Dendritic glutamate receptor channels in rat hippocampal CA3 and CA1 pyramidal neurons.

Abstract

1. Properties of dendritic glutamate receptor (GluR) channels were investigated using fast application of glutamate to outside‐out membrane patches isolated from the apical dendrites of CA3 and CA1 pyramidal neurons in rat hippocampal slices. CA3 patches were formed (15‐76 microns from the soma) in the region of mossy fibre (MF) synapses, and CA1 patches (25‐174 microns from the soma) in the region of Schaffer collateral (SC) innervation. 2. Dual‐component responses consisting of a rapidly rising and decaying component followed by a second, substantially slower, component were elicited by 1 ms pulses of 1 mM glutamate in the presence of 10 microM glycine and absence of external Mg2+. The fast component was selectively blocked by 2‐5 microM 6‐cyano‐7‐nitroquinoxaline‐2,3‐dione (CNQX) and the slow component by 30 microM D‐2‐amino‐5‐phosphonopentanoic acid (D‐AP5), suggesting that the fast and slow components were mediated by the GluR channels of the L‐alpha‐amino‐3‐hydroxy‐5‐methyl‐4‐isoxazolepropionate (AMPA) and NMDA type, respectively. The peak amplitude ratio of the NMDA to AMPA receptor‐mediated components varied between 0.03 and 0.62 in patches from both CA3 and CA1 dendrites. Patches lacking either component were rarely observed. 3. The peak current‐voltage (I‐V) relationship of the fast component was almost linear, whereas the I‐V relationship of the slow component showed a region of negative slope in the presence of 1 mM external Mg2+. The reversal potential for both components was close to 0 mV. 4. Kainate‐preferring GluR channels did not contribute appreciably to the response to glutamate. The responses to 100 ms pulses of 1 mM glutamate were mimicked by application of 1 mM AMPA, whereas 1 mM kainate produced much smaller, weakly desensitizing currents. This suggests that the fast component is primarily mediated by the action of glutamate on AMPA‐preferring receptors. 5. The mean elementary conductance of AMPA receptor channels was about 10 pS, as estimated by non‐stationary fluctuation analysis. The permeability of these channels to Ca2+ was low (approximately 5% of the permeability to Cs+). 6. The elementary conductance of NMDA receptor channels was larger, with a main conductance state of about 45 pS. These channels were 3.6 times more permeable to Ca2+ than to Cs+.(ABSTRACT TRUNCATED AT 400 WORDS)