediated Ca2+ release. We report that ankyrin-B is also enriched in pancreatic beta cells. Ankyrin-B-deficient islets display impaired potentiation of insulin secretion by the muscarinic agonist carbachol, blunted carbachol-mediated intracellular Ca2+- release, and reduced InsP3R stability. Ankyrin-B(+/-) mice also display postprandial hyperglycemia, consistent with impaired parasympathetic potentiation of glucose-stimulated insulin secretion. R1788W mutation of ankyrin-B impairs its function in pancreatic islets and associates with type 2 diabetes in Caucasians and Hispanics. Finally, we have generated knockin mice corresponding to the R1788W and L1622I mutations. Functional characterization of these animals will allow us to better understand the relationship between human ankyrin-B variants and ankyrin-B syndrome.

While learning in response to extrinsic reinforcement is theorized to be driven by dopamine signals that encode the difference between expected and experienced rewards, skills that enable verbal or musical expression can be learned without extrinsic reinforcement. Instead, spontaneous execution of these skills is thought to be intrinsically reinforcing. Whether dopamine signals similarly guide learning of these intrinsically reinforced behaviors is unknown. Juvenile zebra finches are distinguished by their ability to copy the song of an adult tutor, a spontaneous, intrinsically reinforced process. Here, I use the zebra finch as a model system to study the neural mechanisms that operate within a song-specialized region of the basal ganglia (sBG) to enable this remarkable form of motor learning. Using in vivo microdialysis and computational methods to quantify juvenile song development, I first determined that dopamine (DA) signaling in the sBG is necessary for song learning. Using genetically encoded DA sensors and fiber photometry, I showed that DA dynamics in the sBG faithfully track the learned quality of juvenile song performance on a rendition-by-rendition basis. Consequently, my experiments provide compelling evidence that DA functions in the sBG as a reward prediction error-like signal to drive song learning, a process that evolves spontaneously and does not depend on extrinsic reward or punishment. Furthermore, I found that DA release in the sBG is driven not only by inputs from midbrain DA neurons classically associated with reinforcement learning but also by song premotor “cortical” inputs, which act via local cholinergic signaling in the sBG to elevate DA during singing. While I was able to show that both cholinergic and dopaminergic signaling in the sBG are necessary for song learning, I further found that only DA tracks the learned quality of song performance. Therefore, dopamine dynamics in the basal ganglia encode performance quality to drive self-directed and long-term learning of natural behaviors.

Forebrain cholinergic projection systems innervate the entire cortex and hippocampus. These cholinergic systems are involved in a wide range of cognitive and behavioral functions, including learning and memory, attention, and sleep-waking modulation. However, the in vivo physiological mechanisms of cholinergic functions, particularly their fast dynamics and the consequent modulation on the hippocampus and cortex, are not well understood. In this dissertation, I investigated these issues using a number of convergent approaches.

First, to study fast acetylcholine (ACh) dynamics and its interaction with field potential theta oscillations, I developed a novel technique to acquire second-by-second electrophysiological and neurochemical information simultaneously with amperometry. Using this technique on anesthetized rats, I discovered for the first time the tight in vivo coupling between phasic ACh release and theta oscillations on fine spatiotemporal scales. In addition, with electrophysiological recording, putative cholinergic neurons in medial setpal area (MS) were found with firing rate dynamics matching the phasic ACh release.

Second, to further elucidate the dynamic activities and physiological functions of cholinergic neurons, putative cholinergic MS neurons were identified in behaving rats. These neurons had much higher firing rates during rapid-eye-movement (REM) sleep, and brief responses to auditory stimuli. Interestingly, their firing promoted theta/gamma oscillations, or small-amplitude irregular activities (SIA) in a state-dependent manner. These results suggest that putative MS cholinergic neurons may be a generalized hippocampal activation/arousal network.

Third, I investigated the hypothesis that ACh enhances cortical and hippocampal immediate-early gene (IEG) expression induced by novel sensory experience. Cholinergic transmission was manipulated with pharmacology or lesion. The resultant cholinergic impairment suppressed the induction of arc, a representative IEG, suggesting that ACh promotes IEG induction.

In conclusion, my results have revealed that the firing of putative cholinergic neurons promotes hippocampal activation, and the consequent phasic ACh release is tightly coupled to theta oscillations. These fast cholinergic activities may provide exceptional opportunities to dynamically modulate neural activity and plasticity on much finer temporal scales than traditionally assumed. By the subsequent promotion of IEG induction, ACh may further substantiate its function in neural plasticity and memory consolidation.

Examining the coordination of excitatory and inhibitory (E/I) activity within cortical circuits is a fundamental approach to understanding normal and aberrant circuit function. Alteration to this E/I coordination is one of the leading pathophysiological models for several neurological disorders, including epilepsy, schizophrenia, ADHD, and autism spectrum disorders (ASD) (Nelson et al. 2015, Mullins et al. 2016). The overarching goal of this research is to understand the mechanism that regulates excitatory and inhibitory coordination within the cerebellum. Using a range of techniques to monitor and manipulate cortical circuits; I examined how the neuromodulator, acetylcholine, alters E/I activity at the initial input stage of the cerebellar cortex, the granular layer. Additionally, in a collaboration with Dr. Hatten's group at Rockefeller, we investigated the post-migratory role of Astrotactin 2 (ASTN2), a risk gene for autism; and found a role in regulating the surface level expression of synaptic proteins. By investigating how basic circuit function is modified in a transient manner, i.e. neuromodulation, we can reveal mechanisms that facilitate context-dependent learning; moreover, by examining permanent modifications, e.g. ASTN2, to circuit function, we can begin to understand neural mechanisms that underlie both normal synaptic function and the pathophysiology of ASDs.

These studies revealed the following: 1) acetylcholine actively modifies two out of three main nodes - excitatory mossy fiber terminals and inhibitory Golgi cells - within the granule cell layer. This modulation resulted in a bidirectional change in the excitability of granule cells, suggesting that cholinergic circuits within the granule cell layer are well situated to alter information processing in a context-specific manner. 2) ASTN2 binds and regulates the surface expression of multiple synaptic proteins via endocytosis. A truncated form of ASTN2 (ASTN2-JDUP), resulting from copy number variation in its FNIII and MAC/Perforin domains, occurs in patients with neurodevelopmental disorders. Investigations into this JDUP variant revealed changes in the binding affinity to several different binding partners, including neuroligins 1-4. Additionally, conditional overexpression of JDUP or ASTN2 in Purkinje cells, revealed differential changes in postsynaptic glutamatergic and GABAergic activity. Overall, these ASTN2 results lay the foundation for future studies using an ASTN2 loss-of-function mouse model.

Auditory sensations can arise from objects in our environment or from our own actions, such as when we speak or make music. We must able to distinguish such sources of sounds, as well as form new associations between our actions and the sounds they produce. The brain is thought to accomplish this by conveying copies of the motor command, termed corollary discharge signals, to auditory processing brain regions, where they can suppress the auditory consequences of our own actions. Despite the importance of such transformations in health and disease, little is known about the mechanisms underlying corollary discharge in the mammalian auditory system. Using a range of techniques to identify, monitor, and manipulate neuronal circuits, I characterized a synaptic and circuit basis for corollary discharge in the mouse auditory cortex. The major contribution of my studies was to identify and characterize a long-range projection from motor cortex that is responsible for suppressing auditory cortical output during movements by activating local inhibitory interneurons. I used similar techniques to understand how this circuit is embedded within a broader neuromodulatory brain network important for learning and plasticity. These findings characterize the synaptic and circuit mechanisms underlying corollary discharge in mammalian auditory cortex, as well as uncover a broad network interaction potentially used to pattern neural associations between our actions and the sounds they produce.