Meghan C. Eddy

Double P2X2/P2X3 Purinergic Receptor Knockout Mice Do Not Taste NaCl or the Artificial Sweetener SC45647

The P2X ionotropic purinergic receptors, P2X2 and P2X3, are essential for transmission of taste information from taste buds to the gustatory nerves. Mice lacking both P2X2 and P2X3 purinergic receptors (P2X2/P2X3(Dbl-/-)) exhibit no taste-evoked activity in the chorda tympani and glossopharyngeal nerves when stimulated with taste stimuli from any of the 5 classical taste quality groups (salt, sweet, sour, bitter, and umami) nor do the mice show taste preferences for sweet or umami, or avoidance of bitter substances (Finger et al. 2005. ATP signaling is crucial for communication from taste buds to gustatory nerves. Science. 310[5753]:1495-1499). Here, we compare the ability of P2X2/P2X3(Dbl-/-) mice and P2X2/P2X3(Dbl+/+) wild-type (WT) mice to detect NaCl in brief-access tests and conditioned aversion paradigms. Brief-access testing with NaCl revealed that whereas WT mice decrease licking at 300 mM and above, the P2X2/P2X3(Dbl-/-) mice do not show any change in lick rates. In conditioned aversion tests, P2X2/P2X3(Dbl-/-) mice did not develop a learned aversion to NaCl or the artificial sweetener SC45647, both of which are easily avoided by conditioned WT mice. The inability of P2X2/P2X3(Dbl-/-) mice to show avoidance of these taste stimuli was not due to an inability to learn the task because both WT and P2X2/P2X3(Dbl-/-) mice learned to avoid a combination of SC45647 and amyl acetate (an odor cue). These data suggest that P2X2/P2X3(Dbl-/-) mice are unable to respond to NaCl or SC45647 as taste stimuli, mirroring the lack of gustatory nerve responses to these substances.

Behavioral responses to glutamate receptor agonists and antagonists implicate the involvement of brain-expressed mGluR4 and mGluR1 in taste transduction for umami in mice

Recent molecular studies have identified many candidate receptors for umami, typically the taste of monosodium glutamate (MSG). The candidate receptors, including taste-mGluR4, T1R1+T1R3, and truncated mGluR1, respond to MSG in the millimolar concentration range. Expression of brain-expressed mGluR4 and mGluR1 with much higher sensitivities to glutamate has also been reported in taste papillae. To test the involvement of brain-expressed mGluRs in umami taste, we tested glutamate agonists and antagonists at concentration ranges relevant to both types of the receptors using a combination of a detection threshold and conditioned taste aversion (CTA) methods in mice. The detection threshold experiment showed that mice could detect the group III mGluR agonist L(+)-2-amino-4-phosphonobutyrate (L-AP4) taste thresholds at 0.0009-0.0019 mM. Mice conditioned using CTA methods to avoid either MSG or MPG showed aversive responses to MSG with and without amiloride or to MPG, respectively, at concentrations of 0.0001 mM and above. A CTA to L-AP4 or MSG showed comparable concentration-response ranges for L-AP4 and MSG. The Group III mGluR antagonist, (RS)-α-cyclopropyl-4-phosphonophenylglycine (CPPG), and the mGluR1 antagonist, 1-aminoindan-1,5-dicarboxylic acid (AIDA), suppressed aversive responses to glutamate agonists at concentrations between 0.0001 and 100mM in the CTA experiments. Our results suggest the possibility that brain-expressed mGluR4 and mGluR1 may contribute to umami taste in mice.

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

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)-alpha-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.

Medial prefrontal cortex involvement in the expression of extinction and ABA renewal of instrumental behavior for a food reinforcer

Instrumental renewal, the return of extinguished instrumental responding after removal from the extinction context, is an important model of behavioral relapse that is poorly understood at the neural level. In two experiments, we examined the role of the dorsomedial prefrontal cortex (dmPFC) and the ventromedial prefrontal cortex (vmPFC) in extinction and ABA renewal of instrumental responding for a sucrose reinforcer. Previous work, exclusively using drug reinforcers, has suggested that the roles of the dmPFC and vmPFC in expression of extinction and ABA renewal may depend at least in part on the type of drug reinforcer used. The current experiments used a food reinforcer because the behavioral mechanisms underlying the extinction and renewal of instrumental responding are especially well worked out in this paradigm. After instrumental conditioning in context A and extinction in context B, we inactivated dmPFC, vmPFC, or a more ventral medial prefrontal cortex region by infusing baclofen/muscimol (B/M) just prior to testing in both contexts. In rats with inactivated dmPFC, ABA renewal was still present (i.e., responding increased when returned to context A); however responding was lower (less renewal) than controls. Inactivation of vmPFC increased responding in context B (the extinction context) and decreased responding in context A, indicating no renewal in these animals. There was no effect of B/M infusion on rats with cannula placements ventral to the vmPFC. Fluorophore-conjugated muscimol was infused in a subset of rats following test to visualize infusion spread. Imaging suggested that the infusion spread was minimal and mainly constrained to the targeted area. Together, these experiments suggest that there is a region of medial prefrontal cortex encompassing both dmPFC and vmPFC that is important for ABA renewal of extinguished instrumental responding for a food reinforcer. In addition, vmPFC, but not dmPFC, is important for expression of extinction of responding for a food reinforcer. The role of the medial prefrontal cortex in renewal in the original conditioning context may depend in part on control over excitatory context-response or context-(response-outcome) relations that might be learned in acquisition. The role of the vmPFC in expression of extinction may depend on its control over inhibitory context-response or context-(response-outcome) relations that are learned in extinction.

Voluntary exercise improves performance of a discrimination task through effects on the striatal dopamine system

We have previously demonstrated that voluntary exercise facilitates discrimination learning in a modified T-maze. There is evidence implicating the dorsolateral striatum (DLS) as the substrate for this task. The present experiments examined whether changes in DLS dopamine receptors might underlie the exercise-associated facilitation. Infusing a D1R antagonist into the DLS prior to discrimination learning facilitated the performance of nonexercising rats but not exercising rats. Infusing a D2R antagonist impaired the performance of exercising rats but not nonexercising rats. Exercise-associated facilitation of this task may rely on an exercise-induced decrease in D1R and increase in D2R activation in the DLS.

Behavioral studies of umami: tales told by mice and rats.

Psychophysical research with rats and mice has been instrumental in understanding umami taste transduction and perception. Although early studies suggested that an NMDA‐like receptor detected substances that elicit an umami taste, studies using behavioral methods with both rats and mice indicate that the picture is much more complex. When the G protein‐coupled receptor T1R1+T1R3 was discovered, it was believed to be the umami receptor and a more broadly tuned L‐amino acid receptor. However, since then a number of behavioral studies, like molecular and physiological studies, report evidence that other receptors may contribute to umami taste. For example, T1R3 knockout mice (KO) have only slightly elevated detection thresholds for monosodium glutamate (MSG) and L‐alanine. In conditioned taste aversion studies, T1R3 KO mice show bidirectional generalization of the aversion between MSG and L‐alanine, suggesting that these substances have similar tastes. However, these KO mice can discriminate between the tastes of the two substances, indicating other receptors also respond to these amino acids. (RS)‐α‐cycloprophy‐4‐phosphonophenylglycine (CPPG), a potent mGluR4 antagonist, decreases an aversion to MSG in rats while increasing the strength of generalization of the aversion to L‐arginine or L‐serine. These behavioral studies suggest that glutamate can activate several putative receptors, most notably T1R1+T1R3 and taste‐mGluR4, and possibly NMDA‐like receptors or taste‐mGluR1. These receptors generate similar but not identical sensations which, when combined, form a complex perception identified as umami. Further, these studies suggest that afferent signaling from T1R1+T1R3 and taste‐mGluR4 likely combine to generate the taste sensations associated with other L‐amino acids.

A Conditioned Aversion Study of Sucrose and SC45647 Taste in TRPM5 Knockout Mice

Previously, published studies have reported mixed results regarding the role of the TRPM5 cation channel in signaling sweet taste by taste sensory cells. Some studies have reported a complete loss of sweet taste preference in TRPM5 knockout (KO) mice, whereas others have reported only a partial loss of sweet taste preference. This study reports the results of conditioned aversion studies designed to motivate wild-type (WT) and KO mice to respond to sweet substances. In conditioned taste aversion experiments, WT mice showed nearly complete LiCl-induced response suppression to sucrose and SC45647. In contrast, TRPM5 KO mice showed a much smaller conditioned aversion to either sweet substance, suggesting a compromised, but not absent, ability to detect sweet taste. A subsequent conditioned flavor aversion experiment was conducted to determine if TRPM5 KO mice were impaired in their ability to learn a conditioned aversion. In this experiment, KO and WT mice were conditioned to a mixture of SC45647 and amyl acetate (an odor cue). Although WT mice avoided both components of the stimulus mixture, they avoided SC45647 more than the odor cue. The KO mice also avoided both stimuli, but they avoided the odor component more than SC45647, suggesting that while the KO mice are capable of learning an aversion, to them the odor cue was more salient than the taste cue. Collectively, these findings suggest the TRPM5 KO mice have some residual ability to detect SC45647 and sucrose, and, like bitter, there may be a TRPM5-independent transduction pathway for detecting these substances.

Gonadal hormones and voluntary exercise interact to improve discrimination ability in a set-shift task.

Exercise has been demonstrated to improve multiple facets of health, including cognitive function. Rodent studies have suggested that exercise has robust effects on the hippocampus and on tasks that require the hippocampus. However, studies of the effects of exercise in humans often focus on the benefits to cognitive processes that engage areas outside of the hippocampus, such as executive function. Additionally, when exercises cognitive benefits are examined, consideration of both males and females, and gonadal hormones, is rarely made. Here we looked at the interaction of gonadal hormones and exercise in terms of the ability of male and female rats to learn to discriminate rewarded from unrewarded arms in a T maze based on either brightness (white vs. black) or texture (rough vs. smooth) and then to set-shift (a measure of executive function), where this required discrimination is based on the opposite dimension. Gonadectomized or intact males and females had access to running wheels for 2 weeks before being tested. Intact males and females given access to unlocked running wheels performed better at the initial discrimination (Set 1) compared with intact males and females with locked running wheels but not at the set shift (Set 2). No advantage of exercise was observed in gonadectomized rats.

Behavioral comparison of sucrose and l-2-amino-4-phosphonobutyrate (l-AP4) tastes in rats: Does l-AP4 have a sweet taste?

Even though it is generally thought that umami stimuli such as monosodium glutamate (MSG) and sweet stimuli such as sucrose are detected by different taste receptors, these stimuli appear to share taste qualities when amiloride (a sodium channel blocker) is present to reduce the sodium taste. Single fiber recording studies of the facial and glossopharyngeal nerves have shown that encoding of L-2-amino-4-phosphonobutyrate (L-AP4), a potent mGluR4 agonist that elicits a taste quite similar to MSG, may occur in the same fibers that also encode sweet stimuli. This suggests that L-AP4 and sweet substances may activate common receptors or afferent signaling mechanisms. We report results of behavioral experiments that test this hypothesis. In the first study, rats conditioned to avoid sucrose or L-AP4 generalized the aversion to the opposite substance, indicating that both substances elicited similar tastes. However, two taste discrimination experiments showed that rats easily discriminated between sucrose and L-AP4 over a wide range of concentrations, even when the cue function of sodium associated with L-AP4 was reduced by amiloride and neutralized by adding equimolar concentrations of NaCl to sucrose. These data suggest that even though L-AP4 and sucrose elicit similar taste qualities, one or both substances also elicit other taste qualities not shared by the opposite substance. They also suggest that the taste-mGluR4 receptor and the signal pathway activated by L-AP4 are not the same as those activated by sucrose. These data, when combined with fiber recording data, suggest that there is convergence of L-AP4 and sucrose signals at some point early in the gustatory pathway.

Running wheel exercise reduces renewal of extinguished instrumental behavior and alters medial prefrontal cortex neurons in adolescent, but not adult, rats.

Physical exercise in rodents has repeatedly been shown to trigger positive effects on brain function, including increased neurotrophic factors and improved learning and memory. However, most of this work has focused on the adult hippocampus and hippocampal-dependent behavior. Here we examined the effect of running wheel exercise in adult and adolescent male rats on ABA renewal of extinguished instrumental conditioning, in which acquisition occurs in Context A, extinction in Context B, and renewal testing occurs back in Context A. In the first experiment, rats were given unlocked (exercise) or locked (no exercise) running wheel access in their home cages beginning at postnatal Day 30 (adolescent) or postnatal Day 56 (adult). Rats underwent lever-press acquisition in Context A and extinction in Context B. ABA renewal testing took place 2 weeks after the start of running wheel exposure. Nonexercising adolescent rats showed greater ABA renewal than nonexercising adult rats and exercise reduced ABA renewal in adolescents but not adults. ABA renewal depends on medial prefrontal cortex function. In a second experiment, we compared adolescent and adult apical dendrite branch length, branch number, and spine density of medial prefrontal cortex pyramidal neurons after 2 weeks of unlocked or locked running wheel access. The results revealed a higher density of dendritic spines and a lower dendritic branch length in adolescent exercisers than adolescent nonexercisers. Adult exercisers and nonexercisers did not differ. Collectively, these experiments suggest that exercise may have particularly strong effects on adolescent medial prefrontal cortex function and structure.