European Journal zyxwvutsrqp of Neuroscience, Vol. 4, pp. 183 zyxwvuts - zyxw I88 zyxwvut 0 1992 European Neuroscience Association zy Ca2+-MediatedPlateau Potentials in a Subpopulation of lnterneurons in the Ventral Horn of the Turtle Spinal Cord J. Hounsgaard and 0. Kjaerulff Institute of Neurophysiology, Panum Institute, Blegdamsvej 3C, University of Copenhagen, DK-2200 Copenhagen N., Denmark zy Key words: spinal networks, slice preparation Abstract The response properties of interneurons in the ventral horn were studied in transverse slices of segments D8 to S2 from the turtle spinal cord, using the current clamp technique. In about half of the neurons the response properties were dominated by their ability to generate plateau potentials. In these cells the plateau potential could account for delayed onset of spiking and a phase of increasing spike frequency during depolarizing current pulses and for a depolarizing afterpotential following the stimulus. The cells usually received monosynaptic and polysynaptic input from the ipsilateral dorsal root and occasionally from the contralateral root. The plateau potential was insensitive to tetrodotoxin but blocked by nifedipine and by replacing Ca2+ with Co2+ in the medium. It is concluded that the response properties of neurons in the ventral horn outside the motor nucleus have differentiated response properties that may well contribute to spinal motor function. Introduction zyxwvutsrqpo In vertebrates the intrinsic functional capabilities of the spinal motor system have been thought to reside predominantly with the wiring pattern among excitatory and inhibitory ventral horn neurons (Baldissera er al., 1981; Grillner, 1981). At least for motoneurons, however, it is increasingly clear that voltage-sensitive ion channels in the somatic and dendritic membrane also contribute to the formation of spike patterns (Barrett and Barrett, 1976; Schwindt and Crill, 1984; Hounsgaard et al., 1988a) and constitute a potential target for synaptic modulation (Hounsgaard et al., 1988b; Hounsgaard and Kiehn, 1989; Kiehn, 1991). It seems of interest to explore in what form and to what extent postsynaptic response properties contribute to synaptic processing in other cell types in the spinal motor system. First, such properties are known to be of extreme importance in central pattern generators in invertebrates (Harris-Warrick, 1989; Getting, 1989). Secondly, active response properties have been demonstrated in neurons throughout the vertebrate brain (Jahnsen, 1986) and are thought to be of specific functional importance (LlinBs, 1988; Midtgaard and Hounsgaard, 1989). In the present paper we identify a subset of neurons with complex response properties in the proximal part of the ventral horn in the lumbar spinal cord of the turtle. These properties include delayed onset of spiking and initially accelerating firing frequency during depolarizing current pulses, as well as afterdischarge and overt bistability. We find that a nifedipine-sensitive Ca2+-mediated plateau potential is the main factor underlying these properties. The present findings suggest that active postsynaptic response properties, in addition to their role in motoneurons, contribute to the function of spinal networks in general. The results have been presented in an abstract (Kjaerulff and Hounsgaard, 1991). Materials and methods The procedure to obtain and use transverse slice preparations from the turtle spinal cord zyxwv has been described in detail elsewhere (Hounsgaard et af., 1988a; Hounsgaard and Nicholson, 1990). Turtles (Pseudemys scripra and Chrysemys picta, carapace length 10-20 cm) were anaesthetized with zyxwv 50 mg pentobarbitone injected intraperitoneally , and perfused with 0.5 1 normal medium transcardially, followed by isolation of segments zyxwvu Dg-S2 of the lumbar enlargement. Slices cut transversely to a thickness of - 3 mm were transferred to small vials, one for each segment. During dissection and at all later stages, the preparations were kept in medium of the following composition (mM): NaCI, 120; KCl, 5; NaHC03, 15; CaC12, 3; MgCI2, 3; glucose, 20; this solution was saturated with carbogen containing 2 % C02 and 98 % 02. For an experiment a slice was mounted on end in the recording chamber and superfused with medium at room temperature. The funiculi and the dorsal and ventral horns were clearly visible, and, aided by Correspondence to: J. Hounsgaard, as above Received 9 April 1991, revised 30 May 1991, accepted 10 October 1991