Amiloride Enhances Neuronal Excitability of CA1 Pyramidal Neurons in The Anesthetized Rat

Objective: We studied the effects of focal amiloride application on the synaptic function and excitability in rat hippocampal CA1 region, in vivo. Materials and Methods: Field potentials evoked by 0.1Hz fimbrial/commissural stimulation were recorded from hippocampal CA1 stratum pyramidale in rats under urethane anesthesia. Results: Pneumatic application of amiloride (1 mM, 5-15psi, 3s) to str. pyramidale did not change population spike amplitudes but induced multiple population spikes. Amiloride decreased the slope of excitatory postsynaptic potential by 37%, lowered the population spike threshold by 17% and shifted the input/output curve to the left. Amiloride inhibited paired-pulse depression, induced by twin pulse stimulation at 0.1Hz, over interpulse intervals of 10 to 100ms and at a fixed-pulse interval (20ms) of varying stimulus intensity. Brief tetanic stimulation of the fimbria/commissure (100Hz, 70ms) induced a negative wave with 108 +/- 20ms duration and 2.3 +/- 0.1mV amplitude. This wave was followed by irregular discharges of 18.5 +/- 1.6Hz and 2.7 +/- 0.4mV and lasted for 200-300ms. Amiloride increased posttetanic negative wave amplitude and discharge frequency. Conclusion: Our results demonstrate that, amiloride increases postsynaptic excitability in rat hippocampus with a concomitant reduction in neurotransmitter release. These effects of amiloride can be explained by Na+/Ca2+ exchanger and Ca2+ channel inhibition, which results in reduced excitatory neurotransmitter release with more prominent reduction in inhibitory effectiveness leading to hyperexcitability. Additionally, our results also implicate that amiloride and its derivates with neurotransmission enhancing capabilities can be used to improve cognitive functions in dementia resulting from vascular dysfunction of the white matter secondary to diabetes or hypertension.