Activity in rat nucleus tractus solitarius after recovery from sodium deprivation

Amygdalofugal influence on processing of taste information in the nucleus of the solitary tract of the rat

Previous studies have shown that corticofugal input to the first central synapse of the ascending gustatory system, the nucleus of the solitary tract (NST), can alter the way taste information is processed. Activity in other forebrain structures, such as the central nucleus of the amygdala (CeA), similarly influence activation of NST taste cells, although the effects of amygdalofugal input on neural coding of taste information is not well understood. The present study examined responses of 110 NST neurons to 15 taste stimuli before, during, and after electrical stimulation of the CeA in rats. The taste stimuli consisted of different concentrations of NaCl (0.03, 0.1, 0.3 M), sucrose (0.1, 0.3, 1.0 M), citric acid (0.005, 0.01 M), quinine HCl (0.003, 0.03 M), and 0.03 M MSG, 0.1 M KCl, as well as 0.1 M NaCl, 0.01 M citric acid, and 0.03 M MSG mixed with 10 muM amiloride. In 66% of NST cells sampled (73/110) response rates to the majority of effective taste stimuli were either inhibited or augmented. Nevertheless, the magnitude of effect across stimuli was often differential, which provides a neurophysiological mechanism to alter neural coding. Subsequent analysis of across-unit patterns showed that amygdalofugal input plays a role in shaping spatial patterns of activation and could potentially influence the perceptual similarity and/or discrimination of gustatory stimuli by altering this feature of neural coding.

Comparison of somatostatin and corticotrophin-releasing hormone immunoreactivity in forebrain neurons projecting to taste-responsive and non-responsive regions of the parabrachial nucleus in rat

Several forebrain areas have been shown to project to the parabrachial nucleus (PBN) and exert inhibitory and excitatory influences on taste processing. The neurochemicals by which descending forebrain inputs modulate neural taste-evoked responses remain to be established. This study investigated the existence of somatostatin (SS) and corticotrophin-releasing factor (CRF) in forebrain neurons that project to caudal regions of the PBN responsive to chemical stimulation of the anterior tongue as well as more rostral unresponsive regions. Retrograde tracer was iontophoretically or pressure ejected from glass micropipettes, and 7 days later the animals were euthanized for subsequent immunohistochemical processing for co-localization of tracer with SS and CRF in tissue sections containing the lateral hypothalamus (LH), central nucleus of the amygdala (CeA), bed nucleus of the stria terminalis (BNST), and insular cortex (IC). In each forebrain site, robust labeling of cells with distinguishable nuclei and short processes was observed for SS and CRF. The results indicate that CRF neurons in each forebrain site send projections throughout the rostral caudal extent of the PBN with a greater percentage terminating in regions rostral to the anterior tongue-responsive area. For SS, the percentage of double-labeled neurons was more forebrain site specific in that only BNST and CeA exhibited significant numbers of double-labeled neurons. Few retrogradely labeled cells in LH co-expressed SS, while no double-labeled cells were observed in IC. Again, tracer injections into rostral PBN resulted in a greater percentage of double-labeled neurons in BNST and CeA compared to caudal injections. The present results suggest that some sources of descending forebrain input might utilize somatostatin and/or CRF to exert a broad influence on sensory information processing in the PBN.

Two types of inhibitory influences target different groups of taste-responsive cells in the nucleus of the solitary tract of the rat

Electrical stimulation of the chorda tympani nerve (CT; innervating taste buds on the rostral tongue) is known to initiate recurrent inhibition in cells in the nucleus of the solitary tract (NTS, the first central relay in the gustatory system). Here, we explored the relationship between inhibitory circuits and the breadth of tuning of taste-responsive NTS neurons. Initially, NTS cells with evoked responses to electrical stimulation of the CT (0.1 ms pulses; 1 Hz) were tested with each of four tastants (0.1 M NaCl, 0.01 M HCl, 0.01 M quinine and 0.5 M sucrose) in separate trials. Next, the CT was electrically stimulated using a paired-pulse (10-2000 ms interpulse interval; blocks of 100 trials) paradigm. Forty-five (30 taste-responsive) of 51 cells with CT-evoked responses (36 taste-responsive) were tested with paired pulses. The majority (34; 75.6%) showed paired-pulse attenuation, defined as fewer evoked spikes in response to the second (test) pulse compared with the first (conditioning) pulse. A bimodal distribution of the peak of paired-pulse attenuation was found with modes at 10 ms and 50 ms in separate groups of cells. Cells with early peak attenuation showed short CT-evoked response latencies and large responses to relatively few taste stimuli. Conversely, cells with late peak attenuation showed long CT-evoked response latencies and small taste responses with less selectivity. Results suggest that the breadth of tuning of an NTS cell may result from the combination of the sensitivities of peripheral nerve inputs and the recurrent influences generated by the circuitry of the NTS.

Biobehavior of the human love of salt

We are beginning to understand why humans ingest so much salt. Here we address three issues: The first is whether our salt appetite is similar to that in animals, which we understand well. Our analysis suggests that this is doubtful, because of important differences between human and animal love of salt. The second issue then becomes how our predilection for salt is determined, for which we have a partial description, resting on development, conditioning, habit, and dietary culture. The last issue is the source of individual variation in salt avidity. We have partial answers to that too in the effects of perinatal sodium loss, sodium loss teaching us to seek salt, and gender. Other possibilities are suggested. From animal sodium appetite we humans may retain the lifelong enhancement of salt intake due to perinatal sodium loss, and a predisposition to learn the benefits of salt when in dire need. Nevertheless, human salt intake does not fit the biological model of a regulated sodium appetite. Indeed this archetypal 'wisdom of the body' fails us in all that has to do with behavioral regulation of this most basic need.

Gustatory hedonic value: potential function for forebrain control of brainstem taste processing

Among well-nourished populations, eating beyond homeostatic needs when presented with caloric-dense palatable food evidences the assertion that an increasing proportion of consumption is driven by pleasure, not just by the need for calories. This presents a major health crisis because the affective component of foods constitutes a behavioral risk factor that promotes over consumption [Sorensen, L.B., Moller, P., Flint, A., Martens, M., Raben, A., 2003. Effect of sensory perception of foods on appetite and food intake: a review of studies on humans. Int. J. Obes. Relat. Metab. Disord. 27, 1152-1166; Yeomans, M.R., Blundell, J.E., Leshem, M., 2004. Palatability: response to nutritional need or need-free stimulation of appetite? Br. J. Nutr. 92 (Suppl. 1), S3-S14]. Overweight or obese individuals have an increased risk of developing hypertension, stroke, heart disease, chronic musculoskeletal problems, type-2 diabetes, and certain types of cancers [Hill, J.O., Catenacci, V., Wyatt, H.R., 2005. Obesity: overview of an epidemic. Psychiatr. Clin. N. Am. 28, 1-23, vii]. The etiology of obesity is complex involving genetic, metabolic, and behavioral factors, but ultimately results from long-term energy imbalance. Evidence indicates that learned and some forms of unlearned control of ingestive behavior driven by palatability (i.e. hedonic value) are critically dependent on reciprocal interactions between brainstem gustatory nuclei and the ventral forebrain. This review discusses the current understanding of centrifugal control of taste processing in subcortical gustatory nuclei and the potential role of such modulation in hedonic responding.

Responses to taste stimulation in the ventroposteromedial nucleus of the thalamus in rats

Extracellular action potentials were recorded from 73 neurons in the parvicellular division of the ventroposteromedial (VPMpc) nucleus of the thalamus of anesthetized Wistar rats during gustatory, thermal, and tactile stimulation of the whole oral cavity. The stimulus array consisted of 16 room-temperature (23 degrees C) sapid stimuli, distilled water at three temperatures (0, 23, and 37 degrees C), and 0.1 M NaCl at three temperatures (0, 23, and 37 degrees C). Among all 151 neurons isolated in VPMpc, 9% responded exclusively to taste, 33% to taste and temperature, none to taste and touch, but 6% to all three modalities. Discharge rates evoked by the basic tastants were 13.8 +/- 1.6 (SD) spikes/s for 0.1 M NaCl, 9.3 +/- 1.4 spikes/s for 0.01 M HCl, 5.1 +/- 0.9 spikes/s for 0.5 M sucrose, and 4.3 +/- 0.6 spikes/s for 0.01 M quinine HCl. Water evoked mean responses at 0, 23, and 37 degrees C of 9.9 +/- 1.5, 0.6 +/- 0.4, and 1.3 +/- 0.9 spikes/s, respectively. The mean firing rate evoked by 37 and 0 degrees C NaCl was 15.0 +/- 2.4 and 17.0 +/- 2.8 spikes/s, respectively. The exponent of the NaCl concentration-response power function was 0.39. Thalamic taste cells were broadly tuned. The mean breadth-of-tuning coefficient for these 73 gustatory cells was 0.79 +/- 0.02. Two cells responded predominantly with inhibition, which accounted for the majority of inhibitory responses. The taste neurons were statistically divisible into three groups: sodium-oriented (n = 40), acid-oriented (n = 12), and sugar-oriented (n = 17). Four additional bitter-oriented neurons were not closely enough related to be defined as a group and were considered outliers. The sodium-oriented group could be divided into three statistically distinct subgroups, differing in the specificity of their responses to NaCl. With respect to polymodal sensitivity, spontaneous rate, evoked response rates, signal-to-noise ratio, proportions of cells responding best to basic tastants, taste neuron groups, taste spaces, and temporal responses, VPMpc neurons have characteristics that are intermediate between those of parabrachial and cortical gustatory neurons.

Calcium deprivation alters gustatory-evoked activity in the rat nucleus of the solitary tract

Calcium-deprived rats develop a compensatory appetite for substances that contain calcium. To investigate the role of gustatory factors in calcium appetite, we recorded the extracellular activity of single neurons in the nucleus of the solitary tract of calcium-deprived and replete rats. The activity evoked by a broad array of taste stimuli was examined in 51 neurons from replete rats and 47 neurons from calcium-deprived rats. There were no differences between the groups in the responses of all neurons combined. However, neurons with sugar-oriented response profiles gave significantly larger responses to 3, 10, and 100 mM CaCl(2) in the calcium-deprived group than did corresponding cells in the replete group. This difference in taste-evoked responding may underlie an increase in the palatability of CaCl(2) and, in turn, contribute to the expression of calcium appetite.

Molecular neurobiology of ingestive behavior

The concepts and tools of molecular biology may be applied to almost any component of the animal involved in ingestion, but two categories of model system are particularly relevant for molecular analysis: homeostatic regulation of neuropeptide expression in the hypothalamus and neuronal plasticity underlying persistent changes in ingestive behavior. Molecular approaches to these models are reviewed, focusing on our strategy for analyzing conditioned taste aversion learning. Three questions must be answered: Where do the long-term changes occur within the distributed neural network that mediates feeding? This answer reveals the site of neuronal restructuring mediated by gene expression. When does the transition occur from short-term expression to long-term persistence of the change in behavior? This transition reveals the critical time of gene expression. What genes are expressed during the change in behavior? The expression of thousands of genes in discrete subpopulations of cells is likely to be required during critical periods of neuronal restructuring. The identification of these genes is a general challenge for molecular neurobiology. The analysis of ingestive behavior can profit from molecular tools, but ingestion also provides informative models that elucidate the principles of time- and neuron-specific gene expression mediating complex behaviors.

Rapid induction of sodium appetite modifies taste-evoked activity in the rat nucleus of the solitary tract

Sodium-deprived rats develop a salt appetite and show changes in gustatory responses to NaCl in the periphery and brain stem; salt-sensitive neurons respond less to hypertonic NaCl than do corresponding cells in replete controls. By administering DOCA and renin, we generated a need-free sodium appetite quickly enough to permit us to monitor the activity of individual neurons in the nucleus of the solitary tract before and after its creation, permitting a more powerful within-subjects design. Subjects received DOCA pretreatment followed by an intracerebroventricular infusion of renin. In animals that were tested behaviorally, this resulted in elevated intake of 0.5 M NaCl. In neural recordings, renin caused decreased responding to hypertonic NaCl across all neurons and in the salt-sensitive neurons that were most responsive to NaCl before infusion. Most sugar-sensitive cells, in contrast, gave increased phasic responses to NaCl. These results confirm that sodium appetite is accompanied by decreased responding to NaCl in salt-sensitive neurons, complemented by increased activity in sugar-sensitive cells, even when created rapidly and independently of need.

The neural code for taste in the brain stem: response profiles

In the study of the neural code for taste, two theories have dominated the literature: the across neuron pattern (ANP), and the labeled line theories. Both of these theories are based on the observations that taste cells are multisensitive across a variety of different taste stimuli. Given a fixed array of taste stimuli, a cell's particular set of sensitivities defines its response profile. The characteristics of response profiles are the basis of both major theories of coding. In reviewing the literature, it is apparent that response profiles are an expression of a complex interplay of excitatory and inhibitory inputs that derive from cells with a wide variety of sensitivity patterns. These observations suggest that, in the absence of inhibition, taste cells might be potentially responsive to all taste stimuli. Several studies also suggest that response profiles can be influenced by the taste context, defined as the taste stimulus presented just before or simultaneously with another, under which they are recorded. A theory, called dynamic coding, was proposed to account for context dependency of taste response profiles. In this theory, those cells that are unaffected by taste context would provide the signal, i.e., the information-containing portion of the ANP, and those cells whose responses are context dependent would provide noise, i.e., less stimulus specific information. When singular taste stimuli are presented, noise cells would provide amplification of the signal, and when complex mixtures are presented, the responses of the noise cells would be suppressed (depending on the particular combination of tastants), and the ratio of signal to noise would be enhanced.

Acute sodium deficiency reduces gustatory responsiveness to NaCl in the parabrachial nucleus of rats

Sodium-deprived rats ingest excessive amounts of salt solutions at high concentrations which are normally avoided. This phenomenon, salt appetite, is known to be controlled by the sense of taste. In the present study, a possible involvement of the parabrachial nucleus (PBN) in sodium appetite was examined by recording PBN neuron responses to 12 different taste stimuli in sodium-deprived and control groups of urethane anesthetized rats. Sodium deficiency was induced by two injections of furosemide 24 and 22 h before the recording. Taste responses to 0.3 and 0.5 M solutions of NaCl were significantly lower in the sodium-deprived rats than in controls. In gustatory responses to other taste solutions except for quinine hydrochloride solution, no differences were detected between the two groups of the rats. Correlation coefficients of responses among sodium salts and those between 0.5 M NaCl and sweeteners were larger in the sodium-deprived rats than in controls. The results indicate that PBN neurons in sodium-deprived rats are critically implicated in induction of quantitative and qualitative changes of gustatory sense to sodium salts.

Repeated sodium depletion affects gustatory neural responses in the nucleus of the solitary tract of rats

Furosemide sodium depletions were induced repeatedly to determine the effects on gustatory neural responses in the nucleus of the solitary tract (NST) of chronically prepared, but lightly anesthetized, rats. Sodium-replete and sodium-deplete conditions were alternated four times in each rat. When rats were under depleted conditions, the responses to NaCl were significantly greater than in sodium-replete conditions. This effect was attributable primarily to an increase in the magnitude of response of those neurons that responded better to NaCl than to the other standard stimuli (sucrose, citric acid, and quinine hydrochloride). In addition, the largest change in responsiveness of the NaCl-best neurons occurred during the third and fourth sodium depletions. These results are essentially opposite to those reported for NST neurons when sodium appetite is induced by dietary sodium restriction. This suggests that the coding of intensity in the gustatory system is dependent not only on the animal's deprivation condition, but also the method through which the deprivation is produced.