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Effects of dose and of partial body ionizing radiation on taste aversion learning in rats with lesions of the area postrema
Physiology & Behavior, Vol. 32, pp. 119-122. Copyright ©Pergamon Press Ltd., 1984. Printed in the U.S.A.
0031-9384/84 $3.00 + .00
Effects of Dose and of Partial Body Ionizing Radiation on Taste Aversion Learning in Rats with Lesions of the Area Postrema BERNARD
M. R A B I N , * t W A L T E R
A. H U N T * A N D J A C K L E E *
*Behavioral Sciences Department, A r m e d Forces Radiobiology Research Institute, Bethesda, M D 20814 and t D e p a r t m e n t o f Psychology, University o f Maryland Baltimore County, Catonsville, M D 21228 R e c e i v e d 31 M a y 1983 RABIN, B. M., W. A. HUNT AND J. LEE. Effects of dose and of partial body ionizing radiation on taste aversion learning in rats with lesions of the area postrema. PHYSIOL BEHAV 32(1) 119-122, 1984.--The effect of area postrema lesions on the acquisition of a conditioned taste aversion following partial body exposure to ionizing radiation was investigated in rats exposed to head-only irradiation at 100, 200 and 300 tad or to body-only irradiation at 100 and 200 rad. Following head-only irradiation area postrema lesions produced a significant attenuation of the radiation-induced taste aversion at all dose levels, although the rats still showed a significant reduction in sucrose preference. Following body-only exposure, area postrema lesions completely disrupted the acquisition of the conditioned taste aversion. The results are interpreted as indicating that: (a) the acquisition of a conditioned taste aversion following body-only exposure is mediated by the area postrema; and (b) taste aversion learning following radiation exposure to the head-only is mediated by both the area postrema and a mechanism which is independent of the area postrema. Ionizing radiation
Partial body exposure
Conditioned taste aversion
W H E N a novel tasting solution is paired with exposure to ionizing radiation, an animal will avoid further ingestion of that solution at a later time. This avoidance behavior, called a conditioned taste aversion (CTA), may be acquired in a single trial. Previous research has shown that a CTA produced by whole body exposure to 100 rad of ionizing radiation can be attenuated by lesions of the area postrema (AP) [11]. Because the destruction of the AP prevents the emetic response to a variety of systemic toxins in organisms that are capable of emesis [3], and because a radiationinduced CTA can be transferred to the shielded member of a parabiotic pair of rats following irradiation of the exposed member of the pair [6], these results have been interpreted as being consistent with the hypothesis that a radiation-released humoral factor, to which the AP is sensitive, serves as the proximate unconditioned stimulus leading to the acquisition of the radiation-induced CTA. In addition to whole body irradiation, a CTA can also be produced by partial body exposures [5, 15, 16]. In general, partial body irradiation is a less effective unconditioned stimulus than whole body irradiation for producing a CTA. With partial body exposures, irradiation of the abdomen is more effective in producing a CTA than is irradiation of the head, requiring a lower dose of radiation to produce the avoidance behavior [5,16]. While it is possible that differences in the effectiveness o f partial body exposures in producing a CTA simply reflect the release of varying
Area postrema
amounts of a humorai mediator following exposure to ionizing radiation, perhaps due to the irradiation of different amounts of tissue mass, there is the possibility that a CTA produced by body-only exposure may involve different mechanisms than taste aversion learning following head-only exposures. Alternatively, it may be that the brain is less sensitive to ionizing radiation than are other parts of the body, and that, as a result, exposure of the head-only produces smaller behavioral effects. The present experiment was designed to study the role of the AP in the acquisition of a CTA following exposure to ionizing radiation restricted to either the head or to the body. METHOD The surgical and experimental procedures have been described in detail in a previous report [11]. Briefly, the subjects were 119 male Sprague-Dawley derived rats weighing 250-325 g at the time of surgery. The rats were maintained in individual cages with food continually available. Histologically confirmed lesions were placed in the AP of 60 rats by thermal cauterization. Because previous research showed no effects on CTA learning from sham surgery or from sham irradiation in rats with AP lesions [11], the remaining 59 rats served as unoperated controls for the effects of the surgery. After a two-week period to recover from the surgery, rats were placed on a 23.5 hr water deprivation schedule for 10
1Requests for reprints should be addressed to B. M. Rabin, Department of Psychology, University of Maryland Baltimore County, Catonsville, MD 21228.
119
120
RABIN, H U N ' I
days during which water was available for 30 min a day during the early light phase of a 12:12 light:dark cycle. On the conditioning day (day 10), the rats were presented with two calibrated drinking tubes for 30 rain, one containing tap water and the other containing a 10e/~ sucrose solution. Intake of each solution was recorded. Immediately after drinking, all rats were placed in a plastic restraining tube and exposed to gamma radiation using a cobalt-60 source. After exposure, the rats were returned to their home cages. On the test day (day ll), the rats were again presented with calibrated tubes containing water and 10~/c sucrose for 30 rain and intake of each measured. Any rat which did not show a greater sucrose intake than water intake on the conditioning day was excluded from the experiment. The average sucrose intake for all control animals was 20.17 ml while the average intake of the rats with AP lesions was 25.43 ml. For both groups, the sucrose intake was above the levels at which variations in intake have been reported to produce variations in the intensity of a CTA [1]. For the head-only exposures (head and neck exposed), three dose levels of radiation were tested: 100 rad (n=24), 200 rad (n=26), and 300 tad (n=24). Two doses were tested with body-only (thorax to pelvis exposed) irradiation: 100 rad (n =22) and 200 tad (n=23). For each exposure level, half the animals had AP lesions and half were controls, except for the 200 rad body-only group in which 12 had AP lesions and 11 were controls. Dose rate was held constant at 40 rad/min. For radiation exposure, the animals were placed in a plastic restraining tube which was in turn enclosed in a cave made of lead bricks to a minimum thickness of 10 cm. The bricks were drilled to accept the part of the tube containing either the head or the body of the rat. During irradiation the rats were observed with a remote video monitor to verify that the animals did not shift position within the tube. Dosimetry was accomplished using a 3.3 cc Victoreen chamber and using thermoluminescent detectors (LiF TLD 100s). Measurements were made both free in air and at depth using a mouse phantom. Dosimetry indicated less than lC~ scatter within the shielded area. For histological examination, the animals were sacrificed with an overdose of sodium pentobarbital and perfused intracardially with isotonic saline followed by 10% formalin saline. The brains were fixed in formalin, the brainstem cut at 40 p,m and stained with thionin. Only those animals that showed a minimum of 90% destruction of the AP and minimal damage to surrounding structures have been included within the study. Data are presented as preference score, defined as sucrose intake divided by total fluid intake. A preference score greater than 0.50 indicates a greater intake of sucrose solution than of water, and, therefore, a preference for the sucrose solution. For statistical analysis, the preference scores were transformed using the arcsin transformation to normalize the distributions [19]. Overall data analyses were then done using separate 3-way analyses of variance with one repeated factor for the head-only and body-only exposure conditions. Comparison of preference scores within individual dose levels was done using orthogonal comparisons [7]. RESULTS
The results of the head-only exposures are summarized in Fig. 1, which shows that AP lesions produced a significant attenuation of the radiation-induced CTA, although there was a significant reduction in sucrose preference for those
ANI)
I.EI-'.
HEAD ONLY IRRADIATION 1.0
100 RAD
2 0 0 RAD
300 HAD
0.8 0.6 0.4 0.2
Control
AP Lesion
Control
Conditioning [ ]
i
AP Lesion
hi
Control
AP Lesion
Test
FIG. 1. Sucrose preference scores of rats exposed to one of three doses of head only irradiation. Variance bars indicate the standard error.
rats given 200 and 300 rad exposures. The statistical analysis of the data using a 3-way analysis of variance indicated that increasing the radiation dose produced a corresponding significant decrease in sucrose preference, F(2,68)=11.20, p<0.001. At all dose levels, the test day sucrose preference of the control rats differed significantly from the rats with AP lesions, F(1,68)=18.58, p<0.001. While the main effect for the conditioning day/test day comparison was highly significant, F(1,68) = 95.85, p <0.001, indicating a general reduction in sucrose preference as a result of the radiation exposure, significant day x lesion, F(1,68)= 16.50, p <0.001, and day × radiation dose, F(2,68)=5.83, p<0.01, interactions would suggest differences in the pattern of CTA responding across conditions. A more detailed analysis within each dose level using orthogonal comparisons showed that, at each dose level, the test day sucrose preference of the control rats was significantly less than their conditioning day preference. For the animals with AP lesions, only those receiving 100 rad did not show a significant reduction in sucrose preference, F(1,68)=0.022, p>0.10, following exposure. For the rats with AP lesions receiving 200 rad, F(1,68)=9.25, p<0.01, and 300 rad, F(1,68)=6.74, p<0.05, the test day sucrose preference was significantly less than the conditioning day preference. However, at all three dose levels, the test day sucrose preference of the animals with AP lesions was significantly greater than the test day preference of the control rats (100 rad, F(1,68)=5.08, p<0.05; 200 rad, F(I,68)=8.38, p<0.01; 300 rad, F(1,68)=20.88, p<0.001). The results of the body-only irradiations are summarized in Fig. 2, which shows that the rats with AP lesions showed no change in sucrose preference from the conditioning to the test day. The analysis of variance indicated that the two levels of irradiation used, 100 rad and 200 tad, did not produce significantly different effects on sucrose preference, F(1,41)=0.028, p>0.10, and that at both doses the control rats differed significantly from the rats with AP lesions, F(I,41)= 14.29, p<0.001. The main effect for the conditioning day/test day comparison was significant, F(1,41)=22.14, p <0.001, as was the day × lesion interaction, F(I,41)= 19.32, p<0.001, indicating that the pattern of taste aversion responding differed in the control and AP lesioned animals. This difference in the pattern of responding between the control rats and those with lesions of the AP is further clarified by the more detailed individual analysis using or-
P A R T I A L BODY I R R A D I A T I O N AND CTA
121
BODY ONLY IRRADIATION 1.0 0.8
0.6 0.4 0.2
100 RAD
200 RAD
im I
I"
0
Control AP Lesion
Conditioning [ ]
Control AP Lesion Test , ~
FIG. 2. Sucrose preference scores of rats exposed to one of two levels of body only irradiation. Variance bars indicate the standard error.
thogonal comparisons. For the control animals at both dose levels, the test day sucrose preference was significantly less than the conditioning day preference. However, for the animals with AP lesions there was no significant difference between conditioning and test day sucrose preferences (100 tad, F(1,41) =0.002, p>0.10; 200 rad, F(1,41)=0.27,p >0.10). Similarly, at both dose levels, the sucrose preference of the animals with AP lesions was significantly greater than that of the controls (100 rad, F(1,41)=14.30, p<0.001; 200 rad, F(1,41)= 16.22, p <0.001). DISCUSSION The results show that lesions of the AP attenuate the acquisition of a CTA following partial body exposure to ionizing radiation. As such, they support the hypothesis that the mechanisms by which exposure of only the head or body lead to CTA learning are similar. Because the AP functions as the chemoreceptive trigger zone for emesis [3] to monitor the blood and cerebrospinal fluid for potential toxins, the present findings would also be consistent with the hypothesis that some humoral factor to which the AP is sensitive is released by exposure of either the head or body to ionizing radiation. The present results are generally consistent with the results of previous experiments using partial body exposures [5,16] in showing that body-only irradiation produces a CTA at a lower dose than does head-only exposure. The differences between the present results and previous experiments are relatively minor, relating to differences in the specifics of the dose/response relationship between radiation exposure and CTA intensity that result from the use of different doses of radiation, from the exposure of different amounts of tissue, and from the use of different dependent measures [17]. Although the data presented above support the hypothesis that the acquisition of a CTA following irradiation of either head- or body-only is mediated by the AP, these data
also suggest that taste aversion learning following irradiation of the head involves an additional mechanism that is not dependent upon the integrity of the AP. Despite the fact that AP lesions resulted in a significant attenuation of the CTA produced by head-only irradiation, there was still a significant decrease in the sucrose preference shown by these animals that was not observed with the animals given the body-only exposures. This observation suggests that exposure of the head of an organism to radiation may produce a behavioral change by a mechanism in addition to the one mediated by the AP. The continued decrease in sucrose preference in the animals with AP lesions given head-only irradiation could reflect the operation of several possible mechanisms. First, irradiation of the head could additionally result in the activation of sensory receptors. Smith [15] has reviewed studies which indicate that rats are sensitive to olfactory stimulation resulting from radiation exposure, and Reige [12] has shown that blocking the olfactory system of rats with paraffin during irradiation disrupts the acquistion of a radiation-induced CTA. In addition a number of other studies have shown an interaction between olfactory cues and taste in CTA learning [13,18]. It may be possible, therefore, that this olfactory stimulation contributes to the decreased sucrose preference in animals given head-only exposures. However, it has yet to be shown that an olfactory stimulus presented by itself can function as the unconditioned stimulus for a CTA. Alternatively, it is possible that irradiation of the head may have a direct effect on neural activity. Except during development [2, 9, 14], the central nervous system has been considered to be relatively resistant to the effects of ionizing radiation [8]. The present results, in contrast, lead to the speculation that there may be a more direct effect of ionizing radiation on nervous system activity as it affects CTA learning following exposure to radiation. In this regard, it may be noted that exposure to X-rays at doses as low as 15 tad can alter the threshold for electroshock convulsions in adult rats [10] and that the exposure of cats to 300 rad X-irradiation produces changes in the recovery cycle of visual evoked potentials [4]. The present results which might suggest a direct effect of ionizing radiation on nervous system activity in the mature organism must be considered as only suggestive, however, pending further research.
ACKNOWLEDGEMENTS
We wish to acknowledge the support of the computer center facilities of the University of Maryland. This research was conducted according to the principles described in the "Guide for the Care and Use of Laboratory Animals" prepared by the Institute of Laboratory Animal Research, National Research Council. A preliminary report of some of the data has been presented at the Seventh International Congress for Radiation Research, Amsterdam, 1983.
REFERENCES 1. Barker, L. M. CS duration, amount, and concentration effects in conditioned taste aversions. Learn Motivat 7: 265--273, 1976. 2. Ben-Barak, J. The development of the cholinergic system in rat hippocampus following postnatal X-irradiation. Brain Res 226: 171-186, 1981.
3. Borison, H. L. Area postrema: Chemoreceptive trigger zone for vomitting--is that all? Life Sci 14: 1807-1817, 1974. 4. Daugherty, J. H. and C. D. Barnes. Effect of X-irradiation on the interaction of flash-evoked cortical potentials. Radiat Res 49: 359-366, 1972.
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5. Garcia, J. and D. J. Kimeldorf. Some factors which influence radiation-conditioned behavior in rats. Radiat Re~ 12: 719-727, 1960. 6. Hunt, E. L., H. W. Carroll and D. J. Kimeldorf. Humoral mediation of radiation-induced motivation in parabiont rats. Science 150: 1747-1748, 1965. 7. Keppel, G. Design and Analysis: A Researcher's Handbook. Englewood Cliffs, NJ.: Prentice-Hall, 1973. 8. Kimeldorf, D. J. and E. L. Hunt. Ionizing Radiation: Neural Function and Behavior. New York: Academic Press, 1965. 9. Ordy, J. M., K. R. Brizzee, W. P. Dunlap and C. Knight. Effect of prenatal n°Co irradiation on postnatal neural, learning, and hormonal development. Radiat Res 89: 309-324, 1982. 10. Pollack, M. and P. S. Timiras. X-ray dose and electroconvulsive responses in adult rats. Radiat Res 21:111-119, 1964. 11. Rabin, B. M., W. A. Hunt and J. Lee. Attenuation of radiationand drug-induced conditioned taste aversions following area postrema lesions in the rat. Radiat Res 93: 388-394, 1983. 12. Riege, W. H. Possible olfactory transduction of radiationinduced aversion. Psyehon Sei 12: 303-304, 1968.
RABIN, HUNT AND LEE
13. Rusiniak, K. W., C. C. Palmerino and J. Garcia. Potentiation oi odor by taste: Tests of some nonassociative factors..I ('¢~ml;, Physiol Psychol 96: 775-780, 1982. 14. Schneider, B. F. and S. Norton. Postnatal behavioral changes in prenatally irradiated rats. Neurobehav Toxic'ol 1: 193-197, 1979. 15. Smith, J. C. Radiation: Its detection and its effects on taste preferences. In: Progres.~ in Physiological Psyc'halo~,,y, vol 4. edited by E. Stellar and J. M. Sprague. New York: Academic Press, 1971, pp. 53-117. 16. Smith, J. C., G. R. Hollander and A. C. Spector. Taste aversions conditioned with partial body radiation exposures. Physiol Behav 27: 903-913, 1981. 17. Spector, A. C., J. C. Smith and G. R. Hollander. A comparison of dependent measures used to quantify radiation-induced taste aversion. Physial Behav 27: 887-901, 1981. 18. Van Buskirk, R. L. The role of odor in the maintenance of flavor aversion. Physiol Behav 27: 18%193, 1981. 19. Winer, B. J. Statistical Principle,~ in Experimental De.~ign. New York: McGraw-Hill, 1962.
0031-9384/84 $3.00 + .00
Effects of Dose and of Partial Body Ionizing Radiation on Taste Aversion Learning in Rats with Lesions of the Area Postrema BERNARD
M. R A B I N , * t W A L T E R
A. H U N T * A N D J A C K L E E *
*Behavioral Sciences Department, A r m e d Forces Radiobiology Research Institute, Bethesda, M D 20814 and t D e p a r t m e n t o f Psychology, University o f Maryland Baltimore County, Catonsville, M D 21228 R e c e i v e d 31 M a y 1983 RABIN, B. M., W. A. HUNT AND J. LEE. Effects of dose and of partial body ionizing radiation on taste aversion learning in rats with lesions of the area postrema. PHYSIOL BEHAV 32(1) 119-122, 1984.--The effect of area postrema lesions on the acquisition of a conditioned taste aversion following partial body exposure to ionizing radiation was investigated in rats exposed to head-only irradiation at 100, 200 and 300 tad or to body-only irradiation at 100 and 200 rad. Following head-only irradiation area postrema lesions produced a significant attenuation of the radiation-induced taste aversion at all dose levels, although the rats still showed a significant reduction in sucrose preference. Following body-only exposure, area postrema lesions completely disrupted the acquisition of the conditioned taste aversion. The results are interpreted as indicating that: (a) the acquisition of a conditioned taste aversion following body-only exposure is mediated by the area postrema; and (b) taste aversion learning following radiation exposure to the head-only is mediated by both the area postrema and a mechanism which is independent of the area postrema. Ionizing radiation
Partial body exposure
Conditioned taste aversion
W H E N a novel tasting solution is paired with exposure to ionizing radiation, an animal will avoid further ingestion of that solution at a later time. This avoidance behavior, called a conditioned taste aversion (CTA), may be acquired in a single trial. Previous research has shown that a CTA produced by whole body exposure to 100 rad of ionizing radiation can be attenuated by lesions of the area postrema (AP) [11]. Because the destruction of the AP prevents the emetic response to a variety of systemic toxins in organisms that are capable of emesis [3], and because a radiationinduced CTA can be transferred to the shielded member of a parabiotic pair of rats following irradiation of the exposed member of the pair [6], these results have been interpreted as being consistent with the hypothesis that a radiation-released humoral factor, to which the AP is sensitive, serves as the proximate unconditioned stimulus leading to the acquisition of the radiation-induced CTA. In addition to whole body irradiation, a CTA can also be produced by partial body exposures [5, 15, 16]. In general, partial body irradiation is a less effective unconditioned stimulus than whole body irradiation for producing a CTA. With partial body exposures, irradiation of the abdomen is more effective in producing a CTA than is irradiation of the head, requiring a lower dose of radiation to produce the avoidance behavior [5,16]. While it is possible that differences in the effectiveness o f partial body exposures in producing a CTA simply reflect the release of varying
Area postrema
amounts of a humorai mediator following exposure to ionizing radiation, perhaps due to the irradiation of different amounts of tissue mass, there is the possibility that a CTA produced by body-only exposure may involve different mechanisms than taste aversion learning following head-only exposures. Alternatively, it may be that the brain is less sensitive to ionizing radiation than are other parts of the body, and that, as a result, exposure of the head-only produces smaller behavioral effects. The present experiment was designed to study the role of the AP in the acquisition of a CTA following exposure to ionizing radiation restricted to either the head or to the body. METHOD The surgical and experimental procedures have been described in detail in a previous report [11]. Briefly, the subjects were 119 male Sprague-Dawley derived rats weighing 250-325 g at the time of surgery. The rats were maintained in individual cages with food continually available. Histologically confirmed lesions were placed in the AP of 60 rats by thermal cauterization. Because previous research showed no effects on CTA learning from sham surgery or from sham irradiation in rats with AP lesions [11], the remaining 59 rats served as unoperated controls for the effects of the surgery. After a two-week period to recover from the surgery, rats were placed on a 23.5 hr water deprivation schedule for 10
1Requests for reprints should be addressed to B. M. Rabin, Department of Psychology, University of Maryland Baltimore County, Catonsville, MD 21228.
119
120
RABIN, H U N ' I
days during which water was available for 30 min a day during the early light phase of a 12:12 light:dark cycle. On the conditioning day (day 10), the rats were presented with two calibrated drinking tubes for 30 rain, one containing tap water and the other containing a 10e/~ sucrose solution. Intake of each solution was recorded. Immediately after drinking, all rats were placed in a plastic restraining tube and exposed to gamma radiation using a cobalt-60 source. After exposure, the rats were returned to their home cages. On the test day (day ll), the rats were again presented with calibrated tubes containing water and 10~/c sucrose for 30 rain and intake of each measured. Any rat which did not show a greater sucrose intake than water intake on the conditioning day was excluded from the experiment. The average sucrose intake for all control animals was 20.17 ml while the average intake of the rats with AP lesions was 25.43 ml. For both groups, the sucrose intake was above the levels at which variations in intake have been reported to produce variations in the intensity of a CTA [1]. For the head-only exposures (head and neck exposed), three dose levels of radiation were tested: 100 rad (n=24), 200 rad (n=26), and 300 tad (n=24). Two doses were tested with body-only (thorax to pelvis exposed) irradiation: 100 rad (n =22) and 200 tad (n=23). For each exposure level, half the animals had AP lesions and half were controls, except for the 200 rad body-only group in which 12 had AP lesions and 11 were controls. Dose rate was held constant at 40 rad/min. For radiation exposure, the animals were placed in a plastic restraining tube which was in turn enclosed in a cave made of lead bricks to a minimum thickness of 10 cm. The bricks were drilled to accept the part of the tube containing either the head or the body of the rat. During irradiation the rats were observed with a remote video monitor to verify that the animals did not shift position within the tube. Dosimetry was accomplished using a 3.3 cc Victoreen chamber and using thermoluminescent detectors (LiF TLD 100s). Measurements were made both free in air and at depth using a mouse phantom. Dosimetry indicated less than lC~ scatter within the shielded area. For histological examination, the animals were sacrificed with an overdose of sodium pentobarbital and perfused intracardially with isotonic saline followed by 10% formalin saline. The brains were fixed in formalin, the brainstem cut at 40 p,m and stained with thionin. Only those animals that showed a minimum of 90% destruction of the AP and minimal damage to surrounding structures have been included within the study. Data are presented as preference score, defined as sucrose intake divided by total fluid intake. A preference score greater than 0.50 indicates a greater intake of sucrose solution than of water, and, therefore, a preference for the sucrose solution. For statistical analysis, the preference scores were transformed using the arcsin transformation to normalize the distributions [19]. Overall data analyses were then done using separate 3-way analyses of variance with one repeated factor for the head-only and body-only exposure conditions. Comparison of preference scores within individual dose levels was done using orthogonal comparisons [7]. RESULTS
The results of the head-only exposures are summarized in Fig. 1, which shows that AP lesions produced a significant attenuation of the radiation-induced CTA, although there was a significant reduction in sucrose preference for those
ANI)
I.EI-'.
HEAD ONLY IRRADIATION 1.0
100 RAD
2 0 0 RAD
300 HAD
0.8 0.6 0.4 0.2
Control
AP Lesion
Control
Conditioning [ ]
i
AP Lesion
hi
Control
AP Lesion
Test
FIG. 1. Sucrose preference scores of rats exposed to one of three doses of head only irradiation. Variance bars indicate the standard error.
rats given 200 and 300 rad exposures. The statistical analysis of the data using a 3-way analysis of variance indicated that increasing the radiation dose produced a corresponding significant decrease in sucrose preference, F(2,68)=11.20, p<0.001. At all dose levels, the test day sucrose preference of the control rats differed significantly from the rats with AP lesions, F(1,68)=18.58, p<0.001. While the main effect for the conditioning day/test day comparison was highly significant, F(1,68) = 95.85, p <0.001, indicating a general reduction in sucrose preference as a result of the radiation exposure, significant day x lesion, F(1,68)= 16.50, p <0.001, and day × radiation dose, F(2,68)=5.83, p<0.01, interactions would suggest differences in the pattern of CTA responding across conditions. A more detailed analysis within each dose level using orthogonal comparisons showed that, at each dose level, the test day sucrose preference of the control rats was significantly less than their conditioning day preference. For the animals with AP lesions, only those receiving 100 rad did not show a significant reduction in sucrose preference, F(1,68)=0.022, p>0.10, following exposure. For the rats with AP lesions receiving 200 rad, F(1,68)=9.25, p<0.01, and 300 rad, F(1,68)=6.74, p<0.05, the test day sucrose preference was significantly less than the conditioning day preference. However, at all three dose levels, the test day sucrose preference of the animals with AP lesions was significantly greater than the test day preference of the control rats (100 rad, F(1,68)=5.08, p<0.05; 200 rad, F(I,68)=8.38, p<0.01; 300 rad, F(1,68)=20.88, p<0.001). The results of the body-only irradiations are summarized in Fig. 2, which shows that the rats with AP lesions showed no change in sucrose preference from the conditioning to the test day. The analysis of variance indicated that the two levels of irradiation used, 100 rad and 200 tad, did not produce significantly different effects on sucrose preference, F(1,41)=0.028, p>0.10, and that at both doses the control rats differed significantly from the rats with AP lesions, F(I,41)= 14.29, p<0.001. The main effect for the conditioning day/test day comparison was significant, F(1,41)=22.14, p <0.001, as was the day × lesion interaction, F(I,41)= 19.32, p<0.001, indicating that the pattern of taste aversion responding differed in the control and AP lesioned animals. This difference in the pattern of responding between the control rats and those with lesions of the AP is further clarified by the more detailed individual analysis using or-
P A R T I A L BODY I R R A D I A T I O N AND CTA
121
BODY ONLY IRRADIATION 1.0 0.8
0.6 0.4 0.2
100 RAD
200 RAD
im I
I"
0
Control AP Lesion
Conditioning [ ]
Control AP Lesion Test , ~
FIG. 2. Sucrose preference scores of rats exposed to one of two levels of body only irradiation. Variance bars indicate the standard error.
thogonal comparisons. For the control animals at both dose levels, the test day sucrose preference was significantly less than the conditioning day preference. However, for the animals with AP lesions there was no significant difference between conditioning and test day sucrose preferences (100 tad, F(1,41) =0.002, p>0.10; 200 rad, F(1,41)=0.27,p >0.10). Similarly, at both dose levels, the sucrose preference of the animals with AP lesions was significantly greater than that of the controls (100 rad, F(1,41)=14.30, p<0.001; 200 rad, F(1,41)= 16.22, p <0.001). DISCUSSION The results show that lesions of the AP attenuate the acquisition of a CTA following partial body exposure to ionizing radiation. As such, they support the hypothesis that the mechanisms by which exposure of only the head or body lead to CTA learning are similar. Because the AP functions as the chemoreceptive trigger zone for emesis [3] to monitor the blood and cerebrospinal fluid for potential toxins, the present findings would also be consistent with the hypothesis that some humoral factor to which the AP is sensitive is released by exposure of either the head or body to ionizing radiation. The present results are generally consistent with the results of previous experiments using partial body exposures [5,16] in showing that body-only irradiation produces a CTA at a lower dose than does head-only exposure. The differences between the present results and previous experiments are relatively minor, relating to differences in the specifics of the dose/response relationship between radiation exposure and CTA intensity that result from the use of different doses of radiation, from the exposure of different amounts of tissue, and from the use of different dependent measures [17]. Although the data presented above support the hypothesis that the acquisition of a CTA following irradiation of either head- or body-only is mediated by the AP, these data
also suggest that taste aversion learning following irradiation of the head involves an additional mechanism that is not dependent upon the integrity of the AP. Despite the fact that AP lesions resulted in a significant attenuation of the CTA produced by head-only irradiation, there was still a significant decrease in the sucrose preference shown by these animals that was not observed with the animals given the body-only exposures. This observation suggests that exposure of the head of an organism to radiation may produce a behavioral change by a mechanism in addition to the one mediated by the AP. The continued decrease in sucrose preference in the animals with AP lesions given head-only irradiation could reflect the operation of several possible mechanisms. First, irradiation of the head could additionally result in the activation of sensory receptors. Smith [15] has reviewed studies which indicate that rats are sensitive to olfactory stimulation resulting from radiation exposure, and Reige [12] has shown that blocking the olfactory system of rats with paraffin during irradiation disrupts the acquistion of a radiation-induced CTA. In addition a number of other studies have shown an interaction between olfactory cues and taste in CTA learning [13,18]. It may be possible, therefore, that this olfactory stimulation contributes to the decreased sucrose preference in animals given head-only exposures. However, it has yet to be shown that an olfactory stimulus presented by itself can function as the unconditioned stimulus for a CTA. Alternatively, it is possible that irradiation of the head may have a direct effect on neural activity. Except during development [2, 9, 14], the central nervous system has been considered to be relatively resistant to the effects of ionizing radiation [8]. The present results, in contrast, lead to the speculation that there may be a more direct effect of ionizing radiation on nervous system activity as it affects CTA learning following exposure to radiation. In this regard, it may be noted that exposure to X-rays at doses as low as 15 tad can alter the threshold for electroshock convulsions in adult rats [10] and that the exposure of cats to 300 rad X-irradiation produces changes in the recovery cycle of visual evoked potentials [4]. The present results which might suggest a direct effect of ionizing radiation on nervous system activity in the mature organism must be considered as only suggestive, however, pending further research.
ACKNOWLEDGEMENTS
We wish to acknowledge the support of the computer center facilities of the University of Maryland. This research was conducted according to the principles described in the "Guide for the Care and Use of Laboratory Animals" prepared by the Institute of Laboratory Animal Research, National Research Council. A preliminary report of some of the data has been presented at the Seventh International Congress for Radiation Research, Amsterdam, 1983.
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