PHYSIOLOGY OF CHEMORECEPTION 257 



flowing over the chemoreceptor cells (Hodgson 1967). For studies on the 

 fate of a known chemical stimulant of the shark's olfactory system, tritium- 

 labeled d-1 glutamic acid was chosen. 



In experiments conducted in collaboration with Dr. Arland Carsten of 

 the Brookhaven National Laboratory, the labeled glutamic acid was injected 

 into a seawater stream perfusing the nasal sac of an anesthetized nurse or 

 lemon shark. When a bolus of labeled stimulus (2 ml of solution, containing 

 200 [id of radioactivity in glutamic acid) was introduced in the nasal sac, it 

 was found that the pattern of flow through the olfactory sac was no differ- 

 ent than the flow pattern of a dye through the same system (Figure 13). 

 Radioactivity in the outflowing perfusate was measured in samples taken 

 every 5 s during perfusion, and counted in a scintillation counter. The 

 counts rose rapidly after the stimulus was introduced, and fell to zero 

 after 150 to 200 s. (The exact shape of the curves varied according to the 

 anatomy of the olfactory sac in each shark; in the example illustrated in 

 Figure 13, any counts of less than 10 3 are not significant, since the larger 

 amounts approach 1.56 X 10 7 . Also, in a semilog plot, the slight differ- 

 ences at the initial and concluding low ends of the curve are greatly 

 magnified.) 



The lack of tracer activity in the outflow of perfusate after the bolus of 

 labeled stimulus had passed through the nasal sac suggests that seawater 

 flow through the sac is not significantly impeded by the internal folds of 

 the sac, intricate as they are (Plate II). Another possibility is that any stimu- 

 lus bound to the receptor membranes remains there, possibly being metabo- 

 lized or otherwise disposed of in the olfactory epithelia. To check the latter 

 possibility, solutions of radioactively labeled glutamic acid were placed 

 directly in the olfactory sac and allowed to remain there for periods of up 

 to 5 min, after which the sac was flushed with a minimal amount of sea- 

 water and removed. The tissue was fixed and sectioned, and autoradio- 

 graphs were prepared. When the autoradiographs were evaluated, no labeled 

 material was seen in the convoluted membranes containing the chemo- 

 sensory cells. Plate II shows one of the sections, stained with hematoxylin, 

 from this series of autoradiographs. It is remarkable that none of the labeled 

 stimulant, even by chance, remained in or on the olfactory epithelium; the 

 only radioactivity detected was in a few spots in spaces between epithelial 

 folds, not bound to any tissue. 



What these results evidently mean is that there is no prolonged binding 

 of the stimulus to any part of the receptor membranes— at least so far as 

 glutamic acid is concerned. Also, the flushing mechanisms of the olfactory 

 sacs are impressively efficient, with the flow pattern through the sac ap- 

 proaching that of flow through an unobstructed orifice. Such a flow pattern 

 has obvious advantages in facilitating moment-to-moment comparisons of 

 input to the two nasal sacs, and in preventing lingering aftereffects of stimu- 

 lation. Since it is possible that there may be more than one chemosensory 

 mechanism or site, it remains to be seen whether similar flow patterns and 

 lack of detectable binding will be found in tests with other types of labeled 

 and effective chemical stimulants. 



