Cortical spatial aspects of optical intrinsic signals in response to sucrose and NaCl stimuli

Spatially Distributed Representation of Taste Quality in the Gustatory Insular Cortex of Behaving Mice

Visual, auditory, and somatosensory cortices are topographically organized, with neurons responding to similar sensory features clustering in adjacent portions of the cortex. Such topography has not been observed in the piriform cortex, whose responses to odorants are sparsely distributed across the cortex. The spatial organization of taste responses in the gustatory insular cortex (GC) is currently debated, with conflicting evidence from anesthetized rodents pointing to alternative and mutually exclusive models. Here, we rely on calcium imaging to determine how taste and task-related variables are represented in the superficial layers of GC of alert, licking mice. Our data show that the various stimuli evoke sparse responses from a combination of broadly and narrowly tuned neurons. Analysis of the distribution of responses over multiple spatial scales demonstrates that taste representations are distributed across the cortex, with no sign of spatial clustering or topography. Altogether, data presented here support the idea that the representation of taste qualities in GC of alert mice is sparse and distributed, analogous to the representation of odorants in piriform cortex.

Direct Parabrachial-Cortical Connectivity

The parabrachial nucleus (PB) in the upper brain stem tegmentum includes several neuronal subpopulations with a wide variety of connections and functions. A subpopulation of PB neurons projects axons directly to the cerebral cortex, and limbic areas of the cerebral cortex send a return projection directly to the PB. We used retrograde and Cre-dependent anterograde tracing to identify genetic markers and characterize this PB-cortical interconnectivity in mice. Cortical projections originate from glutamatergic PB neurons that contain Lmx1b (81%), estrogen receptor alpha (26%), and Satb2 (20%), plus mRNA for the neuropeptides cholecystokinin (Cck, 48%) and calcitonin gene-related peptide (Calca, 13%), with minimal contribution from FoxP2+ PB neurons (2%). Axons from the PB produce an extensive terminal field in an unmyelinated region of the insular cortex, extending caudally into the entorhinal cortex, and arcing rostrally through the dorsolateral prefrontal cortex, with a secondary terminal field in the medial prefrontal cortex. In return, layer 5 neurons in the insular cortex and other prefrontal areas, along with a dense cluster of cells dorsal to the claustrum, send a descending projection to subregions of the PB that contain cortically projecting neurons. This information forms the neuroanatomical basis for testing PB-cortical interconnectivity in arousal and interoception.

Sensing Senses: Optical Biosensors to Study Gustation

The five basic taste modalities, sweet, bitter, umami, salty and sour induce changes of Ca²⁺ levels, pH and/or membrane potential in taste cells of the tongue and/or in neurons that convey and decode gustatory signals to the brain. Optical biosensors, which can be either synthetic dyes or genetically encoded proteins whose fluorescence spectra depend on levels of Ca²⁺, pH or membrane potential, have been used in primary cells/tissues or in recombinant systems to study taste-related intra- and intercellular signaling mechanisms or to discover new ligands. Taste-evoked responses were measured by microscopy achieving high spatial and temporal resolution, while plate readers were employed for higher throughput screening. Here, these approaches making use of fluorescent optical biosensors to investigate specific taste-related questions or to screen new agonists/antagonists for the different taste modalities were reviewed systematically. Furthermore, in the context of recent developments in genetically encoded sensors, 3D cultures and imaging technologies, we propose new feasible approaches for studying taste physiology and for compound screening.

The coding of valence and identity in the mammalian taste system

The ability of the taste system to identify a tastant (what it tastes like) enables animals to recognize and discriminate between the different basic taste qualities^1,2. The valence of a tastant (whether it is appetitive or aversive) specifies its hedonic value and elicits the execution of selective behaviours. Here we examine how sweet and bitter are afforded valence versus identity in mice. We show that neurons in the sweet-responsive and bitter-responsive cortex project to topographically distinct areas of the amygdala, with strong segregation of neural projections conveying appetitive versus aversive taste signals. By manipulating selective taste inputs to the amygdala, we show that it is possible to impose positive or negative valence on a neutral water stimulus, and even to reverse the hedonic value of a sweet or bitter tastant. Remarkably, mice with silenced neurons in the amygdala no longer exhibit behaviour that reflects the valence associated with direct stimulation of the taste cortex, or with delivery of sweet and bitter chemicals. Nonetheless, these mice can still identify and discriminate between tastants, just as wild-type controls do. These results help to explain how the taste system generates stereotypic and predetermined attractive and aversive taste behaviours, and support the existence of distinct neural substrates for the discrimination of taste identity and the assignment of valence.

The coding of valence and identity in the mammalian taste system

The ability of the taste system to identify a tastant (what it tastes like) enables animals to recognize and discriminate between the different basic taste qualities^1,2. The valence of a tastant (whether it is appetitive or aversive) specifies its hedonic value and elicits the execution of selective behaviours. Here we examine how sweet and bitter are afforded valence versus identity in mice. We show that neurons in the sweet-responsive and bitter-responsive cortex project to topographically distinct areas of the amygdala, with strong segregation of neural projections conveying appetitive versus aversive taste signals. By manipulating selective taste inputs to the amygdala, we show that it is possible to impose positive or negative valence on a neutral water stimulus, and even to reverse the hedonic value of a sweet or bitter tastant. Remarkably, mice with silenced neurons in the amygdala no longer exhibit behaviour that reflects the valence associated with direct stimulation of the taste cortex, or with delivery of sweet and bitter chemicals. Nonetheless, these mice can still identify and discriminate between tastants, just as wild-type controls do. These results help to explain how the taste system generates stereotypic and predetermined attractive and aversive taste behaviours, and support the existence of distinct neural substrates for the discrimination of taste identity and the assignment of valence.

Distribution of Fos-immunoreactive neurons in the gustatory cortex elicited by intra-oral infusion of taste solutions in conscious rats

The location of neurons in the gustatory cortex (GC) activated by intra-oral infusion of solutions in conscious rats was mapped using Fos immunohistochemistry. Groups of adult male Wistar rats (N's = 5) received an infusion of one of the following: dH2O, 0.1 or 1.0 M NaCl, 0.1 or 1.0 M sucrose, 0.32 M MSG (with 100 µM amiloride and 2.5 M inosine 5'-monophosphate), 0.03 M HCl, or 0.003 M QHCl delivered via an intra-oral cannula (0.233 ml/min for 5 min). Unstimulated control rats received no infusion. Taste reactivity (TR) behaviors were videotaped and scored. The number of Fos-immunoreactive (Fos-IR) neurons was counted in eight sections throughout the anterior-posterior extent of the GC in the medial and lateral halves of the granular (GI), dysgranular (DI), and dorsal (AID) and ventral (AIV) agranular insular cortices. Intra-oral infusion of dH2O, NaCl, or sucrose altered the number of Fos-IR neurons in only specific subareas of the GC and the effects of these tastants were concentration-dependent. For example, 1.0 M NaCl increased Fos-IR neurons in the posterior lateral AID and DI and elicited more aversive TR responses than 0.1 M NaCl. Compared to dH2O, infusions of HCl or QHCl increased the total number of Fos-IR neurons in many subareas of the GC throughout its anterior-posterior extent and increased aversive TR behaviors. Linear regression analyses suggested that neurons in the medial AID of the posterior GC may influence aversive behavioral responses to HCl and QHCl while neurons in the posterior lateral AID and DI may play a role in aversive TR responses to 1.0 M NaCl.

Neural processing of gustatory information in insular circuits

The insular cortex is the primary cortical site devoted to taste processing. A large body of evidence is available for how insular neurons respond to gustatory stimulation in both anesthetized and behaving animals. Most of the reports describe broadly tuned neurons that are involved in processing the chemosensory, physiological and psychological aspects of gustatory experience. However little is known about how these neural responses map onto insular circuits. Particularly mysterious is the functional role of the three subdivisions of the insular cortex: the granular, the dysgranular and the agranular insular cortices. In this article we review data on the organization of the local and long-distance circuits in the three subdivisions. The functional significance of these results is discussed in light of the latest electrophysiological data. A view of the insular cortex as a functionally integrated system devoted to processing gustatory, multimodal, cognitive and affective information is proposed.

A gustotopic map of taste qualities in the mammalian brain

The taste system is one of our fundamental senses, responsible for detecting and responding to sweet, bitter, umami, salty, and sour stimuli. In the tongue, the five basic tastes are mediated by separate classes of taste receptor cells each finely tuned to a single taste quality. We explored the logic of taste coding in the brain by examining how sweet, bitter, umami, and salty qualities are represented in the primary taste cortex of mice. We used in vivo two-photon calcium imaging to demonstrate topographic segregation in the functional architecture of the gustatory cortex. Each taste quality is represented in its own separate cortical field, revealing the existence of a gustotopic map in the brain. These results expose the basic logic for the central representation of taste.

Effects of electrical and chemical stimulation of the amygdala on the spontaneous discharge in the insular cortex in rats

Both the amygdala (especially, the basolateral nucleus of the amygdala) and the insular cortex are important for conditioned taste aversion. From the anatomical view point, there are reciprocal connections between the insular cortex and the amygdala. In the present study, we investigated the effect of electrical and chemical stimulation of the amygdala on the spontaneous discharge of the insular cortex neurons in anesthetized rats. In most neurons (10 of 14), spontaneous discharge was decreased after a microinjection of glutamate (Glu). In these neurons, the injection site was within the basolateral nucleus of the amygdala (the basolateral/lateral nuclei). On the other hand, when a gamma amino-butyric acid (GABA) was microinjected into the basolateral nucleus of the amygdala, none of the 5 neurons showed any change in spontaneous discharge. Electrical train stimulation of the basolateral nucleus of the amygdala (100 Hz, 2-6 s) depressed the spontaneous discharge of the neurons in the insular cortex, as in the case of a Glu microinjection. These results indicate that activation of the basolateral nucleus of the amygdala could depress the neuronal activity in the insular cortex. Such results may yield data leading to the elucidation of the neuronal mechanisms of conditioned taste aversion.

The neural processing of taste

Although there have been many recent advances in the field of gustatory neurobiology, our knowledge of how the nervous system is organized to process information about taste is still far from complete. Many studies on this topic have focused on understanding how gustatory neural circuits are spatially organized to represent information about taste quality (e.g., "sweet", "salty", "bitter", etc.). Arguments pertaining to this issue have largely centered on whether taste is carried by dedicated neural channels or a pattern of activity across a neural population. But there is now mounting evidence that the timing of neural events may also importantly contribute to the representation of taste. In this review, we attempt to summarize recent findings in the field that pertain to these issues. Both space and time are variables likely related to the mechanism of the gustatory neural code: information about taste appears to reside in spatial and temporal patterns of activation in gustatory neurons. What is more, the organization of the taste network in the brain would suggest that the parameters of space and time extend to the neural processing of gustatory information on a much grander scale.

Component-dependent urine responses in the rat accessory olfactory bulb

To investigate how pheromonal information is processed in the rat accessory olfactory bulb, we optically imaged intrinsic signals to obtain high-resolution maps of activation induced by urinary stimulation. Application of volatile components in male urine mainly induced activation in the anterior accessory olfactory bulb, irrespective of the sex, whereas volatile female urine elicited activation not only in the anterior but also to some extent in the caudal part of the posterior accessory olfactory bulb of male, but not female, rats. Nonvolatile components of both male and female urine induced activation mainly in the rostral part of the posterior and to a lesser extent in the anterior accessory olfactory bulb, irrespective of the sex. These results indicate that volatile and nonvolatile urinary components activate the anterior and posterior subdivisions of the accessory olfactory bulb, respectively.

Differential spatial representation of taste modalities in the rat gustatory cortex

Discrimination between foods is crucial for the nutrition and survival of animals. Remarkable progress has been made through molecular and genetic manipulations in the understanding of the coding of taste at the receptor level. However, much less is known about the cortical processing of taste sensation and the organizing principles of the gustatory cortex (GC). Using genetic tracing, it has recently been shown that sweet and bitter taste are processed through segregated neuronal circuitries along the gustatory pathway up to the cortical level. This is in disagreement with the evidence that GC neurons recorded in both anesthetized and behaving animals responded to multiple taste modalities (including sweet and bitter). To investigate the functional architecture of the GC in regard to taste modalities, we used in vivo intrinsic optical imaging, a technique that has been successfully applied to explore the organization of other neocortical regions. We found that four of the primary taste modalities (sweet, bitter, salty, and sour) are represented by distinctive spatial patterns but that no region was specific to a single modality. In addition, we found that two tastants of similar hedonic value (pleasant or unpleasant) activated areas with more common regions than two tastants with opposite hedonic value. In summary, we propose that these specific cortical patterns can be used to discriminate among various tastants.

The neurocognitive bases of human multimodal food perception: consciousness

This review explores how we become aware of the (integrated) flavor of food. In recent years, progress has been made understanding the neural correlates of consciousness. Experimental and computational data have been largely based on the visual system. Contemporary neurobiological frameworks of consciousness are reviewed, concluding that neural reverberation among forward- and back-projecting neural ensembles across brain areas is a common theme. In an attempt to extrapolate these concepts to the oral-sensory and olfactory systems involved with multimodal flavor perception, the integration of the sensory information of which into a flavor gestalt has been reviewed elsewhere (Verhagen, J.V., Engelen, L., 2006. The neurocognitive bases of human multimodal food perception: Sensory integration. Neurosci. Biobehav. Rev. 30(5): 613_650), I reconceptualize the flavor-sensory system by integrating it into a larger neural system termed the Homeostatic Interoceptive System (HIS). This system consists of an oral (taste, oral touch, etc.) and non-oral part (non oral-thermosensation, pain, etc.) which are anatomically and functionally highly similar. Consistent with this new concept and with a large volume of experimental data, I propose that awareness of intraoral food is related to the concomitant reverberant self-sustained activation of a coalition of neuronal subsets in agranular insula and orbitofrontal cortex (affect, hedonics) and agranular insula and perirhinal cortex (food identity), as well as the amygdala (affect and identity) in humans. I further discuss the functional anatomy in relation essential nodes. These formulations are by necessity to some extent speculative.

Rapid taste responses in the gustatory cortex during licking

Rapid tastant detection is necessary to prevent the ingestion of potentially poisonous compounds. Behavioral studies have shown that rats can identify tastants in approximately 200 ms, although the electrophysiological correlates for fast tastant detection have not been identified. For this reason, we investigated whether neurons in the primary gustatory cortex (GC), a cortical area necessary for tastant identification and discrimination, contain sufficient information in a single lick cycle, or approximately 150 ms, to distinguish between tastants at different concentrations. This was achieved by recording neural activity in GC while rats licked four times without a liquid reward, and then, on the fifth lick, received a tastant (FR5 schedule). We found that 34% (61 of 178) of GC units were chemosensitive. The remaining neurons were activated during some phase of the licking cycle, discriminated between reinforced and unreinforced licks, or processed task-related information. Chemosensory neurons exhibited a latency of 70-120 ms depending on concentration, and a temporally precise phasic response that returned to baseline in tens of milliseconds. Tastant-responsive neurons were broadly tuned and responded to increasing tastant concentrations by either increasing or decreasing their firing rates. In addition, some responses were only evoked at intermediate tastant concentrations. In summary, these results suggest that the gustatory cortex is capable of processing multimodal information on a rapid timescale and provide the physiological basis by which animals may discriminate between tastants during a single lick cycle.

Rapid taste responses in the gustatory cortex during licking

Rapid tastant detection is necessary to prevent the ingestion of potentially poisonous compounds. Behavioral studies have shown that rats can identify tastants in approximately 200 ms, although the electrophysiological correlates for fast tastant detection have not been identified. For this reason, we investigated whether neurons in the primary gustatory cortex (GC), a cortical area necessary for tastant identification and discrimination, contain sufficient information in a single lick cycle, or approximately 150 ms, to distinguish between tastants at different concentrations. This was achieved by recording neural activity in GC while rats licked four times without a liquid reward, and then, on the fifth lick, received a tastant (FR5 schedule). We found that 34% (61 of 178) of GC units were chemosensitive. The remaining neurons were activated during some phase of the licking cycle, discriminated between reinforced and unreinforced licks, or processed task-related information. Chemosensory neurons exhibited a latency of 70-120 ms depending on concentration, and a temporally precise phasic response that returned to baseline in tens of milliseconds. Tastant-responsive neurons were broadly tuned and responded to increasing tastant concentrations by either increasing or decreasing their firing rates. In addition, some responses were only evoked at intermediate tastant concentrations. In summary, these results suggest that the gustatory cortex is capable of processing multimodal information on a rapid timescale and provide the physiological basis by which animals may discriminate between tastants during a single lick cycle.

Cyclic AMP-dependent attenuation of oscillatory-activity-induced intercortical strengthening of horizontal pathways between insular and parietal cortices

Cyclic AMP (cAMP) is a key intracellular second messenger, and the intracellular cAMP signaling pathway acts to modulate various brain functions. We have previously reported that low-frequency insular cortex stimulation in rat brain slices switches on a voltage oscillator in the parietal cortex that delivers signals horizontally back and forth under caffeine application. The oscillatory activities are N-methyl-D-aspartate (NMDA) receptor-dependent, and the role of oscillation is to strengthen functional intercortical connections. The present study investigated actions of the cAMP signaling pathway on caffeine-induced strengthening of intercortical connections and tried to confirm the role of oscillation on intercortical strengthening by focusing on the cAMP pathway. After induction of parietal oscillation by insular cortex stimulation in caffeine-containing medium, application of membrane-permeable cAMP analog, bromo-cAMP, diminished oscillatory signal delivery from the parietal cortex, but initial insulo-parietal signal propagation remained strong. When oscillatory activities were reduced with co-application of caffeine and bromo-cAMP from the beginning, initial insulo-parietal propagation was established, but amplitudes of propagating wavelets and propagating velocity were reduced. Thus, cAMP-dependent diminution of caffeine-induced NMDA-receptor-dependent oscillatory signal delivery causes attenuation of intercortical strengthening of horizontal pathways between insular and parietal cortices. This finding suggests that the intracellular cAMP signaling pathway has the ability to regulate extracellular communications at the network level, and also that full expression of strengthened intercortical signal communication requires sufficient NMDA-receptor-dependent oscillatory neural activities.

Odor-concentration coding in the guinea-pig piriform cortex

By optical imaging of intrinsic signals, we demonstrated a possible code for odor concentration in the anterior piriform cortex of the guinea-pig. Odor-induced cortical activation, which primarily originated in layer II, appeared in a narrow band beneath the rhinal sulcus over the lateral olfactory tract, corresponding to the dorsal part of the anterior piriform cortex. Lower concentrations activated the rostral region of the band, whereas higher ones generated caudally spreading activation, and the site at which neural activation reached its maximum extent depended upon odor concentration. Different odors with low concentrations generated distinct but somewhat overlapping patterns in the rostral region of the band; the limited extent of cortical activity may be one focal domain for each odor. It was hard to judge, however, that odor-specific domains appeared in the anterior piriform cortex, because the strong stimuli induced largely overlapping patterns. Furthermore, the total area activated increased in proportion to concentrations raised to a power of 0.5-0.9. Importantly, these imaging results were confirmed with unit recordings which indicated a rostro-caudal gradient in odor-sensitivity among cortical neurons. Our results suggest that the dorsal part of the anterior piriform cortex may be associated with odor concentration. Therefore, in addition to recruitment of more olfactory sensory cells and glomeruli in response to stronger stimuli, a rostro-caudal gradient in axonal projections from mitral/tufted cells and/or in association fibers may play an important role in odor-concentration coding in the anterior piriform cortex.

To-and-fro optical voltage signal propagation between the insular gustatory and parietal oral somatosensory areas in rat cortex slices

Taste perception depends not only on special taste information processed in the insular cortex, but also on oral somesthetic processing in the parietal cortex. Many insular cortex neurons show multimodal responsiveness. Such multimodality may be enabled by signal exchange between these two cortices. By using the protocol that we have developed, a synchronized population oscillation of synaptic potentials was induced in the parietal cortex by stimulation to the insular cortex in rat neocortex slices. The spatiotemporal pattern of propagation of this oscillation was studied by recording voltage-sensitive optical signals and field potentials. The first wavelet of the oscillation was propagated from the insular stimulation site to the parietal cortex. However, the second and later wavelets propagated back from the parietal cortex to the insular cortex. The oscillation was detected in the insular cortex as well, but was actually generated in the parietal cortex. Thus, the initial peak of optical signal, sent from the insular to parietal cortex, served to generate oscillatory responses in the parietal cortex, which propagated back to the insular cortex wave-by-wave. We propose that this to-and-fro propagation may be an artificially exaggerated demonstration of an intrinsic mechanism relevant to signal exchange between the parietal and insular cortices.

Age-dependent emergence of a parieto-insular corticocortical signal flow in developing rats

By using the procedure that we developed for inducing population oscillation, it was previously demonstrated that insular cortex stimulation can evoke insulo-parietal field potential propagation and synchronized population oscillation in the parietal cortex in slices obtained from mature rats (27-35 days old). By using the same procedure, we have now studied the reciprocal parieto-insular projection. Parietal cortex stimulation elicited synchronized population oscillation in the parietal--but not insular--cortex in mature tissues. In the insular cortex, the initial wavelet of the oscillation generated by parietal cortex stimulation propagated, but the entire oscillation did not. A prior induction--but not simultaneous occurrence--of oscillation in the parietal cortex sufficed to have this initial wavelet propagate. In immature tissue (9-10 days old), both the parietal cortex oscillation and the parieto-insular propagation were induced only with low [Mg2+]o. This age dependence is exactly the same as we previously observed for the reciprocal insulo-parietal propagation. Given that the parietal cortex receives somatosensory inputs from the oral cavity and the insular cortex receives primarily chemosensory inputs from the same source, the age-dependent changes in the availability of bidirectional signal traffic between these cortices might contribute to the development of multimodal responsiveness of taste neurons.