Gustatory Cortex

The sense of taste guides nutrient consumption and affects overall health and quality of life. One of the main functions of taste is the detection of chemical stimuli that elicit the perception of canonical qualities such as sweet, salty, sour, and bitter. Here, I have used a two-response operant task to essentially ask an animal, “can you taste this?” after damage to a key brain area of the gustatory system. Through the application of psychophysical methods, psychometric taste sensitivity functions were derived for a given taste stimulus. The necessity of arguably the highest-ordered central gustatory region in the rodent, the gustatory cortex (GC), in the maintenance of normal sensitivity to prototypical tastants was assessed by placing large lesions at this brain site. The thalamocortical pathway, in which taste signals ascend from the ventral posteromedial thalamus, parvicelluar layer (VPMpc) to GC, was first hypothesized to be responsible for discriminative taste function by Carl Pfaffmann and colleagues in 1977. Although the delineations of the GC are debated, neurons that are activated when a tastant is delivered onto the tongue have been found and neuronal tracing studies confirm projections from taste thalamus reside here. Although these projections have been described, one aim here was to precisely map the connections from thalamus to GC along its anterior/posterior (AP) axis, with aid of a neuronal tract-tracer. Our prior research has revealed that bilateral GC lesions significantly impair taste sensitivity to salts and quinine but not to sucrose. The range of tastants tested has been extended here to include: a) the polysaccharide, Maltrin, a highly preferred carbohydrate discriminable from sugars, and b) citric acid, a representative compound that in humans generates sour taste. The detectability of these stimuli, as well as NaCl, was tested in rats with neurotoxin-induced (ibotenic acid) GC lesions and in sham-operated controls (SHAM). The effect of unilateral GC lesions on taste detectability was also tested because, although taste deficits are apparent in humans with unilateral damage in insular cortex, there is controversy over which hemisphere is involved. Rats were trained, via operant conditioning, and tested in a gustometer to discriminate a tastant from water in a two-response taste detection task. Psychometric sensitivity functions were derived by lowering the stimulus concentration across test sessions. A lesion mapping system was then used to determine placement and size of left (LGCX), right (RGCX), and bilateral (BGCX) lesions and, in the case of the latter, symmetry as well. Replicating our prior work, for NaCl, there was a significant rightward shift in taste sensitivity between BGCX and SHAM rats and between RGCX and SHAM rats. Similar to sucrose, taste sensitivity to Maltrin was not significantly different between lesion and SHAM groups. Although there was no significant lateral shift in taste sensitivity between surgical groups to citric acid, some declines in BGCX performance relative to SHAM rats were evident for higher concentrations. Together with prior results, complete bilateral lesions in GC do not appear to disrupt sensitivity to carbohydrate stimuli, but do appear to modestly impair sensitivity to quinine and severely impair sensitivity to salts. In the case of the latter stimulus, deficits are evident after unilateral lesions in the right GC. Nevertheless, in all of these cases the rats perform well at higher concentrations. Clearly other central brain regions appear to contribute to sensory-discriminative taste processing in rats. Similar to the controversy over where GC resides within the insular cortex, there are also conflicting reports of where these projections from VPMpc terminate within insular cortex. Here, a fluorescent tract-tracer, DiI, was systematically injected into various sites of GC along the AP axis to quantify the anatomical organization of projections arising from the VPMpc which was analytically partitioned into subfields for quantification of retrogradely labeled neurons. The results confirm previous findings showing that projections to the GC from thalamus arise from the medial portion of VPMpc, whereas projections to the areas dorsal to the conventionally defined GC arise from the more lateral portions of VPMpc. Interestingly, an anterior-posterior organization from the VPMpc to the GC was also found, supporting an orotopic topography in GC suggested in the literature that appears to be roughly maintained along the ascending gustatory pathway in the rat brain. Overall, these experiments have added to the understanding of the functional and neuroanatomical organization of the central gustatory system in the rodent model. However, there is much more work to be done to identify central regions that contribute to sensory-discriminative and other domains of taste function.