AMPA receptor-mediated miniature EPSCs have heterogeneous time courses in orexin neurons

Glutamate plays a predominant role in regulating the activity of orexin neurons that coordinate motivated behaviors, sleep-wake cycle and autonomic functions. To gain more insight into the properties of excitatory transmission to orexin neurons, whole cell patch clamp recordings were made in rat brain slices and quantal analysis of pharmacologically isolated miniature excitatory postsynaptic currents (mEPSCs) was performed. In more than half the orexin neurons examined, mEPSCs showed heterogeneous time course: some mEPSCs had fast rise and decay (fast mEPSC), while some had longer kinetics, smaller amplitude but larger integrated area (slow mEPSC). Other orexin neurons showed low frequency mEPSCs with uniform, fast kinetics. In the former, distribution histogram of 10-90% rise time displayed two peaks, indicating that fast and slow mEPSCs are distinct subgroups. Occasionally fast and slow EPSCs would summate, suggesting that they arise from different pairs of active zones and postsynaptic receptor clusters. A large majority of mEPSCs were mediated by AMPA receptors that are sensitive to GYKI 52466 and DNQX. To determine whether synapses that give rise to fast and slow mEPSCs are differentially modulated, the D1- and D2-like agonists were tested on various parameters of mEPSCs. The agonists altered the frequency as previously reported, but had no effect on the rise, decay or area of mEPSC, suggesting that dopamine affects fast and slow mEPSCs equally. Given the potential physiological impact of EPSC time course on synaptic integration, our study raises an interesting possibility that distinct subset of excitatory synaptic inputs are processed differently by orexin neurons.