Transport of 5-aminolevulinic acid between blood and brain

Abstract

Little is known about the movement of 5-aminolevulinic acid (δ-aminolevulinic acid; ALA) between blood and brain. This is despite the fact that increases in brain ALA may be involved in generating the neuropsychiatric symptoms in porphyrias and that systemic administration of ALA is currently being used to delineate the borders of malignant gliomas. The current study examines the mechanisms involved in the movement of [¹⁴C]ALA across the blood–brain and blood–CSF barriers in the rat. In the adult rat, the influx rate constant (K_i) for [¹⁴C]ALA movement into brain was low (∼0.2 μl/g per min), was unaffected by increasing plasma concentrations of non-radioactive ALA or probenecid (an organic anion transport inhibitor) and, therefore, appears to be a diffusional process. The K_i for [¹⁴C]ALA was 3-fold less than that for [¹⁴C]mannitol, a molecule of similar size. This difference appears to result from a lower lipid solubility rather than saturable [¹⁴C]ALA transport from brain to blood. The K_i for [¹⁴C]ALA for uptake into the neonatal brain was 7-fold higher than in the adult. However, again, this was unaffected by increasing plasma ALA concentrations suggesting a diffusional process. In contrast, at the blood–CSF barrier, there was evidence of carrier-mediated [¹⁴C]ALA transport from blood to choroid plexus and blood to CSF. Both processes were inhibited by administration of non-radioactive ALA and probenecid. However, experiments in choroid plexus epithelial cell primary cultures indicated that transport in these cells was polarized with [¹⁴C]ALA uptake from the apical (CSF) side being about 7-fold greater than uptake from the basolateral (blood) side. In total, these results suggest that the brain is normally fairly well protected from changes in plasma ALA concentration by the very low blood–brain barrier permeability of this compound and by a saturable efflux mechanism present at the choroid plexus.

Introduction

5-Aminolevulinic acid (δ-aminolevulinic acid; ALA) is a precursor of porphyrins and heme, and is important in some CNS disease states. An accumulation of the porphyrin precursors, ALA and/or porphobilinogen, has been suggested as the cause of the neuropsychiatric symptoms that occur in hereditary hepatic porphyrias [28]. Oral administration of ALA has also been used clinically to delineate malignant gliomas [23]. Such oral administration results in a highly selective accumulation of protoporphyrin IX (PpIX) in the glioma which can then be detected by fluorescence microscopy in the operating theater [23], [25].

Despite the physiological and pathophysiological importance of ALA, there is still uncertainty about the degree and mechanisms by which it is transported between blood and brain. Some studies indicate that there is little transport at the blood–brain barrier (BBB), formed by the cerebral capillaries [7], [26], whereas McGillion et al. [12] found relatively rapid transport. Evidence from autoradiography [26] and PpIX fluorescence [24] indicate that there is selective uptake of ALA into the choroid plexus, the site of the blood–CSF barrier, after systemic administration. Studies on isolated choroid plexus also indicate that this tissue is capable of ALA transport [14], but the extent to which transport of ALA from blood to CSF is an important entry mechanism for ALA into brain has not been established.

Because of these uncertainties over blood–brain ALA transport in vivo, the present study in rats was designed to examine: (1) Is the movement of ALA blood to brain or blood to CSF via diffusion or carrier-mediated transport? (2) Is there an saturable clearance mechanism removing ALA from the brain or CSF? (3) Is the movement of ALA from blood to brain altered early in development when brain growth might entail greater heme synthesis?