Elsevier

Neuroscience

Volume 163, Issue 4, 10 November 2009, Pages 1292-1301
Neuroscience

Sensory System
Research Paper
Antagonism of metabotropic glutamate receptor 4 receptors by (RS)-α-cyclopropyl-4-phosphonophenylglycine alters the taste of amino acids in rats

https://doi.org/10.1016/j.neuroscience.2009.07.035Get rights and content

Abstract

T1R1/T1R3, taste-metabotropic glutamate receptor (mGluR) 4 and other taste receptors have been implicated in umami taste perceptionT1R1/T1R3 has also been identified as an l-amino acid receptor. We investigated the possibility that taste-mGluR4 receptors may also play a role in the taste of amino acids in Sprague–Dawley rats using conditioned taste aversion methods. Specifically, we examined whether a taste aversion generalized between l-monosodium glutamate (MSG) and one of three amino acids (glycine, l-serine, and l-arginine), and whether (RS)-α-cyclopropyl-4-phosphonophenylglycine (CPPG), a group III mGluR selective antagonist with a strong binding affinity for mGluR4 receptors, can impact stimulus generalization. Rats showed cross-generalization between MSG and all three amino acids (all mixed with amiloride to block the taste of sodium), although less so for l-arginine than the other two amino acids, suggesting that all of the amino acids shared at least some taste qualities with MSG. However, when 1 mM CPPG was mixed with these amino acids, the strength of the learned taste aversions and cross-generalization for all but glycine were either decreased or increased. The increase in generalization induced by CPPG indicated that the antagonist did not simply reduce the intensity of the stimulus experience but also changed the qualities of the sensory experience. These findings suggest that multiple receptors are involved in amino acid taste and that taste-mGluR4 receptors contribute to the taste of MSG and at least some l-amino acids.

Section snippets

Subjects

One hundred thirty-two naïve male Sprague–Dawley rats at least 90 days old at the start of testing were used for these experiments. The subjects were housed separately in cages within a colony on a 12-h light/dark schedule. The subjects were water deprived 22 h a day and received food ad libitum. Each animal was tested at the same time each day. All experimental procedures with animals conform to the Guide for the Care and Use of Laboratory Animals on the ethical use of animals. All methods

MSG/glycine CPPG experiments

When the rats were conditioned to avoid glycine, the LiCl-injected group drank significantly less glycine (F(1,14)=74.19, P<.001; Fig. 1, top panels) and less MSG (F(1,14)=42.24, P<.001) than saline-injected rats, indicating that the animals had developed the CTA for glycine and that it generalized to MSG. CPPG had no effect on the strength of the CTA to glycine or the generalization of this CTA to MSG. In addition, lick rates were not affected by either CPPG or 75 mM NaCl in water (Table 1).

Discussion

T1R1/T1R3 receptors are thought to be taste receptors for detection of MSG and other l-amino acids (Nelson et al 2002, Li et al 2002, Zhao et al 2003). We hypothesized that mGluR4 receptors expressed in taste specific tissues of the tongue might also contribute to the tastes of other amino acids. We tested this hypothesis by conditioning rats to avoid either MSG or one of three amino acids (glycine, l-serine, l-arginine) and then testing the strength of the CTA and its generalization between

Acknowledgments

This research was supported by NSF IOS 0549167 awarded to E.R.D.

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