Striatal Tonically Active Neurons plays an important role in the study of neurodegenerative diseases. This page provides comprehensive information about this topic, including its mechanisms, significance in disease processes, and therapeutic implications.
Striatal tonically active neurons (TANs), also known as striatal cholinergic interneurons, are a unique population of modulatory neurons in the striatum that play pivotal roles in basal ganglia function. Unlike the majority of striatal neurons that are GABAergic projection neurons, TANs release acetylcholine (ACh) as their primary neurotransmitter, creating a distinct cholinergic signaling layer within the striatal microcircuit. These neurons are characterized by their persistent firing at rest—hence the name "tonically active"—and their crucial involvement in reward learning, movement selection, and attention.
TANs are the sole source of acetylcholine in the striatum and serve as key integrators of cortical, thalamic, and dopaminergic inputs. Their activity signals salient events in the environment and contributes to synaptic plasticity throughout the basal ganglia. Dysfunction of TANs has been implicated in Parkinson's disease, Huntington's disease, and addiction disorders, making them important therapeutic targets.
¶ Classification and Identity
Striatal tonically active neurons are known by several names:
- Cholinergic interneurons: Primary designation reflecting their neurotransmitter
- TANs: Abbreviation commonly used in the literature
- Large aspiny interneurons: Historical term based on morphology
- Type I striatal neurons: Early classification scheme
TANs are distinguished from other striatal interneurons by:
- Neurotransmitter: Acetylcholine (with co-transmission of GABA in some cases)
- Firing pattern: Regular tonic firing at 2-10 Hz at rest
- Morphology: Large soma (20-40 μm), extensive dendritic arborization
- Molecular markers: Choline acetyltransferase (ChAT), vesicular ACh transporter (VAChT)
¶ Anatomy and Morphology
TANs possess distinctive morphological features:
- Soma: Large, polygonal cell body (20-40 μm diameter)
- Dendrites: Thick, aspiny, radiating 200-400 μm from soma
- Axon: Extensive local axonal arborization covering 1-2 mm diameter
The extensive axonal network allows a single TAN to influence thousands of neighboring neurons, creating a widespread cholinergic signal throughout the striatum.
TANs are distributed throughout the striatum:
- Dorsal striatum: Caudate nucleus and putamen (0.5-2% of striatal neurons)
- Ventral striatum: Nucleus accumbens core and shell
- Patch-matrix organization: Slightly higher density in striosomes
TANs exhibit characteristic electrophysiological signatures:
- Resting firing rate: 2-10 Hz continuous firing
- Action potential waveform: Broad (1-2 ms duration), often with prominent afterhyperpolarization
- Input resistance: Moderate (100-200 MΩ)
- Membrane time constant: Slow (10-20 ms)
- Depolarizing sag: Hyperpolarization-activated cyclic nucleotide-gated (HCN) channel activation
TANs integrate diverse synaptic inputs:
- Cortical glutamatergic input: Major excitatory drive from sensorimotor and associative cortex
- Thalamic input: From the centromedian-parafascicular complex
- Dopaminergic input: Modulatory input from substantia nigra pars compacta
- Local GABAergic input: From other interneurons and potentially MSNs
TAN outputs modulate multiple receptor types:
- Muscarinic ACh receptors (M1-M5): G-protein coupled, slow synaptic effects
- Nicotinic ACh receptors: Fast ionotropic receptors on dopamine terminals
- GABA release: Some TANs co-release GABA via vesicular GABA transporter
TANs create a persistent cholinergic tone that modulates:
- MSN excitability: M1 receptor activation increases dendritic excitability
- Dopamine release: Nicotinic receptors on dopaminergic terminals modulate release
- Interneuron networks: Modulation of fast-spiking and LTS interneurons
- Presynaptic terminals: Regulation of glutamate and GABA release
TANs are critical for reward-related plasticity:
- Reward prediction errors: Phasic inhibition during unexpected rewards
- Associative learning: Activity correlates with cue-reward pairing
- Dopamine interaction: Coordinated activity with dopaminergic neurons
- Synaptic plasticity: Cholinergic modulation of corticostriatal synapses
TAN activity influences motor control:
- Movement initiation: Transient pauses in firing preceding movement
- Motor learning: Role in habit formation and skill acquisition
- Action selection: Modulation of competing motor programs
TAN dysfunction in Parkinson's disease:
- Altered firing patterns: Irregular firing and loss of pauses
- Dopamine-acetylcholine imbalance: Therapeutic target for anticholinergics
- Beta oscillations: Role in pathological synchronization
- Treatment: Anticholinergic drugs (trihexyphenidyl, benztropine) remain important
Changes in TAN function in Huntington's disease:
- Early alterations: Firing abnormalities before motor symptoms
- Loss of cholinergic markers: Reduced ChAT expression
- Circuit dysfunction: Contributes to motor and cognitive deficits
TANs in addiction circuitry:
- Dopamine interactions: Nicotinic modulation of dopamine release
- Reward learning: Enhanced cholinergic signaling with drugs of abuse
- Relapse: Activity during cue-induced drug seeking
- Therapeutic target: Nicotinic receptor antagonists for addiction treatment
Evidence for TAN involvement in OCD:
- Altered cholinergic tone: Changes in striatal ACh signaling
- Therapeutic effects: Cholinergic medications affect OCD symptoms
- In vivo extracellular recordings: Single-unit recordings in behaving animals
- In vitro whole-cell patch clamp: Characterization of intrinsic properties
- Juxtacellular labeling: Identification and morphological reconstruction
¶ Genetic and Molecular
- ChAT-Cre transgenic lines: Genetic access to cholinergic neurons
- Optogenetics: Channelrhodopsin for excitation, halorhodopsin for inhibition
- Chemogenetics: DREADDs for long-term manipulation
- Single-cell transcriptomics: Molecular profiling of TAN subtypes
- Fiber photometry: ACh sensors (GRAB-ACh) for monitoring release
- Two-photon microscopy: Calcium imaging in vivo
- FSCV: Fast-scan cyclic voltammetry for ACh detection
- Muscarinic antagonists: M1-selective for Parkinson's, OCD
- Nicotinic agonists: α4β2 and α7 nAChR ligands for cognitive enhancement
- Acetylcholinesterase inhibitors: Limited use due to peripheral effects
- Deep brain stimulation: Effects on TAN activity in PD
- Transcranial focused ultrasound: Potential for targeted modulation
- Cell-specific delivery: AAV vectors for TAN-targeted gene therapy
- ACh sensors: Improved monitoring of cholinergic signaling
- Circuit-specific manipulation: Optogenetic/chemogenetic TAN modulation
Striatal Tonically Active Neurons plays an important role in the study of neurodegenerative diseases. This page provides comprehensive information about this topic, including its mechanisms, significance in disease processes, and therapeutic implications.
The study of Striatal Tonically Active Neurons has evolved significantly over the past decades. Research in this area has revealed important insights into the underlying mechanisms of neurodegeneration and continues to drive therapeutic development.
Historical context and key discoveries in this field have shaped our current understanding and will continue to guide future research directions.
- Calabresi P, et al. (2000) - Electrophysiology of striatal cholinergic interneurons
- Zhou FM, et al. (2002) - Cholinergic interneurons in the striatum
- Cragg SJ, et al. (2006) - Nicotinic modulation of dopamine release in striatum
- Aosaki T, et al. (2010) - Striatal cholinergic interneurons as reward sensors
- Woolf NJ, et al. (1991) - Cholinergic neurons in the rat striatum
- Pisani A, et al. (2005) - Cholinergic control of striatal plasticity