Improvement of mnemonic discrimination with acute light exercise is mediated by pupil-linked arousal in healthy older adults

Physical exercise has positive impacts on hippocampal memory decline with aging. One of the postulated neurobiological mechanisms of the decline is reduced catecholaminergic projections from the locus coeruleus to the hippocampus. Recent human studies revealed that very light exercise rapidly enhances memory and pupil diameter, which suggests that light exercise may improve memory via neural circuits involved in the ascending arousal system, including the locus coeruleus, even in older adults. Thus, we aimed to clarify the effects of a single bout of light-intensity exercise (60% ventilatory threshold) on mnemonic discrimination performance, an index of hippocampal memory function, in healthy older adults using a randomized crossover design. Pupil diameter was measured during exercise as a physiological marker of the ascending arousal system. Discrimination of highly similar stimuli to the targets improved after exercise when compared to the resting control performance. Importantly, causal mediation analysis showed that pupil dilation during exercise mediated the memory improvement. These results suggest that brief light exercise rapidly enhances memory, possibly by upregulating the ascending arousal system.

Disruption of the ascending arousal system and cortical attention networks in post-stroke delirium and spatial neglect

Delirium is an acute attention and cognitive dysfunction, adversely affecting functional outcomes and mortality. As many as half of hospitalized right brain stroke survivors may develop delirium. Further, about 50% of right stroke patients experience spatial neglect, impairing safety and recovery. In this review we explore the brain mechanisms, which may explain the high incidence of delirium and spatial neglect after right-brain stroke. We suggest that brain networks for spatial attention and arousal, composed of ascending projections from the midbrain nuclei and integrating dorsal and ventral cortical and limbic components, may underlie impairments in delirium and spatial neglect. We propose that lateralized deficits in spatial neglect may arise because cortical and limbic components of these functional networks are disproportionally impaired by right-brain strokes, and that spatial neglect may lower the threshold for developing delirium. An improved understanding of the brain basis of delirium and spatial neglect could provide a critical biomarker for initiating preventive care in stroke patients at high risk of hospital morbidity and loss of independence.

Brainstem Monitoring in the Neurocritical Care Unit: A Rationale for Real-Time, Automated Neurophysiological Monitoring

Patients with severe traumatic brain injury or large intracranial space-occupying lesions (spontaneous cerebral hemorrhage, infarction, or tumor) commonly present to the neurocritical care unit with an altered mental status. Many experience progressive stupor and coma from mass effects and transtentorial brain herniation compromising the ascending arousal (reticular activating) system. Yet, little progress has been made in the practicality of bedside, noninvasive, real-time, automated, neurophysiological brainstem, or cerebral hemispheric monitoring. In this critical review, we discuss the ascending arousal system, brain herniation, and shortcomings of our current management including the neurological exam, intracranial pressure monitoring, and neuroimaging. We present a rationale for the development of nurse-friendly-continuous, automated, and alarmed-evoked potential monitoring, based upon the clinical and experimental literature, advances in the prognostication of cerebral anoxia, and intraoperative neurophysiological monitoring.

Syrian hamster neuroplasticity mechanisms fail as temperature declines to 15 °C, but histaminergic neuromodulation persists

Previous research suggests that hippocampal neurons in mammalian hibernators shift their major function from memory formation at euthermic brain temperatures (T _b = ~37 °C) to modulation of hibernation bout duration as T _b decreases. This role of hippocampal neurons during torpor is based in part on in vivo studies showing that histamine (HA) infused into ground squirrel hippocampi lengthened torpor bouts by ~50%. However, it was unclear if HA acted directly on hippocampal neurons or on downstream brain regions via HA spillover into lateral ventricles. To clarify this, we used hippocampal slices to determine if HA would modulate pyramidal neurons at low levels of synaptic activity (as occurs in torpor). We tested the hypotheses that although LTP (a neuroplasticity mechanism) could not be generated at low temperatures, HA (via H2 receptors) would increase population spike amplitudes (PSAs) of Syrian hamster CA1 pyramidal neurons at low stimulation voltages and low temperatures. PSAs were recorded following Schaffer collateral stimulation from subthreshold levels to a maximum response plateau. We found that tetanus evoked LTP at 35 °C but not 15 °C; and at temperatures from 30 to 15 °C, HA significantly enhanced PSA at near threshold levels in slices from non-hibernating hamsters housed in "summer-like" or "winter-like" conditions and from hibernating hamsters. Cimetidine (H2 antagonist) blocked HA-mediated PSA increases in 8 of 8 slices; pyrilamine (H1 antagonist) had no effect in 7 of 8 slices. These results support our hypotheses and show that HA can directly enhance pyramidal neuron excitability via H2 receptors and thus may prolong torpor bouts.