Paul W. Wedeking

Increase in rat brain acetylcholine induced by choline or deanol

Abstract Administration of choline chloride, deanol, or Deaner ® to rats subsequently killed by microwave irradiation caused an increase in the concentration of both choline and acetylcholine in the corpus striatum, indicating that synthesis of brain acetylcholine can be stimulated in vivo by elevating the tissue concentration of its precursor. This finding suggests that the concentration of free choline in brain is below that necessary for a maximal rate of synthesis of acetylcholine, and raises the possibility that the availability of choline in brain may regulate the rate of synthesis of acetylcholine. These results also provide biochemical evidence for the view that the clinical effects of deanol result from its conversion to acetylcholine.

BIODISTRIBUTION OF RADIOLABELED, FORMULATED GADOPENTETATE, GADOTERIDOL, GADOTERATE, AND GADODIAMIDE IN MICE AND RATS

RATIONALE AND OBJECTIVES. The authors studied the long-term distribution of gadolinium (Gd) in mice and rats after the administration of four commercially available magnetic resonance imaging contrast media. The goals were to determine any possible product dissociation in vivo and to evaluate the effects that product excipients had on the tissue distributions. METHODS. Gadolinium-153 (153Gd)-labeled gadopentetate (Magnevist), gadoteridol (ProHance), gadoterate (Dotarem), and gadodiamide (Omniscan) were administered intravenously to mice (0.48 mmol/kg) and rats (0.1 mmol/kg). At various times up to 14 days posttreatment, the residual 153Gd was measured in selected tissues. The tissue distributions obtained were used to make intra- and interchelate distribution evaluations and comparisons regarding tissue clearance and any possible in vivo dissociation of the Gd chelates. RESULTS.Differences were found among the chelates studied relative to the amounts of residual 153Gd present in tissues known to sequester free Gd, particularly in liver and femur at 7 and 14 days after administration, in both mice and rats. The pattern of the 153Gd distribution suggested that the linear chelates, gadopentetate and gadodiamide, dissociated in vivo resulting in more 153Gd present in bone and liver at the longer residence times than in the subjects injected with the macrocyclic chelates, gadoteridol and gadoterate. The only excipient found to affect the distribution profile was calcium(DTPA-BMA); this excipient in formulated gadodiamide decreased the amounts of residual Gd measured in whole body, bone, and liver in mice compared with levels obtained when gadodiamide was injected alone. CONCLUSIONS.The molecular feature found to be most important in differentiating the four chelates evaluated is the presence or absence of a macrocyclic structure. The Gd chelates containing this structure, gadoteridol and gadoterate, have the lowest residual Gd at long residence times in both mice and rats. The order of residual whole body Gd at 14 days (lowest to highest) was: gadoteridol ~ gadoterate ≤ gadopentetategadodiamide. The only excipient that affected the biodistribution was found in the gadodiamide formulation where the addition of 5% calcium (DTPA-BMA) reduced residual Gd to just less than 10 times greater than that found for the macrocyclic chelates with the lowest residual Gd, gadoteridol and gadoterate.

The pharmacology of seizures induced by sensitization with low intensity brain stimulation.

Abstract Daily low-intensity electrical stimulation of selected brain sites causes seizures to gradually develop. Because the limbic area is particularly susceptible to both the sensitization phenomenon and psychotropic drugs, we evaluated several classes of centrally-active agents for activity against these seizures. For comparative purposes, seizures elicited from the amygdala and a nonlimbic site, the sensorimotor cortex, were tested. Rats with chronically-implanted electrodes were stimulated for one min each day with a 50-μA, 60-Hz current. Initially, no overt effects occurred; but eventually seizures wereestablished. Once the seizures stabilized, drug effects on seizure duration were measured. Antidepressive drugs were more potent in suppressing amygdaloid-elicited seizures than cortically-evoked seizures. Both antianxiety and antiepileptic drugs exerted nonselective blockade but the antianxiety agents were extremely potent in comparison to the antiepileptics. Representative neuroleptic drugs failed to block the seizures, even at doses completely debilitating the animals. Stimulants prolonged the seizures but the activity was weak. Thus, the low-intensity brain stimulation (LIBS) procedure has proved to be not only an interesting phenomenon, but one that responds differentially to various classes of CNS drugs.

Pharmacokinetic analysis of blood distribution of intravenously administered 153Gd-labeled Gd(DTPA)2− and 99MTc(DTPA) in rats

Rat plasma distribution data obtained following IV administration of 99mTc(DTPA) alone or after co-administration of 99mTc(DTPA) and 153Gd-labeled Gd(DTPA)2- at 0.001, 0.1, and 1.0 mmol Gd/kg were evaluated using compartmental modeling techniques. A three-compartment open model was found to fit the data significantly better (P less than 0.01) than a two- or four-compartment open model. This model incorporates and links the plasma and urine data and includes a delay to account for the transit time through the kidneys/ureters. The two nonplasma compartments of the model were assumed to be related to rapidly and slowly equilibrating tissues. Tc(DTPA) and Gd(DTPA)2- had nearly identical pharmacokinetic profiles in plasma and the rate constants were essentially the same. No significant dose dependent pharmacokinetic differences were found for the range of Gd(DTPA)2- doses tested. Simulations of the proposed three-compartment model were used to generate concentration-time curves for each of the three compartments.

Increase in tissue concentration of acetylcholine in guinea pigs in vivo induced by administration of choline

Abstract Administration of choline chloride (200 μmoles/kg) intravenously to guinea pigs caused an increase in the concentrations of choline and acetylcholine in adrenals, heart, kidneys, lungs, and liver within 2 min. These results suggest that raising the concentration of choline in plasma will accelerate the formation of acetylcholine in the organs cited. No significant increase in concentration of choline or acetylcholine occurred in brain.

Caenorhabditis elegans as an alternative animal species

Caenorhabditis elegans has proven useful in toxicity testing of known toxicants, but its potential for assessing the toxicity of new pharmaceuticals is relatively unexplored. In this study the procedures used in aquatic testing of toxicants were modified to permit testing of small amounts (<40 mg) of gadolinium-based magnetic resonance imaging (MRI) compounds. Five blinded compounds were tested. The toxicity of these compounds determined using C. elegans was compared to existing mammalian test system data (minimum lethal dose [MLD] values for mice). Four of five compounds tested had the same relative sensitivity with C. elegans as with the mouse test system. Testing with C. elegans is efficient and could markedly reduce the cost of screening potentially useful compounds.

Comparative chemical structure and pharmacokinetics of MRI contrast agents.

Tweedle MF, Eaton SM, Eckelman WC, Gaughan GT, Hagan JJ, Wedeking PW, Yost FJ. Comparative chemical structure and pharmacokinetics of MRI contrast agents. Invest Radiol 1988; 23(Suppl 1):S236‐S239. The blood clearance kinetics of five gadolinium complexes, Gd(L), were determined in rats and the results interpreted in terms of an open two‐compartment pharmacokinetic model. The complexes were tested in vitro for stability in serum and in aqueous solutions of ions that they might encounter in vivo and that might be expected to react with the Gd(L) complexes to produce uncomplexed gadolinium. Reaction with serum was observed in two instances. Chemical structural differences among the chelating ligands appear to govern the overall reactivity of their Gd(L) complexes. It may be inferred from the results that a preferred structural feature of the ligand is the presence of a 12‐membered 1,4,7,10‐tetraaza macrocycle.

Schedule-dependent differences among anti-anxiety drugs

Abstract Five anti-anxiety drugs were administered to rats on either a variable-interval (VI 1-min) schedule, a chained 10-sec differential reinforcement of other behavior (DRO), fixed-ratio (FR 25) procedure, or an FR 15-satiation schedule. Two dose levels, free of effects on behavior not related to lever pressing, for each anxiolytic were selected for testing. On the VI schedule, the anti-anxiety drugs uniformly increased response rates. On the DRO-FR procedure all the anxiolytics increased responding during the DRO component, but produced dissimilar biphasic increases or decreases in FR response rates. The anti-anxiety drugs also produced dissimilar effects on the FR-satiation schedule: chlordiazepoxide, phenobarbital, and meprbamate disrupted satiation (increased responding), diazepam did not affect satiation (no change in responding), and oxazepam facilitated satiation (decreased responding). None of the anxiolytics altered FR response rates in the FR-satiation schedule. The discrepancies recorded suggest that schedule-dependent differences exist between the anti-anxiety drugs studied.

Synthesis, characterization, and imaging performance of a new class of macrocyclic hepatobiliary MR contrast agents.

RATIONALE AND OBJECTIVES To investigate the effect of substituent lipophilicity, substituent position, and overall charge on the hepatobiliary clearance and tolerance of a series of aromatic ring-containing macrocyclic Gd chelates to select a candidate compound for evaluation as a hepatobiliary imaging agent. METHODS Hepatobiliary clearance was studied in rats. Tissue distribution and tolerance were studied in mice. Imaging was performed in cats, rabbits, and Rhesus monkeys using T1-weighted pulse sequences or T1-weighted breath-hold pulse sequences. RESULTS All the compounds were excreted bimodally. Gd-2,5-BPA-DO3A (15d) was found to have the optimal combination of hepatobiliary clearance (47% in rats, 29% in mice) and tolerance (minimum lethal dose 5.0 mmol/kg). Initial imaging studies in cats demonstrated the feasibility of Gd-2,5-BPA-DO3A for hepatic imaging. In rabbits with implanted VX-2 adenocarcinoma as a model for metastatic liver disease, Gd-2,5-BPA-DO3A provided sustained hepatic signal intensity (SI) enhancement and lesion conspicuity over a 120-minute imaging time course. In Rhesus monkeys with normal liver function, Gd-2,5-BPA-DO3A afforded sustained hepatic SI enhancement and a time-dependent increase in gallbladder SI over the entire 90-minute imaging time course. CONCLUSIONS Gd-2,5-BPA-DO3A provides dramatic and sustained SI enhancement of hepatic tissue in cats, rabbits, and Rhesus monkeys that was superior in all respects to the extracellular space MRI agent, Gd-HP-DO3A, that was employed as a control.

Comparison of chlordiazepoxide and food deprivation in rats on a fixed-ratio satiation schedule

Abstract The effects and interactions of injected chlordiazepoxide HCl (CDP) and food deprivation in rats conditioned on a food-reinforced fixed ratio 10-satiation schedule were studied. Increasing the duration of food deprivation had no significant effect on the achievement of satiation. Decreasing the duration of deprivation produced significant decreases in responding for food pellets. Administering CDP elicited increases in responding for more reinforcements at all deprivation levels tested. The CDP effects were attributed to the disinhibition of the mechanisms regulating satiation.