Pharmacological Research Communications, Vol. 15, No. 4, 1983 419

POTENTIATION BY YOHIMBINE OF PENTYLENETETRAZOL-INDUCED SEIZURES IN RATS: ROLE OF ALPHA 2 ADRENERGIC RECEPTORS

M. Lazarova and R. Samanin ~

Istituto di Ricerche Farmacologiche "Mario Negri" Via Eritrea 62, 20157 MILAN, Italy

Received in final form 24 November 1982

SUMMARY

IO mg/kg yohimbine significantly reduced the latency to the first convul-

sion of rats treated with 90 mg/kg pentylenetetrazol (PTZ) and markedly

increased the number of animals showing generalized strong clonic seizures

and dying within i h. All the animals treated with 20mg/kg yohimbine and

PTZ died with i h after repeated episodes of strong generalized clonic

seizures.

Yohimbine's ability to cause tonic seizures and mortality in rats given

70 mg/kg s.c. PTZ was reduced bypretreatment with clonidine (0.5 mg/kg

i.p.). Strong clonic seizures of animals treated with yohimbine + PTZ

were less affected by clonidine treatment. It thus appears that yohimbine

potentiates PTZ-induced seizures through an action on alpha 2 adreno-

receptors, although other mechanisms, e.g. serotonergic ones, seem to

contribute to yohimbine's effect.

INTRODUCTION

It has been recently reported that yohimbine and clonidine respecti-

vely inhibit and intesify electroshock (ECS)-induced seizures in rats

(Crunelli et al., 1981). Since yohimbine inhibits and clonidine activates

inhibitory (alpha 2) adrenoreceptors located on noradrenaline (NA)-containing

neurons (Langer, 1977), these data agree with an inhibitory role of NA in

~Visiting scientist from Institute of Physiology, Bulgarian Academy of Sciences, Sofia, Bulgaria

To whom correspondence should be addressed

0031-6989/83/040419-07/$03.00/0 © 1983 The Italian Pharmacological Society

420 Pharmacologica/ Research Communications, Vol. 15, No. 4, 1983

in various types of seizures (Maynert et al., 1975; Roussinov et al.,

1975; Mason and Corcoran, 1979; Crunelli et al., 1981).

Clonidine, however, was found recently to shorten the duration

(Papanicolau et al., 1982) and inhibit the tonic component of pentylene-

tetrazol (PTZ) induced seizures (Lazarova and Samanin, 1983) suggesting

that alpha 2 adrenergic sites have different roles in ECS- and PTZ-induced

seizures. To further explore this possibility, we studied the effect of

PTZ in animals treated with doses of yohimbine previously found to inhibit

ECS-induced convulsions (Crunelli et al., 1981). The ability of clonidine

to counteract the effect of yohimbine was also assessed to ~onfirm the

involvement of alpha 2 adrenoreceptors confirm.

MATERIALS AND METHODS

Male CD-COBS rats (Charles River, Italy), body weight 175-200 g, were

used. The animals were housed at a constant room temperature (22 ~ I°C)

and relative humidity (60%) with food and water freely available. In one

experiment, the animals were injected subcutaneously with 90 mg/kg PTZ (Knoll AG,

Milan, Italy). This dose was chosen on the basis of preliminary tr~sls in

which approximately 50% of the animals presented tonic or clonic seizures

and was given to reveal signlficsnt reduction or enhancement of'PTZ's

effect. The following parameters were assessed for differences between

control and treated animals: latency to the first convulsion, percentage

of animals showing tonic seizures, weak to moderate generalized clonic

convulsions, strong generalized clonic convulsions (repeated episodes

with screaming and loss of righting reflex for more than 2 minutes),

myoclonic jerks and mortality within I h.

Different groups of rats received intraperitoneal injections of 5, I0

oi 20 mg/kg yohimbine hydrochloride (Aldrich Chemical Company Inc., Beerse,

Belgium) or distilled water and 30 minutes later were injected with PTZ

for seizure testing. In another experiment, yohimbine (iO mg/kg) was

administered 30 minutes before PTZ 70 mg/kg s.c. This dose of PTZ by

itself caused only myoclonic and moderate clonic seizures and was selected

to reveal yohimbine's potentiation of PTZ's effect more clearly. Some

animals received intraperitoneallyO.5 mg/kg clonidine hydrochloride

(Pierrel, Italy) or distilled water together with yohimbine, 30 minutes

before PTZ.

PharmacologicalResearch Communications, Vol. 15. IVo. 4, 1983 421

Data on tonic seizures, generalized clonic seizures and mortality

were statistically analysed by the~ 2 test. Data on latency to the first

convul~ion (experiments with PTZ 70 mg/kg), transformed when appropriate

for homogeneity I/(X+I), were analyzed by ANOVA split-plot design 2x2 for

unrepeated measures. The F test for significant treatment effects was

followed by Tukey's test to compare the experimental groups with their

controls. Data on the effect of yohimbine on latency to the first con-

vulsion in rats treated with 90 mg/kg PTZ were analysed by Dunnett's test.

RESULTS

As shown in Table I, yohimbine 5 mg/kg i.p. had no effect on any

parameter used to assess seizures caused by 90 mg/kg PTZ whereas i0 mg/kg

significantly reduced the latency to the first convulsions and increased

the number of animals showing strong generalized clonic seizures and dying

within i h. This effect was particularly evident with 20 mg/kg yohimbine,

100% of animals dying after repeated very strong generalized clonic

episodes. At this dose yohimbine did not cause any mortality in animals

not given PTZ (data not shown).

Table 2 shows the effect of iO mg/kg yohimbine on PTZ (70 mg/kg s.c.)

with or without concomitant treatment with clonidine. None of the animals

given PTZ alone (controls) showed strong clonic or tonic seizures or died

within I h. After yohimbine + PTZ most animals (80%) showed either tonic

or strong clonic seizures and died within i h. Yohimbine also reduced the

latency to the first convulsion caused by PTZ. iO mg/kg yohimbine caused

by itself no seizures. Pretreatment with clonidine, 0.5 mg/kg i.p., which

by itself did not change the effects of PTZ, significantly reduced tonic

seizures and mortality caused by yohimblne + PTZ treatment with no sinifi-

cant effect on strong clonic seizures. Clonidine significantly counteracted

the yohimbine-induced decrease of latency to the first convulsion caused by

PTZ. The number of clonidine + yohimbine-treated animals showing weak-

moderate clonic seizures was significantly higher than that of animals

which had received yohimbine alone.

DISCUSSION

Previous studies showed that yohimbine and clonidine respectively

inhibit and enhance ECS-induced seizures through an action on alpha 2

Table i

- Effect of yohimbine on PTZ-induced seizures in rats

ro

Treatment

]mteney to

% of rats

(mg/kg i.p.)

the first

showing tonic

convulsion

seizures

(mean+ S.E.)

% of rats

showing generalized

clonic seizures

weak-moderate

strong

% of rats

dying within

lh

Controls

5.8 +

0.6

53.3

33.3

O.O

Yohimbine (5)

5.9 +

0.8

40.0

46.7

13.3

Yohimbine (iO)

3.7 +

O.4 e

40.0

0.0 °

60.0 °

Yohimbine (20)

3.5 +

O.4 e

O.0 °

O.O °

I00.O °

At least 15 rats were used in each group

40 .O

33.3

86

.7

°

I00.O °

Yohimbine was injected intraperitoneally

30 minutes before PTZ (90 mg/kg s.c.)

• p<O.Ol compared with controls (Dunnett's test)

o P 40.05 compared wit~ controls (~2 test)

Table 2 - Effect of yohimbine alone or combined with clonidine on PTZ-induced seizures

Treatment

Latency to

% of rats

% of rats

% of rats

% of rats

(%

(mg/kg i.p.)

the first

convulsion

(mean+._S.E.)

showing tonic

seizures

showing generalized

clonic seizures

weak-moderate

strong

showing

myoclonic

jerks

dying

wi th in

lh

Controls

ii.8 + 0.6

0.0

40.0

0.0

53.3

0.0

m

o

~ Yohimbine (i0)

4.6 + 0.3

46.7

0.0 °

53.3 °

0.0 °

80.0 °

--

~

Clonidine (0.5)

10.8 ~ 0.6

0.0

40.0

0.0

53.3

0.0

~.

Clonidine (0.5)

6.3 + 0 3 ~

13.2 +

60.0 +

26.7

6.7

40.0 +

Yohlmbine (i0)

At least 15 rats were used in each group

Yohimbine and elonidine were administered 30 minutes before PTZ (70 mg/kg s.c.)

p<O.Ol compared with controls (Tukey's test)

p~0.05 interaction

o P < 0.05 compared with controls O~ 2 test)

+ p < 0.05 compared with yohimbine treated animals (~2 test)

~O

424 Pharmacological Research Communications, VoL 15, No. 4, 1983

adrenergic receptors (Crunelli et al., 1981). The potentiation by yohimbi-

ne of PTZ-induced seizures and clonidine's ability to counteract this

effect suggest that alpha 2 adrenoreceptors play different roles in PTZ-

and ECS-induced seizures. A different role of alpha 2 adrenoreceptors has

also been suggested for seizures in quaking mice and audiogenic seizures

since drugs acting on alpha 2 adrenoreceptors caused opposite effects

(Chermat et al., 1981; Horton et al., 1980). It appears therefore that,

depending on the type of seizure examined, alpha 2 adrenergic sites either

facilitate or inhibit the seizures.

Although yohimbine intensified each aspect of PTZ-induced seizures,

only tonic seizures were almost completely abolished by clonidine treatment,

strong generalized clonic seizures being much less affected. This agrees

with the fact that piperoxane, an antagonist on alpha 2 adrenoreceptors

(U'Prichard et al., 1977), was recently found to increase tonic but not

elonic seizures caused by PTZ in rats (Lazarova et el., 1982a). The fact

that clonidine only partially blocked mortality of rats treated with

yOhimbine + PTZ suggests that mechanisms other than blockade of alpha 2

adrenoreceptors contribute to the effect of yohimbine on PTZ.

Besides acting a~ an antagonist on alpha 2 adrenoreceptors, yohimbine

is known to modify serotonin mechanisms in the rat brain (Shaw and Wooley,

1953; Papeschi et al., 1971). In view of the important role of serotonin

in PTZ-induced seizures (De La Torre et al., 1970; Kilian and Frey, 197~;

Roussinov et al., 1975; Lazarova et al., 1980, 1982b) it is possible that

this amine mediates part of the effect of yohimbine on PTZ-induced con-

vulsions. Studies are in progress to verify this possib{lity.

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