| Abstrakt: |
Miura, Masami, Masatomo Yoshioka, Hiroyoshi Miyakawa, Hiroshi Kato, and Ken-Ichi Ito.Properties of calcium spikes revealed during GABAAreceptor antagonism in hippocampal CA1 neurons from guinea pigs. J. Neurophysiol.78: 2269–2279, 1997. Intracellular electrical responses and changes in intracellular calcium concentration ([Ca2+]i) in response to activation of synaptic inputs and to DC injections were recorded simultaneously from CA1 pyramidal neurons (n= 42) in guinea pig hippocampal slices. In the presence of the γ-aminobutyric acid-A (GABAA) receptor antagonists, bicuculline (25 μM) and picrotoxin (10 μM), broad (>20 ms) all-or-none spikes were induced by activation of synaptic inputs (20 pulses, 30 Hz) and were accompanied by a simultaneous rapid and large rise in [Ca2+]i(34 of 34 cells). By contrast, direct depolarizing current (0.7 nA, 1 s) induced spikes having short duration, during which time the spike firing pattern was observed not to be significantly affected. When Na+channels were blocked by QX-314 applied intracellularly through the recording microelectrode in the presence of GABAAreceptor antagonists, broad spikes were frequently generated by activation of synaptic inputs (32 of 33 cells). These broad spikes were blocked by Cd2+(200 μM) or in Ca2+-free medium (6 of 6 cells) but were resistant to either tetrodotoxin (TTX; 1 μM; 6 of 6 cells) or QX-314, whereas short-duration spikes were blocked by both TTX andQX-314. Based on these findings we conclude that broad and short-duration spikes are Ca2+and Na+spikes, respectively. To investigate the properties of the Ca2+spikes, antagonists of a voltage-operated Ca2+channel were applied to the evoked responses. Nifedipine (30 μM), a L-type Ca2+channel blocker, suppressed the generation of Ca2+spikes, whereas Ni2+(100 μM), theT- and R-type Ca2+channel blocker, and ω-agatoxin-IVA (ω-Aga-IVA, 60 nM), a P-type Ca2+channel blocker, had little effect on the generation of Ca2+spikes. Nifedipine suppressed the rise in [Ca2+]iinduced by synaptic inputs up to 26% of the control in the soma and 18–32% in the dendrites (n= 5), respectively, whereas Ni2+suppressed the rise by 12–27% (n= 5) in both soma and dendrites. ω-Aga-IVA showed little effect (less than a 10% change; n= 7). These results suggest that the GABAAinhibitory system tonically suppresses dendritic Ca2+spikes, and the L-type Ca2+channel plays a major role in the generation of Ca2+spikes and in Ca2+influx. |
