Synaptic and extrasynaptic transmission of kidney-related neurons in the rostral ventrolateral medulla.

Gao H, Derbenev AV. J Neurophysiol. 2013 Dec;110(11):2637-47 In this paper, they wanted to look at the possibility of tonic excitatory and inhibitory currents in the rvlm that would regulate activity. To do this, they applied pseudorabies with a GFP reporter to the kidney cortex, then approximately 4 days later, obtained brainstem slices containing the RVLM from infected rats. They could then do whole cell recording on fluorescent cells, confident that they were examining kidney-related presympathetic neurons. What they found was that the cells exhibited spontaneous excitatory currents that were sensitive to AP-5 and CNQX, meaning that both NMDA and AMPA channels are spontaneously active. They also saw that glutamate mediates a persistent and tonic current when examining the cells under current-clamp. When they looked at GABA-A-mediated currents, they found that tonic and phasic currents were sensitive to different antagonists, with phasic inhibition being due mostly to gabazine sensitive currents, while tonic inhibition was due mostly to bicuculline-sensitive currents. The interesting part of this paper is that glutamate seems to be causing tonic excitation through ionotropic channels, an effect I haven't really heard of before. They don't know where it comes from or if it's synaptic or extrasynaptic receptors, but they know it's not from presynaptic release of glutamate because TTX (which blocks action potential-induced neurotransmitter release) didn't affect the excitatory currents. They pointed out the interesting possibility that this tonic excitation may be due to glial cell releasing glutamate on to extrasynaptic receptors, which is not what people usually think of when they think of glutamate excitation. -DH