Calcium channel dynamics limit synaptic release in response to prosthetic stimulation with sinusoidal waveforms.

Extracellular electric stimulation with sinusoidal waveforms has been shown to allow preferential activation of individual types of retinal neurons by varying stimulus frequency. It is important to understand the mechanisms underlying this frequency dependence as a step toward improving methods of preferential activation. In order to elucidate these mechanisms, we implemented a morphologically realistic model of a retinal bipolar cell and measured the response to extracellular stimulation with sinusoidal waveforms. We compared the frequency response of a passive membrane model to the kinetics of voltage-gated calcium channels that mediate synaptic release. The passive electrical properties of the membrane exhibited lowpass filtering with a relatively high cutoff frequency (nominal value = 717 Hz). This cutoff frequency was dependent on intra-axonal resistance, with shorter and wider axons yielding higher cutoff frequencies. However, we found that the cutoff frequency of bipolar cell synaptic release was primarily limited by the relatively slow opening kinetics of L- and T-type calcium channels. The cutoff frequency of calcium currents depended nonlinearly on stimulus amplitude, but remained lower than the cutoff frequency of the passive membrane model for a large range of membrane potential fluctuations. These results suggest that while it may be possible to modulate the membrane potential of bipolar cells over a wide range of stimulus frequencies, synaptic release will only be initiated at the lower end of this range.

Spatial relation between microbleeds and amyloid deposits in amyloid angiopathy.

Advanced cerebrovascular ß-amyloid deposition (cerebral amyloid angiopathy, CAA) is associated with cerebral microbleeds, but the precise relationship between CAA burden and microbleeds is undefined. We used T2*-weighted magnetic resonance imaging (MRI) and noninvasive amyloid imaging with Pittsburgh Compound B (PiB) to analyze the spatial relationship between CAA and microbleeds. On coregistered positron emission tomography (PET) and MRI images, PiB retention was increased at microbleed sites compared to simulated control lesions (p = 0.002) and declined with increasing distance from the microbleed (p < 0.0001). These findings indicate that microbleeds occur preferentially in local regions of concentrated amyloid and support therapeutic strategies aimed at reducing vascular amyloid deposition.

MRI markers of small vessel disease in lobar and deep hemispheric intracerebral hemorrhage.

BACKGROUND AND PURPOSE: MRI evidence of small vessel disease is common in intracerebral hemorrhage (ICH). We hypothesized that ICH caused by cerebral amyloid angiopathy (CAA) or hypertensive vasculopathy would have different distributions of MRI T2 white matter hyperintensity (WMH) and microbleeds. METHODS: Data were analyzed from 133 consecutive patients with primary supratentorial ICH and adequate MRI sequences. CAA was diagnosed using the Boston criteria. WMH segmentation was performed using a validated semiautomated method. WMH and microbleeds were compared according to site of symptomatic hematoma origin (lobar versus deep) or by pattern of hemorrhages, including both hematomas and microbleeds, on MRI gradient recalled echo sequence (grouped as lobar only-probable CAA, lobar only-possible CAA, deep hemispheric only, or mixed lobar and deep hemorrhages). RESULTS: Patients with lobar and deep hemispheric hematoma had similar median normalized WMH volumes (19.5 cm versus 19.9 cm(3), P=0.74) and prevalence of >or=1 microbleed (54% versus 52%, P=0.99). The supratentorial WMH distribution was similar according to hemorrhage location category; however, the prevalence of brain stem T2 hyperintensity was lower in lobar hematoma versus deep hematoma (54% versus 70%, P=0.004). Mixed ICH was common (23%). Patients with mixed ICH had large normalized WMH volumes and a posterior distribution of cortical hemorrhages similar to that seen in CAA. CONCLUSIONS: WMH distribution is largely similar between CAA-related and non-CAA-related ICH. Mixed lobar and deep hemorrhages are seen on MRI gradient recalled echo sequence in up to one fourth of patients; in these patients, both hypertension and CAA may be contributing to the burden of WMH.