Human phenotypes and animal knockout models of genetic autonomic disorders

Preserved functional autonomic phenotype in adult mice overexpressing moderate levels of human alpha-synuclein in oligodendrocytes

Mice overexpressing human alpha‐synuclein in oligodendrocytes (MBP1‐α‐syn) recapitulate some key functional and neuropathological features of multiple system atrophy (MSA). Whether or not these mice develop severe autonomic failure, which is a key feature of human MSA, remains unknown. We explored cardiovascular autonomic regulation using long‐term blood pressure (BP) radiotelemetry and pharmacological testing. We instrumented 12 MBP1‐α‐syn mice and 11 wild‐type mice aged 9 months for radiotelemetry. Animals were tested with atropine, metoprolol, clonidine, and trimethaphan at 9 and 12 months age. We applied spectral and cross‐spectral analysis to assess heart rate (HR) and BP variability. At 9 months of age daytime BP (transgenic: 101 ± 2 vs. wild type: 99 ± 2 mmHg) and HR (497 ± 11 vs. 505 ± 16 beats/min) were similar. Circadian BP and HR rhythms were maintained. Nighttime BP (109 ± 2 vs. 108 ± 2 mmHg) and HR (575 ± 15 vs. 569 ± 14 beats/min), mean arterial BP responses to trimethaphan (−21 ± 8 vs. −10 ± 5 mmHg, P = 0.240) and to clonidine (−8 ± 3 vs. −5 ± 2 mmHg, P = 0.314) were similar. HR responses to atropine (+159 ± 24 vs. +146 ± 22 beats/min), and to clonidine (−188 ± 21 vs. −163 ± 33 beats/min) did not differ between strains. Baroreflex sensitivity (4 ± 1 vs. 4 ± 1 msec/mmHg) and HR variability (total power, 84 ± 17 vs. 65 ± 21 msec²) were similar under resting conditions and during pharmacological testing. Repeated measurements at 12 months of age provided similar results. In mice, moderate overexpression of human alpha‐synuclein in oligodendrocytes is not sufficient to induce overt autonomic failure. Additional mechanisms may be required to express the autonomic failure phenotype including higher levels of expression or more advanced age.

Mice overexpressing human alpha‐synuclein in oligodendrocytes recapitulate key functional and neuropathological features of multiple system atrophy (MSA). Whether or not these mice model MSA abnormalities in cardiovascular autonomic regulation is unknown. Therefore, we explored cardiovascular autonomic regulation using long‐term blood pressure radiotelemetry combined with detailed pharmacological testing in mice overexpressing moderate levels of human alpha‐synuclein under the control of the oligodendrocyte‐specific murine myelin basic protein promoter. The major finding was that the moderate overexpression of human alpha‐synuclein in oligodendrocytes was not sufficient to induce overt autonomic failure in this mouse model.

Desvenlafaxine Succinate Identifies Novel Antagonist Binding Determinants in the Human Norepinephrine Transporter

Desvenlafaxine succinate (DVS) is a recently introduced antagonist of the human norepinephrine and serotonin transporters (hNET and hSERT, respectively), currently in clinical development for use in the treatment of major depressive disorder and vasomotor symptoms associated with menopause. Initial evaluation of the pharmacological properties of DVS (J Pharmacol Exp Ther 318:657-665, 2006) revealed significantly reduced potency for the hNET expressed in membranes compared with whole cells when competing for [(3)H]nisoxetine (NIS) binding. Using hNET in transfected human embryonic kidney-293 cells, this difference in potency for DVS at sites labeled by [(3)H]NIS was found to distinguish DVS, the DVS analog rac-(1-[1-(3-chloro-phenyl)-2-(4-methylpiperazin-1-yl)-ethyl]cyclohexanol (WY-46824), methylphenidate, and the cocaine analog 3beta-(4-iodophenyl)tropane-2beta-carboxylic acid methyl ester (RTI-55) from other hNET antagonists, such as NIS, mazindol, tricyclic antidepressants, and cocaine. These differences seem not to arise from preparation-specific perturbations of ligand intrinsic affinity or antagonist-specific surface trafficking but rather from protein conformational alterations that perturb the relationships between distinct hNET binding sites. In an initial search for molecular features that differentially define antagonist binding determinants, we document that Val148 in hNET transmembrane domain 3 selectively disrupts NIS binding but not that of DVS.

Specific derivatization of the vesicle monoamine transporter with novel carrier-free radioiodinated reserpine and tetrabenazine photoaffinity labels

Two iodophenylazide derivatives of reserpine and one iodophenylazide derivative of tetrabenazine have been synthesized and characterized as photoaffinity labels of the vesicle monoamine transporter (VMAT2). These compounds are 18-O-[3-(3'-iodo-4'-azidophenyl)-propionyl]methyl reserpate (AIPPMER), 18-O-[N-(3'-iodo-4'-azidophenethyl)glycyl]methyl reserpate (IAPEGlyMER), and 2-N-[(3'-iodo-4'-azidophenyl)-propionyl]tetrabenazine (TBZ-AIPP). Inhibition of [3H]dopamine uptake into purified chromaffin granule ghosts showed IC50 values of approximately 37 nM for reserpine, 83 nM for AIPPMER, 200 nM for IAPEGlyMER, and 2.1 microM for TBZ-AIPP. Carrier-free radioiodinated [125I]IAPEGlyMER and [125I]TBZ-AIPP were synthesized and used to photoaffinity label chromaffin granule membranes. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed specific [125I]IAPEGlyMER labeling of a polypeptide that migrated as a broad band (approximately 55-90 kDa), with the majority of the label located between 70 and 80 kDa. The labeling by [125I]IAPEGlyMER was blocked by 100 nM reserpine, 10 microM tetrabenazine, 1 mM serotonin, and 10 mM (-)-norepinephrine and dopamine. Analysis of [125I]TBZ-AIPP-labeled chromaffin granule membranes by SDS-PAGE and autoradiography demonstrated specific labeling of a similar polypeptide, which was blocked by 1 microM reserpine and 10 microM tetrabenazine. Incubation of [125I]TBZ-AIPP-photolabeled chromaffin granule membranes in the presence of the glycosidase N-glycanase shifted the apparent molecular weight of VMAT2 to approximately 51 kDa. These data indicate that [125I]IAPEGlyMER and [125I]TBZ-AIPP are effective photoaffinity labels for VMAT2.

Wavelet methods for spike detection in mouse renal sympathetic nerve activity

Abnormal autonomic nerve traffic has been associated with a number of peripheral neuropathies and cardiovascular disorders prompting the development of genetically altered mice to study the genetic and molecular components of these diseases. Autonomic function in mice can be assessed by directly recording sympathetic nerve activity. However, murine sympathetic spikes are typically detected using a manually adjusted voltage threshold and no unsupervised detection methods have been developed for the mouse. Therefore, we tested the performance of several unsupervised spike detection algorithms on simulated murine renal sympathetic nerve recordings, including an automated amplitude discriminator and wavelet-based detection methods which used both the discrete wavelet transform (DWT) and the stationary wavelet transform (SWT) and several wavelet threshold rules. The parameters of the wavelet methods were optimized by comparing basal sympathetic activity to postmortem recordings and recordings made during pharmacological suppression and enhancement of sympathetic activity. In general, SWT methods were found to outperform amplitude discriminators and DWT methods with similar wavelet coefficient thresholding algorithms when presented with simulations with varied mean spike rates and signal-to-noise ratios. A SWT method which estimates the noise level using a "noise-only" wavelet scale and then selectively thresholds scales containing the physiologically important signal information was found to have the most robust spike detection. The proposed noise-level estimation method was also successfully validated during pharmacological interventions.