Temporal Coding of Intensity of NaCl and HCl in the Nucleus of the Solitary Tract of the Rat

Abstract

. Sensory neurons are generally tuned to a subset of stimulus qualities within their sensory domain and manifest this tuning by the relative size of their responses to stimuli of equal intensity. However, response size alone cannot unambiguously signal stimulus quality, since response size also depends on stimulus intensity. Thus a common problem faced by sensory systems is that response size (e.g., spike count) confounds stimulus quality and intensity. Here, using the gustatory system as a model, we asked whether temporal firing characteristics could disambiguate these axes. To address this question, we recorded taste responses of single neurons in the nucleus of the solitary tract (NTS, the first central gustatory relay) in anesthetized rats to a range of concentrations of NaCl and HCl and their binary mixtures. To assess the contribution of the temporal characteristics of the response to discrimination among tastants, a family of metrics that quantifies the similarity of two spike trains in terms of spike count and spike timing was used. Results showed that the spike count produced by different taste qualities and different concentrations overlapped in most cells, implying that information conveyed by spike count is imprecise. Multidimensional scaling analysis of taste responses using similarity of temporal characteristics showed that different taste qualities, intensities, and mixtures formed distinct clusters in this "temporal coding" taste space and were arranged in a logical order. Thus the temporal structure of taste responses in single cells in the NTS can simultaneously convey information about both taste quality and intensity. I N T R O D U C T I O NIn all sensory systems, individual cells are tuned to respond selectively to a certain set of stimuli. The variety of tuning curves across cells spans and defines the broader stimulus domain and enables the identification and discrimination of different stimuli. However, changes in stimulus intensity generally broaden those tuning curves and may produce confusion between a change in stimulus intensity and a change in identity. When the tuning (specificity) is narrow, the identity of the neuron can signal the identity of the stimulus (e.g., pitch, color, taste quality, etc.) and the relative firing rate can indicate intensity (e.g., loudness, brightness, concentration). In a system such as gustation, where most cells respond well to more than one taste quality (sweet, sour, salty, bitter, and perhaps umami), stimuli of different taste qualities can evoke equivalent firing rates if the concentrations are just right. As a result, in most cases firing rate alone cannot convey an unambiguous message about taste quality, especially in broadly tuned neurons.In many studies of taste-responsive cells in the CNS, groups of cells are defined by the stimulus that evokes the "best" or most robust response when exemplars of each basic taste quality are presented at moderate concentrations. Even though most cells are multisensitive across taste qualities, several researc...

Central Representation of Postingestive Chemosensory Cues in Mice That Lack the Ability to Taste

“…NaCl stimulation clearly evokes electrophysiological responses in rostral parts of the nTS (Travers and Norgren, 1995; McCaughey, 2007; Kang and Lundy, 2010; Chen et al, 2011) suggesting that c-Fos expression does not entirely coincide with electrical activity. Thus, lack of c-Fos reactivity does not necessarily indicate an absence of responsiveness.…”

c-Fos Expression in the Nucleus of the Solitary Tract in Response to Salt Stimulation in Rats

“…Direct electrical recordings from the NST were also conducted and revealed the different temporal coding for tastants and their concentration [5]. The c-Fos immunohistochemical study also shows the spatial coding of tastant.…”

“…Furthermore, visceral taste information is conveyed to the NST from the gastrointestinal tract [3]. This taste signaling to the NST was well verified using c-Fos, an immediate early gene family [4], and electrical recordings [5] for various tastants.…”

In Vivo Bioelectronic Tongue

“…14.9). Chen and colleagues found a new path to resolve this dilemma when taking the temporal coding into consideration [53]. Metric space analysis, as they defined, not only pays attention to spike counts in certain time bins, but also temporal feature of spikes.…”