Biology subjects

McMahon, D. G.

Publications and source records attributed to McMahon, D. G..

11 recordsLinked to original sources

bioRxiv2025

Photoperiodic Regulation of SK Channels in Dorsal Raphe Serotonin Neurons

Day length, or seasonal photoperiod, shapes mood and affective behaviors but the neural mechanisms underlying these effects are still being defined. Serotonin neurons of the dorsal raphe nucleus (DRN) are critical regulators of affective behaviors and photoperiod modulates their excitability and ongoing activity. Here, we investigated the influence of seasonal photoperiod on the function and expression of small conductance calcium-activated potassium (SK) channels which mediate the afterhyperpolarizing potential (AHP) in dorsal raphe serotonin neurons. Building on previous work demonstrating that photoperiod modulates serotonergic excitability and behavior, we hypothesized that day length influences SK channel activity, thereby contributing to differences in neuronal excitability observed between Long, Equinox, and Short photoperiod conditions. Using multi-electrode array recording of DRN slices we found a significant dose-dependent increase in spike rate to the application of the SK channel inhibitor apamin, indicating that SK channels indeed influence the spike rate of dorsal raphe serotonin neurons. In addition, DRN neurons in slices from Long photoperiod mice exhibited less pronounced responses to apamin relative to those from Short photoperiod mice, suggesting reduced function or expression of SK channels in Long photoperiod. Indeed, whole-cell recordings demonstrated that SK channel-mediated AHP currents were reduced in Long photoperiod mice. However, there were no significant differences in expression levels of the SK3 subunit (Kcnn3) in DRN serotonin neurons across photoperiod conditions as determined by single molecule fluorescence in situ hybridization. Overall, these findings indicate that photoperiod modulates SK channel function in DRN serotonin neurons likely at a post-transcriptional level. This study advances our understanding of how seasonal cues influence intrinsic neuronal properties and provides a mechanistic link between photoperiod, serotonergic excitability, and mood-related behaviors. The identification of SK channels as modulators of photoperiodic effects may offer novel therapeutic targets for mood disorders associated with dysregulated serotonin signaling.

neuroscience↗

bioRxiv2024

SCN gene expression plasticity in response to photoperiod

Seasonal daylength, or circadian photoperiod, is a pervasive environmental signal that profoundly influences physiology and behavior. In mammals, the central circadian clock resides in the suprachiasmatic nuclei (SCN) of the hypothalamus where it receives retinal input and synchronizes, or entrains, organismal physiology and behavior to the prevailing light cycle. The process of entrainment induces sustained plasticity in the SCN, but the molecular mechanisms underlying SCN plasticity are incompletely understood. Entrainment to different photoperiods persistently alters the timing, waveform, period, and light resetting properties of the SCN clock and its driven rhythms. To elucidate novel molecular mechanisms of photoperiod plasticity, we performed RNAseq on whole SCN dissected from mice raised in Long (LD 16:8) and Short (LD 8:16) photoperiods. Fewer rhythmic genes were detected in Long photoperiod and in general the timing of gene expression rhythms was advanced 4-6 hours. However, a few genes showed significant delays, including Gem. There were significant changes in the expression clock-associated gene Timeless and in SCN genes related to light responses, neuropeptides, GABA, ion channels, and serotonin. Particularly striking were differences in the expression of the neuropeptide signaling genes Prokr2 and Cck, as well as convergent regulation of the expression of three SCN light response genes, Dusp4, Rasd1, and Gem. Transcriptional modulation of Dusp4 and Rasd1, and phase regulation of Gem, are compelling candidate molecular mechanisms for plasticity in the SCN light response through their modulation of the critical NMDAR-MAPK/ERK-CREB/CRE light signaling pathway in SCN neurons. Modulation of Prokr2 and Cck may critically support SCN neural network reconfiguration during photoperiodic entrainment. Our findings identify the SCN light response and neuropeptide signaling gene sets as rich substrates for elucidating novel mechanisms of photoperiod plasticity.

neuroscience↗

bioRxiv2023

Neanderthal Introgression Shaped Human Circadian Traits

IntroductionWhen the ancestors of modern Eurasians migrated out of Africa and interbred with Eurasian archaic hominins, namely Neanderthals and Denisovans, DNA of archaic ancestry integrated into the genomes of anatomically modern humans. This process potentially accelerated adaptation to Eurasian environmental factors, including reduced ultra-violet radiation and increased variation in seasonal dynamics. However, whether these groups differed substantially in circadian biology, and whether archaic introgression adaptively contributed to human chronotypes remains unknown. ResultsHere we traced the evolution of chronotype based on genomes from archaic hominins and present-day humans. First, we inferred differences in circadian gene sequences, splicing, and regulation between archaic hominins and modern humans. We identified 28 circadian genes containing variants with potential to alter splicing in archaics (e.g., CLOCK, PER2, RORB, RORC), and 16 circadian genes likely divergently regulated between present-day humans and archaic hominins, including RORA. These differences suggest the potential for introgression to modify circadian gene expression. Testing this hypothesis, we found that introgressed variants are enriched among eQTLs for circadian genes. Supporting the functional relevance of these regulatory effects, we found that many introgressed alleles have associations with chronotype. Strikingly, the strongest introgressed effects on chronotype increase morningness, consistent with adaptations to high latitude in other species. Finally, we identified several circadian loci with evidence of adaptive introgression or latitudinal clines in allele frequency. ConclusionsThese findings identify differences in circadian gene regulation between modern humans and archaic hominins and support the contribution of introgression via coordinated effects on variation in human chronotype. SIGNIFICANCE STATEMENTInterbreeding between humans and Neanderthals created the potential for adaptive introgression as humans moved into environments that had been populated by Neanderthals for hundreds of thousands of years. Here we discover lineage-specific genetic differences in circadian genes and their regulatory elements between humans and Neanderthals. We show that introgressed alleles are enriched for effects on circadian gene regulation, consistently increase propensity for morningness in Europeans, and show evidence of adaptive introgression or associations between latitude and frequency. These results expand our understanding of how the genomes of humans and our closest relatives responded to environments with different light/dark cycles, and demonstrate a coordinated contribution of admixture to human chronotype in a direction that is consistent with adaptation to higher latitudes.

evolutionary biology↗

bioRxiv2022

DNA methylation is a widespread mechanism of light-induced circadian clock period plasticity

The suprachiasmatic nucleus (SCN) of the hypothalamus is a principal light-responsive circadian clock that adjusts circadian rhythms in mammalian physiology and behavior to changes in external light signals. Although mechanisms underlying how light acutely resets the timing of circadian rhythms have been characterized, it remains elusive how light signals induce lasting changes in circadian period, so-called period after-effects. Here we have found that the period after-effects on circadian behavior of changing photoperiods are blocked by application of DNA methyltransferase inhibitors directed to the SCN. At the level of single light pulses that act as clock-resetting stimulations, pharmacologically inhibiting DNA methylation in the SCN significantly attenuates period after-effects following acute phase shifts in behavioral rhythms in vivo, and blocks period after-effects on clock gene rhythms in the isolated ex vivo SCN. Acute clock resetting shifts themselves, however, do not appear to require DNA methylation at the SCN and behavioral levels, in contrast to subsequent period plasticity. Our results indicate that DNA methylation in the SCN mediates light-induced period after-effects in response to photoperiods, and single light pulses, and together with previous studies showing that DNA methylation in the SCN is essential for period after-effects of non-24hr light cycles (T-cycles), suggest that DNA methylation in the SCN is a widespread mechanism of light-induced circadian period plasticity.

neuroscience↗

bioRxiv2022

Reactive astrocytes transduce blood-brain barrier dysfunction through a TNFα-STAT3 signaling axis and secretion of alpha 1-antichymotrypsin

Astrocytes are critical components of the neurovascular unit that support blood-brain barrier (BBB) function in brain microvascular endothelial cells (BMECs). Transformation of astrocytes to a reactive state in response to injury and disease can be protective or harmful to BBB function, but the underlying mechanisms for these effects remain mostly unclear. Using a human induced pluripotent stem cell (iPSC)-derived coculture model of BMEC-like cells and astrocytes, we found that tumor necrosis factor alpha (TNF) transitions astrocytes to an inflammatory reactive state through activated STAT3 signaling, whereby the resultant astrocytes disrupt passive BBB function and induce vascular cell adhesion molecule 1 (VCAM-1) expression in the BMEC-like cells. These associations between inflammatory reactive astrocytes, STAT3 activation, and vascular VCAM-1 expression were corroborated in human postmortem tissue. Bioinformatic analyses coupled with CRISPR interference techniques in the iPSC model revealed that inflammatory reactive astrocytes transduce BBB disruption in part through SERPINA3, which encodes alpha 1-antichymotrypsin (1ACT), a secreted serine protease inhibitor associated with aging, neuroinflammation, and Alzheimers disease. In murine ex vivo cortical explant cultures, shRNA-mediated silencing of Serpina3n in astrocytes reduced vascular VCAM-1 expression after TNF challenge. Further, direct treatment with recombinant Serpina3n in both ex vivo explant cultures and the brain in vivo (via intracerebroventricular injection into wild-type mice) was sufficient to induce vascular VCAM-1 expression and reduce tight junction integrity. Overall, our results define the TNF-STAT3 signaling axis as a driver of an inflammatory reactive astrocyte subtype responsible for BBB dysfunction. Our results also identify 1ACT as an explicit mediator of BBB damage and suggest that inhibition of 1ACT expression or activity could represent a therapeutic avenue for reversing BBB deficits in aging and neurodegenerative disease.

neuroscience↗

bioRxiv2021

Optogenetic stimulation of VIPergic SCN neurons induces photoperiodic changes in the mammalian circadian clock

Circadian clocks play key roles in how organisms respond to and even anticipate seasonal change in day length, or photoperiod. In mammals, photoperiod is encoded by the central circadian pacemaker in the brain, the suprachiasmatic nucleus (SCN). The subpopulation of SCN neurons that secrete the neuropeptide VIP mediate the transmission of light information within the SCN neural network, suggesting a role for these neurons in circadian plasticity in response to light information that has yet to be directly tested. Here, we used in vivo optogenetic stimulation of VIPergic SCN neurons followed by ex vivo PERIOD 2::LUCIFERASE (PER2::LUC) bioluminescent imaging to test whether activation of this SCN neuron sub-population can induce SCN network changes that are hallmarks of photoperiodic encoding. We found that optogenetic stimulation designed to mimic a long photoperiod indeed altered subsequent SCN entrained phase, increased the phase dispersal of PER2 rhythms within the SCN network, and shortened SCN free-running period - similar to the effects of a true extension of photoperiod. Optogenetic stimulation also induced analogous changes on related aspects of locomotor behavior in vivo. Thus, selective activation of VIPergic SCN neurons induces photoperiodic network plasticity in the SCN which underpins photoperiodic entrainment of behavior.

neuroscience↗

bioRxiv2020

Circadian photoperiod alters TREK-1 channel function and expression in dorsal raphe serotonergic neurons

Seasonal daylength has been linked to the development and prevalence of mood disorders, however, the neural mechanisms underlying this relationship remain unknown. Previous work in our laboratory has shown that developmental exposure to seasonal photoperiods has enduring effects on the activity of mouse dorsal raphe serotonergic neurons, their intrinsic electrical properties, as well as on depression and anxiety-related behaviors. Here we focus on the possible ionic mechanisms that underlie the observed photoperiodic programming of the electrophysiological properties of serotonin neurons, focusing on the twin-pore K+ channels TREK-1 and TASK-1 that set resting membrane potential and regulate excitability. Using multielectrode array recordings in ex vivo dorsal raphe slices, we examined the effects of pharmacological inhibition of these channels on the spike rates of serotonin neurons of mice from different photoperiods. Pharmacological inhibition of TREK-1 significantly increased spike frequency in Short and Equinox photoperiod cohorts, but did not further elevate the firing rate in slices from Long photoperiod mice, suggesting that TREK-1 function is reduced in Long photoperiods. In contrast, inhibition of TASK-1 resulted in increases in firing rates across all photoperiods, suggesting that it contributes to setting excitability, but is not regulated by photoperiod. To examine if photoperiod impacts transcriptional regulation of TREK-1, we quantified Kcnk2 mRNA levels specifically in dorsal raphe 5-HT neurons using triple-label RNAscope. We found that Long photoperiod significantly reduced levels of Kcnk2 in serotonin neurons co-expressing Tph2, and Pet-1, Photoperiodic effects on the function and expression of TREK-1 were blocked in melatonin 1 receptor knockout (MT-1KO) mice, consistent with previous findings that MT-1 signaling is necessary for photoperiodic programming of dorsal raphe 5-HT neurons. Taken together these results indicate that photoperiodic regulation of TREK-1 expression and function plays a key role in photoperiodic programming the excitability of dorsal raphe 5-HT neurons.

neuroscience↗

bioRxiv2020

Photoperiodic Effects on Monoamine Signaling & Gene Expression Throughout Development in the Serotonin & Dopamine Systems

Photoperiod or the duration of daylight has been implicated as a risk factor in the development of mood disorders. The dopamine and serotonin systems are impacted by photoperiod and are consistently associated with affective disorders. Hence, we evaluated, at multiple stages of postnatal development, the expression of key dopaminergic (TH) and serotonergic (Tph2, SERT, and Pet-1) genes, and midbrain monoamine content in mice raised under control Equinox (LD 12:12), Short winter-like (LD 8:16), or Long summerlike (LD 16:8) photoperiods. Focusing in early adulthood, we evaluated the midbrain levels of these serotonergic genes, and also assayed these gene levels in the dorsal raphe nucleus (DRN) with RNAScope. Mice that developed under Short photoperiods demonstrated elevated midbrain TH expression levels, specifically during perinatal development compared to mice raised under Long photoperiods, and significantly decreased serotonin and dopamine content throughout the course of development. In adulthood, Long photoperiod mice demonstrated decreased midbrain Tph2 and SERT expression levels and reduced Tph2 levels in the DRN compared Short photoperiod mice. Thus, evaluating gene x environment interactions in the dopaminergic and serotonergic systems during multiple stages of development may lead to novel insights into the underlying mechanisms in the development of affective disorders.

neuroscience↗

bioRxiv2020

Integrating Long-term Optogenetic Stimulation and Bioluminescence Recording to Study Neural Plasticity Induced by Circadian Entrainment

How daily clocks in the brain are set by light to local environmental time and encode the seasons is not fully understood. The suprachiasmatic nucleus (SCN) is a central circadian clock that orchestrates physiology and behavior in tune with daily and seasonal light cycles. Here we have found that optogenetically simulated light input to explanted SCN changes the waveform of the molecular clockworks from sinusoidal in free-running conditions to highly asymmetrical shapes with accelerated synthetic (rising) phases and extended degradative (falling) phases marking clock advances and delays at simulated dawn and dusk. Daily waveform changes are sufficient to entrain to simulated winter and summer photoperiods, and to non-24h periods. SCN imaging further reveals that acute waveform shifts are greatest in the ventrolateral SCN, while period effects are greatest in the dorsomedial SCN. Thus, circadian entrainment is encoded by SCN clock gene waveform changes that arise from spatiotemporally distinct intrinsic responses within the SCN neural network.

neuroscience↗

bioRxiv2020

Neonicotinoids Disrupt Circadian Rhythms and Sleep in Honey Bees

Honey bees are critical pollinators in ecosystems and agriculture, but their numbers have significantly declined. Declines in pollinator populations are thought to be due to multiple factors including habitat loss, climate change, increased vulnerability to disease and parasites, and pesticide use. Neonicotinoid pesticides are agonists of insect nicotinic cholinergic receptors, and sub-lethal exposures are linked to reduced honey bee hive survival. Honey bees are highly dependent on circadian clocks to regulate critical behaviors, such as foraging orientation and navigation, time-memory for food sources, sleep, and learning/ memory processes. Because circadian clock neurons in insects receive light input through cholinergic signaling we tested for effects of neonicotinoids on honey bee circadian rhythms and sleep. Neonicotinoid ingestion by feeding over several days results in neonicotinoid accumulation in the bee brain, disrupts circadian rhythmicity in many individual bees, shifts the timing of behavioral circadian rhythms in bees that remain rhythmic, and impairs sleep. Neonicotinoids and light input act synergistically to disrupt bee circadian behavior, and neonicotinoids directly stimulate wake-promoting clock neurons in the fruit fly brain. Neonicotinoids disrupt honey bee circadian rhythms and sleep, likely by aberrant stimulation of clock neurons, to potentially impair honey bee navigation, time-memory, and social communication.

animal behavior and cognition↗

bioRxiv2020

Distinct components of photoperiodic light are differentially encoded by the mammalian circadian clock

Seasonal light cycles influence multiple physiological functions and are mediated through photoperiodic encoding by the circadian system. Despite our knowledge of the strong connection between seasonal light input and downstream circadian changes, less is known about the specific components of seasonal light cycles that are encoded and induce persistent changes in the circadian system. Using combinations of three T cycles (23, 24, 26 hr.) and two photoperiods per T cycle (Long and Short, with duty cycles scaled to each T cycle), we investigate after-effects of entrainment to these six light cycles. We measure locomotor behavior duration (), period ({tau}), and entrained phase angle ({Psi}) in vivo, and SCN phase distribution ({sigma}{phi}), {tau}, and {Psi} ex vivo in order to refine our understanding of critical light components for influencing particular circadian properties. We find that photoperiod and T cycle length both drive determination of in vivo {Psi} but differentially influence after-effects in and {tau}, with photoperiod driving changes in and photoperiod length and T cycle length combining to influence {tau}. Using skeleton photoperiods, we demonstrate that in vivo {Psi} is determined by both parametric and non-parametric components, while changes in are driven non-parametrically. Within the ex vivo SCN, we find that {Psi} and {sigma}{phi} of the PER2::LUCIFERASE rhythm follow closely with their likely behavioral counterparts ({Psi} and of the locomotor activity rhythm), while also confirming previous reports of {tau} after-effects of gene expression rhythms showing negative correlations with behavioral {tau} after-effects in response to T cycles. We demonstrate that within-SCN {sigma}{phi} changes, thought to underly changes in vivo, are induced primarily non-parametrically. Taken together, our results demonstrate distinct components of seasonal light input differentially influence {Psi}, , and {tau}, and suggest the possibility of separate mechanisms driving the persistent changes in circadian behaviors mediated by seasonal light.

neuroscience↗