Terahertz and far-infrared synchrotron spectroscopy and global modeling of methyl mercaptan, CH3(32)SH.

In this work, terahertz and Fourier transform far-infrared (FTFIR) synchrotron spectra of methyl mercaptan, CH(3)SH, have been investigated in order to provide new laboratory information for enhanced observations of this species in interstellar molecular clouds and star-forming regions. Like its methanol cousin, methyl mercaptan has particularly rich spectra associated with its large-amplitude internal rotation that extend throughout the THz and FIR regions. We have recorded new spectra for CH(3)SH from 1.1-1.5 and 1.790-1.808 THz at the University of Cologne as well as high-resolution FTFIR synchrotron spectra from 50-550 cm(-1) at 0.001 cm(-1) resolution on the far-IR beam-line at the Canadian Light Source. Assignments are reported for rotational quantum numbers up to J ≈ 40 and K ≈ 15, and torsional states up to v(t) = 2 for the THz measurements and v(t) = 3 for the FTFIR observations. The THz and FTFIR measurements together with literature results have been combined in a global analysis of a dataset comprising a total of 1725 microwave and THz frequencies together with ~18000 FTFIR transitions, ranging up to v(t) = 2 and J(max) = 30 for MW∕THz and 40 for FTFIR. The global fit employs 78 torsion-rotation parameters and has achieved a weighted standard deviation of ~1.1. A prediction list (v(t) ≤ 2, J ≤ 45 and K ≤ 20) has been generated from the model giving essentially complete coverage of observable CH(3)(32)SH transitions within the bandwidths of major new astronomical facilities such as HIFI (Heterodyne Instrument for the Far Infrared) on the Herschel Space Observatory, ALMA (Atacama Large Millimeter Array), SOFIA (Stratospheric Observatory For Infrared Astronomy) and APEX (Atacama Pathfinder Experiment) to close to spectroscopic accuracy.

Perception of airborne odors by loggerhead sea turtles.

Sea turtles are known to detect chemical cues, but in contrast to most marine animals, turtles surface to breathe and thus potentially have access to olfactory cues both in air and in water. To determine whether sea turtles can detect airborne chemical cues, captive loggerhead turtles (Caretta caretta) were placed into a circular, water-filled arena in which odorants could be introduced to the air above the water surface. Air that had passed across the surface of a cup containing food elicited increased activity, diving and other behavior normally associated with feeding. By contrast, air that had passed across the surface of an identical cup containing distilled water elicited no response. Increases in activity during food odor trials occurred only after turtles surfaced to breathe and peaked in the first post-breath minute, implying that the chemical cues eliciting the responses were unlikely to have been detected while the turtles were under water. These results provide the first direct evidence that sea turtles can detect airborne odors. Under natural conditions, this sensory ability might function in foraging, navigation or both.

Dichroic filters to protect milliwatt far-infrared detectors from megawatt ECRH radiation.

Dichroic filters have been used to shield effectively the far infrared (FIR) detectors at the interferometer/polarimeter on TEXTOR. The filters consist of metal foils with regular holes, the hole diameter, the mutual spacing and the thickness of the foils are chosen to transmit radiation at the design frequency with transmission >90%. The attenuation at the low frequency end of the bandpass filter is about 30 dB per octave, the high frequency transmission is between 20% and 40%. The filters have been used to block the stray radiation from the megawatt microwave heating beam to the detectors of the FIR interferometer, operating with power on the detector in the milliwatt range. If required, the low frequency attenuation can be still enhanced, without compromising the transmission in the passband. The FIR interferometer used for plasma density and position control is no longer disturbed by electromagnetic waves used for plasma heating.

Application of superlattice multipliers for high-resolution terahertz spectroscopy.

Frequency multipliers based on superlattice (SL) devices as nonlinear elements have been developed as radiation sources for a terahertz (THz) laboratory spectrometer. Input frequencies of 100 and 250 GHz from backward wave oscillators have been multiplied up to the 11th harmonic, producing usable frequencies up to 2.7 THz. Even at these high frequencies the output power is sufficient for laboratory spectroscopy. Comparisons to conventional high-resolution microwave spectroscopy methods reveal several superior features of this new device such as very high line frequency accuracies, broadband tunability, high output power levels at odd harmonics of the input frequency up to high orders, and a robust applicability.

PET imaging of brain acetylcholinesterase using [11C]CP-126,998, a brain selective enzyme inhibitor.

PET and [(11)C]CP-126,998, an N-benzylpiperidinebenzisoxazole, were used to image brain acetylcholinesterase (AChE) distribution in healthy controls before and after administration of 5 mg donepezil p.o., a reversible AChE inhibitor. Logan plots were used to compute distribution volumes (V(T)). The V(T) of [(11)C]CP-126,998 was highest in the basal ganglia and cerebellum and lowest in the cerebral cortex, thalamus, amygdala, and hippocampus. The regional V(T) values correlated well with AChE concentration measured in vitro. Donepezil, given 4 h before PET scanning, induced a substantial inhibition of [(11)C]CP-126,998 binding (43-62%) in all brain regions when compared to the baseline PET study. The results of this study indicate that PET imaging of [(11)C]CP-126,998 may be useful in quantifying the distribution of regional brain AChE. This new PET radiotracer may potentially be employed in the diagnosis and treatment of patients with disorders of cholinergic neurotransmission, such as Alzheimer's disease.

Noninvasive assessment of aromatic L-amino acid decarboxylase activity in aging rhesus monkey brain in vivo.

The effect of aging on aromatic L-amino acid decarboxylase (AAAD) activity in rhesus monkey striatum was assessed in vivo using PET imaging. Two analogs of L-DOPA, 6-fluoro-m-tyrosine (FMT) and 6-fluoro-L-DOPA (FDOPA), were used to image rhesus monkeys of various ages. Results show that when the animals were grouped between young (3-11 years) and aged (25-37 years), FDOPA uptake in the older animals showed a 21% decline (P < 0.0005), while FMT uptake in young and older animals were not different. On the other hand, when individual uptake values were plotted vs. age, linear regression analysis showed FDOPA uptake similarly declined with age (r = -0.84, P < 0.001) while FMT uptake increased with age (r = 0.66, P < 0.05). Since FMT pharmacokinetics has been shown to be unaffected by metabolic steps occurring after the AAAD step, while FDOPA traces all the steps involved in L-DOPA metabolism, FMT is a suitable tracer to assess AAAD activity while FDOPA traces dopamine turnover. Based on these tracer characteristics, this study found that AAAD activity is maintained or increased in the aging rhesus monkey striatum while the FDOPA uptake decreases with age consistent with age-related declines in neuronal mechanisms whose overall effect is increased striatal dopamine turnover and clearance. Furthermore, comparison of results of this study with previous studies support the notion that the effect of aging in the dopamine system is different from that of MPTP-induced parkinsonism.

Measurement of dopamine release with continuous infusion of [11C]raclopride: optimization and signal-to-noise considerations.

UNLABELLED: PET studies with [11C]raclopride provide an indirect measure of changes in synaptic dopamine. Previously, we used the bolus-plus-infusion (B/I) method to assess dopamine response from the percentage change in binding potential (deltaBP) before and after administration of amphetamine. The goal of this work is to optimize the measurement of changes in neurotransmitter with the B/I method by choosing the optimal timing for pre- and poststimulus scanning. METHODS: Two sources of variability in deltaBP were considered: within-subject and between-subject noise. A noise model based on a phantom study and human data was used to evaluate the within-subject noise. For between-subject noise, simulated time--activity curves were generated from measured [11C]raclopride input functions. Optimal timing to measure deltaBP was determined and applied to human data. RESULTS: According to the simulation study, the optimal scan times for pre-and poststimulus scans were 39-50 and 58-100 min, respectively. The optimal timing resulted in a 28% noise reduction compared with the original timing. By applying the optimal timing to human studies, the statistical significance of the difference in deltaBP between patients with schizophrenia and healthy volunteers increased from P = 0.038 to 0.012. CONCLUSION: Careful assessment of the sources of noise in receptor imaging studies can increase the sensitivity of the B/I method for the detection of biologic signals.

Assessment of dynamic neurotransmitter changes with bolus or infusion delivery of neuroreceptor ligands.

To describe the effect of endogenous dopamine on [11C]raclopride binding, we previously extended the conventional receptor ligand model to include dynamic changes in neurotransmitter concentration. Here, we apply the extended model in simulations of neurotransmitter competition studies using either bolus or bolus-plus-infusion (B/I) tracer delivery. The purpose of this study was (1) to develop an interpretation of the measured change in tracer binding in terms of underlying neurotransmitter changes, and (2) to determine tracer characteristics that maximize sensitivity to neurotransmitter release. A wide range of kinetic parameters was tested based on existing reversible positron emission tomography tracers. In simulations of bolus studies, the percent reduction in distribution volume (deltaV) caused by a neurotransmitter pulse was calculated. For B/I simulations, equilibrium was assumed, and the maximum percent reduction in tissue concentration (deltaC) after neurotransmitter release was calculated. Both deltaV and deltaC were strongly correlated with the integral of the neurotransmitter pulse. The values of deltaV and deltaC were highly dependent on the kinetic properties of the tracer in tissue, and deltaV could be characterized in terms of the tissue free tracer concentration. The value of deltaV was typically maximized for binding potentials of approximately 3 to 10, with deltaC being maximized at binding potentials of approximately 1 to 2. Both measures increased with faster tissue-to-blood clearance of tracer and lower nonspecific binding. These simulations provide a guideline for interpreting the results of neurotransmitter release studies and for selecting radiotracers and experimental design.

Kinetic modeling of [11C]raclopride: combined PET-microdialysis studies.

The in vivo binding of D2 receptor ligands can be affected by agents that alter the concentration of endogenous dopamine. To define a more explicit relation between dopamine and D2 receptor binding, the conventional compartment model for reversible ligands has been extended to account for a time-varying dopamine pulse. This model was tested with [11C]raclopride positron emission tomography and dopamine microdialysis data that were acquired simultaneously in rhesus monkeys. The microdialysis data were incorporated into the model assuming a proportional relation to synaptic dopamine. Positron emission tomography studies used a bolus-plus-infusion tracer delivery with amphetamine given at 40 minutes to induce dopamine release. The extended model described the entire striatal time-activity curve, including the decrease in radioactivity concentration after an amphetamine-induced dopamine pulse. Based on these results, simulation studies were performed using the extended model. The simulation studies showed that the percent decrease in specific binding after amphetamine measured with the bolus-plus-infusion protocol correlates well with the integral of the postamphetamine dopamine pulse. This suggests that changes in specific binding observed in studies in humans can be interpreted as being linearly proportional to the integral of the amphetamine-induced dopamine pulse.

Graphical analysis of 6-fluoro-L-dopa trapping: effect of inhibition of catechol-O-methyltransferase.

UNLABELLED: Graphical methods to analyze tracer time-course data allow reliable quantitation of the rate of incorporation of tracer from plasma into a "trapped" kinetic component, even when the details of the kinetic model are unknown. Applications of the method over long time periods often expose the slow reversibility of the trapping process. In the extended graphical method, both trapping rate and a presumed first-order loss rate constant are estimated simultaneously from the time-course data. METHODS: We applied the extended graphical method to 6-fluoro-L-dopa (6-FD), simultaneously estimating the rate of uptake (Ki) and the rate constant for loss from the trapped component (K(loss)) in a single fitting procedure. We applied this approach to study the effects of two catechol-O-methyl-transferase inhibitors on the kinetics of 6-FD in cynomolgus monkeys. RESULTS: Inhibition of peripheral O-methylation with either inhibitor, confirmed by high-performance liquid chromatography analysis of labeled compounds in arterial plasma, had no significant effect on Ki, in agreement with previously reported studies. In contrast, tolcapone, a catechol-O-methyl-transferase inhibitor, having central effects in addition to peripheral effects at the dosage used, decreased K(loss) by 40% from control values (p < 0.002), whereas nitecapone, which has no known central activity, had no significant effect. CONCLUSION: This method provides insight into the neurochemical basis for the kinetic behavior of 6-FD in both health and disease and may be used to define the action of centrally active drugs that influence the metabolism of dopamine.

In vivo muscarinic binding selectivity of (R,S)- and (R,R)-[18F]-fluoromethyl QNB.

We have developed a multistep radiochemical synthesis of two diastereomers of quinuclidinyl-4-[18F]-fluoromethyl-benzilate ([18F]-FMeQNB), a high-affinity ligand for muscarinic acetylcholine receptors. Previously, we have shown that the nonradioactive (R,R)-diastereomer displays an eightfold selectivity for M1 over M2 while the nonradioactive (R,S)-diastereomer displays a sevenfold selectivity for M2 over M1 in vitro. This paper reports the results of in vivo comparison studies. In the rat, uptake of (R,S)-[18F]-FMeQNB was nearly uniform in all brain regions following the concentration of M2 subtype. The uptake was reduced by 36-54% in all brain regions on coinjection with 50 nmol of unlabeled ligand. An injection of (R,S)-[18F]-FMeQNB followed at 60 min by injection of unlabeled ligand and subsequent sacrifice at 120 min displaced 30-50% of radioactivity in the pons, medulla, and cerebellum, which contain a high proportion of M2 subtype. The most dramatic displacement and inhibition of uptake on coinjection of (R,S)-[18F]-FMeQNB was observed in the heart. In rhesus monkey, the compound showed prolonged uptake and retention in the brain. In the blood, the parent compound degraded rapidly to a single radiolabeled polar metabolite believed to be fluoride. Within 30 min the parent compound represented less than 5% of the plasma activity. Displacement with (R)-QNB was generally slow, but was more rapid from those tissues which contain a higher proportion of M2 subtype. The results are consistent with the hypothesis that (R,S)-[18F]-FMeQNB is M2 selective in vivo. On the other hand, (R,R)-[18F]-FMeQNB showed higher uptake in those brain regions containing a higher concentration of M1 subtype. Uptake in the heart at 60 min was much lower than that observed with the (R,S)-diastereomer. Inhibition of uptake on coinjection with unlabeled (R,S)-FMeQNB is only significant in the heart, thalamus, and pons. Inhibition of uptake on coinjection with unlabeled (R,R)-FMeQNB is quite uniform in all brain regions. Displacement with (R)-QNB shows a more varying amount displaced. These results are consistent with (R,R)-[18F]-FMeQNB being M1 selective in vivo.

Evaluation of fluorinated m-tyrosine analogs as PET imaging agents of dopamine nerve terminals: comparison with 6-fluoroDOPA.

UNLABELLED: Fluorinated m-tyrosine analogs were evaluated as PET imaging agents and compared with 6-fluoroDOPA in the visualization of dopamine nerve terminals. METHODS: The three m-tyrosine analogs, 6-[18F]fluoro-L-m-tyrosine (6-FMT), 2-[18F]fluoro-L-m-tyrosine (2-FMT) and 6-[18F]fluoro-fluoromethylene-DL-m-tyrosine (6-F-FMMT), were prepared via electrophilic radiofluorination using [18F]acetylhypofluorite. These three analogs, as well as 6-[18F]fluoro-L-DOPA (6-FD), were injected into sets of rhesus monkeys, and serial PET images were acquired. Plasma samples were collected at different times after tracer administration, and metabolite analyses were done using high-performance liquid chromatography (HPLC). RESULTS: Visual inspection of the PET images obtained using these four tracers showed that the best image contrast was obtained with 6-FMT. Patlak analysis with a reference tissue input function yielded a mean uptake rate constant for 6-FMT of 0.019 min-1, a value twice those for the other tracers including 6-FD. CONCLUSION: These results demonstrate the superiority of 6-[18F]FMT in visualizing dopamine terminals in the rhesus monkey brain and suggest that 6-[18F]FMT is the tracer of choice in the assessment of dopamine metabolism in the living human brain.

Affinities of dopamine analogs for monoamine granular and plasma membrane transporters: implications for PET dopamine studies.

Affinities of dopamine (DA) analogs to both granular and plasma membrane uptake transporters were measured in vitro by inhibition of [3H]DA uptake in bovine chromaffin granule ghosts and C6 glial cells transfected with cDNA for the rat presynaptic dopamine transporter, respectively. Five amines were studied: DA, 6-fluorodopamine (6FDA), m-tyramine (MTA), 6-fluoro-m-tyramine (6FMTA), and beta-fluoromethylene-m-tyramine (FMMTA). Direct uptake of 18F labeled 6FDA and 6FMTA was also measured in the chromaffin granule system and compared with [3H]DA uptake. Results show that the transporter affinities of 6FDA and MTA were similar to that of DA in both transport systems while affinities of 6FMTA and FMMTA were lower. Furthermore while the direct uptake of DA and FDA in chromaffin granules were essentially identical and significantly reserpine-inhibitable, the direct uptake of 6FMTA was about 15-fold less and only minimally sensitive to reserpine pretreatment. Thus, although vesicular protection and reuptake may influence the turnover of FDA in 6-fluoroDOPA studies, they are unlikely to be important determinants of the kinetics of the slowly clearing components in studies with either 6-fluoro-m-tyrosine (6FMT) or 6-fluoro-beta-fluoro-methylene-m-tyrosine (6FFMMT), the bioprecursors of 6FMTA and 6-fluoro-FMMTA, respectively. These results are consistent with the finding that the longterm component in 6FMT PET studies is 6-fluoro-hydroxyphenylacetic acid (6FHPAC), which can be explained by the lack of vesicular protection of 6FMTA from MAO oxidation.

Suppression of simian immunodeficiency virus replication by human immunodeficiency virus type 1 trans-dominant negative rev mutants.

We demonstrate that trans-dominant negative rev mutants are able to suppress simian immunodeficiency virus provirus replication in both transient cotransfection assays and stably transduced HUT 78 cells. These studies suggest that the efficacy of trans-dominant rev strategies in reducing viral burden may be evaluated in a simian immunodeficiency virus-rhesus macaque animal model.

Assessing genetic-based therapies for AIDS using the simian immunodeficiency virus.

A plasmid encoding the full-length infectious molecular proviral clone of SIVmac239 was generated. Virus derived from cells transfected with this clone replicated to high levels and was cytopathic for some transformed human CD4+ cell lines and primary rhesus macaque peripheral blood mononuclear cells. Since replication of SIV requires the functional expression of the viral encoded rev protein, transient co-transfection studies were initiated with the infectious proviral clone and a well-characterized trans-dominant negative HIV-1 rev mutant.

Evaluation of 3-[18F]fluoro-alpha-fluoromethyl-p-tyrosine as a tracer for striatal tyrosine hydroxylase activity.

3-[18F]Fluoro-alpha-fluoromethyl-p-tyrosine (3-F-FMPT) was evaluated as a tracer for CNS tyrosine hydroxylase (TH) activity in rodents and in a rhesus monkey. Results of in vitro experiments using rat striatal homogenates showed that the introduction of fluorine into the 3-phenyl position did not significantly alter the ability of FMPT to act as a TH-activated L-aromatic amino acid decarboxylase (L-AAAD) inhibitor. These studies further showed that 3-F-FMPT-induced L-AAAD inhibition was dose-dependent. Furthermore, striatal homogenates prepared from rats pretreated with the potent TH inhibitor alpha-methyl-p-tyrosine was found to have diminished 3-F-FMPT-induced L-AAAD inhibition. However, despite these promising in vitro results, the biodistribution of this compound in mice showed low brain uptake and fast clearance through the kidneys. A PET study using a Rhesus monkey injected with 3-[18F]F-FMPT confirmed the results obtained in mice, i.e. negligible brain uptake but high localization in the bladder. We conclude that 3-[18F]F-FMPT would not be useful as a tracer for cerebral TH activity.

Effect of galanin on food intake in rats: involvement of lateral and ventromedial hypothalamic sites.

Galanin has previously been reported to elicit feeding in satiated animals when injected into the hypothalamic paraventricular nucleus. It is not known, however, 1) whether this action is due to activation of feeding signals or suppression of satiety signals or both or 2) whether other hypothalamic regions such as the lateral hypothalamus (LH) or the ventromedial hypothalamus (VMH) are involved in this action. The effects of galanin on food intake were therefore examined in satiated and in fasted rats both after intracerebroventricular injection (0.1, 1, and 10 micrograms/10 microliters) and after microinjection (1 and 5 micrograms/0.5 microliters) into the LH and VMH. Twenty minutes after intracerebroventricular injection, galanin significantly and dose dependently augmented food intake by up to sevenfold in freely feeding rats and by up to 79% in fasted animals. The galanin-induced augmentation of cumulative food intake up to 2 h after injection was due to the initial increase in food consumption during the 0 to 20-min interval. This suggests that galanin acts by activation of feeding behavior and not by suppression of satiety signals in these fasted animals, in which satiety signals are presumably not initially operative. Twenty minutes after intrahypothalamic injections into both the LH and VMH, galanin (5 micrograms) significantly increased food consumption, fivefold in freely feeding rats and 30-35% in fasted rats. Thus stimulation of feeding by centrally injected galanin also involves loci within the LH and VMH.

Visualization of dopamine nerve terminals by positron tomography using [18F]fluoro-beta-fluoromethylene-m-tyrosine.

[18F]-6-Fluoro-beta-fluoromethylene-m-tyrosine ([18F]FFMMT) was evaluated as a potential imaging agent for dopamine nerve terminals using positron emission tomography (PET). Biodistribution and time course of this tracer in mice after i.p. injection was consistent with the distribution of dopamine. PET imaging studies involving rhesus macaques showed specific uptake in the dopamine-rich caudate-putamen region. This specific localization was blocked by inhibiting the enzyme L-aromatic amino acid decarboxylase and the transport of the tracer into brain was shown to be stereospecific. These results show the promise of L-[18F]FFMMT as a PET tracer in monitoring degeneration of the CNS dopamine system.