258 C. PFAFFMANN 



4. In many situations, the preferential response to NaCi after depriva- 

 tion is immediate and thus appears to depend on some change in the taste 

 or response to the taste of the stimulus without prior learning. 



5. Efforts to assess the locus or nature of these changes, whether in 

 peripheral receptor sensitivity or in central neural mechanisms, point to 

 central processes. 



Our early studies of sensitivity (Pfaffmann and Bare, 1950) indicated 

 that the receptor threshold for NaCl in adrenalectomized rats was not 

 different from normal and that the receptor threshold itself fell consider- 

 ably below the behavior preference threshold of normal animals. Since 

 that time, conditioning studies in rats (Carr, 1952 ; Harriman and MacLeod 

 1953) have confirmed the fact that thresholds of normal and adrenalec- 

 tomized rats are not different and Koh and Teitelbaum (1961) have re- 

 cently shown a general correspondence between the thresholds obtained by 

 conditioning methods and those observed electrophysiologically. But, 

 since our early recording studies were concerned only with threshold values, 

 Dr. Marvin Nachman* (1962b) and I have recently re-examined the chorda 

 tympani responses in salt deprived animals over a wide range of NaCl 

 concentrations and for a number of other suprathreshold stimuli. In 

 particular, we wished to determine whether relative changes occurred in 

 the response to NaCl compared to other salts and taste stimuli. 



In place of adrenalectomy, we employed a salt deficient diet for a 20-day 

 period, half of the animals on the diet being used in behavioral tests to 

 show that an enhanced NaCl preference had been established. A two- 

 bottle, 2 hr test was used in which each animal was pre-trained to sample 

 and drink from two drinking tubes mounted on the front of a small in- 

 dividual cage. Six rats in each group, experimental and control, were pre- 

 trained and then tested while 22 hr thirsty for the preference between dis- 

 tilled water and 0.4 m NaCl solution. This salt concentration is quite aver- 

 sive to the normal rat. 



Figure 1 shows the mean cumulative intakes in the test and indicates that 

 the control animals took very httle salt and mostly water, whereas the 

 deprived rats in the first 15 min took more salt than water. Later in the 

 period, the intake of water by experimental animals showed a significant 

 rise whereas salt drinking continued at a reasonably regular rate. This late 

 rise in water intake probably reflects a post-ingestive or thirst effect pro- 

 duced by the hypertonic salt. Although the method of plotting exaggerates 

 this late rise, it is equally evident with a Hnear time scale. Close examina- 

 tion of the early portions of the water and salt curves indicates that the 

 first acceleration of water drinking occurs at about 5 min, which could 

 mean that this is the first sign of an immediate post-ingestive effect per- 

 haps different from the later thirst effect. Six other rats were tested for 



* Special Public Health Service Postdoctoral Fellow. 



