Proceedings of The Physiological Society
University of Oxford (2011) Proc Physiol Soc 23, PC294
Nitric Oxide (NO) Modulates the Excitability of Magnocellular Neurons of the Supraoptic Nucleus (SON) during changes in Plasma Osmolality
M. P. da Silva1, R. R. Ventura2, W. A. Varanda1
1. Physiology, University of Sao Paulo, Ribeirao Preto, Sao Paulo, Brazil. 2. Physiology, University of Parana, Curitiba, Parana, Brazil.
Magnocellular neurons of the supraoptic nucleus are responsible for the synthesis of both vasopressin (VP) and oxytocin (OT). Several studies have shown that nitric NO plays a neuromodulatory role on the excitability of these neurons, leading to inhibition of OT and VP release. This effect is related to increases in plasma osmolality, which also leads to an augmented expression in nitric oxide synthase. In this study, we investigated the role of this nitrergic modulation on the electrical properties of magnocellular neurons subjected to a hypertonic stimulus. We used the patch clamp technique to evaluate action potentials and ionic currents in coronal brain slices of Wistar rats containing the SON. Recordings were made in a control (C) situation (Artificial Cerebro Spinal Fluid (ACSF), 300 mosm/Kg.H2O), followed by a hypertonic stimulus (H) (ACSF plus 30 mM manitol, 330 mosm/Kg.H2O) and hypertonic stimulus plus 500 microM L-Arginine (H+LA) or 100 microM L-NAME (H+LN). Hypertonicity induces an increase in the firing frequency of the neurons which can be reversed by L-Arginine (C=3.4 ± 0.7 Hz; H=4.9 ± 0.6 Hz and H+LA=4.1 ± 0.5 Hz ; n=7, P<0.05). As expected L-NAME produced an additional increase in frequency (C=2.1 ± 0.6 Hz; H=3.1 ± 0.5 Hz and H+LN=3.5 ± 0.6 Hz; n= 7, P<0.05). L-Arginine also hyperpolarized the neurons (C=-57.3 ± 2.7, H=-56.8 ± 2.6 and H+LA=-61.2 ± 2.9 mV; n=6, P<0.05) and this effect was reversed by L-NAME (C= -60.3±1.8, H= -56.9 ± 2 and H+LN= -58.3 ± 2.1 mV; n=5, P<0.05). Changes in membrane potential were also observed in the Hyperpolarization After Potential (HAP), were H+LA induced an increase in its peak (C=-74.1 ± 1.7, H=-70.9 ± 1.9 and H+LA=-73 ± 1.7 mV; n=6, P<0.05). L-NAME produced the opposite response (C=-72.9 ± 3.1, H=-71.1 ± 3.6 and H+LN=-68.5 ± 4.1 mV; n=6, P<0.05). We further investigated whether these alterations were due to nitrergic effects on hyperpolarization activated cation currents (Ih). Our results showed that L-Arginine significantly decreased the steady state values of Ih (measured at -120 mV) in relation to control (C=-24.87 ± 1.38; LA= -12.39 ± 3.87 pA; n= 6, P<0.05). L-NAME induced an increase in Ih (C=-12.39 ± 3.87; LN=-26.59 ± 9.47 pA; n=6, P<0.05) as well as hypertonicity alone (C=-24.11 ± 10.34 vs H=-72.43 ± 13.58 pA). Curiously, 30 microM ODQ (a blocker of cGMP) increases Ih (from -18.7 ± 6.7 to - 44.5 ±4.6 pA; n=4, P<0.05) and ODQ plus L-Arginine decreased Ih to -15.4 ± 3.7 pA (n=4, P<0.05). In conclusion, our results showed that the excitability of magnocellular neurons can be modulated by NO during changes in plasma osmolality, and that this modulation is essentially determined by an effect on Ih currents.
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