- Email: [email protected]
The discriminative stimulus properties of mepyramine
Life Sciences, Vol. 37, pp. P r i n t e d in the U.S.A.
1145-1153
THE D I S C R I M I N A T I V E
P e r g a m o n Press
STIMULUS
PROPERTIES
OF M E P Y R A M I N E 1
Jason M. W h i t e D e p a r t m e n t of P s y c h o l o g y Monash University Clayton, V i c t o r i a 3168. Australia. (Received in final form July 19, 1985) Summary Rats w e r e trained to d i s c r i m i n a t e i.p. injections of m e p y r a m i n e (10.0 mg/kg) from saline. C o r r e c t responses on one of the two levers w e r e r e i n f o r c e d w i t h access to a solution of s w e e t e n e d c o n d e n s e d milk. A level of at least 27 correct responses in the first 30 (90%) was required in c o n s e c u t i v e saline and m e p y r a m i n e training sessions before a test s e s s i o n was conducted. A m o n g s t the antihistamines, mepyramine, t r i p e l e n n a m i n e and c h l o r p h e n i r a m i n e all p r o d u c e d d o s e - r e l a t e d r e s p o n d i n g on the m e p y r a m i n e lever, reaching a m a x i m u m of over 90% m e p y r a m i n e - a p p r o p r i a t e responses. Both t r i p e l e n n a m i n e and c h l o r p h e n i r a m i n e were more potent than the training drug. D i p h e n h y d r a m i n e and p r o m e t h a z i n e p r o d u c e d high levels of m e p y r a m i n e - a p p r o p r i a t e responses, but the 90% level was not reached w i t h any dose tested. The d e x t r o r o t a t o r y isomer of chlorpheniramine, which binds with high affinity to H lreceptors, was a p p r o x i m a t e l y twice as potent as the racemic mixture. The anticholinergic, scopolamine, and the local anaesthetic, procaine, p r o d u c e d only low levels of m e p y r a m i n e a p p r o p r i a t e responses. However, 10.0 m g / k g of the antid e p r e s s a n t i m i p r a m i n e produced over 90% m e p y r a m i n e - a p p r o p r i a t e responses. These results suggest that a d i s c r i m i n a t i o n can be formed on the basis of the specific H l - r e c e p t o r a n t a g o n i s t activity of mepyramine. S e d a t i o n is a p r o m i n e n t effect of a n t i h i s t a m i n e s in man (i). It had been difficult to d e t e r m i n e w h e t h e r this effect was due to the h i s t a m i n e antagonisl actions of these drugs, or to some other p r o p e r t y such as their anticholinergic or local a n a e s t h e t i c actions. However, recent e v i d e n c e has shown that among the a n t i h i s t a m i n e s there is a good c o r r e l a t i o n b e t w e e n their p o t e n c y in p r o d u c i n g sedation and their a f f i n i t y for one type of h i s t a m i n e receptor in th~ central nervous s y s t e m (CNS) (2). On the basis of such findings it is now thought that a n t a g o n i s t activity at central H l - r e c e p t o r s is r e s p o n s i b l e for th~ sedative effects of these drugs (3).
i.
This i n v e s t i g a t i o n R e s e a r c h Fund.
was supported
by a grant from the M o n a s h Special
0024-3205/85 $3.00 + .00 Copyright (c) 1985 P e r g a m o n Press Ltd.
1146
D i s c r i m i n a t i v e E f f e c t s of M e p y r a m i n e
Vol.
37, No.
12, 1985
Unfortunately there is little research on o t h e r CNS e f f e c t s of antihistamines. T h e y h a v e b e e n r e p o r t e d to p r o d u c e b e h a v i o u r a l c h a n g e s in animals. T h e s e i n c l u d e d e c r e a s e s in a g g r e s s i o n in m i c e (4) and m u r i c i d e in rats (5), i n c r e a s e s in n o n - r e i n f o r c e d r e s p o n d i n g of rats f o l l o w i n g t e r m i n a t i o n of r e i n f o r c e m e n t (6) and i n c r e a s e s in the r e s p o n s e rates of s q u i r r e l m o n k e y s r e i n f o r c e d a c c o r d i n g to a f i x e d - i n t e r v a l s c h e d u l e (7). However, t h e s e f i n d i n g s are not a l w a y s easy to interpret. M a n y of the b e h a v i o u r a l c h a n g e s are not s p e c i f i c to the a n t i h i s t a m i n e s (psychomotor stimulants decrease aggressive b e h a v i o u r a n d i n c r e a s e f i x e d - i n t e r v a l r e s p o n d i n g , for e x a m p l e ) a n d o n l y in some c a s e s is t h e r e any e v i d e n c e that the e f f e c t s a r e m e d i a t e d t h r o u g h a n t a g o n ist a c t i v i t y at H l - r e c e p t o r s (e.g. 7). In the o t h e r s it is l i k e l y that some o t h e r a c t i o n of the a n t i h i s t a m i n e s , s u c h as t h e i r a n t i c h o l i n e r g i c activity, is r e s p o n s i b l e (e.g. 4). To d a t e t h e r e has b e e n l i t t l e i n v e s t i g a t i o n of the d i s c r i m i n a t i v e s t i m u l u s e f f e c t s of the a n t i h i s t a m i n e s . D r u g d i s c r i m i n a t i o n m a y be a p a r t i c u l a r l y a p p r o p r i a t e t e c h n i q u e as it has p r o v e n u s e f u l for i s o l a t i n g p h a r m a c o l o g i c a l l y s p e c i f i c CNS e f f e c t s of a v a r i e t y of drugs (8). T h e m a i n aim of the p r e s e n t s t u d y was to s h o w that a n i m a l s c o u l d be t r a i n e d to d i s c r i m i n a t e a r e l a t i v e l y low d o s e of a n t i h i s t a m i n e f r o m s a l i n e and to p r o v i d e e v i d e n c e that t h e s e d i s c r i m i n a t i v e e f f e c t s r e s u l t f r o m a c t i o n at h i s t a m i n e receptors. The training d r u g was m e p y r a m i n e ( p y r i l a m i n e ) . T h i s c o m p o u n d is o f t e n r e g a r d e d as the p r o t o t y p e H 1 a n t a g o n i s t and is r e l a t i v e l y d e v o i d of a n t i c h o l i n e r g i c and local a n a e s t h e t i c a c t i v i t y (9). S e v e r a l o t h e r a n t i h i s t a m i n e s w e r e t e s t e d to d e t e r m i n e w h e t h e r they p r o d u c e d m e p y r a m i n e - l i k e d i s c r i m i n a t i v e effects. To e n s u r e that anticholinergic or local a n a e s t h e t i c a c t i o n s w e r e not m e d i a t i n g the d i s c r i m i n a t i v e e f f e c t s of m e p y r a m i n e , s c o p o l a m i n e and p r o c a i n e w e r e a l s o tested. F u r t h e r c o n f i r m a t i o n of the r o l e of H 1 - r e c e p t o r s was s o u g h t by c o m p a r i n g the p o t e n c y of d - c h l o r p h e n i r a m i n e and the r a c e m i c m i x t u r e , d l - c h l o r p h e n i r a m i n e . O n l y the d e x t r o r o t a t o r y i s o m e r is a c t i v e at H 1 - r e c e p t o r s (i0). T h e final test i n v o l v e d a c o m p o u n d not c l a s s e d as an a n t i h i s t a m i n e , but w h i c h has b e e n s h o w n to be a p o t e n t a n t a g o n i s t at H 1 - r e c e p t o r s : the a n t i d e p r e s s a n t , i m i p r a m i n e (i0, ii). Methods Subjects Six m a l e W i s t a r h o o d e d rats, b r e d in the P s y c h o l o g y D e p a r t m e n t at M o n a s h U n i v e r s i t y s e r v e d as subjects. T h e a n i m a l s w e r e m a i n t a i n e d at 85% of t h e i r f r e e - f e e d i n g w e i g h t s (244-261 gms) a n d w e r e h o u s e d in i n d i v i d u a l cages w i t h free a c c e s s to water. Apparatus T w o i d e n t i c a l o p e r a n t c h a m b e r s w e r e used. In e a c h t h e r e was a small r e c e s s e d a r e a in the m i d d l e of one wall. A 1.5 cm d i a m e t e r h o l e in this a r e a a l l o w e d a d i p p e r to d e l i v e r 0.15 mls of liquid. O n e a c h s i d e of the d i p p e r was a s t a n d a r d rat lever. T h e c h a m b e r s was i l l u m i n a t e d by 3W f l u o r e s c e n t lamps and w e r e e n c l o s e d in s o u n d - and l i g h t - a t t e n u a t i n g cubicles. Programming the c o n t i n g e n c i e s and r e c o r d i n g of d a t a w e r e d o n e w i t h s o l i d s t a t e c o n t r o l equipment. Procedure E a c h a n i m a l was t r a i n e d to o b t a i n a 1:3 d i l u t i o n of s w e e t e n e d c o n d e n s e d m i l k (Nestle) w i t h tap water. D i s c r i m i n a t i o n t r a i n i n g then began. In any t r a i n i n g s e s s i o n r e s p o n s e s on o n l y one lever w e r e r e i n f o r c e d . For half the
Vol.
37, No.
12, 1985
Discriminative
Effects
of M e p y r a m i n e
1147
rats left lever responses were r e i n f o r c e d if s a l i n ~ had been a d m i n i s t e r e d and right lever responses if the t r a i n i n g drug (10.0 m g / k g mepyramine) had been administered; for the other half the right was a p p r o p r i a t e after saline and the left after mepyramine. In the initial training sessions the inapprop r i a t e lever was c o v e r e d and every response on the a p p r o p r i a t e lever was reinforced. Both levers w e r e then made a v a i l a b l e and the r e i n f o r c e r s w e r e d e l i v e r e d a c c o r d i n g to v a r i a b l e - r a t i o (VR) schedules. The schedule size was gradually increased t h r o u g h the training period up to a m a x i m u m of VR30. At this stage it was c h a n g e d so that the first r e i n f o r c e r was d e l i v e r e d according to a f i x e d - r a t i o 30 schedule and all r e m a i n i n g reinforcers a c c o r d i n g to the VR30 schedule. Apart from o c c a s i o n a l deviations, saline and m e p y r a m i n e were a d m i n i s t e r e d on a l t e r n a t e days. Each training session was of 20 min duration. An average of 41 training sessions was r e q u i r e d before testing could begin. A test session was c o n d u c t e d if at least 27 out of the first 30 responses w e r e m a d e on the a p p r o p r i a t e lever in c o n s e c u t i v e saline and m e p y r a m i n e t r a i n i n g sessions. The test session was then c o n d u c t e d on the f o l l o w i n g day. In these sessions the animal was removed after c o m p l e t i n g thirty responses, w i t h o u t r e c e i v i n g reinforcement. The number of responses made on each lever was recorded. A single session was c o n d u c t e d each day, five days per week. All drugs and saline were a d m i n i s t e r e d 15 min prior to the start of the session. The six animals were tested w i t h graded doses of mepyramine, but four subjects were used for each of the other drugs. Both the order of drugs tested and the order of doses within each drug series was v a r i e d b e t w e e n animals. The data are e x p r e s s e d as the mean p e r c e n t a g e of responses c o m p l e t e d on the p y r i l a m i n e - a p p r o p r i a t e lever out of a total of thirty. A drug was c o n s i d e r e d to have s u b s t i t u t e d for the training drug if it resulted in a mean of at least 90% (27) p y r i l a m i n e - a p p r o p r i a t e responses. Each animal c o m p l e t e d thirty responses in each test session unless o t h e r w i s e noted. Drugs The drugs used in this study were m e p y r a m i n e maleate, tripelennamine hydrochloride, diphenhydramine hydrochloride, p r o m e t h a z i n e hydrochloride, s c o p o l a m i n e hydrobromide, p r o c a i n e h y d r o c h l o r i d e and i m i p r a m i n e h y d r o c h l o r i d e (all from Sigma C h e m i c a l Co., St. Louis, Me USA), d - c h l o r p h e n i r a m i n e m a l e a t e and d l - c h l o r p h e n i r a m i n e m a l e a t e (both d o n a t e d by Essex Laboratories, Sydney, Australia). All drugs were d i s s o l v e d in 0.9% saline. To avoid lesions induced by the antihistamines, drug solutions and saline were a d m i n i s t e r e d i.p. in a v o l u m e of 10.0 ml/kg b. wt. All doses are expressed in terms of the free base. Results The d i s c r i m i n a t i v e effects of graded doses of m e p y r a m i n e are shown in Fig. i. L o w doses (0.03-0.1 mg/kg) of the drug p r o d u c e d p r e d o m i n a n t l y salineappropriate responding, i n t e r m e d i a t e doses (0.3 - 3.0 mg/kg) p r o d u c e d both m e p y r a m i n e - and s a l i n e - a p p r o p r i a t e responding, w h i l e the training dose (10.0 mg/kg) e n g e n d e r e d a m e a n of 94% m e p y r a m i n e - a p p r o p r i a t e responding. The data from individual subjects for this and all other compounds tested appear in Table i. They show that while some subjects r e s p o n d e d almost solely on either the salineor the m e p y r a m i n e - a p p r o p r i a t e lever, others distributed their responses b e t w e e n the two. The effects of three other a n t i h i s t a m i n e s are also shown in Fig. i. T r i p e l e n n a m i n e p r o v e d to be more p o t e n t than m e p y r a m i n e : f o l l o w i n g a d m i n i s t r a t i o n of 3.0 mg/kg of this c o m p o u n d all animals r e s p o n d e d
1148
Discriminative
Effects
of M e p y r a m i n e
completely on the m e p y r a m i n e - a p p r o p r i a t e a p p r o p r i a t e r e s p o n d i n g graded down to near
o~
37, No.
12,
1985
lever. The level of m e p y r a m i n e zero w i t h the 0.1 mg/kg dose.
100
L.~ Z
O
MEPYRAMINE
D Z
•
TRIPELENNAMINE
•
DIPHENHYDRAMINE
•
PROMETHAZINE
o
Vol.
80
(./) I.I.I
III
c,,U.J
~-
60
II
e,-
C~
,-,
40
!
I..I.I
Z l,-.Wl
<
20
oc >.I.I.I
5"
_
w
I
I
I
I
I
S
0.03
0.1
0.3
1.0
I
I
I
3.0 5.6 10.0 17.5
DOSE OF DRUG (mglkg) FIG.
I
i.
Discriminative effects of g r a d e d doses of mepyramine, tripelennamine, diphenhydramine and promethazine in rats trained to discriminate mepyramine (10.0 mg/kg) from saline. For each dose, the p r o p o r t i o n of the thirty responses of the test session c o m p l e t e d on the mepyramineappropriate lever is shown; all other responses were c o m p l e t e d on the s a l i n e - a p p r o p r i a t e lever. The points are means obtained from single observations in each of four animals, except for m e p y r a m i n e w h e r e the data came from all six animals. An a s t e r i s k (*) indicates that one animal was unable to c o m p l e t e the required thirty responses, and the point is the mean of the remaining three.
Vol.
37, No. 12,
1985
Discriminative
Effects
of M e p y r a m i n e
1149
Two of the antihistimates, diphenhydramine and promethazine, engendered high levels of m e p y r a m i n e - a p p r o p r i a t e responding, but did not satisfy the substitution criterion of 90%. With d i p h e n h y d r a m i n e the m a x i m u m level was 82% at the 17.5 mg/kg dose. Only three of the four animals completed 30 responses at this dose. A dose of 5.6 mg/kg of p r o m e t h a z i n e produced 89% mepyramine-appropriate responding, just short of the criterion. Interestingly, the higher dose, 10.0 mg/kg produced a lower level of m e p y r a m i n e - a p p r o p r i a t e responding, mainly due to one subject. Only three of the four completed the 30 responses.
TABLE Results
mepyramine
tripelennamine
diphenhyramine
promethazine
* 1-6
S 0 0 0 1 3 10
1 3 0 0 0
From Individual Subjects Showing The Percentage Mepyramine-Appropriate Responses
33 04 05 01 02 0 3 17 13 20 17 0 20 3 0 0 0 13 3 97 7 0 I0 0 100 10 13 20 i0 100 83 100 100 93 93
0.i 73 104 0.3 23 83 1.0 93 93 3.0 i00 i00 1.0 3.0 10.0 17.5
05 06 3 0 0 90 I00 I00
01 10 3 04 0 3 80 80 i00 I00 47 i00 I00
1.0 01 3.0 100 5.6 93 10.0 90
1
35 3 7 *
32 10 5 76 27 100 3 67 97 100 87 * 3
36dl-chlorphen - 0.3 0 0 40 27 93 93
iramine
01
1.0 0 3.0 100 5.6 90
Of
173
04 10 3 97
7 47 100
476 i0 100 100
d-chlorpheniramine
0.3 1.0 3.0 5.6
04 106 01 73 40 100 0 97 97 97 i00 i00 97 i00 I00 i00
scopolamine
0.03 0.i 0.3
31 3 33
imipramine
1.0 3.0 5.6 10.0
procaine
3.0 10.0 30.0 56.0
02
3 0 87 32 0 0 17
0 0 43
2
04
97 100 I00 04 30 I0 27
04
57 23 13 5
7 0 100 05 0 7 *
0 0 *
5
06
100 70 97 06 3 i00 17
Subject was unable to complete the session Denotes which of the six subjects
The effects of the antihistamine d -chlorpheniramine and the racemic mixture d l - c h l o r p h e n i r a m i n e appear in Fig. 2. Both substituted for the training drug, but d - c h l o r p h e n i r a m i n e was approximately twice as potent as the racemic mixture. A dose of 5.6 mg/kg of dl -chlorpheniramine was required to produce over 90% m e p y r a m i n e - a p p r o p r i a t e responding, but only 3.0 mg/kg of d-chlorpheniramine.
1150
Discriminative
o~
1001
Effects
of M e p y r a m i n e
Vol.
37, No.
12, 1985
0 d- CHLORPHENIRAMINE @ dr- CHLORPHENIRAMINE
Z Z
80
L~ ¢-y ,.,
60
I--
~O-
40
a_ .< !
Z ~-
.<
20
!
I
!
I
0.3
1.0
30
56
DOSE OF DRUG (mglkg) FIG.
2
D i s c r i m i n a t i v e effects of g r a d e d doses of d - c h l o r p h e n i r a m i n e and dl-chlorpheniramine in rats trained to d i s c r i m i n a t e mepyramine (10.0 mg/kg) from saline. All other details are the same as in Fig i.
Three drugs not u s u a l l y c l a s s i f i e d as a n t i h i s t a m i n e s w e r e also tested. Fig. 3. shows that graded doses of s c o p o l a m i n e and p r o c a i n e p r o d u c e d some mepyramine-appropriate responding, but the m a x i m u m levels of 33% and 29%, respectively, fell well short of the 90% criterion. In both cases, the h i g h e s t dose tested was the m a x i m u m which still e n a b l e d the m a j o r i t y of animals to c o m p l e t e the session. In contrast, i m i p r a m i n e s u b s t i t u t e d for the training drug : a i0.0 mg/kg dose of this compound p r o d u c e d a mean of 96% m e p y r a m i n e a p p r o p r i a t e responding. All animals c o m p l e t e d the required thirty responses at this dose.
Vol.
=~
37, No.
12, 1985
Discriminative
Effects
of M e p y r a m l n e
100 • SCOPOLAMINE
L~ Z
o z
•
80
l
PROCAINE
&IMIPRAMINE
0 a..
,.,
I---
1151
60
0
! W
Z
x
.< ¢Y
20
L~ ~"
0
I
0.03
I
I
I
0.1
0.3
1.0
I
I
I
3.0 5.6 10.0
I
I
30.0 56.0
DOSE OF DRUG ( m g / k g ) Fi@.
3
D i s c r i m i n a t i v e effects of graded doses of scopolamine, p r o c a i n e and imipramine in rats trained to d i s c r i m i n a t e m e p y r a m i n e (10.0 mg/kg) from saline. All other details are the same as in Fig. i.
Discussion A r e l a t i v e l y large number of sessions was required to train the animals to d i s c r i m i n a t e m e p y r a m i n e from saline. This agrees with an earlier report (12) in w h i c h rats were trained to d i s c r i m i n a t e either mepyramine, diphenh y d r a m i n e or d i m e n h y d r i n a t e from saline. The actual number of sessions was lower than the number required here, but the training doses were much highez and a d i f f e r e n t task (T-maze shock escape) was used. Using the present
1152
Discriminative
Effects
of M e p y r a m i n e
Vol.
37, No.
12,
1985
methods, it is likely that fewer sessions w o u l d be r e q u i r e d if a higher training dose of m e p y r a m i n e was used. In a p r e l i m i n a r y study, many fewer sessions w e r e required to train animals to d i s c r i m i n a t e i0.0 m g / k g of t r i p e l e n n a m i n e from saline. A l t h o u g h there may be some d i f f e r e n c e in the p r o p e r t i e s of tripelennamine and mepyramine, the s h o r t e r t r a i n i n g time was p r o b a b l y due simply to the g r e a t e r p o t e n c y of t r i p e l e n n a m i n e as a d i s c r i m i n a t i v e stimulus. Antihistamines come from a wide v a r i e t y of chemical classes. In this study four d i f f e r e n t a n t i h i s t a m i n e s from three chemical classes w e r e tested for their m e p y r a m i n e - l i k e discriminative effects. Both t r i p e l e n n a m i n e (an ethylenediamine like mepyramine) and c h l o r p h e n i r a m i n e (an alkylamine) subs t i t u t e d for the training drug. In c o m m o n w i t h m e p y r a m i n e these two have only m i l d s e d a t i v e a c t i v i t y and little or no a n t i c h o l i n e r g i c a c t i v i t y (9). A d m i n i s t r a t i o n of the larger doses of d i p h e n h y d r a m i n e (an ethanolamine) and p r o m e t h a z i n e (a p h e n o t h i a z i n e ) r e s u l t e d in high levels of m e p y r a m i n e a p p r o p r i a t e r e s p o n d i n g w h i c h fell just short of the c r i t e r i o n for substitution. These two c o m p o u n d s have m i l d a n t i c h o l i n e r g i c a c t i v i t y w h i l e d i p h e n h y d r a m i n e has a m o d e r a t e level and p r o m e t h a z i n e a somewhat higher level of s e d a t i v e activity (9). It is p o s s i b l e that the a n t i c h o l i n e r g i c a c t i v i t y of these compounds, not c h a r a c t e r i s t i c of the training drug, may have m a s k e d their mepyramine-like effects. It was also apparent, particularly with promethazine, that the animals had d i f f i c u l t y c o m p l e t i n g the 30 responses r e q u i r e d in the test s e s s i o n f o l l o w i n g a d m i n i s t r a t i o n of the higher doses. From informal o b s e r v a t i o n it was clear that their general a c t i v i t y was being i n f l u e n c e d by these drugs : m o v e m e n t was slower and less precise. In contrast, there were no o b s e r v a b l e effects of a d m i n i s t e r i n g mepyramine, tripelennamine or c h l o r p h e n i r a m i n e . W h e t h e r the b e h a v i o u r a l changes were due to a n t i c h o l i n e r gic or some other a c t i o n of d i p h e n h y d r a m i n e and p r o m e t h a z i n e can not be determ i n e d at this point. The third major p r o p e r t y of the a n t i h i s t a m i n e s , their local a n a e s t h e t i c activity, did not seem to i n f l u e n c e their d i s c r i m i n a t i v e effects. Of the five used in this study, three (mepyramine, d i p h e n h y d r a m i n e and t r i p e l e n n a m i n e ) have some local a n a e s t h e t i c effect, w i t h t r i p e l e n n a m i n e having the greatest e f f i c a c y (9). This p r o p e r t y did not c l e a r l y c o r r e l a t e w i t h the d i s c r i m i n a t i v e effects of the a n t i h i s t a m i n e s . The r e l a t i v e u n i m p o r t a n c e of local a n a e s t h e t i c a c t i v i t y was also s u p p o r t e d by the f a i l u r e of p r o c a i n e to e n g e n d e r m u c h m e p y r a m i n e - a p p r o p r i a t e r e s p o n d i n ~ This result, together w i t h the low level of m e p y r a m i n e - a p p r o p r i a t e r e s p o n d i n g p r o d u c e d by s c o p o l a m i n ~ d e m o n s t r a t e s the p h a r m a c o l o g i c a l s p e c i f i c i t y of the d i s c r i m i n a t i v e effects of mepyramine. Both drugs w e r e tested up to the m a x i m u m doses w h i c h could be used before gross b e h a v i o u r a l disturbances prevented the animals from c o m p l e t i n g the 30 responses of the test session. Clearly, if either local a n a e s t h e t i c or a n t i c h o l i n e r g i c a c t i v i t y was the basis for the d i s c r i m i n a t i v e effects of m e p y r a m i n e then at least one of these c o m p o u n d s should have s u b s t i t u t e d for the training drug. By itself, the fact that m e p y r a m i n e - l i k e discriminative effects are not p r o d u c e d by either a local a n a e s t h e t i c or an a n t i c h o l i n e r g i c is not s u f f i c i e n t to show that these effects are m e d i a t e d t h r o u g h a c t i v i t y at histamine receptors. M e p y r a m i n e may be acting on some other, unknown, neural substrate. However, the g r e a t e r p o t e n c y of the d e x t r o r o t a t o r y isomer of c h l o r p h e n i r a m i n e over the racemic m i x t u r e provides c o n v i n c i n g e v i d e n c e that H l - r e c e p t o r a n t a g o n i s t a c t i v i t y is critical. T h ~ d e x t r o r o t a t o r y isomer has far g r e a t e r p o t e n c y as an inhibitor of specific [ H ] m e p y r a m i n e binding than the l e v o r o t a t o r y isomer (i0). Thus the d e x t r o r o t a t o r y isomer should be approximately twice as potent as the racemic m i x t u r e - as was o b s e r v e d here. This confirms the s t e r e o s p e c i f i c i t y of the a n t i h i s t a m i n e s in their d i s c r i m i n a t i v e effects as well as their b i n d i n g to H I- receptors. It should be noted that such s t e r e o s p e c i f i c i t y is not o b s e r v e d with all ections of a n t i h i s t a m i n e s .
Vol.
37, No.
12, 1985
Discriminative
For example, the c h a r a c t e r i s t i c changes istration of a n t i h i s t a m i n e s are p r o d u c e d by m i x t u r e w i t h a p p r o x i m a t e l y equal p o t e n c y u n l i k e l y to be m e d i a t e d t h r o u g h activity at
Effects
of M e p y r a m i n e
1153
in EEG a c t i v i t y following adminl - c h l o r p h e n i r a m i n e and the racemic (13). Effects such as these are Hl-receptors.
A p a r t i c u l a r l y interesting finding to emerge from this study was that i m i p r a m i n e p r o d u c e d m e p y r a m i n e - l i k e d i s c r i m i n a t i v e effects. Antidepressants have a v a r i e t y of actions in the CNS : they block reuptake of n o r a d r e n a l i n e and serotonin and are antagonists at ~-adrenergic, serotonergic, m u s c a r i n i c and h i s t a m i n e r g i c receptors (9). They have been shown to be potent inhibitors of [ 3H] m e p y r a m i n e b i n d i n g (10,11). However, it has been s u g g e s t e d that their histamine antagonist activity may not m e d i a t e their antidepressant effects, but may be r e s p o n s i b l e for their sedative effects (ii). This could be c o n f i r m e d if it could be shown that amongst a range of a n t i d e p r e s s a n t s the order of p o t e n c y in p r o d u c i n g mepyramine-like discriminative effects was highly c o r r e l a t e d with their p o t e n c y in p r o d u c i n g sedation, but only w e a k l y c o r r e l a t e d w i t h their potency as antidepressants. References i.
2. 3.
4. 5. 6. 7. 8.
9. i0. ii. 12. 13.
W.W. DOUGLAS in The P h a r m a c o l o g i c a l Basis of Therapeutics, 6th ed. (A.G. Gilman, L.S. Goodman and A. Gilman, eds.) pp. 608-650, Macmillan, New York (1980). T.T. QUACH, A.M. DUCHEMIN, C. ROSE and J.C. SCHWARTZ, Euro, J. Pharmacol. 60, 391-392 (1979). J.C. SCHWARTZ, T.T. QUACH and M. GARBAG in Receptors as s u p r a m o l e c u l a r Entities. (G. Biggio, E. Costa, G.L. Gessa and P.F. Spano, eds.) pp. 359420, P a r g a m o n Press, O x f o r d (1983). A. BARNETT, J.B. MALICK and R.I. TABER, P s y c h o p h a r m a c o l o g i a . 19, 359-365 (1971). Z.P. HOROVITZ, J.J. PIALA, J.P. HIGH, J.C. BURKE and R.C. LEAF, Int. J. N e u r o p h a r m a c o l . 5, 405-411 (1966). M.C. G E R A L D and R.P. MAICKEL, Br. J. Pharmac. 44, 462-471 (1972). J.W. McKEARNEY, P s y c h o p h a r m a c o l o g y . 77, 156-158 (1982). C.R. S C H U S T E R and R.L. BALSTER in A d v a n c e s in B e h a v i o r a l P h a r m a c o l o g y Vol. i. (T. T h o m p s o n and P.B. Dews, ed.) pp. 86-138, A c a d e m i c Press, New York (1977). W.C. BOWMAN and M.J. RAND, T e x t b o o k of Pharmacology. 2nd ed. Blackwell, O x f o r d (1980). R.S.L. CHANG, V.T. TRAN and S.H. SNYDER, J. Neurochem. 32, 1653-1663 (1979). H. HALL and S. OGREN, Life Sci. 34, 597-605 (1984). D.A. OVERTON, Arch. int. Pharmacodyn. 232, 221-226 (1978). C.L. FAINGOLD and C.A. BERRY, Arch. int. Pharmacodyn. 199, 213-218 (1972).
1145-1153
THE D I S C R I M I N A T I V E
P e r g a m o n Press
STIMULUS
PROPERTIES
OF M E P Y R A M I N E 1
Jason M. W h i t e D e p a r t m e n t of P s y c h o l o g y Monash University Clayton, V i c t o r i a 3168. Australia. (Received in final form July 19, 1985) Summary Rats w e r e trained to d i s c r i m i n a t e i.p. injections of m e p y r a m i n e (10.0 mg/kg) from saline. C o r r e c t responses on one of the two levers w e r e r e i n f o r c e d w i t h access to a solution of s w e e t e n e d c o n d e n s e d milk. A level of at least 27 correct responses in the first 30 (90%) was required in c o n s e c u t i v e saline and m e p y r a m i n e training sessions before a test s e s s i o n was conducted. A m o n g s t the antihistamines, mepyramine, t r i p e l e n n a m i n e and c h l o r p h e n i r a m i n e all p r o d u c e d d o s e - r e l a t e d r e s p o n d i n g on the m e p y r a m i n e lever, reaching a m a x i m u m of over 90% m e p y r a m i n e - a p p r o p r i a t e responses. Both t r i p e l e n n a m i n e and c h l o r p h e n i r a m i n e were more potent than the training drug. D i p h e n h y d r a m i n e and p r o m e t h a z i n e p r o d u c e d high levels of m e p y r a m i n e - a p p r o p r i a t e responses, but the 90% level was not reached w i t h any dose tested. The d e x t r o r o t a t o r y isomer of chlorpheniramine, which binds with high affinity to H lreceptors, was a p p r o x i m a t e l y twice as potent as the racemic mixture. The anticholinergic, scopolamine, and the local anaesthetic, procaine, p r o d u c e d only low levels of m e p y r a m i n e a p p r o p r i a t e responses. However, 10.0 m g / k g of the antid e p r e s s a n t i m i p r a m i n e produced over 90% m e p y r a m i n e - a p p r o p r i a t e responses. These results suggest that a d i s c r i m i n a t i o n can be formed on the basis of the specific H l - r e c e p t o r a n t a g o n i s t activity of mepyramine. S e d a t i o n is a p r o m i n e n t effect of a n t i h i s t a m i n e s in man (i). It had been difficult to d e t e r m i n e w h e t h e r this effect was due to the h i s t a m i n e antagonisl actions of these drugs, or to some other p r o p e r t y such as their anticholinergic or local a n a e s t h e t i c actions. However, recent e v i d e n c e has shown that among the a n t i h i s t a m i n e s there is a good c o r r e l a t i o n b e t w e e n their p o t e n c y in p r o d u c i n g sedation and their a f f i n i t y for one type of h i s t a m i n e receptor in th~ central nervous s y s t e m (CNS) (2). On the basis of such findings it is now thought that a n t a g o n i s t activity at central H l - r e c e p t o r s is r e s p o n s i b l e for th~ sedative effects of these drugs (3).
i.
This i n v e s t i g a t i o n R e s e a r c h Fund.
was supported
by a grant from the M o n a s h Special
0024-3205/85 $3.00 + .00 Copyright (c) 1985 P e r g a m o n Press Ltd.
1146
D i s c r i m i n a t i v e E f f e c t s of M e p y r a m i n e
Vol.
37, No.
12, 1985
Unfortunately there is little research on o t h e r CNS e f f e c t s of antihistamines. T h e y h a v e b e e n r e p o r t e d to p r o d u c e b e h a v i o u r a l c h a n g e s in animals. T h e s e i n c l u d e d e c r e a s e s in a g g r e s s i o n in m i c e (4) and m u r i c i d e in rats (5), i n c r e a s e s in n o n - r e i n f o r c e d r e s p o n d i n g of rats f o l l o w i n g t e r m i n a t i o n of r e i n f o r c e m e n t (6) and i n c r e a s e s in the r e s p o n s e rates of s q u i r r e l m o n k e y s r e i n f o r c e d a c c o r d i n g to a f i x e d - i n t e r v a l s c h e d u l e (7). However, t h e s e f i n d i n g s are not a l w a y s easy to interpret. M a n y of the b e h a v i o u r a l c h a n g e s are not s p e c i f i c to the a n t i h i s t a m i n e s (psychomotor stimulants decrease aggressive b e h a v i o u r a n d i n c r e a s e f i x e d - i n t e r v a l r e s p o n d i n g , for e x a m p l e ) a n d o n l y in some c a s e s is t h e r e any e v i d e n c e that the e f f e c t s a r e m e d i a t e d t h r o u g h a n t a g o n ist a c t i v i t y at H l - r e c e p t o r s (e.g. 7). In the o t h e r s it is l i k e l y that some o t h e r a c t i o n of the a n t i h i s t a m i n e s , s u c h as t h e i r a n t i c h o l i n e r g i c activity, is r e s p o n s i b l e (e.g. 4). To d a t e t h e r e has b e e n l i t t l e i n v e s t i g a t i o n of the d i s c r i m i n a t i v e s t i m u l u s e f f e c t s of the a n t i h i s t a m i n e s . D r u g d i s c r i m i n a t i o n m a y be a p a r t i c u l a r l y a p p r o p r i a t e t e c h n i q u e as it has p r o v e n u s e f u l for i s o l a t i n g p h a r m a c o l o g i c a l l y s p e c i f i c CNS e f f e c t s of a v a r i e t y of drugs (8). T h e m a i n aim of the p r e s e n t s t u d y was to s h o w that a n i m a l s c o u l d be t r a i n e d to d i s c r i m i n a t e a r e l a t i v e l y low d o s e of a n t i h i s t a m i n e f r o m s a l i n e and to p r o v i d e e v i d e n c e that t h e s e d i s c r i m i n a t i v e e f f e c t s r e s u l t f r o m a c t i o n at h i s t a m i n e receptors. The training d r u g was m e p y r a m i n e ( p y r i l a m i n e ) . T h i s c o m p o u n d is o f t e n r e g a r d e d as the p r o t o t y p e H 1 a n t a g o n i s t and is r e l a t i v e l y d e v o i d of a n t i c h o l i n e r g i c and local a n a e s t h e t i c a c t i v i t y (9). S e v e r a l o t h e r a n t i h i s t a m i n e s w e r e t e s t e d to d e t e r m i n e w h e t h e r they p r o d u c e d m e p y r a m i n e - l i k e d i s c r i m i n a t i v e effects. To e n s u r e that anticholinergic or local a n a e s t h e t i c a c t i o n s w e r e not m e d i a t i n g the d i s c r i m i n a t i v e e f f e c t s of m e p y r a m i n e , s c o p o l a m i n e and p r o c a i n e w e r e a l s o tested. F u r t h e r c o n f i r m a t i o n of the r o l e of H 1 - r e c e p t o r s was s o u g h t by c o m p a r i n g the p o t e n c y of d - c h l o r p h e n i r a m i n e and the r a c e m i c m i x t u r e , d l - c h l o r p h e n i r a m i n e . O n l y the d e x t r o r o t a t o r y i s o m e r is a c t i v e at H 1 - r e c e p t o r s (i0). T h e final test i n v o l v e d a c o m p o u n d not c l a s s e d as an a n t i h i s t a m i n e , but w h i c h has b e e n s h o w n to be a p o t e n t a n t a g o n i s t at H 1 - r e c e p t o r s : the a n t i d e p r e s s a n t , i m i p r a m i n e (i0, ii). Methods Subjects Six m a l e W i s t a r h o o d e d rats, b r e d in the P s y c h o l o g y D e p a r t m e n t at M o n a s h U n i v e r s i t y s e r v e d as subjects. T h e a n i m a l s w e r e m a i n t a i n e d at 85% of t h e i r f r e e - f e e d i n g w e i g h t s (244-261 gms) a n d w e r e h o u s e d in i n d i v i d u a l cages w i t h free a c c e s s to water. Apparatus T w o i d e n t i c a l o p e r a n t c h a m b e r s w e r e used. In e a c h t h e r e was a small r e c e s s e d a r e a in the m i d d l e of one wall. A 1.5 cm d i a m e t e r h o l e in this a r e a a l l o w e d a d i p p e r to d e l i v e r 0.15 mls of liquid. O n e a c h s i d e of the d i p p e r was a s t a n d a r d rat lever. T h e c h a m b e r s was i l l u m i n a t e d by 3W f l u o r e s c e n t lamps and w e r e e n c l o s e d in s o u n d - and l i g h t - a t t e n u a t i n g cubicles. Programming the c o n t i n g e n c i e s and r e c o r d i n g of d a t a w e r e d o n e w i t h s o l i d s t a t e c o n t r o l equipment. Procedure E a c h a n i m a l was t r a i n e d to o b t a i n a 1:3 d i l u t i o n of s w e e t e n e d c o n d e n s e d m i l k (Nestle) w i t h tap water. D i s c r i m i n a t i o n t r a i n i n g then began. In any t r a i n i n g s e s s i o n r e s p o n s e s on o n l y one lever w e r e r e i n f o r c e d . For half the
Vol.
37, No.
12, 1985
Discriminative
Effects
of M e p y r a m i n e
1147
rats left lever responses were r e i n f o r c e d if s a l i n ~ had been a d m i n i s t e r e d and right lever responses if the t r a i n i n g drug (10.0 m g / k g mepyramine) had been administered; for the other half the right was a p p r o p r i a t e after saline and the left after mepyramine. In the initial training sessions the inapprop r i a t e lever was c o v e r e d and every response on the a p p r o p r i a t e lever was reinforced. Both levers w e r e then made a v a i l a b l e and the r e i n f o r c e r s w e r e d e l i v e r e d a c c o r d i n g to v a r i a b l e - r a t i o (VR) schedules. The schedule size was gradually increased t h r o u g h the training period up to a m a x i m u m of VR30. At this stage it was c h a n g e d so that the first r e i n f o r c e r was d e l i v e r e d according to a f i x e d - r a t i o 30 schedule and all r e m a i n i n g reinforcers a c c o r d i n g to the VR30 schedule. Apart from o c c a s i o n a l deviations, saline and m e p y r a m i n e were a d m i n i s t e r e d on a l t e r n a t e days. Each training session was of 20 min duration. An average of 41 training sessions was r e q u i r e d before testing could begin. A test session was c o n d u c t e d if at least 27 out of the first 30 responses w e r e m a d e on the a p p r o p r i a t e lever in c o n s e c u t i v e saline and m e p y r a m i n e t r a i n i n g sessions. The test session was then c o n d u c t e d on the f o l l o w i n g day. In these sessions the animal was removed after c o m p l e t i n g thirty responses, w i t h o u t r e c e i v i n g reinforcement. The number of responses made on each lever was recorded. A single session was c o n d u c t e d each day, five days per week. All drugs and saline were a d m i n i s t e r e d 15 min prior to the start of the session. The six animals were tested w i t h graded doses of mepyramine, but four subjects were used for each of the other drugs. Both the order of drugs tested and the order of doses within each drug series was v a r i e d b e t w e e n animals. The data are e x p r e s s e d as the mean p e r c e n t a g e of responses c o m p l e t e d on the p y r i l a m i n e - a p p r o p r i a t e lever out of a total of thirty. A drug was c o n s i d e r e d to have s u b s t i t u t e d for the training drug if it resulted in a mean of at least 90% (27) p y r i l a m i n e - a p p r o p r i a t e responses. Each animal c o m p l e t e d thirty responses in each test session unless o t h e r w i s e noted. Drugs The drugs used in this study were m e p y r a m i n e maleate, tripelennamine hydrochloride, diphenhydramine hydrochloride, p r o m e t h a z i n e hydrochloride, s c o p o l a m i n e hydrobromide, p r o c a i n e h y d r o c h l o r i d e and i m i p r a m i n e h y d r o c h l o r i d e (all from Sigma C h e m i c a l Co., St. Louis, Me USA), d - c h l o r p h e n i r a m i n e m a l e a t e and d l - c h l o r p h e n i r a m i n e m a l e a t e (both d o n a t e d by Essex Laboratories, Sydney, Australia). All drugs were d i s s o l v e d in 0.9% saline. To avoid lesions induced by the antihistamines, drug solutions and saline were a d m i n i s t e r e d i.p. in a v o l u m e of 10.0 ml/kg b. wt. All doses are expressed in terms of the free base. Results The d i s c r i m i n a t i v e effects of graded doses of m e p y r a m i n e are shown in Fig. i. L o w doses (0.03-0.1 mg/kg) of the drug p r o d u c e d p r e d o m i n a n t l y salineappropriate responding, i n t e r m e d i a t e doses (0.3 - 3.0 mg/kg) p r o d u c e d both m e p y r a m i n e - and s a l i n e - a p p r o p r i a t e responding, w h i l e the training dose (10.0 mg/kg) e n g e n d e r e d a m e a n of 94% m e p y r a m i n e - a p p r o p r i a t e responding. The data from individual subjects for this and all other compounds tested appear in Table i. They show that while some subjects r e s p o n d e d almost solely on either the salineor the m e p y r a m i n e - a p p r o p r i a t e lever, others distributed their responses b e t w e e n the two. The effects of three other a n t i h i s t a m i n e s are also shown in Fig. i. T r i p e l e n n a m i n e p r o v e d to be more p o t e n t than m e p y r a m i n e : f o l l o w i n g a d m i n i s t r a t i o n of 3.0 mg/kg of this c o m p o u n d all animals r e s p o n d e d
1148
Discriminative
Effects
of M e p y r a m i n e
completely on the m e p y r a m i n e - a p p r o p r i a t e a p p r o p r i a t e r e s p o n d i n g graded down to near
o~
37, No.
12,
1985
lever. The level of m e p y r a m i n e zero w i t h the 0.1 mg/kg dose.
100
L.~ Z
O
MEPYRAMINE
D Z
•
TRIPELENNAMINE
•
DIPHENHYDRAMINE
•
PROMETHAZINE
o
Vol.
80
(./) I.I.I
III
c,,U.J
~-
60
II
e,-
C~
,-,
40
!
I..I.I
Z l,-.Wl
<
20
oc >.I.I.I
5"
_
w
I
I
I
I
I
S
0.03
0.1
0.3
1.0
I
I
I
3.0 5.6 10.0 17.5
DOSE OF DRUG (mglkg) FIG.
I
i.
Discriminative effects of g r a d e d doses of mepyramine, tripelennamine, diphenhydramine and promethazine in rats trained to discriminate mepyramine (10.0 mg/kg) from saline. For each dose, the p r o p o r t i o n of the thirty responses of the test session c o m p l e t e d on the mepyramineappropriate lever is shown; all other responses were c o m p l e t e d on the s a l i n e - a p p r o p r i a t e lever. The points are means obtained from single observations in each of four animals, except for m e p y r a m i n e w h e r e the data came from all six animals. An a s t e r i s k (*) indicates that one animal was unable to c o m p l e t e the required thirty responses, and the point is the mean of the remaining three.
Vol.
37, No. 12,
1985
Discriminative
Effects
of M e p y r a m i n e
1149
Two of the antihistimates, diphenhydramine and promethazine, engendered high levels of m e p y r a m i n e - a p p r o p r i a t e responding, but did not satisfy the substitution criterion of 90%. With d i p h e n h y d r a m i n e the m a x i m u m level was 82% at the 17.5 mg/kg dose. Only three of the four animals completed 30 responses at this dose. A dose of 5.6 mg/kg of p r o m e t h a z i n e produced 89% mepyramine-appropriate responding, just short of the criterion. Interestingly, the higher dose, 10.0 mg/kg produced a lower level of m e p y r a m i n e - a p p r o p r i a t e responding, mainly due to one subject. Only three of the four completed the 30 responses.
TABLE Results
mepyramine
tripelennamine
diphenhyramine
promethazine
* 1-6
S 0 0 0 1 3 10
1 3 0 0 0
From Individual Subjects Showing The Percentage Mepyramine-Appropriate Responses
33 04 05 01 02 0 3 17 13 20 17 0 20 3 0 0 0 13 3 97 7 0 I0 0 100 10 13 20 i0 100 83 100 100 93 93
0.i 73 104 0.3 23 83 1.0 93 93 3.0 i00 i00 1.0 3.0 10.0 17.5
05 06 3 0 0 90 I00 I00
01 10 3 04 0 3 80 80 i00 I00 47 i00 I00
1.0 01 3.0 100 5.6 93 10.0 90
1
35 3 7 *
32 10 5 76 27 100 3 67 97 100 87 * 3
36dl-chlorphen - 0.3 0 0 40 27 93 93
iramine
01
1.0 0 3.0 100 5.6 90
Of
173
04 10 3 97
7 47 100
476 i0 100 100
d-chlorpheniramine
0.3 1.0 3.0 5.6
04 106 01 73 40 100 0 97 97 97 i00 i00 97 i00 I00 i00
scopolamine
0.03 0.i 0.3
31 3 33
imipramine
1.0 3.0 5.6 10.0
procaine
3.0 10.0 30.0 56.0
02
3 0 87 32 0 0 17
0 0 43
2
04
97 100 I00 04 30 I0 27
04
57 23 13 5
7 0 100 05 0 7 *
0 0 *
5
06
100 70 97 06 3 i00 17
Subject was unable to complete the session Denotes which of the six subjects
The effects of the antihistamine d -chlorpheniramine and the racemic mixture d l - c h l o r p h e n i r a m i n e appear in Fig. 2. Both substituted for the training drug, but d - c h l o r p h e n i r a m i n e was approximately twice as potent as the racemic mixture. A dose of 5.6 mg/kg of dl -chlorpheniramine was required to produce over 90% m e p y r a m i n e - a p p r o p r i a t e responding, but only 3.0 mg/kg of d-chlorpheniramine.
1150
Discriminative
o~
1001
Effects
of M e p y r a m i n e
Vol.
37, No.
12, 1985
0 d- CHLORPHENIRAMINE @ dr- CHLORPHENIRAMINE
Z Z
80
L~ ¢-y ,.,
60
I--
~O-
40
a_ .< !
Z ~-
.<
20
!
I
!
I
0.3
1.0
30
56
DOSE OF DRUG (mglkg) FIG.
2
D i s c r i m i n a t i v e effects of g r a d e d doses of d - c h l o r p h e n i r a m i n e and dl-chlorpheniramine in rats trained to d i s c r i m i n a t e mepyramine (10.0 mg/kg) from saline. All other details are the same as in Fig i.
Three drugs not u s u a l l y c l a s s i f i e d as a n t i h i s t a m i n e s w e r e also tested. Fig. 3. shows that graded doses of s c o p o l a m i n e and p r o c a i n e p r o d u c e d some mepyramine-appropriate responding, but the m a x i m u m levels of 33% and 29%, respectively, fell well short of the 90% criterion. In both cases, the h i g h e s t dose tested was the m a x i m u m which still e n a b l e d the m a j o r i t y of animals to c o m p l e t e the session. In contrast, i m i p r a m i n e s u b s t i t u t e d for the training drug : a i0.0 mg/kg dose of this compound p r o d u c e d a mean of 96% m e p y r a m i n e a p p r o p r i a t e responding. All animals c o m p l e t e d the required thirty responses at this dose.
Vol.
=~
37, No.
12, 1985
Discriminative
Effects
of M e p y r a m l n e
100 • SCOPOLAMINE
L~ Z
o z
•
80
l
PROCAINE
&IMIPRAMINE
0 a..
,.,
I---
1151
60
0
! W
Z
x
.< ¢Y
20
L~ ~"
0
I
0.03
I
I
I
0.1
0.3
1.0
I
I
I
3.0 5.6 10.0
I
I
30.0 56.0
DOSE OF DRUG ( m g / k g ) Fi@.
3
D i s c r i m i n a t i v e effects of graded doses of scopolamine, p r o c a i n e and imipramine in rats trained to d i s c r i m i n a t e m e p y r a m i n e (10.0 mg/kg) from saline. All other details are the same as in Fig. i.
Discussion A r e l a t i v e l y large number of sessions was required to train the animals to d i s c r i m i n a t e m e p y r a m i n e from saline. This agrees with an earlier report (12) in w h i c h rats were trained to d i s c r i m i n a t e either mepyramine, diphenh y d r a m i n e or d i m e n h y d r i n a t e from saline. The actual number of sessions was lower than the number required here, but the training doses were much highez and a d i f f e r e n t task (T-maze shock escape) was used. Using the present
1152
Discriminative
Effects
of M e p y r a m i n e
Vol.
37, No.
12,
1985
methods, it is likely that fewer sessions w o u l d be r e q u i r e d if a higher training dose of m e p y r a m i n e was used. In a p r e l i m i n a r y study, many fewer sessions w e r e required to train animals to d i s c r i m i n a t e i0.0 m g / k g of t r i p e l e n n a m i n e from saline. A l t h o u g h there may be some d i f f e r e n c e in the p r o p e r t i e s of tripelennamine and mepyramine, the s h o r t e r t r a i n i n g time was p r o b a b l y due simply to the g r e a t e r p o t e n c y of t r i p e l e n n a m i n e as a d i s c r i m i n a t i v e stimulus. Antihistamines come from a wide v a r i e t y of chemical classes. In this study four d i f f e r e n t a n t i h i s t a m i n e s from three chemical classes w e r e tested for their m e p y r a m i n e - l i k e discriminative effects. Both t r i p e l e n n a m i n e (an ethylenediamine like mepyramine) and c h l o r p h e n i r a m i n e (an alkylamine) subs t i t u t e d for the training drug. In c o m m o n w i t h m e p y r a m i n e these two have only m i l d s e d a t i v e a c t i v i t y and little or no a n t i c h o l i n e r g i c a c t i v i t y (9). A d m i n i s t r a t i o n of the larger doses of d i p h e n h y d r a m i n e (an ethanolamine) and p r o m e t h a z i n e (a p h e n o t h i a z i n e ) r e s u l t e d in high levels of m e p y r a m i n e a p p r o p r i a t e r e s p o n d i n g w h i c h fell just short of the c r i t e r i o n for substitution. These two c o m p o u n d s have m i l d a n t i c h o l i n e r g i c a c t i v i t y w h i l e d i p h e n h y d r a m i n e has a m o d e r a t e level and p r o m e t h a z i n e a somewhat higher level of s e d a t i v e activity (9). It is p o s s i b l e that the a n t i c h o l i n e r g i c a c t i v i t y of these compounds, not c h a r a c t e r i s t i c of the training drug, may have m a s k e d their mepyramine-like effects. It was also apparent, particularly with promethazine, that the animals had d i f f i c u l t y c o m p l e t i n g the 30 responses r e q u i r e d in the test s e s s i o n f o l l o w i n g a d m i n i s t r a t i o n of the higher doses. From informal o b s e r v a t i o n it was clear that their general a c t i v i t y was being i n f l u e n c e d by these drugs : m o v e m e n t was slower and less precise. In contrast, there were no o b s e r v a b l e effects of a d m i n i s t e r i n g mepyramine, tripelennamine or c h l o r p h e n i r a m i n e . W h e t h e r the b e h a v i o u r a l changes were due to a n t i c h o l i n e r gic or some other a c t i o n of d i p h e n h y d r a m i n e and p r o m e t h a z i n e can not be determ i n e d at this point. The third major p r o p e r t y of the a n t i h i s t a m i n e s , their local a n a e s t h e t i c activity, did not seem to i n f l u e n c e their d i s c r i m i n a t i v e effects. Of the five used in this study, three (mepyramine, d i p h e n h y d r a m i n e and t r i p e l e n n a m i n e ) have some local a n a e s t h e t i c effect, w i t h t r i p e l e n n a m i n e having the greatest e f f i c a c y (9). This p r o p e r t y did not c l e a r l y c o r r e l a t e w i t h the d i s c r i m i n a t i v e effects of the a n t i h i s t a m i n e s . The r e l a t i v e u n i m p o r t a n c e of local a n a e s t h e t i c a c t i v i t y was also s u p p o r t e d by the f a i l u r e of p r o c a i n e to e n g e n d e r m u c h m e p y r a m i n e - a p p r o p r i a t e r e s p o n d i n ~ This result, together w i t h the low level of m e p y r a m i n e - a p p r o p r i a t e r e s p o n d i n g p r o d u c e d by s c o p o l a m i n ~ d e m o n s t r a t e s the p h a r m a c o l o g i c a l s p e c i f i c i t y of the d i s c r i m i n a t i v e effects of mepyramine. Both drugs w e r e tested up to the m a x i m u m doses w h i c h could be used before gross b e h a v i o u r a l disturbances prevented the animals from c o m p l e t i n g the 30 responses of the test session. Clearly, if either local a n a e s t h e t i c or a n t i c h o l i n e r g i c a c t i v i t y was the basis for the d i s c r i m i n a t i v e effects of m e p y r a m i n e then at least one of these c o m p o u n d s should have s u b s t i t u t e d for the training drug. By itself, the fact that m e p y r a m i n e - l i k e discriminative effects are not p r o d u c e d by either a local a n a e s t h e t i c or an a n t i c h o l i n e r g i c is not s u f f i c i e n t to show that these effects are m e d i a t e d t h r o u g h a c t i v i t y at histamine receptors. M e p y r a m i n e may be acting on some other, unknown, neural substrate. However, the g r e a t e r p o t e n c y of the d e x t r o r o t a t o r y isomer of c h l o r p h e n i r a m i n e over the racemic m i x t u r e provides c o n v i n c i n g e v i d e n c e that H l - r e c e p t o r a n t a g o n i s t a c t i v i t y is critical. T h ~ d e x t r o r o t a t o r y isomer has far g r e a t e r p o t e n c y as an inhibitor of specific [ H ] m e p y r a m i n e binding than the l e v o r o t a t o r y isomer (i0). Thus the d e x t r o r o t a t o r y isomer should be approximately twice as potent as the racemic m i x t u r e - as was o b s e r v e d here. This confirms the s t e r e o s p e c i f i c i t y of the a n t i h i s t a m i n e s in their d i s c r i m i n a t i v e effects as well as their b i n d i n g to H I- receptors. It should be noted that such s t e r e o s p e c i f i c i t y is not o b s e r v e d with all ections of a n t i h i s t a m i n e s .
Vol.
37, No.
12, 1985
Discriminative
For example, the c h a r a c t e r i s t i c changes istration of a n t i h i s t a m i n e s are p r o d u c e d by m i x t u r e w i t h a p p r o x i m a t e l y equal p o t e n c y u n l i k e l y to be m e d i a t e d t h r o u g h activity at
Effects
of M e p y r a m i n e
1153
in EEG a c t i v i t y following adminl - c h l o r p h e n i r a m i n e and the racemic (13). Effects such as these are Hl-receptors.
A p a r t i c u l a r l y interesting finding to emerge from this study was that i m i p r a m i n e p r o d u c e d m e p y r a m i n e - l i k e d i s c r i m i n a t i v e effects. Antidepressants have a v a r i e t y of actions in the CNS : they block reuptake of n o r a d r e n a l i n e and serotonin and are antagonists at ~-adrenergic, serotonergic, m u s c a r i n i c and h i s t a m i n e r g i c receptors (9). They have been shown to be potent inhibitors of [ 3H] m e p y r a m i n e b i n d i n g (10,11). However, it has been s u g g e s t e d that their histamine antagonist activity may not m e d i a t e their antidepressant effects, but may be r e s p o n s i b l e for their sedative effects (ii). This could be c o n f i r m e d if it could be shown that amongst a range of a n t i d e p r e s s a n t s the order of p o t e n c y in p r o d u c i n g mepyramine-like discriminative effects was highly c o r r e l a t e d with their p o t e n c y in p r o d u c i n g sedation, but only w e a k l y c o r r e l a t e d w i t h their potency as antidepressants. References i.
2. 3.
4. 5. 6. 7. 8.
9. i0. ii. 12. 13.
W.W. DOUGLAS in The P h a r m a c o l o g i c a l Basis of Therapeutics, 6th ed. (A.G. Gilman, L.S. Goodman and A. Gilman, eds.) pp. 608-650, Macmillan, New York (1980). T.T. QUACH, A.M. DUCHEMIN, C. ROSE and J.C. SCHWARTZ, Euro, J. Pharmacol. 60, 391-392 (1979). J.C. SCHWARTZ, T.T. QUACH and M. GARBAG in Receptors as s u p r a m o l e c u l a r Entities. (G. Biggio, E. Costa, G.L. Gessa and P.F. Spano, eds.) pp. 359420, P a r g a m o n Press, O x f o r d (1983). A. BARNETT, J.B. MALICK and R.I. TABER, P s y c h o p h a r m a c o l o g i a . 19, 359-365 (1971). Z.P. HOROVITZ, J.J. PIALA, J.P. HIGH, J.C. BURKE and R.C. LEAF, Int. J. N e u r o p h a r m a c o l . 5, 405-411 (1966). M.C. G E R A L D and R.P. MAICKEL, Br. J. Pharmac. 44, 462-471 (1972). J.W. McKEARNEY, P s y c h o p h a r m a c o l o g y . 77, 156-158 (1982). C.R. S C H U S T E R and R.L. BALSTER in A d v a n c e s in B e h a v i o r a l P h a r m a c o l o g y Vol. i. (T. T h o m p s o n and P.B. Dews, ed.) pp. 86-138, A c a d e m i c Press, New York (1977). W.C. BOWMAN and M.J. RAND, T e x t b o o k of Pharmacology. 2nd ed. Blackwell, O x f o r d (1980). R.S.L. CHANG, V.T. TRAN and S.H. SNYDER, J. Neurochem. 32, 1653-1663 (1979). H. HALL and S. OGREN, Life Sci. 34, 597-605 (1984). D.A. OVERTON, Arch. int. Pharmacodyn. 232, 221-226 (1978). C.L. FAINGOLD and C.A. BERRY, Arch. int. Pharmacodyn. 199, 213-218 (1972).