2 TYPES OF KAINATE RESPONSE IN CULTURED RAT HIPPOCAMPAL-NEURONS

1. Two different types of kainate response were recorded in cultured rat hippocampal neurons with the use of the whole-cell and outside-out configurations of the patch-clamp technique. 2. There was an outward rectification in the current-voltage (I-V) plot of the kainate-induced current (type I response) in relatively large neurons bearing a morphological resemblance to young pyramidal cells. In smaller neurons with, elliptical somata and fine neurites, the kainate response was characterized by a remarkable inward rectification in the I-V plot of the kainate-induced current and a significant permeability to Ca2+ (type II response). 3. Both type I and type II responses were negligible below 2-mu-M and almost saturated at 500-mu-M kainate. The concentrations producing half-maximal responses and the Hill coefficients were 68 mu-M and 1.76 and 56-mu-M and 1.21 for type I and type II responses, respectively. Both responses were suppressed similarly by the non-N-methyl-D-aspartate (NMDA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX). 4. The mean single-channel conductance (gamma) of the type II kainate response was estimated, from the relation between the whole-cell mean currents and current variances, to be 8.7 pS. The power spectrum for the current noise was fitted with the sum of two Lorentzians with cutoff frequencies (fc) of 61.1 +/- 1.4 and 327.8 +/- 10.5 Hz (n = 12). In the type I kainate response, gamma was only 2.2 pS, and the power spectrum was fitted with the sum of two Lorentzians with fc values of 30.5 +/- 0.9 and 168.4 +/- 2.5 Hz (n = 7). 5. In outside-out patches excised from neurons showing the type 11 response, kainate evoked directly resolvable single-channel events of which conductances ranged between 4 and 36 pS, whereas in patches from neurons showing the type I response, it produced noise currents that were accompanied by few discrete single-channel currents.