Catherine McLellan

In vitro binding properties and autoradiographic imaging of 3-iodobenzamide ([125I]-IBZM): a potential imaging ligand for D-2 dopamine receptors in spect

The in vitro binding properties of the [125I] labeled benzamide (S(-)-N-[(1-ethyl-2-pyrrolidinyl)-methyl]-2-hydroxy-3-iodo-6-methoxy- benzamide, IBZM) were determined in bovine and mouse caudate membrane homogenates and by autoradiography of mouse brain slices. [125I]-IBZM binding is saturable and reversible with a Bmax of 373 +/- 51 fmol/mg protein and a Kd of 3.1 +/- 0.62 nM (mean +/- SD, Scatchard analyses) and 0.56 nM as calculated by association and dissociation time constants. In competition experiments, Ki values for the D-2 antagonists YM-09151-2 and spiperone are 4 orders of magnitude lower than the Ki value for the D-1 antagonist SCH-23390 and S(-)-IBZM is ten-fold more potent than R(+)-IBZM. [125I]-IBZM has a low affinity for serotonin S-2 and for alpha receptors. Therefore, it is a highly selective ligand for dopamine D-2 receptors. Autoradiographic images of brain sections incubated with [125I]-IBZM show the dopamine D-2 receptors of the striatum, nucleus accumbens and olfactory tubercle with a high ratio of specific to nonspecific binding. Thus, S(-)-IBZM, when labeled with [123I], may be useful for in vivo imaging of dopamine D-2 receptors by single photon emission computerized tomography (SPECT).

New rapid analysis method demonstrates differences in 6-[18F]fluoro-l-dopa plasma input curves with and without carbidopa and in hemi-MPTP lesioned monkeys

Kinetic modeling of the PET tracer 6-[18F]fluoro-L-dopa ([18F]Dopa), used to measure presynaptic dopamine function, requires the accurate determination of the plasma input curve. We have developed a new method that uses alumina extraction preceded by cation and anion exchange resins to determine the parent compound, [18F]Dopa and its critical metabolite 3-O-methyl-6-[18F]fluoro-L-dopa. Using this method we found that carbidopa increases the plasma input of [18F]Dopa while decreasing the rate of metabolite formation, and that previous drug treatment can significantly effect [18F]Dopa metabolism.

Correlation of changes in α2-adrenoceptor number and locomotor responses to clonidine following clorgyline discontinuation

[3H]‐clonidine binding in vitro and the locomotor response to clonidine in vivo were studied over an eight week period following four weeks of treatment with the monoamine oxidase‐inhibiting antidepressant, clorgyline (1 mg kg−1 day−1). Long‐term clorgyline administration caused decreases in responsiveness to clonidine and in the number of α2‐adrenoceptors; these changes reverted towards pretreatment values very gradually over an eight week period following discontinuation of the drug. This study provides some of the first detailed evidence regarding the slow return of adaptional changes following discontinuation of an antidepressant drug in animals and has implications for understanding some delayed drug interactions associated with MAO‐inhibiting antidepressants in man.

6-(18F)Fluoro-L-DOPA and Cerebral Blood Flow in Unilaterally MPTP-Treated Monkeys

Intravenous administration of 15O-labeled water and 6-[18F]-L-fluorodopa were used to assess abnormal striatal activity in monkeys after long-term recovery of unilateral lesions of the dopaminergic nigro-striatal system induced by the neurotoxin MPTP. PET data were examined in relation to behavioral and biological parameters. Cerebral blood flow and 6-[18F]-L-DOPA uptake were found to be significantly reduced in the lesioned striatum, compared to the unaffected side and to normal controls. There was no correlation between cerebral blood flow and any of the behavioral parameters. The uptake rate constant of 18F-DOPA from blood to striatum and the ratios of striatum to occipital areas were highly correlated to the concentrations of homovanillic acid in the cerebrospinal fluid of the same animals but not to the rotational behavior. This MPTP-inducedmodei of striatal dopamine deficiency in primates presents similarities with idiopathic Parkinsons disease and may be used to evaluate the effects of dopaminergic lesions and transplants on brain function.

Preparation and biological evaluation of 18F-labeled benzamide analogs as potential dopamine D2 receptor ligands

Three 18F-labeled benzamide derivatives were prepared and evaluated as potential ligands to study the dopamine D2 receptor phenomenon. The compounds are analogs of iodobenzamide, eticlopride and raclopride and are labeled with an N-2-[18F]fluoroethyl functionality on the pyrrolidine ring. The compounds were tested in vitro for binding affinity and found to exhibit somewhat lower affinity than the non-fluorinated analog. In vivo distribution studies revealed that all compounds were more highly bound to plasma proteins than was raclopride. In addition, compartmentation of radioactivity demonstrated nonspecific binding to be the predominate retention in the brain as reflected by the low caudate to cerebellum ratios for these compounds. These three 18F-labeled benzamide derivatives are inferior to raclopride and iodobenzamide for studies of the D2 receptor system using positron emission tomography.

6-hydroxydopamine pretreatment effects on α- and β-adrenergic receptor adaptation to clorgyline

SummaryThe effects of 6-hydroxydopamine (6-OHDA) lesions on brain adrenergic receptor adaptation to 21 days of treatment with the selective monoamine oxidase type A (MAO-A) inhibitor clorgyline were studied in rats. 6-OHDA pretreatment effectively blocked the decrease in α1-, α2- and β-adrenergic receptor densities observed in response to clorgyline treatment. In saline-treated rats, 6-OHDA reduced norepinephrine (NE) to 8% of control levels, modestly reduced dopamine (DA) to 67% of controls, but did not affect serotonin (5HT) levels in the cortex, Clorgyline administration to shams increased NE and 5HT to 239% and 160% of their respective control levels, but did not effect DA levels. 6-OHDA lesions attenuated clorgylines effect on cortical NE levels but not 5HT. The results suggest that β-adrenergic receptor adaptation to MAOIs as with tricyclic antidepressants is a response to an increase in catecholamine receptor occupancy, and that a similar molecular mechanism is responsible for the observed clorgyline induced changes in α-adrenergic receptors.

The effects of glutaraldehyde cross-linking on the function of the adenylate cyclase complex of turkey erythrocytes

Glutaraldehyde appears to preferentially effect the activation processes of the adenylate cyclase complex of turkey erythrocyte membranes. The primary effect of low concentration (0.01 percent, 0.05 percent, and 0.1 percent) glutaraldehyde membrane treatment is to decrease catecholamine-stimulated cAMP formation. The effect can be blocked by prior activation of the system with isoproterenol + p[NH]ppG. 0.6 percent glutaraldehyde treatment of membranes has substantial effects on both F(-)- and catecholamine-stimulated cAMP production. The effects are blocked by prior activation of the adenylate cyclase complex with NaF, but not by isoproterenol + p[NH]ppG. Glutaraldehyde at these concentrations has no effect on Mn++-stimulated cAMP formation. The data is discussed with respect to the organization of the major macromolecular components of the adenylate cyclase complex as it exists within the native membrane prior to and following activation of the system.