Selective Dopamine Receptor Stimulation Differentially Affects [H]Arachidonic Acid Incorporation, a Surrogate Marker for Phospholipase A2-Mediated Neurotransmitter Signal Transduction, in a Rodent Model of Parkinson’s Disease

Our laboratory has developed a technique whereby radiolabeled long-chain fatty acids are injected intravenously in awake rats to pulse-label brain lipids, mainly phospholipids, to measure regional brain lipid metabolism by autoradiography. The brain incorporation of [H]arachidonic acid ([H]AA), a polyunsaturated fatty acid, may reflect regional changes in neurotransmitter signal transduction using phospholipase A2. Using this radiotracer, we examined the brain dopamine system in rats with a chronic unilateral 6-hydroxydopamine lesion of the substantia nigra pars compacta, a model of Parkinson’s disease. Four weeks after lesioning, rats received either vehicle; SKF38393 or quinpirole (LY-171,555) (D1and D2-dopaminelike agonists, respectively); or (1)-butaclamol (D1/D2 antagonist) followed by either vehicle, SKF38393, or quinpirole. They then were infused with [H]AA and their brains processed for autoradiography. SKF38393 increased [H]AA incorporation into the lesioned side compared with the intact side in the caudate putamen, somatosensory and motor cortices and subthalamic nucleus, but decreased incorporation in the ipsilateral ventrolateral thalamus. Quinpirole increased ipsilateral [H]AA incorporation in the caudate putamen and somatosensory and motor cortices, and decreased it in the ventrolateral thalamus. (1)-Butaclamol blocked this effect. The data suggest up-regulation in basal ganglia and cortical dopamine circuits mediated by phospholipase A2 ipsilateral to the substantia nigra lesion. It has been established that the regional cerebral metabolic rate for glucose (rCMRglc) is a useful marker for regional brain activity and it can be accurately measured with quantitative autoradiography in animals using 2-[1C]deoxy-D-glucose ([C]2DG) as a tracer (Sokoloff, 1977; Sokoloff et al., 1977). An analogous approach for examining regional brain lipid metabolism also has been developed. This “fatty acid technique” uses intravenously administered radiolabeled long-chain fatty acids to pulse-label brain lipids, mainly phospholipids, in awake animals (Robinson et al., 1992). In this manner one measures a regional “incorporation coefficient”, k*, of the radiolabeled plasma fatty acid into stable brain lipids using quantitative autoradiography. Different fatty acid tracers pulse-label different stereospecific numbered (sn) positions in different phospholipids. In the case of labeled arachidonic acid (AA), an unsaturated (20:4, n-6) fatty acid, it is incorporated mainly into the sn-2 position of phosphatidylinositol and phosphatidylcholine (DeGeorge et al., 1989; Fonlupt et al., 1994) and it has been shown that this incorporation is affected by acute or chronic alterations of functional brain activity (DeGeorge et al., 1991; Nariai et al., 1991; Wakabayashi et al., 1994, 1995; Rabin et al., 1998). Moreover, this incorporation is independent of cerebral blood flow (Chang et al., 1997) and can be inhibited by the phospholipase A2 (PLA2) inhibitor manoalide (Jones et al., 1996; Grange et al., 1998). These data indicate that functional activity in brain circuits that contain receptors linked to PLA2 activation can be revealed as patterns of regional radiolabeled AA incorporation. Dopamine receptor stimulation, in particular stimulating the D2 subtype, results in increased AA release and this effect is seemingly in part PLA2-dependent (Kanterman et al., 1991; Piomelli et al., 1991; Schinelli et al., 1994; Vial and Piomelli, 1995). Parkinson’s disease is a disabling neurological disorder that arises because of dysfunction of brain circuits involving These studies were carried out under the authority of a memorandum of understanding between the United States National Institutes of Health and Food and Drug Administration. ABBREVIATIONS: rCMRglc, regional cerebral metabolic rates for glucose; 2DG, 2-deoxy-D-glucose; PLA2, phospholipase A2; 6-OHDA, 6-hydroxydopamine; AA, arachidonic acid; GABA, g-aminobutyric acid. 0022-3565/01/2963-1074–1084 THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS Vol. 296, No. 3 U.S. Government work not protected by U.S. copyright 3310/887797 JPET 296:1074–1084, 2001 Printed in U.S.A. 1074 at A PE T Jornals on Jauary 8, 2018 jpet.asjournals.org D ow nladed from the basal ganglia. Degeneration of dopaminergic neurons from the substantia nigra pars compacta results in dopamine deficiency that ultimately leads to the signs and symptoms of the disease (Ehringer and Hornykiewicz, 1960). Pharmacological therapeutic strategies to treat Parkinson’s disease involve administering drugs that increase dopamine levels in dopaminergic synapses or reduce inhibition of dopaminergic neurons; the overall goal is to increase dopaminergic signaling in basal ganglia circuits (Narabayashi et al., 1993; Kohler and Paulson, 1995). Parkinson’s disease can be modeled in rats by injecting 6-hydroxydopamine (6-OHDA) unilaterally into the substantia nigra pars compacta to lesion meso-striatal dopamine neurons (Gerlach and Riederer, 1996). This model allows direct comparisons of experimental treatments on the ipsilateral (lesioned) and contralateral (intact) side of the brain in individual animals. Sagar and Snodgrass (1980) and Wooten and Collins (1981) studied rCMRglc in this model using [C]2DG autoradiography. They noted small regional asymmetries in brain metabolism between the lesioned and intact sides of rats receiving saline vehicle treatment. Subsequently, Hayakawa et al. (1998) used the fatty acid technique in the unilateral 6-OHDA lesion model. In contrast to the earlier studies using [C]2DG, they detected robust increases in [H]AA incorporation into basal ganglia structures ipsilateral to the lesion that included globus pallidus and caudate putamen, as well as affecting several anterior cerebral cortical regions. Following the earlier 2DG studies, Trugman and Wooten (1987) examined the effects of the selective dopaminergic agonists SKF38393 (a D1 receptor agonist) and quinpirole (LY171,555; a D2 receptor agonist) on rCMRglc in chronic unilaterally 6-OHDA-lesioned rats. They showed D1 and D2 agonist stimulated rCMRglc in a number of defined brain regions compared with vehicle treatment. They also demonstrated differential effects of D1 and D2 agonists on rCMRglc in the entopeduncular nucleus and substantia nigra pars compacta. In view that D2 dopamine receptor stimulation appears to be linked to PLA2 activation and arachidonic acid release (Kanterman et al., 1991; Piomelli et al., 1991; Schinelli et al., 1994; Vial and Piomelli, 1995) and of significant differences in [H]AA incorporation between the lesioned and intact sides in unilaterally substantia nigra-lesioned rats (Hayakawa et al., 1998), we decided to evaluate in this model the effects of selective dopamine D1 and D2 receptor stimulation by SKF38393 and quinpirole, respectively, on [H]AA incorporation. The results suggest general disinhibition of basal ganglia and cortical circuits containing neurotransmitters whose receptors’ signal transduction mechanism is PLA2 and that there are differences in responsiveness of these circuits to D1 and D2 receptor stimulation. An abstract of this work has been published (Hayakawa et al., 1997). Materials and Methods Animals. The studies were carried out with male Sprague-Dawley rats (Taconic Farms, Germantown, NY). They were maintained in a vivarium with controlled temperature, humidity, and light cycle (on 6:00 AM–6:00 PM) and had access ad libitum to standard pelleted laboratory chow and fresh tap water. The experiments were conducted in accordance with and approved by the National Institute of Child Health and Human Development Animal Care and Use Committee, Protocol #95-029 (Guide for the Care and Use of Laboratory Animals, National Institutes of Health Publication 86-23). Drugs. [H]Arachidonic acid was purchased from Moravek Biochemicals (Brea, CA). S-(1)-Apomorphine HCl, 6-hydroxydopamine HBr, (1)-butaclamol HCl, (2)-quinpirole (LY-171,555), and R-(1)SKF38393 were obtained from Research Biochemicals International (Natick, MA). Pentobarbital sodium was purchased from Richmond Veterinary Supply Co. (Richmond, VA). L-Ascorbic acid, desipramine HCl, HEPES, and fatty acid-free bovine serum albumin, propylene glycol, and Tween 80 were purchased from Sigma Chemical Co. (St.