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Endogenous glutamyl peptides and extracellular magnesium modulate glutamatergic neurotransmission
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141 ENDOGENOUS GLUTAMYL PEPTIDES AND EXTRACELLULAR MAGNESIUM MODULATE GLUTAMATERGIC NEUROTRANSMISSION V. Varga, I. Holopalnen, R. JanHky, K.E.O. ~kerman, PirJo Saransaari and S.S. OJa, Tampere Brain Research Center, Department of Biomedical Sciences, University of Tampere, and Department of Biochemistry and Pharmacy, ~bo Akademi, Turku, Finland. Glutamate acts at different receptor subtypes gating channels penetrable to mono- and divalent cations. The mechanism and regulation of cation fluxes are not fully understood. The NMDAmediated responses show voltage-dependent Mg block. Glutamate, kalnate and NMDA effects are also antagonized by D-glutamyl and aspartyl peptides. We now further analyzed effects of Mg and certain glutamyl peptldes. Glutamate and its agonists depolarized cultured cerebellar granule cells and increased dose-dependently the influx and intracellular levels of free calcium. The glutamate-, NMDA- and quisqualate-stimulated influx of Ca was inhibited by extracellular Mg but not the influx evoked by kainate. Several peptides also inhibited stimulated Ca influx and opposed elevation of intracellular free Ca. y-L-Glutamylglutamate and -aspartate were the most effective ones in normal and low-Ms (0.i mM) media, respectively. The NMDA-activated influx of Ca was potentiated by 0.O1 mM glycine only in Mg-free medium. Although inhibiting the influx of Ca, y-L-glutamylaspartate and -taurine tended to increase the potassium- and kainate-evoked release of glycine and taurine, possibly through interaction with NMDA receptors. Both basal and evoked release of glutamate, D-aspartate, taurine and glycine were slightly enhanced in low-Ms medium. Our data bespeak Mg-modulated interaction of endogenous glutamyl peptides with glutamatergic neurotransmission (Supported by the Emll Aaltonen Foundation). 142 MAGNESIUM POTENTIATES OUISOUALATE EFFECTS ON THE VESTIBULAR SYSTEM AFFERENTS_ Rosario Vega, Enrique Soto and Ma. E u g e n i a Perez. Universidad Aut6noma de P u e b l a - ICUAP, P.O. B o x 406, P u e b l a , Pue, M e x i c o . S t u d i e s of M g ÷÷ e f f e c t s o n t h e r e s p o n s e e v o k e d by e x c i t a t o r y a m i n o a c i d s (EAA), have shown that non-NMDA r e c e p t o r c u r r e n t s a r e not a f f e c t e d by M g ÷+. H o w e v e r , studying the e f f e c t s of M g ÷÷ u p o n v e s t i b u l a r s y s t e m p r i m a r y a f f e r e n t r e s p o n s e s e l i c i t e d by EAA agonists, we have found t h a t extracellular M g ÷+ exerts a significant influence upon OA evoked responses. The electrical a c t i v i t y of the semicircular canal vestibular system afferents of t h e axolotl (Ambystoma tigrinum) w e r e r e c o r d e d e x t r a c e l l u l a r l y . D r u g s w e r e ejected from a p i p e t t e in v o l u m e s of 20 ul in a lO ml bath. In t h e p r e s e n c e of a Ringer with normal M g ÷÷ c o n c e n t r a t i o n (I mM), K A (0.I uM to 0.I mM, N = 49) a n d OA (0.01 u M to 0.I mM, N = 33) induced a monotonically increasing dose d e p e n d e n t e x c i t a t o r y e f f e c t . H o w e v e r , w h e n a M g ÷+ f r e e R i n g e r s o l u t i o n w a s u s e d in t h e p e r f u s i o n bath, t h e d o s e r e s p o n s e c u r v e for O A (N = 79) w a s n o n - l i n e a r , with a significant reduction of t h e maximum effect, a n d t h e a p p a r i t i o n of a plateau between 1 and i0 uM. In c o n t r a s t t h e K A (N = 25) d o s e r e s p o n s e c u r v e was unchanged. These results indicate that e x t r a c e l l u l a r M g ÷÷ concentration potentiates the excitatory effects of OA upon t h e v e s t i b u l a r s y s t e m a f f e r e n t f i b e r s . T h i s a c t i o n of M g ÷" is t h e o p p o s i t e to t h a t r e p o r t e d f o r N M D A r e s p o n s e s a n d its m e c h a n i s m r e m a i n s to be e x p l a i n e d .
143 MODULATION OF THE PRESYNAPTIC AND POSTSYNAPTIC COMPONENTS OF NORADRENERGIC TRANSMISSION BY KINDLING. A. Vezzani, R. Serafini, C. Bendotti, M. Rizzi, G. Vigan8 and R. Samanin, Istituto di R~cerche Farmacologiche "MARIO NEGRI", Milano, 20157 ITALY. It has been shown that norepinephrine (NE) stimulates glutamate (GLu) release in the dentate gyrus (DG) and enhances Glu-induced excitation on CA1 cells through B-adrenoceptors. Inhibitory actions of NE (~l-mediated) have also been reported in the hippocampus. We have investigated whether during kindling development modifications in noradrenergic neurotransmission occurred associated with up-regulation of the NMDA-receptors. Rats were kindled in the dorsal left hippocampus (LH) and sacrificed 48 h after the 12th stimulation (stage 2) or one week after the last stage 5 seizures. Following field electrical stimulation (2-10 Hz for 2 min) at stage 5, NE release from hippocampal slices was enhanced by 30% in respect to controls (C) (rats implanted with electrodes but not stimulated) at all frequencies and decreased by 20% in DG, These differences were restricted to the LH and were not statistically significant. The stimulatory effect of NE (I-IOOO~M) on PI turnover in slices from both hippocampi at stage 5 was significantly enhanced by 70% (p
141 ENDOGENOUS GLUTAMYL PEPTIDES AND EXTRACELLULAR MAGNESIUM MODULATE GLUTAMATERGIC NEUROTRANSMISSION V. Varga, I. Holopalnen, R. JanHky, K.E.O. ~kerman, PirJo Saransaari and S.S. OJa, Tampere Brain Research Center, Department of Biomedical Sciences, University of Tampere, and Department of Biochemistry and Pharmacy, ~bo Akademi, Turku, Finland. Glutamate acts at different receptor subtypes gating channels penetrable to mono- and divalent cations. The mechanism and regulation of cation fluxes are not fully understood. The NMDAmediated responses show voltage-dependent Mg block. Glutamate, kalnate and NMDA effects are also antagonized by D-glutamyl and aspartyl peptides. We now further analyzed effects of Mg and certain glutamyl peptldes. Glutamate and its agonists depolarized cultured cerebellar granule cells and increased dose-dependently the influx and intracellular levels of free calcium. The glutamate-, NMDA- and quisqualate-stimulated influx of Ca was inhibited by extracellular Mg but not the influx evoked by kainate. Several peptides also inhibited stimulated Ca influx and opposed elevation of intracellular free Ca. y-L-Glutamylglutamate and -aspartate were the most effective ones in normal and low-Ms (0.i mM) media, respectively. The NMDA-activated influx of Ca was potentiated by 0.O1 mM glycine only in Mg-free medium. Although inhibiting the influx of Ca, y-L-glutamylaspartate and -taurine tended to increase the potassium- and kainate-evoked release of glycine and taurine, possibly through interaction with NMDA receptors. Both basal and evoked release of glutamate, D-aspartate, taurine and glycine were slightly enhanced in low-Ms medium. Our data bespeak Mg-modulated interaction of endogenous glutamyl peptides with glutamatergic neurotransmission (Supported by the Emll Aaltonen Foundation). 142 MAGNESIUM POTENTIATES OUISOUALATE EFFECTS ON THE VESTIBULAR SYSTEM AFFERENTS_ Rosario Vega, Enrique Soto and Ma. E u g e n i a Perez. Universidad Aut6noma de P u e b l a - ICUAP, P.O. B o x 406, P u e b l a , Pue, M e x i c o . S t u d i e s of M g ÷÷ e f f e c t s o n t h e r e s p o n s e e v o k e d by e x c i t a t o r y a m i n o a c i d s (EAA), have shown that non-NMDA r e c e p t o r c u r r e n t s a r e not a f f e c t e d by M g ÷+. H o w e v e r , studying the e f f e c t s of M g ÷÷ u p o n v e s t i b u l a r s y s t e m p r i m a r y a f f e r e n t r e s p o n s e s e l i c i t e d by EAA agonists, we have found t h a t extracellular M g ÷+ exerts a significant influence upon OA evoked responses. The electrical a c t i v i t y of the semicircular canal vestibular system afferents of t h e axolotl (Ambystoma tigrinum) w e r e r e c o r d e d e x t r a c e l l u l a r l y . D r u g s w e r e ejected from a p i p e t t e in v o l u m e s of 20 ul in a lO ml bath. In t h e p r e s e n c e of a Ringer with normal M g ÷÷ c o n c e n t r a t i o n (I mM), K A (0.I uM to 0.I mM, N = 49) a n d OA (0.01 u M to 0.I mM, N = 33) induced a monotonically increasing dose d e p e n d e n t e x c i t a t o r y e f f e c t . H o w e v e r , w h e n a M g ÷+ f r e e R i n g e r s o l u t i o n w a s u s e d in t h e p e r f u s i o n bath, t h e d o s e r e s p o n s e c u r v e for O A (N = 79) w a s n o n - l i n e a r , with a significant reduction of t h e maximum effect, a n d t h e a p p a r i t i o n of a plateau between 1 and i0 uM. In c o n t r a s t t h e K A (N = 25) d o s e r e s p o n s e c u r v e was unchanged. These results indicate that e x t r a c e l l u l a r M g ÷÷ concentration potentiates the excitatory effects of OA upon t h e v e s t i b u l a r s y s t e m a f f e r e n t f i b e r s . T h i s a c t i o n of M g ÷" is t h e o p p o s i t e to t h a t r e p o r t e d f o r N M D A r e s p o n s e s a n d its m e c h a n i s m r e m a i n s to be e x p l a i n e d .
143 MODULATION OF THE PRESYNAPTIC AND POSTSYNAPTIC COMPONENTS OF NORADRENERGIC TRANSMISSION BY KINDLING. A. Vezzani, R. Serafini, C. Bendotti, M. Rizzi, G. Vigan8 and R. Samanin, Istituto di R~cerche Farmacologiche "MARIO NEGRI", Milano, 20157 ITALY. It has been shown that norepinephrine (NE) stimulates glutamate (GLu) release in the dentate gyrus (DG) and enhances Glu-induced excitation on CA1 cells through B-adrenoceptors. Inhibitory actions of NE (~l-mediated) have also been reported in the hippocampus. We have investigated whether during kindling development modifications in noradrenergic neurotransmission occurred associated with up-regulation of the NMDA-receptors. Rats were kindled in the dorsal left hippocampus (LH) and sacrificed 48 h after the 12th stimulation (stage 2) or one week after the last stage 5 seizures. Following field electrical stimulation (2-10 Hz for 2 min) at stage 5, NE release from hippocampal slices was enhanced by 30% in respect to controls (C) (rats implanted with electrodes but not stimulated) at all frequencies and decreased by 20% in DG, These differences were restricted to the LH and were not statistically significant. The stimulatory effect of NE (I-IOOO~M) on PI turnover in slices from both hippocampi at stage 5 was significantly enhanced by 70% (p