Influences of ethanol and temperature on sucrose-evoked response of gustatory neurons in the hamster solitary nucleus

Abstract

Taste-responsive neurons in the nucleus of the solitary tract (NST), the first gustatory nucleus, often respond to thermal or mechanical stimulation. Alcohol, not a typical taste modality, is a rewarding stimulus. In this study, we aimed to investigate the effects of ethanol (EtOH) and/or temperature as stimuli to the tongue on the activity of taste-responsive neurons in hamster NST. In the first set of experiments, we recorded the activity of 113 gustatory NST neurons in urethane-anesthetized hamsters and evaluated responses to four basic taste stimuli, 25% EtOH, and 40°C and 4°C distilled water (dH₂O). Sixty cells responded to 25% EtOH, with most of them also being sucrose sensitive. The response to 25% EtOH was significantly correlated with the sucrose-evoked response. A significant correlation was also observed between sucrose- and 40°C dH₂O-and between 25% EtOH- and 40°C dH₂O-evoked firings. In a subset of the cells, we evaluated neuronal activities in response to a series of EtOH concentrations, alone and in combination with 32 mM sucrose (EtOH/Suc) at room temperature (RT, 22°C-23°C), 40°C, and 4°C. Neuronal responses to EtOH at RT and 40°C increased as the concentrations increased. The firing rates to EtOH/Suc were greater than those to EtOH or sucrose alone. The responses were enhanced when solutions were applied at 40°C but diminished at 4°C. In summary, EtOH activates most sucrose-responsive NST gustatory cells, and the concomitant presence of sucrose or warm temperatures enhance this response. Our findings may contribute to elucidate the neural mechanisms underlying appetitive alcohol consumption.

Keywords: Electrophysiology; Ethanol; In vivo; Solitary nucleus; Sucrose.

Fig. 1. Stimulus-evoked firings (impulses/s) of 113 nucleus of the solitary tract (NST) neurons, in response to four basic taste stimuli, 25% ethanol (EtOH), and distilled water (dH₂O) at 40°C and 4°C in order from top to bottom panels.

The last panel shows the mean number of spikes during 5 sec of dH₂O at a room temperature (baseline activity) before taste stimulus. Net taste responses, unaffected by somatosensory or thermal aspects of the test solutions, were calculated as a mean number of spikes during the first 5 sec of each taste stimulus minus the baseline activity of same neuron. Each neuron was classified according to the taste stimulus that was the most effective in causing it to respond (best stimulus) and cells are arranged along the abscissa according to their best stimulus, with cells 1–33 being sucrose-best (Sb: red), 34–51 NaCl-best (Nb: blue), 52–54 citric acid-best (Cb: yellow), and 55–60 QHCl-best (Qb: green) in EtOH-responsive groups (A). Similarly, 53 cells are arranged: 3 Sb, 16 Nb, 15 Cb, and 19 Qb are arranged in EtOH-non-responsive groups (B). Within each best-stimulus group, cells are arranged according to the magnitude of the response to their best stimulus. The response profile for any one cell in the figure can be read from top to bottom.

Fig. 2. Comparison of the mean firing rate (× SE) of nucleus of the solitary tract (NST) neurons in response to four taste stimuli, 25% ethanol (EtOH), and distilled water (dH₂O) at 40°C (40) and 4°C (4) between EtOH-responsive and EtOH-non-responsive neurons.

The last bars on the right of the figure indicate the baseline activities at room temperature. The solid bars indicate mean responses of EtOH-responsive neurons and open bars represent those of EtOH-non-responsive cells. For sucrose (S), 25% EtOH (E), and 40°C dH₂O stimulus, net responses in EtOH-responsive neurons were significantly larger than those in EtOH-non-responsive cells (*p < 0.001, t-test). In comparison, citric acid (C)-, and QHCl (Q)-evoked firings were significantly larger in EtOH-non-responsive group (**p < 0.005, t-test). There were no differences across taste stimuli between EtOH-responsive and EtOH-non-responsive cells for NaCl (N) and 4°C dH₂O stimuli.

Fig. 3. Response profiles for various stimuli of a typical QHCl-best neuron (NST43Q) belonging to ethanol (EtOH)-non-responsive cells.

Neuronal firings for each tastant during the 10-sec stimulus are shown in filled bars, and the pre- and post-rinse periods with distilled water (dH₂O) for 5 sec are shown in open bars. The stimuli applied to the anterior tongue were sucrose (S), NaCl (N), citric acid (C), QHCl (Q), 3, 5, 10, 15, 25, and 40% of EtOH, and dH₂O at 40°C and 4°C. This neuron only responded to 32 mM QHCl as a taste stimulation.

Fig. 4. A representative peri-stimulus time histogram (1-ms bins) of the impulses in a sucrose-best neuron (NST95S) in ethanol (EtOH)-responsive cells.

Taste stimuli are a series of EtOH (A) and a mixture of EtOH and 32 mM sucrose (EtOH/Suc) (B). Evoked firings were increased in a dose-dependent manner for the stimulations with EtOH or EtOH/Suc. The stimulation with EtOH/Suc produced larger responses than with EtOH alone. (C) Mean responses (± SE) of 15 nucleus of the solitary tract (NST) gustatory neurons to a series of 6 concentrations of EtOH (open circle) and EtOH/Suc (solid circle) stimulations at room temperature. Stimulus-evoked firings were increased in a dose-dependent manner. Responses to EtOH/Suc stimulation were greater than those to EtOH alone.

Fig. 5. Mean responses (× SE) of 3 nucleus of the solitary tract (NST) gustatory neurons to a series of 6 concentrations of ethanol (EtOH) (open symbol) and EtOH/Suc (solid symbol) at 40°C (circle) and 4°C (rectangle).

Neuronal responses to serial concentrations of EtOH alone or with sucrose stimulation at 40°C were similar to those shown at room temperature. However, this trend at 40°C was not shown at 4°C.