ISSN:
1432-1106
Keywords:
Calcium
;
Dentate gyrus
;
Hippocampus
;
Ion-selective microelectrodes
;
Long-term potentiation
;
N-methyl-D-aspartate
;
Norepinephrine
;
Plasticity
;
Quisqualate
Source:
Springer Online Journal Archives 1860-2000
Topics:
Medicine
Notes:
Summary Mechanisms of action of norepinephrine (NE) on dentate gyrus granule cells were studied in rat hippocampal slices using extra- and intracellular recordings and measurements of stimulus and amino acid-induced changes in extracellular Ca2+ and K+ concentration. Bath application of NE (10–50 μM) induced long-lasting potentiation of perforant path evoked potentials, and markedly enhanced high-frequency stimulus-induced Ca2+ influx and K+ efflux, actions blocked by β-receptor antagonists and mimicked by β agonists. Enhanced Ca2+ influx was primarily postsynaptic, since presynaptic Δ [Ca2+]0 in the stratum moleculare synaptic field was not altered by NE. Interestingly, the potentiation of both ionic fluxes and evoked population potentials were antagonized by the N-methyl-D-aspartate (NMDA) receptor antagonist 2-amino-5-phosphonovalerate (APV). Furthermore, NE selectively enhanced the Δ[Ca2+]0, Δ[K+]0 and extracellular slow negative field potentials elicited by iontophoretically applied NMDA, but not those induced by the excitatory amino acid quisqualate. These results suggest that granule cell influx of Ca2+ through NMDA ionophores is enhanced by NE via β-receptor activation. In intracellular recordings, NE depolarized granule cells (4.8±1.1 mV), and increased input resistance (RN) by 34±6.5%. These actions were also blocked by either the β-antagonist propranolol or specific β 1-blocker metoprolol. Moreover, the depolarization and RN increase persisted for long periods (93±12 min) after NE washout. In contrast, while NE, in the presence of APV, still depolarized granule cells and increased RN, APV made these actions quickly reversible upon NE washout (16±9 min). This suggested that NE induction of long-term, but not short-term, plasticity in the dentate gyrus requires NMDA receptor activation. NE may be enhancing granule cell firing by some combination of blockade on the late Ca2+-activated K+ conductance and depolarization of granule cells, both actions that can bring granule cells into a voltage range where NMDA receptors are more easily activated. Furthermore, NE also elicited activity-independent long-lasting depolarization and RN increases, which required functional NMDA receptors to persist.
Type of Medium:
Electronic Resource
URL:
http://dx.doi.org/10.1007/BF00249605
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