Faculty Bibliography

KEY POINTS:receptors, probably on bipolar cell axon terminals. The GABAergic masking inhibition appears independent of dopaminergic circuitry that has been shown also to affect RGC sensitivity. The results indicate a novel mechanism whereby inhibition controls the sensitivity of different cohorts of RGCs. This can limit and thereby ensure that appropriate signals are carried centrally in scotopic conditions when sensitivity rather than acuity is crucial. ABSTRACT:The responses of rod photoreceptors, which subserve dim light vision, are carried through the retina by three independent pathways. These pathways carry signals with largely different sensitivities. Retinal ganglion cells (RGCs), the output neurons of the retina, show a wide range of sensitivities in the same dark-adapted conditions, suggesting a divergence of the rod pathways. However, this organization is not supported by the known synaptic morphology of the retina. Here, we tested an alternative idea that the rod pathways converge onto single RGCs, but inhibitory circuits selectively mask signals so that one pathway predominates. Indeed, we found that application of GABA receptor blockers increased the sensitivity of most RGCs by unmasking rod signals, which were suppressed. Our results indicate that inhibition controls the threshold responses of RGCs under dim ambient light. This mechanism can ensure that appropriate signals cross the bottleneck of the optic nerve in changing stimulus conditions.

The study of transmitter interactions in reward and motor pathways in the brain, including the striatum, requires methodology to detect stimulus-driven neurotransmitter release events. Such methods exist for dopamine, and have contributed to the understanding of local and behavioral factors that regulate dopamine release. However, factors that regulate release of another key transmitter in these pathways, acetylcholine (ACh), are unresolved, in part because of limited temporal and spatial resolution of current detection methods. We have optimized a voltammetric method for detection of local stimulus-evoked ACh release using enzyme-coated carbon-fiber microelectrodes and fast-scan cyclic voltammetry. These electrodes are based on the detection of H2O2 generated by the actions of acetylcholine esterase and choline oxidase, and reliably respond to ACh in a concentration-dependent manner. Methods for enzyme coating were optimized for mechanical stability that allowed for their use in ex vivo brain slices. We report here the first quantitative assessment of extracellular ACh concentration after local electrical stimulation in dorsal striatum in slices from control mice. The selective detection of ACh under these conditions was confirmed by showing that the response detected in the control slices was absent in slices from mice bred to lack ACh synthesis in the forebrain. These electrodes represent a new tool to study ACh and ACh-dopamine interactions with micrometer spatial resolution.

STUDY OBJECTIVES: Emerging evidence suggests a role for sleep in contributing to the progression of Alzheimer disease (AD). Slow wave sleep (SWS) is the stage during which synaptic activity is minimal and clearance of neuronal metabolites is high, making it an ideal state to regulate levels of amyloid beta (Abeta). We thus aimed to examine relationships between concentrations of Abeta42 in the cerebrospinal fluid (CSF) and measures of SWS in cognitively normal elderly subjects. METHODS: Thirty-six subjects underwent a clinical and cognitive assessment, a structural MRI, a morning to early afternoon lumbar puncture, and nocturnal polysomnography. Correlations and linear regression analyses were used to assess for associations between CSF Abeta42 levels and measures of SWS controlling for potential confounders. Resulting models were compared to each other using ordinary least squared linear regression analysis. Additionally, the participant sample was dichotomized into "high" and "low" Abeta42 groups to compare SWS bout length using survival analyses. RESULTS: A significant inverse correlation was found between CSF Abeta42 levels, SWS duration and other SWS characteristics. Collectively, total SWA in the frontal lead was the best predictor of reduced CSF Abeta42 levels when controlling for age and ApoE status. Total sleep time, time spent in NREM1, NREM2, or REM sleep were not correlated with CSF Abeta42. CONCLUSIONS: In cognitively normal elderly, reduced and fragmented SWS is associated with increases in CSF Abeta42, suggesting that disturbed sleep might drive an increase in soluble brain Abeta levels prior to amyloid deposition.

Understanding how the structure of cognition arises from the topographical organization of the cortex is a primary goal in neuroscience. Previous work has described local functional gradients extending from perceptual and motor regions to cortical areas representing more abstract functions, but an overarching framework for the association between structure and function is still lacking. Here, we show that the principal gradient revealed by the decomposition of connectivity data in humans and the macaque monkey is anchored by, at one end, regions serving primary sensory/motor functions and at the other end, transmodal regions that, in humans, are known as the default-mode network (DMN). These DMN regions exhibit the greatest geodesic distance along the cortical surface-and are precisely equidistant-from primary sensory/motor morphological landmarks. The principal gradient also provides an organizing spatial framework for multiple large-scale networks and characterizes a spectrum from unimodal to heteromodal activity in a functional metaanalysis. Together, these observations provide a characterization of the topographical organization of cortex and indicate that the role of the DMN in cognition might arise from its position at one extreme of a hierarchy, allowing it to process transmodal information that is unrelated to immediate sensory input.

BACKGROUND: AMPAkines augment the function of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors in the brain to increase excitatory outputs. These drugs are known to relieve persistent pain. However, their role in acute pain is unknown. Furthermore, a specific molecular and anatomic target for these novel analgesics remains elusive. METHODS: The authors studied the analgesic role of an AMPAkine, CX546, in a rat paw incision (PI) model of acute postoperative pain. The authors measured the effect of AMPAkines on sensory and depressive symptoms of pain using mechanical hypersensitivity and forced swim tests. The authors asked whether AMPA receptors in the nucleus accumbens (NAc), a key node in the brain's reward and pain circuitry, can be a target for AMPAkine analgesia. RESULTS: Systemic administration of CX546 (n = 13), compared with control (n = 13), reduced mechanical hypersensitivity (50% withdrawal threshold of 6.05 +/- 1.30 g [mean +/- SEM] vs. 0.62 +/- 0.13 g), and it reduced depressive features of pain by decreasing immobility on the forced swim test in PI-treated rats (89.0 +/- 15.5 vs. 156.7 +/- 18.5 s). Meanwhile, CX546 delivered locally into the NAc provided pain-relieving effects in both PI (50% withdrawal threshold of 6.81 +/- 1.91 vs. 0.50 +/- 0.03 g; control, n = 6; CX546, n = 8) and persistent postoperative pain (spared nerve injury) models (50% withdrawal threshold of 3.85 +/- 1.23 vs. 0.45 +/- 0.00 g; control, n = 7; CX546, n = 11). Blocking AMPA receptors in the NAc with 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione inhibited these pain-relieving effects (50% withdrawal threshold of 7.18 +/- 1.52 vs. 1.59 +/- 0.66 g; n = 8 for PI groups; 10.70 +/- 3.45 vs. 1.39 +/- 0.88 g; n = 4 for spared nerve injury groups). CONCLUSIONS: AMPAkines relieve postoperative pain by acting through AMPA receptors in the NAc.