Olfactory responses in the gustatory area of the parabrachial pons

Electrophysiological responses to olfactory and gustatory stimuli were recorded in the same neural elements in the parabrachial nuclei (PbN) of rats under Flaxedil. Responses to olfactory stimuli varied in temporal pattern and magnitude from responses to gustatory stimuli. Intravenous infusion of Nembutal while recording from one unit appeared to alter the pattern of responsiveness across stimuli. Most importantly the response to one olfactory stimulus (vanilla) was eliminated by this procedure. Olfactory-gustatory elements were located in the lateral portion of the taste-responsive area of the PbN, just below the brachium conjunctivum. These results suggest that the PbN receives input from both olfactory and gustatory pathways and that these pathways may converge on the same neural elements within the PbN. This convergence may be part of a neurophysiological substrate for the perception of flavor.     

Elucidating coding of taste qualities with the taste modifier miraculin in the common marmoset

To investigate the relationships between the activity in different types of taste fibers and the gustatory behavior in marmosets, we used the taste modifier miraculin, which in humans adds a sweet taste quality to sour stimuli. In behavioral experiments, we measured marmosets' consumption of acids before and after tongue application of miraculin. In electrophysiological experiments responses of single taste fibers in chorda tympani and glossopharyngeal nerves were recorded before and after tongue application of miraculin. We found that after miraculin marmosets consumed acids more readily. Taste nerve recordings showed that after miraculin taste fibers which usually respond only to sweeteners, S fibers, became responsive to acids. These results further support our hypothesis that the activity in S fibers is translated into a hedonically positive behavioral response.     

The neural code for taste in the nucleus of the solitary tract of the rat: effects of adaptation

Adaptation of the tongue to NaCl, HCl, quinine or sucrose was used as a tool to study the stability and organization of response profiles in the nucleus of the solitary tract (NTS). Taste responses in the NTS were recorded in anesthetized rats before and after adaptation of the tongue to NaCl, HCl, sucrose or quinine. Results showed that the magnitude of response to test stimuli following adaptation was a function of the context, i.e., adaptation condition, in which the stimuli were presented. Over half of all taste responses were either attenuated or enhanced following the adaptation procedure: NaCl adaptation produced the most widespread, non-stimulus-selective cross-adaptation and sucrose adaptation produced the least frequent cross-adaptation and the most frequent enhancement of taste responses. Adaptation to quinine cross-adapted to sucrose and adaptation to HCl cross-adapted to quinine in over half of the units tested. The adaptation procedure sometimes unmasked taste responses where none were present beforehand and sometimes altered taste responses to test stimuli even though the adapting stimulus did not itself produce a response. These effects demonstrated a form of context-dependency of taste responsiveness in the NTS and further suggest a broad potentiality in the sensitivity of NTS units across taste stimuli. Across unit patterns of response remained distinct from each other under all adaptation conditions. Discriminability of these patterns may provide a neurophysiological basis for residual psychophysical abilities following adaptation.     

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.     

Botulinum toxin in the therapy of gustatory sweating

Three patients suffering from gustatory sweating following trauma to the preuricular region from a bullet wound or parotid gland surgery were treated by intracutaneous injection of botulinum toxin A. Within 2 weeks, gustatory sweating in the area injected completely ceased in all patients with no side-effects. The efficacy of treatment was confirmed by repeated Minor’s iodine starch tests. So far, sweating has not recurred during a follow-up period of up to 8 months. Botulinum toxin appears to be a promising new drug for the treatment of this autonomic disorder.     

Partial recovery of gustatory function after neurol tissue transplantation to the lesioned gustatory neocortex

The ability of homotypic cortical tissue grafts to induce recovery of function after a gustatory neocortex (GN) lesion was studied using the conditioned taste aversion (CTA) paradigm. On acquisition day, 26 GN-lesioned and 8 sham-lesioned rats were presented with a saccharin solution, followed by an injection of the illness-inducing agent lithium chloride (LiCl). On the test day, 2 days later, saccharin was presented again. The GN-lesioned rats showed significantly less aversion to saccharin on the test day, indicating that the lesion impaired their ability to form taste-illness association. Nine of the lesioned rats were then bilaterally transplanted with fetal GN tissue. Nine weeks after the transplantation, the rats were presented with a LiCl solution, which served as both a tastant and an illness-inducing agent. An NaCl solution, which tasted very similar to the LiCl solution, was used to test the CTA to salt 3 days later. The nontransplanted rats consumed significantly more LiCl than the transplanted and sham-operated rats on the acquisition day, but both transplanted and nontransplanted rats consumed more NaCl than sham-operated rats on the test day. Nissl and Golgi stainings showed numerous somata and extensive arborization of neurons within the grafts. The results indicate that fetal GN grafts can restore the ability to integrate gustatory and visceral inputs but not to form long-lasting taste-illness associations.     

Testing a neural coding hypothesis using surrogate data

Determining how a particular neuron, or population of neurons, encodes information in their spike trains is not a trivial problem, because multiple coding schemes exist and are not necessarily mutually exclusive. Coding schemes generally fall into one of two broad categories, which we refer to as rate and temporal coding. In rate coding schemes, information is encoded in the variations of the average firing rate of the spike train. In contrast, in temporal coding schemes, information is encoded in the specific timing of the individual spikes that comprise the train. Here, we describe a method for testing the presence of temporal encoding of information. Suppose that a set of original spike trains is given. First, surrogate spike trains are generated by randomizing each of the original spike trains subject to the following constraints: the local average firing rate is approximately preserved, while the overall average firing rate and the distribution of primary interspike intervals are perfectly preserved. These constraints ensure that any rate coding of information present in the original spike trains is preserved in the members of the surrogate population. The null-hypothesis is rejected when additional information is found to be present in the original spike trains, implying that temporal coding is present. The method is validated using artificial data, and then demonstrated using real neuronal data.     

Genetic tracing of the gustatory and trigeminal neural pathways originating from T1R3-expressing taste receptor cells and solitary chemoreceptor cells

We established transgenic mouse lines expressing a transneuronal tracer, wheat germ agglutinin (WGA), under the control of mouse T1R3 gene promoter/enhancer. In the taste buds, WGA transgene was faithfully expressed in T1R3-positive sweet/umami taste receptor cells. WGA protein was transferred not laterally to the synapse-bearing, sour-responsive type III cells in the taste buds but directly to a subset of neurons in the geniculate and nodose/petrosal ganglia, and further conveyed to a rostro-central region of the nucleus of solitary tract. In addition, WGA was expressed in solitary chemoreceptor cells in the nasal epithelium and transferred along the trigeminal sensory pathway to the brainstem neurons. The solitary chemoreceptor cells endogenously expressed T1R3 together with bitter taste receptors T2Rs. This result shows an exceptional signature of receptor expression. Thus, the t1r3-WGA transgenic mice revealed the sweet/umami gustatory pathways from taste receptor cells and the trigeminal neural pathway from solitary chemoreceptor cells.     

Microinjection of bicuculline into the central nucleus of the amygdala alters gustatory responses of the rat parabrachial nucleus

The central amygdaloid nucleus (CeA) receives projection from the parabrachial nucleus (PBN) gustatory neurons and descendingly projects to the PBN, and taste responses in the PBN are significantly affected by stimulation or lesion of the CeA. To examine whether the GABA receptors within the CeA are involved in this modulation, the effects of microinjection of bicuculline, a GABA(A)-selective antagonist, into the CeA on the activities of PBN taste neurons were observed by using extracellular recording technique. In general, after bicuculline was administered to ipsilateral CeA, the responses of PBN neurons to four tastants all increased, with the magnitudes significantly higher than those obtained before drug administration (P<0.01), respectively. However, after bicuculline was delivered into the contralateral CeA, only the responses to NaCl, HCl and QHCl increased. According to the best-stimulus category, 47% NaCl-best (8/17), 64% HCl-best (7/11), 80% QHCl-best (4/5), and 33% sucrose-best (1/3) increased their responses to at least one basic taste stimulus after GABA(A) receptors within the ipsilateral CeA were blocked. After contralteral CeA injection, more NaCl-best neurons (6/8) increased responses than that after ipsilateral CeA injection, but other best-stimulus units showed no differences before and after drug injection into the contralateral CeA. Analyses of across-unit patterns indicated that the correlation coefficient of responses between NaCl and sucrose was apparently higher after drug administration to the CeA. However, after drug injection into the contralateral CeA, the correlations between NaCl and the other three tastants were higher than those before. These results indicate that the GABA(A) receptors within the CeA play an important role in modulating the gustatory activities of PBN neurons.     

Population coding in spike trains of simultaneously recorded retinal ganglion cells

To achieve a better understanding of the parallel information processing that takes place in the nervous system, many researchers have recently begun to use multielectrode techniques to obtain high spatial- and temporal-resolution recordings of the firing patterns of neural ensembles. Apart from the complexities of acquiring and storing single unit responses from large numbers of neurons, the multielectrode technique has provided new challenges in the analysis of the responses from many simultaneously recorded neurons. This paper provides insights into the problem of coding/decoding of retinal images by ensembles of retinal ganglion cells. We have simultaneously recorded the responses of 15 ganglion cells to visual stimuli of various intensities and wavelengths and analyzed the data using discriminant analysis. Models of stimulus encoding were generated and discriminant analysis used to estimate the wavelength and intensity of the stimuli. We find that the ganglion cells we have recorded from are non-redundant encoders of these stimulus features. While single ganglion cells are poor classifiers of the stimulus parameters, examination of the responses of only a few ganglion cells greatly enhances our ability to specify the stimulus wavelength and intensity. Of the parameters studied, we find that the rate of firing of the ganglion cells provides the most information about these stimulus parameters, while the timing of the first action potential provides almost as much information. While we are not suggesting that the brain is using these variables, our results show how a population of sensory neurons can encode stimulus features and suggest that the brain could potentially deduce reliable information about stimulus features from response patterns of retinal ganglion cell populations.     

Lateral parabrachial nucleus lesions in the rat: long-and short-duration gustatory preference tests

The present study reports two experiments that evaluated the influence of bilateral ibotenic acid lesions of the viscerosensory neurons in the lateral parabrachial nucleus (LPBN) on intake of four prototypical taste stimuli (sucrose, sodium chloride, citric acid, and quinine hydrochloride). In the 24-h, two-bottle tests of Experiment 1, rats with lesions of the LPBN were severely impaired in their concentration-dependent consumption of sucrose, displayed a mild disturbance of sodium chloride intake, and drank normal amounts of citric acid and quinine hydrochloride. These lesion-induced deficits were less pronounced when assessed with the 15-min, 1-bottle tests of Experiment 2. The results suggest that destruction of the viscerosensory neurons within the LPBN disrupt the processing of gastrointestinal feedback.     

Differential projections from gustatory responsive regions of the parabrachial nucleus to the medulla and forebrain

The present study combined extracellular electrophysiology with anterograde and retrograde tracing techniques to determine efferent projections from taste responsive sites within the parabrachial nucleus (PBN). Taste activity was recorded from two distinct regions of the PBN, the waist region consisting of the ventrolateral (VL) and central medial (CM) subnuclei, and the external region, consisting of the external medial (EM) and external lateral (EL) subnuclei. Ascending and descending projections from these two regions differed. Small biotinylated dextran injections placed in taste responsive sites in the waist area produced a prominent descending projection to the medullary parvocellular reticular formation, a projection nearly non-existent from the external region. Differences in ascending projections were more subtle. Projections to the thalamus were bilateral in all cases, however, the waist region had a larger ipsilateral thalamic projection than the external region and the external region had a larger contralateral projection compared to the waist. Central nucleus of amygdala (CNA) projections from the waist area were primarily from posterior tongue responsive sites in VL and terminated in the central medial and lateral CNA subnuclei; external region projections were distributed to the capsular region of CNA. Both the external and waist region projected to substantia innominata (SI). Different efferent projections from the two gustatory responsive regions of the PBN may reflect functional specialization of PBN subnuclei. Descending projections from orally responsive sites in the waist area project to the lateral parvocellular reticular formation, a region implicated in brainstem circuitry underlying consummatory components of ingestive function. The external region, contains cells responsive to pain and oral aversive stimuli, but does not apparently contribute directly to local brainstem functions. Rather, forebrain pathways appear critical to the expression of external region functions.     

Association of extracellular acetylcholinesterase with gustatory nerve terminal fibers in the nucleus of the solitary tract

Acetylcholinesterase (AChE) staining is associated with terminal fields of the glossopharyngeal and chorda tympani nerves in the nucleus of the solitary tract (NST) [7]. To address AChE function at these sites, the location of the staining was examined at the fine structural level in combination with the labeling of chorda tympani nerve fibers with biotinylated dextran in golden Syrian hamsters. AChE staining was located in the endoplasmic reticulum of geniculate ganglion neuronal somata, and extracellularly, surrounding labeled chorda tympani terminal fibers and boutons in the NST. Neuronal profiles adjacent to these labeled fibers were stained less intensely, whereas most non-adjacent profiles were unstained. The location of staining is consistent with the secretion of AChE into the extracellular space by primary afferent chorda tympani fibers. AChE staining was reduced in the dextran-labeled chorda tympani fibers and terminals as well as adjacent non-labeled profiles 2 weeks following nerve transection and dextran application. The distribution of staining outside synapses and the loss of staining following denervation is suggestive of a non-cholinergic role for AChE in the intact gustatory system.     

Opioid modulation of taste responses in the nucleus of the solitary tract

Gustatory processing within the medulla is modulated by a number of physiologic and experiential factors. Several neurotransmitters, including excitatory amino acids, GABA, and substance P, are involved in synaptic processing within the rostral portion of the nucleus of the solitary tract (NST). Endogenous opiates have been implicated in the regulation of feeding behavior and in taste palatability and gustatory responses in the parabrachial nuclei are reduced by systemic morphine. In the present experiments, extracellular recording of neuronal activity within the NST in response to taste input was combined with local microinjection of met-enkephalin (Met-ENK) and naltrexone (NLTX) to determine the effect of these agents on gustatory activity. The anterior tongue was stimulated with anodal current pulses to determine the time course of drug action (n=85 cells) and with prototypical taste stimuli (0.032 M sucrose, NaCl, and quinine hydrochloride, and 0.0032 M citric acid) to investigate the effects of these opioid compounds on taste-evoked responses (n=80 cells). Among these 165 taste-responsive neurons in the NST, the activity of 39 (23.6%) was suppressed by Met-ENK. These effects were dose-dependent and blockable by NLTX, which alone was without effect, suggesting that opiates do not maintain a tonic inhibitory influence. Immunohistochemical experiments demonstrated both micro - and delta-opioid receptors within the gustatory portion of the NST; previous studies had shown numerous fiber terminals containing Met-ENK. These data suggest that endogenous opiates play an inhibitory role in gustatory processing within the medulla.     

Aversive taste stimuli facilitate extracellular acetylcholine release in the insular gustatory cortex of the rat: a microdialysis study

The release of extracellular acetylcholine (ACh) in the insular gustatory cortex of conscious rats during taste stimulation was measured using the microdialysis technique. The mean basal release of ACh before stimulation was 273 ± 21 fmol/10 μl (mean ± S.E.M., n = 25). Intraorally applied taste stimuli or distilled water significantly increased the release of ACh. Among them, infusion of 0.001 M quinine HCl produced a marked increase in the release of ACh up to 355% of baseline levels. Infusion of 0.01 M saccharin to the subjects that had acquired an aversion to this taste also caused a prominent increase in ACh up to 343% of basal levels. In contrast, saccharin infusion to the naive subjects moderately increased ACh up to 243% of baseline. Water infusion resulted in the smallest increase in ACh up to 175% of baseline. Although intraoral infusions of quinine or distilled water caused a significant increase in ACh in the parietal cortex, the magnitude of increased ACh was smaller than that in the gustatory cortex. These results suggest that ACh release in the insular gustatory cortex is related to behavioral expression to aversive taste stimuli.     

Context‐dependent coding and gain control in the auditory system of crickets

Sensory systems process stimuli that greatly vary in intensity and complexity. To maintain efficient information transmission, neural systems need to adjust their properties to these different sensory contexts, yielding adaptive or stimulus‐dependent codes. Here, we demonstrated adaptive spectrotemporal tuning in a small neural network, i.e. the peripheral auditory system of the cricket. We found that tuning of cricket auditory neurons was sharper for complex multi‐band than for simple single‐band stimuli. Information theoretical considerations revealed that this sharpening improved information transmission by separating the neural representations of individual stimulus components. A network model inspired by the structure of the cricket auditory system suggested two putative mechanisms underlying this adaptive tuning: a saturating peripheral nonlinearity could change the spectral tuning, whereas broad feed‐forward inhibition was able to reproduce the observed adaptive sharpening of temporal tuning. Our study revealed a surprisingly dynamic code usually found in more complex nervous systems and suggested that stimulus‐dependent codes could be implemented using common neural computations.     

Temporal Cognition in Children with Autistic Spectrum Disorders: Tests of Diachronic Thinking

Impaired diachronic thinking-(the propensity and capacity to think about events spreading across time)-was demonstrated in a 2-Phase study in which children with autism were compared with age and ability matched controls. Identical tests of diachronic thinking were administered in both phases of the study, but to different participant groups, with the same results. The marked impairments shown are therefore robust. Various non-temporal explanations of the findings were eliminated by the results of control tasks in Phase 2. Diachronic thinking did not correlate with verbal or non-verbal ability, age, or mentalising ability, consistent with other evidence of the specificity of diachronic thinking ability. Possible causes of impaired diachronic thinking in autism are discussed.     

Trigeminal modulation of gustatory neurons in the nucleus of the solitary tract

Electrophysiological methods were used to investigate the effects of trigeminal nerve stimulation or transection on responses of single gustatory neurons in the nucleus of the solitary tract (NTS) to tastants (NaCl, sucrose, citric acid, monosodium glutamate) in pentobarbital-anesthetized rats. Unilateral transection of the lingual nerve, or the mandibular branch of the trigeminal nerve, resulted in significant reductions (by 21 and 29%, respectively; P<0.01) in tastant-evoked responses, with no further effect following bilateral transection. Electrical stimulation of the central cut end of the mandibular nerve directly excited nine of 14 gustatory NTS units. For these units, central mandibular stimulation facilitated the tastant-evoked responses in six, depressed responses in three, and had no effect in five. Facilitation of tastant-evoked responses peaked 4 min after mandibular stimulation and recovered within 8 min. Electrical stimulation of the peripheral cut end of the mandibular nerve significantly reduced tastant-evoked responses in nine other NTS units, with a maximal reduction at 4 min post-stimulation followed by recovery. Stimulation of the superior cervical sympathetic ganglion did not affect NTS tastant-evoked responses. These results suggest the presence of complex central modulation of NTS neurons by trigeminal afferents, as well as a peripheral depressant effect on gustatory processing possibly mediated via neuropeptide release from trigeminal nerve endings in the tongue.     

Intensity-invariant coding in the auditory system

The auditory system faithfully represents sufficient details from sound sources such that downstream cognitive processes are capable of acting upon this information effectively even in the face of signal uncertainty, degradation or interference. This robust sound source representation leads to an invariance in perception vital for animals to interact effectively with their environment. Due to unique nonlinearities in the cochlea, sound representations early in the auditory system exhibit a large amount of variability as a function of stimulus intensity. In other words, changes in stimulus intensity, such as for sound sources at differing distances, create a unique challenge for the auditory system to encode sounds invariantly across the intensity dimension. This challenge and some strategies available to sensory systems to eliminate intensity as an encoding variable are discussed, with a special emphasis upon sound encoding.