5-HT modulates multiple conductances in immature rat rostral ventrolateral medulla neurones in vitro

Whole-cell patch-clamp recordings were made from rostral ventrolateral medulla (RVLM) neurones of brainstem slices from 8- to 12-day-old rats. In the presence of tetrodotoxin (0·5 μM), 5-HT (50 μM) elicited an outward current (I5-HT,outward) (10/44 neurones) associated with an increase in membrane conductance, and an inward current (I5-HT,inward) (29/44 neurones) accompanied by a decrease or no significant change in membrane conductance. The steady-state I-V relationship of I5-HT,outward showed an inward rectification; the 5-HT-induced current, which reversed at -87·9 ± 3·0 mV, was suppressed by 0·1 mM Ba2+. Two types of steady-state I-V relationship for I5-HT,inward were noted: type I I5-HT,inward was characterized by a significant decrease in membrane conductance and reversed at a potential close to or negative to the theoretical K+ equilibrium potential (EK), -94 mV, in 8/17 neurones; type II I5-HT,inward was not associated with a significant change in membrane conductance and was relatively independent of membrane potential. Both type I and type II I5-HT,inward were significantly reduced in a low [Na+]o solution. In this solution, I5-HT,inward decreased with hyperpolarization and had a linear steady-state I-V relationship with a reversal potential of approximately -110 mV. The reversal potential of type I I5-HT,inward shifted to about -80 mV as the [K+]o was increased from 3·1 to 7·0 mM in low [Na+]o solution. The type II I5-HT,inward did not reverse at the estimated EK in the same solution. While not affected by externally applied Cs+ (1 mM), I5-HT,inward was significantly smaller in RVLM neurones patched with Cs+-containing electrodes; the current reversed at -11·9 ± 6·4 mV in 8/15 responsive neurones. It may be concluded that in rat RVLM neurones 5-HT increases an inwardly rectifying K+ conductance which may underlie the I5-HT,outward and that a combination of varying degrees of K+ conductance decrease and a Cs+-insensitive, non-selective cation conductance increase may account for the two types of conductance change associated with I5-HT,inward. Rostral ventrolateral medulla (RVLM) neurones are a group of neurones situated in the ventral quadrant of the rostral medulla that are thought to play an important role in cardiovascular, respiratory and nociceptive functions (Dampney, 1994). RVLM neurones receive a moderately dense network of 5-HT-immunoreactive fibres arising from dorsal raphe nuclei (Steinbusch, 1981; Vertes & Kocsis, 1994). The importance of the central serotonergic system, especially the B3, B7 and B8 raphe nuclei, in cardiovascular function has been recognized (Chalmers et al. 1988). More importantly, microiontophoretic application of 5-HT or 5-HT receptor agonists to RVLM neurones increased or decreased the blood pressure, heart rate and sympathetic outflow that are mediated by 5-HT2- and 5-HT1A-like receptors (Gillis et al. 1989; Lovick, 1989a,b; Mandal et al. 1990; Nosjean & Guyenet, 1991; Helke et al. 1992). In addition, the selective 5-HT2 antagonist ketanserin has been proposed as an innovative drug therapy for systemic hypertension and other cardiovascular disorders (Frishman et al. 1995). 5-HT reportedly interacts with seven subtypes of cell surface receptor (Hoyer et al. 1994). Operationally, except for 5-HT3 receptors which are ligand-gated cation channels, all are G-protein coupled receptors. Activation of different types of 5-HT receptor results in a decrease and/or increase of membrane conductance to several ions (Bobker & Williams, 1990). Ion channels associated with 5-HT1 and 5-HT2 receptors appear to be diverse (Bobker & Williams, 1990). For example, activation of 5-HT1A receptors opens an inwardly rectifying K+ conductance in several types of central neurone (Colino & Halliwell, 1987; Williams et al. 1988; Pan et al. 1993; Penington et al. 1993; Okuhara & Beck, 1994). On the other hand, stimulation of 5-HT1A receptors may inhibit Ca2+ currents in a number of neurones including rat dorsal raphe (Penington & Kelly, 1990), hypoglossal (Bayliss et al. 1995) and dorsal root ganglion (Mar et al. 1994). While activation of 5-HT2 receptors is generally associated with a decrease of K+ conductance, the type(s) of K+ conductance appears to vary with different neurones. Thus, activation of 5-HT2 receptors decreases a leak K+ current in facial motoneurones (Larkman & Kelly, 1992), an inwardly rectifying K+ conductance in rat nucleus accumbens neurones (North & Uchimura, 1989) and an outwardly rectifying K+ conductance in rat sympathetic preganglionic neurones (Pickering et al. 1994). On the other hand, 5-HT has been shown to depolarize several types of central neurone by augmenting a hyperpolarization-activated, Cs+-sensitive, non-selective cationic current, Ih (Bobker & Williams, 1989; Pape & McCormick, 1989; Takahashi & Berger, 1990; Larkman & Kelly, 1992). Recently we have reported that 5-HT interacting with 5-HT2- and 5-HT1A-like receptors depolarizes and hyperpolarizes RVLM neurones of brainstem slices of immature rats (Hwang & Dun, 1998). The present study was undertaken to characterize further the ionic mechanisms underlying these two responses. Some of the results have been published as an abstract (Hwang & Dun, 1997).