Douglas Mental Health
University Institute, Montreal,
Canada
Schizophrenia may reflect a sensitization of
dopaminergic (DA) function. Apomorphine (Apo), a
DA receptor agonist, induces both sensitization
and tolerance of DA function in rodents
depending on dose intervals. We investigated
sensitization and tolerance to Apo in healthy
male volunteers. After a period of
acclimatization to the experimental setting (Day
1) subjects were assigned randomly to two
groups: Group A subjects received seven
injections of placebo (physiological saline)
(PLA) and Group B subjects received seven
injections of Apo HCl (7 mug/kg sc) under
double-blind conditions at 2 h intervals
commencing at 0930 hours (Day 2) after an
overnight fast. Twelve hours after the seventh
injection, i.e. on Day 3, after an overnight
fast all subjects received an injection of Apo.
Serial samples of blood commencing at 0900 hours
were drawn after the first and last injection in
both groups for assay of growth hormone (GH),
prolactin (PRL) and cortisol by
radioimmunoassay; sleepiness was measured using
the Analog Sleepiness Rating Scale and yawning
recorded by video recorder. The GH response in
Group B (N = 8) was (a) decreased after the
eighth injection of Apo compared with the first
injection of Apo (P = 0.03) and (b) decreased
after the eighth injection of Apo compared with
the first injection of Apo in Group A (N = 10)
(P = 0.001). The number of yawns in Group B was
significantly decreased after the eighth
injection of Apo compared with the first
injection of Apo (P = 0.042). PRL, cortisol and
sleepiness were not significantly different
between the first and eighth injection of Apo.
Sensitization was not observed in any of the
measures studied. These results are compatible
with induction of acute tolerance of DA-mediated
GH and yawning responses. The method used
provides a safe pharmacological paradigm to
examine plasticity of DA mechanisms in man.
Results are discussed in the context of possible
therapeutic implications for schizophrenia.
Introduction
The dopamine (DA) hypothesis (Kapur and
Remington 2001; Kapur and Mamo 2003) implicates
an enhancement of DA function in the
pathophysiology of schizophrenia (Breier et al.
1997; Abi-Dargham et al. 1998) at least with
respect to positive symptoms (Davis et al.
1991). Direct and indirect clinical evidence is
compatible with a role for neurochemical
sensitization (DA supersensitivity) underlying
the enhancement (Lieberman et al. 1997). The
psychosis induced in normal subjects by
amphetamine, cocaine, methylphenidate, and
related psychostimulants (Angrist 1983) which
increase DA function, is similar to
schizophrenia in cross-sectional clinical
features, course and response to neuroleptics
(Sato et al. 1992). Recurrence of the psychosis
is prompt with either subsequent exposure to the
same drug, even with a lower dose than initially
used (Sato et al. 1983), or with psychological
stressors (Sato 1979) even after many years of
abstinence. A single dose of amphetamine can
activate psychotic symptoms (Lieberman et al.
1990) in doses that are subpsychotogenic in
nonschizophrenic subjects; this may reflect an
enhanced sensitivity to DA (Lieberman et al.
1987).
These observations suggest that prior
repeated administration of drugs that increase
DA function induce long-term sensitization to
these agents and that sensitization underlies or
causes the psychotic state to recur. Also, they
suggest that there may be a common
neurobiological mechanism underlying
schizophrenia and stimulant-induced psychosis
(Sato et al. 1992). Virtually all CNS stimulants
which induce psychosis induce increased
locomotor activity and stereotyped behavior in
rodents as well as behavioral augmentation of
these behaviors following repeated
administration (i.e. sensitization) (Segal and
Schuckit 1983) by increasing DA function.
Sensitization of brain DA function to repeated
stimulation with pharmacological agents,
electrical stimulation or environmental
stressors has been proposed as an animal model
to understand the neurobiological basis of
schizophrenia (Lieberman et al. 1990; Sato et
al. 1992; Akiyama et al. 1994; Glenthoj et al.
1993; Glenthoj 1995).
Recently, Seeman et al. (2005) have shown
that there is an increase in density of D2
receptors in the high-affinity state (D2high) in
DA behavioral supersensitivity induced by a
variety of experimental techniques:
administration of amphetamine, phencyclidine,
quinpirole, long-term antipsychotics; ethanol
withdrawl; neonatal hippocampal lesions; rats
born by Caesarean Section; gene knockouts. The
authors propose there are multiple pathways
leading to psychosis including multiple gene
mutations, drug abuse or brain injury all of
which may converge via D2high to elicit
psychotic symptoms. Apomorphine (Apo), a DA
receptor agonist in animals and man (Lal 1988;
Tsang and Lal 1977; Nair et al. 1982) with a
high affinity for D1-like and D2-like receptors
(Seeman and Van Tol 1994) and which stimulates
DA receptors at doses which do not affect
noradrenaline (Butcher and Anden 1969) or
serotonin (Lal et al. 1972a) turnover, induces
both sensitization and tolerance of DA function
in rodents.
The same response may show sensitization or
tolerance to Apo depending on dose interval.
Repeated injections of Apo at intervals of 24 h
(Gancher et al. 1996a) or 48 h (Deschaies et al.
1984; Castro et al. 1985) increase Apo-induced
rotational behavior in the rat with an
unilateral lesioned nigrostriatal tract and also
increase Apo-induced locomotor activity when
given at intervals of 48 h (Castro et al. 1985)
to 7 days (Mattingly et al. 1989). When Apo is
given at intervals at 2 h (Castro et al. 1985)
tolerance is induced. Continuous sc infusion of
Apo abolishes Apo-induced rotational behavior in
the mouse (Winkler and Weiss 1986).
Sensitization to amphetamines or Apo show
similarities. Both require stimulation of D1
receptors and may require participation of
excitatory amino acid receptors (Akiyama et al.
1994; Mattingly et al. 1991; Druhan et al.
1993).
Sensitization to Apo results in cross
sensitization to amphetamine and cocaine, and
animals sensitized to amphetamine or cocaine
cross-sensitize to Apo (Bedingfield et al.
1996). The psychotomimetic properties of
amphetamines limit the use of such agents to
study sensitization and tolerance of DA function
in humans. Also, such agents affect not only DA
but also noradrenaline and serotonin systems.
Unlike amphetamine, there is no convincing
evidence that Apo induces psychosis (Lal and de
la Vega 1975; De´patie and Lal 2001). Apo
stimulates growth hormone (GH) secretion (Lal et
al. 1972b), decreases prolactin (PRL) secretion
(Lal et al. 1973; Martin et al. 1974) and
induces yawning (Lal et al. 1987) and sedation
(Feldman et al. 1945) via stimulation of D2
receptors (Nair et al. 1982; Serra et al. 1987;
Melis et al. 1987, Corsini et al. 1981). We
investigated sensitization and tolerance of DA
function in normal male volunteers by examining
the GH, PRL, yawning and sedative responses to
repeated injections of Apo.
Discussion
Few data are available on sensitization and
tolerance to Apo in normal subjects. In 24
healthy male subjects, Isaacs and MacArthur
(1954) gave two injections of Apo HCl (1 mg sc)
a few days apart and noted that the nausea or
emetic effect was more severe after the first
injection than after the second injection in
nine subjects, less severe after the first
injection in five subjects and no difference in
ten. Szechtman et al. (1988) gave 10.7 lg/kg Apo
HCl sc to 5 healthy volunteers at 2 week
intervals for a total of 12 injections. The mean
data for the first six trials were compared with
those of the last six trials. The GH response
and number of yawns were unchanged though the
onset of yawning time and peak yawning advanced.
Subjective feelings of nausea and warmth, but
not sedation, showed tolerance. In the present
study in normal subjects the GH and yawning
responses following seven injections of Apo at 2
h intervals were reduced when the subjects were
tested with a challenge dose of Apo 12 h after
the last injection.
Cortisol levels after the first and eighth
injection of Apo were not significantly
different. This suggests that stress was not a
factor in accounting for the difference. These
observations are compatible with the induction
of tolerance. The finding that the GH response
to Apo preceded by seven injections of PLA
(Group A) was significantly greater than the GH
response to Apo preceded by seven injections of
Apo (Group B) but not significantly different
from the first injection of Apo in Group B
further supports this view. However, the
possibility that the decreased GH response to
repeated injections of Apo was caused by a
decrease in pituitary GH reserve cannot be
excluded. The yawning response to Apo preceded
by seven injections of placebo (Group A) was not
significantly different from the yawning
response to Apo preceded by seven injections of
Apo (Group B). This might suggest a non-specific
effect of the injection schedule per se
accounted for the apparent tolerance.
However, the yawning response to Apo after
seven injections of PLA (Group A) was not
significantly different from the response to the
first injection of Apo in Group B. In the mouse
repeated injections of Apo induces its own
metabolism (Kaul and Conway 1971). Such an
effect might account for the diminished GH and
yawning response to Apo. In the present study
the concentration of plasma Apo was too low to
be measured by radioimmunoassay (Gancher et al.
1989). However, Gancher et al. (1992, 1995)
showed that in parkinsonian patients there was
no change in Apo pharmacokinetics during 3
months of waking hour sc Apo-infusion and that
after Apo sc infusion for 6&endash;31 h there
was no decrease in Apo levels comparing pre and
post Apo-infusion Apo challenge tests. Aside
from the absence of pharmacokinetic data, our
study is limited by the small number of
subjects. In the present study, subjects were
only examined for tolerance 12 h after the last
dose of Apo so that the number of injections
required to induce tolerance, the duration of
tolerance after cessation of injections or
whether continued injections of Apo would
sustain tolerance and for how long are unknown.
Chemical denervation supersensitivity of DA
receptors is believed to play a role in tardive
dyskinesia. In a single case study of tardive
dyskinesia, Apo given every 2&endash;6 h had an
initial beneficial effect but over a 2- to
4-week period tolerance to the antidyskinetic
effect was noted; tolerance to the emetic effect
developed within 48 h.
Several studies have examined tolerance to
the therapeutic effects of Apo on motor symptoms
of Parkinson's disease and on L-dopa-induced
dyskinesias. Tolerance to the antiparkinsonian
effect has been reported following repeated
injections of Apo given 15 min after the
beneficial effect has worn off (Grandas and
Obeso 1989) or following infusion of Apo for
periods of 4&endash;6 h (Gancher et al. 1996b).
Grandas et al. (1992) noted tolerance to the
antiparkinsonian effects of sc Apo when given at
an injection interval of 2 h but not when the
interval was extended to 4 h. No tolerance to
the antiparkinsonian effect was noted following
chronic sc infusion given during the waking
hours administered for periods of 3 months
(Gancher et al. 1995) or 1&endash;5 years
(Hughes et al. 1993). In Parkinson's disease,
L-dopa induces dyskinesias which are believed to
result from a sensitization process as a
consequence of pulsatile intermittent exogenous
DA stimulation (Katzenschlager et al.
2005).
Continuous waking day sc infusion of Apo
improves L-dopa induced dyskinesias (Colzi et
al. 1998; Kan˙ovsky´ et al. 2002;
Katzenschlager et al. 2005) within 6 months
without significant tolerance or loss of
antiparkinsonian effects (Hughes et al. 1993;
Kan˙ovsky´ et al. 2002). Though part of
this might be due to the reduction in L-dopa
administered, challenge tests with L-dopa and
Apo confirm the decrease in dyskinesia
(Katzenschlager et al. 2005). Dopaminergic
sensitization is believed to underlie the
pathophysiology of schizophrenia, at least with
the development of positive symptoms (see
''Introduction''). In a dose-response study of
Apo-induced GH secretion in acute schizophrenia,
Mu¨ller-Spahn et al. (1998) showed that
there is an increased receptor sensitivity of
the DA system controlling GH secretion. Brown et
al. (1988) noted that in some patients with
schizophrenia studied on seven or more
occasions, the GH response to Apo was
significantly correlated with positive symptom
scores. In a subgroup of 5 patients with
schizophrenia who received Apo on 12 consecutive
trials at intervals of 2 weeks to 2 months the
GH response decreased over time. This suggests
tolerance. However, 3 of these 5 patients who
were followed for more than 12 trials then
showed an increased responsivity. To account for
this the authors suggested that the
desensitizing effect of Apo is time limited. If
sensitization of DA function subserves the
development of schizophrenia, then Apo might be
expected to improve schizophrenia if given in an
appropriate doseinterval regimen or by
continuous administration. Though some studies
point to an antischizophrenic effect of Apo
(Smith et al. 1977; Tamminga et al. 1978;
Corsini et al. 1977) a review of the topic found
no convincing evidence from published data for
an anti-psychotic effect (Lal 1988).
However, none of the studies was designed to
address the DA sensitization hypothesis of
schizophrenia. Eleven of the 13 studies involved
observation following a single parenteral dose
of Apo. In a further study, Tamminga et al.
(1986) investigated the effect of a single dose
of an analog of Apo, namely,
N-n-propyl-norapomorphine (NPA), in nine
patients with schizophrenia. The Brief
Psychiatric Rating Scale (BPRS) scores were
significantly reduced by NPA compared with
placebo. Unfortunately, the occurrence of nausea
may have unblinded the study. Nine different
patients received NPA for 3 weeks in increasing
doses from a twice daily schedule to a three
times daily schedule. There was no change in the
BPRS. The authors interpreted their findings as
indicating tolerance to the antipsychotic
effects of NPA. However, in this 3-week study
there was no data provided to show an initial
antipsychotic effect so that it is difficult to
conclude there was tolerance. The dose interval
and duration of NPA administration may have been
suboptimal. Chronic subcutaneous infusion of Apo
in schizophrenia has not been investigated.
Hypothalamic DA circuits subserving GH secretion
and yawning are distinct from the
mesocorticolimbic DA system which is assumed to
play a role in the pathophysiology of
schizophrenia. Thus, changes in hypothalamic DA
function subserving GH secretion and yawning may
not reflect changes in DA function in pathways
relevant to schizophrenia.
Accordingly, care is required in
extrapolating data from one DA system to
another. Though changes in GH response to Apo
have been noted in schizophrenia, the findings
have not been consistent (see Lal 1987, for
review). However, Pandey et al. (1977) found an
increase in the GH response to Apo in acute
schizophrenia but not in chronic schizophrenia.
Also, Mu¨ller- Spahn et al. (1998), using
graded doses of Apo, provided evidence for an
increase in DA receptor sensitivity mediating GH
secretion in acute schizophrenia. The present
study in the context of animal studies, longterm
clinical benefit of Apo in the management of
L-dopa induced dyskinesias and the implication
of sensitization of DA function in schizophrenia
point to a need to reassess possible therapeutic
benefit for Apo in the treatment of
schizophrenia, especially in those patients with
positive symptoms who are treatment resistant.
Also the present study suggests a safe
pharmacological paradigm to examine plasticity
of DA systems in man.
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Eur
J Neurosci.
2007;26(9):2532-258
Ontogenetic
quinpirole treatment produces long-lasting
decreases in the expression of Rgs9, but
increases Rgs17 in the striatum, nucleus
accumbens and frontal
cortex
Maple AM, Perna MK, Parlaman JP,
Stanwood GD, Brown RW.
Department of
Psychology, East Tennessee State University,
Johnson City, USA.
Ontogenetic treatment of rats with the
dopamine D(2)-like receptor agonist quinpirole
produces a significant increase in dopamine D(2)
receptor sensitivity that persists throughout
the animal's lifetime, a phenomenon known as
D(2) priming. The present study was designed to
investigate the effects of priming of the D(2)
receptor on the expression of three different
members of the regulator of G-protein signaling
(RGS) family: Rgs4, Rgs9 and Rgs17. Male
offspring were ontogenetically treated with
quinpirole or saline from postnatal days (P)1-21
and raised to adulthood.
On approximately P65, animals were given an
acute quinipirole injection (0.1 mg/kg) and the
number of yawns was recorded for 1 h
after the injection. Yawning has been
shown to be a behavioural event mediated by the
dopamine D(2)/D(3) receptor. Animals
ontogenetically treated with quinpirole
demonstrated a significant 2.5-fold increase in
yawning as compared to controls. Rgs transcripts
were analysed through in situ hybridization
several weeks later.
Rats ontogenetically treated with quinpirole
demonstrated a significant decrease in Rgs9
expression in the frontal cortex, but a more
robust decrease in the striatum and nucleus
accumbens as compared to controls. Regarding
Rgs17, ontogenetic quinpirole produced a modest
but significant increase in expression in the
same brain areas. There were no significant
differences in Rgs4 expression produced by drug
treatment in any of the brain regions
analysed.
This study demonstrates that ontogenetic
quinpirole treatment, which results in priming
of the D(2) receptor, results in significant
decreases in Rgs9, which has been shown to
regulate G-protein coupling to D(2)
receptors.
Brain
Res.
2008;1200:66-77.
Adulthood
olanzapine treatment fails to alleviate
decreases of ChAT and BDNF RNA expression in
rats quinpirole-primed as
neonates
Brown RW, Perna MK, Maple AM, Wilson
TD, Miller BE.
Department of
Psychology, East Tennessee State University,
Johnson City, USA.
Neonatal quinpirole (dopamine D(2)/D(3)
agonist) treatment to rats has been shown to
increase dopamine D(2) receptor sensitivity
throughout the animal's lifetime. Male and
female Sprague-Dawley rats were neonatalally
treated with quinpirole (1 mg/kg) from postnatal
days (P) 1-21 and raised to adulthood. Beginning
on P62, rats were administered the atypical
antipsychotic olanzapine (2.5 mg/kg) twice daily
for 28 days.
Starting 1 day after the end of olanzapine
treatment, animals were behaviorally tested on
the place and match-to-place version of the
Morris water maze (MWM) over seven consecutive
days, and a yawning behavioral test was
also performed to test for sensitivity of the
D(2) receptor 1 day following MWM testing.
Similar to results from a past study, olanzapine
alleviated cognitive impairment on the MWM place
version and increases in yawning produced by
neonatal quinpirole treatment.
Brain tissue analyses showed that neonatal
quinpirole treatment resulted in a significant
decrease of hipppocampal ChAT and BDNF RNA
expression that were unaffected by adulthood
olanzapine treatment, although adulthood
olanzapine treatment produced a significant
increase in cerebellar ChAT RNA expression.
There were no significant effects of drug
treatment on NGF RNA expression in any brain
area. These results show that neonatal
quinpirole treatment produced significant
decreases of protein RNA expression that is
specific to the hippocampus.
Although olanzapine alleviated cognitive
deficits produced by neonatal quinpirole
treatment, it did not affect expression of
proteins known to be important in cognitive
performance.
