Intrastriatal administration of 5-aminolevulinic acid induces convulsions and body asymmetry through glutamatergic mechanisms

Angiotensin II increases the firing activity of pallidal neurons and participates in motor control in rats

The globus pallidus has emerged as a crucial node in the basal ganglia motor control circuit under both healthy and parkinsonian states. Previous studies have shown that angiotensin II (Ang II) and angiotensin subtype 1 receptor (AT1R) are closely related to Parkinson's disease (PD). Recent morphological study revealed the expression of AT1R in the globus pallidus of mice. To investigate the functions of Ang II/AT1R on the globus pallidus neurons of both normal and parkinsonian rats, electrophysiological recordings and behavioral tests were performed in the present study. Electrophysiological recordings showed that exogenous and endogenous Ang II mainly excited the globus pallidus neurons through AT1R. Behavioral tests further demonstrated that unilateral microinjection of Ang II into the globus pallidus induced significantly contralateral-biased swing in elevated body swing test (EBST), and bilateral microinjection of Ang II into the globus pallidus alleviated catalepsy and akinesia caused by haloperidol. AT1R was involved in Ang II-induced behavioral effects. Immunostaining showed that AT1R was expressed in the globus pallidus of rats. On the basis of the present findings, we concluded that pallidal Ang II/AT1R alleviated parkinsonian motor deficits through activating globus pallidus neurons, which will provide a rationale for further investigations into the potential of Ang II in the treatment of motor disorders originating from the basal ganglia.

Mechanisms of Neuronal Damage in Acute Hepatic Porphyrias

Porphyrias are a group of congenital and acquired diseases caused by an enzymatic impairment in the biosynthesis of heme. Depending on the specific enzyme involved, different types of porphyrias (i.e., chronic vs. acute, cutaneous vs. neurovisceral, hepatic vs. erythropoietic) are described, with different clinical presentations. Acute hepatic porphyrias (AHPs) are characterized by life-threatening acute neuro-visceral crises (acute porphyric attacks, APAs), featuring a wide range of neuropathic (central, peripheral, autonomic) manifestations. APAs are usually unleashed by external "porphyrinogenic" triggers, which are thought to cause an increased metabolic demand for heme. During APAs, the heme precursors δ-aminolevulinic acid (ALA) and porphobilinogen (PBG) accumulate in the bloodstream and urine. Even though several hypotheses have been developed to explain the protean clinical picture of APAs, the exact mechanism of neuronal damage in AHPs is still a matter of debate. In recent decades, a role has been proposed for oxidative damage caused by ALA, mitochondrial and synaptic ALA toxicity, dysfunction induced by relative heme deficiency on cytochromes and other hemeproteins (i.e., nitric oxide synthases), pyridoxal phosphate functional deficiency, derangements in the metabolic pathways of tryptophan, and other factors. Since the pathway leading to the biosynthesis of heme is inscribed into a complex network of interactions, which also includes some fundamental processes of basal metabolism, a disruption in any of the steps of this pathway is likely to have multiple pathogenic effects. Here, we aim to provide a comprehensive review of the current evidence regarding the mechanisms of neuronal damage in AHPs.

Acute porphyrias - A neurological perspective

Acute hepatic porphyrias (AHP) can cause severe neurological symptoms involving the central, autonomic, and peripheral nervous system. Due to their relative rarity and their chameleon-like presentation, delayed diagnosis and misdiagnosis are common. AHPs are genetically inherited disorders that result from heme biosynthesis enzyme deficiencies and comprise four forms: acute intermittent porphyria (AIP), variegate porphyria (VP), hereditary coproporphyria (HCP), and ALA-dehydratase porphyria (ALADP). Depending on the clinical presentation, the main differential diagnoses are Guillain-Barré syndrome and autoimmune encephalitis. Red flags that could raise the suspicion of acute porphyria are neurological symptoms starting after severe (abdominal) pain, in association with reddish urine, hyponatremia or photodermatitis, and the presence of encephalopathy and/or axonal neuropathy. We highlight the diagnostic difficulties by presenting three cases from our neurological intensive care unit and give a comprehensive overview about the diagnostic findings in imaging, electrophysiology, and neuropathology.

In vivo Bidirectional Modulation of Cannabinoid on the Activity of Globus Pallidus in Rats

Endocannabinoids are bioactive substances which participate in central motor control. The globus pallidus (GP) is a major nucleus in the basal ganglia circuit, which plays an important function in movement regulation. Both cannabinoid receptor type 1 (CB₁R) and cannabinoid receptor type 2 (CB₂R) are expressed in the GP suggesting GP as a main action area of endocannabinoids. To investigate the direct electrophysiological and behavioral effects of cannabinoids in GP, in vivo single unit extracellular recordings and behavioral tests were performed in rats. Administration of WIN 55,212-2 exerted three neuronal response patterns from all sampled neurons of GP, including (1) increase of the firing rate; (2) decrease of the firing rate; (3) increase and then decrease of the firing rate. Selectively blocking CB₁R by AM 251 decreased the firing rate and increased the firing rate. Selectively blocking CB₂R by AM 630 did not change the firing rate significantly, which suggested that endocannabinoids modulated the spontaneous firing activity of pallidal neurons mainly via CB₁R. Furthermore, co-application of AM 251, but not AM 630, blocked WIN 55,212-2-induced modulation of firing activity of pallidal neurons. Finally, both haloperidol-induced postural behavioral test and elevated body swing test (EBST) showed that unilateral microinjection of WIN 55,212-2 mainly induced contralateral-biased swing and deflection behaviors. Meanwhile, AM 251 produced opposite effect. The present in vivo study revealed that cannabinoids produced complicated electrophysiological and behavioral effects in the GP, which further demonstrated that the GP is a major functional region of endocannabinoid.

Hypothesis: Metabolic targeting of 5-aminolevulinate synthase by tryptophan and inhibitors of heme utilisation by tryptophan 2,3-dioxygenase as potential therapies of acute hepatic porphyrias

Metabolic targeting of liver 5-aminolevulinate synthase (5-ALAS) by inhibition of heme utilisation by tryptophan (Trp) 2,3-dioxygenase (TDO) or the use of tryptophan is proposed as a therapy of acute hepatic porphyrias. 5-ALAS, the rate-limiting enzyme of heme biosynthesis, is under negative feedback control by a small regulatory heme pool in the hepatic cytosol. Acute porphyric attacks, precipitated by fasting, certain hormones and some drugs, involve induction of 5-ALAS secondarily to depletion of the above pool, and the resultant elevation of 5-ALA levels initiates the abdominal and neurological symptoms of attacks. By utilising the regulatory heme, cytosolic TDO undermines the feedback control, thus allowing 5-ALAS induction to occur, e.g. upon glucocorticoid induction of TDO during fasting (starvation) and exogenous glucocorticoid administration. Currently, glucose therapy is the preferred strategy for reversing moderate attacks induced by fasting (calorie restriction), with more severe attacks being treated by intravenous heme preparations. Reversal of fasting-induced attacks by glucose is explained by the previously demonstrated reversal of increased heme utilisation by TDO. Inhibitors of this utilisation are therefore potential therapeutic targets in acute attacks and also for maintenance of a symptomless state. Existing TDO inhibitors other than glucose include allopurinol, nicotinamide and recently developed potent inhibitors such as LM10 used in cancer therapy. Based on studies in rats, the hypothesis predicts that the safety or otherwise of drugs in the hepatic porphyrias is determined by their ability to inhibit TDO utilisation of heme under basal conditions or after glucocorticoid induction or heme activation of TDO, in parallel with reciprocal changes in 5-ALAS induction. Tryptophan is also proposed as a potential therapy of acute attacks either alone or as an adjunct to the recently proposed 5-ALAS1 gene silencing. Trp increases heme biosynthesis by enhancing 5-ALA dehydratase activity and, based on a Trp-5-ALA model presented herein, Trp offers several advantages over heme therapy, namely rapid conversion of 5-ALA into heme, a greatly enhanced heme availability, a near complete inhibition of 5-ALAS induction, assumed rapid clearance of 5-ALA and hence accelerated resolution of symptoms of attacks, and finally provision of the neuroprotective metabolite kynurenic acid to neutralise the neurological symptoms. The hypothesis also addresses heme regulation in species lacking the TDO free apoenzyme and its glucocorticoid induction mechanism and proposes detailed assessment of heme biosynthesis in these species. Detailed proposals for testing the hypothesis are presented.

Psychiatric Aspects of Acute Porphyria: a Comprehensive Review

PURPOSE OF REVIEW

The aim of this paper is to review psychiatric manifestations, comorbidities, and psychopharmacological management in individuals with acute porphyria (AP).

RECENT FINDINGS

Recent literature begins to clarify associations between AP, schizophrenia, bipolar disorder, and other psychopathology. Broad psychiatric symptoms have been associated to acute porphyria (AP) and correspond to a spectrum of heterogeneous manifestations such as anxiety, affective alterations, behavioral changes, personality, and psychotic symptoms. These symptoms may be difficult to identify as being related to porphyria since symptoms may arise at any time during the disease process. In addition, these patients may present psychiatric conditions secondary to the disease, such as adjustment disorder and substance use disorders. Timely diagnosis and appropriate treatment of psychiatric manifestations positively impact the course of the disease.

The Subthalamic Neurons are Activated by Both Orexin-A and Orexin-B

The subthalamic nucleus is an important nucleus in the indirect pathway of the basal ganglia circuit and therefore is involved in motor control under both normal and pathological conditions. Morphological studies reveal that the subthalamic nucleus receives relatively dense orexinergic projections originating from the hypothalamus. Both orexin-1 (OX₁) and orexin-2 (OX₂) receptors are expressed in the subthalamic nucleus. To explore the functions of orexinergic system in the subthalamic nucleus, extracellular electrophysiological recordings and behavioral tests were performed in the present study. Exogenous application of orexin-A significantly increased the spontaneous firing rate from 5.70 ± 0.66 Hz to 9.87 ± 1.18 Hz in 64.00% subthalamic neurons recorded. OX₁ receptors are involved in orexin-A-induced excitation. Application of orexin-B increased the firing rate from 7.47 ± 0.92 Hz to 11.85 ± 1.39 Hz in 80.95% subthalamic neurons recorded, entirely through OX₂ receptors. Both OX₁ and OX₂ receptor antagonists decreased the firing rate in 43.75% and 62.50% subthalamic neurons recorded respectively, suggesting the involvement of endogenous orexinergic system in the control of spontaneous firing activity. Further elevated body swing test revealed that microinjection of orexins and the receptor antagonists into the subthalamic nucleus induced contralateral-biased swing and ipsilateral-biased swing, respectively. Taken together, the present study suggests that orexins play important roles in the subthalamic nucleus which may provide further evidence for the involvement of subthalamic orexinergic tone in Parkinson's disease.

SIGNIFICANCE

Previous morphological studies indicate that the subthalamic nucleus receives orexinergic innervation and expresses both OX₁ and OX₂ receptors. Using in vivo multibarrel electrophysiological recordings, the present study revealed that exogenous application of orexin-A and orexin-B increased the spontaneous firing rate of the subthalamic neurons through OX₁ and OX₂ receptors. Endogenous orexinergic system was involved in the control of spontaneous firing of the subthalamic neurons. Further behavioral test revealed that intrasubthalamic application of orexins and the receptor antagonists induced biased swing behavior. The present study may provide further evidence for the involvement of subthalamic orexinergic tone in Parkinson's disease.

Orexin-A increases the activity of globus pallidus neurons in both normal and parkinsonian rats

Orexin is a member of neuropeptides which was first identified in the hypothalamus. The globus pallidus is a key structure in the basal ganglia, which is involved in both normal motor function and movement disorders. Morphological studies have shown the expression of both OX1 and OX2 receptors in the globus pallidus. Employing single unit extracellular recordings and behavioural tests, the direct in vivo electrophysiological and behavioural effects of orexin-A in the globus pallidus were studied. Micro-pressure administration of orexin-A significantly increased the spontaneous firing rate of pallidal neurons. Correlation analysis revealed a negative correlation between orexin-A induced excitation and the basal firing rate. Furthermore, application of the specific OX1 receptor antagonist, SB-334867, decreased the firing rate of pallidal neurons, suggesting that endogenous orexinergic systems modulate the firing activity of pallidal neurons. Orexin-A increased the excitability of pallidal neurons through both OX1 and OX2 receptors. In 6-hydroxydopamine parkinsonian rats, orexin-A-induced increase in firing rate of pallidal neurons was stronger than that in normal rats. Immunostaining revealed positive OX1 receptor expression in the globus pallidus of both normal and parkinsonian rats. Finally, postural test showed that unilateral microinjection of orexin-A led to contralateral deflection in the presence of systemic haloperidol administration. Further elevated body swing test revealed that pallidal orexin-A and SB-334867 induced contralateral-biased swing and ipsilateral-biased swing respectively. Based on the electrophysiological and behavioural findings of orexin-A in the globus pallidus, the present findings may provide a rationale for the pathogenesis and treatment of Parkinson's disease.

Syzygium jambos and Solanum guaraniticum show similar antioxidant properties but induce different enzymatic activities in the brain of rats

Syzygium jambos and Solanum guaraniticum are both employed in Brazil as medicinal plants, even though their potential toxicity is not well established and they are frequently misused. The aim of this study was investigate the effect of the aqueous leaf extracts of both plants on δ-aminolevulinate dehydratase (δ-ALA-D) and acetylcholinesterase (AChE) activities and the antioxidant action against oxidative damage induced by sodium nitroprusside in rats, using in vitro assays. In addition, the presence of gallic, caffeic and chlorogenic acids, as well as rutin, quercetin and kaempferol as bioactive compounds in the extracts was identified by HPLC and their levels quantified. The antioxidant activities of both extracts were assessed by their capabilities to scavenge nitric oxide and to inhibit lipid peroxidation. Only Syzygium jambos presented thiol-peroxidase-like activity. Although neither extract affected the AChE activity, the aqueous extract of Solanum guaraniticum inhibited brain δ-ALA-D activity, suggesting a possible impairment effect on the central nervous system. Our results showed that both extracts exhibited efficient free radical scavenger activity and are an interesting source of bioactive compounds, justifying their use in folk medicine, although Solanum guaraniticum extract could have neurotoxicity properties and we therefore suggest that its use should be restricted to ensure the health of the population.

High doses of 2,2'-dithienyl diselenide cause systemic toxicity in rats: an in vitro and in vivo study

Organoselenium compounds have important pharmacological properties. However, these compounds can cause toxicity, typically related to oxidation of endogenous thiols. The aim of this study was to investigate whether 2,2'-dithienyl diselenide (DTDS) has potential toxicity in vitro and in vivo. Therefore, sulfhydryl-containing enzyme activities, δ-aminolevulinic acid dehydratase (δ-ALA-D) and Na(+) -K(+) -ATPase were used to predict DTDS toxicity in rat brain homogenate in vitro. In in vivo experiments, a DTDS administration (50 or 100 mg kg(-1) , p.o.) to rats was performed and toxicological parameters were determined. DTDS inhibited δ-ALA-D (IC50 2 µm) and Na(+) -K(+) -ATPase (IC50 17 µm) activities in vitro. The inhibitory effect of DTDS on δ-ALA-D and Na(+) -K(+) -ATPase activities was restored by dithiothreitol. DTDS (5-25 µm) elicited a thiol oxidase-like activity. In vivo, DTDS (50 and 100 mg kg(-1) ) caused systemic toxicity, evidenced by a decrease in water and food intakes and body weight gain, as well as the death of rats. DTDS at the dose of 100 mg kg(-1) increased plasma alanine and aspartate aminotransferase activities and decreased urea levels. At 50 and 100 mg kg(-1) , it increased lipid peroxidation levels. At the highest dose, DTDS inhibited δ-ALA-D activity. By contrast, Na(+) -K(+) -ATPase activity and antioxidant defense were not altered in the brains of rats exposed to DTDS. In conclusion, interaction with the cisteinyl residues seems to mediate the inhibitory effect of DTDS on sulfhydryl-containing enzymes in vitro. In addition, high oral doses of DTDS induce toxicity in rats.

Delta-aminolevulinic dehydratase is a proteasome interacting protein

The proteasome interacts with a large number of proteins which regulate specific cellular functions. The focus of this study is to examine the proteasome interaction with Delta-aminolevulinate dehydratase (ALAD). ALAD is involved in the heme biosynthesis pathway and was co-isolated, with the 20S proteasome using several chromatographic purification steps. The MALDI-TOF mass spectrometry analysis identified this proteasome co-isolated protein as ALAD. When the proteasome was isolated using density-gradient centrifugation, ALAD was also found in the 26S proteasome fractions. It co-isolated with the 20S more than with the 26S proteasome. Furthermore, immunoprecipitated ALAD stained positive with antibodies to proteasome subunits. These results indicate that ALAD might interact with the proteasome. It is possible that ALAD is involved in modulating proteasome activity. When purified proteasomes were incubated with ALAD it was found that ALAD changes proteasome activity in a dose dependent manner. This indicates that ALAD may play a significant role in regulating proteasome activity. The data supports the hypothesis that ALAD, an important enzyme for heme synthesis, is also important as a proteasome interacting protein.

Effects of ethanol on the proteasome interacting proteins

Proteasome dysfunction has been repeatedly reported in alcoholic liver disease. Ethanol metabolism end-products affect the structure of the proteasome, and, therefore, change the proteasome interaction with its regulatory complexes 19S and PA28, as well as its interacting proteins. Chronic ethanol feeding alters the ubiquitin-proteasome activity by altering the interaction between the 19S and the 20S proteasome interaction. The degradation of oxidized and damaged proteins is thus decreased and leads to accumulation of insoluble protein aggregates, such as Mallory-Denk bodies. Ethanol also affects the immunoproteasome formation. PA28a/b interactions with the 20S proteasome are decreased in the proteasome fraction isolated from the liver of rats fed ethanol chronically, thus affecting the cellular antigen presentation and defense against pathogenic agents. Recently, it has been shown that ethanol also affects the proteasome interacting proteins (PIPs). Interaction of the proteasome with Ecm29 and with deubiquitinating enzymes Rpn11, UCH37, and Usp14 has been found to decrease. However, the two UBL-ubiquitin-associated domain (UBA) PIPs p62 and valosin-containing protein are upregulated when the proteasome is inhibited. The increase of these UBL-UBA proteins, as well as the increase in Hsp70 and Hsp25 levels, compensated for the proteasome failure and helped in the unfolding/docking of misfolded proteins. Chronic alcohol feeding to rats causes a significant inhibition of the proteasome pathway and this inhibition results from a decreases of the interaction between the 20S proteasome and the regulatory complexes, PIPs, and the ubiquitin system components.

5-Aminolevulinate and 4, 5-dioxovalerate ions decrease GABAA receptor density in neuronal cells, synaptosomes and rat brain

Porphyrias are heme-associated metabolic disorders such as intermittent acute porphyria (IAP) and lead poisoning, where 5-aminolevulinate (ALA) accumulates. Effects of ALA on the CNS have been explained by ALA binding to GABA(A) receptors, followed by receptor lesions from oxyradicals and 4, 5-dioxovalerate (DOVA) generated from metal-catalyzed ALA oxidation by oxygen. We have characterized the effects of ALA and DOVA on GABA(A) receptor density in synaptosomes and neurons in vitro and also in brains of rats treated with ALA or succinylacetone methyl ester (SAME), a tyrosine catabolite derivative able to induce ALA accumulation. Radiolabeling assays revealed that following exposure to DOVA the concentration of synaptosomal GABAergic sites decreased by approximately 50%. Pretreatment with DOVA resulted in less GABA(A) receptor density in P19 and WERI cells and altered cell morphology. Furthermore, exposure to DOVA also induced a 5-fold increase in WERI cell mortality rate. Treatment with ALA resulted in loss of neuronal morphology and decrease of GABA(A) density in P19 neuronal cells. ALA and SAME treatment diminished the density of GABAergic receptors in the habenular complex and the parabigeminal nucleus of rat brain as studied by immunohistochemical procedures. Our results strongly suggest that ALA- and DOVA-promoted damage to GABA(A) receptors may contribute to the neurological manifestations of AIP and plumbism.

Effects of extracellular δ-aminolaevulinic acid on sodium currents in acutely isolated rat hippocampal CA1 neurons

The effects of delta-aminolaevulinic acid (ALA) on voltage-gated sodium channel (VGSC) currents (I(Na)) in acutely isolated hippocampal CA1 neurons from 10- to 12-day-old Wistar rats were examined by using the whole-cell patch-clamp technique under voltage-clamp conditions. ALA from 0.01 microm to 20 microm was applied to the recorded neurons. Low concentrations of ALA (0.01-1.0 microM) increased I(Na) amplitude, whereas high concentrations of ALA (5.0-20.0 microM) decreased it. The average I(Na) amplitude reached a maximum of 117.4 +/- 3.9% (n = 9, P < 0.05) with 0.1 microM ALA, and decreased to 78.1 +/- 3.8% (n = 13, P < 0.05) with 10 microm ALA. ALA shifted the steady-state activation and inactivation curves of I(Na) in the hyperpolarizing direction with different V0.5, suggesting that ALA could depress the opening threshold of the voltage-gated sodium channel (VGSC) and thus increase the excitability of neurons through facilitating the opening of VGSC. The time course of recovery from inactivation was significantly prolonged at both low and high concentrations of ALA, whereas either low or high concentrations of ALA had no significant effect on the attenuation of I(Na) during stimulation at 5 Hz, indicating that the effect of ALA on VGSC is state-independent. Furthermore, we found that application of ascorbic acid, which blocks pro-oxidative effects in neurons, could prevent the increase of I(Na) amplitude at low concentrations of ALA. Baclofen, an agonist of GABAb receptors, induced some similar effects to ALA on VGSC, whereas bicuculline, an antagonist of GABAa receptors, could not prevent ALA-induced effects on VGSC. These results suggested that ALA regulated VGSC mainly through its pro-oxidative effects and GABAb receptor-mediated effects.

Effect of δ-aminolevulinic acid treatment on N-methyl-d-aspartate receptor at different ages in the rat brain

We report here the effects of the chronic treatment with the oxidant agent delta-aminolevulinic acid (ALA) on the N-methyl-D-aspartate (NMDA) receptors in 4-, 12- and 24-month-old male Wistar rats. ALA was administered daily for 15 days (40 mg/kg i.p). The study was performed by membrane homogenate binding and autoradiography, using tritiated 5-methyl-10, 11-dihydro-5H-dibenzo(a,d)cycloheptan-5,10-imine maleate ([3H]MK-801). [3H]MK-801 binding was significantly decreased in most areas studied (cortex and hippocampus) at all ages in treated rats with respect to their controls. Furthermore, Western blot assays were performed using antibodies against the NMDA receptor NR2A subunit, which is widely distributed in the brain, mainly in cortex and hippocampus. In cortex but not in hippocampus, the ALA treatment induced significant decreases in the amounts of NR2A subunit in 12- and 24-month-old animals. We conclude that chronic treatment with ALA is able to induce NMDA receptor decreases in an age-independent way and that NR2A subunit seems to be involved in these decreases in cerebral cortex, but not in the other structures studied.

Transport of 5-aminolevulinic acid between blood and brain

Little is known about the movement of 5-aminolevulinic acid (delta-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 [(14)C]ALA across the blood-brain and blood-CSF barriers in the rat. In the adult rat, the influx rate constant (K(i)) for [(14)C]ALA movement into brain was low ( approximately 0.2 microl/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 [(14)C]ALA was 3-fold less than that for [(14)C]mannitol, a molecule of similar size. This difference appears to result from a lower lipid solubility rather than saturable [(14)C]ALA transport from brain to blood. The K(i) for [(14)C]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 [(14)C]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 [(14)C]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.

Inhibition of adenylate cyclase activity by 5-aminolevulinic acid in rat and human brain

The effect of the haem precursor 5-aminolevulinic acid (ALA) on the production of cyclic adenosine-monophosphate (cAMP) by rat cerebellar membranes was investigated. It was found that ALA dose-dependently decreased cAMP levels (maximal inhibition of 38%, at 1 mM), due to an inhibition of basal adenylate cyclase activity. ALA also inhibited fluoride- and Gpp(NH)p-stimulated, but not the forskolin-stimulated adenylate cyclase activity. 5-Aminovaleric acid (an inhibitor of GABA(B) receptors) did not prevent the inhibition, indicating that it was not mediated by the activation of the G(i)-protein coupled GABA(B) receptor. In addition, the nucleotide binding site of G-protein appeared not to be affected by ALA since it did not inhibit [3H]Gpp(NH)p binding to our membrane preparation. Antioxidants (glutathione, ascorbate and trolox) completely prevented the inhibition indicating that ALA effect was mediated by an oxidative damage of adenylate cyclase. ALA also inhibited the activity of adenylate cyclase in membranes isolated from rat cortex and striatum and from human cortex. These results may be of value in understanding the neurochemical mechanisms underlying the neurotoxic effects of ALA.

5-Aminolevulinic acid inhibits [3H]muscimol binding to human and rat brain synaptic membranes

The interaction of 5-aminolevulinic acid (ALA) with GABA(A) receptors has been proposed to underlie the neurological dysfunctions of ALA-accumulating disorders, such as acute intermittent porphyria. The effects of ALA on [3H]muscimol binding to human and rat cerebral cortical membranes were compared. ALA (0.1-10 mM) significantly inhibited the binding of [3H]muscimol (12 nM), with a similar potency in rat and human membranes (IC50 = 199 vs. 228 microM, respectively). Kinetical analysis revealed that ALA (1 mM) significantly increased the Kd and decreased the Bmax of [3H]muscimol to both rat (100 and 50%, respectively) and human (200 and 40%, respectively) membranes, indicating a mixed-type inhibition. The similarity in the potency and mechanism of the ALA-induced inhibition of muscimol binding in rat and human membranes indicate that rat studies are useful to evaluate the neurotoxic properties of ALA towards the human GABAergic system, and may help to understand the pathophysiology of porphyria.