Brain incorporation of [1–11C]arachidonate in normocapnic and hypercapnic monkeys, measured with positron emission tomography

doi:10.1016/S0006-8993(97)00088-7

Brain Research

Volume 755, Issue 1, April 1997, Pages 74-83

https://doi.org/10.1016/S0006-8993(97)00088-7 Get rights and content

Abstract

Positron emission tomography (PET) was used to determine brain incorporation coefficients k^* of [1-¹¹C]arachidonate in isofluraneanesthetized rhesus monkeys, as well as cerebral blood flow (CBF) using [¹⁵O]water. Intravenously injected [1-¹¹C]arachidonate disappeared from plasma with a half-life of 1.1 min, whereas brain radioactivity reached a steady-state by 10 min. Mean values of k^* were the same whether calculated by a single-time point method at 20 min after injection began, or by least-squares fitting of an equation for total brain radioactivity to data at all time points. k^* equalled 1.1–1.2 × 10^–4 ml · s^–1 · g^–1 in gray matter and was unaffected by a 2.6-fold increase in CBF caused by hypercapnia. These results indicate that brain incorporation of [1-¹¹C]arachidonate can be quantified in the primate using PET, and that incorporation is flow-independent.

Introduction

Phospholipids are major components of biological membranes and participate in signal transduction and neuroplastic changes [4,15,17,18,42,48]. To elucidate roles of phospholipids in brain structure and function, our laboratory has developed an in vivo method that involves injecting a radiolabeled long-chain fatty acid intravenously, and subsequently measuring regional brain radioactivity within brain phospholipids [13], [14], [15],37,40,42]. Rates of incorporation into and turnover within phospholipids can be calculated for specific fatty acids, using appropriate operational equations. Rapid disappearance of an injected radiolabel from blood, combined with its rapid entry and distribution within brain phospholipids, leads to pulse-labeling of specific sites on brain phospholipids. In awake rats, radioactive palmitate labels mainly the sn-1 position of brain phosphatidylcholine, radioactive arachidonate the sn-2 positions of phosphatidylinositol and phosphatidylcholine, and radioactive docosahexaenoate the sn-2 positions of phosphatidylethanolamine and phosphatidylcholine. Thus, a combination of the three labeled fatty acids can be used to examine brain phospholipid metabolism in vivo, with regard to brain structure and function.

The fatty acid method has elucidated phospholipid metabolism in experimental brain tumors, ischemia and in a cranial nerve nucleus following nerve resection [33,34,36,51,55]. In awake rats, cholinergic stimulation by the M1 muscarinic agonist arecoline increased incorporation of labeled arachidonate and labeled docosahexaenoate into sn-2 positions of phospholipids, without changing labeled palmitate incorporation, suggesting that the two polyunsaturated tracers participate in M1-mediated signal transduction whereas saturated labeled palmitate does not [14]. As labeled arachidonate and labeled docosahexaenoate are specifically incorporated into synaptic membrane phospholipids following arecoline administration [24], and as their incorporation is reduced by inhibiting phospholipase A₂ (Grange et al., unpubl. obs) these tracers are in vivo markers of phospholipase A₂-mediated signal transduction [40]. Arecoline has been shown to increase incorporation of [1-¹⁴C]arachidonate into the cerebral cortex ipsilateral to a chronic unilateral lesion of the nucleus basalis in rats, consistent with upregulation of post-synaptic signal transduction involving phospholipase A₂ [35].

We wished to see if we could use labeled arachidonate to examine brain phospholipid metabolism using positron emission tomography (PET) in humans. As a preliminary step in this effort, we determined in the present study rates of incorporation of the positron-labeled [1-¹¹C]arachidonate [11] into brain regions of anesthetized monkeys, using PET. We made measurements under normocapnic and hypercapnic conditions, to assure that labeled fatty acid incorporation into brain as measured with PET is independent of cerebral blood flow (CBF) [14,27,40,42,55]. With flow independence, incorporation must reflect only intrinsic brain metabolism. An abstract of part of this work has been published [1].

Section snippets

Radiochemistry

Ether solutions (0.25 M) of the magnesium bromide of (all Z) 1-bromononadeca-4,7,10,13-tetraene were prepared and reacted with [¹¹C]CO₂ within 18 h, as previously described [12]. Starting with 1205 ± 228 mCi(n = 11) of [¹¹C]CO₂ at the end of bombardment, 252 ± 178 mCi·(n = 11) of the desired [1-¹¹C]arachidonic acid could be prepared routinely at the end of synthesis in approximately 35 min. The final product was formulated in 10 ml 8% (v/v) serum in 0.9% saline, sonicated at 40°C for 3 min, and

Results

Mean arterial pCO₂ in isoflurane-anesthetized monkeys ranged from 32 to 37 mmHg during normocapnia and from 68 to 76 mmHg during hypercapnia. Mean CBF in each condition, determined using [¹⁵O]water and PET, equalled 31.8 ± 3.0 (S.D.) ml 100 g^–1-min^–1 and 84.0 ± 17.2 ml 100 g^–1-min^–1, respectively. Thus, breathing 5% CO₂ in air increased CBF 2.6-fold.

Fig. 1 illustrates mean total plasma organic radioactivity, and plasma organic minus [¹¹C]CO₂ radioactivity representing only [1-¹¹C] arachidonate [14,26],

Discussion

This paper presents a noninvasive quantitative method to determine regional cerebral incorporation coefficients k^* of a positron-labeled long chain fatty acid, [1-¹¹C] arachidonate, in a primate. Values of k^* for whole brain and for gray and white matter regions were determined using PET in anesthetized monkeys, by a single-time point analysis and by least-squares fitting of an equation representing total brain radioactivity as a function of time. k^* did not differ significantly between the two

Acknowledgements

We thank John Bacher D.V.M. for his expert assistance and advice in surgical procedures.

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Research reportBrain incorporation of [1–11C]arachidonate in normocapnic and hypercapnic monkeys, measured with positron emission tomography

Abstract

Introduction

Section snippets

Radiochemistry

Results

Discussion

Acknowledgements

Trends Neurosci.

J. Lipid. Res.

Anal. Biochem.

J. Biol. Chem.

Brain Res.

Brain Res

Brain Res. Rev.

Am. Heart J.

Neurochem. Int.

Brain incorporation of 11C-arachidonate in normocapnic and hypercapnic monkeys: A PET study

Abstr. Soc. Neurosci.

Incorporation of [1-carbon-11] palmitate in monkey brain using PET

J. Nucl. Med.

Cerebral blood volume maps of gliomas: Comparison with tumor grade and histological findings

Radiology

Measurement of regional cerebral pH in human subjects using continuous inhalation of 11CO2 and positron emission tomography

J. Cereb. Blood Flow Metab.

Evaluation of the 11CO2 positron emission tomographic method for measuring brain pH. I. pH changes measured in states of altered PCO2

J. Cereb. Blood Flow Metab.

Measurement of brain pH using 11CO2 and positron emission tomography

J. Cereb. Blood Flow Metab.

Tomographic measurement of LCBF with high-resolution PET and H215O: Comparison of methods

J. Cereb. Blood Flow Metab.

Radiosynthesis of [1-C-11]-arachidonic acid

J. Nucl. Med.

Radiosynthesis of 1-[11C] polyhomoallylic fatty acids

J. Label. Comp. Radiopharmacol.

Adrenergic and cholinergic stimulation of arachidonate and phosphatidate metabolism in cultured astroglial cells

Neurochem. Res.

Arecoline-stimulated brain incorporation of intravenously administered fatty acids in unanesthetized rats

J. Neurochem.

Intravenous injection of [1-14C] arachidonate to examine regional brain lipid metabolism in unanesthetized rats

J. Neurosci. Res.

Role of phospholipases in generating lipid second messengers in signal transduction

FASEB J.

Receptor activation and inositol lipid hydrolysis in neural tissues

J. Neurochem.

Lack of capillary recruitment in the brains of awake rats during hypercapnia

J. Cereb. Blood Flow Metab.

Effect of a phospholipase A2 inhibitor on arachidonate incorporation into brain lipid in vivo

J. Neurochem.

Specific activity of brain palmitoyl-CoA pool provides rates of incorporation of palmitate in brain phospholipids in awake rats

J. Neurochem.

Local cerebral blood volume response to carbon dioxide in man

Circ. Res.

Research report
Brain incorporation of [1–¹¹C]arachidonate in normocapnic and hypercapnic monkeys, measured with positron emission tomography

Brain incorporation of ¹¹C-arachidonate in normocapnic and hypercapnic monkeys: A PET study

Measurement of regional cerebral pH in human subjects using continuous inhalation of ¹¹CO₂ and positron emission tomography

Evaluation of the ¹¹CO₂ positron emission tomographic method for measuring brain pH. I. pH changes measured in states of altered P_CO2

Measurement of brain pH using ¹¹CO₂ and positron emission tomography

Tomographic measurement of LCBF with high-resolution PET and $H_{2}^{15} O$ : Comparison of methods

Intravenous injection of [1-¹⁴C] arachidonate to examine regional brain lipid metabolism in unanesthetized rats