Institute of Pharmacology
and Toxicology, Medical Academy,
Magdeburg
Apomorphine-induced yawning was completely
suppressed in animals treated with 5 nmol
[D-Pro4]casomorphin (CM) (ICV), 10 nmol
[D-Phe3]CM (ICV) or 10 nmol
[D-Pip4]CM (ICV). The
apomorphine-induced yawning was also decreased,
by des-Tyr analogs, but only by about 50%.
Physostigmine (0.15 mg/kg, IP) induced yawning.
The physostigmine-induced yawning was suppressed
by 5 nmol [D-Pro4]CM and 10 nmol
[D-Phe3]CM. Both
[des-Tyr-D-Phe3]CM and
[des-Tyr-D-Pip4]CM were without effect,
whereas [des-Tyr-D-Pro4]CM increased
significantly the physostigmine-induced yawning.
The results suggest that dopaminergic
transmission can be modulated by
beta-casomorphin derivatives, thus resulting in
a decrease in yawning. In the case of the
des-tyrosine derivatives, we can assume a
dopaminergic modulation, too. An increase in
serotonergic activity might be supposed for
[des-Tyr-D-Pro4]CM.
B-CASOMORPHIN( l-5) (CM)
(Tyr-Pro-Phe-Pro-Gly), a partial sequence of
bovine $-casein, exhibits analgesic potency due
to a particular affinity to opiate receptors.
Modified derivatives have been obtained by
substituting D-amino acids, which showed a
considerable increase in receptor affinity and
an enhanced resistance to enzymatic degradation
resulting in an augmented analgesic action.
Further pharmacological studies demonstrated
not only antinociceptive properties of these
casomorphins, but also several other effects,
which suggested influences on dopaminergic
mechanisms. To elucidate whether these actions
are indirectly mediated by opiate receptors or
due to direct influences, we investigated the
influence of casomorphin derivatives with high
analgesic potency on the yawning behavior of
rats, comparing their effects also with those of
their des-tyrosine analogues, which exhibit no
affinity to opiate receptors.
Yawning behavior can be induced by dopamine
receptor agonists, cholinergic agonists, as well
as peptide hormones.
Since yawning is elicited only by small doss
of the dopamine agonists, it has been proposed
that this response may be mediated via dopamine
autoreceptors. The stimulation of dopamine
autoreceptors and the resulting attenuation of
dôpamine release disinhibits a cholinergic
link, thus inducing yawning.
This supposed dopaminergic control of
yawning offers a useful method to investigate
the influence of casomorphins on dopaminergic
mechanisms and was used in the present
study.
DISCUSSION
After the first description of APO-induced
yawning by Mogiinicka and Klimek (19), the
influence of several neuroactive drugs on this
dopaminergic mechanism was investigated.
Not only mixed dopamine D1/D2 receptor
agonists like apomorphine and peribedil
(15,19,28,3 1) and highly selective dopamine D2
receptor agonists (4,13-15,34) but also
muscarinic receptor agonists (27,29,32,34)
elicited yawning in rats.
The yawning behavior induced by dopamine
receptor agonists is blocked by both dopamine D2
receptor antagonists and muscarinic receptor
antagonists (4,9,14,19,29,30,33,34). However,
yawning is not inhibited by the peripheral
anticholinergic agent methylscopolamine or by
mecamylamine, a nicotinergic receptor blocking
agent. In contrast, the yawning induced by D2
receptor agonists is not blocked by dopamine D1
receptor antagonists (14,32,34).
Therefore, it is hypothesized that yawning
involves dopamine D2 receptor stimulation and
central muscarinic receptor activation
(4,15,32,34). On the other hand, yawning was
also prevented by the selective dopamine D1
receptor antagonist SCH 23390. This finding
suggested that yawning may not be mediated by
autoreceptors, but by a sensitive population of
postsynaptically located dopamine receptors
(13,20,24-26).
All these results obtained indicate a close
interaction between cliolinergic and
dopaminergic transmitter systems in yawning and
a modulating influence of adrenergic and also
serotonergic mechanisms. Therefore, it has to be
taken into account that the pharmacological
influence of drug-induced yawning is due to
different mechanisms of action and cannot
exclusively be attributed to dopaminergic
mechanisms. Hence, a hypothetical model of a
central control mechanism includes a
dopaminergic inhibitory-cholinergic exitatory
link (9,29,31,32).
Our results show a dose-dependent inhibition
of the APOinduced yawning by casomorphin
derivatives with opioid activity. These
dose-response curves suggest an effect of
casomorphin derivatives on dopamine-mediated
processes. The APO-induced yawning could be
inhibited by the peptides at presynaptic
dopamine receptors antagonizing the inhibition
of dopamine release by small APO doses. On the
basis of their biochemical investigations,
Kammerer et ai. postulated an indirect
dopaminergic action of [D-Pro4]CM as
well (10). They demonstrated an inhibition of
dopamine reuptake and an enhanced K'-stimulated
release of dopamine by [D-Pro4]CM. A
blockade of dopamine receptors was only found
using much higher doses than with haloperidol
(11). Such a presynaptic dopaminergic
disinhibition may possibly al'o decrease the
tone of the cholinergic
system activated by physostigmine, thus
leading to an inhibition of the
physostigmine-induced yawning. Another mode of
action of the casomorphin derivatives could be
due to the interactions with opiate receptors.
Like the tyrosine-containing casomorphin
derivatives, morphine inhibited the
apomorphine-induced yawning completely. However,
only the morphine effect was suppressed by
naloxone (8). Therefore, a corresponding action
of [D-Pro4]CM on the opiate receptor
seems to be most unlikely.
The dopaminergic transmission can be
modulated by casomorphin derivatives, thus
resulting in a decrease in yawning. The decrease
in PHYSO-induced yawning by 5 nmol
[DPro4]CM and 10 nmol [D-Phe3JCM may
originate from a possible cholinolytic action,
too.
The effect of des-tyrosine casomorphins
differs in some respect from that of the
tyrosine-containing casomorphins. They show a
U-shaped dose-response curve as well as an
altogether lower effect.
[des-Tyr-D-Phe3]CM and
[des-Tyr-D-Pip4JCM were not found to
influence the PHYSO-induced yawning. However,
[desTyr-D-Pro4]CM increased the
physostigmine-induced yawning significantly.
Nickolson and Berendsen (21) found differences
in the effect between the neuroleptic-like
[des-Tyr--yjendorphin and haloperidol (5).
Whereas haloperidol completely blocked the
APO-induced yawning,
[des-Tyr-y]endorphin inhibited yawning
only partially. The dose-dependent inhibition of
yawning showed a similar U-shaped curve like the
inhibition by our investigated des-Tyr
casomorphins. Contrary to haloperidol, which
inhibited yawning in each case, the small
yawning rate
obtained with very low APO doses was
potentiated by [des-Tyr-y]endorphin.
Nickolson and Berendsen concluded that low doses
of [des-Tyr-yJendorphin sensitize
presynaptic receptors whereas high doses
sensitize postsynaptic sites of action
(21).
Possibly the serotonergic modulation may
play an important role in the yawning behavior.
Yawning induced by dopaminergic drugs can be
potentiated by decreasing activity of
serotonergic neurons, whereas the
physostigmine-induced yawning was attenuated.
Serotonergic activation had the contrary effect
(6,15).
For instance, Holmgren and Urba-Holmgren
have found that apomorphine yawning is inhibited
but physostigmine yawning is enhanced or not
influenced by the 5-HT-uptake blocker citalopram
(9). There was a comparable influence of
[des-Tyr-DPro4]CM supported by
biochemical results in which an inhibition of
serotonin reuptake into striatal synaptosomes
could be demonstrated (Schräder, personal
communication).
Summarizing our own results in the light of
these data and interpretations, it is assumed
that casomorphin derivatives containing 1
tyrosine modulate dopaminergic transmission.
However, it cannot be completely excluded that
this effect involves a cholinolytic action
resulting in an inhibition of yawning. At least
for [D-Pro4JCM, an opioid-like action can be
excluded, too. Furthermore, an opiate-like
effect can be ruled out with regard to
des-tyrosine derivatives by reason of the
missing Nterminal tyrosine. Consequently, a
modulation of dopaminergic transmission can be
assumed without excluding a serotonergic
activation by [des-Tyr-D-Pro4]CM.
