Pharmacology Biochemistry & Behavior, Vol. 3, pp. 795-797. Copyright O 1975 by ANKHO International Inc. All rights of reproduction in any form reserved. Printed in the U.S.A.
Effects of Lithium Chloride on Muricidal Behavior in Rats J O H N R U S H A N D J. M E N D E L S 1
Depression Research Unit, Department o f Psychiatry, University o f Pennsylvania and Veterans Administration Hospital, Philadelphia, Pennsylvania 19104 ( R e c e i v e d 10 M a r c h 1 9 7 5 ) RUSH, J. AND J. MENDELS. Effects of lithium chloride on muricidal behavior in rats. PHARMAC. BIOCHEM. BEHAV. 3(5) 795-797, 1975. - Lithium chloride at two different doses (1 mEq/kg and 2 mEq/kg) IP BID for 10 days failed to inhibit muricidal behavior in rats. Lithium chloride at the higher dose caused neurotoxicity in 6 of 11 rats as measured by the rotorod. These dosages generated serum levels of 0.70 and 1.00 mEq/L respectively. The same behavior was blocked by imipramine HC1 at an ED 5 o of 8.5 mg/kg 45 rain following a single IP injection without evidence ofneurotoxicity by the rotorod method. These results indiciate that lithium chloride is unlike the tricyclic agents in the muricide test. Therefore, if its clinical antidepressant activity is substantial, it may be most effective in a neurochemically different class of depressives than the tricyclics. Lithium
Muricide
Depression
Antidepressant Drugs
A N T I D E P R E S S A N T drugs, as well as some a n t i h i s t a m i n e s a n d s t i m u l a n t s [ 1 , 3 ] , i n h i b i t m u r i c i d e ( m o u s e killing b y rats) at doses w h i c h do n o t cause n e u r o t o x i c i t y , e.g. as m e a s u r e d b y the r o t o r o d p e r f o r m a n c e [ 1 0 ] . In c o n t r a s t , drugs t h a t are n o t effective in t h e t r e a t m e n t o f d e p r e s s i o n (e.g. sedative h y p n o t i c s a n d m a j o r t r a n q u i l i z e r s ) i n h i b i t m u r i c i d e o n l y at doses w h i c h also p r o d u c e n e u r o t o x i c i t y [ 1 0 ] . F o r these reasons, selective i n h i b i t i o n of m u r i c i d e has b e e n suggested as o n e test for p r e d i c t i n g possible clinical a n t i d e p r e s s a n t activity o f n e w drugs [ 11 ]. While t h e role o f l i t h i u m in the t r e a t m e n t o f clinical depression r e m a i n s controversial, t h e r e is evidence t h a t it m a y act as an a n t i d e p r e s s a n t in a s u b g r o u p o f depressed p a t i e n t s [4, 9, 15, 17, 1 9 ] . The effect o f l i t h i u m o n m u r i c i d e is u n c e r t a i n . Sheard [23] has r e p o r t e d t h a t a relatively high dose (5 m E q / k g ) o f l i t h i u m blocks p a r a - c h l o r o p h e n y l a l a n i n e i n d u c e d muricide. It is possible t h a t this dose o f l i t h i u m m a y cause significant n e u r o t o x i c i t y , b u t this e f f e c t was n o t e x a m i n e d for. E i c h e l m a n a n d T h o a [5] r e p o r t e d t h a t l i t h i u m (0.8 m E q / k g for 14 c o n s e c u t i v e days) failed to b l o c k m u r i c i d a l b e h a v i o r in L o n g Evans rats b u t did n o t r e p o r t specific data, i n c l u d i n g s e r u m l i t h i u m levels or e v a l u a t i o n o f n e u r o t o x i c ity. T h e c u r r e n t e x p e r i m e n t was u n d e r t a k e n t o d e t e r m i n e t h e effect of clinically e q u i v a l e n t doses of l i t h i u m on m u r i c i d a l behavior. METHOD
Sprague-Dawley ( H o l t z m a n ) male rats b y a m o d i f i c a t i o n o f t h e m e t h o d o f food l i m i t a t i o n ( [ 2 0 ] a n d Paul, p e r s o n a l communication). Initially, t h e animals were given ad lib access to food and w a t e r for 10 days. Each a n i m a l was t h e n allowed access t o 10 g o f Purina R a t C h o w for 1 hr each day for 10 c o n s e c u t i v e days. On each o f the n e x t 5 days, while food l i m i t a t i o n c o n t i n u e d , each a n i m a l was given 2 e x p o s u r e s t o a mouse. T h e e x p o s u r e s were s e p a r a t e d b y 30 m i n a n d lasted for a m a x i m u m o f 5 m i n if the rat failed to kill. If the rat killed, t h e m o u s e was r e m o v e d f r o m the cage w i t h i n 15 sec o f t h e kill. A muricidal rat was defined as o n e w h i c h killed mice o n t h e last 5 c o n s e c u t i v e exposures. T h e rats were given ad lib access to f o o d a n d w a t e r for the r e m a i n d e r o f t h e e x p e r i m e n t . T h e i r weights r e t u r n e d t o baseline values a f t e r 10 days. O n t h e t e n t h day following r e t u r n to ad lib food, t w o a d d i t i o n a l e x p o s u r e s t o mice were allowed. Rats w h i c h killed b o t h mice were designated as muricidal. These rats were t h e n divided i n t o 2 equal sized groups a n d were used for t h e study. T w o g r o u p s of rats receiving d i f f e r e n t doses of l i t h i u m chloride were studied. T h e initial weight for t h e rats in G r o u p 1 was 398 -+ 6.2 g. ( m e a n + S D) a n d for the rats in G r o u p 2 was 2 8 4 -+ 8.7 g. ( m e a n + S D). T h e first group o f 7 muricidal rats received 1 m E q / k g LiC1 IP b.i.d, for 10 c o n s e c u t i v e days. A c o n t r o l g r o u p (N = 7) received an e q u i v a l e n t v o l u m e o f n o r m a l saline IP b.i.d, over the same t i m e period. T h e s e c o n d g r o u p o f muricidal rats received 2 m E q / k g LiC1 IP b.i.d, for 10 days. Eleven c o n t r o l animals received t h e same v o l u m e of saline. No rat was e x p o s e d t o mice d u r i n g t h e i n j e c t i o n period.
Animals and Procedure Muricidal b e h a v i o r was i n d u c e d in i n d i v i d u a l l y h o u s e d
~Reprint requests to be addressed to J. Mendels, M. D., Veterans Administration Hospital, University and Woodland Avenues, Philadelphia, PA 19104. 795
796
RUSH
Two hours following the last (20th) injection, each animal was exposed to 1 mouse for 5 min, and the percentage of killers for each group was calculated. Subsequently, the rats were tested on the r o t o r o d for 3 min following a 2 min acclimatization period [2,10]. Rats which fell more than 3 times were labelled neurotoxic. The animals were then guillotined and blood was collected for measurement of serum lithium concentration [6]. RESULTS F o o d limitation induced muricidal behavior in 48 percent of the animals by the end o f the food limitation period. After 10 days of their receiving food ad lib, 36 percent o f the original group continued to be muricidal. Every killer rat exhibited muricidal behavior within 2 min following exposure to a mouse, even after return to ad lib food access. During the food limitation procedure 22 percent of the animals died. These results compare favorably with those of Paul et al. [20]. Using a similar procedure with Sprague-Dawley rats, they reported a 43 percent muricide induction rate at the end of the food limitation period. At the conclusion of re-exposure to ad lib food, the muricide rate was 38 percent. The death rate in their study was 32 percent. The serum lithium level obtained for the group treated with 1 m E q / k g LiC1 b.i.d, was 0.70 +- 0.02 (mean +- S D) m E q / L . For the group given 2 m E q / k g LiC1 b.i.d, the serum lithium level was 1.00 -+ 0.11 m E q / L . A l t h o u g h a diuresis and increased water c o n s u m p t i o n was observed in the lithium treated animals, there were no deaths in either the lithium or saline treated groups during or following the injection period. The effect of the two dosage schedules of lithium on muricidal behavior and rotorod performance is shown in Table 1. There was no alteration in either behavior in the animals who received 1 m E q / k g LiC1 or saline. In the group receiving 2 m E q / k g LiC1 b.i.d., there was a disappearance of muricidal behavior in 3 of 11 rats. T w o of these animals, as well as 4 which continued to exhibit muricidal activity, were n e u r o t o x i c according to the rotorod test. One saline treated animal failed to display muricide following the injections.
AND
MENDELS
Thus, LiCI 2 m E q / k g b.i.d, produced serum levels equivalent to clinically therapeutic levels. At this dose, a significant reduction in r o t o r o d performance was found ( p < 0 . 0 5 , Fishers Exact Probability Test) compared to the control group. However, there was no significant selective inhibition of muricide o t h e r than that accounted for by neurotoxicity. A second brief e x p e r i m e n t was c o n d u c t e d to ensure that the muricidal behavior produced by our experimental procedure was indeed susceptible to tricyclic antidepressant inhibition [10]. For this purpose, 4 muricidal rats were tested for r o t o r o d performance and muricidal behavior 45 min following a single IP injection of imipramine hydrochloride. Since the reported EDso for imipramine HC1 was 8.0 mg/kg, 2 dosages were tested for, namely 7.0 mg/kg and 10.0 mg/kg. At the lower dose no inhibition of muricide or alteration in r o t o r o d performance was noted. At the higher dose, all 4 animals failed to display muricidal behavior without a d e c r e m e n t in rotorod performance. The EDs o of 8.5 mg/kg is compatible with that reported elsewhere [10]. DISCUSSION A l t h o u g h the effectiveness of lithium as an antidepressant remains controversial, recent reviews [15, 16, 17] suggest that a specific subgroup of depressed patients do respond to lithium. In addition, in a crossover study we found that 9 depressed patients who failed to respond to either lithium or desmethylimipramine did respond to the other drug [ 1 9 ] . The results of this and related studies suggest that a search for neurochemical differences between tricyclic and lithium responding depressed patients might be of value. Our findings indicate that lithium does not act like the tricyclic antidepressants [10,1 l] or electroconvulsive therapy [25] in inhibiting muricidal behavior at nonn e u r o t o x i c doses. Further, at doses equivalent to those k n o w n to be therapeutic in humans, [18,22], no selective inhibition of muricide occurs. These results support the findings of Eichelman and T h o a [5] who reported that lithium at 0.8 m E q / k g for 14 days was ineffective in blocking muricide in Long-Evans rats. However, it is likely from our data that the dose they
TABLE 1 THE EFFECT OF TWO DIFFERENT DOSES OF LITHIUM CHLORIDE ON MURICIDAL AND ROTOROD BEHAVIORS
Group
N
Serum Li*
Muricide Inhibited
Neurotoxic t
1 mEq/kg LiC1
7
0.70 + 0.02
0
0
Saline Control
7
0,00
0
0
2 mEq/kg LiC1
11
1.00 ± 0.11
3
6
Saline Control
11
0.00
1
1
*X ± S.D.
~Neurotoxicity was determined by rotorod performance
LITHIUM
AND
MURICIDAL
BEHAVIOR
797
used failed t o g e n e r a t e s e r u m levels e q u i v a l e n t t o t h o s e k n o w n t o be t h e r a p e u t i c in m a n . T h e lack o f a g r e e m e n t w i t h Sheard [23] m a y be d u e t o the high dose (5 m E q / k g ) he used to i n h i b i t P C P A - i n d u c e d muricide. This dose is likely t o be n e u r o t o x i c b y t h e r o t o r o d . O u r findings are c o n s i s t e n t w i t h t h o s e o f G e y e r e t al. [7] w h o r e p o r t e d t h e failure o f l i t h i u m t o p o t e n t i a t e t h e b e h a v i o r a l a n d cardiovascular effects o f y o h i m b i n e in c o n s c i o u s dogs at plasma c o n c e n t r a t i o n s o f 1 . 2 9 - 1 . 5 4 m E q / L . This effect o f y o h i m b i n e p o t e n t i a t i o n has b e e n r e p o r t e d to be c h a r a c t e r i s t i c of clinically useful tricyclic a n t i d e p r e s s a n t s or m o n o a m i n e oxidase i n h i b i t o r s [ 1 3 , 2 1 ] . Using a d i f f e r e n t a p p r o a c h , G o l d s t e i n a n d his associates have s h o w n t h a t l i t h i u m a n d several tricyclic a n t i d e p r e s sants all p o t e n t i a t e d t h e i n h i b i t o r y e f f e c t of n o r e p i n e p h r i n e o n e v o k e d p o s t g a n g l i o n i c p o t e n t i a l s in t h e s u p e r i o r cervical ganglion o f t h e cat [ 1 4 , 2 4 ] .
A n a l t e r n a t i v e c o n c l u s i o n suggested b y t h e data is t h a t the m u r i c i d e m o d e l is a l i m i t e d p r e d i c t o r o f a n t i d e p r e s s a n t activity in general. F o r e x a m p l e , s o m e investigators have suggested t h a t t h i o r i d a z i n e has a n t i d e p r e s s a n t activity in some n e u r o t i c depressed o u t p a t i e n t p o p u l a t i o n s [ 1 2 ] . However, in doses up to 4 0 m g / k g IP in rats, this drug failed t o i n h i b i t m u r i c i d e w i t h o u t causing n e u r o t o x i c i t y [8 ].
ACKNOWLEDGMENTS We wish to thank Dr. Luci Paul, Temple University and Dr. Alan Frazer for advice and assistance and Mr. Michael Kane and Mr. Lata Nemecek for their technical assistance. This study is supported by Research Funds from the Veterans Administration and NIMH Grant No. 5-T01-MH 11178-05.
REFERENCES 1. Barnett, A., R. I. Taber and R. E. Roth. Activity of antihistamines in laboratory antidepressant tests. Int. J. Pharmac. 8: 73-79, 1969. 2. Dunham, N. W. and T. S. Miya. A note on a simple device for detecting neurological deficit in rats and mice. J. Am. Pharm. Ass. (Sci. Edn.) 46: 208-209, 1957. 3. Dunlop, E. The use of antidepressants and stimulants. Mod. Treat. 2: 5 4 3 - 5 6 8 , 1965. 4. Dyson, W. L. and J. Mendels. Lithium and depression. Curt. ther. Res. 10: 6 0 1 - 6 0 8 , 1968. 5. Eichelman, B. S. and N. B. Thoa. The aggressive monoamines. Biol. Psychiat. 6: 143-164, 1973. 6. Frazer, A., S. K. Secunda and J. Mendels. A method for the determination of sodium, potassium, magnesium and lithium concentrations in erythrocytes. Clinica chim. Acta 36: 499-509, 1972. 7. Geyer, H., I. Sanghvi and S. Gershon. Exploration of the antidepressant potential of lithium. Psychopharmacolog~a 28: 107-113, 1973. 8. Gogerty, J., personal communication. Director, Pharmacology and Biochemistry Section, Sandoz Pharmaceuticals, Hanover, New Jersey. 9. Goodwin, F. K., D. L. Murphy and W. E. Bunney. Lithium carbonate treatment in depression and mania. Archs gen. Psychiat. Chicago 21: 4 8 6 - 4 9 6 , 1969. 10. Horovitz, Z. P., J. J. Piala, J. P. High, J. C. Burke and R. C. Leaf. Effects of drugs on the mouse-killing (muricide) test and its relationship to amygdaloid function. Int. J. Neuropharm. 5: 405-411, 1966. 11. Horovitz, Z. P., P. W. Raggozzino and R. C. Leaf. Selective block of rat mouse-killing behavior by antidepressants. Life Sci. 4: 1909-1912, 1965. 12. Klein, D. F. and J. M. Davis. Diagnosis and Drug Treatment o f Psychiatric Disorders. Baltimore, Williams and Wilkins, 1969. 13. Lang, W. J. and S. Gershon. Effects of psychoactive drugs on yohimbine induced responses in conscious dogs. Archs Int. Pharmacodyn. Ther. 142: 4 5 7 - 4 7 2 , 1963.
14. Malseed, R. T., G. V. Rossi and F. J. Goldstein. Potentiation of monoaminergic activity in peripheral ganglia by tricyclic antidepressants. Eur. J. Pharmac. 20: 34-39, 1972. 15. Mendels, J. Lithium and depression. In: Lithium: Its Role in Psychiatric Research and Treatment, edited by S. Gershon and B. Shopsin. New York: Raven Press, 1973, pp. 253-267. 16. Mendels, J. Lithium in the treatment of depressive states. In: Lithium Research and Therapy, edited by N. Johnson. New York: Academic Press, 1975, in press. 17. Mendels, J. and A. Frazer. Intracellular lithium concentration and clinical response - Towards a membrane theory of depression J. Psychiat. Res. 10: 9 - 1 8 , 1973. 18. Mendels, J. and S. K. Secunda, editors. Lithium. New York: Gordon and Breach, 1972. 19. Mendels, J., S. K. Secunda and W. L. Dyson. A controlled study of the antidepressant effects of lithium. Archs Gen. Psychiat. Chicago 26: 154-157, 1972. 20. Paul, L., W. M. Miley and R. Baenninger. Mouse killing by rats: Roles of hunger and thirst in its initiation and maintenance. J. comp. physiol. Psychol. 76: 242-249, 1971. 21. Sanghvi, I., E. Bindler and S. Gershon. The clinical evaluation of a new animal method for the prediction of clinical antidepressant activity. Life Sci. 8: 99-106, 1969. 22. Schou, M. Lithium in psychiatric therapy and prophylaxis. J. Psychiat. Res. 6: 6 7 - 9 5 , 1968. 23. Sheard, M. H. Behavioral effects of p-chlorophenylalanine in rats: Inhibition of lithium. Communs behav. Biol. 5: 7 1 - 7 3 , 1970. 24. Therani, J. B., G. V. Rossi and F. J. Goldstein. Effect of lithium and rubidium on the ganglionic inhibiting action of norepinephrine. Life ScL 15: 525-537, 1974. 25. Vogel, J. and D. R. Haubrich. Chronic administration of electroconvulsive shock effects on mouse-killing activity and brain monoamines in rats. Physiol. Behav. l l : 725-728, 1973.
Effects of Lithium Chloride on Muricidal Behavior in Rats J O H N R U S H A N D J. M E N D E L S 1
Depression Research Unit, Department o f Psychiatry, University o f Pennsylvania and Veterans Administration Hospital, Philadelphia, Pennsylvania 19104 ( R e c e i v e d 10 M a r c h 1 9 7 5 ) RUSH, J. AND J. MENDELS. Effects of lithium chloride on muricidal behavior in rats. PHARMAC. BIOCHEM. BEHAV. 3(5) 795-797, 1975. - Lithium chloride at two different doses (1 mEq/kg and 2 mEq/kg) IP BID for 10 days failed to inhibit muricidal behavior in rats. Lithium chloride at the higher dose caused neurotoxicity in 6 of 11 rats as measured by the rotorod. These dosages generated serum levels of 0.70 and 1.00 mEq/L respectively. The same behavior was blocked by imipramine HC1 at an ED 5 o of 8.5 mg/kg 45 rain following a single IP injection without evidence ofneurotoxicity by the rotorod method. These results indiciate that lithium chloride is unlike the tricyclic agents in the muricide test. Therefore, if its clinical antidepressant activity is substantial, it may be most effective in a neurochemically different class of depressives than the tricyclics. Lithium
Muricide
Depression
Antidepressant Drugs
A N T I D E P R E S S A N T drugs, as well as some a n t i h i s t a m i n e s a n d s t i m u l a n t s [ 1 , 3 ] , i n h i b i t m u r i c i d e ( m o u s e killing b y rats) at doses w h i c h do n o t cause n e u r o t o x i c i t y , e.g. as m e a s u r e d b y the r o t o r o d p e r f o r m a n c e [ 1 0 ] . In c o n t r a s t , drugs t h a t are n o t effective in t h e t r e a t m e n t o f d e p r e s s i o n (e.g. sedative h y p n o t i c s a n d m a j o r t r a n q u i l i z e r s ) i n h i b i t m u r i c i d e o n l y at doses w h i c h also p r o d u c e n e u r o t o x i c i t y [ 1 0 ] . F o r these reasons, selective i n h i b i t i o n of m u r i c i d e has b e e n suggested as o n e test for p r e d i c t i n g possible clinical a n t i d e p r e s s a n t activity o f n e w drugs [ 11 ]. While t h e role o f l i t h i u m in the t r e a t m e n t o f clinical depression r e m a i n s controversial, t h e r e is evidence t h a t it m a y act as an a n t i d e p r e s s a n t in a s u b g r o u p o f depressed p a t i e n t s [4, 9, 15, 17, 1 9 ] . The effect o f l i t h i u m o n m u r i c i d e is u n c e r t a i n . Sheard [23] has r e p o r t e d t h a t a relatively high dose (5 m E q / k g ) o f l i t h i u m blocks p a r a - c h l o r o p h e n y l a l a n i n e i n d u c e d muricide. It is possible t h a t this dose o f l i t h i u m m a y cause significant n e u r o t o x i c i t y , b u t this e f f e c t was n o t e x a m i n e d for. E i c h e l m a n a n d T h o a [5] r e p o r t e d t h a t l i t h i u m (0.8 m E q / k g for 14 c o n s e c u t i v e days) failed to b l o c k m u r i c i d a l b e h a v i o r in L o n g Evans rats b u t did n o t r e p o r t specific data, i n c l u d i n g s e r u m l i t h i u m levels or e v a l u a t i o n o f n e u r o t o x i c ity. T h e c u r r e n t e x p e r i m e n t was u n d e r t a k e n t o d e t e r m i n e t h e effect of clinically e q u i v a l e n t doses of l i t h i u m on m u r i c i d a l behavior. METHOD
Sprague-Dawley ( H o l t z m a n ) male rats b y a m o d i f i c a t i o n o f t h e m e t h o d o f food l i m i t a t i o n ( [ 2 0 ] a n d Paul, p e r s o n a l communication). Initially, t h e animals were given ad lib access to food and w a t e r for 10 days. Each a n i m a l was t h e n allowed access t o 10 g o f Purina R a t C h o w for 1 hr each day for 10 c o n s e c u t i v e days. On each o f the n e x t 5 days, while food l i m i t a t i o n c o n t i n u e d , each a n i m a l was given 2 e x p o s u r e s t o a mouse. T h e e x p o s u r e s were s e p a r a t e d b y 30 m i n a n d lasted for a m a x i m u m o f 5 m i n if the rat failed to kill. If the rat killed, t h e m o u s e was r e m o v e d f r o m the cage w i t h i n 15 sec o f t h e kill. A muricidal rat was defined as o n e w h i c h killed mice o n t h e last 5 c o n s e c u t i v e exposures. T h e rats were given ad lib access to f o o d a n d w a t e r for the r e m a i n d e r o f t h e e x p e r i m e n t . T h e i r weights r e t u r n e d t o baseline values a f t e r 10 days. O n t h e t e n t h day following r e t u r n to ad lib food, t w o a d d i t i o n a l e x p o s u r e s t o mice were allowed. Rats w h i c h killed b o t h mice were designated as muricidal. These rats were t h e n divided i n t o 2 equal sized groups a n d were used for t h e study. T w o g r o u p s of rats receiving d i f f e r e n t doses of l i t h i u m chloride were studied. T h e initial weight for t h e rats in G r o u p 1 was 398 -+ 6.2 g. ( m e a n + S D) a n d for the rats in G r o u p 2 was 2 8 4 -+ 8.7 g. ( m e a n + S D). T h e first group o f 7 muricidal rats received 1 m E q / k g LiC1 IP b.i.d, for 10 c o n s e c u t i v e days. A c o n t r o l g r o u p (N = 7) received an e q u i v a l e n t v o l u m e o f n o r m a l saline IP b.i.d, over the same t i m e period. T h e s e c o n d g r o u p o f muricidal rats received 2 m E q / k g LiC1 IP b.i.d, for 10 days. Eleven c o n t r o l animals received t h e same v o l u m e of saline. No rat was e x p o s e d t o mice d u r i n g t h e i n j e c t i o n period.
Animals and Procedure Muricidal b e h a v i o r was i n d u c e d in i n d i v i d u a l l y h o u s e d
~Reprint requests to be addressed to J. Mendels, M. D., Veterans Administration Hospital, University and Woodland Avenues, Philadelphia, PA 19104. 795
796
RUSH
Two hours following the last (20th) injection, each animal was exposed to 1 mouse for 5 min, and the percentage of killers for each group was calculated. Subsequently, the rats were tested on the r o t o r o d for 3 min following a 2 min acclimatization period [2,10]. Rats which fell more than 3 times were labelled neurotoxic. The animals were then guillotined and blood was collected for measurement of serum lithium concentration [6]. RESULTS F o o d limitation induced muricidal behavior in 48 percent of the animals by the end o f the food limitation period. After 10 days of their receiving food ad lib, 36 percent o f the original group continued to be muricidal. Every killer rat exhibited muricidal behavior within 2 min following exposure to a mouse, even after return to ad lib food access. During the food limitation procedure 22 percent of the animals died. These results compare favorably with those of Paul et al. [20]. Using a similar procedure with Sprague-Dawley rats, they reported a 43 percent muricide induction rate at the end of the food limitation period. At the conclusion of re-exposure to ad lib food, the muricide rate was 38 percent. The death rate in their study was 32 percent. The serum lithium level obtained for the group treated with 1 m E q / k g LiC1 b.i.d, was 0.70 +- 0.02 (mean +- S D) m E q / L . For the group given 2 m E q / k g LiC1 b.i.d, the serum lithium level was 1.00 -+ 0.11 m E q / L . A l t h o u g h a diuresis and increased water c o n s u m p t i o n was observed in the lithium treated animals, there were no deaths in either the lithium or saline treated groups during or following the injection period. The effect of the two dosage schedules of lithium on muricidal behavior and rotorod performance is shown in Table 1. There was no alteration in either behavior in the animals who received 1 m E q / k g LiC1 or saline. In the group receiving 2 m E q / k g LiC1 b.i.d., there was a disappearance of muricidal behavior in 3 of 11 rats. T w o of these animals, as well as 4 which continued to exhibit muricidal activity, were n e u r o t o x i c according to the rotorod test. One saline treated animal failed to display muricide following the injections.
AND
MENDELS
Thus, LiCI 2 m E q / k g b.i.d, produced serum levels equivalent to clinically therapeutic levels. At this dose, a significant reduction in r o t o r o d performance was found ( p < 0 . 0 5 , Fishers Exact Probability Test) compared to the control group. However, there was no significant selective inhibition of muricide o t h e r than that accounted for by neurotoxicity. A second brief e x p e r i m e n t was c o n d u c t e d to ensure that the muricidal behavior produced by our experimental procedure was indeed susceptible to tricyclic antidepressant inhibition [10]. For this purpose, 4 muricidal rats were tested for r o t o r o d performance and muricidal behavior 45 min following a single IP injection of imipramine hydrochloride. Since the reported EDso for imipramine HC1 was 8.0 mg/kg, 2 dosages were tested for, namely 7.0 mg/kg and 10.0 mg/kg. At the lower dose no inhibition of muricide or alteration in r o t o r o d performance was noted. At the higher dose, all 4 animals failed to display muricidal behavior without a d e c r e m e n t in rotorod performance. The EDs o of 8.5 mg/kg is compatible with that reported elsewhere [10]. DISCUSSION A l t h o u g h the effectiveness of lithium as an antidepressant remains controversial, recent reviews [15, 16, 17] suggest that a specific subgroup of depressed patients do respond to lithium. In addition, in a crossover study we found that 9 depressed patients who failed to respond to either lithium or desmethylimipramine did respond to the other drug [ 1 9 ] . The results of this and related studies suggest that a search for neurochemical differences between tricyclic and lithium responding depressed patients might be of value. Our findings indicate that lithium does not act like the tricyclic antidepressants [10,1 l] or electroconvulsive therapy [25] in inhibiting muricidal behavior at nonn e u r o t o x i c doses. Further, at doses equivalent to those k n o w n to be therapeutic in humans, [18,22], no selective inhibition of muricide occurs. These results support the findings of Eichelman and T h o a [5] who reported that lithium at 0.8 m E q / k g for 14 days was ineffective in blocking muricide in Long-Evans rats. However, it is likely from our data that the dose they
TABLE 1 THE EFFECT OF TWO DIFFERENT DOSES OF LITHIUM CHLORIDE ON MURICIDAL AND ROTOROD BEHAVIORS
Group
N
Serum Li*
Muricide Inhibited
Neurotoxic t
1 mEq/kg LiC1
7
0.70 + 0.02
0
0
Saline Control
7
0,00
0
0
2 mEq/kg LiC1
11
1.00 ± 0.11
3
6
Saline Control
11
0.00
1
1
*X ± S.D.
~Neurotoxicity was determined by rotorod performance
LITHIUM
AND
MURICIDAL
BEHAVIOR
797
used failed t o g e n e r a t e s e r u m levels e q u i v a l e n t t o t h o s e k n o w n t o be t h e r a p e u t i c in m a n . T h e lack o f a g r e e m e n t w i t h Sheard [23] m a y be d u e t o the high dose (5 m E q / k g ) he used to i n h i b i t P C P A - i n d u c e d muricide. This dose is likely t o be n e u r o t o x i c b y t h e r o t o r o d . O u r findings are c o n s i s t e n t w i t h t h o s e o f G e y e r e t al. [7] w h o r e p o r t e d t h e failure o f l i t h i u m t o p o t e n t i a t e t h e b e h a v i o r a l a n d cardiovascular effects o f y o h i m b i n e in c o n s c i o u s dogs at plasma c o n c e n t r a t i o n s o f 1 . 2 9 - 1 . 5 4 m E q / L . This effect o f y o h i m b i n e p o t e n t i a t i o n has b e e n r e p o r t e d to be c h a r a c t e r i s t i c of clinically useful tricyclic a n t i d e p r e s s a n t s or m o n o a m i n e oxidase i n h i b i t o r s [ 1 3 , 2 1 ] . Using a d i f f e r e n t a p p r o a c h , G o l d s t e i n a n d his associates have s h o w n t h a t l i t h i u m a n d several tricyclic a n t i d e p r e s sants all p o t e n t i a t e d t h e i n h i b i t o r y e f f e c t of n o r e p i n e p h r i n e o n e v o k e d p o s t g a n g l i o n i c p o t e n t i a l s in t h e s u p e r i o r cervical ganglion o f t h e cat [ 1 4 , 2 4 ] .
A n a l t e r n a t i v e c o n c l u s i o n suggested b y t h e data is t h a t the m u r i c i d e m o d e l is a l i m i t e d p r e d i c t o r o f a n t i d e p r e s s a n t activity in general. F o r e x a m p l e , s o m e investigators have suggested t h a t t h i o r i d a z i n e has a n t i d e p r e s s a n t activity in some n e u r o t i c depressed o u t p a t i e n t p o p u l a t i o n s [ 1 2 ] . However, in doses up to 4 0 m g / k g IP in rats, this drug failed t o i n h i b i t m u r i c i d e w i t h o u t causing n e u r o t o x i c i t y [8 ].
ACKNOWLEDGMENTS We wish to thank Dr. Luci Paul, Temple University and Dr. Alan Frazer for advice and assistance and Mr. Michael Kane and Mr. Lata Nemecek for their technical assistance. This study is supported by Research Funds from the Veterans Administration and NIMH Grant No. 5-T01-MH 11178-05.
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