Temperature and amiloride alter taste nerve responses to Na+, K+, and NH4+ salts in rats
Introduction
It is generally accepted that passive Na+ channels represent the primary transduction pathway for sodium and lithium salts [10,11,13,25]. Both psychophysical and electrophysiological studies indicate that amiloride-sensitive Na channels are critical for NaCl detection. Amiloride reduced the integrated chorda tympani nerve responses to NaCl in rats [5], mice [7], hamsters [12], and monkeys [11], reduced single fiber responses of the chorda tympani nerve to NaCl in rats [20] and hamsters [13], and increased the consumption of NaCl in rats [3] and hamsters [13]. However, a recent study in our laboratory and accumulating evidence from other studies indicate that these Na+ channels may also play a role in nonsodium salt transduction [2,16,18,20] as well as in acid transduction [8].
For example, amiloride was found to suppress both whole nerve and single fiber responses to KCl [16,20] and single fiber responses to CaCl2 [20]in rat chorda tympani. In canine chorda tympani, amiloride inhibited the integrated nerve responses to RbCl, CsCl, KCl, and NH4Cl [18]. Moreover, patch-clamp studies demonstrate that K+ ions permeate amiloride-sensitive Na channels in frog taste cells [2] and h ions permeate these channels in hamster taste cells [8]. Taken as a whole, amiloride may exert relatively specific or nonspecific effects on salt taste depending on the species under investigation and the analysis procedure for assessing the effects of amiloride suppression. One purpose of the present study was to use pharmacological and whole nerve electrophysiological methods to reexamine the cation selectivity of amiloride inhibition using a novel stimulus delivery apparatus [15] and a more sensitive procedure for accessing the inhibitory action of amiloride [13,16].
Neural activation by salt stimuli is temperature dependent and this temperature dependency seems to be linked with amiloride-sensitive Na+ channels. In rats and cats, taste nerve responses to NaCl, KCl, and NH4Cl decrease as temperature decreases below the temperature range where maximal responses occur (e.g. NaCl and KCl: 25–35°C; NH4Cl: 35–45°C) [19,27,28]. Moreover, amiloride has been shown to inhibit the integrated responses of the chorda tympani nerve to NaCl stimulation more at lower temperatures (10°C) than at warmer temperatures (30°C) [17]. The magnitude of amiloride suppression was similar within the temperature range of 25–35 C. How temperature influences amiloride-sensitive Na channels is open for investigation.
A second purpose of the present study was to investigate the effects of temperature on amiloride suppression of nerve responses to various sodium and nonsodium salts. In a recent study in mice, amiloride was found to inhibit the integrated responses of the chorda tympani to NaCl stimulation more at lower temperatures (12°C) than at warmer temperatures (24°C); however, amiloride was without effect on taste nerve responses to KCl at either temperature [21].
The present findings indicate that amiloride significantly inhibits the integrated responses of chorda tympani nerve to all sodium and nonsodium salts at both 25 and 35°C. In addition, we show that the temperature dependence of amiloride suppression on salt induced nerve responses is related to the cation and not the anion.
Section snippets
Subjects
Recordings were obtained from the whole chorda tym-pani nerve of 19 adult male rats weighing 250–450 g [Sprague-Dawley, CrL:CD (SD) BR, Charles River Breeding Laboratories]. Rats were housed in transparent plastic cages, a maximum of two per cage, in a temperature-controlled colony room on a 12–12 h light-dark cycle with lights on at 0500 h. All animals had free access to Purina Rat Chow 5001 and deionized-distilled water ad libitum. All preparations began approximately 4 h into the animals light
Experiment 2
In Experiment 1, 100 µm amiloride-HCl significantly suppressed the integrated responses of the whole chorda tympani nerve to a range of sodium, potassium, and ammonium salts. Thus, the present findings suggest that potassium and ammonium ions may permeate passive Na+ channels in rat taste receptor cells. However, 100 µm amiloride also inhibits other membrane proteins [14]. Consequently, the suppression of sodium, potassium, and ammonium salt responses may arise, in part, from amiloride action on
Acknowledgements
We thank Dr. Rick Hyson for his useful comments on an earlier version of this manuscript. The National Institute of Health, Grants HL-38630 and DC-02641, supported the present research.
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