mise à jour du
6 juillet 2008
J Neural Transm
Induction of tolerance of dopaminergic
responses in man
Lal S, Thavundayil JX, Ng Ying Kin NM,
Dai X, Schwartz G, Montoya A.
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.
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.
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.

Eur J Neurosci.
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.
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.