Influence of atropine on taste-stimulated parotid secretion

Conditioning of human salivary flow using a visual cue for sour candy

OBJECTIVE

Although the "mouthwatering" to sight, smell, or thought of food is commonly accepted in food and nutrition research, the concept of mouthwatering and human salivary flow conditioning is not well accepted in salivary research. The objective of this study was to revisit whether human salivary flow could be classically conditioned to a previously neutral stimulus.

DESIGN

Sour candy or a non-food control in opaque containers were presented to healthy participants (n = 8). Simple images were consistently paired with container contents. Participants viewed the images for 15 s, then opened the containers and ate (candy) or did not eat (non-food control) the contents. This was repeated 14 times (7 of each stimulus). Order was semi-randomized to ensure one candy and one non-food were presented as the first two and last two stimuli. Saliva was collected with cotton dental rolls during these presentations (first two and last two) after viewing the image for 15 s, but before opening the container.

RESULTS

Participants were successfully conditioned to increase salivary flow in response to the image that predicted candy, as demonstrated by greater weight of saliva in response to 1) the candy-paired image than the non-food-paired image, and 2) the candy-paired image at the end of the first visit compared with the beginning (when the image had no meaning). However, the effect was attenuated during the second visit.

CONCLUSIONS

We demonstrate classical conditioning of human salivary flow is achievable, but the effect may not persist to a second visit.

Do non-nutritive sweeteners influence acute glucose homeostasis in humans? A systematic review

The human body associates sensory cues with metabolic consequences. Exposure to sweet-tasting sugars - even in the absence of ingestion - triggers physiological responses that are associated with carbohydrate digestion, absorption and metabolism. These responses include the release of insulin and incretin hormones, which work to reduce blood glucose. For this reason, non-nutritive sweeteners (NNS) have been posited to trigger similar physiological responses and reduce postprandial blood glucose concentrations. The first part of this review presents a brief overview of sweet taste receptor activation in the oral cavity and gastrointestinal tract and the ensuing physiological responses related to glucose homeostasis. The second part of this review contains a systematic literature review that tested the hypothesis that NNS use improves glucose regulation postprandially. Studies were grouped based on sweet taste receptor stimulation paradigms, including pre-ingestive stimulation, ingestion of NNS alone, co-ingestion of NNS with foods, and using NNS as preloads to influence subsequent blood glucose excursions. In summary, the review found that NNS triggered physiological responses, albeit inconsistently, yet failed to significantly lower blood glucose levels in almost all studies.

Nutritional implications of the cephalic-phase salivary response

The rapid release of saliva following cognitive or sensory stimulation is termed the cephalic phase salivary response. Optimal methods and conditions for measuring the cephalic phase salivary response have not been established. Consequently, determination of the extent to which the large inter and intra-individual variability reflect biological or procedural influences is not possible. The nutritional significance of the response may be great due to the multiple roles played by saliva in food selection and digestion. The cephalic phase salivary response has been proposed as an objective index of various individual states (e.g. hunger) and characteristics (e.g. dietary restraint, extroversion), but its value for such purposes requires further verification.

Gustatory-salivary reflex: neural activity of sympathetic and parasympathetic fibers innervating the submandibular gland of the hamster

Electrophysiological experiments were performed to clarify the neural control mechanisms subserving gustatory-salivary reflex in anesthetized and decerebrate hamsters. Efferent neural activities of postganglionic sympathetic and preganglionic parasympathetic fibers, innervating the submandibular gland, were recorded when taste stimuli were infused into the oral cavity. Neural activities of primary gustatory afferents were also recorded from the chorda tympani (innervating the anterior part of the tongue) and the glossopharyngeal nerve (innervating the posterior part of the tongue). The parasympathetic fibers showed a low rate of spontaneous discharges (about 0.3 Hz), and responded tonically in an excitatory manner to taste stimulation. The magnitude of parasympathetic activity was highly correlated with the magnitude of gustatory afferent responses of the chorda tympani rather than that of the glossopharyngeal nerve. On the other hand, the sympathetic fibers showed irregular burst discharges (1.5 burst/s), and the rate of burst discharges was increased in response to high concentrations of HCl (0.03 M) or NaCl (1 M) solutions. Deafferentation experiments suggest that the parasympathetic activity is mainly influenced by gustatory information via the chorda tympani, while the sympathetic activity can be evoked by both the chorda tympani and glossopharyngeal nerve.

Odorant stimulation of secretory and neural processes in the salamander olfactory mucosa

Topical application of the odorants guaiacol (10(-3) mol/l, 1-30 min) and 2-isobutyl-3-methoxypyrazine (IBMP, 10(-5)-10(-3) mol/l, 15 min) caused time- and concentration-dependent reductions in the secretory granule content of acinar cells of the superficial Bowman's glands (sBG) and moderate to extensive vacuolation in acinar cells of sBG and deep olfactory glands (dG). Topical application of 9.8 mg/ml scopolamine 10 min before 10(-4) mol/l IBMP significantly reduced the amount of secretory granule depletion from sBG compared to that seen with IBMP alone and resulted in less extensive vacuolation in sBG and dG acinar cells. The i.p. injection of 42 mg/kg propranolol 10 min before topical application of 10(-4) mol/l IBMP had no effect on the action of IBMP. Guaiacol and IBMP also had time- and concentration-dependent effects on the secretory activity of sustentacular cells in the olfactory epithelium. The protrusion of secretory material into the mucociliary matrix that covers the epithelial surface and vacuolation within the secretory material resulted from odorant application. Scopolamine and propranolol had no effects on the action of IBMP on sustentacular cell secretory activity. When applied in the vapor phase, guaiacol elicited action potentials recorded from individual olfactory receptor neurons; the impulse frequency was concentration-dependent and showed tonic and phasic components when the duration of stimulation was varied. Low to moderate concentrations of IBMP delivered in the vapor phase evoked monophasic negative slow voltage transients recorded from the surface of the olfactory mucosa. The amplitudes of these transients increased with increasing stimulus concentrations. Higher concentrations or longer stimulus durations evoked longer-latency positive-voltage generating processes and negative afterpotentials. The properties of the electrophysiological responses to both odorants were characteristic of responses evoked by a wide variety of 'typical' odorants.