adenosine-3--5--cyclic-phosphorothioate and alpha-methyl-4-carboxyphenylglycine

Antagonists at presynaptic muscarinic autoreceptors increase endogenous acetylcholine (ACh) release and enhance cognition but little is known regarding their actions on plasticity at glutamatergic synapses. Here the mechanisms of the persistent enhancement of hippocampal excitatory transmission induced by the M2/M4 muscarinic ACh receptor antagonist methoctramine were investigated in vivo. The persistent facilitatory effect of i.c.v. methoctramine in the CA1 region of urethane-anesthetized rats was mimicked by gallamine, an M2 receptor antagonist, supporting a role for this receptor subtype. Neither the N-methyl-D-aspartate (NMDA) receptor antagonists D-(-)-2-amino phosphonopentanoic acid (d-AP5) and memantine, nor the metabotropic glutamate receptor subtype 1a antagonist (S)-(+)-alpha-amino-4-carboxy-2-methylbenzeneacetic acid (LY367385) significantly affected the methoctramine-induced persistent synaptic enhancement, indicating a lack of requirement for these glutamate receptors. The selective kinase inhibitors Rp-adenosine-3', 5'-cyclic monophosphorothioate (Rp-cAMPS) and the myrostylated pseudosubstrate peptide, Myr-Ser-Ile-Tyr-Arg-Arg-Gly-Ala-Arg-Arg-Trp-Arg-Lys-Leu-OH (ZIP), were used to investigate the roles of protein kinase A (PKA) and the atypical protein kinase C, protein kinase Mzeta (PKM zeta), respectively. Remarkably, pretreatment with either agent prevented the induction of the persistent synaptic enhancement by methoctramine and post-methoctramine treatment with Rp-cAMPS transiently reversed the enhancement. These findings are strong evidence that antagonism of M2 muscarinic ACh receptors in vivo induces an NMDA receptor-independent persistent synaptic enhancement that requires activation of both PKA and PKM zeta.

Topics: Animals; Benzoates; Cyclic AMP; Cyclic AMP-Dependent Protein Kinases; Data Interpretation, Statistical; Diamines; Electrophysiology; Excitatory Amino Acid Antagonists; Glycine; Hippocampus; In Vitro Techniques; Injections, Intraventricular; Long-Term Potentiation; Male; Parasympatholytics; Protein Kinase C; Rats; Rats, Wistar; Receptor, Muscarinic M2; Receptor, Muscarinic M4; Receptors, G-Protein-Coupled; Receptors, Metabotropic Glutamate; Receptors, Muscarinic; Receptors, N-Methyl-D-Aspartate; Synapses; Synaptic Transmission; Thionucleotides

The possibility that protein kinase C (PKC) is involved in the induction of N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP) at CA1 synapses in the hippocampus has been the subject of considerable investigation. However, many of the conclusions have been drawn from the use of relatively nonspecific PKC inhibitors. In the present study we have examined the role of PKC in tetanus-induced LTP of AMPA receptor-mediated synaptic transmission in hippocampal slices obtained from adult rats. In particular, we have investigated the possible role of PKC in a molecular switch process that is triggered by the synaptic activation of metabotropic glutamate receptors and regulates the induction of LTP. We find that the three PKC inhibitors examined, chelerythrine, Ro-31-8220 and Gö 6983, all block the setting of the molecular switch at concentrations consistent with inhibition of PKC. In contrast, these inhibitors are without affect on the induction of LTP, even when applied in very much higher concentrations. A PKA inhibitor, Rp-cAMPS, had no effect on either process. We suggest that neither PKC nor PKA is required to induce LTP at this synapse. However, PKC is involved in the regulation of LTP induction, via the molecular switch process.

Topics: Alkaloids; Animals; Benzoates; Benzophenanthridines; Cyclic AMP; Electric Stimulation; Excitatory Amino Acid Antagonists; Female; Glycine; Hippocampus; In Vitro Techniques; Indoles; Long-Term Potentiation; Neuronal Plasticity; Phenanthridines; Protein Kinase C; Rats; Rats, Wistar; Receptors, AMPA; Receptors, N-Methyl-D-Aspartate; Synapses; Synaptic Transmission; Thionucleotides

Neurons of the avian cochlear nucleus, nucleus magnocellularis (NM), are activated by glutamate released from auditory nerve terminals. If this stimulation is removed, the intracellular calcium ion concentration ([Ca2+]i) of NM neurons rises and rapid atrophic changes ensue. We have been investigating mechanisms that regulate [Ca2+]i in these neurons based on the hypothesis that loss of Ca2+ homeostasis causes the cascade of cellular changes that results in neuronal atrophy and death. In the present study, video-enhanced fluorometry was used to monitor changes in [Ca2+]i stimulated by agents that mobilize Ca2+ from intracellular stores and to study the modulation of these responses by glutamate. Homobromoibotenic acid (HBI) was used to stimulate inositol trisphosphate (IP3)-sensitive stores, and caffeine was used to mobilize Ca2+ from Ca2+-induced Ca2+ release (CICR) stores. We provide data indicating that Ca2+ responses attributable to IP3- and CICR-sensitive stores are inhibited by glutamate, acting via a metabotropic glutamate receptor (mGluR). We also show that activation of C-kinase by a phorbol ester will reduce HBI-stimulated calcium responses. Although the protein kinase A accumulator, Sp-cAMPs, did not have an effect on HBI-induced responses. CICR-stimulated responses were not consistently attenuated by either the phorbol ester or the Sp-cAMPs. We have previously shown that glutamate attenuates voltage-dependent changes in [Ca2+]i. Coupled with the present findings, this suggests that in these neurons mGluRs serve to limit fluctuations in intracellular Ca2+ rather than increase [Ca2+]i. This system may play a role in protecting highly active neurons from calcium toxicity resulting in apoptosis.

Topics: Adenylyl Cyclase Inhibitors; Adenylyl Cyclases; Alanine; Animals; Benzoates; Caffeine; Calcium; Calcium Channel Blockers; Calcium Channels; Calcium Signaling; Chelating Agents; Chick Embryo; Cochlear Nucleus; Cyclic AMP; Cycloleucine; Cysteine; Egtazic Acid; Enzyme Inhibitors; Excitatory Amino Acid Agonists; Excitatory Amino Acid Antagonists; Fluorescent Dyes; Fura-2; Gallic Acid; Glutamic Acid; Glycine; Ibotenic Acid; Inositol 1,4,5-Trisphosphate Receptors; Ion Channel Gating; Neurons; Neuroprotective Agents; Neurotransmitter Agents; Patch-Clamp Techniques; Phosphodiesterase Inhibitors; Receptors, Cytoplasmic and Nuclear; Ryanodine; Second Messenger Systems; Thionucleotides