Min Wang

Inflammation plays a large role in the etiology of the late onset, sporadic form of Alzheimer’s disease (AD), yet these critical factors are not adequately modeled in mice where inflammatory mechanisms often differ widely from primates. In contrast, aging rhesus macaques offer a powerful translational model for investigating how advancing age and inflammation initiate early-stage pathology in sporadic AD, and for evaluating preventive therapeutic strategies. Unlike rodents, macaques possess highly developed association cortices with magnified calcium signaling, human-like inflammatory responses, and are naturally homozygous for ApoE-ε4—factors that together contribute to the spontaneous emergence of tau and amyloid pathology alongside cognitive decline. Critically, macaques allow the detection of early, soluble forms of hyperphosphorylated tau (pTau), including pT217Tau, which rapidly dephosphorylates postmortem and is rarely observable in human brain tissue outside of biopsies. New findings reveal that soluble pTau is neurotoxic and capable of propagating pathology across cortical networks, with elevated pT217Tau in plasma. Growing evidence points to age-related inflammatory signaling as a key driver of calcium dysregulation, which in turn promotes tau hyperphosphorylation, amyloid-β (Aβ) accumulation, synapse loss and autophagic degeneration. Both GCPII (glutamate carboxypeptidase II) and kynurenic acid inflammatory signaling have expanded roles in the primate association cortices that contribute to cognitive deficits. Pharmacological interventions in aged macaques demonstrate that targeting inflammation and restoring calcium homeostasis can significantly reduce pTau pathology with minimal side effects—highlighting a promising path for early intervention in AD.

Cognitive deficits from dorsolateral prefrontal cortex (dlPFC) dysfunction are common in neuroinflammatory disorders, including long-COVID, schizophrenia and Alzheimer’s disease, where impairments are correlated with kynurenine inflammatory signaling. Kynurenine synthesis from tryptophan is increased under conditions of inflammation, then further metabolized to kynurenic acid (KYNA) in brain, where it blocks NMDA and α7-nicotinic receptors (nic-α7Rs). These receptors are essential for neurotransmission in dlPFC, suggesting that KYNA may contribute to higher cognitive deficits in these disorders. The current study employed several methods to examine the expression of KYNA and its synthetic enzyme, KAT II, in primate dlPFC, and to determine its effects on working memory-related dlPFC neuronal firing and cognitive functioning in aging macaques with naturally-occurring neuroinflammation. We found that KYNA, its synthetic enzyme, KAT II, and the gene encoding KAT II (AADAT), have greatly expanded expression in macaque and human dlPFC in both glia and neurons, with AADAT especially prominent in primate neurons compared to rodent PFC. In macaques, like humans, plasma kynurenine/tryptophan ratios increased with age, consistent with age-related increasing inflammation. Local application of KYNA onto dlPFC neurons markedly reduced the delay-related firing needed for working memory via actions at NMDA and nic-α7Rs, while inhibition of KAT II enhanced neuronal firing in aged macaques. Systemic administration of agents that reduce KYNA production similarly improved cognitive performance in aged monkeys. These data show that KYNA inflammatory signaling expands in primate dlPFC, and that inhibition of kynurenine-KYNA production may provide a powerful therapeutic avenue for treating higher cognitive deficits in neuroinflammatory disorders.

Summary: Cognitive deficits from dorsolateral prefrontal cortex (dlPFC) dysfunction are common in neuroinflammatory disorders, including long-COVID, schizophrenia and Alzheimer’s disease, and have been correlated with kynurenine inflammatory signaling. Kynurenine is further metabolized to kynurenic acid (KYNA) in brain, where it blocks NMDA and α7-nicotinic receptors (nic-α7Rs). These receptors are essential for neurotransmission in dlPFC, suggesting that KYNA may cause higher cognitive deficits in these disorders. The current study found that KYNA and its synthetic enzyme, KAT II, have greatly expanded expression in primate dlPFC in both glia and neurons. Local application of KYNA onto dlPFC neurons markedly reduced the delay-related firing needed for working memory via actions at NMDA and nic-α7Rs, while inhibition of KAT II enhanced neuronal firing in aged macaques. Systemic administration of agents that reduce KYNA production similarly improved cognitive performance in aged monkeys, suggesting a therapeutic avenue for the treatment of cognitive deficits in neuroinflammatory disorders.

Sustained cognitive deficits are a common and debilitating feature of “long COVID”, but currently there are no FDA-approved treatments. The cognitive functions of the dorsolateral prefrontal cortex (dlPFC) are the most consistently afflicted by long COVID, including deficits in working memory, motivation, and executive functioning. COVID-19 infection greatly increases kynurenic acid (KYNA) and glutamate carboxypeptidase II (GCPII) in brain, both of which can be particularly deleterious to PFC function. KYNA blocks both NMDA and nicotinic-alpha-7 receptors, the two receptors required for dlPFC neurotransmission, and GCPII reduces mGluR3 regulation of cAMP-calcium-potassium channel signaling, which weakens dlPFC network connectivity and reduces dlPFC neuronal firing. Two agents approved for other indications may be helpful in restoring dlPFC physiology: the antioxidant N-acetyl cysteine inhibits the production of KYNA, and the α2A-adrenoceptor agonist guanfacine regulates cAMP-calcium-potassium channel signaling in dlPFC and is also anti-inflammatory. Thus, these agents may be helpful in treating the cognitive symptoms of long COVID.

The prefrontal cortex underlies our high order cognitive abilities and is the target of projections from many neuromodulatory nuclei. The dorsolateral prefrontal cortex is particularly critical for rule representation and working memory, or the ability to hold information “in mind” in the absence of sensory input. Emerging evidence supports a prominent and permissive role for acetylcholine in these excitatory circuits, through actions at cholinergic nicotinic receptors. Here we review the involvement of acetylcholine in working memory via actions at nicotinic receptors.

Noradrenergic (NE) α1-adrenoceptors (α1-ARs) contribute to arousal mechanisms and play an important role in therapeutic medications such as those for the treatment of posttraumatic stress disorder (PTSD). However, little is known about how α1-AR stimulation influences neuronal firing in the dorsolateral prefrontal cortex (dlPFC), a newly evolved region that is dysfunctional in PTSD and other mental illnesses. The current study examined the effects of α1-AR manipulation on neuronal firing in dlPFC of rhesus monkeys performing a visuospatial working memory task, focusing on the “delay cells” that maintain spatially tuned information across the delay period. Iontophoresis of the α1-AR antagonist HEAT (2-{[β-(4-hydroxyphenyl)ethyl]aminomethyl}-1-tetralone) had mixed effects, reducing firing in a majority of neurons but having nonsignificant excitatory effects or no effect in remaining delay cells. These data suggest that endogenous NE has excitatory effects in some delay cells under basal conditions. In contrast, the α1-AR agonists phenylephrine and cirazoline suppressed delay cell firing and this was blocked by coadministration of HEAT. These results indicate an inverted-U dose response for α1-AR actions, with mixed excitatory actions under basal conditions and suppressed firing with high levels of α1-AR stimulation such as with stress exposure. Immunoelectron microscopy revealed α1-AR expression presynaptically in axons and axon terminals and postsynaptically in spines, dendrites, and astrocytes. It is possible that α1-AR excitatory effects arise from presynaptic excitation of glutamate release, whereas postsynaptic actions suppress firing through calcium–protein kinase C opening of potassium channels on spines. The latter may predominate under stressful conditions, leading to loss of dlPFC regulation during uncontrollable stress.SIGNIFICANCE STATEMENTNoradrenergic stimulation of α1-adrenoceptors (α1-ARs) is implicated in posttraumatic stress disorder (PTSD) and other mental disorders that involve dysfunction of the prefrontal cortex, a brain region that provides top-down control. However, the location and contribution of α1-ARs to prefrontal cortical physiology in primates has received little attention. This study found that α1-ARs are located near prefrontal synapses and that α1-AR stimulation has mixed effects under basal conditions. However, high levels of α1-AR stimulation, as occur with stress, suppress neuronal firing. These findings help to explain why we lose top-down control under conditions of uncontrollable stress when there are high levels of noradrenergic release in brain and why blocking α1-AR, such as with prazosin, may be helpful in the treatment of PTSD.

Persistent activity generated in the PFC during the delay period of working memory tasks represents information about stimuli held in memory and determines working memory performance. Alternative models of working memory, depending on the rhythmicity of discharges or exclusively on short-term synaptic plasticity, are inconsistent with the neurophysiological data. Dual Perspectives Companion Paper:Working Memory: Delay Activity, Yes! Persistent Activity? Maybe Not, by Mikael Lundqvist, Pawel Herman, and Earl K. Miller