Relationships between spontaneous discharge rates and taste responses of the dog thalamic neurons

Response characteristics to the 4 taste stimuli (NaC1, tartaric acid, sucrose and quinine-HCl) and spontaneous discharge rates of taste neurons in the dog thalamus were examined. Response patterns of 72 thalamic neurons were somewhat complex, involving excitation (on type, on-off type and off type) and inhibition (short-term type and long-term type). A comparison among the responses to the 4 taste stimuli renders low response discharge rates and a high incidence of inhibitory responses to quinine-HCl. Measuring of the breadth of responsiveness showed that the thalamic taste neurons have varying sensitivities to the 4 taste stimuli and neurons responding best to quinine-HCl were more broadly tuned than those to each of the other 3 stimuli. These neurons also have varying spontaneous discharge rates and showed some relationship between the spontaneous rates and responses to taste stimuli. Neurons which showed inhibitory responses to stimuli have relatively high spontaneous rates. Neurons with higher spontaneous rates tend to be more narrowly tuned to the 4 taste stimuli. These relationships appeared not only as a general tendency of all the thalamic neurons but also as a characteristic of each of the 3 individual neurons which changed the spontaneous rates and breadth of responsiveness during long time recordings of neural discharges.