Date of Award

8-2016

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Program

Biomedical Sciences

Track

Neuroscience

Research Advisor

John D. Boughter, Jr., Ph.D.

Committee

Malinda E.C. Fitzgerald, Ph.D. Kristin M. Hamre, Ph.D. Jeffrey D. Steketee, Ph.D.

Abstract

Humans can distinguish at least five different taste qualities, sour, salty, bitter, sweet, and umami (the savory taste of certain amino acids). In neuroscience research, behavioral testing is used to measure the ability of rodents (including inbred mice) to discriminate between the different taste qualities. Taste reactivity and two-bottle preference are behavioral tests that are utilized to investigate different aspects of taste. These tests involve either voluntary or forced consumption of taste stimuli, respectively. Either test can be used to infer the preference and palatability of the stimulus consumed by an animal.

In order to understand the basis of taste behavior, one must understand the organization of the taste pathway. As an organism consumes a particular food or fluid, it first binds to or activates taste receptors or channels located inside taste buds found in the oral cavity. This transduction event then produces a cascade of neuronal activation via sensory nerves that innervate the taste buds –branches of three cranial nerves (VII, IX, and X). These cranial nerves then synapse centrally in the nucleus of the solitary tract (NST) where the relayed taste information is kept relatively segregated from visceral input (which arrives via cranial nerve X).

From this point, the taste information is relayed to the parabrachial nucleus (PBN) in the pons, where the taste and visceral information now overlap. The PBN has not been studied as extensively as the NST in terms of taste representation, especially in regards to umami taste. A few recent studies have indicated that taste neurons in the PBN respond to sweet and synergistic umami (i.e. a combination of glutamate and a ribonucleotide) stimuli in a similar manner, providing a rationale for further study of the representation of these taste stimuli in this area.

Sweet and umami taste share a common G-protein-coupled taste receptor subunit, T1R3, that responds in combination with either T1R1 to transduce umami stimuli or T1R2 to transduce sweet stimuli. Aside from sharing a common taste receptor, previous studies using pharmacological manipulations, electrophysiology, conditioned taste aversion (CTA), and discrimination studies have shown a strong functional link between sweet and umami taste in rodents. Compounds found to be sweet taste inhibitors either entirely or partially block the nerve response to the prototypical umami stimulus monosodium glutamate (MSG), as well as a synergistic mixture of MSG combined with the cyclic nucleotide inosine monophosphate (IMP). When the epithelial sodium channel blocker amiloride is combined with MSG, both rats and mice have difficulty determining the difference between this umami stimulus and sucrose. Overall, it appears that some umami stimuli appear to be perceived as sucrose-like in rodents, which differs dramatically from the human perception of umami stimuli. Although umami taste has not been studied as comprehensively in mice as it has been in rats, it is important to investigate due to the widespread use of a variety of genetic mouse models in taste research. Along with using behavioral models, one might gauge the uniqueness of sweet and umami stimuli using an anatomical technique, such as visualization of the immediate early gene c-fos in PBN neurons. In fact, previous research has indicated stimulation with different taste qualities produces distinctive c-fos patterns in the PBN. For this current research study, my first hypothesis was that since previous studies suggested the similarity between sweet and umami compounds in C57BL/6J (B6) mice; stimuli of both taste qualities would produce similar levels of preference, consumption, and levels of taste reactivity behaviors. Secondly, I hypothesized that taste stimulation with either sweet (sucrose) or umami (monopotassium glutamate; MPG, or the synergistic mixture of MPG+IMP) stimuli would produce a similar c-fos expression pattern in sweet and umami stimuli, and this would also be distinct from the c-fos expression patterns elicited by both the bitter stimulus, quinine hydrochloride (QHCl) and water.

Overall, the preference tests revealed that both sucrose and umami stimuli (especially MSG+IMP) were preferred and consumed at a similarly high level in B6 mice. However, the taste reactivity test did not yield any insight into whether the sweet and umami taste stimuli were perceived as similar. However, taste reactivity to the bitter stimulus, QHCl, was easily distinguishable from the other tested taste stimuli. Using c-fos immunohistochemistry to visualize neuronal activation, I then compared staining patterns of activation evoked by: water, QHCl, sucrose, saccharin, MPG, and MPG+IMP in subdivisions of the PBN in B6 mice, as well as a few other non-taste brainstem areas (locus coeruleus and mesencephalic nucleus of the trigeminal nerve). Results showed that quinine elicited significantly less c-fos positive nuclei in the entire dorsal lateral (DL) subnucleus compared to water. A few other significant effects of the tastant stimuli were found in the rostral portion of the waist, central lateral (CL), and DL PBN subnuclei, but distinct c-fos representations were not found for each stimulus tested. To determine if tastant effects might have been subtler in terms of cell density or patterning; and

therefore, could have been missed using normal cell counting methods, I decided to use a three-dimensional mapping approach to examine c-fos expression in the PBN. Results of this new mapping approach suggest its potential usage in future studies.

ORCID

http://orcid.org/0000-0002-7317-8307

DOI

10.21007/etd.cghs.2016.0413

Included in

Neurosciences Commons

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