To the editor

In a recent paper in Nature Neuroscience, Chafke and Bourque1 assert that “mechanisms underlying osmoreception [in osmosensory neurons] are understood,” and more specifically, that a stretch-inactivated cation channel (SIC) is a point of “molecular convergence” for osmotransduction and peptide-induced excitation. An enticing idea, but the data do not stand up to scrutiny.

Gd3+, with its many side effects on calcium2, potassium3, sodium3, non-selective4 and cation-selective5 channels, is a problematic diagnostic tool6. Nevertheless, the evidence offered for “molecular convergence” is that 10 μM Gd3+ and 100 μM Gd3+ shorten the mean open times (MOT) of peptide-stimulated and “control” SICs by similar percentages. This MOT comparison is made against previously published and highly variable findings from which “representative excerpts”8,9 were 10–100 ms bursts, unlike the present events (Fig. 3a).

Figure 6, illustrating peptide-stimulated events modulated by 100 μM Gd3+, is disconcerting for several reasons. The 100 μM Gd3+ produces a fast-flickery block of cation channels (including SICs in mammalian muscle7), yet the single-channel events illustrating Gd3+ block are distinctly larger than control currents. Earlier work9 showed reduced single-channel amplitude with Gd3+, and these old data constitute the controls for Fig. 6. The finding that the MOT for channel activity occurring in long bursts8,9 is similar to the MOT of openings that seldom occur in bursts (the new data, Fig 3a) is hardly a flag that identical proteins produced both responses. Also, Gd3+ interacts strongly with many anions, including proteins10, so whole-cell responses to 200 μM Gd3+ (Fig. 6d), which may reflect action on the peptides and/or multiple channel types, are not grounds for concluding that identical channels carry peptide-stimulated and osmotransducing currents.

Ongoing channel activity can be easily characterized as ‘stretch-inactivating’ by applying brief pulses of pipette suction11, but Chafke and Bourque never use this direct approach to show peptide-activated channels are SICs. Instead, they present one dose–response curve in which the steady-state Popen is approximately 2-fold higher at 0 cm H2O than at any other pressure in a range of 175 cm H2O. The two-fold dynamic range for SIC activity is unimpressive. We are assured that “in 12 patches… Popen of peptide stimulated channels could be modified by changing hydrostatic pressure”. This is not data—nor is the allusion to three [other] patches in which “channel Popen varied as a bell-shaped function of pipette pressure.” Statistics are needed, as well as more current traces.

How reliable is the bell-shaped response? Previously8,9 the authors renormalized applied pressures so that “0 cm H2O” did not signify atmospheric pressure, but rather the pressure at which NPopen was maximum. Therefore, from Fig. 5b one cannot infer that membrane tension increased on either side of 0 cm H2O (ref. 11). Furthermore, channels in cell-attached patches can experience uncontrolled variations in kinetics, so that normalizing data records to the highest activity inevitably leads to bell-shaped curves, particularly when the dynamic range is only two.

In previous papers claiming that SICs underlie osmotransduction8,9, the reported pressures were about 50-fold smaller (±2 cm versus ±100 cm H2O; Fig 6b). Several years ago, we alerted Dr. Bourque that the published pressures seemed extremely low—but this discrepancy goes unmentioned and certainly invalidates any comparison between the present data and the older data (Fig. 6b).

We are not convinced that “…mechanisms underlying osmoreception are understood…” and we disagree that the new data support a “molecular convergence.”