Reduction of the Scopolamine-Induced Impairment of Passive-Avoidance Performance by σ Receptor Agonist in Mice

Physiology & Behavior, Vol. 61, No. 2, pp. 257–264, 1997 Copyright q 1997 Elsevier Science Inc. Printed in the USA. All rights reserved 0031-9384/97 $17.00 / .00

PII S0031-9384(96)00447-7

Reduction of the Scopolamine-Induced Impairment of Passive-Avoidance Performance by s Receptor Agonist in Mice TOSHIHIKO SENDA, KIYOSHI MATSUNO, 1 TETSUYA KOBAYASHI AND SHIRO MITA New Drug Research, Central Research Laboratories, Santen Pharmaceutical Co., Ltd., 3-9-19, Shimoshinjo, Higashiyodogawa, Osaka 533, Japan Received 12 December 1995 SENDA, T., K. MATSUNO, T. KOBAYASHI AND S. MITA. Reduction of the scopolamine-induced impairment of passiveavoidance performance by s receptor agonist in mice. PHYSIOL BEHAV 61(2) 257–264, 1997.—We examined the ameliorating effects of several s receptor agonists on scopolamine-induced memory impairment in mice. Scopolamine was administered IP 30 min before the training session. Each s receptor agonist was administered 60 min before or immediately after the training session, or 60 min before the retention test in the passive-avoidance performance experiments. ( / )-N-Allylnormetazocine (( / )-SKF10,047), a prototype s1 receptor agonist, showed an ameliorating effect on the scopolamine-induced memory impairment in these 3 administration schedules, and ( 0 )-SKF-10,047, a stereoisomer with low affinity for the s1 receptor subtype, failed to reduce this memory impairment in mice. In addition, 1,3-di(2-tolyl)guanidine (DTG) and ( / )-3-(3-hydroxyphenyl)-N-(1-propyl)piperizine (( / )-3-PPP), nonselective s receptor agonists, did not affect this memory impairment. Physostigmine, an acetylcholinesterase (AChE) inhibitor, alleviated the scopolamine-induced memory impairment in all these drug administration schedules. In addition, ( / )-SKF-10,047-induced antiamnesic effect was antagonized by the concurrent administration of haloperidol, a s receptor antagonist, or N,N-dipropyl-2-(4-methoxy-3-(2-phenylethoxy)phenyl)ethylamine monohydrochloride (NE-100), a selective s1 receptor antagonist. These findings indicate that the s1 receptor agonist has ameliorating effects on all phases of learning and memory processes. This profile of s1 receptor agonist is similar to that of an AChE inhibitor. Copyright q 1997 Elsevier Science Inc. s Receptor agonists ( / )-SKF-10,047 ( 0 )-SKF-10,047 DTG Scopolamine Memory impairment Passive-avoidance performance

s1 Receptor subtype

(13). Although ( / )-[ 3H]-SKF-10,047 bound to the s1 receptor subtype, [ 3H]-DTG recognized both s1 and s2 receptor subtypes (24). Therefore, it was suggested that the mechanism of the increment in extracellular ACh level induced by receptor agonists was mediated through the s1 receptor subtype. Moreover, it was suggested that ( / )-SKF-10,047 was predominantly involved in the cortical cholinergic system of rats, because it did not affect the cortical extracellular 3,4-dihydroxyphenylacetic acid (DOPAC) level when administered at the same dose ranges (12). Taking all these observations into consideration, s receptor agonists, particularly s1 receptor agonists, appear to be predominantly involved in the activation of central cholinergic transmission. The central cholinergic system has been reported to play an important role in learning and memory (1,4,28). Therefore, it was suggested that s1 receptor agonists may be effective in improving experimental memory impairment in animals. In fact, it was reported that the s1 receptor agonists, ( / )-SKF-10,047 and ( / )-pentazocine, ameliorated the impairment of spontaneous alternation tasks caused by ( / )-MK-801, an N-methyl-D-aspartate

CENTRAL s receptors have been reported to be involved in several neuronal transmission mechanisms in the central nervous system (6,30,34). Particularly, in the cholinergic system, ( {)N-allylnormetazocine (({)-SKF-10,047), a prototype s receptor agonist (8), has been reported to enhance stimulation-evoked acetylcholine (ACh) release in guinea pig cerebral slices (27). In addition, ( / )-SKF-10,047, a selective isomer for the s1 receptor subtype (8), also potentiated KCl-evoked ACh release in rat hippocampal slices (10). Subsequently, we reported that the s receptor agonists, ( / )-SKF-10,047, ( {)-pentazocine, 1,3di(2-tolyl)guanidine (DTG) and ( / )-3-(3-hydroxyphenyl)-N(1-propyl)piperizine (( / )-3-PPP), dose-dependently increased extracellular ACh levels in the rat frontal cortex (12,13) and hippocampus (16), and that the activating effect of ( / )-SKF10,047 against the central cholinergic system was antagonized by haloperidol, a prototype s receptor antagonist (13,16). In addition, we showed that the order of abilities of the s receptor agonists to increase the extracellular ACh level in rat frontal cortex was positively correlated with that of their affinities for ( / )[ 3H]-SKF-10,047 binding sites, but not those of [ 3H]-DTG 1

( / )-3-PPP Mice

To whom requests for reprints should be addressed.

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SENDA ET AL. TABLE 1 THE STEP-DOWN LATENCIES (SDL) IN THE TRAINING SESSION OF PASSIVE-AVOIDANCE PERFORMANCE IN THE EXPERIMENT OF THE EFFECTS OF (/)SKF-10,047 ((/)-SKF), (0)-SKF, DTG, (/)-3-PPP,AND PHYSOSTIGMINE (PHY) ON SCOPOLAMINE (SCOP)-INDUCED MEMORY IMPAIRMENT Pretraining Admin. Treatment (mg/kg)

n

SDL

Posttraining Admin.

Preretention Admin.

n

n

SDL

SDL

Vehicle (VEH) SCOP 0.75 /(/)-SKF 0.025 /(/)-SKF 0.05 /(/)-SKF 0.1 /(/)-SKF 0.25 /(/)-SKF 0.5 [Statistical value] VEH SCOP 0.75 /(0)-SKF 0.05 /(0)-SKF 0.1 /(0)-SKF 0.25 /(0)-SKF 0.5 [Statistical value] VEH SCOP 0.75 /DTG 0.5 /DTG 1.0 /DTG 2.5 /DTG 5.0 /DTG 10.0 [Statistical value] VEH SCOP 0.75 /(/)-3-PPP 0.25 /(/)-3-PPP 0.5 /(/)-3-PPP 1.0 /(/)-3-PPP 2.5 [Statistical value]

27 5 (4–8) 27 9 (6–14) 17 5 (4–8) 18 6 (4–9) 24 5 (5–9) 24 8 (4–12) 22 6 (4–10) [H(6) Å 10.57] 22 10 (5–15) 21 8 (5–13) 18 7 (4–14) 16 8 (5–19) 18 6 (5–10) 17 4 (4–10) [H(5) Å 6.33] 28 10 (5–18) 26 8 (4–12) 20 9 (5–18) 20 8 (6–14) 20 7 (4–13) 20 5 (5–9) 15 5 (4–8) [H(6) Å 9.64] 26 5 (4–12) 28 5 (4–8) 18 6 (3–9) 19 8 (4–10) 18 8 (4–11) 14 7 (5–11) [H(5) Å 3.03]

18 18 18 18 18 – –

7 7 8 6 9

(4–8) (3–13) (5–16) (4–9) (7–13) – – [H(4) Å 9.09] 24 9 (6–17) 24 9 (5–16) 19 11 (8–19) 19 10 (5–17) 14 10 (4–14) 14 6 (3–12) [H(5) Å 7.00] 27 6 (5–10) 29 5 (3–14) 23 6 (4–11) 21 10 (6–16) 20 9 (6–12) 17 9 (5–11) 18 6 (4–11) [H(6) Å 9.04] 26 6 (4–8) 29 5 (3–9) 20 7 (5–9) 19 7 (4–9) 19 8 (6–12) 19 8 (5–13) [H(5) Å 6.76]

5 (4–9) 5 (4–6) 6 (4–9) 8 (7–8) 6 (4–10) 5 (4–9) 6 (4–12) [H(6) Å 5.35] 22 7 (4–11) 21 6 (4–8) 16 8 (5–13) 18 6 (3–8) 19 6 (4–13) 15 9 (4–12) [H(5) Å 1.92] 26 5 (4–9) 25 5 (4–6) 19 4 (3–7) 19 6 (4–9) 19 6 (5–8) 15 4 (4–9) 16 7 (5–8) [H(6) Å 7.91] 26 5 (4–9) 25 5 (4–6) 19 6 (5–13) 20 7 (4–13) 19 6 (4–11) 16 7 (4–8) [H(5) Å 5.64]

VEH SCOP 0.75 /PHY 0.1 /PHY 0.25 /PHY 0.5 [Statistical value]

23 23 15 16 12

16 16 15 15 15

6 (4–10) 7 (5–10) 7 (6–13) 8 (6–14) 8 (6–10) [H(4) Å 1.81]

16 16 15 19 14

6 (4–9) 10 (8–16) 7 (4–16) 10 (7–17) 9 (4–13) [H(4) Å 6.61]

26 25 16 16 19 19 19

9 (5–11) 6 (4–10) 8 (5–16) 11 (6–14) 4 (3–11) [H(4) Å 6.67]

Values of SDL are expressed as median and interquartile ranges. p values were more than 0.05 in all experiments; n, number of mice used.

receptor antagonist, and by carbon monoxide (CO) in mice (19,20). We also reported that 1-(3,4-dimethoxyphenethyl)-4(3-phenylpropyl)piperazine dihydrochloride (SA4503), a novel s1 receptor agonist (18), ameliorated the impairment of passiveavoidance performance caused by scopolamine, an antagonist of the muscarinic cholinoceptor (17). Additionally, we showed that ( / )-SKF-10,047, at doses of 0.1 Ç0.5 mg/kg, reduced the impairment of passive-avoidance performance induced by p-chloroamphetamine (PCA), a serotonin releaser, in mice, when administered before and immediately after the training session and before the retention test, respectively (15). Because learning and memory consist of at least three different stages (i.e., acquisition, consolidation and retrieval phases) (21,26), these findings suggest that ( / )-SKF-10,047 improves the PCA-induced memory impairment at all phases of learning and memory.

As described above, s1 receptor agonists ameliorated the experimental memory impairment in rodents, and we previously reported that ( / )-SKF-10,047, at doses of 0.1 Ç2.5 mg/kg, did not affect the scopolamine-induced memory impairment in mice when administered immediately after the training session (14). However, the effects of other administration schedules of ( / )SKF-10,047 on scopolamine-induced memory impairment have not been examined. Therefore, in the present study, we minutely examined the ameliorating effects of pretraining (acquisition phase) or preretention (retrieval phase) administration of ( / )SKF-10,047 on scopolamine-induced memory impairment in mice. Consequently, ( / )-SKF-10,047, at a dose of 0.05 mg/kg, ameliorated scopolamine-induced memory impairment when administrated before the training session. Therefore, we further examined the effect of posttraining (consolidation phase) admin-

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FIG. 1. Effects of ( / )-SKF-10,047 (left panel) and ( 0 )-SKF-10,047 (right panel) administered SC 60 min before (pretraining) (A) and immediately after (posttraining) (B) session or 60 min before the retention test (C) on scopolamine (SCOP)-induced memory impairment in the passive-avoidance performance in mice. Scopolamine was administered IP 30 min prior to the training session. The retention test was performed 24 h after the training session. Values are expressed as medians and interquartile ranges. The numbers of mice in each group are indicated in parentheses. For further details, see Methods. Statistical values of Kruskal–Wallis test were as follows. ( / )-SKF-10,047: H(6) Å 53.71, p õ 0.01 (pretraining), H(4) Å 56.58, p õ 0.01 (posttraining), H(6) Å 69.40, p õ 0.01 (preretention); ( 0 )-SKF-10,047: H(5) Å 47.35, p õ 0.01 (pretraining), H(5) Å 37.91, p õ 0.01 (posttraining), H(5) Å 45.89, p õ 0.01 (preretention). *p õ 0.05 and ** p õ 0.01 compared to vehicle (VEH) /VEH Group, ## p õ 0.01 compared to SCOP/VEH Group (Dunn or Tukey-type test).

istration of ( / )-SKF-10,047 at doses of less than 0.1 mg/kg on scopolamine-induced memory impairment. We also compared the effects of ( / )-SKF-10,047 with those of ( 0 )-SKF-10,047, a ligand with low affinity for the s1 receptor subtype (24,34), and of DTG and ( / )-3-PPP, nonselective agonists of the s receptors (24,34,35). Moreover, we examined whether or not the ( / )-SKF-10,047-induced antiamnesic effect was mediated by the s1 receptor subtype.

METHOD

Animals Groups of 5 male ddY mice ( Nihon SLC, Shizuoka, Japan ) , weighing 25 – 35 g, were housed in plastic cages with free access to food and water in a controlled environment ( 23 { 17C and 55 { 10% humidity ) , with a 12-h light-dark cycle ( light on between 0700 h and 1900 h ) . The animals were used

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SENDA ET AL.

FIG. 2. Effects of DTG (left panel) and ( / )-3-PPP (right panel) administered SC 60 min before (pretraining) (A) and immediately after (posttraining) (B) or 60 min before the retention test (C) on scopolamine (SCOP)-induced memory impairment in the passive-avoidance performance in mice. For further details, see legend to FIG. 1. Statistical values of Kruskal–Wallis test were as follows. DTG: H(6) Å 63.57, p õ 0.01 (pretraining), H(6) Å 54.95, p õ 0.01 (posttraining), H(6) Å 60.83, p õ 0.01 (preretention); ( / )-3-PPP: H(5) Å 44.65, p õ 0.01 (pretraining), H(5) Å 40.99, p õ 0.01 (posttraining), H(5) Å 61.96, p õ 0.01 (preretention). *p õ 0.05 and ** p õ 0.01 compared to vehicle (VEH) /VEH Group (Dunn-type test).

following at least a 7-day period of adaptation to the laboratory conditions. Passive-Avoidance Performance Apparatus. The apparatus for testing step-down type passiveavoidance learning performances consisted of a 25 1 25 1 30 cm Plexiglas rectangular box featuring a grid floor, 2.1 mm stainless-steel rods set 7.5 mm apart, with a wooden platform 4.5 1 4.5 1 3.0 cm in one corner of the grid floor (14,15).

Training session. Each mouse was gently placed on the wooden platform. When the mouse stepped off the platform and placed all its paws on the grid floor, a 0.6-mA scrambled electric footshock was delivered for 2 s. Animals with a step-down latency within the criterion range (step-down latency: 3–30 s) were then used for the retention test. This training procedure was carried out between 1100 h and 1800 h. Retention test. Twenty-four hours after the training session, each mouse was again placed on the platform, and the step-down latency was recorded up to a maximum cutoff time

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FIG. 3. Antagonisms by s receptor antagonists, haloperidol (left panel) and NE-100 (right panel), of the ( / )-SKF-10,047-induced antiamnesic effect on scopolamine (SCOP)-induced memory impairment in the passive-avoidance performance in mice. ( / )-SKF10,047 (SC) and/or haloperidol (HAL, IP) or NE-100 (p.o.) was administered immediately after the training session. For further details, see legend to FIG. 1. Statistical values of Kruskal–Wallis test were as follows. Antagonism by haloperidol: H(6) Å 53.21, p õ 0.01; antagonism by NE-100: H H(6) Å 67.14, p õ 0.01. *p õ 0.05 and **p õ 0.01 compared to vehicle (VEH) /VEH/VEH Group; ## p õ 0.01 compared to SCOP/VEH/VEH Group, $p õ 0.05, and $$p õ 0.01 compared to SCOP/ ( / )-SKF-10,047/VEH Group (Tukey-type test).

of 300 s. The retention test was also carried out between 1100 h and 1800 h. Drug Administration Drug preparation. ( / )-SKF-10,047 (Research Biochemicals Inc. (RBI), Natick, MA), ( 0 )-SKF-10,047 (RBI), ( / )-3-PPP (RBI), physostigmine (Sigma, St. Louis, MO), haloperidol (Sumitomo Pharmaceutical Co., Ltd., Osaka, Japan) and scopolamine (Tokyo Kasei, Tokyo, Japan) were dissolved in saline. TABLE 2 THE STEP-DOWN LATENCIES (SDL) IN THE TRAINING SESSION OF PASSIVE-AVOIDANCE PERFORMANCE IN THE EXPERIMENT OF THE ANTAGONISM BY HALOPERIDOL (HAL) OR NE-100 (NE) ON THE ANTIAMNESIC EFFECT OF (/)-SKF-10,047 ((/)-SKF) Treatment (mg/kg)

SDL

/VEH /VEH /VEH /HAL 0.1 /HAL 0.25 /HAL 0.25 /HAL 0.25

15 15 15 15 15 15 15

6 5 7 7 6 6 6

(4–7) (4–16) (6–12) (5–9) (4–10) (4–13) (4–8)

/VEH /VEH /VEH /NE 0.25 /NE 0.5 /NE 0.5 /NE 0.5

15 15 15 15 15 15 15

7 8 6 8 9 8 7

(6–7) (5–11) (5–11) (5–12) (4–12) (5–14) (4–8)

Statistical Analysis The results are expressed in terms of medians and interquartile ranges. Statistical comparisons were made with the Kruskal– Wallis test, a nonparametric analysis of variance (ANOVA), followed by a Dunn-type test (the number of animals is not equal) or a Tukey-type test (the number of animals is equal). In all statistical evaluations, p õ 0.05 was used as the criterion for statistical significance.

Values of SDL were expressed as median and interquartile ranges. p values were more than 0.05 in both experiments; n, number of mice used.

Throughout the study, none of the drug treatments affected the step-down latency of passive-avoidance performance in the training session (Tables 1 and 2).

Vehicle (VEH) SCOP 0.75 SCOP 0.75 SCOP 0.75 SCOP 0.75 SCOP 0.75 VEH [Statistics] VEH SCOP 0.75 SCOP 0.75 SCOP 0.75 SCOP 0.75 SCOP 0.75 VEH [Statistics]

/VEH /VEH /(/)-SKF 0.05 /(/)-SKF 0.05 /(/)-SKF 0.05 /VEH /VEH [F(6) Å 5.67] /VEH /VEH /(/)-SKF 0.05 /(/)-SKF 0.05 /(/)-SKF 0.05 /VEH /VEH [F(6) Å 2.13]

n

DTG (RBI) was initially dissolved in 1 N HCl and neutralized with 1 N NaOH, then diluted in saline. N,N-dipropyl-2-(4-methoxy-3-(2-phenylethoxy)phenyl)ethylamine monohydrochloride (NE-100, synthesized in our laboratory) was suspended in 1% methylcellulose. All drugs were given at a dose of 0.1 ml/10 g body weight. Scopolamine-induced memory impairment. Scopolamine (0.75 mg/kg) was administered IP 30 min prior to the training session (14). Previously, we reported that s receptor agonists and physostigmine ameliorated the PCA-induced memory impairment with no change in the response to electric footshock (15). Therefore, the doses and drug administration schedules of s receptor agonists and physostigmine in the present study were according to the experimental schedules of that report. Namely, ( / )- and ( 0 )-SKF-10,047, DTG, and ( / )-3-PPP were injected subcutaneously 60 min before or immediately after the training session or 60 min before the retention test, respectively. Physostigmine was administered IP according to the same schedule as the s receptor agonists. To examine the effects of antagonists on the antiamnesic effect of ( / )-SKF-10,047, we used haloperidol, a nonspecific s receptor antagonist, or NE-100, a selective s1 receptor antagonist (2,22). Each antagonist was administered immediately after the training session IP or orally, respectively. Control animals were treated at the corresponding times with vehicle instead of drugs.

RESULTS

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SENDA ET AL. retention test, which indicated a disruption in the retention performance of the passive-avoidance response (Figs. 1 A, B, C–4 A, B, C). ( / )-SKF-10,047 reduced the scopolamine-induced memory impairment when administered 60 min before or immediately after the training session or 60 min before the retention test (Fig. 1). Significant antiamnesic effects in pre- or posttraining session or preretention test administration of ( / )-SKF-10,047 were observed at a dose of 0.05, 0.05, or 0.1 mg/kg, respectively. On the other hand, ( 0 )-SKF-10,047, at doses of 0.05Ç0.5 mg/kg, did not ameliorate this memory impairment when administered in all schedules used (Fig. 1). In addition, pre- or posttraining session or preretention test administration of DTG, at doses of 0.5Ç10 mg/kg, or ( / )-3-PPP, at doses of 0.25Ç2.5 mg/kg, did not reduce this memory impairment (Fig. 2). Effects of s Receptor Antagonists of the ( / )-SKF-10,047Induced Antiamnesic Effect on Scopolamine-Induced Memory Impairment To clarify whether or not the ameliorating effect of ( / )-SKF10,047 was via the central s receptor, the antagonistic effects of the s receptor antagonists, haloperidol and NE-100, were studied. The antiamnesic effect of ( / )-SKF-10,047 (0.05 mg/kg) on scopolamine-induced memory impairment when it was administered immediately after the training session was significantly antagonized by the concurrent administration of haloperidol or NE-100 (Fig. 3). Neither haloperidol nor NE-100 had any effect on the step-down latency, when administered immediately after the training session. Effect of Physostigmine on Scopolamine-Induced Memory Impairment Physostigmine, an AChE inhibitor, reduced the scopolamineinduced memory impairment with all drug administration schedules used. Significant antiamnesic effects of pre- or posttraining session or preretention test administration of physostigmine were observed at doses of 0.25, 0.1 or 0.25 mg/kg, respectively. (Fig. 4). DISCUSSION

FIG. 4. Effects of physostigmine (PHY) administered IP 60 min before (pretraining) (A) and immediately after (posttraining) (B) or 60 min before the retention test (C) on scopolamine (SCOP)-induced memory impairment in passive-avoidance performance in mice. For further details, see legend to FIG. 1. Statistical values of Kruskal–Wallis test were as follows: H(4) Å 45.62, p õ 0.01 (pretraining), H(4) Å 44.48, p õ 0.01 (posttraining), H(4) Å 48.37, p õ 0.01 (preretention). *p õ 0.05 and **p õ 0.01 compared to vehicle (VEH) /VEH group, ##p õ 0.01 compared to SCOP/VEH group (Dunn-type test).

Effects of ( / )-SKF-10,047, ( 0 )-SKF-10,047, DTG, and ( / )3-PPP on Scopolamine-Induced Memory Impairment Vehicle-treated mice that received an electric shock during the training session showed a prolonged step-down latency in the retention test. Scopolamine administered 30 min before the training session significantly shortened the step-down latency in the

The present study demonstrated that the pre- or posttraining session or preretention test administration of ( / )-SKF-10,047 significantly reduced scopolamine-induced memory impairment in mice, as does physostigmine. In addition, ( / )-SKF-10,047induced antiamnesic effect was mediated by s1 receptor subtype, because ( 0 )-SKF-10,047, DTG, and ( / )-3-PPP failed to reduce the same memory impairment in all administration schedules and the ameliorating effect of ( / )-SKF-10,047 was completely antagonized by s1 receptor antagonist. As described earlier, the central cholinergic system plays an important role in memory functions. Activation of cholinergic transmission by cholinomimetics such as AChE inhibitors has been reported to reduce experimental memory impairment in rodents (5,25), and in patients with Alzheimer’s disease (3,32). In agreement with these reports, the present study showed that pre- or posttraining session or preretention test administration of physostigmine, an AChE inhibitor, alleviated scopolamine-induced memory impairment in mice. Previously, we also reported that the AChE inhibitors, physostigmine and tetrahydroaminoacridine, showed antiamnesic effects on the memory impairment induced by PCA, a serotonin releaser, using these administration schedules in mice (15). These results suggest that activation of the central cholinergic system improved the memory impairment

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at the acquisition, consolidation, and retrieval stages of learning and memory processes. Similar to AChE inhibitors, the present study showed that ( / )-SKF-10,047 significantly reduced scopolamine-induced memory impairment when administered at these schedules. This finding was in agreement with our previous reports that ( / )-SKF-10,047 reduced the PCA-induced memory impairment using these administration schedules in mice (15), and that the preretention test administration of ( / )-SKF-10,047 ameliorated scopolamine-induced memory impairment in rats (16). These results suggest that ( / )-SKF-10,047 facilitates the acquisition, consolidation and retrieval stages of learning and memory processes, as do AChE inhibitors, and that these ( / )SKF-10,047 effects were independent from the mechanisms of induction of memory impairments and from species differences. Moreover, the possibility that ( / )-SKF-10,047-induced antiamnesic effect was mediated through the activation of the cholinergic system can be considered, because scopolamine, an amnesic agent used in the present study, was a muscarinic cholinoceptor antagonist, and the antiamnesic effect of ( / )SKF-10,047 was similar to that of AChE inhibitor. This speculation was supported by our previous findings that ( / )-SKF10,047 increased extracellular ACh level in rat frontal cortex and hippocampus (11–13,16) and that the ( / )-SKF-10,047-induced ameliorating effect against PCA-induced memory impairment was antagonized by scopolamine and hemicholionium-3 (14). In addition, the present results suggest that the antiamnesic effect of ( / )-SKF-10,047 involved central s receptors, particularly s1 receptor subtype, because ( / )-SKF-10,047-induced antiamnesic effect was antagonized by haloperidol and NE-100, and ( 0 )-SKF-10,047 did not ameliorate the scopolamine-induced memory impairment. Reportedly, haloperidol antagonizes the ( / )-SKF-10,047- or DTG-induced colonic motor response to eating (9) and duodenal bicarbonate output in rats (23), and the ( / )-SKF-10,047-induced increase in the latency of the N126 peak in visually evoked potential recordings in rabbits (29,33). In addition, haloperidol and NE-100 antagonized the ( / )-SKF10,047-induced head-weaving behavior in rats (22). Moreover, the binding studies showed that haloperidol bound to both s1 and s2 receptor subtypes with the same potency (2,7), that NE-100 had high affinity for the s1 receptor subtype and negligible affinity for other receptors, such as dopaminergic, serotonergic, and phencyclidine receptors (22), and that the affinity of NE-100 for s1 receptor subtype was about 55 times more potent than that for the s2 receptor subtype (2). These findings indicated that haloperidol was a s receptor antagonist and that NE-100 was a selective s1 receptor antagonist, respectively. In addition, s re-

ceptors were characterized by the stereoselectivities of benzomorphans, such as SKF-10,047. ( / )-Benzomorphans bound to the s1 receptor subtype with high affinity, although its ( 0 )-isomers bound with low affinity (24,31,34). In the present study, it was showed that ( 0 )-SKF-10,047 failed to reduce the scopolamine-induced memory impairment. Therefore, we assume that ( / )-SKF-10,047-induced antiamnesic effect was mediated by s receptor, particularly s1 receptor subtype. This consideration was supported by others’ findings that: 1. ( / )-SKF-10,047 ameliorated the memory impairment elicited by both the treatment of MK-801 and the exposure of CO, but ( 0 )-isomer did not affect it (19,20), 2. our previous findings that ( / )SKF-10,047 ameliorated the PCA- and scopolamine-induced amnesia in mice and rats, although its ( 0 )-isomer did not affect it (15,16), and 3. that both ( / )-SKF-10,047-induced reduction of memory impairment and increment of extracellular ACh level were antagonized by haloperidol (16). Moreover, the present study showed that DTG and ( / )-3PPP did not ameliorate scopolamine-induced memory impairment. The differences between ( / )-SKF-10,047 and other s receptor agonists might be considered to be due to the selectivity for the s receptor subtype. Reportedly, DTG binds both s1 and s2 receptor subtypes with high affinity (24), and ( / )-3-PPP binds to not only s1 receptor subtypes, but also other s receptor subtypes (35). In fact, we previously showed that ( / )-SKF10,047 activates the cholinergic system, not only in the cortex (12,13), but also in the hippocampus (11,16), in agreement with others’ findings (10,27). On the other hand, DTG has been reported to decrease the KCl-induced ACh release in rat hippocampal slices (10). We also showed that DTG did not increase the extracellular ACh level in rat hippocampus (11). These findings suggested that selective s1 receptor agonists would be the most appropriate agents among s receptor agonists for reducing memory impairment. This speculation was supported by our previous findings that the magnitude of the recovery from PCA-induced memory impairment elicited by ( / )-SKF-10,047 was superior to that elicited by DTG or ( / )-3-PPP (15), and that the activation of the cortical cholinergic systems induced by s receptor agonists involved the benzomorphan-type s ( s1 ) receptor subtype (13). In conclusion, the present study showed that pre- or posttraining session or preretention test administration of ( / )-SKF10,047 reduced scopolamine-induced memory impairment in mice, similar to AChE inhibitors. In addition, this study suggested that the antiamnesic effect of ( / )-SKF-10,047 was mediated by central s1 receptor subtype.

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