Effect of gap junction blocker beta-glycyrrhetinic acid on taste disk cells in frog

Abstract

A gap junction blocker, 18beta-glycyrrhetinic acid (beta-GA), increased the membrane resistance of Ia, Ib and II/III cells of frog taste disk by 50, 160, and 300 M Omega, respectively, by blocking the gap junction channels and hemichannels. The amplitudes of gustatory depolarizing potentials in the disk cells for 4 basic taste stimuli were reduced to 40-60% after intravenous injection of beta-GA at 1.0 mg/kg. beta-GA of 1.0 mg/kg did not affect the resting potentials and the reversal potentials for tastant-induced depolarizing potentials in any taste disk cells. The percentage of cells responding to each of 4 basic taste stimuli and varying numbers of 4 taste qualities did not differ between control and beta-GA-treated taste disk cells. This implies that gustatory depolarizing response profiles for 4 basic taste stimuli were very similar in control and beta-GA-treated taste disk cells. It is concluded that beta-GA at 1.0 mg/kg reduced the amplitude of gustatory depolarizing potentials in taste disk cells by strongly blocking depolarizing currents flowing through the gap junction channels and hemichannels, but probably weakly affected the gustatory transduction mechanisms for 4 taste stimuli.

Fig. 1

Three step-potential changes of resting membrane potentials in taste disk cells by a microelectrode after intravenous injection of β-GA. Left arrow shows penetration of microelectrode into a Ia cell, and middle and right arrows show that into a Ib, and a II/III cell. Vertically deflected potential pulses denote amplitudes of input resistances of cells. β-GA was injected at 1.0 mg/kg

Fig. 2

Relationships between doses of β-GA and amplitudes of input resistances in taste disk cells. β-GA was injected i. v. at doses of 0.2–2.0 mg/kg body weight. Vertical bars in this and other figures denote SEM and numerals number of cells tested. Control input resistance was 24 ± 2 MΩ (N = 65) in Ia cells, 34 ± 3 MΩ (N = 65) in Ib cells, and 50 ± 4 MΩ (N = 65) in II/III cells

Fig. 3

Time course of input resistance changes in taste disk cells following injection of β-GA. a Continuous recording of input resistance changes in a Ib cell following intravenous injection of β-GA at 1.0 mg/kg. b–d Time courses of input resistances in Ia (b), Ib (c), and II/III cell (d) following β-GA injection. At time 0 β-GA was injected at 1.0 mg/kg

Fig. 4

Relationship between doses of β-GA and resting potentials in taste disk cells. Numerals above squares are number of taste disks tested

Fig. 5

Relationships between membrane potentials and gustatory responses induced by taste stimuli in taste disk cells following β-GA injection. a Change in 1 M NaCl-induced depolarizing potential in a Ib cell by altering membrane potential. b–e Relationships between amplitude of membrane potentials and amplitude of taste stimulus-induced responses in 4 Ib cells. b 1 M NaCl. c 1 mM acetic acid. d 10 mM Q-HCl. e 1 M sucrose. Data in a are plotted in b. β-GA was injected at 1.0 mg/kg

Fig. 6

Amplitudes of gustatory depolarizing responses of taste disk cells before (control) and after injection of β-GA. a Ia cells. b Ib cells. c II/III cells. Taste stimuli were 1 M NaCl (N), 1 mM acetic acid (A), 10 mM Q-HCl (Q), and 1 M sucrose (S). β-GA was injected at 1.0 mg/kg

Fig. 7

Changes in membrane resistances of taste disk cells in control and β-GA-injected tongue induced by gustatory stimulation. a Recording of membrane resistance changes of a II/III cell by 4 basic taste stimuli. b Ia cell. c Ib cell. d II/III cell. Taste stimuli: 1 M NaCl (N), 1 mM acetic acid (A), 10 mM Q-HCl (Q), 1 M sucrose (S). β-GA was injected at 1.0 mg/kg

Fig. 8

Percentage of taste disk cells responding to each of four basic stimuli before (control) and after β-GA injection. a Ia cells. b Ib cells. c II/III cells. Taste stimuli: 1 M NaCl (N). 1 mM acetic acid (A), 10 mM Q-HCl (Q), 1 M sucrose (S). β-GA was injected at 1.0 mg/kg. Control data in Ia and Ib cells were from Sato et al. (2008). Number of cells tested was 11 Ia, 11 Ib, and 31 II/III cells in control tongues, and 14 Ia, 22 Ib, and 18 II/III cells in β-GA-injected tongues

Fig. 9

Percentage of taste disk cells responding to various numbers of 4 basic taste qualities before (control) and after β-GA injection. a Ia cells. b Ib cells. c II/III cells. Taste stimuli: 1 M NaCl (N), 1 mM acetic acid (A), 10 mM Q-HCl (Q), 1 M sucrose (S). β-GA was injected at 1.0 mg/kg. All percentage patterns in Ia, Ib, and II/III cells were obtained using same data described in Fig. 8

Fig. 10

Gustatory response profile of β-GA-treated II/III cells for 4 basic taste qualities. Taste stimuli: 1 M NaCl, 1 mM acetic acid, 10 mM Q-HCl, and 1 M sucrose. β-GA was injected at 1.0 mg/kg