Hrvatin S, Sun S, Wilcox OF, Yao H, Lavin-Peter AJ, Cicconet M, Assad EG, Palmer ME, Aronson S, Banks AS, Griffith EC, Greenberg ME. Neurons that regulate mouse torpor. Nature 2020;583(7814):115-121. doi: 10.1038/s41586-020-2387-5.

 

Background: Mammalian torpor and hibernation are adaptive states that allow survival during food scarcity and cold by greatly reducing body temperature and metabolism. Although thermoregulatory mechanisms in the hypothalamus are well characterized, the neural basis of how homeothermic animals actively suppress these mechanisms to enter hypothermic states such as torpor remains unclear.

 

Hypothesis: This study tested the hypothesis that specific neurons in the hypothalamic preoptic area control the initiation and maintenance of fasting-induced torpor in mice.

 

Methods: The authors used a fasting-induced mouse model of torpor combined with whole-brain activity mapping, genetic labeling (FosTRAP) of neurons active during torpor, and chemogenetic reactivation using Gq-DREADDs. They identified active brain regions through FOS expression and systematically tested the sufficiency and necessity of these neurons in inducing torpor. Single-nucleus RNA sequencing characterized molecular identities, and calcium imaging using fiber photometry (FP3002) monitored neuronal activity during natural torpor.

 

Results: Chemogenetic stimulation of neurons activated during natural torpor, specifically in the anteroventral medial and lateral preoptic area (avMLPA), was sufficient to induce torpor-like hypothermia and inactivity in fed mice. Single-cell profiling revealed glutamatergic Adcyap1-expressing neurons as key regulators. Silencing these neurons impaired natural torpor, while calcium recordings showed their activity tightly correlated with torpor onset and maintenance.

 

Conclusions: This study identified glutamatergic Adcyap1-positive neurons in the avMLPA as core regulators of torpor in mice. Their activation alone reproduced torpor’s physiological features, and their inhibition disrupted it. These findings define a specific hypothalamic circuit governing entry into hypometabolic states, providing a foundation for exploring mechanisms that could enable controlled hypothermia in non-hibernating species.

Barik A, Sathyamurthy A, Thompson J, Seltzer M, Levine A, Chesler A. A spinoparabrachial circuit defined by Tacr1 expression drives pain. Elife 2021;10:e61135. doi: 10.7554/eLife.61135.

 

Background: Pain perception depends on spinal projection neurons that relay nociceptive information to the brain. Among these, neurons expressing the neurokinin 1 receptor (Tacr1) and its ligand Substance P (Tac1) are known to contribute to pain and itch processing, yet the specific pathways and central targets mediating persistent pain are incompletely understood. Identifying how distinct subsets of these neurons shape behavioral responses to sustained versus acute noxious stimuli is crucial for understanding chronic pain mechanisms.

 

Hypothesis: This study tested the hypothesis that Tacr1-expressing projection neurons in the spinal cord communicate with Tacr1-expressing neurons in the superior lateral parabrachial nucleus (PBN-SL) to drive behavioral responses associated with persistent pain.

 

Methods: The authors used Tacr1-Cre transgenic mice combined with viral-mediated chemogenetic and optogenetic tools to selectively activate or silence Tacr1-expressing neurons in the spinal cord and PBN-SL. Fiber photometry (FP3002) measured population calcium activity during nociceptive and pruritic stimuli. Behavioral assays assessed nocifensive, grooming, and itch-related behaviors following manipulation of these circuits.

 

Results: Chemogenetic activation of spinal Tacr1 neurons induced robust, localized nocifensive behaviors mimicking persistent pain and suppressed itch responses. Tracing experiments identified dense projections to the PBN-SL. PBN-SLTacr1 neurons responded selectively to sustained, but not acute, noxious stimuli. Activation of these neurons heightened pain behaviors and suppressed itch, while their silencing markedly reduced responses to prolonged noxious stimuli without affecting reflexive responses.

 

Conclusions: This study revealed a defined spinoparabrachial pathway in which spinal and parabrachial Tacr1-expressing neurons cooperate to encode ongoing pain. This circuit amplifies behavioral responses to persistent noxious input while simultaneously inhibiting itch, highlighting its role in chronic pain processing.

Flanigan ME, Aleyasin H, Li L, Burnett CJ, Chan KL, LeClair KB, Lucas EK, Matikainen-Ankney B, Durand-de Cuttoli R, Takahashi A, Menard C, Pfau ML, Golden SA, Bouchard S, Calipari ES, Nestler EJ, DiLeone RJ, Yamanaka A, Huntley GW, Clem RL, Russo SJ. Orexin signaling in GABAergic lateral habenula neurons modulates aggressive behavior in male mice. Nat Neurosci 2020;23(5):638-650. doi: 10.1038/s41593-020-0617-7.

 

Background: Aggression is a feature of several neuropsychiatric disorders and may arise from dysregulation in brain reward systems. In mice, some individuals find aggression rewarding and seek opportunities to fight. The lateral habenula (LHb), which processes aversive and rewarding stimuli, has been linked to aggression, but its local circuitry remains unclear. Evidence of a small GABAergic population expressing glutamic acid decarboxylase 2 (GAD2) suggests possible inhibitory control within the LHb.

 

Hypothesis: This study tested the hypothesis that orexinergic projections from the lateral hypothalamus activate GAD2-expressing neurons in the LHb through orexin receptor 2 (OxR2) signaling, thereby modulating aggressive behavior and the rewarding properties of aggression in male mice.

 

Methods: The authors used a combination of fiber photometry (FP3001), optogenetics, chemogenetics, electrophysiology and molecular analyses in male mice. They monitored LHb activity during aggression tests, manipulated specific neuronal populations and examined how orexin signaling influenced LHb GAD2 neuron activity and aggression-related behaviors, including conditioned place preference for aggression-paired contexts.

 

Results: Aggressive encounters were associated with decreased overall LHb activity but increased activity in GAD2 LHb neurons. Optogenetic activation of GAD2 neurons or orexin terminals in the LHb enhanced aggression and aggression reward, while their inhibition or OxR2 knockdown reduced both. GAD2 neurons provided local inhibition within the LHb, and orexin directly excited these cells via OxR2.

 

Conclusions: These findings reveal a novel orexin-sensitive inhibitory microcircuit within the LHb that promotes aggression by suppressing overall LHb activity. The study identifies orexin signaling in LHb GAD2 neurons as a key mechanism linking hypothalamic arousal systems to the motivational control of aggressive behavior.

O’Neal TJ, Bernstein MX, MacDougall DJ, Ferguson SM. A conditioned place preference for heroin is signaled by increased dopamine and direct pathway activity and decreased indirect pathway activity in the nucleus accumbens. J Neurosci 2022;42(10):2011-2024. doi: 10.1523/JNEUROSCI.1451-21.2021.

 

Background: Addiction arises when neutral cues acquire motivational significance through repeated pairing with drug exposure. The nucleus accumbens (NAc) integrates dopaminergic and striatal inputs to encode reward-related learning, but how heroin alters NAc signaling remains unclear. Direct (dMSN) and indirect (iMSN) pathway medium spiny neurons have opposing roles in motivation and action selection, yet their specific contributions to heroin reinforcement are not well defined.

 

Hypothesis: This study tested the hypothesis that heroin conditioned place preference (CPP) is driven by increased dopamine and dMSN activity and decreased iMSN activity in the NAc, forming an imbalance that encodes the reinforcing value of heroin-paired contexts.

 

Methods: The authors used fiber photometry (FP3001) to measure dopamine and calcium activity in NAc dMSNs and iMSNs during heroin CPP in male and female rats. Neural activity was recorded during conditioning and test sessions as rats entered or exited heroin- or saline-paired chambers. Additional experiments examined whether buprenorphine pretreatment affected heroin CPP and NAc Fos expression.

 

Results: Rats developed robust CPP and showed increased NAc Fos activation. During heroin conditioning, dopamine and dMSN signals increased while iMSN signals decreased before entry into heroin-paired contexts, with reversed patterns before exit. Across conditioning, dopamine and dMSN activity sensitized, and iMSN activity declined. Buprenorphine pretreatment blocked CPP formation and reduced NAc Fos levels.

 

Conclusions: Heroin reward learning depends on enhanced dopamine and direct pathway activity with concurrent suppression of indirect pathway signaling in the NAc. This imbalance encodes heroin-cue associations, while buprenorphine prevents their development.

Lange G, Gnazzo F, Faust RP, Beeler JA. Accumbal dopamine and acetylcholine dynamics during psychostimulant sensitization. bioRxiv [Preprint]. 2025;2025.03.29.646091. doi: 10.1101/2025.03.29.646091.

 

Background: Behavioral sensitization to repeated psychostimulant exposure reflects neural adaptations implicated in addiction. Dopamine (DA) signaling in the nucleus accumbens (NAcc) is central to this process, but striatal acetylcholine (ACh), released by cholinergic interneurons (CINs), may also contribute through reciprocal interactions with DA. However, how psychostimulants alter ACh dynamics and DA–ACh coupling across repeated exposures has not been fully characterized.

 

Hypothesis: This study hypothesized that repeated psychostimulant exposure would differentially alter dopamine and acetylcholine signaling in the NAcc, producing sensitized changes dependent on dopamine D2 receptors expressed on cholinergic interneurons.

 

Methods: The authors used dual-color fiber photometry (GRAB-rDA and GRAB-ACh sensors; FP3002) to record simultaneous DA and ACh activity in the NAcc shell of freely moving mice. Animals received repeated injections of cocaine, amphetamine or saline, followed by a drug-challenge session; a separate cohort of CIN-specific D2R knockout mice underwent the same protocol.

 

Results: Repeated psychostimulant exposure enhanced locomotor activity and produced sensitized increases in slow extracellular DA while reducing the amplitude and frequency of phasic DA transients. ACh transients were suppressed in amplitude and frequency, effects that sensitized across sessions. Psychostimulants weakened DA–ACh correlations and coherence without altering their temporal alignment. CIN-specific D2R knockout mice displayed delayed behavioral sensitization and failed to show DA or ACh sensitization despite preserved acute responses.

 

Conclusions: Repeated psychostimulant exposure induces sensitized, opposing adaptations in NAcc dopamine and acetylcholine signaling. D2 receptors on cholinergic interneurons are essential for neuromodulator sensitization but not for the expression of behavioral sensitization itself.

Hollon NG, Williams EW, Howard CD, Li H, Traut TI, Jin X. Nigrostriatal dopamine signals sequence-specific action-outcome prediction errors. Curr Biol 2021;31(23):5350-5363.e5. doi: 10.1016/j.cub.2021.09.040.

 

Background: Dopamine neurons are thought to encode prediction errors during Pavlovian learning by signaling mismatches between expected and received rewards. However, whether dopamine similarly signals discrepancies in action-outcome associations during self-initiated, goal-directed behavior has been unclear. Understanding how dopamine encodes action-outcome prediction errors is essential for linking reinforcement learning theories to the neural mechanisms of instrumental behavior.

 

Hypothesis: This study tested the hypothesis that nigrostriatal dopamine signals sequence-specific prediction errors for action-outcome associations, such that dopamine activity would be suppressed when a reward is the expected result of a learned action or action sequence.

 

Methods: The authors trained mice expressing channelrhodopsin in dopamine neurons to perform lever-pressing tasks for optogenetic intracranial self-stimulation (opto-ICSS) while recording subsecond dopamine release with fast-scan cyclic voltammetry and fiber photometry (FP3002). They compared dopamine responses during self-initiated stimulation and temporally matched passive playback sessions. Additional tests included omission, delay and magnitude probe trials, as well as a left-right lever-press sequence task to assess hierarchical control of learned actions.

 

Results: Dopamine release was markedly lower when stimulation resulted from the mouse’s own action than when delivered noncontingently. Omission of expected outcomes produced transient dopamine dips below baseline, consistent with negative prediction errors. These effects were action-specific, temporally restricted to the expected outcome and extended to learned action sequences, showing suppression only for reinforced sequences and not for other movements.

 

Conclusions: Nigrostriatal dopamine encodes action-outcome prediction errors that are specific to learned actions and sequences. This hierarchical and temporally precise modulation suggests dopamine contributes to goal-directed learning and behavioral control.

Murphy KR, Farrell JS, Gomez JL, Stedman QG, Li N, Leung SA, Good CH, Qiu Z, Firouzi K, Butts Pauly K, Khuri-Yakub BPT, Michaelides M, Soltesz I, de Lecea L. A tool for monitoring cell type-specific focused ultrasound neuromodulation and control of chronic epilepsy. Proc Natl Acad Sci USA 2022;119(46):e2206828119. doi: 10.1073/pnas.2206828119.

 

Background: Temporal lobe epilepsy often resists drug treatment, creating a need for noninvasive approaches to control deep brain activity. Focused ultrasound (FUS) offers a promising tool for neuromodulation, but its effects on specific neuronal cell types are unclear. Since activating hippocampal parvalbumin (PV) interneurons can suppress seizures, determining whether FUS can selectively modulate inhibitory versus excitatory neurons is key to developing targeted therapies.

 

Hypothesis: This study tested the hypothesis that FUS can modulate neural activity in a cell type–specific manner and that optimized parameters could preferentially activate inhibitory PV interneurons while suppressing excitatory neurons, thereby reducing epileptiform activity.

 

Methods: Based on an FP3002 system the authors developed a fiber Photometry Coupled focused Ultrasound System (PhoCUS) that combines optical recording with ultrasound stimulation in freely moving mice. Using genetically encoded calcium indicators, they simultaneously monitored activity of excitatory and inhibitory neurons in the hippocampus during varied FUS protocols. Whole-brain metabolic effects were assessed with positron emission tomography (PET), and therapeutic efficacy was tested in a kainate model of chronic temporal lobe epilepsy.

 

Results: PhoCUS allowed precise monitoring of FUS-induced activity. A 900 Hz, 20% duty cycle protocol selectively increased PV interneuron activity while suppressing excitatory neurons. PET imaging confirmed localized inhibition within the hippocampus, and the same protocol significantly reduced epileptiform spikes in epileptic mice.

 

Conclusions: Focused ultrasound can achieve cell type–specific neuromodulation and suppress epileptiform activity. PhoCUS provides a versatile platform for developing noninvasive, targeted ultrasound therapies for epilepsy and other brain disorders.