Search

Significance When an animal forms a memory, synapses in specific brain pathways change their strength. Pinpointing which synapses and pathways are modulated in any given learning paradigm, however, is technically challenging and needs to be performed one candidate connection at a time. Here we present SYNPLA, a tool to quickly detect strengthened synapses in genetically or anatomically defined pathways across the brain. To do so, we exploit the temporary translocation of AMPA receptor GluA1 into newly strengthened synapses. Using an assay that can identify proteins less than 40 nm away, we label only synapses that contain both GluA1 and a presynaptic protein exogenously expressed in a specific pathway. SYNPLA thus provides a pathway- and synapse-specific screening tool for memory formation.

The advent of endothermy, which is achieved through the continuous homeostatic regulation of body temperature and metabolism1,2, is a defining feature of mammalian and avian evolution. However, when challenged by food deprivation or harsh environmental conditions, many mammalian species initiate adaptive energy-conserving survival strategies—including torpor and hibernation—during which their body temperature decreases far below its homeostatic set-point3–5. How homeothermic mammals initiate and regulate these hypothermic states remains largely unknown. Here we show that entry into mouse torpor, a fasting-induced state with a greatly decreased metabolic rate and a body temperature as low as 20 °C6, is regulated by neurons in the medial and lateral preoptic area of the hypothalamus. We show that restimulation of neurons that were activated during a previous bout of torpor is sufficient to initiate the key features of torpor, even in mice that are not calorically restricted. Among these neurons we identify a population of glutamatergic Adcyap1-positive cells, the activity of which accurately determines when mice naturally initiate and exit torpor, and the inhibition of which disrupts the natural process of torpor entry, maintenance and arousal. Taken together, our results reveal a specific neuronal population in the mouse hypothalamus that serves as a core regulator of torpor. This work forms a basis for the future exploration of mechanisms and circuitry that regulate extreme hypothermic and hypometabolic states, and enables genetic access to monitor, initiate, manipulate and study these ancient adaptations of homeotherm biology. A specific neuronal population in the medial and lateral preoptic area of the hypothalamus regulates entry into torpor in mice.

The posterior parietal cortex (PPC) performs many functions, including decision-making and movement control. It remains unknown which input and output pathways of PPC support different functions. We addressed this issue in mice, focusing on PPC neurons projecting to the dorsal striatum (PPC-STR) and the posterior secondary motor cortex (PPC-pM2). Projection-specific, retrograde labeling showed that PPC-STR and PPC-pM2 represent largely distinct subpopulations, with PPC-STR receiving stronger inputs from association areas and PPC-pM2 receiving stronger sensorimotor inputs. Two-photon calcium imaging during decision-making revealed that the PPC-STR population encodes history-dependent choice bias more strongly than PPC-pM2 or general PPC populations. Furthermore, optogenetic inactivation of PPC-STR neurons or their terminals in STR decreased history-dependent bias, while inactivation of PPC-pM2 neurons altered movement kinematics. Therefore, PPC biases action selection through its STR projection, while controlling movements through PPC-pM2 neurons. PPC may support multiple functions through parallel subpopulations, each with distinct input-output connectivity.

Recording the activity of a group of neurons in a freely-moving animal is a challenging undertaking. Moreover, as the brain is dissected into smaller and smaller functional subgroups, it becomes paramount to record from projections and/or genetically-defined subpopulations of neurons. Fiber photometry is an accessible and powerful approach that can overcome these challenges. By combining optical and genetic methodologies, neural activity can be measured in deep brain structures by expressing genetically-encoded calcium indicators, which translate neural activity into an optical signal that can be easily measured. The current protocol details the components of a multi-fiber photometry system, how to access deep brain structures to deliver and collect light, a method to account for motion artifacts, and how to process and analyze fluorescent signals. The protocol details experimental considerations when performing single and dual color imaging, from either single or multiple implanted optic fibers.

The neural mechanisms conferring reduced motivation, as observed in depressed individuals, is poorly understood. Here, we examine in rodents if reduced motivation to exert effort is controlled by transmission from the lateral habenula (LHb), a nucleus overactive in depressed-like states, to the rostromedial tegmental nucleus (RMTg), a nucleus that inhibits dopaminergic neurons. In an aversive test wherein immobility indicates loss of effort, LHb→RMTg transmission increased during transitions into immobility, driving LHb→RMTg increased immobility, and inhibiting LHb→RMTg produced the opposite effects. In an appetitive test, driving LHb→RMTg reduced the effort exerted to receive a reward, without affecting the reward's hedonic property. Notably, LHb→RMTg stimulation only affected specific aspects of these motor tasks, did not affect all motor tasks, and promoted avoidance, indicating that LHb→RMTg activity does not generally reduce movement but appears to carry a negative valence that reduces effort. These results indicate that LHb→RMTg activity controls the motivation to exert effort and may contribute to the reduced motivation in depression.