The Journal of Neurophysiology Vol. 83 No. 5 May 2000, pp. 2526-2532
Copyright ©2000 by the American Physiological Society

Sweet Taste Transduction in Hamster: Role of Protein Kinases

Department of Anatomy and Neurobiology, Colorado State University, Fort Collins 80523; and Rocky Mountain Taste and Smell Center, University of Colorado Health Sciences Center, Denver, Colorado 80262

Varkevisser, Brian and Sue C. Kinnamon. Sweet Taste Transduction in Hamster: Role of Protein Kinases. J. Neurophysiol. 83: 2526-2532, 2000. Two different second-messenger pathways have been implicated in sweet taste transduction: sugars produce cyclic AMP (cAMP), whereas synthetic sweeteners stimulate production of inositol 1,4,5-tris-phosphate (IP₃) and diacylglycerol (DAG). Both sugars and sweeteners depolarize taste cells by blocking the same resting K⁺ conductance, but the intermediate steps in the transduction pathways have not been examined. In this study, the loose-patch recording technique was used to examine the role of protein kinases and other downstream regulatory proteins in the two sweet transduction pathways. Bursts of action currents were elicited from ~35% of fungiform taste buds in response to sucrose (200 mM) or NC-00274-01 (NC-01, 200 µM), a synthetic sweetener. To determine whether protein kinase C (PKC) plays a role in sweet transduction, taste buds were stimulated with the PKC activator PDBu (10 µM). In all sweet-responsive taste buds tested (n = 11), PDBu elicited burst of action currents. In contrast, PDBu elicited responses in only 4 of 19 sweet-unresponsive taste buds. Inhibition of PKC by bisindolylmaleimide I (0.15 µM) resulted in inhibition of the NC-01 response by ~75%, whereas the response to sucrose either increased or remained unchanged. These data suggest that activation of PKC is required for the transduction of synthetic sweeteners. To determine whether protein kinase A (PKA) is required for the transduction of sugars, sweet responses were examined in the presence of the membrane-permeant PKA inhibitor H-89 (10 and 19 µM). Surprisingly, H-89 did not decrease responses to either sucrose or NC-01. Instead, responses to both compounds were increased in the presence of the inhibitor. These data suggest that PKA is not required for the transduction of sugars, but may play a modulatory role in both pathways, such as adaptation of the response. We also examined whether Ca²⁺-calmodulin dependent cAMP phosphodiesterase (CaM-PDE) plays a role in sweet taste transduction, by examining responses to sucrose and synthetic sweeteners in the presence of the CaM-PDE inhibitor W-7 (100 µM). Inhibition resulted in an increase in the response to sucrose, whereas the response to NC-01 remained unchanged. These data suggest that the pathways for sugars and sweeteners are negatively coupled; the Ca²⁺ that is released from intracellular stores during stimulation with synthetic sweeteners may inhibit the response to sucrose by activation of CaM-PDE.