Institute of Experimental
Pharrnacology and Toxicology,
University of Cagliari
Italy
It is now well established on the basis of
biochemical and pharmacological evidence that
brain amine (DA) can act on at least two types
of receptors, termed D1 and D2. Available
evidence indicates that the two receptors are
distinct molecular entities, utilize different
transducing units and have a different
distribrution the brain .
In spite of these differences, the
behavioural roles of D1 and D2 receptors appear
strikingly similar as deduced from the effects
of their selective pharmacological blockade.
Thus, in normal animals the central effects of
SCH 23390, a selective D1 receptor
antagonist,are similar to those of selective D2
antagonists. These properties include the
ability to induce catalepsy, depress conditioned
avoidance responses and block the hypermotility,
stereotypy and the hypothermie effects of D1/D2
agonists like apomorphine and of specific D2
agonists like pergolide, LY 17155 and RU 24213
as well as yawning induced by apomorphine. In
addition, SCH 23390, similarly to D2
antagonists, stimulates DA release in terminal
dopaminergic areas as a result of stimulation of
firing activity.
On the other hand clear-cut differences
between D1 and D2 blockade can be shown for
those effects which involve an action on DA
autoreceptors; thus SCH 23390, in contrast with
D2 antagonists, fails to antagonize the sedative
actions and the in vitro inhibition of DA
release elicited by D2 agonists. Other
differences between the central effects of SCH
23390 and D2 antagonists refer to onset and
duration of action, which are much shorter in
the case of SCH 23390 and are best explained by
pharmacokinetic differences.
From this evidence the furictions of central
D1 receptors appear hardly distinguishable from
those of central postsynaptic D2 receptors.
Strangely enough, selective stimulation of
central D1 receptors in normal rats by the
reportedly selective D1 agonist SKF 38393
results in behavioural effects which are quite
unlike those of selective stimulation of central
D2 receptors. Thus, in normal rats, SKF 38393
induces grooming and rearing and upward sniffing
but not the high intensity stereotypy items
(confined downward sniffing, licking and
gnawing) evoked by D1/D2 agonists or by pure D2
agonists. To further complicate this issue, D1
receptor stimulation in denervated animals is
fully able to evoke the complete behavioural
syndrome of D2 receptor stimulation including
licking and gnawing. Moreover, after
dopaminergic denervation, SCH 23390 loses the
ability to block the behavioural stimulation
elicited by central specific D2 agonists and
acquires full D1 specificity. A tentative
explanation for these results has been put
forward by postulating that D1 and D2 receptors
are 'coupled' in a reciprocally facilitatory
manner in normal animals but become 'uncoupled'
after DA denervation. Such coupling is not
easily translated into molecular terms. The idea
of a reciprocally facilitatory influence between
D1 and D2 at the molecular level contrasts with
the evidence that D1 and D2 receptors exert
opposite, reciprocally inhibit effects on the
production of cAMP. On the other hand, the
interpretation of the mechanism of the results
by Morelli and Di Chiara (1984) , Arnt (1985a,b)
and Mueller (1985) is complicated by the fact
that denervation could have resulted not only in
interruption of DA transmission but also in
supersensitivity of D1 and D2 receptors.
In order to circumvent these difficulties
have reexamined the changes in sensitivity to SC
23390 blockade of D2-mediated effects after
interruption of DA transmission. We selected
conditions (6 h reserpine pretreatment) in which
absence of supersensitivity to the agonist could
documented and used an easily quantifiable
behaviour like yawning as a behavioural index
central D2 receptor activation. In the normal
rat this behaviour appears to result from D2
recept activation .[...]
Discussion
In agreement with previous studies, BHT 920,
LY 171 555 and (+)3PPP elicited yawning in rats.
While LY 171555 and (+)3PPP, as previously
reported for apomorphine, showed a bell-shaped
dose-response curve, BHT 920 showed a monophasic
dose-response with a plateau at its maximum.
These differences are clearly related to the
emergence of competing behavioural stimulation
induced by LY 171555 and by (+)3PPP which is
absent after BHT 920. Such differences might be
related to the nature of the interaction of LY
171555, (+)3PPP and BHT 920 with the receptors
responsible for their ability to induce yawning.
These receptors appear to be of the D2 type. In
fact, available evidence indicates that BHT 920,
LY 171555 and (+)3PPP are specific agonists of
D2 receptors, as indicated by their low affinity
for D1 receptors and lack of stimulatory
influence on DA-sensitive adenylate cyclase
compared to their high affinity for D2 receptors
labeled by spiperone and their inhibitory effect
on DA-sensitive adenylate cyclase.
Behavioural and biochermical evidence
suggests that, while BHT 920 is a partial
agonist, LY 171555 and (+)3PPP are full agonists
at D2 receptors. Thus BHT 920 can stimulate in
vivo postsynaptic DA receptors rendered
supersensitive by 6-OHDA denervation, while in a
normally innervated condition it elicits
behavioural inhibition by stimulation of DA
autoreceptors.
In contrast, LY 171555 and (+)3PPP elicit
biphasic inhibitory-stimulatory behavioural
effects in normal animals. Moreover, LY 171555
behaves as a full agonist of D2 receptors. Thus,
the differences in the shape of the dose
response curve for the yawning induced by LY
171555 and (+)3PPP as compared to that for BHT
920 are related to the fact that the first two
are full agonists and the third is a partial
agonist of D2 receptors.
Pretreatment with low doses of the specific
D1 receptor antagonist SCH 23390 reduced the
ability of BHT 920, LY 171555 and (+)3PPP to
induce yawning in normal rats. This effect was
dose-related but did not exceed a 50% reduction
of the incidence of yawning. SCH 23390 at the
doses which antagonized D2 agonist-induced
yawning did induce catalepsy. We exclude a
causal relation between these two effects since,
for example, BHT 920 was fully effective to
elicit yawning also when given in doses of
50-200mg/kg to reserpinized rats which are
themselves fully cataleptic. On the other hand,
apomorphine as well as LY 171555 after 16 h
reserpine pretreatment was more potent to elicit
yawning in spite of the fact that the animals
were cataleptic. Finally, kainate lesions of
file caudate, which aholish the ability of SCH
23390 to induce catalepsy, did not affected
property to block the yawning induced by B
920.
We have previously reported that SCH 23 was
capable of antagonizing yawning induced
apomorphine. We then
concluded that this effect was not due to
stimulation of DA autoreceptors. The present
data confirm this suggestion and extend it to DA
agonists specific for D2 receptors. Because
apomorphine can act on both D1 and D2 receptors
we suggested that yawning be mediated at least
in part by stimulation of receptors by
exogenously administered agonist.
Such an explanation cannot be applied to
yawning induced by BHT 920, LY 171555 (+)3PPP
since it is highly unlikely that these drugs
interact with D1 receptors at the doses
eliciting yawning. It is also unlik on the basis
of binding studies, that SCH 2313 interacts with
D2 receptors at the doses which were active
against D2 agonist-induced yawing in the present
study. We are therefore left with the
possibility that SCH 23390 antagonizes a
postsynaptic receptor-mediated effect by
blocking D1 receptors, which are under tonic
stimulation by endgenous DA.
Endogenous DA, by acting on D1 receptors
would thus exert a permissive-facilitatory role
for the expression of yawning induced by D2
receptor agonists. If this hypothesis is
correct, abolition of endogenous DA transmission
should exert effects on D2 agonist-induced
yawning similar those of D1 blockade by SCH
23390, and abolish the effectiveness of D1
blockade by SCH23390 in reducing D2
agonist-induced yawing by a similar extent and
this reduction is similar to that elicied by SCH
23390 pretreatment. Moreover, in
reserpine-pretreated rats SCH 23390 loses the
property od reducing D2 agonist-induced
yawning.
One might argue that, since SCH 23392 is a
potent antagonist of 5 HT2 receptors and
reserpine is an effective depletor of 5HT.
blockade of endogenous 5HT transmission might be
the mechanisin by which SCH 23390 interferes
with the behavioural expression by D2 receptor
activation. This possibility is excluded by the
fact that blockade of 5HT receptors by
antagonists devoid of effects on D1 receptors,
e.g. ketanserin and metergoline, fails to affect
D2 agonist-induced yawning in normal rats.
Although reserpine depletes noradrenaline
(NA) in addition to DA and 5HT, it seems
unlikely that inhibition by SCH 23390 of DA
agonist induced yawning is related to
NA-receptor blockade since SCH 23390 is a very
poor ligand of alpha-receptors both in vivo and
in vitro.
Our finding that SCH 23390 loses the ability
to block D2-mediated effects in reserpinized
rats is line with the results of Morelli and Di
Chiara. However, in contrast with these studies
reserpine treatment now resulted in reduction
rather than potentiation of responses to DA
agonists. Indeed, the 6 h reserpine pretreatment
in our experiments is quite different from the
conditions in the other studies. Morelli used a
16 h reserpine pretreatt while Arnt and Breese
and Mueller used chronic 6-OHDA lesions or
repeated times daily reserpine pretreatment.
Breese and ueller (1985) included an experiment
performed after 24 h reserpine pretreatment in
order to extende the role of supersensitivity.
However evidence obtained by us shows that even
16 h reserpine pretreatment induces
supersensitivity to agonist. Indeed, Breese and
Mueller (1985) report that LY 55 elicits less
hypermotility in animals thus pretreated than in
normal animals, probably because of the
expression of confined stereotypies (sniffng,
licking and gnawing) which was indicative of an
increased response to the agonis. As we have
shown, there is no evidence of supersensitivitv
after 6 h reserpine pretreatment reserpinized
rats actually showed a reduction of yawning.
This could not be explained by the emergence of
competing behavioural items since the animals
remained motionless in the intervals between
yawns.
Arnt (1985a,b), having purposely induced a
supersensitivity to the agonists and shown that
in these conditions SCH 23390 loses the ability
to block D2-mediated behavioural stimulation,
postulated that D1 and D2 receptors are coupled
in a normally innervated system and uncoupled as
a result of denervation supersensitivity. We
have now provided evidence that denervation
itself, without receptor supersensitivity, is
sufficient to abolish the relationship between
D1 and D2 receptors. We therefore postulate that
D1 and D2 receptors are located either on
different neurons or on different areas of the
same neurons or even in different areas but
along the neural circuit mediating
DA-agonist-induced yawning.
Whatever might be the specific neural
mechanism by which endogenous DA input on to D1
receptors exerts a permissive-facilitatory role
on the behavioural expression of D2 receptor
stimulation, this hypothesis accounts not only
for the results of the present study but also
for certain observations reported in the
literature which could not be explained
convincingly in terms of current concepts. Thus,
as pointed out in the Introduction, a D1
antagonist like SCH 23390 reduces most, if not
all, the known behavioural effects of D2
agonists (e.g. hypermotility, stereotypy,
hypothermia) attributable to an action on
postsynaptic DA receptors. Moreover, a D1
antagonist like SCH 23390 produces a cataleptic
state which appears both phenomenologically and
pharmacologiçally similar to that induced
by haloperidol or by pure D2 antagonists. This
suggests that blockade of D1 receptors impairs
D2 receptor stimulation not only by exogenous D2
agonists but also by endogenous DA.
On the other hand our hypothesis can
explainthe observation that depletion of brain
DA by short-term reserpine and a-methyltyrosine
pretreatment abolishes the stimulant effects of
bromocriptine., a D2 agonist devoid of intrinsic
activity on D1 receptors. Since bromocriptine is
devoid of DA-releasing properties in vivo, as
estimated by brain dialysis in freely moving
rats, it is unlikely that the DA dependence of
its stimulant properties is due to some
amphetamine- or cocaine-like property. On the
other hand, bromocriptine-induced stimulant
effects in DA-depleted rats can be reinstated by
low doses of D1/D2 agonists like L-DOPA or
apomorphine .
Whether these properties of bromocriptine
extend to other ergot and non-ergot D2 agonists
when hypermotility-stereotyped behaviour is
taken as a measure of central postsynatic D2
receptor stimulation will be studied next.
-Morelli
M et al Antagonism of apomorphine-induced
yawning by SCH 23390: Evidence against the
autoreceptor hypothesis Psychopharmacology 1986;
89; 259-260
