Hydrolysis of inositol phospholipids by phospholipase C is initiated by either receptor stimulation or opening of Ca2+ channels. This was once thought to be the sole mechanism to produce the diacylglycerol that links extracellular signals to intracellular events through activation of protein kinase C. It is becoming clear that agonist-induced hydrolysis of other membrane phospholipids, particularly choline phospholipids, by phospholipase D and phospholipase A2 may also take part in cell signaling. The products of hydrolysis of these phospholipids may enhance and prolong the activation of protein kinase C. Such prolonged activation of protein kinase C is essential for long-term cellular responses such as cell proliferation and differentiation.

https://science.sciencemag.org/content/sci/258/5082/607.full.pdf

Intracellular signaling by hydrolysis of phospholipids and activation of protein kinase C

The ionotropic glutamate receptors are ligand-gated ion channels that mediate the vast majority of excitatory neurotransmission in the brain. The cloning of cDNAs encoding glutamate receptor subunits, which occurred mainly between 1989 and 1992 ([Hollmann and Heinemann, 1994][1]), stimulated this

/pdf/the-glutamate-receptor-ion-channels-560mntczdb.pdf

The glutamate receptor ion channels

The application of molecular cloning technology to the study of the glutamate receptor system has led to an explosion of knowledge about the structure, expression, and function of this most important fast excitatory transmitter system in the mammalian brain. The first functional ionotropic glutamate receptor was cloned in 1989 (Hollmann et al 1989) , and the results of this molecular-based approach over the past three years are the focus of this review. We discuss the implications of and the new questions raised by this work-which is probably only a glance at this fascinating and complex signaling system found in brains from the snails to man. Glutamate receptors are found throughout the mammalian brain, where they constitute the major excitatory transmitter system. The longest-known and best-studied glutamate receptors are ligand-gated ion channels, also called ionotropic glutamate receptors , which are permeable to cations. They have traditionally been classified into three broad subtypes based upon pharmaco­ logical and electrophysiological data: a-amino-3-hydroxy-5-methyl-4isoxazole propionate (AMPA) receptors, kainate (KA) receptors , and N-methyl-D-aspartate (NMDA) receptors. Recently, however, a family of G protein-coupled glutamate receptors , which are also called metabotropic glutamate or transl -aminocyclopentanel ,3-dicarboxylate (tACPD) recep­ tors, was identified (Sugiyama et al 1987) . (For reviews of the classification and the pharmacological and electrophysiological properties of glutamate receptors see Mayer & Westbrook 1987, Collingridge & Lester 1989, Honore 1989, Monaghan et al 1989, Wroblewski & Danysz 1 989, Hansen &

Cloned Glutamate Receptors

Recent work has shown that the hippocampus contains a class of receptors for the excitatory amino acid glutamate that are activated by N-methyl-D-aspartate (NMDA) and that exhibit a peculiar dependency on membrane voltage in becoming active only on depolarization. Blockade of these sites with the drug aminophosphonovaleric acid (AP5) does not detectably affect synaptic transmission in the hippocampus, but prevents the induction of hippocampal long-term potentiation (LTP) following brief high-frequency stimulation. We now report that chronic intraventricular infusion of D,L-AP5 causes a selective impairment of place learning, which is highly sensitive to hippocampal damage, without affecting visual discrimination learning, which is not. The L-isomer of AP5 did not produce behavioural effects. AP5 treatment also suppressed LTP in vivo. These results suggest that NMDA receptors are involved in spatial learning, and add support to the hypothesis that LTP is involved in some, but not all, forms of learning.

Selective impairment of learning and blockade of long-term potentiation by an N -methyl-D-aspartate receptor antagonist, AP5

Developmental and regional expression in the rat brain and functional properties of four NMDA receptors.

The responses of vertebrate neurones to glutamate involve at least three receptor types. One of these, the NMDA receptor (so called because of its specific activation by N-methyl-D-aspartate), induces responses presenting a peculiar voltage sensitivity. Above resting potential, the current induced by a given dose of glutamate (or NMDA) increases when the cell is depolarized. This is contrary to what is observed at classical excitatory synapses, and recalls the properties of 'regenerative' systems like the Na+ conductance of the action potential. Indeed, recent studies of L-glutamate, L-aspartate and NMDA-induced currents have indicated that the current-voltage (I-V) relationship can show a region of 'negative conductance' and that the application of these agonists can lead to a regenerative depolarization. Furthermore, the NMDA response is greatly potentiated by reducing the extracellular Mg2+ concentration [( Mg2+]o) below the physiological level (approximately 1 mM). By analysing the responses of mouse central neurones to glutamate using the patch-clamp technique, we have now found a link between voltage sensitivity and Mg2+ sensitivity. In Mg2+-free solutions, L-glutamate, L-aspartate and NMDA open cation channels, the properties of which are voltage independent. In the presence of Mg2+, the single-channel currents measured at resting potential are chopped in bursts and the probability of opening of the channels is reduced. Both effects increase steeply with hyperpolarization, thereby accounting for the negative slope of the I-V relationship of the glutamate response. Thus, the voltage dependence of the NMDA receptor-linked conductance appears to be a consequence of the voltage dependence of the Mg2+ block and its interpretation does not require the implication of an intramembrane voltage-dependent 'gate'.

Magnesium gates glutamate-activated channels in mouse central neurones

Protein kinase C modulation of NMDA currents: an important link for LTP induction

1 Changes in intracellular Ca2+ concentration ([Ca2+]i) induced by activation of GABAA receptors (synaptic stimulation or application of the GABAA agonist isoguvacine) were studied on pyramidal cells and interneurons from hippocampal slices of rats from two age groups (postnatal days (P) 2-5 and P12-13) using the fluorescent dye fluo-3 and a confocal laser scanning microscope Cells were loaded with the dye either intracellularly, using patch pipettes containing fluo-3 in the internal solution, or extracellularly, using pressure pulses applied to an extracellular pipette containing the permeant dye fluo-3 AM 2 Interneurons and pyramidal cells from P2-5 slices loaded with fluo-3 AM responded by an increase in [Ca2+]i to isoguvacine and to glutamate, in contrast to cells from P12-13 slices which responded to glutamate but not to isoguvacine 3 The isoguvacine-induced rise in [Ca2+]i was reversibly blocked by bath application of the GABAA receptor antagonist bicuculline (20 microM), suggesting the specific involvement of GABAA receptors The sodium channel blocker tetrodotoxin (TTX, 1 microM in the bath) did not prevent the isoguvacine-induced rise in [Ca2+]i 4 The isoguvacine-induced rise in [Ca2+]i was reversibly blocked by bath application of the calcium channel blocker D600 (50 microM) suggesting the involvement of voltage-dependent Ca2+ channels 5 Electrical stimulation of afferent fibres induced a transient increase in [Ca2+]i in neonatal pyramidal cells and interneurons (P5) loaded non-invasively with fluo-3 AM This elevation of [Ca2+]i was reversibly blocked by bicuculline (20 microM) but not by APV (50 microM) and CNQX (10 microM) 6 During simultaneous electrophysiological recording in the current-clamp mode and [Ca2+]i monitoring from P5 pyramidal cells, electrical stimulation of afferent fibres, in the presence of APV (50 microM) and CNQX (10 microM), caused synaptic depolarization accompanied by a few action potentials and a transient increase in [Ca2+]i In voltage clamp (-70 mV) however, there was no increase in [Ca2+]i following synaptic stimulation, showing that it is depolarization dependent 7 Using a non-invasive method of [Ca2+]i monitoring, we demonstrate here that in neonatal (P2-5) hippocampus, GABA is an excitatory neurotransmitter which can cause an elevation of [Ca2+]i in interneurons and pyramidal cells via activation of voltage-dependent Ca2+ channels This action may underlie the trophic role of GABA in hippocampal development

https://physoc.onlinelibrary.wiley.com/doi/pdf/10.1113/jphysiol.1995.sp020882

Synaptic GABAA activation induces Ca2+ rise in pyramidal cells and interneurons from rat neonatal hippocampal slices.

1 The whole-cell and outside-out configurations of the patch-clamp method were used to investigate the properties of the channels activated by N-methyl-D-aspartate (NMDA channels) in mouse central neurones in culture Recording was made in Mg2+-free solutions 2 In the whole-cell recording mode the currents induced by both NMDA and L-glutamate were accompanied by a large increase in noise In both cases the noise power spectra were well fitted by single Lorentzian functions and the corresponding mean time constant, tau, was about 6 ms at room temperature The single-channel conductance, gamma n, estimated from the ratio of the noise variance to the total current, varied between 22 and 40 pS 3 Endogenous amino acids known to activate NMDA receptors (L-glutamate, L-aspartate, L-cysteine sulphinate and quinolinate) as well as exogenous NMDA agonists such as ibotenate and trans-2,3-piperidine dicarboxylate (trans-PDA) all produced similar responses In particular, analysis of the current noise yielded tau values between 4 and 8 ms in all cases 4 NMDA responses were antagonized by 2-amino-5-phosphonovalerate (APV) without any effect on gamma n or tau values measured by noise analysis; NMDA responses were also diminished by D-alpha-aminoadipate and cis-2,3-piperidine dicarboxylate 5 In outside-out patches, it was observed that the single-channel current amplitude varies linearly as a function of membrane potential between -80 and +60 mV The reversal potential is near 0 mV NMDA channels are permeable to Na+, K+ and Cs+, but blocked by choline The single-channel conductance, gamma e, varies between 40 and 50 pS at room temperature 6 The NMDA channels open in bursts of short openings interrupted by brief closures At -60 mV, the closures had a mean duration, tc, of 04 +/- 02 ms The mean channel open time, to, was 59 +/- 10 ms for NMDA and 53 +/- 17 ms for L-glutamate The mean burst duration, tb, was 105 +/- 07 ms for NMDA and 85 +/- 20 ms for L-glutamate 7 When the temperature was increased between 14 and 24 degrees C, the NMDA channel conductance increased with a Q10 of 16 while the mean open time decreased with a Q10 close to 2 8 The NMDA channel showed, in addition to the 'main' conductance state (40-50 pS), smaller conductance states of 15 and 35 pS(ABSTRACT TRUNCATED AT 400 WORDS)

Piotr Bregestovski

Papers

Magnesium gates glutamate-activated channels in mouse central neurones

Protein kinase C modulation of NMDA currents: an important link for LTP induction

Synaptic GABAA activation induces Ca2+ rise in pyramidal cells and interneurons from rat neonatal hippocampal slices.

N‐methyl‐D‐aspartate‐activated channels of mouse central neurones in magnesium‐free solutions.

Selective activation of quisqualate metabotropic receptor potentiates NMDA but not AMPA responses.