Metabotropic glutamate receptors, transmitter output and fatty acids: studies in rat brain slices

Abstract

The effects of (1S,3R)‐1‐aminocyclopentane‐1,3‐dicarboxylic acid (1S,3R‐ACPD), a non‐selective agonist of the metabotropic glutamate receptors (mGluRs), have been studied in rat cortical and striatal slices by measuring the depolarization‐induced output of D‐[3H]‐aspartate (D‐[3H]‐Asp) and of [3H]‐glutamate ([3H]‐Glu), neosynthesized from [3H]‐glutamine. In cortical slices, 1S,3R‐ACPD potentiated the depolarization‐induced (KCl, 30 mM) output of both D‐[3H]‐Asp and [3H]‐Glu. The potentiation, obtained at 300 μm 1S,3R‐ACPD was 65 ± 6% for D‐[3H]‐Asp and 56 ± 10% for [3H]‐Glu. Conversely, in striatal slices, 1S,3R‐ACPD reduced the depolarization‐induced transmitter output. The reduction, obtained at 300 μm of the agonist, was 60 ± 8% for D‐[3H]‐Asp and 50 ± 5% for neosynthesized [3H]‐Glu. Bovine serum albumin (BSA, 15 μm), which is able to bind locally produced fatty acids, completely eliminated the potentiating effect 1S,3R‐ACPD had on D‐[3H]‐Asp output from cortical slices. Low concentrations of arachidonic acid (1–10 μm) or of oleic acid (1–10 μm) added to BSA‐containing perfusion medium, restored this potentiating effect. BSA, however, had no effect on the inhibitory action of 1S,3R‐ACPD in striatal slices. Bromophenacyl bromide (100 μm), an inhibitor of phospholipase A2, and RG80267 (100 μm), an inhibitor of diacylglycerol lipase, have been shown to inhibit fatty acid production. These compounds prevented the potentiating effect of 1S,3R‐ACPD on D‐[3H]‐Asp‐output in cortical slices. Indomethacin (100 μm), an inhibitor of cyclo‐oxygenases, plus nordihydroguaiaretic acid (100 μm), an inhibitor of lipoxygenases, increased D‐[3H]‐Asp‐output in cortical slices perfused with BSA‐containing medium. These experiments suggest that the mGluR‐mediated potentiation of transmitter output requires the availability of unsaturated fatty acids, such as arachidonic or oleic acids, in cortical slices. In contrast, the mGluR‐induced inhibition of transmitter output is not dependent upon fatty acid availability in striatal slices. The requirement of both unsaturated fatty acids and 1S,3R‐ACPD in the facilitation of transmitter exocytosis may play an important role in the regulation of synaptic plasticity.

Regulation of Neurotransmitter Release by Metabotropic Glutamate Receptors

“…Although these results at first seem to concur with the studies showing augmentation of glutamate release in synaptosomal preparations in the presence of low concentrations of arachidonic acid, different stimuli were used by the two groups to evoke neurotransmitter release. Although Lombardi et al (1994, 1996) used 30 m M KCl in their study, this clamped depolarization was found to be unresponsive to either 1 S ,3 R ‐ACPD or protein kinase C manipulation in the Herrero et al (1992) study using synaptosomes. However, this discrepancy might be due to time‐course or methodological differences, such as measurement of prelabeled versus endogenous amino acids or the use of different preparations (Lombardi et al, 1993, 1994, 1996).…”

“…Although Lombardi et al (1994, 1996) used 30 m M KCl in their study, this clamped depolarization was found to be unresponsive to either 1 S ,3 R ‐ACPD or protein kinase C manipulation in the Herrero et al (1992) study using synaptosomes. However, this discrepancy might be due to time‐course or methodological differences, such as measurement of prelabeled versus endogenous amino acids or the use of different preparations (Lombardi et al, 1993, 1994, 1996). In particular, there is likely to be a considerable involvement of glial cells in the responses in brain slices, but not the synaptosome preparations (Winder and Conn, 1996).…”

“…It is probable that these contrasting effects of 1 S ,3 R ‐ACPD reflect the activation of group I and II mGlu receptors in the cortex and striatum, respectively. The augmentation of the KCl response seemed to be dependent on the presence of endogenous fatty acids, as the potentiation was prevented by inhibitors of fatty acid formation and phospholipase A 2 (Lombardi et al, 1994, 1996). Although these results at first seem to concur with the studies showing augmentation of glutamate release in synaptosomal preparations in the presence of low concentrations of arachidonic acid, different stimuli were used by the two groups to evoke neurotransmitter release.…”

Interactions of Carbon Monoxide and Metabotropic Glutamate Receptor Groups in the Nucleus Tractus Solitarii of Rats

“…Presynaptic mGluRs regulating the release of excitatory amino acids have been demonstrated in in vitro release studies in various brain slice and synaptosomal preparations (Herrero et al, 1994;Lombardi et al, 1994Lombardi et al, , 1996East et al, 1995). The presynaptic mGluRs are likely to be localized on glutamatergic vagal afferent terminals in the nucleus tractus solitarii (Lawrence and Jarrott, 1994), whereas postsynaptic mGluRs could be associated with GABAergic interneurons (Glaum and Miller, 1993a;Beart et al, 1994) or aspartateutilizing neurons/terminals (Beart et al, 1994) in the nucleus tractus solitarii.…”

Synaptic Synthesis, Dephosphorylation, and Degradation

“…Cortical-subcortical coronal frontal slices (anteriorly to the lateral ventricles) were then cut at a thickness of 200 m with a Leica VT1000S vibratome and maintained at 32°C for 30 min to allow functional recovery and later kept at 25°C in the same buffer. Recovered slices were transferred in a perfusion chamber and perfused (1 ml/min) at room temperature for 5 min, 20 min, or 1 h with a gassed (95% 0 2 /5% CO 2 ) solution (1.25 mM KH 2 PO 4 , 1.3 mM MgSO 4 , 2.5 mM CaCl 2 , 17.6 mM NaHCO 3 , 10 mM D-glucose, pH 7.4) containing isomolar low or high [K ϩ ] (125 mM NaCl, 3 mM KCl or 98 mM NaCl, 30 mM KCl, respectively) in accordance with a previously published protocol (14). After perfusion, the slides were rapidly lysed in a potter with 200 l of lysis buffer (50 mM Tris-HCl, pH 7.4, 150 mM NaCl, 1% Triton X-100, 2 mM EDTA, 1 mM DTT, 1 mM PMSF, 1 mM NaF, 1 mM Na 3 VO 4 , phosphatase inhibitor mixture (Roche), and protease inhibitor mixture (Sigma)).…”