Direct and indirect regulation of a single ion channel.

The activity of ion channels is subjected to intense regulation. Changes in ion channel activity occur when a ligand, such as a neurotransmitter, acts upon a cell by one of two rather distinct pathways: ionotropic regulation, where a ligand alters the activity of a channel by binding to a receptor site on the extracellular surface of the channel protein itself (Unwin, 1993), and metabotropic regulation, where a ligand binds to a different receptor, generating second messenger molecules that act on the channel either directly or through enzyme-mediated phosphorylation (Raymond et al. 1993;Wickham & Clapham, 1995). Traditionally, it was believed that an ion channel could not be regulated in the same manner both ionotropically and metabotropically. This concept is challenged by the study of Ali et al. (1998) in this issue of The Journal of Physiology. In many cases, serotonin (5-HT) and dopamine have similar mechanisms of action on cells, usually operating through G-protein coupled receptors. Ali et al. (1998) investigated the ability of both serotonin and dopamine to enhance the activity of a chloride (Cl−) channel present in pressure-sensitive neurons of the leech. The effect of either transmitter on Cl− channel activity was very similar, in that both substances increased the open probability; however, the manner in which the transmitters achieved this effect was entirely different. For this study, single-channel Cl− currents were recorded in cell-attached patches. Serotonin increased the activity of the channel only when included in the pipette solution, but not when bath applied. Dopamine enhanced channel activity when applied to the cell membrane either via the pipette solution or, more effectively, when bath applied. Moreover, the effect of dopamine, but not serotonin, could be blocked by an inhibitor of cAMP-dependent protein kinase A (PKA). These data together with earlier studies (Lessmann & Dietzel, 1995) lead the authors to suggest that serotonin binds directly to the Cl− channel to increase its activity ionotropically, whereas dopamine enhances activity metabotropically by binding to a separate receptor, generating cAMP and activating PKA to phosphorylate some protein, most likely the channel itself (Fig. 1). One could conclude that this Cl− channel is a ligand-gated channel that opens following serotonin binding, but is also regulated in the same manner, at least phenomenologically, by PKA-dependent phosphorylation. Figure 1 The open probability of a chloride channel is enhanced by serotonin (5-HT) ionotropically and by dopamine (DA) metabotropically. The second messengers liberated by dopamine could, in principle, have additional effects. It is important to distinguish between the seemingly ubiquitous mechanism of ion channel modulation (see below) and the form of regulation reported by Ali et al. (1998). An excellent contrast is provided by examining the amino acid receptors. These channels open upon binding of molecules such as glutamate to a site(s) on the receptor-channel complex. A number of other transmitters that stimulate second messenger pathways have been shown to enhance or depress the response to amino acid stimulation (Raymond et al. 1993). However, it is important to note that these modulators fail to affect activity when a ligand is not bound to the channel. The situation is similar for the voltage-gated channels, where the response to voltage changes can be altered markedly by second messengers and protein phosphorylation (Wickman & Clapham, 1995; Jonas & Kaczmarek, 1996). On the other hand, Ali et al. (1998) have uncovered an intrinsically different example of ion channel regulation where an increase in open probability of an ion channel is achieved through the independent pathways of ionotropic (serotonin) and metabotropic (dopamine) stimulation. The findings have general implications for both ion channel study and neuronal signalling. If the increased open probability produced by serotonin and dopamine is due to a similar biophysical mechanism, it would suggest that ligand binding and phosphorylation may have access to the same portion(s) of the channel protein responsible for gating, despite the fact that binding and phosphorylation occur on extra- and intracellular faces of the protein, respectively. Dual regulation may be important in controlling the temporal aspects of synaptic transmission. The second messengers employed by dopamine would certainly take longer to act, and would probably last longer, than the serotonin effect. Rapid signalling could come in the form of input from presynaptic serotonergic neurons, while a longer lasting signal could be produced by a neuron secreting dopamine. Dual regulation could also allow the same form of electrical activity to be either coupled or uncoupled to cellular events, such as changes in gene transcription or cytoskeletal re-arrangements. Activation of the channel by serotonin would be uncoupled from such events, whereas the changes in cAMP and PKA produced by dopamine could trigger a host of cellular changes beyond that of Cl− channel regulation (see Fig. 1). In summary, this paper expands our understanding of ion channel physiology, showing that a single ion channel species can be regulated, in essentially the same manner, both ionotropically and metabotropically.