Dendritic Ca²⁺ accumulations and metabotropic glutamate receptor activation associated with an n‐methyl‐d‐aspartate receptor‐independent long‐term potentiation in hippocampal CA1 neurons

Abstract

Bathing hippocampal slices in the potassium channel blocker tetraethylammonium (TEA), while stimulating the Schaffer collaterals at a low frequency, induces Ca(2+)-dependent, N-methyl-D-aspartate (NMDA) receptor-independent long-term potentiation of synaptic transmission (LTPk) in CA1 neurons. We have combined ratio imaging of fura-2 and mag-fura-5 in hippocampal CA1 neurons with intracellular and field recordings to evaluate postsynaptic Ca2+ changes that occur in the induction of LTPk. Test stimuli were applied at 0.05 Hz to stratum radiatum in the presence of the NMDA receptor antagonists D,L-2-amino-5-phosphonovaleric acid (100 microM) or MK-801 (10 microM). During TEA exposure (15-25 mM; 10 min), cells fired prolonged action potentials both spontaneously and in response to test stimuli resulting in transient, micromolar Ca2+ accumulations in both somata and dendrites. The initial EPSP slope, measured 60 min after TEA wash-out, was potentiated to approximately 200% of control. The Ca2+ channel blocker nimodipine (10 microM) greatly reduced Ca2+ transients in both magnitude and duration and prevented LTPk induction. Pretreatment of slices with compounds that block metabotropic glutamate receptor (mGluR)-stimulated phosphoinositide hydrolysis, L-2-amino-3-phosphonopropionic acid (L-AP3, 50-200 microM) or L-aspartate-beta-hydroxamate (50-100 microM), as well as protein kinase C (PKC) inhibitors (sphingosine, 20 microM; RO-31-8220, 0.2 microM; or calphostin C, 2 microM) also blocked LTPk. Ca2+ transients were unaffected by L-AP3 or RO-31-8220. These findings suggest that Ca2+ influx through voltage-gated channels and co-activation of PKC by mGluRs are both necessary for induction of LTPk. Activation of mGluRs must also occur in NMDA receptor-dependent induction paradigms, but is possibly of lesser importance owing to the much greater gating of Ca2+ directly into the dendritic spines.

Localized Loss of Ca2+Homeostasis in Neuronal Dendrites Is a Downstream Consequence of Metabolic Compromise during Extended NMDA Exposures

“…To further address this point, we also tested the effects of doubling the incubation time (to 40 min) and found that the characteristics of propagating dendritic Ca 2ϩ signals (described below) were unchanged, still occurring with mean delay of 35-40 min and propagating from apical dendrites. We have made estimates of indicator concentration at the soma (based on fluorescence intensity comparisons with neurons extensively dialyzed with known indicator concentrations) and conclude a concentration ϳ200 M, which is consistent with previous reports (Petrozzino and Connor, 1994). Ca 2ϩ and Na ϩ measurements.…”

A Long-Lasting Calcium-Activated Nonselective Cationic Current Is Generated by Synaptic Stimulation or Exogenous Activation of Group I Metabotropic Glutamate Receptors in CA1 Pyramidal Neurons

“…Changes of [Ca 2ϩ ] i were monitored in the soma of pyramidal neurons with a confocal laser scanning microscope simultaneously to the recording of I AC PD (performed in the presence of the superfusing medium A). As previously described in cultured hippocampal neurons and CA1 pyramidal cells (Mayer and Miller, 1990;Frenguelli et al, 1993;Jaffe and Brown, 1994;Petrozzino and Connor, 1994;Shirasaki et al, 1994), 1S,3R-ACPD induced a significant rise of [Ca 2ϩ ] i of 49 Ϯ 14% (V H ϭ Ϫ60 mV; n ϭ 4, p Ͻ 0.05) (Fig. 2).…”

Long-term potentiation and evidence for novel synaptic association in CA1 stratum oriens of rat hippocampus.

“…Second, LTP induced by intracellular Ca 2 + release from a photosensitive caged compound develops slowly to test stimulations (Malenka et al 1988). Third, bath application of the K + channel blocker tetraethylammonium (TEA) induces slowly developing NMDA-independent LTP (Aniksztejn and Ben-Ari 1991;Petrozzino and Connor 1994). Induction of this LTP requires mGluR activation and synaptic input (Petrozzino and Connor 1994).…”

“…Third, bath application of the K + channel blocker tetraethylammonium (TEA) induces slowly developing NMDA-independent LTP (Aniksztejn and Ben-Ari 1991;Petrozzino and Connor 1994). Induction of this LTP requires mGluR activation and synaptic input (Petrozzino and Connor 1994). Fourth, repeated postsynaptic depolarizing pulses produce LTP in the presence of APV if the pulses are applied during 2-Hz synaptic stimulations (KuUmann et al 1992).…”