Proceedings of The Physiological Society
University of Manchester (2010) Proc Physiol Soc 19, C114
Nitric oxide is an activity-dependent homeostatic regulator of neuronal intrinsic excitability
J. R. Steinert1, H. Tong1, M. D. Haustein1, I. D. Forsythe1
1. MRC Toxicology Unit, Leicester, United Kingdom.
Voltage-gated potassium channels (Kv) are widely expressed in the central nervous system and serve to regulate excitability, action potential (AP) waveform and firing rates (Rudy & McBain, 2001). Homeostatic mechanisms act to tune neuronal ion channel expression to network activity, thereby refining synaptic integration and AP output. It is increasingly apparent that glutamatergic signalling modulates intrinsic excitability of target neurons through nitrergic signalling in the auditory brainstem (Steinert et al., 2008). We exploited glutamatergic signalling at the auditory brainstem and hippocampus to examine effects of nitric oxide (NO) on K+ channels in vitro. CBA/Ca mice (P12-P16) were killed by decapitation in accordance to the UK Animals (Scientific Procedures) Act 1986. Brain slices were prepared (Steinert et al., 2008), whole cell patch recordings performed at 36°C from CA3 pyramidal and principal neurons of the medial nucleus of the trapezoid body (MNTB) and postsynaptic APs were elicited by stimulation using a bipolar electrode. Prolonged synaptic stimulation (1hr, 10Hz Poisson-distributed inter-stimulus interval) reduced postsynaptic excitability and shortened AP waveforms, so improving AP fidelity during synaptic trains in both CA3 and MNTB neurons. This was accompanied by AMPAR- or NMDAR-mediated potentiation of postsynaptic high voltage-activated K+ channels in MNTB (Ctrl:23±1nA vs 59±4nA* at 50mV) and CA3 (Ctrl:21±3nA vs 37±3nA* at 50mV), respectively which was suppressed by nNOS inhibition (10μM 7- nitroindazole) and mimicked by NO donors (1hr, 100μM sodium nitroprusside or PapaNONOate; 48±3nA* at 50mV). Potentiated currents in both brain regions were TEA (1mM) insensitive but blocked by r-stromatoxin-1 (300nM) suggesting a suppression of Kv3 currents while Kv2 conductances were potentiated. The absence of this modulation in the MNTB from nNOS or Kv2.2 KO mice confirmed the signaling pathway. If this signaling is relevant to in vivo conditions, then we would predict increased Kv currents in neurons recorded at short intervals after animal sacrifice. At the shortest interval (within 15min) Kv currents were up to 4-fold larger than those in neurons from control slices and decayed rapidly (τ=15min) to resting ‘control’ levels. Simultaneously, Na+ currents increased in the same neurons with a similar time course. Prolonged synaptic activity improved fidelity due to induction of a Kv2.2-mediated inter-AP hyperpolarisation (7.9±1.3mV*) which is absent in nNOS and Kv2.2 KO mice, suggesting a physiological role of this activity-driven NO-dependent homeostatic control of postsynaptic excitability by tuning K+ conductances in different brain regions to complement other forms of neuronal plasticity such as synaptic scaling. Data are mean±SEM. Significance was tested using two-tailed Student's t-test and differences considered significant at *p<0.05.
Where applicable, experiments conform with Society ethical requirements