Properties of interneurones in the intermediolateral cell column of the rat spinal cord: Role of the potassium channel subunit Kv3.1

Sympathetic preganglionic neurones located in the intemediolateral cell col umn (IML) are subject to inputs descending from higher brain regions, as we ll as strong influences from local interneurones. Since interneurones in th e IML have been rarely studied directly we examined their electrophysiologi cal and anatomical properties. Whole cell patch clamp recordings were made from neurones in the IML of 250 mum slices of the thoracic spinal cord of t he rat at room temperature. Action potential durations of interneurones (4. 2 +/- 0.1 ms) were strikingly shorter than those of sympathetic preganglion ic neurones (9.4 +/- 0.2 ms) due to a more rapid repolarisation phase. Low concentrations of tetraethylammonium. chloride (TEA) (0.5 mM) or 4-aminopyr idine (4-AP) (30 muM) affected interneurones but not sympathetic preganglio nic neurones by prolonging the action potential repolarisation as well as d ecreasing both the afterhypolarisation amplitude and firing frequency. Foll owing recordings, neurones sensitive to TEA and 4-AP were confirmed histolo gically as interneurones with axons that ramified extensively in the spinal cord, including the IML and other autonomic regions. In contrast, all cell s that were insensitive to TEA and 4-AP were confirmed as sympathetic prega nglionic, neurones. Both electrophysiological and morphological data are th erefore consistent with the presence of the voltage-gated potassium channel subunit Kv3.I in interneurones but not sympathetic preganglionic neurones. Testing this proposal immunohistochemically revealed that Kv3.1b was local ised in low numbers of neurones within the IML but in higher numbers of neu rones on the periphery of the IML. Kv3.1b-expressing neurones were not symp athetic preganglionic neurones since they were not retrogradely labelled fo llowing intraperitoneal injections of Fluorogold. Since Kv3.2 plays a simil ar role to Kv3.I we also tested for the presence of Kv3.2 using immunohisto chemistry, but failed to detect it in neuronal somata in the spinal cord. T hese studies provide electrophysiological and morphological data on interne urones in the IML and indicate that the channels containing the Kv3.1 subun it are important in setting the firing pattern of these neurones. (C) 2001 IBRO. Published by Elsevier Science Ltd. All rights reserved.