The dorsal vagal nucleus (DVN), the nucleus of the solitary tract (NTS) and the area postrema (AP) are essential for diverse autonomic functions, including the processing of visceral satiety signals. We have analysed the distribution of KATP currents within these areas as a marker for intrinsic metabolic sensitivity of these neurones. Furthermore, we have correlated the presence of KATP channels with other electrical properties of vagal neurones.
Brainstem slices (200 µm) were obtained from 18- to 25-day-old Sprague-Dawley rats after terminal anaesthesia (Sagatal; 60 mg kg-1; I.P.) and transcardial perfusion with Na+-free solution. After recovery at 34 °C for 30 min, slices were kept in ACSF at room temperature. Whole cell recordings were established with borosilicate capillaries (2-4 MV) filled with (mM): 120 potassium gluconate, 1 NaCl, 1 MgCl2, 1 CaCl2, 10 Hepes, 10 EGTA, pH 7.2 and K2ATP.
Thirty-three per cent of DVN (n = 54), 54 % of NTS (n = 35) and all AP neurones (n = 8) possessed functional KATP channels, as examined with KATP channel openers (diazoxide (200 µM), NNC 55-9216 (100 µM)), or by tolbutamide (500 µM) block of spontaneous, or cyanide-induced (1 mM), KATP currents. These were observed in 46 % of all recordings independent of the pipette ATP concentration (0-5 mM). However, cells expressing KATP channels opened these spontaneously in 11 of 16 recordings with ATP-free pipette solution, in 2 of 19 with 1 mM ATP, and in no recordings with higher ATP concentrations.
NNC55-9216 is a specific opener for KATP channels containing SUR1, but not SUR2A or SUR2B (Dabrowski et al. 2002). KATP currents that were sensitive to diazoxide, but not to NNC 55-9216 were not observed. This suggests that most brainstem KATP channels are of the SUR1 type.
The majority of DVN and NTS neurones have slowly inactivating K+ currents. Seventy-five per cent of DVN and 56 % of NTS neurones without these currents also lacked KATP currents. In contrast, H-type inward currents were only observed in a small subpopulation of cells (10 out of 97), but 9 of these showed KATP currents. T-type Ca2+ currents were observed in 5 cells; 3 of these had KATP currents.
Whilst KATP channels are abundant in these regions, only 8 of 123 neurones tested opened KATP channels in response to short term hypoglycaemia.
In summary, KATP channels are abundant in parasympathetic neurones, but probably less so in efferent than in afferent neurones. Their presence alone is not indicative of glucose sensitivity. The results also suggest that the intracellular ATP concentration of these cells remains mainly governed by cell metabolism in whole cell recordings.
This work was supported by an MRC Career Development Award to S.T. and by Novo Nordisk A/S.