Effect of cholecystokinin on taste responsiveness in rats

The Functional and Neurobiological Properties of Bad Taste

The gustatory system serves as a critical line of defense against ingesting harmful substances. Technological advances have fostered the characterization of peripheral receptors and have created opportunities for more selective manipulations of the nervous system, yet the neurobiological mechanisms underlying taste-based avoidance and aversion remain poorly understood. One conceptual obstacle stems from a lack of recognition that taste signals subserve several behavioral and physiological functions which likely engage partially segregated neural circuits. Moreover, although the gustatory system evolved to respond expediently to broad classes of biologically relevant chemicals, innate repertoires are often not in register with the actual consequences of a food. The mammalian brain exhibits tremendous flexibility; responses to taste can be modified in a specific manner according to bodily needs and the learned consequences of ingestion. Therefore, experimental strategies that distinguish between the functional properties of various taste-guided behaviors and link them to specific neural circuits need to be applied. Given the close relationship between the gustatory and visceroceptive systems, a full reckoning of the neural architecture of bad taste requires an understanding of how these respective sensory signals are integrated in the brain.

Neurons with diverse phenotypes project from the caudal to the rostral nucleus of the solitary tract

The nucleus of the solitary tract is a potential site for taste-visceral interactions. Connections from the caudal, visceral area of the nucleus (cNST) to the rostral, gustatory zone (rNST) have been described, but the phenotype of cells giving rise to the projection(s) and their distribution among rNST subdivisions are unknown. To determine these characteristics of the intrasolitary pathway, we injected pan-neuronal and floxed AAV viruses into the cNST of mice expressing cre in glutamatergic, GABAergic, or catecholaminergic neurons. Particular attention was paid to the terminal field distribution in rNST subdivisions by simultaneously visualizing P2X2 localized to gustatory afferent terminals. All three phenotypically identified pathways terminated in rNST, with the density greatest for glutamatergic and sparsest for catecholaminergic projections, observations supported by retrograde tracing. Interestingly, cNST neurons had more prominent projections to rNST regions medial and ventral to P2X2 staining, i.e., the medial and ventral subdivisions. In addition, GABAergic neurons projected robustly to the lateral subdivision and adjacent parts of the reticular formation and spinal trigeminal nucleus. Although cNST neurons also projected to the P2X2-rich central subdivision, such projections were sparser. These findings suggest that cNST visceral signals exert stronger excitatory and inhibitory influences on local autonomic and reflex pathways associated with the medial and ventral subdivisions compared to weaker modulation of ascending pathways arising from the central subdivision and ultimately destined for the forebrain.

A novel slice preparation to study medullary oromotor and autonomic circuits in vitro

BACKGROUND

The medulla is capable of controlling and modulating ingestive behavior and gastrointestinal function. These two functions, which are critical to maintaining homeostasis, are governed by an interconnected group of nuclei dispersed throughout the medulla. As such, in vitro experiments to study the neurophysiologic details of these connections have been limited by spatial constraints of conventional slice preparations.

NEW METHOD

This study demonstrates a novel method of sectioning the medulla so that sensory, integrative, and motor nuclei that innervate the gastrointestinal tract and the oral cavity remain intact.

RESULTS

Immunohistochemical staining against choline-acetyl-transferase and dopamine-β-hydroxylase demonstrated that within a 450 μm block of tissue we are able to capture sensory, integrative and motor nuclei that are critical to oromotor and gastrointestinal function. Within slice tracing shows that axonal projections from the NST to the reticular formation and from the reticular formation to the hypoglossal motor nucleus (mXII) persist. Live-cell calcium imaging of the slice demonstrates that stimulation of either the rostral or caudal NST activates neurons throughout the NST, as well as the reticular formation and mXII.

COMPARISON WITH EXISTING METHODS

This new method of sectioning captures a majority of the nuclei that are active when ingesting a meal. Tradition planes of section, i.e. coronal, horizontal or sagittal, contain only a limited portion of the substrate.

CONCLUSIONS

Our results demonstrate that both anatomical and physiologic connections of oral and visceral sensory nuclei that project to integrative and motor nuclei remain intact with this new plane of section.

Melanocortin-4 receptor-null mice display normal affective licking responses to prototypical taste stimuli in a brief-access test

We tested whether MC4R null mice display altered gustatory function relative to wild-type controls that may contribute to the characteristic hyperphagia and obesity associated with this gene deletion. Mice were tested for their licking responses to prototypical taste solutions (sucrose, NaCl, quinine, citric acid) in series of daily 30-min sessions in which a range of concentrations of each tastant was available in randomized blocks of 5-s trials. Notwithstanding some minor deviations, the concentration-response functions of the MC4R null and wild-type mice were basically the same for all of the prototypical compounds tested here. Thus, taste-based appetitive and avoidance behavior is expressed in the absence of the MC4 receptor, demonstrating that this critical component in the melanocortin system is not required for normal affective gustatory function to be maintained.

Neural plasticity in the gustatory system

Sensory systems adapt to changing environmental influences by coordinated alterations in structure and function. These alterations are referred to as plastic changes. The gustatory system displays numerous plastic changes even in receptor cells. This review focuses on the plasticity of gustatory structures through the first synaptic relay in the brain. Unlike other sensory systems, there is a remarkable amount of environmentally induced changes in these peripheral-most neural structures. The most consistent and largest changes occur to stimuli that also impact on homeostatic systems, especially when the environmental manipulation is instituted during early development.

Neuropeptides modulate rat chorda tympani responses

The neuropeptide leptin has been shown to selectively modulate rat chorda tympani (CT) responses to sweet tastants. To explore whether other neuropeptides can modulate such responses, rat whole nerve CT responses to NaCl, HCl, quinine HCl, and sucrose were measured while administering cholecystokinin-8 (CCK-8), substance P(4-11) (SP(4-11)), or calcitonin gene-related peptide (CGRP). To avoid possible confounding effects on CT responses that take long times to develop, such as those that arise from intraperitoneal injections, we investigated the effects of the above peptides injected into the ipsilateral lingual artery (LA) on CT nerve responses during the initial seconds after a tastant was placed on the tongue. We found that CT responses to NaCl and HCl were increased by CCK-8 and decreased by CGRP. SP(4-11) had no noticeable effect. Peptide-induced CT responses to quinine HCl or sucrose were too small to accurately detect. These data suggest that at short latencies, after local infusion via the LA, neuropeptides can alter CT responses in a peptide-specific manner.

Integration of gastric distension and gustatory responses in the parabrachial nucleus

Palatable gustatory stimuli promote feeding, whereas gastric distension generally inhibits this behavior. We explored a neural basis for integration of these opposing sensory signals by evaluating the effect of gastric distension on gustatory responses in the parabrachial nucleus (PBN) of anesthetized rats. Sixteen percent of 92 taste cells were coactivated; they responded to independent taste or gastric distension stimulus application. Modulation of taste responses by distension was more prevalent; taste responses declined 37% in response to distension in 25% of the cells and increased by 46% in 10% of cells. Across the whole population, however, the suppressive effect of distension on taste responses was small (6%). The incidence of modulation did not vary as a simple hedonic function of gustatory sensitivity, i.e., similar proportions of sucrose-, citric-acid-, and QHCl-best, but not NaCl-best, neurons were modulated by gastric distension. Coactivated, modulated, and nonmodulated gustatory-responsive cells were intermingled in the gustatory zone of the caudal PBN. The suppression of PBN taste responses by visceral stimulation may reflect a mechanism for satiation and further implicates the PBN in the control of ingestive function.