Preferential cholinergic excitation of corticopontine neurons.

KEY POINTS: Phasic activation of M1 muscarinic receptors generates transient inhibition followed by longer lasting excitation in neocortical pyramidal neurons. Corticopontine neurons in the mouse prefrontal cortex exhibit weaker cholinergic inhibition, but more robust and longer lasting excitation, than neighbouring callosal projection neurons. Optogenetic release of endogenous ACh in response to single flashes of light (5 ms) preferentially enhances the excitability of corticopontine neurons for many tens of seconds. Cholinergic excitation of corticopontine neurons involves at least three ionic mechanisms: suppression of KV 7 currents, activation of the calcium-dependent non-specific cation conductance underlying afterdepolarizations, and activation of what appears to be a calcium-sensitive but calcium-permeable non-specific cation conductance. Preferential cholinergic excitation of prefrontal corticopontine neurons may facilitate top-down attentional processes and behaviours. ABSTRACT: Pyramidal neurons in layer 5 of the neocortex comprise two broad classes of projection neurons: corticofugal neurons, including corticopontine (CPn) neurons, and intratelencephalic neurons, including commissural/callosal (COM) neurons. These non-overlapping neuron subpopulations represent discrete cortical output channels contributing to perception, decision making and behaviour. CPn and COM neurons have distinct morphological and physiological characteristics, and divergent responses to modulatory transmitters such as serotonin and acetylcholine (ACh). To better understand how ACh regulates cortical output, in slices of mouse prefrontal cortex (PFC) we compared the responsivity of CPn and COM neurons to transient exposure to exogenous or endogenous ACh. In both neuron subtypes, exogenous ACh generated qualitatively similar biphasic responses in which brief hyperpolarization was followed by longer lasting enhancement of excitability. However, cholinergic inhibition was more pronounced in COM neurons, while excitatory responses were larger and longer lasting in CPn neurons. Similarly, optically triggered release of endogenous ACh from cholinergic terminals preferentially and persistently (for ∼40 s) enhanced the excitability of CPn neurons, but had little impact on COM neurons. Cholinergic excitation of CPn neurons involved at least three distinct ionic mechanisms: suppression of KV 7 channels (the 'M-current'), activation of the calcium-dependent non-specific cation conductance underlying afterdepolarizations, and activation of what appears to be a calcium-sensitive but calcium-permeable non-specific cation conductance. Our findings demonstrate projection-specific selectivity in cholinergic signalling in the PFC, and suggest that transient release of ACh during behaviour will preferentially promote corticofugal output.

Phosphorylation-dependent proteome of Marcks in ependyma during aging and behavioral homeostasis in the mouse forebrain.

Ependymal cells (ECs) line the ventricular surfaces of the mammalian central nervous system (CNS) and their development is indispensable to structural integrity and functions of the CNS. We previously reported that EC-specific genetic deletion of the myristoylated alanine-rich protein kinase C substrate (Marcks) disrupts barrier functions and elevates oxidative stress and lipid droplet accumulation in ECs causing precocious cellular aging. However, little is known regarding the mechanisms that mediate these changes in ECs. To gain insight into Marcks-mediated mechanisms, we performed mass spectrometric analyses on Marcks-associated proteins in young and aged ECs in the mouse forebrain using an integrated approach. Network analysis on annotated proteins revealed that the identified Marcks-associated complexes are in part involved in protein transport mechanisms in young ECs. In fact, we found perturbed intracellular vesicular trafficking in cultured ECs with selective deletion of Marcks (Marcks-cKO mice), or upon pharmacological alteration to phosphorylation status of Marcks. In comparison, Marcks-associated protein complexes in aged ECs appear to be involved in regulation of lipid metabolism and responses to oxidative stress. Confirming this, we found elevated signatures of inflammation in the cerebral cortices and the hippocampi of young Marcks-cKO mice. Interestingly, behavioral testing using a water maze task indicated that spatial learning and memory is diminished in young Marcks-cKO mice similar to aged wildtype mice. Taken together, our study provides first line of evidence for potential mechanisms that may mediate differential Marcks functions in young and old ECs, and their effect on forebrain homeostasis during aging.

Mining sick: Creatively unsettling normative narratives about industry, environment, extraction, and the health geographies of rural, remote, northern, and Indigenous communities in British Columbia.

Rural, remote, northern, and Indigenous communities on Turtle Island are routinely-as Cree Elder Willie Ermine says-pathologized. Social science and health scholarship, including scholarship by geographers, often constructs Indigenous human and physical geographies as unhealthy, diseased, vulnerable, and undergoing extraction. These constructions are not inaccurate: peoples and places beyond urban metropoles on Turtle Island live with higher burdens of poor health; Indigenous peoples face systemic violence and racism in colonial landscapes; rural, remote, northern, and Indigenous geographies are sites of industrial incursions; and many rural and remote geographies remain challenging for diverse Indigenous peoples. What, however, are the consequences of imagining and constructing people and places as "sick"? Constructions of "sick" geographies fulfill and extend settler (often European white) colonial narratives about othered geographies. Rural, remote, northern, and Indigenous geographies are discursively "mined" for narratives of sickness. This mining upholds a sense of health and wellness in southern, urban, Euro-white-settler imaginations. Drawing from multi-year, relationship-based, cross-disciplinary qualitative community-informed experiences, and anchored in feminist, anti-colonial, and anti-racist methodologies that guided creative and humanities-informed stories, this paper concludes with different stories. It unsettles settler-colonial powers reliant on constructing narratives about sickness in others and consequently reframes conversations about Indigenous well-being and the environment.

Les communautés autochtones, nordiques et rurales de Turtle Island sont, comme le dit l'aîné cri Willie Ermine, couramment considérées comme pathogènes. Le discours professoral en sciences sociales et en santé, y compris chez les géographes, conçoit souvent les géographies autochtones, tant humaines que physiques, comme étant malsaines, malades, vulnérables et soumises à l'extraction. Ces conceptions ne sont pas erronées: les gens et les endroits en dehors des agglomérations urbaines sur Turtle Island sont davantage exposés à un état de santé précaire, les Autochtones font face à une violence et un racisme systémique dans les milieux coloniaux, les géographies autochtones, nordiques et rurales sont le siège d'incursions industrielles et de nombreux contextes territoriaux ruraux et éloignés continuent de présenter un défi pour diverses populations autochtones. Toutefois, quelles sont les conséquences d'imaginer et de concevoir les gens et les endroits comme étant « malades ¼? Le concept de territoires « malades ¼ favorise et consolide les récits coloniaux (souvent le blanc européen) sur l'état d'autres territoires, lesquels seraient dans une situation plus favorable. Les géographies autochtones, nordiques et rurales sont alors « minées ¼ de manière discursive par les récits sur leur caractère pathogène. Ce minage soutient une impression de santé et de bien­être dans l'imaginaire colonial, blanc européen et urbain des populations du sud. S'inspirant d'expériences communautaires qualitatives éclairées interdisciplinaires pluriannuelles axées sur les relations et ancrées dans les méthodologies féministes, anticoloniales et antiracistes qui ont guidé les discours créatifs inspirés par les sciences humaines, le présent texte se termine avec des récits différents. Ceux­ci déstabilisent les pouvoirs coloniaux qui se fondent sur le concept de territoire pathogène et, par conséquent, ré­interprètent les perceptions et les affirmations sur l'environnement et le bien­être des Autochtones.

Freeze-frame imaging of synaptic activity using SynTagMA.

Information within the brain travels from neuron to neuron across billions of synapses. At any given moment, only a small subset of neurons and synapses are active, but finding the active synapses in brain tissue has been a technical challenge. Here we introduce SynTagMA to tag active synapses in a user-defined time window. Upon 395-405 nm illumination, this genetically encoded marker of activity converts from green to red fluorescence if, and only if, it is bound to calcium. Targeted to presynaptic terminals, preSynTagMA allows discrimination between active and silent axons. Targeted to excitatory postsynapses, postSynTagMA creates a snapshot of synapses active just before photoconversion. To analyze large datasets, we show how to identify and track the fluorescence of thousands of individual synapses in an automated fashion. Together, these tools provide an efficient method for repeatedly mapping active neurons and synapses in cell culture, slice preparations, and in vivo during behavior.

Implicitly learned suppression of irrelevant spatial locations.

How do we ignore a salient, irrelevant stimulus whose location is predictable? A variety of studies using instructional manipulations have shown that participants possess the capacity to exert location-based suppression. However, for the visual search challenges we face in daily life, we are not often provided explicit instructions and are unlikely to consciously deliberate on what our best strategy might be. Instead, we might rely on our past experience-in the form of implicit learning-to exert strategic control. In this paper, we tested whether implicit learning could drive spatial suppression. In Experiment 1, participants searched displays in which one location contained a target, while another contained a salient distractor. An arrow cue pointed to the target location with 70 % validity. Also, unbeknownst to the participants, the same arrow cue predicted the distractor location with 70 % validity. Results showed facilitated RTs to the predicted target location, confirming target enhancement. Critically, distractor interference was reduced at the predicted distractor location, revealing that participants used spatial suppression. Further, we found that participants had no explicit knowledge of the cue-distractor contingencies, confirming that the learning was implicit. In Experiment 2, to seek further evidence for suppression, we modified the task to include occasional masked probes following the arrow cue; we found worse probe identification accuracy at the predicted distractor location than control locations, providing converging evidence that observers spatially suppressed the predicted distractor locations. These results reveal an ecologically desirable mechanism of suppression, which functions without the need for conscious knowledge or externally guided instructions.

Abbreviated exposure to hypoxia is sufficient to induce CNS dysmyelination, modulate spinal motor neuron composition, and impair motor development in neonatal mice.

Neonatal white matter injury (nWMI) is an increasingly common cause of cerebral palsy that results predominantly from hypoxic injury to progenitor cells including those of the oligodendrocyte lineage. Existing mouse models of nWMI utilize prolonged periods of hypoxia during the neonatal period, require complex cross-fostering and exhibit poor growth and high mortality rates. Abnormal CNS myelin composition serves as the major explanation for persistent neuro-motor deficits. Here we developed a simplified model of nWMI with low mortality rates and improved growth without cross-fostering. Neonatal mice are exposed to low oxygen from postnatal day (P) 3 to P7, which roughly corresponds to the period of human brain development between gestational weeks 32 and 36. CNS hypomyelination is detectable for 2-3 weeks post injury and strongly correlates with levels of body and brain weight loss. Immediately following hypoxia treatment, cell death was evident in multiple brain regions, most notably in superficial and deep cortical layers as well as the subventricular zone progenitor compartment. PDGFαR, Nkx2.2, and Olig2 positive oligodendrocyte progenitor cell were significantly reduced until postnatal day 27. In addition to CNS dysmyelination we identified a novel pathological marker for adult hypoxic animals that strongly correlates with life-long neuro-motor deficits. Mice reared under hypoxia reveal an abnormal spinal neuron composition with increased small and medium diameter axons and decreased large diameter axons in thoracic lateral and anterior funiculi. Differences were particularly pronounced in white matter motor tracts left and right of the anterior median fissure. Our findings suggest that 4 days of exposure to hypoxia are sufficient to induce experimental nWMI in CD1 mice, thus providing a model to test new therapeutics. Pathological hallmarks of this model include early cell death, decreased OPCs and hypomyelination in early postnatal life, followed by dysmyelination, abnormal spinal neuron composition, and neuro-motor deficits in adulthood.