A noninvasive stimulus onset device for use in gustatory research.

A simple device for precise, noninvasive measurement of stimulus onset time in gustatory research is described. This device utilizes an ultrafast, self-heated, micro-thermistor to sense the cooling effect of the fluid stream that bathes the tongue.

Convergence of lingual and palatal gustatory neural activity in the nucleus of the solitary tract.

The responses of 54 neurons to independent sapid stimulation of 4 taste receptor subpopulations associated with: (1) anterior tongue; (2) nasoincisor ducts; (3) soft palate; and (4) foliate papillae were recorded from the nucleus of the solitary tract (NST) of the Rat. Neurons responding to stimulation of receptor subpopulations in the anterior oral cavity (anterior tongue or nasoincisor ducts) were located more rostrally in the NST than neurons responding to stimulation of receptor subpopulations in the posterior oral cavity (soft palate or foliate papillae). Half of the sampled neurons responded exclusively to stimulation of one receptor subpopulation with the remaining neurons responsive to stimulation of two or more receptor subpopulations. The most common pattern of convergence observed was between responses arising from stimulation of the taste buds on the anterior tongue and those associated with the nasoincisor ducts of the hard palate. The sensitivity of NST neurons to anterior tongue and nasoincisor duct stimulation with the 4 standard taste stimuli was determined. When stimulating the anterior tongue, the order of effectiveness was NaCl greater than HCl greater than sucrose greater than quinine hydrochloride (QHCl). When the nasoincisor ducts were tested, however, the order of stimulus effectiveness was strikingly different: sucrose was the best stimulus, followed by HCl, NaCl, and QHCl. If both the anterior tongue and nasoincisor ducts are included, stimulation of taste receptors in the anterior oral cavity of the rat produces good responses to stimuli representing 3 of the 4 classical taste qualities: sweet, salty, and sour.

Specificity of acquired aversions to taste qualities in hamsters and rats.

Nine groups of rats and 27 groups of hamsters (n = 12/group) each tasted 1 from among 27 different solutions before receiving an ip injection of apomorphine, then were tested for aversions to 4 solutions prototypic of human beings' four taste qualities (S, sucrose; N, NaCl; H, Hcl; Q, quinine hydrochloride). With most of the solutions that are described as sweet by humans employed as a conditional stimulus (CS), the rodents acquired an S aversion (exceptions occurring for some artificial sweeteners). With CSs described as either predominantly salty or sour by humans, the rodents acquired an N aversion in the former case or an H aversion in the latter case; a Q aversion was also acquired with two (including H) of the three CSs described as sour. With most of the CSs described as predominantly bitter or as having a considerable bitter component by humans, the rodents acquired a Q aversion (as well as a weaker H aversion). Considerable parallels among the taste sensations of humans, rats, and hamsters are indicated. Patterns of activity evoked across four classes of peripheral gustatory neurons (those responding best to lingual stimulation with S, N, H, or Q) in rodents when the CSs were applied to the tongue were similar to the patterns of aversions across the four test stimuli for the CSs. This suggests that these four neural channels mediate the sensations evoked by S, N, H, and Q in rats and hamsters, perhaps even in human beings.

The pontine taste area in the rat.

The pontine taste area relays gustatory information from the rostral pole of the solitary nucleus to both the thalamus and ventral forebrain. An electrophysiological investigation of this area was carried out in 3 stages. First, multiunit responses from the dorsal pons were mapped using sapid, thermal, and tactile stimuli applied to the anterior tongue. The gustatory zone lies within and just dorsal and ventral to the brachium conjunctivum as it enters the pons from the cerebellum. Second, gustatory stimuli were applied independently to the anterior and posterior tongue to determine whether receptors in both fields are represented in the pons. Responses with characteristics similar to those obtained from the glossopharyngeal nerve were located on the dorsal edge of the pontine gustatory zone. More ventrally the responses from the posterior tongue mimicked anterior tongue responses, but were of lesser amplitude than the largest anterior responses occurring at the ventral edge of the gustatory zone. Third, 71 single units were isolated in the dorsal pons, and tested for sensitivity to gustatory stimulation of the anterior and posterior tongue separately. More than half the units responded to gustatory stimuli--some from the anterior tongue alone, some from the posterior alone, but most responded to stimuli applied to either field. In the latter instance 7 of 10 units tested continued to respond after anesthetizing the chorda tympani with Xylocaine instilled into the middle ear, thus demonstrating a true glossopharyngeal input. This proves that gustatory information from two distinct receptive fields may converge on the same central neuron.

Taste nerve fibers: a random distribution of sensitivities to four tastes.

The numbers of rat glossopharyngeal and chorda tympani fibers responding to one, two, three, or four taste stimuli of different quality (sodium chloride, hydrochloric acid, quinine, and sucrose) and to each of the six possible pairs of these stimuli can be predicted if there are four independent sensitivities randomly distributed among innervating fibers.

Olfactory input to the hypothalamus: electrophysiological evidence.

Electrical stimulation of the rat's olfactory bulb or lateral olfactory tract elicited unit discharges in the region of the medial forebrain bundle of the lateral hypothalamus, with latencies of 4 to 25 milliseconds. Unit responses in this area were driven by odors in preparations that were paralyzed to prevent breathing artifacts.