Synaptic-like axo-axonal transmission from striatal cholinergic interneurons onto dopaminergic fibers

Subtypes of neuronal nicotinic acetylcholine receptors (nAChRs) are constructed from numerous subunit combinations that compose channel-receptor complexes with varied functional and pharmacological characteristics. Structural and functional diversity and the broad presynaptic, postsynaptic, and nonsynaptic locations of nAChRs underlie their mainly modulatory roles throughout the mammalian brain. Presynaptic and preterminal nicotinic receptors enhance neurotransmitter release, postsynaptic nAChRs contribute a small minority of fast excitatory transmission, and nonsynaptic nAChRs modulate many neurotransmitter systems by influencing neuronal excitability. Nicotinic receptors have roles in development and synaptic plasticity, and nicotinic mechanisms participate in learning, memory, and attention. Decline, disruption, or alterations of nicotinic cholinergic mechanisms contribute to dysfunctions such as epilepsy, schizophrenia, Parkinson's disease, autism, dementia with Lewy bodies, Alzheimer's disease, and addiction.

Here we describe a protocol for advanced CUBIC (Clear, Unobstructed Brain/Body Imaging Cocktails and Computational analysis). The CUBIC protocol enables simple and efficient organ clearing, rapid imaging by light-sheet microscopy and quantitative imaging analysis of multiple samples. The organ or body is cleared by immersion for 1-14 d, with the exact time required dependent on the sample type and the experimental purposes. A single imaging set can be completed in 30-60 min. Image processing and analysis can take <1 d, but it is dependent on the number of samples in the data set. The CUBIC clearing protocol can process multiple samples simultaneously. We previously used CUBIC to image whole-brain neural activities at single-cell resolution using Arc-dVenus transgenic (Tg) mice. CUBIC informatics calculated the Venus signal subtraction, comparing different brains at a whole-organ scale. These protocols provide a platform for organism-level systems biology by comprehensively detecting cells in a whole organ or body.

The dopamine projection from ventral tegmental area (VTA) to nucleus accumbens (NAc) is critical for motivation to work for rewards, and reward-driven learning. How dopamine supports both functions is unclear. Dopamine spiking can encode prediction errors, vital learning signals in computational theories of adaptive behavior. By contrast, dopamine release ramps up as animals approach rewards, mirroring reward expectation. This mismatch might reflect differences in behavioral tasks, slower changes in dopamine cell spiking, or spike-independent modulation of dopamine release. Here we compare spiking of identified VTA dopamine cells with NAc dopamine release in the same decision-making task. Cues indicating upcoming reward increased both spiking and release. Yet NAc core dopamine release also covaried with dynamically-evolving reward expectations, without corresponding changes in VTA dopamine cell spiking. Our results suggest a fundamental difference in how dopamine release is regulated to achieve distinct functions: broadcast burst signals promote learning, while local control drives motivation.