no
relation to extracellular strital levels of
dopamine
Stahle L, Ungerstedt U
Department of pharmacology,
Karolinska Institute. Sockholm, Sweden
Dopamine (DA) autoreceptors are believed to
autoregulate the activity of DA neurons by
inhibiting release of DA, inhibiting tyrosine
hydroxylase, decreasing DA utilisation and
reducing the fuing of DA neurons. The
autoreceptors are assumed to be more sensitive
to treatment with DA agonists than the
postsynaptic receptors. It bas also been
hypothesised that behavioural effects induced by
low doses of DA agonists are mediated by
stimulation of DA autoreceptors. Thus, the
behavioural effects of DA agonists such as
yawning and motor inhibition have been used to
screen for autoreceptor active substances.
However, it bas been suggested that DA
agonist-induced yawning, sedation and
suppression of exploratory behaviour are not
mediated by stimulation of DA
autoreceptors.
The present study was undertaken to further
investigate the hypothesis that the behavioural
effects of low doses of DA agonists are mediated
by a reduction of the extracellular levels of DA
(from hereon referred to as the autoreceptor
hypothesis). This was done by comparing the
effects on extracellular levels of DA of
apomorphine (APO), the tyrosine hydroxylase
inhibitor a-methyldl-p-tyrosine (a-MPT) and the
monoamine storage blocker reserpine. Microalysis
was used to sample the extracellular DA. The
results from the microalysis experiments were
then compared to behavioural effects of the
drugs. The findings suggest that the behavioural
effects of low doses of DA agonists are not
correlated to the extracellular levels of DA.
Preliminary accounts of the present work have
been published elsewhere. [...]
Discussion : The idea that low doses
of DA agonists selectively stimulate DA
autoreceptors was originally put forward by
Carlsson and Strömborn and also by
Ljungberg and Ungerstedt and DiChiara et al. The
hypothesis is based on the finding that low
doses of DA agonists inhibit tyrosine
hydroxylase without inducing stereotyped
behaviour which has been subsequently
substantiated. The fact that APO in the low-dose
range also suppresses exploration, and that the
effects of a wide range of DA agonists on
behaviour and synthesis are strongly correlated,
suggested that suppression of exploration also
is mediated by DA autorecepters. Similar
arguments were used to formulate the same
hypothesis for yawning behaviour.
Other evidence supporting the autoreceptor
hypothesis bas been put forward. Yawning
behaviour can be suppressed by pretreatment (4
bouts) with reserpine. This was interpreted as a
maximal reduction of synaptic DA to which the
effect of APO could not be added. Surprisingly,
Yamada and Furukawa found that reserpine (24
bouts pretreatment time) enhanced yawning which
they interpreted as an additive effect of APO
and reserpine. In both cases the findings were
taken as evidence in favour of the autoreceptor
hypothesis. Serra et al. reported that reserpine
alone induces yawning at 24-hr pretreatment, but
not at 4 hr which they interpreted as an
argument against the autoreceptor
hypothesis.
Dourish and Hutson found that 6-OHDA lesions
of striatal DA terminals abolish yawning
behaviour induced by APO 0. 1 mg/kg. The authors
interpreted their findings as due to a loss of
autoreceptors. Similarly, recent observations by
Stoessel et al. show that even smaller doses of
APO did not induce yawning in rats with
6-OHDA-induced substantia nigra lesions as would
have been expected if postsynaptic receptors had
mediated the response. However, intrastriatal
injections of DA agonists elicit yawning in rats
with local 6-OHDA lesions at the injection site.
Hence, it cannot be excluded that the results of
Dourish and Hutson and Stoessel et al. are
caused by nonspecific blockade of the expression
of yawning, e.g., through behavioural
competition. Recently, Melis et al. suggested
that yawning is mediated by hypothalamic
DA.
The present study was undertaken to further
test the hypothesis that suppression of
exploration and induction of yawning by low
doses of DA agonists are mediated by stimulation
of DA autoreceptors. From the autoreceptor
hypothesis a number of predictions follow. In
the present study three such predictions were
tested:
1 . The relation between the synaptic levels
of DA and behaviour should be the same if DA
levels are reduced by an agonist or by some
other drug, e.g., a-MPT (a tyrosine hydroxylase
inhibitor) or reserpine (a monoamine storage
blocker).
2. The reduction of synaptic levels of DA
and the behavioural effects of DA agonists
should have a similar time-course.
3. The dose-response curve for the
behavioural effects of a DA agonist should be
shifted to the left (i.e, the response should be
enhanced) by pretreatment with a-MPT.
Effects of APo, a-MPT or reserpine on
behaviour and extracellular dopamine
levels
It was expected, from the autoreceptor
hypothesis, that when the extracellular levels
of DA are reduced to a given level by APO,
reserpine or a-MPT, the behavioural effects
should be similar. The results from the present
study show that a-MPT (50 and 100 mg/kg),
reserpine (2 mg/kg) and APO (0.05 mgfkg) all
reduce the extracellular levels of DA to between
40% and 60% of basal levels. However, a-MPT (50
and 100 mg/kg) did not suppress exploration.
Reserpine (2 mg/kg) had no effect on behaviour
after 3 hr, but suppressed exploration after 4
hr. APO (0.05 mg/kg) suppressed exploratory
behaviour. Pergolide (0.005 mg/kg), which has
previously been shown to suppress exploration
(48), had no effect on extracellular levels of
DA in the striatum. Thus, it was not possible to
demonstrate the presence of a relation between
reduced extracellular levels of DA and
suppression of exploratory behaviour.
The fact that reserpine (2 mg/kg) did affect
behaviour 4 hr after administration may be taken
as evidence supporting the autoreceptor
hypothesis with respect to suppression of
exploration, though not for yawning because no
yawning was induced by reserpine. It is,
however, not easy to explain why there is a
drastic change in behaviour between 3 and 4 hr
after injection, while there is only a small
decline in extracellular DA levels. A possible
explanation is that the effect of reserpine is
uneven and that, in some critical synapses,
there is a large loss at 3-4 hr after injection.
Another possibility is that reduced
noradrenaline neurotransmission contributes to
the suppressive effect on exploration by
reserpine.
In the case of yawning behaviour, the
discrepancy between the effects on behaviour and
neurochemistry of DA agonists and a-MPT and
reserpine is even more obvious since neither
a-MPT nor reserpine can induce yawning behaviour
within the first 5 hr postinjection. In
addition, it has been reported that no DA
antagonist induces yawning behaviour and the
same result bas been obtained in this laboratory
for SCH 23390 and raclopride. In this connection
it is noteworthy that both yawning and
suppression of exploration can be elicited by
dopamine agonists in the presence of amphetamine
and that neuroleptic drugs can elicit yawning in
rats treated with high doses of dopamine
agonists or amphetamine.
The dialysis experiments in the present
study were performed in the striatum. We have
recently found that the effects of APO on DA
levels are smaller in the accumbens and the
frontal cortex than in the striatum, while
alphaMPT has approximately the same effect in
the acumbens and the striatum. Hence, a regional
variation in the sensitivity to APO is not
likely to account for the present findings. This
is important since it has been suggested that
suppression of exploration is mediated by the
nucleus accumbens.
Time-Response to APO, Pergolide and otMPT
on Behaviour and Extracellular Dopamine
Levels
The time-course for suppression of
exploration and induction of yawning by APO
(0.05 µg) were found to have a more rapid
onset and a shorter duration than the effect on
extracelluIar DA levels A similar discrepancy in
the time-course for yawning and changes in DA
levels was obtained with pergolide (0.02 mg/kg)
and with EMD 23448 and BHT 920. The time-course
for the effects of APO on exploration are
consistent with findings in mice. Interestingly,
it has been shown that the levels of APO in the
rat brain follow a time-course similar to that
of the behavioural effects. Thus, there is no
direct relation in time between behavioural
effects induced by DA agonists and the reduction
of DA levels.
In a previously published article, we
tentatively explained this discrepancy in
time-course between behavioural and
neurochemical effects as due to rapid changes in
autoreceptor sensitivity. However, such a
mechanism does not seem to explain the present
findings because we have found that the yawning
induced by repeated APO administration shows
only slight desensitisation and suppression of
exploration shows no desensitisation. Neither is
it likely that changes in DA levels measured by
means of microdialysis are delayed because of
slow diffusion of DA into the probe. This
conclusion is supported by the present finding
that the suppression of exploration following
a-MPT had a slower onset than the decrease of DA
levels, ie., the opposite to the findings with
APO. Thus, it is concluded that the observed
discrepancy in time-course between changes in
behaviour and changes in the extracellular
levels of DA reflects a true discrepancy in
time.
Effects of a-MPT, on APO-Induced Yawning
and Suppression of Exploration
Pretreatment with a-MPT did not shift the
dose-response curve for APO-induced yawning
(Fig. 4). The effect of aMPT on suppression of
exploration added to that of APO, but not to
pergolide. It was predicted that the yawning
dose-response curve would be shifted to the left
and that a-MPT per se would induce yawning.
Thus, it seems likely that induction of yawning
by DA agonists is independent of the
extracellular level of DA. The same conclusion
was drawn by Scheel-Krüger on the basis of
6-OHDA lesions in dorsal striatum and local
injections of DA agonists in the lesioned area.
The reason why die effect of a-MPT on
suppression of exploration adds to that of APO,
but not pergolide is not clear.
In conclusion, the present study
shows that behavioural effects of low doses of
DA agonists are not related to a reduction of
the extracellular levels of DA. If the
extracellular levels of DA reflect the synaptic
levels of DA, this may be interpreted as
evidence against the autoreceptor hypothesis. It
cannot be excluded that autoreceptor mediated
effects of a different nature, such as changes
in release of co-transmitters or changes in
firing pattern, account for the behavioural
effects of low doses of DA agonists. However, we
would like to propose that these behavioural
effects are mediated by a population of
postsynaptic DA receptors being more sensitive
to DA agonists than other postsynaptic receptor
populations such as the recepters that mediate
stereotyped behaviour following APO. Several
explanations for the high receptor sensitivity
are possible such as a large amount of spare
receptors or that the receptor is
pharmacologically different from other DA
receptors. This hypothesis has the advantage
that it is reasonably simple to test.
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MR Dissociation of autoreceptor activation
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M et al Antagonism of apomorphine-induced
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L Do autoreceptors mediate dopamine
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