Regulation of phosphoinositide turnover in neonatal rat cerebral cortex by group I- and II- selective metabotropic glutamate receptor agonists

1. The interactive effects of different metabotropic glutamate (mGlu) receptor subtypes to regulate phosphoinositide turnover have been studied in neonatal rat cerebral cortex and hippocampus by use of agonists and antagonists selective between group I and II mGlu receptors.
2. The group II-selective agonist 2R,4R-4-aminopyrrolidine-2,4-dicarboxylate (2R,4R-APDC; 100 μM) had no effect on basal total inositol phosphate ([³H]-InsP_x) accumulation (in the presence of Li⁺) in myo-[³H]-inositol pre-labelled slices, but enhanced the maximal [³H]-InsP_x response to the group I-selective agonist (S)-3,5-dihydroxyphenylglycine (DHPG) by about 100% in both hippocampus and cerebral cortex. In cerebral cortex the enhancing effect of 2R,4R-APDC occurred with respect to the maximal responsiveness and had no effect on EC₅₀ values for DHPG (-log EC₅₀ (M): control, 5.56±0.05; +2R,4R-APDC, 5.51±0.08). 2R,4R-APDC also caused a significant enhancement of the DHPG-stimulated inositol 1,4,5-trisphosphate (Ins(1,4,5)P₃) mass response over an initial 0–300 s time-course.
3. The enhancing effects of 2R,4R-APDC on DHPG-stimulated [³H]-InsP_x accumulation were observed in both the presence and nominal absence of extracellular Ca²⁺, and irrespective of whether 2R,4R-APDC was added before, simultaneous with, or subsequent to DHPG. Furthermore, increasing the tissue cyclic AMP concentration up to 100 fold had no effect on DHPG-stimulated Ins(1,4,5)P₃ accumulation in the absence or presence of 2R,4R-APDC.
4. 2R,4R-APDC and (2S, 1′R, 2′R, 3′R)-2-(2,3-dicarboxylcyclopropyl)glycine (DCG-IV), the latter agent in the presence of MK-801 to prevent activation of NMDA-receptors, each inhibited forskolin-stimulated cyclic AMP accumulation by about 50%, with respective EC₅₀ values of 1.3 and 0.04 μM (-log EC ₅₀ (M): 2R,4R-APDC, 5.87±0.09; DCG-IV, 7.38±0.05). In the presence of DHPG (30 μM), 2R,4R-APDC and DCG-IV also concentration-dependently increased [³H]-InsP_x accumulation with respective EC₅₀ values of 4.7 and 0.28 μM (-log EC₅₀ (M): 2R,4R-APDC, 5.33±0.04; DCG-IV, 6.55±0.09) which were 3–7 fold rightward-shifted relative to the adenylyl cyclase inhibitory responses.
5. The group II-selective mGlu receptor antagonist {"type":"entrez-nucleotide","attrs":{"text":"LY307452","term_id":"1257780057","term_text":"LY307452"}}LY307452 (30 μM) caused parallel rightward shifts in the concentration-effect curves for inhibition of forskolin-stimulated adenylyl cyclase, and enhancement of DHPG-stimulated [³H]-InsP_x accumulation, by 2R,4R-APDC yielding similar equilibrium dissociation constants (K_ds, 3.7±1.1 and 4.1±0.4 μM respectively) for each response.
6. The ability of 2R,4R-APDC to enhance receptor-mediated [³H]-InsP_x accumulation appeared to be agonist-specific; thus although DHPG (100 μM) and the muscarinic cholinoceptor agonist carbachol (10 μM) stimulated similar [³H]-InsP_x accumulations, only the response to the former agonist was enhanced by co-activation of group II mGlu receptors.
7. These data demonstrate that second messenger-generating phosphoinositide responses stimulated by group I mGlu receptors are positively modulated by co-activation of group II mGlu receptors in cerebral cortex and hippocampus. The data presented here are discussed with respect to the possible mechanisms which might mediate the modulatory activity, and the physiological and pathophysiological significance of such crosstalk between mGlu receptors.