In Vivo Imaging of Venous Side Cerebral Small-Vessel Disease in Older Adults: An MRI Method at 7T.

BACKGROUND AND PURPOSE: Traditional neuroimaging markers of small-vessel disease focus on late-stage changes. We aimed to adapt a method of venular assessment at 7T for use in older adults. We hypothesized that poorer venular morphologic characteristics would be related to other small-vessel disease neuroimaging markers and a higher prevalence of small-vessel disease-Alzheimer disease risk factors. MATERIALS AND METHODS: Venules were identified in periventricular ROIs on SWI and defined as tortuous or straight. The tortuosity ratio was defined as total tortuous venular length divided by total straight venular length. White matter hyperintensity burden (visually rated from 0 to 3) and the number of microbleeds (0, 1, >1) were determined. Differences in tortuous and straight venular lengths were evaluated. Relationships with demographic variables, allele producing the e4 type of apolipoprotein E (APOE4), growth factors, pulse pressure, physical activity, and Modified Mini-Mental State Examination were assessed via Spearman correlations. RESULTS: Participants had 42% more tortuous venular tissue than straight (median, 1.42; 95% CI, 1.13-1.62). APOE4 presence was associated with a greater tortuosity ratio (ρ = 0.454, P = .001), and these results were robust to adjustment for confounders and multiple comparisons. Associations of the tortuosity ratio with sex and vascular endothelial growth factor did not survive adjustment. Associations of the tortuosity ratio with other variables of interest were not significant. CONCLUSIONS: Morphologic measures of venules at 7T could be useful biomarkers of the early stages of small-vessel disease and Alzheimer disease. Longitudinal studies should examine the impact of apolipoprotein E and vascular endothelial growth factor on the risk of venular damage.

Network science and the effects of music preference on functional brain connectivity: from Beethoven to Eminem.

Most people choose to listen to music that they prefer or 'like' such as classical, country or rock. Previous research has focused on how different characteristics of music (i.e., classical versus country) affect the brain. Yet, when listening to preferred music--regardless of the type--people report they often experience personal thoughts and memories. To date, understanding how this occurs in the brain has remained elusive. Using network science methods, we evaluated differences in functional brain connectivity when individuals listened to complete songs. We show that a circuit important for internally-focused thoughts, known as the default mode network, was most connected when listening to preferred music. We also show that listening to a favorite song alters the connectivity between auditory brain areas and the hippocampus, a region responsible for memory and social emotion consolidation. Given that musical preferences are uniquely individualized phenomena and that music can vary in acoustic complexity and the presence or absence of lyrics, the consistency of our results was unexpected. These findings may explain why comparable emotional and mental states can be experienced by people listening to music that differs as widely as Beethoven and Eminem. The neurobiological and neurorehabilitation implications of these results are discussed.

Evaluating the impact of spatio-temporal smoothness constraints on the BOLD hemodynamic response function estimation: an analysis based on Tikhonov regularization.

Recently we have proposed the use of Tikhonov regularization with temporal smoothness constraints to estimate the BOLD fMRI hemodynamic response function (HRF). The temporal smoothness constraint was imposed on the estimates by using second derivative information while the regularization parameter was selected based on the generalized cross-validation function (GCV). Using one-dimensional simulations, we previously found this method to produce reliable estimates of the HRF time course, especially its time to peak (TTP), being at the same time fast and robust to over-sampling in the HRF estimation. Here, we extend the method to include simultaneous temporal and spatial smoothness constraints. This method does not need Gaussian smoothing as a pre-processing step as usually done in fMRI data analysis. We carried out two-dimensional simulations to compare the two methods: Tikhonov regularization with temporal (Tik-GCV-T) and spatio-temporal (Tik-GCV-ST) smoothness constraints on the estimated HRF. We focus our attention on quantifying the influence of the Gaussian data smoothing and the presence of edges on the performance of these techniques. Our results suggest that the spatial smoothing introduced by regularization is less severe than that produced by Gaussian smoothing. This allows more accurate estimates of the response amplitudes while producing similar estimates of the TTP. We illustrate these ideas using real data.

Estimation of false discovery rates for wavelet-denoised statistical parametric maps.

Correction for multiple comparisons in neuroimaging data is an important area of research. Recently, wavelet-based methods have gained popularity and have been reported to achieve better sensitivity compared to spatial domain methods. However, these techniques produce smoothed statistical maps which are difficult to interpret. The generated maps have to be thresholded again in the spatial domain to delineate active from inactive regions. The selection of a proper threshold satisfying the required error rate control is not straightforward. In this paper, a framework is proposed for thresholding wavelet-denoised maps in which a rejection region is fixed, and the achieved false discovery rate (FDR) is estimated. This approach provides a meaningful strategy to choose thresholds for wavelet-denoised statistical parametric maps (SPMs). Two FDR estimation algorithms were used to assess the achieved error rate control when thresholding wavelet filtered SPMs at various rejection regions. Their performance was evaluated using both simulated and resting fMRI data. The proposed framework was also applied on in vivo data.

Properties of cholinergic responses in isolated parapodial muscle fibers of Aplysia.

The parapodial neuromuscular junction in the marine snail Aplysia brasiliana is a model synapse for the investigation of neural modulation. The parapodial muscle fibers are innervated by cholinergic motoneurons and by serotonergic modulatory cells. The physiological properties of voltage-gated currents of the muscle membranes and the effects of serotonin on these currents have been published previously. However, the pharmacological properties of the cholinergic receptors have not been investigated. Acetylcholine (ACh) applied exogenously to dissociated muscle fibers produces a response with a reversal potential of about -52 mV; the resting membrane potential of the average muscle fiber is approximately -56 mV. ACh induces variable responses (depolarizations or hyperpolarizations) in individual cells, but the transmitter never causes a depolarization adequate to produce muscle contraction. We demonstrate that the ACh response is the result of the activation of two distinct receptors. One receptor is linked to a chloride channel and induces a hyperpolarization with a reversal potential near -70 mV. This receptor is activated selectively by suberyldicholine and by nicotine and is antagonized by curare but not by hexamethonium. The second response, presumably caused by increased conductance to mixed cations, results in muscle fiber depolarization with a reversal potential near -35 mV and does induce muscle contraction. This receptor is activated by methylcarbamylcholine and selectively blocked by hexamethonium; atypically, this receptor is not activated by nicotine nor by carbachol. The depolarizing, cation-selective receptors likely are associated with identified excitatory cholinergic motoneurons the activity of which typically results in muscle contractions because the reversal potential for this ACh response is more depolarized than the activation threshold for voltage-gated calcium channels in these fibers. The hyperpolarizing, chloride-selective receptors may be associated with inhibitory motoneurons; such motoneurons have yet to be identified, but their presence is inferred because of the occurrence of spontaneous inhibitory junctional potentials recording from muscle fibers in situ. Muscle fiber responses to exogenously applied ACh reflect the relative contribution of each receptor type in each muscle fiber.

Serotonergic modulation of a voltage-gated calcium current in parapodial swim muscle from Aplysia brasiliana.

Here we describe the effects of serotonin (5-HT) on dissociated parapodial muscle fibers from Aplysia brasiliana. 5-HT has previously been implicated as a modulatory transmitter at the parapodial neuromuscular junction. Exogenously applied or endogenously released 5-HT increases the amplitude of motoneuron-induced excitatory junctional potentials and contractions in parapodial muscle. Exogenously applied 5 microM 5-HT increases the amplitude of a voltage-gated inward calcium current in isolated muscle fibers by an average of 42% in response to a voltage step from -70 to -10 mV. The amplitude of the inward current was increased at all voltages tested, with the peak increase occurring between -30 and -20 mV. The dihydropyridine calcium channel antagonist nifedipine (10 microM) blocked this effect of 5-HT. The data indicate that 5-HT increases a previously identified calcium current in parapodial muscle fibers that is similar to the vertebrate L-type current. Although several types of K+ channels exist in these fibers, including Ca(2+)-dependent K+ channels, the results suggest that 5-HT has little effect on these currents. Parapodial muscle contractions during swimming behavior occur in response to bursts of motoneuron action potentials that produce graded muscle depolarizations that occur over a 1- to 2-s period rather than being instantaneous or rapid responses as might be produced by one or two action potentials or a brief voltage step. With the use of 1-s voltage ramps, we attempted to mimic physiological depolarization and demonstrate that 5-HT is able to increase the amplitude of the inward calcium current. The data presented in this paper provide evidence that 5-HT increases the Ca2+ current, which may be one mechanism by which 5-HT modulates muscle contractions during swim behavior.

Parapodial swim muscle in Aplysia brasiliana. I. Voltage-gated membrane currents in isolated muscle fibers.

1. We describe voltage-gated membrane currents present in single muscle fibers dissociated from the parapodia (swim appendages) of the marine gastropod mollusk Aplysia brasiliana. These muscles are utilized in swimming behavior and their activity is modulated by serotonin. It is necessary to characterize the innate membrane properties of these fibers before defining the mechanism of action of serotonin in facilitating muscle fiber responses to motoneuron input. 2. Freshly dissociated parapodial muscle fibers appear by morphological criteria to be a uniform population with an average length of 240 microns and width of 15 microns. The average resting potential of all fibers is -56 mV and the fibers contract in response to elevated extracellular K+ concentration or intracellular depolarization. 3. Muscle membrane currents were studied by single-electrode voltage clamp with the use of intracellular microelectrodes. The muscle fibers were found to fall into one of two groups, which we have classified as type I and type II, the former having two voltage-gated outward K+ currents and a small, less frequently seen Ca2+ current. Type II fibers display the same two K+ currents, a prominent Ca2+ current and, in addition, two Ca(2+)-dependent K+ currents, the latter described in a companion paper. 4. Membrane currents were characterized using 1-s voltage ramps and several voltage step protocols, including ones for analyzing K+ tail currents. Both fiber types had similar current-voltage relationships and input resistance of > or = 60 - 300 M omega. The current-voltage curves were quite flat at potentials more negative than resting potential, with no evidence of a voltage-gated, inwardly rectified (anomalous) potassium current. Outward K+ currents and a Ca2+ current were seen to appear at a threshold of near -40 mV. 5. Because type I fibers had no apparent Ca(2+)-activated K+ currents, the two voltage-gated outward K+ currents were most conveniently studied in these fibers. Compared with type II fibers, type I fibers display a relatively slowly rising total outward current with depolarization comprised of a delayed rectifier current and a transient A current (IA). These two currents were distinguished by slightly different thresholds for activation, by inactivation properties of IA, and by their partially selective sensitivity to tetraethylammonium and 4-aminopyridine. 6. Although contraction of all parapodial muscle fibers is dependent on extracellular Ca2+, an inward Ca2+ current was detected in only about one third of type I fibers, and the current was small. A similar and more prominent Ca2+ current was observed in all type II fibers and was analyzed more fully in these cells. This current had an activation threshold near -40 mV and peaked between -10 and 0 mV. It displayed little inactivation with depolarization steps of 80-200 ms, was blocked in the absence of Ca2+ or in the presence of Co2+, and was present, although not enhanced, when Ba2+ was substituted for Ca2+. This current was completely blocked by the dihydropyridine nifedipine (10 microM), and is therefore similar to an L-type Ca2+ current. 7. The voltage-gated membrane currents described in parapodial muscle fibers provide a framework for analyzing possible mechanisms by which serotonin facilitates neuromuscular output. This facilitatory mechanism will provide a better understanding of the role of serotonin in controlling locomotion.

Parapodial swim muscle in Aplysia brasiliana. II. Ca(2+)-dependent K+ currents in isolated muscle fibers and their blockade by chloride substitutes.

1. We describe here the properties of two Ca(2+)-dependent K+ currents found in type II muscle fibers dissociated from the parapodia (swim appendages) of the marine snail Aplysia brasiliana. 2. Type II parapodial muscle fibers display three voltage-dependent currents that are also seen in type I fibers, a delayed rectifier current [IK(V)], a transient A current (IA), and a prominent L-type Ca2+ current. In addition, type II fibers also have two outward K+ currents, a transient, inactivating one and a slower, noninactivating one [IK(Ca,t) and IK(Ca,s), respectively], that are Ca2+ dependent. The expression of these currents in normal type II fibers generally produces a waveform of total outward current that is faster to peak than the total outward current seen in response to voltage steps in type I fibers and that does not inactivate at the end of an 80-ms voltage step. 3. Both IK(Ca,t) and IK(Ca,s) are absent when external Ca2+ is eliminated or when extracellular Ca2+ concentration ([Ca2+]o) is substituted with 10 mM Co2+ or Ba2+. Their threshold for activation is around -40 mV. IK(Ca,t) peaks rapidly and then inactivates, but IK(Ca,s) rises slowly and does not inactivate for as long as 200 ms. Both currents, like IK(V) and IA, are sensitive to tetraethylammonium and 4-aminopyridine and are not readily separated from either the voltage-gated currents or from one another by these pharmacological agents. 4. Tail current analysis from depolarized voltage steps in varying (K+]o demonstrates that these currents are carried by K+ ions and not by Cl-. 5. An unexpected finding, however, is that these Ca(2+)-dependent K+ currents are blocked by standard Cl- ion substitutes, such as methanesulfonate, isethionate, and propionate. IK(Ca,s) is slightly more sensitive to these Cl- substitutes than is IK(Ca,t). The chloride blocker 4,4'-diisothiocyantastilbene-2,2'disulfonic acid also partially blocked the Ca(2+)-dependent K+ currents. 6. The presence of these Ca(2+)-dependent K+ currents in type II fibers may contribute to a more rapid repolarization following depolarization-induced contractions. In contrast to type I fibers, which have smaller calcium current and no Ca(2+)-activated K+ currents, type II muscle cells may function more like "fast" fibers and relax more rapidly.

Identification and characterization of cerebral ganglion neurons that induce swimming and modulate swim-related pedal ganglion neurons in Aplysia brasiliana.

1. We have identified and characterized a family of several pairs of neurons in the cerebral ganglion of Aplysia brasiliana that are capable of inducing, maintaining, or modulating a motor program that underlies swim locomotion in this marine mollusk. We have operationally defined these cells as command neurons (CNs) for swimming. 2. The command cells occur in bilateral pairs in the cerebral ganglion and make direct and indirect outputs to neurons in the pedal ganglia, including motor neurons, a central pattern generator circuit, and modulatory neurons that enhance muscle contractions during swimming. Several of the CNs are sufficient individually to induce the swim motor program (SMP), all receive sensory feedback from the periphery, and several interconnect with other swim-related CNs. 3. Tonic discharges of approximately 10 Hz in CN types 1-3 (CN1-CN3) are capable of eliciting the oscillatory, phasic SMP as recorded in peripheral nerves that innervate the swim appendages, the parapodia. CN1, CN2, and CN3 make monosynaptic excitatory connections onto ipsilateral, contralateral, and bilateral pedal swim-modulatory neurons [parapodial opener-phase (POP) cells], respectively; and each command cell type activates the pedal central pattern generator (CPG), leading to sustained phasic output of motor neurons and POP cells. 4. Tonic firing of CN4 causes weak activation of the SMP contralaterally. These neurons occur as two pairs of neurons in each cerebral hemiganglion, with mutual electrical and chemical synaptic interconnections. CN4 cells also excite CN1 and CN2 cells. Thus CN4 is classified as a higher-order swim command cell type. 5. Command cells classified as types 5-8 (CN5-CN8), although not capable of inducing the SMP individually, nonetheless have strong synaptic connections with pedal POP cells and/or with other command neurons. These command cells may excite or inhibit follower cells on the same or opposite sides of the preparation and modulate the swim output. 6. All the command cells tested received strong input from mechanical stimulation, either stretch or pinching, of either parapodium. Mechanosensory input from the parapodia was shown to depend on the presence of the pedal ganglion, but not the pleural. Sensory stimulation activated command cells and motor neurons, but POP cells received input from sensory stimuli only through the cerebral ganglion, probably via command cells. The effects of applied mechanosensory stimuli could be entirely mimicked by motor neuron-induced contractions of the parapodia.