2. Introduction:
When presented with an unfamiliar food source, an organism relies on external olfactory and
gustatory information to access the liability that would accrue if the food were to be ingested. Early
experience, defined to be the period before complete maturation of an organism, is more influential
to an organisms’ individual preference for differing food palates than the period after this point
(Scott, 1962). This distinction does not remove the opportunity for an organism to alter its
preference for a food after maturity. A change in taste preference--post maturity--has been linked
to an external cultural influence rather than a biochemical influence that is seen before maturation
(Erickson, 2008). It is necessary to distinguish the three terms that are going to be reoccurring
throughout the entirety of this review; taste, aroma, and flavor-as they relate to preference. Taste
refers to the five traditional senses/receptors that are found on the tongue; bitter, sweet, sour, salt,
and umami. Aroma is the odor perceived when volatile compounds enter the nose and are
absorbed. Flavor is the combination of the gustatory and olfactory chemosensory stimuli, and it is
the integration of that information which formulates a flavor preference. In this paper we will look
at the underlining influence of olfactory and gustatory information integration and the role these
sensory systems have in instilling a flavor preference.
My research:
Over the course of three months I looked at the caudate putamen (CPu), piriform cortex (PC), the
agranular insular area (AIp), and the gustatory area (GU) in the left and right hemisphere of adult
female rats. These rats were commercially bought by the Neurobiology and Anatomy department
at Wake Forest Baptist Health. Before being obtained for the experiment, rats were stored in a dry
environment, 21◦±3◦ and kept in large kennels. The female rats cohabited with male rats and had
free access to a source of food pellets and water. Upon arrival to the lab, each rat was on a restricted
diet, but water was still of unlimited access and kept in individual kennels. After 24 h, rats
underwent surgery before the start of the behavioral trials. Rats were anesthetized throughout the
duration of the procedure. The goal of surgery was to implant six needles approximately 1.2mm
to 1.6mm dorsal to the most anterior portion of the brain. These needles were coated with Dil,
GFP, and DAPI immunofluorescent proteins. The Dil tag coated the tip of the needle and was used
to mark the location and point of application of the DAPI and GFP tags. The GFP was designed to
tag the excited neurons within the CPu, PC, AIp and GU which were expressed during the behavior
trials. The DAPI was designed to tag all neurons within the left and right hemispheres within this
3. range of recording. Through this method, we worked to distinguish the point in which the neurons
in the central taste and central gustatory cortices began communicating with other portions of the
brain. It is from this point, we hope to define a specific neural pathway particular to flavor
preference. Upon finding this region, if one such pathway exist, we expect to define this region
and manipulate flavor preference in lieu of some gustatory and/or olfactory sensory information.
Significant portion of the semester was spent trying to isolate the position in the brain in which the
nuclei of the neurons, which were originally tagged (at the point of application), stopped
expressing fluorescence in the nuclei of the neuron but maintained expression elsewhere-this is an
indication of signaling. From the data collected, we found inconclusive results due to an excessive
amount of GFP and DAPI tags. While we were able to accurately target the regions mentioned
above, the dyes spread further than anticipated causing uninterested regions of the brain to express
the protein. This inhibits any data collected in this experiment from being utilized or translated
into the field. However, this has allowed for a verification into the experimental scheme and allows
for an opportunity to adjust the applied quantities of staining proteins and to obtain more concrete
results.
Olfactory epithelium and the olfactory bulb:
The olfactory epithelium lines the uppermost region of the nasal cavity. Within this lining contains
bipolar primary receptor neurons that respond to the odorants that are present in air. This sensory
information is directly translated to the brain via axonal connections to the olfactory bulb, located
at the ventral region of the brain. From this region there is an accumulation of neutrophil and cells
that process, sends, and receives olfactory information. The piriform cortex is a primary area
involved in the perception and learning of olfactory sensory information. Much of this information
is then relayed to secondary cortices involved in olfactory information. While the olfactory bulbs’
primary role is to synthesize olfactory information, it is said to play a secondary role in nonspecific
mechanisms involving arousal and excitation in the forebrain (Sheperd et al., 1981). This leads to
an evolutionary purpose for aroma preference as a means of determining how hazardous a food
item is without having the need to ingest. However, this perspective does not explain the behavior
regarding intake of nutrient-less foods, or diets containing trans-fats and are high in sugar content.
Gustation:
The oral cavity contains a range of taste buds and receptors that function to detect taste, which has
been broken down into five subtypes; bitter, sweet, salt, sour, and umami. On the surface of the
4. lips and the tongue are nerve endings which respond to hot and cold temperatures. These receptors
receive sensory input which leads to depolarization at the apical end of the bud traveling down to
the basal end leading to a depolarization and release of neurotransmitters to the post-synaptic
neuronal cells. There are three pathways in which taste sensory information is able to travel
through the brain, via cranial nerves VII-facial nerve, IX-glossopharyngeal, and X-Vagus. These
neurons travel to the brain to relay information to the primary and secondary taste cortices.
Together these nerves interpret sensory information regarding the regulation and secretion of
saliva, digestive fluids, chewing, and the ionic composition of foods. These neurons project to a
multitude of regions within the brain including the thalamus, primary taste cortices-AIp and GU-,
and the orbitofrontal cortex (Breslin, A.S. P 2013). It is from the projection from these regions we
believe the taste and smell sensory systems overlap and rely information. The overlap between
these regions is not debated, however, we aim to develop a skeletal pathway to define as flavor.
Flavor and preference:
The interaction between the taste and olfactory sensory systems creates what we define as flavor.
Upon ingestion of foods, vertebrate have an innate bodily response to its biochemical component
which can either lead to a positive (source of energy) or aversive response (nausea, diarrhea, etc.)
(Loper, B.H, et al. 2015). It is significantly more efficient to condition adult male rats to respond
to an aversive stimulus, which led to nausea, than conditioning rats using a positive stimulus,
cocaine (Vollbrecht J. P. et al. 2015). However, this does not explain the trend in the western world
for humans to intake foods which lead to obesity, heart problems, and/or digestive complications.
The previous examples are considerably long-term effects compared to the conditions studied in
previous experiments; this discrepancy has led to numerous projects looking at the interaction
between motivation and hunger as opposed to the functional influence of this behavior (Sugar et
al. 2005). It’s important to note that foods are unable to be identified on the sole basis of smell or
on the sole basis of taste. To correctly identify a particle of food, subjects must place the food on
the tongue and obtain a secondary olfactory sensory input. This secondary olfactory information
plays a critical role in flavor preference (Mozell et al. 1969). Before gustatory information reaches
the brain, it travels through the greater palatine nerve which has terminal axons in the nasal cavity
and terminal axons leading to three cranial nerves mentioned previously. I propose this connection
to the nasal cavity is essential to the creation of flavor preference. The taste sensory information
travels to the cranial nerves via the palatine nerve by an electric potential--not by a chemical
5. means--which is characteristic of odors present in air. I propose that this action potential leads to
differential activation that mimics the binding of odors to the bipolar neurons in the upper
epithelium lining, and that it is this type of stimulation which is essential for the communication
between the olfactory and taste cortices and thus the construction of a flavor preference.
Conclusion:
Despite the lapse in data from this semester, what we have found has enabled us to ask more
questions in regards to the magnitude in which the olfactory and gustatory sensory systems
interact. In our future research we plan to use a more precise method of injection which will lead
to clearer data and thus definite interpretations. There is no doubt that these two sensory systems
interact, but we are unsure whether these systems are highly influenced by environmental stimuli
or by an unknown neural pathway-that being what we worked to deconstruct this semester. Upon
this distinction, we hope to address the mechanisms that construct and alter flavor preference.
6. References:
Breslin, A.S.P (2013). An Evolutionary Perspective on Food Review and Human Taste. NIH.
23(9), R409-R418.
Erickson, P.R. (2008). A study of the science of taste: On the origins and influence of the core
ideas. Cambridge. 31, 63-66.
Getchell, T. V., and Shepard, G.M. (1978). Adaptive properties of olfactory receptor analysed with
odour pulses of varying durations. J.Physiol. 282: 541-560.
Loper, B.H., La Sala, M., Dotson, C., and Steinle, N (2015). Taste perception, associated hormonal
modulation, and nutrient intake. Nutr Rev. 73(2), 83-87.
Mozell, N. M., Smith, B.P. Smith, P.E., Sullivan, R.L., and Swender, P. (1969). Nasal chemo-
reception in flavor identification. Arch. Otolaryngol. 90, 131-137.
Scott, J.P. (1962). Critical periods in behavioral development. Science 138, 949-958.
Sugai, T., Yoshimura, H., and Onoda, Norihiko. (2005). Functional Reciprocal Connections
between Olfactory and Gustatory Pathways. Chemse. 30, i166-i167.
Vollbrecht, J.P., Nobile, W.C., Chafferfon, M.A., Jutkiewicz M.E., and Ferrario, R.C (2015). Pre-
existing differences in motivation for food and sensitivity to cocaine-induced locomotion in
obesity-prone rats. Elsvier. 152, 151-160.
