The lactate receptor, G-protein-coupled receptor 81/hydroxycarboxylic acid receptor 1: Expression and action in brain.

We have proposed that lactate is a "volume transmitter" in the brain and underpinned this by showing that the lactate receptor, G-protein-coupled receptor 81 (GPR81, also known as HCA1 or HCAR1), which promotes lipid storage in adipocytes, is also active in the mammalian brain. This includes the cerebral neocortex and the hippocampus, where it can be stimulated by physiological concentrations of lactate and by the HCAR1 agonist 3,5-dihydroxybenzoate to reduce cAMP levels. Cerebral HCAR1 is concentrated on the postsynaptic membranes of excitatory synapses and also is enriched at the blood-brain barrier. In synaptic spines and in adipocytes, HCAR1 immunoreactivity is also located on subplasmalemmal vesicular organelles, suggesting trafficking to and from the plasma membrane. Through activation of HCAR1, lactate can act as a volume transmitter that links neuronal activity, cerebral blood flow, energy metabolism, and energy substrate availability, including a glucose- and glycogen-saving response. HCAR1 may contribute to optimizing the cAMP concentration. For instance, in the prefrontal cortex, excessively high cAMP levels are implicated in impaired cognition in old age, fatigue, stress, and schizophrenia and in the deposition of phosphorylated tau protein in Alzheimer's disease. HCAR1 could serve to ameliorate these conditions and might also act through downstream mechanisms other than cAMP. Lactate exits cells through monocarboxylate transporters in an equilibrating manner and through astrocyte anion channels activated by depolarization. In addition to locally produced lactate, lactate produced by exercising muscle as well as exogenous HCAR1 agonists, e.g., from fruits and berries, might activate the receptor on cerebral blood vessels and brain cells.

Lactate receptor sites link neurotransmission, neurovascular coupling, and brain energy metabolism.

The G-protein-coupled lactate receptor, GPR81 (HCA1), is known to promote lipid storage in adipocytes by downregulating cAMP levels. Here, we show that GPR81 is also present in the mammalian brain, including regions of the cerebral neocortex and hippocampus, where it can be activated by physiological concentrations of lactate and by the specific GPR81 agonist 3,5-dihydroxybenzoate to reduce cAMP. Cerebral GPR81 is concentrated on the synaptic membranes of excitatory synapses, with a postsynaptic predominance. GPR81 is also enriched at the blood-brain-barrier: the GPR81 densities at endothelial cell membranes are about twice the GPR81 density at membranes of perivascular astrocytic processes, but about one-seventh of that on synaptic membranes. There is only a slight signal in perisynaptic processes of astrocytes. In synaptic spines, as well as in adipocytes, GPR81 immunoreactivity is located on subplasmalemmal vesicular organelles, suggesting trafficking of the protein to and from the plasma membrane. The results indicate roles of lactate in brain signaling, including a neuronal glucose and glycogen saving response to the supply of lactate. We propose that lactate, through activation of GPR81 receptors, can act as a volume transmitter that links neuronal activity, cerebral energy metabolism and energy substrate availability.

Serotonin transporters in dopamine transporter imaging: a head-to-head comparison of dopamine transporter SPECT radioligands 123I-FP-CIT and 123I-PE2I.

UNLABELLED: Current SPECT radioligands available for in vivo imaging of the dopamine transporter (DAT) also show affinity for monoamine transporters other than DAT, especially the serotonin transporter (SERT). The effect of this lack of selectivity for in vivo imaging is unknown. In this study, we compared the SPECT radioligands (123)I-2-ß-carbomethoxy-3ß-(4-iodophenyl)-N-(3-fluoropropyl)nortropane ((123)I-FP-CIT) and (123)I-N-(3-iodoprop-2E-enyl)-2-ß-carbomethoxy-3ß-(4-methylphenyl) nortropane ((123)I-PE2I), which has a 10-fold higher selectivity than (123)I-FP-CIT for DAT versus SERT [corrected]. METHODS: Sixteen healthy individuals were scanned in random order with both radioligands. The radioligands were administered according to standard recommendations: (123)I-FP-CIT was given as a bolus injection, and the ratio between the striatum and reference tissue was measured after 3 h. (123)I-PE2I was administered in a bolus-infusion setup, and the nondisplaceable binding potential (BP(ND)) was measured after 2 h. To assess the contribution of SERT to the overall SPECT signal, SERT was blocked by intravenous citalopram in 6 of the individuals. RESULTS: The striatum-to-reference ratio - 1 of (123)I-FP-CIT was on average 18% higher than the striatal BP(ND) of (123)I-PE2I. Equal doses of radioactivity resulted in 3 times higher counting rates for (123)I-FP-CIT than for (123)I-PE2I, both in target and in reference brain regions. Citalopram infusion led to significant reductions in both striatal (22.8% ± 20.4%, P < 0.05) and thalamic (63.0% ± 47.9%, P < 0.05) (123)I-FP-CIT binding ratios, whereas BP(ND) of (123)I-PE2I was unaltered. Likewise, blocking of SERT led to increased (21% ± 30.1%, P < 0.001) plasma (123)I-FP-CIT, probably as a result of significant blocking of peripheral SERT binding sites. By contrast, plasma (123)I-PE2I remained stable. CONCLUSION: (123)I-FP-CIT and (123)I-PE2I had approximately the same target-to-background ratios, but per injected megabecquerel, (123)I-FP-CIT gave rise to 3-fold higher cerebral counting rates. We found that (123)I-FP-CIT, but not (123)I-PE2I, brain images have a highly interindividual but significant signal contribution from SERT. Whether the SERT signal contribution is of clinical importance needs to be established in future patient studies.