Differential effects of scopolamine and mecamylamine on passive avoidance behavior

Life Sciences Vol . 11, Part I, pp. 189-179, 1972 . Printed in Great Britain

Pergamon Press

DIFFERENTIAL EFFECTS OF S~CAPOLAMINE AND MECAMYLAMINE ON PASSIVE AVOIDANCE BEHAVIOR Stanley D . Glick and Stuart Greenetein The Herbert M. Singer Laboratory of Neurosciences and Addictive Diseases, Beth Israel Medical Center, New York, N .Y . and Department of Pharmacology, Mount Sinai School of Medicine New York, N .Y .

(Received 3 December 1971 ; in final form 3 January 1972)

Summary . Scopolamine and mecamylamine were found to have different relative potencies for imparing ac quisition, retention and retrieval of passive avoidance behavior . Although each drug showed state-dependent effects when administered prior to both training and retest, no evidence was obtained for cross statedependency between the two drugs . Scopolamine and mecamylamine were found to synergize when administered in combination prior to training but antagonize each other when administered in combination after training . These results suggest that central muscarinic and nicotinic cholinergic mechanisms have separable behavioral functions . The role of cholinergic mechanisms in associative processes has become an increasingly active area of investigation (3) . Cholinergic blocking agents have been found to impair learning and memory in several different behavioral contests (1,8,10) . Although electrophyeiological data have indicated the existence of both muscarinic and nicotinic cholinergic synapses in the brain (6),

only a few behavioral studies have sought to

analyze such distinctions .

Stein and Seifter (9) reported

that drinking elicited by cholinergic stimulation of the hypothalamus was selectively a muscarinic effect .

More re-

cently, Chiapetta and Jarvik (2) found that scopolamine, a muscarinic blocking agent and mecamylamine, a nicotinic blocking agent produced similar impairments of one-trial passive avoidance learning ; however, locomotor activity

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Cholinergic Mechanism ani Passive Avoidancé

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was reduced by mecamylamine but not by scopolamine . present study,

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In the

the effects of scopolamine and mecamylamine were

further compared in terms of relative potencies for impairing acquisition (pre-training drug administration), retention (posttraining drug administration) and retrieval (pre-retest drug administration) .

Tests for state-dependency (7) and cross etate-

dependency between the two drugs were also conducted . General Method

Subjects The Ss wére naive female CF 1 mice, 8 to 10 weeks old .

They

were housed 11-12 per cage and were provided with ad libitum access to food and water .

Different naive mice were used for

each experiment .

Apparatus The apparatus was similar to that described in detail by Jarvik ~ Ropp (5) .

Briefly, the trough-shaped step-through

apparatus consisted of a small and large compartment, with a small (3 .2 cm high, 2 .7 cm wide) opening connecting the two compartments .

The small compartment was transparent and was

lighted by a Tensor light . was kept dark .

The large compartment wasopaque and

Two parallel steel plates formed the floor of the

small compartment (8 cm z 3 .2 cm) .

The floor of the large com-

partment (18 cm x 3 .2 cm) consisted of two pairs of plates bent up to form the sidewalk .

Adjacent plates were 2-3 mm apart .

Both compartments were covered with Plexiglas lids .

Solid state

logic and timing circuits were connected to the three pairs of

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Cholinergic Mechanism and Passive Avoidance

plates so that latencies were recorded and shock contingencies were programmed automatically .

When a mouse stood in the

apparatus, it always bridged a pair of plates . Conditioning proçedure A mouse was placed into the small compartment of the stepthrough apparatus facing away from the large compartment . bridged a pair of plates, a timer started .

When it

When the mouse walked

into the large compartment and bridged the rear plates, the timer stopped and the mouse received a 350 microA foot shock . This conditioning trial was terminated when the mouse escaped back to the saall~lighted compartment from which it was then reaoved .

All retest trials were conducted with the same pro-

cedure anal always occurred 24 hours following the conditioning trial (a 600 sec . ceiling was employed for retest) .

The Wilcoxon

rank sum test (two tailed) for unpaired replicates (12) was used to compare the latencies of experimental and control mice in all eaperiments . Drug Administration Scopolamine hydrobromide and mecamylamine hydrochloride in doses of 10-30 mg/kg were dissolved in physiological saline and administered intraperitoneally in volumes of approximately 0 .1 ml . Control injections consisted of comparable volumes of saline alone . All drug dtreataents described as pre-training or pre-retest were conducted 30 minutes prior to testing .

Drug treatments de-

scribed as "immediate" poet-training were completed withip 30 seconds after Craining . PART A The first three ezperiments were concerned with determining the relative potencies of scopolamine and mecamylamine on passive avoidance retention when . drug administration was conducted prior

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to training, aftertraining or prior to retest . E~_erinent 1 :

Pre-training

Procedure Five groups of mice with 12 mice in each group were administered either scopolamine (10 or 30 mg/kg), mecamylamine (10 or 30 mg/kg) or physiological saline prior to training .

Results and Discuasioa Table 1 shows that both scopolamine and mecamylamine significantly impaired retention of the passive avoidance response . TABLE 1 Drug Aflminietration Prior to Training Median Latencies (Seconds) Initial

Retest

Scopolamine 10 mg/kg

25 .6

94 .2**

Scopolamine 30 mg/kg

28 .4

44 .2**

Mecamylamine 10 mg/kg

32 .8

Mecamylamine 30 mg/kg

70 .6*

Saline

20 .5

222 .8* 50 .8** 600+

*,**~eignificantly differçnt from saline at pc.05 and .O1 respectively . Only mecamylamine at a dose of 30 mg/kg significantly increased initial latencies .

The only discrepancy between these

results and those of Chiapetta b Jarvik (2) is that the 10 mg/kg dosage of scopolamine had a significantly (p~G .05) larger effect on retest than the same dosage of mecamylamine .

However, the

30 mg/kg dosages of the two drugs had similar effects (difference

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173

not significant at p7" 1) . Bzpe~iment 2 :

Pont-t raining

Procedure Five groups of mice with 11-12 mice in each group were administered either ecopolaaine (10 or 30 ng/kg), necamylaaine (10 or 30 mg/kg) ur physiological saline immediately after training .

Three additional groups of mice were administered

etopolamine (10 mg/kg), macamylanine (10 kg/mg) or physiological saline 4 hours after training . Results and Discussion Table 2 shows that both scopolamine and aecamylamine significantly impaired passive avoidance retention when administered immediately after training but not when administered 4 hours after training . TABLE 2 Drug Administration After Training Median Betest Latencies (Seconds) Training-injection intervals Immediate Scopolamine 10 mg/kg

333 .4*

Scopolamine 30 ng/kg

306 .5*

Mecamylanine 10 mg/kg

256 .5*

Mecanylamine 30 mg/kg

204 .1*

Saline

573 .7

4 hours 57g .2 592 .1 583 .2

*significantly leas than saline at pß .05 . The scopolamine teeults replicate those reported bq Glick 6 Ziaaerberg (4) .

In contrast to the pre-training results,

the 30 mg/kg dosage of mecamylamine had a significantly (pß .05) larger effect than the sane dosage of scopolamine .

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Cholinergic Mechairiam and Passive Avoidance

Experiment 3 :

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Pre-retest

Procedure Five groups of mice with 11-12 mice in each group were administered either scopolamine (10 or 30 mg/kg), mecamylamine (10 or 30 mg/kg) or physiological saline prior to retest . Results and Discussion Table 3 shows that scopolamine significantly impaired passive avoidance retention when administered prior to retest, mecamylamine, however, had no significant effect at either dose . TABLE 3 Drug Administration Prior to Retest Median Retest Latencies (Seconds) Scopolamine 10 mg/kg

259 .5*

Scopolamine 30 mg/kg

146 .4*

Mecamylamine 10 mg/kg

476 .8

Mecamylamine 30 mg/kg

600+

Saline

600+ *significantly less than saline at p< .05 .

Since mecamylamine decreases activity

(viz . Experiment 1), the

lack of a pre-retest effect may be partially due to the activity decrement . PART B The next three experiments were concerned with determining if the differential effects of scopolamine and mecamylamine found in Part A are a function of separate cholinergic processes . This objective was approached through a state-dependency model . Both scopolamine and mecamylamine have been shown to produce

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175

state dependent phenomena, i .e . the e££ect o£ administering each drug prior to training would not be apparent if the same animals were administered the same drug prior to retentiog (7) . The most important question here is whether scopolamine and mecamylamine show cross state-dependency, i .e . will administration of one drug prior to training and the other drug prior to retest produce a greater or lesser deficit than adainistering This same paradigm was applied to the poet-

either drug alone? traiaiag effects .

State-dependency of poet-trial drug effects

had not previouelq been investigated, although recent data have indicated that retrograde amnesic effects of electroconvulaive shock may be state-dependent (11) .

Finally, the eaent to which

simultaneous administration of scopolamine and mecamylamine interact was deterained for both pre-training and poet-training effects . E~eriment 4 :

Pre-training State-dependency

Procedure Five groups of mice with 12 mice in each group were administered either scopolamine (10 mg/kg), mecamylamine (10 mg/kg) or physiological saline prior to training and retest . The five groups had the following respective training-retest injections ; scopolamine-scopolamine, mecamylamine-aecamylamine, scopolamine-mecamylamine, aecamylaaine-acopolaaine and ealinesaline . Results and Discussion Table 4 shows that only the groups receiving different drugs prior to training and retest had significantly impaired passive avoidance retention .

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TABLE 4 Drug Administration Prior to Both Training and Retest Median Retest Latencies (S conds) Scopolamine-Scopolamine

428 .6

Mecamylamine-Mecamylamine

600+

Scopolamine-Mecamylamine

158 .5*

Mecamylamine-Scopolamine

135 .6*

Saline-Saline

569 .4 *significantly less than saline at p x.05 .

State dependency was therefore shown for each drug alone but This lack of cross state-dependency between

not between drugs .

scopolamine and mecamylamine suggests that two separate cholinergic mechanisms are involved in passive avoidance learning . Eaperiment 5 :

Poet -training State-Dependency

Procedure Five groups of mice with 11-12 mice in each group were administered either scopolamine (10 mg/kg), mecamylamine (10 mgkg) or physiological saline immediately after training and prior to retest .

The drug sequences of the five groups were

the same as those in the preceding experiment . Results and Discussion Table 5 shows that all drugged groups had significantly impaired passive avoidance retention .

No evidence for atate-

dependency between poet-training and retest drug states was therefore obtained .

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177

TABLE 5 Drug Administration Immediately After Training and Prior to Retest . Median Retest Latencies (seconda) Scopolamine-Scopolamine

105 .1*

Mecamylamine-Mecamylamine

238 .4*

Scopolamine-Mecamylamine

288 .9*

Mecamylamine-Scopolamine

169 .9*

Saline-Saline

594 .1

*~aignificantly less than saline at pG .05 . Ezperiment 6 : Intera ction s Procedure Pour groups of mice with 12 mice in each group were administered either a combination of scopolamine (10 mg/kg) and mecamylamine (10 mg/kg) or physiological saline either prior to training or immediately after training . Results and Discussion As shown in Table 6, only the pre-training drug combination significantly impaired passive avoidance retention . TABLE 6 Effect of Drug Combination (scopolamine 10 mg/kg + mecamylamine 10 mg/kg) Administered Either Prior to Training or Immediately After Training .

Median Retest Latencies (Seconds)

Before training : Combination Saline

21 .8* 600 +

After training : Combination Saline

440 .5

580 .4 *significantly leas than saline at pç .001 .

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Furthermore, when compared to the results of Experiments 1 and 2, scopolamine and mecamylamine synergized when administered prior to training (combinationCeither drug alone at pß .05) but antagonized each other when administered immediately after training (combination mecamylamine at pf .05 ; combination scopolamine not significant) . Considered together, all of the results of this study demonstrate that scopolamine and mecamylamine impair the acquisition and retention of passive avoidance behavior by different mechanisms .

The two drugs had different relative

potencies depending on time of administration, showed no cross state-dependency and interacted in complex ways . Although any interpretation of all these effects can only be speculative, the results do at least strongly suggest that muscarinic and nicotinic synapses in the brain have different behavioral functions .

In view of Chiapetta and Jarvik's

(2) negative

findings with methylscopolamine and hexamethonium, it appears unlikely that peripheral actions of scopolamine and mecamylamine were responsible for the present findings .

Perhaps the two

proposed central cholinergic mechanisms represent one way in which redundancy of information may be encoded . Acknowledgement This work was supported by a grant to Mount Sinai School of Medicine by Herbert M . Singer .

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