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Effects of medetomidine, an α-2 adrenoceptor agonist, and atipamezole, an α-2 antagonist, on spatial memory performance in adult and aged rats
BEHAVIORAL AND NEURAL BIOLOGY 58, 113--119 (1992)
Effects of Medetomidine, an oz-2Adrenoceptor Agonist, and Atipamezole, an e-2 Antagonist, on Spatial Memory Performance in Adult and Aged Rats SYNNOVE CARLSON, H E I K K I TANILA, P I A R-~MA, ERNST M E C K E , AND A N T T I PERTOVAARA 1
Department of Physiology, University of Helsinki, Siltavuorenpenger 20 J, 00170 Helsinki, Finland
(DR) task (Arnsten & Gotdman-Rakic, 1985; Arnsten, Cai, & Goldman-Rakic, 1988) and in delayed matching-to-sample performance after clonidine administration (Jackson & Buccafusco, 1991, Arnsten & Goldman-Rakic, 1990). Also, contradictory effects on DR performance in monkeys have been reported after clonidine administration (Davis, Callahan, & Doens, 1988). Medetomidine is a highly selective and potent agonist at both pre- and postsynaptic a-2 adrenoceptors (Virtanen, Savola, Saano, & Nyman, 1988). The ~-2/~-1 receptor binding selectivity ratio of medetomidine is 1620 compared with 220 of clonidine (Virtanen et al., 1988). The pharmacological effects of medetomidine can be antagonized by atipamezole, a novel, highly selective ~-2 adrenoceptor antagonist (Virtanen, Savola, & Saano, 1989). In the current work we attempted to study the effect of the highly selective a-2 adrenergic agonist medetomidine on the working memory of adult and aged rats. Furthermore, the preliminary reports indicating that atipamezole too may have some beneficial effects on cognitive functions of aged rats (Nieminen, Airaksinen, & MacDonald, 1990; Sirvi6, Riekkinen, Valjakka, Halonen, & Riekkinen, 1990) encouraged us to study its effects on the spatial memory performance of the same rats. The memory performance was studied using a variable delay, spatial delayed alternation (DA) task performed in an open T-maze. The effects of the drugs on the task performance were evaluated first when the rats were adults and again when they were aged.
The effects of a novel, highly selective a-2 agonist, medetomidine, and its antagonist, atipamezole, were studied on the working memory of rats performing a spatial delayed alternation task. Testing was performed in two stages, at the age of 8.3 months (mean) and again when the rats were 17.6 months (mean). A low dose (3 ftg/kg) and a high dose (30 ~g/kg) of medetomidine improved the performance of the old rats in the memory task but had no effect on the young rats. The dose-response curve of medetomidine resembles that of guanfacine, another a-2 agonist. At the low dose of medetomidine (3 ftg/kg) the animals showed no signs of sedation. Since medetomidine even at a low dose has a beneficial effect on the memory performance of old rats, it could be a good candidate for the treatment of age-associated memory dysfunction, v 1992 Academic Press, Inc. There is growing evidence that the central noradrenergic function declines with aging in several species (Leslie, Loughlin, Sternberg, McGaugh, Young, & Zornetzer, 1985; Boyajian & Leslie, 1987; Estes & Simpkins, 1980; Ponzio, Brunello, & Algeri, 1978; Goldman-Rakic & Brown, 1981) and that this decline may be associated with the age-related rnemory loss (Leslie et al., 1985; Barnes 1979; Barnes, Nadel, & Honig, 1980; Gage, Dunnett, & BjSrklund, 1984; Rapp, Rosenberg, & Gallagher, 1987; Pelleymounter, Smith, & Gallagher, 1987; Ponzio et al., 1978; Bartus, Fleming, & Johnson, 1978; Walker, Kitt, Struble, Wagster, Price, & Cork, 1988). a-2 adrenergic agonists like clonidine and guanfacine have been reported to improve the perf~rmance of aged monkeys in a delayed response
METHODS
1 We t h a n k J u k k a Veilahti for assistance in statistical analysis. The study was supported by grants from the Paulo Foundation and from the Science and Research Foundation of Farmos Ltd. Turku. Correspondence and reprint requests should be addressed to S. Carlson.
Eight male Wistar rats (weight 355-490 g) were obtained from the Finnish National Laboratory Animal Centre. The animals were housed individually 113 0163-1047/92 $5.00 Copyright © 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.
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in standard rack-mounted cages. They had free access to water, and food was available at all times except during delayed alternation training and testing when the rats were maintained at 85% body wt. The rats were on a 12-h light-dark cycle. Training and testing was conducted during the light cycle in a quiet room in the animal ward. Medetomidine (Domitor 1 mg/ml) and atipamezole (Antisedan 5 mg/ml) (both synthetized by the Farmos Group Ltd., Turku, Finland) and 0.9% saline (control) were administered im. Testing was conducted in an open T-maze constructed of plywood and painted green throughout. The walls of the maze were 15 cm high. The length of the stem was 76 cm and the length of the arms was 57 cm, including a food recess which formed an angle at the ends of the arms such that the food reward was not visible when the rat entered the arms. During delayed alternation testing, correct responses were rewarded by a small piece of food pellet (ca. 100 mg) placed in the food recess. The starting compartment in the stem and the arms of the T-maze could be closed by a guillotine door. For 1 week prior to spontaneous alternation testing the rats were handled daily for 5 min by the experimenter and their weights were measured. The training of the rats was performed according to the method described by Nonneman and Kolb (1979). Following the adjustment period the rats were placed on 2 days in the T-maze for 5 rain where they were allowed to walk around freely. Pretraining consisted of 10 trials per day in the T-maze until the rat ate the food reward on each trial. At this stage a reward was placed at the ends of both arms of the T-maze. The rat was adjusted to wait for 3 s in the starting compartment before the guillotine door was opened. If the rat consecutively entered the same arm two times, the arm was blocked by the guillotine door. Training then proceeded to the delayed alternation phase with a 0-s delay. After the first choice when the food had been at both ends of the arms, the reward was placed only to the end of the arm that had not been chosen on the first trial. Daily training consisted of 10 trials. If the rat entered the same arm twice in a row, the reward was left in its place until the rat chose the correct arm. When the rat succeeded in making 80% correct choices of the daily trials, a 10-s delay was introduced. During the delay period the rat was kept in a round bucket without any view to the T-maze. The moment the rat entered the bucket determined the onset of the delay. This delay was maintained until the rat performed 80% of the trials correctly. On subsequent days the delay was gradually in-
ET AL.
creased until the rat's performance dropped near chance level. Alternating delays were then introduced. The number of daily trials was increased to 20, and four different delays were chosen according to the performance of the individual rat. The shortest delay for any rat was 10 s. The longest delay was the one preceding the delay in which the performance had dropped to chance level. The other two delays were adjusted individually such that the performance of the rats was about 65% correct choices of the 20 trials of the day. The rats were tested with the drugs first when they were adults (age range 6.5-10.5 months, mean 8.3 months) and again when they were aged (age range 17-18 months, mean 17.6 months). Before the second period of testing was started the performance level of the rats in the DA task was checked and the delays were again adjusted such that the rats succeeded in performing correctly in about 65% of the daily trials. The effects on the DA performance of three doses of medetomidine (3, 10, and 30 t~g/kg) and four doses of atipamezole (0.3, 0.6, 1, and 3 mg/kg) were compared with the performance on saline. Each dose of the drugs and the saline control of the drug were given once at both stages of testing (young and aged). Thus both medetomidine and atipamezole had one saline control. The order of giving the drug doses and their saline control was randomized and the tester was unaware of the nature of the injection. The interval between the testing of the different doses of the drug and their saline control was about 24 h. All doses were administered im 15 min prior to the testing. The delayed alternation performance on the drug was compared with matched saline control. The number of trials correct on drug was subtracted from the number of trials correct on saline. The difference score was then multiplied by 5% as each trial constituted 5% of the total number of trials. The results were analyzed statistically using one-way analysis of variance for repeated measures (1-ANOVA-R), and when the result was statistically significant, a post hoc analysis was performed with paired t test. RESULTS During the training period one rat was replaced by another animal because of slow learning and reluctance to eat the rewards. Table 1 illustrates the number of sessions (a session consisted of 10 trials) needed by individual rats to adjust to the Tmaze and to start eating rewards during every trial (pretraining) (mean 9.3 sessions, SD = 0.5, n = 7)
EFFECTS OF MEDETOMIDINE ON MEMORY TABLE 1 Total Number of Sessions of Seven Individual Rats during Pretraining Period and Delayed Alternation (DA) Training Rat
Pretraining (n)
DA training (n)
R1 R2 R3 G1 G2 G3 G4
10 9 10 9 9 9 9
31 30 31 30 30 31 30
115 Atl pamezole
N=8
30 c 20 oc; 10
/
z E o
-10 u
-20
Age.
-30
[ ] 7-11months • 17-18months
Note. n, n u m b e r of sessions.
0.3
and the number of sessions needed before alternating delays were introduced (mean 30.4 sessions, SD = 0.5, n -- 7). These data are not available for the rat that replaced the poorly performing rat. However, the training of this animal was similar to that of the other seven rats. When tested for the first time, the variable delays for all rats were the same: 10 s, 2, 5, and 7 min. When they were tested in the second stage at older age, the delays were 10 s, 2.8 min (mean), 10.9 min (mean), and 14.3 rain (mean). With comparable delays the rats at older age tended to make more errors than at younger age (Fig. 4, 1-ANOVA-R, F(1, 26) -8.75, p < .007). At the age of 7-11 months there was no statistically significant improvement or impairment in the DA performance with either medetomidine or atipamezole (Figs. 1 and 2) at the doses used in this study. it
Medetomidine
N=8
30
2
~ lO 7 :E
:g
~= -10 j~ -20
Age.
[ ] 7-11mont hs • 17-18months
g -30 i
3
110
30
Dose IJg/kg~
F I G . 1. Effects of medetomidine on delayed a l t e r n a t i o n t a s k performance in r a t s as y o u n g adults a n d at older age. Vertical lines indicate SEM. *p < .02, **p < .002 (one-sample t test).
O6
10
3.0 Dose mg/kg
FIG. 2. Effects of atipamezole on delayed alternation t a s k performance in r a t s as y o u n g adults a n d at older age. Vertical
lines indicate SEM.
When the rats were aged, medetomidine improved significantly the performance in the memory task (1-ANOVA-R, F(7, 24) = 6.49, p < .003) (Fig. 3), whereas atipamezole had no statistically significant effect on the task performance (F(7, 32) = 1.80, p < .2). In post hoc analysis a significant improvement in the task performance was found with medetomidine at doses 3/~g/kg (tdep(7) ---- 4.82, p < .002) and 30/~g/kg (tdep(7) ---- 3 . 0 5 , p < .02) (Fig. 1). With 10 t~g/kg of medetomidine the performance of the rats in the DA task did not differ significantly from their performance on saline (tdep(7) = 1.52, p < .2). With 3 and 1 0 / ~ g / k g of medetomidine, no signs of sedation could be observed in the rats. With 30 tLg/kg of medetomidine the rats showed clear signs of sedation: their movements were slow and they walked clumsily. However, they were not asleep and were able to perform the DA task and eat the rewards. The number of errors in the task performance increased with increasing length of delay both when the rats were young (1-ANOVA-R, F(3, 45) = 7.00, p < .001) and when they were old (F(3, 45) = 9.88, p < 0.0001) (Fig. 4). To study whether the beneficial effect of medetomidine doses 3 and 30 t~g/kg was related to the length of the delay, we compared the performances of the old rats at these drug doses in the shortest and longest delays of the task. Medetomidine at 3 t~g/kg diminished the number of errors regardless of the length of the delay. Medetomidine at 30 t~g/kg had its clearest effect on the shortest delay (1-sample t test, p < .04) (Fig. 5). However, the number of errors in the shortest delay was small also on saline.
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-~ 3O 12 0 (9
10
S
0
Z
2O
E
8
0 L.
ILl
Delay:
mm,
q)
6
¢-
~ 10 o4 N ¢-
2
~
0
MED 3 I
I
I
I
C
3
10
3o
IJg/kg
DOSE FIG. 3. The performances of individual rats at age 17.6 months (mean) in the DA task at different doses of medetomidine and matched saline control.
DISCUSSION A low (3 ~g/kg) and a high dose (30 ~g/kg) of medetomidine improved the performance of the aged rats in the spatial memory task. The highest dose of medetomidine (30 /~g/kg) also produced clear signs of sedation in the rats. The dose-response
i er
2
i
!
!
1
2
3 DELAY
FIG. 4. Comparable delays in the DA task of the rats as
young (0) and as aged (&) plotted against the numberof errors. Delay 1 is the shortest delay. The vertical lines indicate SEM. The differencein the number of errors in differentdelays was statistically significantbetween the two groups.
MED 3 0
T r e a t m e n t FIG. 5. Relation of the beneficial effects of medetomidine (3 and 30 ~g/kg on the length of the delay in the spatial memory performance of old rats. Delays I and IV refer to the shortest and longest delays, respectively. (*) One-sample t test, t = 2.65, p < .04.
profile of medetomidine resembles that of guanfacine described in the study by Arnsten et al. (1988). In a low and moderate dose range, guanfacine improved the performance of monkeys in a delayed response task. At a higher dose, the performance did not differ from that of saline control. However, at a still higher dose of guanfacine, which also produced sedation of the animals, the performance improved again. The dose-response curve of medetomidine, as well as of guanfacine, is quite different from that of clonidine, which at a low dose impaired and at higher doses improved the performance of old monkeys in a DR task (Arnsten & GoldmanRakic, 1985; Arnsten et al., 1988, 1990). Guanfacine is a more selective ~-2 adrenergic agonist than clonidine (Arnsten et al., 1988), and the a-2/a-1 receptor binding selectivity of medetomidine is 1620 compared to 220 of clonidine (Virtanen et al., 1988). Contradictory results concerning the effects of a2 agonists on memory performance of old animals have also been reported. Davis et al. (1988) could not improve the performance of old monkeys in a DR task with clonidine, and SirviS, Riekkinen, Vajanto, Koivisto, and Riekkinen (1991) reported that a low dose of guanfacine did not improve the performance of old rats in a water-maze test. It has been suggested that the beneficial effects of these drugs on memory performance may be related to the testing situation and become evident especially
EFFECTS OF MEDETOMIDINEON MEMORY when the possibility of distraction is greater and when long delays are used (Davis et al., 1988; Jackson & Buccafusco, 1991). It is also possible that a2 agonists improve working memory but not reference memory (Sirvi6 et al., 1991). Medetomidine improved the task performance of the aged rats only. This finding is in concert with Lhe growing evidence that monoaminergic and cho[inergic neurotransmitter levels decline with in,creasing age (Luine, Bowling, & Hearns, 1990; Luine & Hearns, 1990). Decreased noradrenaline (NA) levels have been found in aged rats in basal forebrain nuclei and in cortical areas (Luine et al., 1990). Also, 5-HT and dopamine decrease with increasing age in these areas and in the hippocampus and striatum (dopamine) (Luine et al., 1990). Several studies have demonstrated a correlation between such age-related changes of neurotransmitter ]levels and impaired memory performance. It has been shown that impaired T-maze performance of aged rats correlates inversely with the levels of NA J[n the cortex (Markowska, Stone, Ingram, Reynolds, Gold, Conti, Pontecorvo, Wenk, & Olton, 1989). Age associated cell loss in the locus coeruleus has been shown to correlate with impairment in a stepthrough inhibitory avoidance task in mice (Leslie et al., 1985), and locus coeruleus tissue transplantation into the third ventricle of aged rats improved their performance in the inhibitory avoidance task (Collier, Gash, & Sladek, 1988). It has been well documented that lesions in the medial prefrontal cortex impair DA performance in a T-maze (Wikmark, Divac, & Weiss, 1973; Larsen & Divac, 1978; Nonneman & Kolb, 1979; Thomas & Brito, 1980; Brito, Thomas, Davis, & Gingold, 11982). The importance of the septohippocampal systern for the performance of delayed alternation tasks in rats has also been well recognized. Hippocampal lesions disrupt spontaneous alternation performance (Isseroff, 1979). Decreased cholinergic activity, induced by septal lesions (Brito & Brito, 1990), c,ombined nucleus basalis and medial septal lesion (Wenk, Hughey, Boundy, Kim, Walker, & Olton, 1987), and the administration of scopolamine directly into the prefrontal cortex or hippocampus (Dunnett, Wareham, & Torres, 1990) have also been shown to impair severely the DA performance of rats. On the other hand, destruction of the locus coeruleus or depletion of brain NA by DSP4 injections results in only moderate (60-s delays) or no (shorter delays) impairment of DA performance (Wenk et al., 1987). Several authors have pointed out the possible imp,ortance of interactions between the different neu-
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rotransmitter systems (cholinergic and monoaminergic) involved in memory processes. There is evidence indicating that cortical cholinergic release is modulated by NA, a mechanism that has been suggested to operate also at the hippocampal level (Beani, Bianchi, Giacomelli, & Tamberi, 1978; Vizi, 1980; Ammassari-Teule, Maho, & Sara, 1991). Interaction of the noradrenergic system with other neurotransmitter systems has also been shown in behavioral studies. Decker and Callagher (1987) showed that NA depletion further impaired the scopolamine-induced disruption of radial arm maze performance of rats. Suppression of septal dopaminergic neurons causes impairment in spontaneous alternation behavior in rats, but this behavior remains unaffected when the dopaminergic lesion is accompanied by a septal noradrenergic lesion (Taghzouti, Le Moal, & Simon, 1991). Moreover, Luine et al. (1990) showed that in aged rats the NA content declines in many brain regions but only the decline in nucleus basalis correlated significantly with impaired performance in radial arm maze. The beneficial effect of medetomidine in the spatial memory task performance of aged rats may be best understood on the basis of interactions between different neurotransmitter systems. It has earlier been suggested that the beneficial effect of a-2 adrenergic agonists on the memory performance of aged animals is mediated through postsynaptic a-2 adrenergic receptors in the frontal cortex (Brozoski, Brown, Rosvold, & Goldman, 1979; Arnsten & Goldman-Rakic, 1985), probably of the Ri-type (Rauwolscine insensitive) (Arnsten et al., 1988), whereas several other effects of a-2 adrenergic agonists (e.g., hypotension and sedation) may be mediated through the Rs-subtype (Rauwolscine sensitive) of these receptors. On the other hand, low doses of clonidine have been shown to act preferentially at the presynaptic level by reducing the NA release (Starke, 1977; Starke & Altmann, 1973). In line with this, Ammassari-Teule et al. (1991) showed that small doses of clonidine attenuate spatial learning defects produced by a partial fornix section in rats. The authors suggested that small doses of clonidine could affect via presynaptic a-2 adrenergic receptors and thus could increase indirectly the release of acetylcholine at the hippocampal level by decreasing the release of NA. It may also be argued that the beneficial effect of a-2 adrenergic agonist medetomidine on memory performance is not mediated solely via a-2 adrenergic receptors since medetomidine, which is an imidazole derivative (Savola, Ruskoaho, Puurunen, Salonen, & K~irki, 1986), also binds to the imidazole
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CARLSON ET AL.
receptors of t h e cerebral cortex (Wikberg, U h l e n , & Chajlani, 1991). F u r t h e r studies are n e e d e d to clarify the m e c h a n i s m s of action of a-2 a d r e n e r g i c agonists on t h e b r a i n of aged a n i m a l s . The ability of m e d e t o m i d i n e to i m p r o v e the perf o r m a n c e of old r a t s in a m e m o r y t a s k a t a low dose in which no sedation of the a n i m a l s is e v i d e n t m a k e s it a good c a n d i d a t e for t h e t r e a t m e n t of age-associated m e m o r y dysfunction. A l t h o u g h A N O V A did not show a significant t r e a t m e n t effect of atipamezole on t h e p e r f o r m a n c e of t h e r a t s in the DA task, it should be n o t e d t h a t t h e h i g h e s t dose of a t i p a m e z o l e (3 m g / k g ) gave a n i m p r o v e m e n t in t h e t a s k p e r f o r m a n c e s i m i l a r to t h a t produced by t h e h i g h e s t m e d e t o m i d i n e dose. The s a m e dose of a t i p a m e z o l e h a s b e e n r e p o r t e d to i m p r o v e t h e p e r f o r m a n c e of old r a t s in t h e passive a v o i d a n c e and a p p e t i t i v e spatial d i s c r i m i n a t i o n m a z e t a s k s (Sirvi5 et al., 1990; N i e m i n e n et al., 1990). Atipamezole, like o t h e r a-2 a n t a g o n i s t s , increases t h e r e l e a s e of n o r a d r e n a l i n e from c e n t r a l n o r a d r e n e r g i c n e u r o n s by blocking p r e s y n a p t i c rel e a s e - i n h i b i t i n g receptors (Scheinin, MacDonald, & Scheinin, 1988). I n t r a p e r i t o n e a l injection of atipamezole causes a d o s e - d e p e n d e n t increase in t h e cent r a l t u r n o v e r of n o r a d r e n a l i n e as reflected by increased levels of its m a j o r metabolites. T h e i n c r e a s e r e a c h e d t h e level of significance a t doses 3 m g / k g and h i g h e r (Scheinin et al., 1988). H i g h doses of a t i p a m e z o l e also increase t h e t u r n o v e r of o t h e r a m i n e s (DA, 5-HT) (Scheinin et al., 1988). Also, depletion of d o p a m i n e in t h e p r e f r o n t a l cortex impairs d e l a y e d a l t e r n a t i o n p e r f o r m a n c e both in monk e y s a n d r a t s (Brozoski et al., 1979; Simon, Scatton, & Le Moal, 1980). Thus, it is possible t h a t t h e improved p e r f o r m a n c e of aged r a t s in cognitive t a s k s produced by h i g h doses of a t i p a m e z o l e could be med i a t e d b y these systems. H o w e v e r , m o r e r e s e a r c h is n e e d e d u n t i l final conclusions can be m a d e a b o u t t h e effects of h i g h doses of a t i p a m e z o l e on cognitive functions of aged rats. REFERENCES Ammassari-Teule, M., Maho, C., & Sara, S. J. (1991). Clonidine reverses spatial learning deficits and reinstates O frequencies in rats with partial foruix section. Behavioural Brain Research, 45, 1-8. Arnsten, A. F. T., Cai, J. X., & Goldman-Rakic, P. S. (1988). The a-2 adrenergic agonist guanfacine improves memory in aged monkeys without sedative or hypotensive side effects: Evidence for a-2 receptor subtypes. Journal of Neuroscience, 8, 4287-4298. Arnsten, A. F. T., & Goldman-Rakic, P. S. (1985). a-2 adrenergic mechanisms in prefrontal cortex associated with cognitive
decline in aged nonhuman primates. Science, 230, 12731276. Arnsten, A. F. T., & Goldman-Rakic, P. S. (1990). Analysis of a2 adrenergic agonist effects on the delayed nonmatch-tosample performance of aged rhesus monkeys. Neurobiology of Aging, 11, 583-590. Barnes, C. A. (1979). Memory deficits associated with scenescence: A neurophysiological and behavioral study in the rat. Journal of Comparative Physiology and Psychology, 93, 74104. Barnes, C. A. L., Nadel, L., & Honig, W. K. (1980). Spatial memory deficit in scenescent rats. Canadian Journal of Psychology, 34, 29-39. Bartus, R. T., Fleming, D., & Johnson, H. R. (1978). Aging in the rhesus monkey: Debilitating effects on short-term memory. Journal of Gerontology, 33, 858-871. Bartus, R. T., Dean, R. L., & Fleming, D. L. (1988). Aging in the rhesus monkey: Effects on visual discrimination, learning and reversal learning. Journal of Gerontology, 34, 209219. Beani, L. K., Bianchi, C., Giacomelli, A., & Tamberi, F. (1978). Noradrenaline inhibition of acetylcholine release from guinea-pig brain. European Journal of Pharmacology, 48, 179-193. Boyajian, C. L., & Leslie, F. M. (1987). Pharmacological evidence for a-2-adrenoceptor heterogeneity: Differential binding properties of [~H]rauwolscine and [3H]idazoxan in rat brain. Journal of Pharmacology and Experimental Therapy, 241, 1079-1091. Brito, G. N. O., Thomas, G. J., Davis, B. J., & Gingold, S. I. (1982). Prelimbic cortex, mediodorsal thalamus, septum, and delayed alternation in rats. Experimental Brain Research, 46, 52-58. Brito, G. N. O., & Brito, L. S. O. (1990). Septohippocampal system and the prelimbic sector of frontal cortex: A neuropsychological battery analysis in the rat. Behavioural Brain Research, 3, 127-146. Brozoski, T. J., Brown, R. M., Rosvold, H. E., & Goldman, P. S. (1979). Cognitive deficit caused by regional depletion of dopamine in prefrontal cortex of rhesus monkey. Science, 205, 929-932. Collier, T. J., Gash, D. M., & Sladek, J. R. (1988). Transplantation of norepinephrine neurons into aged rats improves performance of a learned task. Brain, 448, 77-87. Davis, R. E., Callahan, M. J., & Doens, D. A. (1988). Clonidine disrupts aged-monkey delayed response performance. Drug Development Research, 12, 279-286. Decker, M. W., & Gallagher, M. (1987). Scopolamine-disruption of radial arm maze performance: Modification by noradrenergic depletion. Brain Research, 417, 59-69. Dunnett, S. B., Wareham, A. T., & Torres, E. M. (1990). Cholinergic blockade in prefrontal cortex and hippocampus disrupts short-term memory in rats. NeuroReport, 1, 61-64. Estes, K. S., & Simpkins, J. W. (1980). Age-related alterations in cathecolamine concentrations in discrete preoptic area and hypothalamic regions in the male rat. Brain Research, 194, 556-560. Gage, F. H., Dunnett, S. B., & Bj6rklund, A. (1984). Spatial learning and motor deficits in aged rats. Neurobiology of Aging, 5, 43-48. Goldman-Rakic, P. S., & Brown, R. M. (1981). Regional changes
EFFECTS OF MEDETOMIDINE ON MEMORY of monoamines in cerebral cortex and subcortical structures of aging rhesus monkeys. Neuroscience, 6, 177-187. Isseroff, A. (1979). Limited recovery of spontaneous alternation after extensive hippocampal damage: Evidence for a major memory impairment. Experimental Neurology, 64, 284-294. Jackson, W. J., & Buccafusco, J. J. (1991). Clonidine enhances delayed matching-to-sample performance of young and aged monkeys. Pharmacology, Biochemistry and Behavior, 39, 7984. Larsen, J. K., & Divac, I. (1978). Selective ablations within the prefrontal cortex of the rat and performance of delayed alternation. Physiological Psychology, 6(1), 15-17. Leslie, F. M., Loughlin, S. E., Sternberg, D. B., McGaugh, J. L., Young, L. E., & Zornetzer, S. F. (1985). Noradrenergic changes and memory loss in aged mice. Brain Research, 359, 292-299. Luine, V., Bowling, D., & Hearns, M. (1990). Spatial memory deficits in aged rats: Contributions of monoaminergic systems. Brain Research, 537, 271-278. Luine, V., & Hearns, M. (1990). Spatial memory deficits in aged rats: Contributions of the cholinergic system assessed by CHAT. Brain Research, 523, 321-324. Markowska, A. L., Stone, W. S., Ingram, D. K., Reynolds, J., Gold, P. E., Conti, L. H., Ponteccorvo, M. J., Wenk, G. L., & Olton, D. S. (1989). Individual differences in aging: Behavioral and neurobiological correlates. Neurobiology of Aging, 10, 31-43. Nieminen, S. A., Airaksinen, M. M., & McDonald, E. (1990). Modulation of behaviour and memory by an a-2 adrenoceptor antagonist, atipamezole, in rats. European Journal of Neuroscience Supplement, 13, 145. Nonneman, A. J., & Kolb, B. (1979). Functional recovery after serial ablation of prefrontal cortex in the rat. Physiology and Behavior, 22, 895-901. Pelleymounter, M. A., Smith, M. Y., & Gallagher, M. (1987). Spatial learning impairments in aged rats trained with a salient configuration of stimuli. Psychobiology, 15, 248-254. Ponzio, F., Brunello, N., & Algeri, S. (1978). Cathecolamine synthesis in brain of aging rats. Journal of Neurochemistry, 30, 1617-1620. Rapp, P. R., Rosenberg, R., & Gallagher, M. (1987). An evaluation of spatial information processing in the aged rats. Behavioral Neuroscience, 101, 3-12. Savola, J.-M., Ruskoaho, H , Puurunen, J., Salonen, J. S., & K~irki, M. T. (1986). Evidence for medetomidine as a selective and potent agonist at a-2 adrenoceptors. Journal of Autonomic Pharmacology, 5, 275. Scheinin, H., MacDonald, E., & Scheinin, M. (1988). Behavioral and neurochemical effects of atipamezole, a novel a-2 adrenoceptor antagonist. European Journal of Pharmacology, 151, 35-42. Scheinin, H., Virtanen, R., MacDonald, E., Lammintausta, R., & Scheinin, M. (1989). Medetomidine---A novel a-2 adre-
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noceptor agonist: A review of its pharmacodynamic effects.
Progress in Neuro-Psychopharmacology & Biological Physiology, 13, 635-651. Simon, H., Scatton, B., & Le Meal, M. (1980). Dopaminergic A 10 neurones are involved in cognitive functions. Nature, 288, 150-151. Sirvi5, J., Riekkinen, P., Valjakka, A., Halonen, T., & Riekkinen, P. J. (1990). The effects of a-2 adrenergic drugs on cortical and hippocampal electrical activity and learning/memory in rats with cognitive dysfunction. Society for Neuroscience Abstracts, 16, 1236. SirviS, J., Riekkinen, P., Vajanto, I., Koivisto, E., & Riekkinen, P. J. (1991). The effects of guanfacine, a-2 agonist, on the performance of young and aged rats in spatial navigation task. Behavioral and Neural Biology, 56, 101-107. Starke, K. (1977). Regulation of noradrenaline release by presynaptic receptor system. Review of Physiological Biochemistry and Pharmacology, 77, 1-24. Starke, K., & Altmann, K. P. (1973). Inhibition of noradrenergic neurotransmission by clonidine: an action on prejunctional a-receptors. Neuropharmacology, 12, 339-347. Taghzouti, K., Le Meal, M., & Simon, H. (1991). Suppression of noradrenergic innervation compensates for behavioral deficits by lesion of dopaminergic terminals in the lateral septum. Brain Research, 552, 124-128. Thomas, G. J., & Brito, G. N. O. (1980). Recovery of delayed alternation in rats after lesions in medial frontal cortex and septum. Journal of Comparative and Physiological Psychology, 94, 808-818. Virtanen, R., Savola, J.-M., & Saano, V. (1989). Highly selective and specific antagonism of central and peripheral a-2 adrenoceptors by atipamezole. Archives of International Pharmacodynamics and Therapy, 297, 190-204. Virtanen, R., Savola, J.-M., Saano, V., & Nyman, L. (1988). Characterization of the selectivity, specificity and potency ofmedetomidine as an ~-2 adrenoceptor agonist. European Journal of Pharmacology, 150, 9-14. Vizi, E. S. (1980). Modulation of cortical release of acetylcheline by noradrenaline released from nerves arising from the rat locus coeruleus. Neuroscience, 5, 2139-2144. Walker, L. C., Kitt, C. A., Struble, R. G., Wagster, M. V., Price, D. L., & Cork, L. C. (1988). The neural basis of memory decline in aged monkeys. Neurobiology of Aging, 9, 657-666. Wenk, G., Hughey, D., Boundy, V., Kim, A., Walker, L., & Olton, D. (1987). Neurotransmitters and memory: Role of cholinergic, serotonergic and noradrenergic systems. Behavioral Neuroscience, 101, 325-332. Wikberg, J. E. S., Uhlen, S., & Chajlani, V. (1991). Medetomidine stereosiomers delineate two closely related subtypes of idazoxan (imidazoline) I-receptors in the guinea pig. European Journal of Pharmacology, 193, 335-340. Wikmark, R. G. E., Divac, I., & Weiss, R. (1973). Retention of spatial delayed alternation in rats with lesions in the frontal lobes. Brain, Behavior & Evolution, 8, 329-339.
Effects of Medetomidine, an oz-2Adrenoceptor Agonist, and Atipamezole, an e-2 Antagonist, on Spatial Memory Performance in Adult and Aged Rats SYNNOVE CARLSON, H E I K K I TANILA, P I A R-~MA, ERNST M E C K E , AND A N T T I PERTOVAARA 1
Department of Physiology, University of Helsinki, Siltavuorenpenger 20 J, 00170 Helsinki, Finland
(DR) task (Arnsten & Gotdman-Rakic, 1985; Arnsten, Cai, & Goldman-Rakic, 1988) and in delayed matching-to-sample performance after clonidine administration (Jackson & Buccafusco, 1991, Arnsten & Goldman-Rakic, 1990). Also, contradictory effects on DR performance in monkeys have been reported after clonidine administration (Davis, Callahan, & Doens, 1988). Medetomidine is a highly selective and potent agonist at both pre- and postsynaptic a-2 adrenoceptors (Virtanen, Savola, Saano, & Nyman, 1988). The ~-2/~-1 receptor binding selectivity ratio of medetomidine is 1620 compared with 220 of clonidine (Virtanen et al., 1988). The pharmacological effects of medetomidine can be antagonized by atipamezole, a novel, highly selective ~-2 adrenoceptor antagonist (Virtanen, Savola, & Saano, 1989). In the current work we attempted to study the effect of the highly selective a-2 adrenergic agonist medetomidine on the working memory of adult and aged rats. Furthermore, the preliminary reports indicating that atipamezole too may have some beneficial effects on cognitive functions of aged rats (Nieminen, Airaksinen, & MacDonald, 1990; Sirvi6, Riekkinen, Valjakka, Halonen, & Riekkinen, 1990) encouraged us to study its effects on the spatial memory performance of the same rats. The memory performance was studied using a variable delay, spatial delayed alternation (DA) task performed in an open T-maze. The effects of the drugs on the task performance were evaluated first when the rats were adults and again when they were aged.
The effects of a novel, highly selective a-2 agonist, medetomidine, and its antagonist, atipamezole, were studied on the working memory of rats performing a spatial delayed alternation task. Testing was performed in two stages, at the age of 8.3 months (mean) and again when the rats were 17.6 months (mean). A low dose (3 ftg/kg) and a high dose (30 ~g/kg) of medetomidine improved the performance of the old rats in the memory task but had no effect on the young rats. The dose-response curve of medetomidine resembles that of guanfacine, another a-2 agonist. At the low dose of medetomidine (3 ftg/kg) the animals showed no signs of sedation. Since medetomidine even at a low dose has a beneficial effect on the memory performance of old rats, it could be a good candidate for the treatment of age-associated memory dysfunction, v 1992 Academic Press, Inc. There is growing evidence that the central noradrenergic function declines with aging in several species (Leslie, Loughlin, Sternberg, McGaugh, Young, & Zornetzer, 1985; Boyajian & Leslie, 1987; Estes & Simpkins, 1980; Ponzio, Brunello, & Algeri, 1978; Goldman-Rakic & Brown, 1981) and that this decline may be associated with the age-related rnemory loss (Leslie et al., 1985; Barnes 1979; Barnes, Nadel, & Honig, 1980; Gage, Dunnett, & BjSrklund, 1984; Rapp, Rosenberg, & Gallagher, 1987; Pelleymounter, Smith, & Gallagher, 1987; Ponzio et al., 1978; Bartus, Fleming, & Johnson, 1978; Walker, Kitt, Struble, Wagster, Price, & Cork, 1988). a-2 adrenergic agonists like clonidine and guanfacine have been reported to improve the perf~rmance of aged monkeys in a delayed response
METHODS
1 We t h a n k J u k k a Veilahti for assistance in statistical analysis. The study was supported by grants from the Paulo Foundation and from the Science and Research Foundation of Farmos Ltd. Turku. Correspondence and reprint requests should be addressed to S. Carlson.
Eight male Wistar rats (weight 355-490 g) were obtained from the Finnish National Laboratory Animal Centre. The animals were housed individually 113 0163-1047/92 $5.00 Copyright © 1992 by Academic Press, Inc. All rights of reproduction in any form reserved.
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in standard rack-mounted cages. They had free access to water, and food was available at all times except during delayed alternation training and testing when the rats were maintained at 85% body wt. The rats were on a 12-h light-dark cycle. Training and testing was conducted during the light cycle in a quiet room in the animal ward. Medetomidine (Domitor 1 mg/ml) and atipamezole (Antisedan 5 mg/ml) (both synthetized by the Farmos Group Ltd., Turku, Finland) and 0.9% saline (control) were administered im. Testing was conducted in an open T-maze constructed of plywood and painted green throughout. The walls of the maze were 15 cm high. The length of the stem was 76 cm and the length of the arms was 57 cm, including a food recess which formed an angle at the ends of the arms such that the food reward was not visible when the rat entered the arms. During delayed alternation testing, correct responses were rewarded by a small piece of food pellet (ca. 100 mg) placed in the food recess. The starting compartment in the stem and the arms of the T-maze could be closed by a guillotine door. For 1 week prior to spontaneous alternation testing the rats were handled daily for 5 min by the experimenter and their weights were measured. The training of the rats was performed according to the method described by Nonneman and Kolb (1979). Following the adjustment period the rats were placed on 2 days in the T-maze for 5 rain where they were allowed to walk around freely. Pretraining consisted of 10 trials per day in the T-maze until the rat ate the food reward on each trial. At this stage a reward was placed at the ends of both arms of the T-maze. The rat was adjusted to wait for 3 s in the starting compartment before the guillotine door was opened. If the rat consecutively entered the same arm two times, the arm was blocked by the guillotine door. Training then proceeded to the delayed alternation phase with a 0-s delay. After the first choice when the food had been at both ends of the arms, the reward was placed only to the end of the arm that had not been chosen on the first trial. Daily training consisted of 10 trials. If the rat entered the same arm twice in a row, the reward was left in its place until the rat chose the correct arm. When the rat succeeded in making 80% correct choices of the daily trials, a 10-s delay was introduced. During the delay period the rat was kept in a round bucket without any view to the T-maze. The moment the rat entered the bucket determined the onset of the delay. This delay was maintained until the rat performed 80% of the trials correctly. On subsequent days the delay was gradually in-
ET AL.
creased until the rat's performance dropped near chance level. Alternating delays were then introduced. The number of daily trials was increased to 20, and four different delays were chosen according to the performance of the individual rat. The shortest delay for any rat was 10 s. The longest delay was the one preceding the delay in which the performance had dropped to chance level. The other two delays were adjusted individually such that the performance of the rats was about 65% correct choices of the 20 trials of the day. The rats were tested with the drugs first when they were adults (age range 6.5-10.5 months, mean 8.3 months) and again when they were aged (age range 17-18 months, mean 17.6 months). Before the second period of testing was started the performance level of the rats in the DA task was checked and the delays were again adjusted such that the rats succeeded in performing correctly in about 65% of the daily trials. The effects on the DA performance of three doses of medetomidine (3, 10, and 30 t~g/kg) and four doses of atipamezole (0.3, 0.6, 1, and 3 mg/kg) were compared with the performance on saline. Each dose of the drugs and the saline control of the drug were given once at both stages of testing (young and aged). Thus both medetomidine and atipamezole had one saline control. The order of giving the drug doses and their saline control was randomized and the tester was unaware of the nature of the injection. The interval between the testing of the different doses of the drug and their saline control was about 24 h. All doses were administered im 15 min prior to the testing. The delayed alternation performance on the drug was compared with matched saline control. The number of trials correct on drug was subtracted from the number of trials correct on saline. The difference score was then multiplied by 5% as each trial constituted 5% of the total number of trials. The results were analyzed statistically using one-way analysis of variance for repeated measures (1-ANOVA-R), and when the result was statistically significant, a post hoc analysis was performed with paired t test. RESULTS During the training period one rat was replaced by another animal because of slow learning and reluctance to eat the rewards. Table 1 illustrates the number of sessions (a session consisted of 10 trials) needed by individual rats to adjust to the Tmaze and to start eating rewards during every trial (pretraining) (mean 9.3 sessions, SD = 0.5, n = 7)
EFFECTS OF MEDETOMIDINE ON MEMORY TABLE 1 Total Number of Sessions of Seven Individual Rats during Pretraining Period and Delayed Alternation (DA) Training Rat
Pretraining (n)
DA training (n)
R1 R2 R3 G1 G2 G3 G4
10 9 10 9 9 9 9
31 30 31 30 30 31 30
115 Atl pamezole
N=8
30 c 20 oc; 10
/
z E o
-10 u
-20
Age.
-30
[ ] 7-11months • 17-18months
Note. n, n u m b e r of sessions.
0.3
and the number of sessions needed before alternating delays were introduced (mean 30.4 sessions, SD = 0.5, n -- 7). These data are not available for the rat that replaced the poorly performing rat. However, the training of this animal was similar to that of the other seven rats. When tested for the first time, the variable delays for all rats were the same: 10 s, 2, 5, and 7 min. When they were tested in the second stage at older age, the delays were 10 s, 2.8 min (mean), 10.9 min (mean), and 14.3 rain (mean). With comparable delays the rats at older age tended to make more errors than at younger age (Fig. 4, 1-ANOVA-R, F(1, 26) -8.75, p < .007). At the age of 7-11 months there was no statistically significant improvement or impairment in the DA performance with either medetomidine or atipamezole (Figs. 1 and 2) at the doses used in this study. it
Medetomidine
N=8
30
2
~ lO 7 :E
:g
~= -10 j~ -20
Age.
[ ] 7-11mont hs • 17-18months
g -30 i
3
110
30
Dose IJg/kg~
F I G . 1. Effects of medetomidine on delayed a l t e r n a t i o n t a s k performance in r a t s as y o u n g adults a n d at older age. Vertical lines indicate SEM. *p < .02, **p < .002 (one-sample t test).
O6
10
3.0 Dose mg/kg
FIG. 2. Effects of atipamezole on delayed alternation t a s k performance in r a t s as y o u n g adults a n d at older age. Vertical
lines indicate SEM.
When the rats were aged, medetomidine improved significantly the performance in the memory task (1-ANOVA-R, F(7, 24) = 6.49, p < .003) (Fig. 3), whereas atipamezole had no statistically significant effect on the task performance (F(7, 32) = 1.80, p < .2). In post hoc analysis a significant improvement in the task performance was found with medetomidine at doses 3/~g/kg (tdep(7) ---- 4.82, p < .002) and 30/~g/kg (tdep(7) ---- 3 . 0 5 , p < .02) (Fig. 1). With 10 t~g/kg of medetomidine the performance of the rats in the DA task did not differ significantly from their performance on saline (tdep(7) = 1.52, p < .2). With 3 and 1 0 / ~ g / k g of medetomidine, no signs of sedation could be observed in the rats. With 30 tLg/kg of medetomidine the rats showed clear signs of sedation: their movements were slow and they walked clumsily. However, they were not asleep and were able to perform the DA task and eat the rewards. The number of errors in the task performance increased with increasing length of delay both when the rats were young (1-ANOVA-R, F(3, 45) = 7.00, p < .001) and when they were old (F(3, 45) = 9.88, p < 0.0001) (Fig. 4). To study whether the beneficial effect of medetomidine doses 3 and 30 t~g/kg was related to the length of the delay, we compared the performances of the old rats at these drug doses in the shortest and longest delays of the task. Medetomidine at 3 t~g/kg diminished the number of errors regardless of the length of the delay. Medetomidine at 30 t~g/kg had its clearest effect on the shortest delay (1-sample t test, p < .04) (Fig. 5). However, the number of errors in the shortest delay was small also on saline.
116
CARLSON ET AL.
-~ 3O 12 0 (9
10
S
0
Z
2O
E
8
0 L.
ILl
Delay:
mm,
q)
6
¢-
~ 10 o4 N ¢-
2
~
0
MED 3 I
I
I
I
C
3
10
3o
IJg/kg
DOSE FIG. 3. The performances of individual rats at age 17.6 months (mean) in the DA task at different doses of medetomidine and matched saline control.
DISCUSSION A low (3 ~g/kg) and a high dose (30 ~g/kg) of medetomidine improved the performance of the aged rats in the spatial memory task. The highest dose of medetomidine (30 /~g/kg) also produced clear signs of sedation in the rats. The dose-response
i er
2
i
!
!
1
2
3 DELAY
FIG. 4. Comparable delays in the DA task of the rats as
young (0) and as aged (&) plotted against the numberof errors. Delay 1 is the shortest delay. The vertical lines indicate SEM. The differencein the number of errors in differentdelays was statistically significantbetween the two groups.
MED 3 0
T r e a t m e n t FIG. 5. Relation of the beneficial effects of medetomidine (3 and 30 ~g/kg on the length of the delay in the spatial memory performance of old rats. Delays I and IV refer to the shortest and longest delays, respectively. (*) One-sample t test, t = 2.65, p < .04.
profile of medetomidine resembles that of guanfacine described in the study by Arnsten et al. (1988). In a low and moderate dose range, guanfacine improved the performance of monkeys in a delayed response task. At a higher dose, the performance did not differ from that of saline control. However, at a still higher dose of guanfacine, which also produced sedation of the animals, the performance improved again. The dose-response curve of medetomidine, as well as of guanfacine, is quite different from that of clonidine, which at a low dose impaired and at higher doses improved the performance of old monkeys in a DR task (Arnsten & GoldmanRakic, 1985; Arnsten et al., 1988, 1990). Guanfacine is a more selective ~-2 adrenergic agonist than clonidine (Arnsten et al., 1988), and the a-2/a-1 receptor binding selectivity of medetomidine is 1620 compared to 220 of clonidine (Virtanen et al., 1988). Contradictory results concerning the effects of a2 agonists on memory performance of old animals have also been reported. Davis et al. (1988) could not improve the performance of old monkeys in a DR task with clonidine, and SirviS, Riekkinen, Vajanto, Koivisto, and Riekkinen (1991) reported that a low dose of guanfacine did not improve the performance of old rats in a water-maze test. It has been suggested that the beneficial effects of these drugs on memory performance may be related to the testing situation and become evident especially
EFFECTS OF MEDETOMIDINEON MEMORY when the possibility of distraction is greater and when long delays are used (Davis et al., 1988; Jackson & Buccafusco, 1991). It is also possible that a2 agonists improve working memory but not reference memory (Sirvi6 et al., 1991). Medetomidine improved the task performance of the aged rats only. This finding is in concert with Lhe growing evidence that monoaminergic and cho[inergic neurotransmitter levels decline with in,creasing age (Luine, Bowling, & Hearns, 1990; Luine & Hearns, 1990). Decreased noradrenaline (NA) levels have been found in aged rats in basal forebrain nuclei and in cortical areas (Luine et al., 1990). Also, 5-HT and dopamine decrease with increasing age in these areas and in the hippocampus and striatum (dopamine) (Luine et al., 1990). Several studies have demonstrated a correlation between such age-related changes of neurotransmitter ]levels and impaired memory performance. It has been shown that impaired T-maze performance of aged rats correlates inversely with the levels of NA J[n the cortex (Markowska, Stone, Ingram, Reynolds, Gold, Conti, Pontecorvo, Wenk, & Olton, 1989). Age associated cell loss in the locus coeruleus has been shown to correlate with impairment in a stepthrough inhibitory avoidance task in mice (Leslie et al., 1985), and locus coeruleus tissue transplantation into the third ventricle of aged rats improved their performance in the inhibitory avoidance task (Collier, Gash, & Sladek, 1988). It has been well documented that lesions in the medial prefrontal cortex impair DA performance in a T-maze (Wikmark, Divac, & Weiss, 1973; Larsen & Divac, 1978; Nonneman & Kolb, 1979; Thomas & Brito, 1980; Brito, Thomas, Davis, & Gingold, 11982). The importance of the septohippocampal systern for the performance of delayed alternation tasks in rats has also been well recognized. Hippocampal lesions disrupt spontaneous alternation performance (Isseroff, 1979). Decreased cholinergic activity, induced by septal lesions (Brito & Brito, 1990), c,ombined nucleus basalis and medial septal lesion (Wenk, Hughey, Boundy, Kim, Walker, & Olton, 1987), and the administration of scopolamine directly into the prefrontal cortex or hippocampus (Dunnett, Wareham, & Torres, 1990) have also been shown to impair severely the DA performance of rats. On the other hand, destruction of the locus coeruleus or depletion of brain NA by DSP4 injections results in only moderate (60-s delays) or no (shorter delays) impairment of DA performance (Wenk et al., 1987). Several authors have pointed out the possible imp,ortance of interactions between the different neu-
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rotransmitter systems (cholinergic and monoaminergic) involved in memory processes. There is evidence indicating that cortical cholinergic release is modulated by NA, a mechanism that has been suggested to operate also at the hippocampal level (Beani, Bianchi, Giacomelli, & Tamberi, 1978; Vizi, 1980; Ammassari-Teule, Maho, & Sara, 1991). Interaction of the noradrenergic system with other neurotransmitter systems has also been shown in behavioral studies. Decker and Callagher (1987) showed that NA depletion further impaired the scopolamine-induced disruption of radial arm maze performance of rats. Suppression of septal dopaminergic neurons causes impairment in spontaneous alternation behavior in rats, but this behavior remains unaffected when the dopaminergic lesion is accompanied by a septal noradrenergic lesion (Taghzouti, Le Moal, & Simon, 1991). Moreover, Luine et al. (1990) showed that in aged rats the NA content declines in many brain regions but only the decline in nucleus basalis correlated significantly with impaired performance in radial arm maze. The beneficial effect of medetomidine in the spatial memory task performance of aged rats may be best understood on the basis of interactions between different neurotransmitter systems. It has earlier been suggested that the beneficial effect of a-2 adrenergic agonists on the memory performance of aged animals is mediated through postsynaptic a-2 adrenergic receptors in the frontal cortex (Brozoski, Brown, Rosvold, & Goldman, 1979; Arnsten & Goldman-Rakic, 1985), probably of the Ri-type (Rauwolscine insensitive) (Arnsten et al., 1988), whereas several other effects of a-2 adrenergic agonists (e.g., hypotension and sedation) may be mediated through the Rs-subtype (Rauwolscine sensitive) of these receptors. On the other hand, low doses of clonidine have been shown to act preferentially at the presynaptic level by reducing the NA release (Starke, 1977; Starke & Altmann, 1973). In line with this, Ammassari-Teule et al. (1991) showed that small doses of clonidine attenuate spatial learning defects produced by a partial fornix section in rats. The authors suggested that small doses of clonidine could affect via presynaptic a-2 adrenergic receptors and thus could increase indirectly the release of acetylcholine at the hippocampal level by decreasing the release of NA. It may also be argued that the beneficial effect of a-2 adrenergic agonist medetomidine on memory performance is not mediated solely via a-2 adrenergic receptors since medetomidine, which is an imidazole derivative (Savola, Ruskoaho, Puurunen, Salonen, & K~irki, 1986), also binds to the imidazole
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CARLSON ET AL.
receptors of t h e cerebral cortex (Wikberg, U h l e n , & Chajlani, 1991). F u r t h e r studies are n e e d e d to clarify the m e c h a n i s m s of action of a-2 a d r e n e r g i c agonists on t h e b r a i n of aged a n i m a l s . The ability of m e d e t o m i d i n e to i m p r o v e the perf o r m a n c e of old r a t s in a m e m o r y t a s k a t a low dose in which no sedation of the a n i m a l s is e v i d e n t m a k e s it a good c a n d i d a t e for t h e t r e a t m e n t of age-associated m e m o r y dysfunction. A l t h o u g h A N O V A did not show a significant t r e a t m e n t effect of atipamezole on t h e p e r f o r m a n c e of t h e r a t s in the DA task, it should be n o t e d t h a t t h e h i g h e s t dose of a t i p a m e z o l e (3 m g / k g ) gave a n i m p r o v e m e n t in t h e t a s k p e r f o r m a n c e s i m i l a r to t h a t produced by t h e h i g h e s t m e d e t o m i d i n e dose. The s a m e dose of a t i p a m e z o l e h a s b e e n r e p o r t e d to i m p r o v e t h e p e r f o r m a n c e of old r a t s in t h e passive a v o i d a n c e and a p p e t i t i v e spatial d i s c r i m i n a t i o n m a z e t a s k s (Sirvi5 et al., 1990; N i e m i n e n et al., 1990). Atipamezole, like o t h e r a-2 a n t a g o n i s t s , increases t h e r e l e a s e of n o r a d r e n a l i n e from c e n t r a l n o r a d r e n e r g i c n e u r o n s by blocking p r e s y n a p t i c rel e a s e - i n h i b i t i n g receptors (Scheinin, MacDonald, & Scheinin, 1988). I n t r a p e r i t o n e a l injection of atipamezole causes a d o s e - d e p e n d e n t increase in t h e cent r a l t u r n o v e r of n o r a d r e n a l i n e as reflected by increased levels of its m a j o r metabolites. T h e i n c r e a s e r e a c h e d t h e level of significance a t doses 3 m g / k g and h i g h e r (Scheinin et al., 1988). H i g h doses of a t i p a m e z o l e also increase t h e t u r n o v e r of o t h e r a m i n e s (DA, 5-HT) (Scheinin et al., 1988). Also, depletion of d o p a m i n e in t h e p r e f r o n t a l cortex impairs d e l a y e d a l t e r n a t i o n p e r f o r m a n c e both in monk e y s a n d r a t s (Brozoski et al., 1979; Simon, Scatton, & Le Moal, 1980). Thus, it is possible t h a t t h e improved p e r f o r m a n c e of aged r a t s in cognitive t a s k s produced by h i g h doses of a t i p a m e z o l e could be med i a t e d b y these systems. H o w e v e r , m o r e r e s e a r c h is n e e d e d u n t i l final conclusions can be m a d e a b o u t t h e effects of h i g h doses of a t i p a m e z o l e on cognitive functions of aged rats. REFERENCES Ammassari-Teule, M., Maho, C., & Sara, S. J. (1991). Clonidine reverses spatial learning deficits and reinstates O frequencies in rats with partial foruix section. Behavioural Brain Research, 45, 1-8. Arnsten, A. F. T., Cai, J. X., & Goldman-Rakic, P. S. (1988). The a-2 adrenergic agonist guanfacine improves memory in aged monkeys without sedative or hypotensive side effects: Evidence for a-2 receptor subtypes. Journal of Neuroscience, 8, 4287-4298. Arnsten, A. F. T., & Goldman-Rakic, P. S. (1985). a-2 adrenergic mechanisms in prefrontal cortex associated with cognitive
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