Response properties of cerebellar neurons to stimulation of the frog glossopharyngeal nerve and tongue

The cerebellum receives information from many kinds of sensory organs (muscle, somatosensory, auditory, vestibular, visual) as well as from the autonomic system. The cerebellum presumably has a role in the control of tongue movement and salivary secretion. However, the relationship between cerebellar neuron activity and tongue sensation has not been investigated previously. In the present study, negative cerebellar field potentials in the molecular layer and single unit responses of Purkinje cells induced by electrical stimulation of the bullfrog glossopharyngeal (IXth) nerve or tongue surface were investigated. The interaction between IXth nerve stimulation and natural (taste and touch) stimulation of the tongue in their effects on cerebellar neuron activity were investigated. The negative field potentials were potentiated by a brief train of electrical pulses to the tongue or IXth nerve. With electrical stimulation of the tongue surface, several fungiform papillae were needed to elicit cerebellar field potentials. The latency of Purkinje cells following IXth nerve stimulation was 44.4-53.6 msec for complex spikes, whereas for simple spikes two maxima were seen, with mean values at 33.9-36 msec and 96.8 msec. A preceding electrical stimulation of the IXth nerve depressed the negative field potentials or Purkinje cell complex spike responses induced by test stimulation of the IXth nerve. These depressive effects were also seen following a preceding natural stimulation of the tongue and were dependent upon the type of preceding stimulation. The depressive effects were produced by preceding stimulation with NaCl, CaCl2, water, and touch, but not with quinine and acetic acid stimulation. These results clearly demonstrate that gustatory and tactile signals from the tongue can influence cerebellar neuron activity.     

Bitter-responsive brainstem neurons: Characteristics and functions

The sensation that humans describe as “bitter” is evoked by a large group of chemically diverse ligands. Bitter stimuli are avoided by a range of species and elicit reflex rejection, behaviors considered adaptations to the toxicity of many of these compounds. We review novel evidence for neurons that are narrowly tuned to bitter ligands at the initial stages of central processing. These “B-best” neurons in the nucleus of the solitary tract (NST) and parabrachial nucleus (PBN) respond to multiple types of bitter stimuli and exhibit average responses to bitter tastants that are 6-8 times larger than to moderate concentrations of compounds representing other qualities. However, in the PBN B-best units are appreciably activated by intense salt and acid. Neurons broadly sensitive to salts and acids (“AN” neurons) also responded to bitter stimuli. This sensitivity appeared restricted to stronger intensities of ionic bitters, as cycloheximide remained ineffective across concentrations. In addition to chemosensitive profile, B-best neurons were also distinctive with regard to their posterior receptive fields, long latencies, slow firing rates and projection status. Compared to B-best NST cells, those in the PBN received increased convergence from anterior and posterior receptive fields and responded to a greater number of bitter stimuli. We conclude that B-best neurons likely contribute to pathways underlying gaping, aversive hedonic quality and taste coding. The differential responsiveness of B-best and AN neurons to ionic and nonionic bitter ligands also suggests a potential substrate for discrimination within this quality.     

A comparison of voluntary salt-intake behavior in Nax-gene deficient and wild-type mice with reference to peripheral taste inputs

The Na(x) channel, a subfamily of voltage-gated sodium channels, is thought to be a specific sodium receptor in the central nervous system. Our previous study revealed that Na(x)-gene-deficient mice consumed excessive amounts of NaCl even under water-deprived conditions. In the present study, to investigate whether the peripheral taste inputs are involved in the abnormal intake of salt in Na(x)-deficient mice (homo), voluntary intake of various taste solutions in homo and wild-type mice (wild) was examined under non-deprived conditions. Homo showed a higher preference for 0.15 M NaCl solution than wild. Preference ratios for other basic tastants were identical between groups. Transection of the chorda tympani (CT) or the glossopharyngeal (GP) nerve had little effect on salt-intake behavior in homo and wild. Although combined transection of the superior laryngeal (SL) and GP nerves decreased NaCl intake in homo but not in wild, there were no differences in preference ratios for NaCl in homo before and after SL+GP transection. On the other hand, preference ratios for NaCl in wild tended to increase after combined SL and GP transection. Consequently, preference ratios for NaCl after SL+GP transection were no different between homo and wild. While electrophysiological responses of the CT and the GP to various taste solutions were indistinguishable between homo and wild, those of the SL to NaCl in homo were smaller than those in wild only at lower concentrations (0.01 and 0.03 M). Thus, chemosensory inputs from the oro-pharyngeal regions had little effect on abnormal salt intake in homo, if any. From these results, it is suggested that the higher preference for NaCl in homo is mainly due to the lack of Na(x) channels in the central nervous system.     

Voltage-gated channels involved in taste responses and characterizing taste bud cells in mouse soft palates

Taste bud cells (TBCs) on soft palates differ from those on tongues in innervation and chemosensitivity. We investigated voltage-gated channels involved in the taste responses of TBCs on mouse soft palates under in-situ tight-seal voltage/current-clamp conditions. Under the cell-attached mode, TBCs spontaneously fired action currents, which were blocked by application of 1 microM TTX to TBC basolateral membranes. Firing frequencies increased in response to taste substances applied to TBC receptor membranes. Under the whole-cell clamp mode, as expected, TBCs produced various voltage-gated currents such as TTX-sensitive Na+ currents (INa), outward currents (Iout) including TEA-sensitive and insensitive currents, inward rectifier K+ currents (Iir), and Ca2+ currents including T-type, P/Q-type, and L-type Ca2+ currents. We classified TBCs into three types based on the magnitude of their voltage-gated Na+ currents and membrane capacitance. HEX type (60% of TBCs examined) was significantly larger in Na+ current magnitude and smaller in membrane capacitance than LEX type (23%). NEX type (17%) had no Na+ currents. HEX type was equally distributed within single taste buds, while LEX type was centrally distributed, and NEX type was peripherally distributed. There were correlations between these electrophysiological cell types and morphological cell types determined by three-dimensional reconstruction. The present results show that soft palate taste buds contain TBCs with different electrophysiological properties, and suggest that their co-operation is required in taste transduction.     

The contribution of gustatory nerve input to oral motor behavior and intake-based preference. I. Effects of chorda tympani or glossopharyngeal nerve section in the rat

Two-bottle intake tests and taste reactivity (TR) tests were used to reveal whether changes in ingestive behavior would follow bilateral section of either the chorda tympani (CT) or the glossopharyngeal (GP) nerve. Rats received two-bottle intake tests to compare 24-h ingestion of water to that of NaCl, MgCl2, quinine, or sucrose. Prior to each long-term intake test, rats received a 1 min, 1 ml intraoral infusion of the same chemical stimulus. Ingestive and aversive oral motor responses elicited by these 1 ml infusions were videotaped and subsequently analyzed. GP-section did not alter quinine or sucrose preference; overall, preference of MgCl2 and NaCl was also similar to controls. In contrast, TR tests in GP-sectioned rats revealed that most quinine, MgCl2 and NaCl stimuli elicited significantly fewer aversive oral motor responses. In addition, the latency of aversive responses to these 3 chemical stimuli was increased for these rats. Intake-based preference tests failed to show any difference between rats with CT nerve section and controls. In TR tests, however, CT-sectioned rats displayed significantly fewer ingestive oral motor responses to NaCl, MgCl2, and quinine than controls. Neither sucrose intake nor sucrose-elicited TR were altered by CT or GP nerve section. This report confirms the failure of long-term intake tests to uncover behavioral deficits following the section of gustatory nerves. In contrast, the use of a different behavioral test makes clear for the first time that gustatory nerve section has dramatic consequences on ingestive behavior. The examination of taste elicited oral motor behaviors reveals a coherent and nerve specific pattern of neurological deficit following peripheral nerve section.     

Response to NaCl taste in mixture with HCl by sodium deficient rats

Sodium deficient, adrenalectomized rats and nondeficient control rats were offered, for 20 min, a mixture of 0.1 M NaCl and 0.02 N HCl. The sodium deficient rats drank substantial amounts whereas control animals were essentially indifferent to the mixture. Further tests on the sodium deficient rats with a stronger NaCl concentration in the mixture did not alter the amount drunk, which was about $\text{1}\text{3}$ the amount ingested of singly presented 0.1 NaCl. These results suggest that the sodium deficient rat can recognize the similarity of the mixture to NaCl but that the other mixture component influences the behavioral outcome in a way other than simple intensity suppression.     

Sex differences in odor identification ability: a cross-cultural analysis

To ascertain the generality of a sex difference noted in odor identification ability, the University of Pennsylvania Smell Identification Test (UPSIT) was administered to four groups of subjects: Black Americans (n = 438), White Americans (n = 1559), Korean Americans (n = 106), and Native Japanese (n = 308). The women of all four groups outperformed the men to the same relative degree. The Korean American group performed better than the Black and White American groups, which, in turn, outperformed the Native Japanese. Analyses of the proportions of subjects correctly answering each of the test items revealed considerable similarity of relative item difficulty among the subject groups. Taken together, these data suggest that sex differences in odor identification ability are probably not due to ethnic or cultural factors, per se.