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Dopamine receptor-mediated hypothermia induced in rats by (+)-, but not by (—)-3-PPP
European Journal of Pharmacology, 107 (1985) 299-304
299
Elsevier
D O P A M I N E R E C E P T O R - M E D I A T E D H Y P O T H E R M I A INDUCED IN RATS BY ( + ) - , BUT N O T BY ( - ) - 3 - P P P
STEPHAN HJORTH 1.,, ARVID CARLSSON 1, DAVID CLARK 1, KJELL SVENSSON 1 and DOMINGO SANCHEZ 2 1 Department of Pharmacology, and 20rg. Chem. Unit, Department of Pharmacology, University of GMeborg, P.O. Box 33031, S- 400 33 Gf~teborg, Sweden
Received 26 June 1984, revised MS received29 August 1984, accepted2 October 1984
S. HJORTH, A. CARLSSON, D. CLARK, K. SVENSSON and D. SANCHEZ, Dopamine receptor-mediated hypothermia induced in rats by ( + )-, but not by ( - )-3-PPP, European J. Pharmacol. 107 (1985) 299-304. The novel dopaminergic agents (+)- and (-)-3-PPP were evaluated for their effects upon thermoregulation in rats maintained at room temperature ( - 22°C). Although approximately 30 times less potent than apomorphine, ( +)-3-PPP induced a clearcut, dose-dependent and haloperidol/pimozide-reversible hypothermia. In contrast, the (-)-enantiomer per se lacked a significant effect upon rat body temperature. However, (-)-3-PPP clearly attenuated apomorphine-induced hypothermia. Simultaneous biochemical investigations confirmed the presence of central dopamine (DA) agonist and antagonist properties for (+)- and (-)-3-PPP, respectively, at the doses employed. The results are compared to the agonist and antagonist effects of the 3-PPP enantiomers in various other central DA receptor systems. Particular reference is made to the recent hypothesis by Carlsson (J. Neural Transm. 57 (1983) 309, relating agonist intrinsic activity to the DA receptor responsiveness state, in turn determined by the endogenous tone. Based on the findings with (+)- and (-)-3-PPP it is suggested that DA receptors mediating hypothermia in the rat may be more akin to 'normosensitive' postsynaptic than to highly 'agonist-responsive' autoreceptors. 3-PPP enantiomers
Dopamine receptors
Thermoregulation
1. Introduction
Dopamine (DA) appears to play an important modulatory role in the maintenance of thermoregulation (e.g. Cox, 1979). Thus, hypothermia is generally found in rats and mice treated with central DA receptor agonists (for refs. see Clark, 1979). Ample evidence also demonstrates that such hypothermia is readily antagonised by pretreatment with DA receptor blockers such as pimozide and haloperidol, confirming the dopaminergic nature of the response (Cox, 1979). Although it seems generally accepted that, in the rat, the medial preoptic hypothalamic area represents a critical target locus, DA receptors in other brain regions may also participate in the thermoregulatory events
* To whom all corresoondenceshould be addressed. 0014-2999/85/$03.30 © 1985 ElsevierSciencePublishers B.V.
Rat
(e.g. Brown et al., 1982). The relative involvement of DA auto- vs. postsynaptic receptors in body temperature control remains to be established. Recently, we have detailed the neurochemical and behavioural properties of the novel dopaminergic agent 3-(3-hydroxyphenyl)-N-n-propylpiperidine, 3-PPP and its enantiomers (for review see Hjorth, 1983). Whilst both are able to stimulate DA autoreceptors, ( + ) - and ( - ) - 3 - P P P concomitantly exert agonist and antagonist actions, respectively, on normosensitive postsynaptic DA receptors. However, not only the (+)-, but also the ( - )-enantiomer display significant agonist properties on presumed non-synaptic DA receptors, i.e. r e d u c t i o n of p l a s m a p r o l a c t i n levels in reserpine-pretreated rats (Eriksson et al,, 1983), rotation in unilaterally substantia nigra-lesioned (6-OH-DA) rats (Arnt et al., 1983) and emesis induction in dogs (Arnt et al., 1983; G. Paalzow,
300 personal communication). As pointed out by Carlsson (1983), it appears likely that the responses obtained in the different experimental models are related not only to the neuroanatomical location but also to the (basal) functional state of the receptors involved (see further Discussion section). That is, the agonist properties of (-)-3PPP and similar agents are more clearly evident in systems assumed to possess highly responsive DA receptors. By extension the hypothesis might also imply that, depending on the prior functional state, receptors may differ in their relative susceptibility to sensitisation vs. down-regulation of responsiveness. Interestingly, based on their resistance to supersensitivity induction and their easy desensitisation DA receptors mediating DA-agonist hypothermia have been suggested to be in a state of 'maximal functional sensitivity' (Schwartz et al., 1978). In view of the considerations, and in order to further characterise the overall pharmacological profiles of (+)- and (-)-3-PPP, the effects of the compounds on rat body temperature were investigated.
2. Materials and methods
2.1. Animals Male Sprague-Dawley rats (200-300 g; ALAB, Stockholm, Sweden) were used throughout the studies. The animals were kept under controlled standard environmental conditions (temp. 25°C; humidity -60%; lights on 6 a.m.-8 p.m.) for at least a week after arrival in the Department until used in the experiments. Food pellets (Ewos, Srdert~ilje, Sweden) and tap water were allowed ad lib.
2.3. Dissections and biochemical analyses Brain dissections were carried out according to Carlsson and Lindqvist (1973). The determinations of monoamines and their metabolites in the limbic (containing e.g. the olfactory tubercles, nucleus accumbens and nucleus amygdaloideus centralis) and striatal brain portions were performed by standard HPLC (EC detection) techniques or slight modifications thereof (Svensson and Lindberg, in preparation). 2.4. Drugs The drugs used in the study were (+)- and (-)-3-PPP HC1 (Wikstri3m et al., 1984), apomorphine HCI x 1/2 H20 (Sandoz, Basle, Switzerland), pimozide and haloperidol (Leo, Helsingborg, Sweden). Pimozide and haloperidol were dissolved in a minimal quantity of glacial acetic acid and made up to volume with 5.5% glucose solution, whereas the other compounds were dissolved in physiological (0.9%) saline (in the case of apomorphine a few grains of ascorbic acid were added to prevent oxidation). The injection volume was 5 ml/kg. For information on dosage, routes of administration and time schedules, see figure and table texts. 2.5. Statistics Student's t-test was employed and was preceded by an analysis of variance where appropriate; probability levels ~<5% were considered significant. 3. Results
3.1. Body temperature 2.2. Temperature measurements Body temperatures were monitored thermoelectrically (Yellow Springs Instr. Co. Inc., Ohio, U.S.A.; YSI Model 43 TK) 20-30 s after inserting the thermistor probe 6 cm into the rectum of lightly manually restrained rats. The experiments were always carried out between 8 and 12 a.m. and at ambient temperatures of 21-23°C.
Table 1 and fig. 1 show that (+)-, but not ( - )-3-PPP (6.8 mg = 27/~mol/kg s.c.), induced a clearcut hypothermia (1.2-2.1°C) in rats maintained at an ambient temperature of 21-23°C. Although not significantly different, this hypothermic response tended to be somewhat more pronounced at 60 than at 30 rain after administration, possibly indicating a contribution from a potential
301 TABLE 1 Effects of ( + )- and ( - )-3-PPP upon the rat rectal temperature. Interaction with haloperidol. Rats were given haloperidol (0.~ m g / k g i.p.) 30 rain prior to ( + ) - or ( - ) - 3 - P P P (6.8 m g / k g s.c.). Controls received appropriate vehicle injections at coi:responding time intervals. Rectal temperatures were monitored immediately before (t = 0) and 30 rain after the s.c, injections and immediately before killing (i.e. corresponding to 30, 60 and 90 min after haloperidol administration). The brains were subsequently dissected and analysed for monoamine and metabolite levels (see table 2 and Results section). Statistics: analysis of variance followed by t-test: a p < 0.001 from corresponding vehicle-treated group, and b p < 0.005 from corresponding (+)-3-PPP (only) group. Treatment
Vehicle Haloperidol(H) (+)-3-PPP H+(+)-3-PPP (-)-3-PPP H+(-)-3-PPP
Treatment interval (rain)
n
0
30
38.1+-0.11 38.35:0.07 38.2+-0.04 38.15:0.30 38.25:0.12 38.25:0.17
38.2+-0.15 38.1+0.09 4 38.3+0.12 38.2+0.11 4 37.1+0.20 a 36.1+0.25 a 4 38.2+0.24 b 38.4+0.20 b 4 38.05:0.11 38.25:0.13 4 37.95:0.17 38.25:0.15 4
60
active metabolite(s). In a separate experiment it was shown that pimozide pretreatment (0.5 mg/kg i.p~, 4 h before) blocked the hypothermic response to (+)-3-PPP (6.8 mg/kg s.c.) administration (temp. change 0-60 min; (+)-3-PPP: -1.60_+ 0.23°C, vs. pimozide/(+)-3-PPP: +0.08 _+ 0.11°C; n = 4-5, P < 0.001). Likewise, haloperidol pretreatment (0.5 mg/kg i.p., 30 rain before) completely abolished (+)-3-PPP-induced hypothermia, while alone or in combination with (-)-3-PPP it failed to influence rat body temperature (table 1). From the dose-response experiments in fig. 1 it is seen that lower doses of (+)-3-PPP were either completely ineffective (0.43-1.7 mg/kg s.c.) or produced, at best, a trend to hypothermia (3.4 mg/kg s.c.). No statistically significant temperature changes were, however, obtained over the entire time period studied (15-90 rain post-injection; data not shown). In contrast, the higher doses of (+)-3PPP (6.8 and 13.6 mg/kg s.c.) produced a clearcut °C
z~°C 0
58.5
-0.5
58.0
\\o_
:
.
\\ ~ o ~ / 9.
•
:
~°
. /
o/
zx ,//A
• 10
57.5 -1.5
57.0
\,,/
- 2.0
0.03 '
0.1 '
05'
I.'o
30 '
I0.0 ' 30.0 ' mg/kcj s.c.
Fig. 1. Dose-dependent hypothermia induced in the rat by apomorphine and (+)-3-PPP. The graph was prepared from data obtained in four separate experiments. Absolute temperature changes in individual vehicle (0.9% NaCl)-treated control animals ranged between - 0 . 3 and +0.5°C over the time period studies (data not shown; cf. also table 1). Rats were given apomorphine (0.03-1.0 m g / k g s.c.) (O ©) or ( + )3-PPP (0.43-13.6 m g / k g s.c.) (zx zx), The rectal temperatures were monitored immediately before and 30 min after drug injection. The mean absolute temperature changes (A°C) 0-30 min + S . E M . ( n = 3-8) are shown. Statistical comparisons: Student's t-test: *** P < 0.001 and ** P < 0.01 from corresponding control values.
56.5 ~)
I
I
20
I
410
I
610
I
810 min.
Fig. 2. ( - ) - 3 - P P P antagonism of apomorphine-induced hypothermia. Rats were given apomorphine (0.3 or 1.0 mg/kg) and ( - )-3-PPP (6.8 mg/kg) simultaneously s.c. on either side of the neck region. Controls received vehicle (0.9% NaCI) instead of ( - ) - 3 - P P P . The rectal temperatures were monitored immediately before and at 10 min intervals after drug injection. The mean temperatures (°C) are shown; n = 5 at each time point, standard errors omitted for clarity. O O Apomorphine 0.3/NaC1, • • apomorphine 0 . 3 / ( - )-3PPP, zx zx apomorphine 1.0/NaCI, • • apomorphine 1 . 0 / ( - ) - 3 - P P P . Statistical comparisons: Student's t-test: *** P < 0.001, **(*) P < 0.005, ** P < 0.01 and * P < 0.05 vs. corresponding apomorphine/NaCl treatment group.
302 TABLE 2 3-PPP effects on brain DA metabolism. Interaction with haloperidol. Treatment
Limbic
Striatum
DA
DOPAC
HVA (ng/g)
DA
DOPAC
HVA (ng/g)
Vehicle Haloperidol(H) (+)-3-PPP H+(+)-3-PPP (-)-3-PPP H +(-)-3-PPP
2140+_ 91 1791.+ 55 a 2171-+ 94 1684_+66 a'b 2048-+ 15 1827_+170a
370+_ 9 990.+ 31 a 223.+ 15 a
243.+41 622_+16a 108+ 5 ~ 634___43a'b 244.+16 592±95 a'b
7079.+506 6096-+305 7365.+510 5582_+318~'b 6667.+260 5967_+335
980.+ 28 3590-+ 92 a 635+ 30 a 3047_+352~'b 1463_+149a 3246±400 ~'b
769.+ 92 2242_+ 92 361+_ 44 a 2320_+ 66 a'b 1088_+94 a 2404±123 ~'b
784-+ 66 a'b
386_+ 11 794_+102a'b
n 4 4 4 4 4 4
For experimental detail see legend to table 1. Statistical comparisons (P < 0.05 or less): a significantly different from vehicle-treated. and b significantly different from corresponding 3-PPP (no haloperidol)-treated group. hypothermic response which was maximal (2.52.6°C) at about 1.5 h and was evident for approximately 4 h (13.6 m g / k g s.c.; data not shown). Apomorphine was roughly 30 times more potent than ( + ) - 3 - P P P in inducing hypothermia in the rat. Thus, while 0.03 m g / k g (s.c.) of apomorphine failed to alter body temperature, 0.1-1.0 m g / k g caused a dose-dependent hypothermia. The duration of the hypothermic action of apomorphine (1.0 m g / k g s.c.) was approximately 60-70 rain and the nadir ( - 1 . 7 ° C ) was reached about 30 min after administration. While ( - ) - 3 - P P P (0.43-6.8 m g / k g s.c.) did not significantly alter rat body temperature (table 1 and unpublished data), it clearly reduced DA agonist-induced hypothermia. Thus, the temperature decrease elicited by apomorphine (0.3 or 1.0 m g / k g , s.c.) was partly, though significantly, attenuated by concurrent treatment with ( - ) - 3 - P P P (6.8 m g / k g s.c.) (fig. 2). While the time course remained essentially unaltered, the amplitude of the apomorphine-induced hypothermic effect was blunted by ( - ) - 3 - P P P treatment. Thus, at 20-40 and 20-50 min after administration ( - ) - 3 - P P P significantly antagonised the decreases in body temperature caused by 0.3 and 1.0 m g / k g of apomorphine, respectively. The somewhat delayed ( - )-3-PPP antagonism had been seen during studies of the ability of ( - ) - 3 - P P P to counteract apomorphine-induced locomotor hyperactivity (Hjorth et al., 1983, and unpublished data). The body temperature experiments described do not differentiate between drug effects on heatgenerating vs. heat-loss systems as they were carried out only at ambient temperatures of - 22°C.
3.2. Brain monoamines and monoamine metabolites
Immediately following completion of the experiment shown in table 1, the rats were decapitated and the brain regions analysed for dopamine (DA) and its metabolites (DOPAC and HVA). As shown in table 2, ( - ) - 3 - P P P did not change the limbic but increased the striatal levels of D O P A C and HVA. On the other hand, in both regions these levels were decreased by ( + ) - 3 - P P P and increased by haloperidol. The (+)-3-PPP-induced D O P A C and HVA reductions were prevented by haloperidol. In fact, except for the slightly lower (P < 0.025) limbic D O P A C values seen after ( + )3 - P P P / h a l o p e r i d o l vs. haloperidol only, the D O P A C and HVA values in all haloperidol-treated groups were similar. The brain DA levels remained essentially unchanged by the treatments depicted. However minor (16-22%), and in four our of six cases significant, decreases occurred in the haloperidol-treated groups. Simultaneous assessments of the noradrenaline, 5-hydroxytryptamine and 5hydroxyindole acetic acid (5-HIAA) levels revealed no changes in any of the treatment groups (data not shown).
4. Discussion
The present study showed that (+)-3-PPP, similarly to the well-known DA agonist apomorphine, could induce a clearcut haloperidol/pimozide-reversible hypothermia when administered to rats maintained at room temperature ( - 2 2 ° C ) . In contrast, its enantiomeric twin, ( - ) - 3 - P P P , failed
303 to alter rat body temperature per se but significantly counteracted the apomorphine-induced hypothermic response. Consistent with these data are our observations that (-)-3-PPP was capable of antagonising the temperature-lowering effect of (-)-3-PPP (Hjorth, 1983; D.C., S.H. and K.S., unpublished results). Furthermore, confirming and extending previous findings, the biochemical results obtained in the present study (see table 2) may also be taken to illustrate the presence of dopaminergic agonist and antagonist properties of (+)- and (-)-3-PPP, respectively (Hjorth, 1983; Hjorth et al., 1983), at the doses used. While (+)-3-PPP appears to act as a full dopaminergic agonist in most experimental situations (e.g. Hjorth, 1983; cf. however, Arnt et al., 1983), the action of its (-)-counterpart is more complex. Thus, (-)-3-PPP displays either full agonist, partial agonist or weak antagonist effects depending on the test model applied. Generally, however, its DA agonistic properties seem to predominate in tests assessing presumed non-synaptic and/or 'supersensitive' DA receptor activity (cf. Introduction). Carlsson (1983) has recently proposed that the degree of previous agonist occupancy determines DA receptor responsiveness, and thereby in part the intrinsic activity of agents interacting with the receptors. That is, at DA receptors adapted to low or high agonist occupancy, compounds such as (-)-3-PPP would act as an agonist or antagonist, respectively. The (hypothalamic?) DA receptors implicated in the DA agonist-induced hypothermic response have been suggested to be in a state of 'maximal functional sensitivity' (Schwartz et al., 1978), possibly related to a minimum of endogenous tone (Cox and Lee, 1979). Given the adaptational characteristics of these receptors (easily desensitised but resistant to supersensitivity induction; Schwartz et al., 1978), they might be expected to fall into the highly agonist-responsive DA receptor category (exemplified by DA autoreceptors and non-synaptic receptors), according to Carlsson (1983). Similarly to the agonist activity displayed on the latter receptors (Arnt et al., 1983; Eriksson et al., 1983) both 3-PPP enantiomers should thus, like apomorphine, be able to elicit a clearcut hypothermic response in non-pretreated
rats. However, hypothermia was induced by (+)(>/13 /~mol = 3.4 mg/kg s.c.) but not by (-)-3PPP (3.4-27 /~mol/kg s.c.). This should be compared to the potencies for both enantiomers, e.g. in reducing plasma prolactin levels in reserpinised rats (EDs0s - 2/~mol/kg s.c.; Eriksson et al., 1983) or in producing contralateral rotation in unilaterally substantia nigra 6-OH-DA-lesioned rats (EDs0s 0.77-2.1 ~tmol/kg s.c.; Arnt et al., 1983). These responses are considered to reflect activation of presumed non-synaptic and 'denervationsupersensitive' postsynaptic, respectively, DA receptors. The doses used in the present studies thus fall well within the supramaximal range for eliciting responses from highly agonist-sensitive central DA receptors. Yet, only the (+), but not the ( - ) enantiomer induced the expected DA agonist response - hypothermia - under these conditions. It may therefore be inferred that the DA 'hypothermia' receptors do not belong to the highly DA agonist-responsive receptor category. Instead, (-)-3-PPP behaved as an antagonist, counteracting the apomorphine-induced hypothermic action. Similarly, (+)- and (-)-3-PPP act as agonist and antagonist, respectively, at 'normosensitive', low agonist-responsive (Carlsson, 1983) postsynaptic DA receptors associated with motor activation (Hjorth et al., 1983) and control of striatal ACh release (Plantj6 et al., 1983). It is also important to note the close correspondence between the (+)-3-PPP doses inducing dopaminergic behavioural (motor) activation, and those required for hypothermia (i.e. >/3.4 mg/kg s.c.). On the other hand, lower doses of the agent (0.43-1.7 mg/kg s.c.) failed to alter the body temperature of the rats (present data), despite clearly being sufficient to cause dose-dependent reductions in central DA synthesis and spontaneous locomotor activity (Hjorth et al., 1983). Similarly, a low 'DA autoreceptor' dose of apomorphine (0.03 mg/kg s.c.; cf. e.g. Argiolas et al., 1982; Montanaro et al., 1983) was without hypothermic effect. Apomorphine at a dose of 0.1 mg/kg (s.c.) induced significant but mild hypothermia, whereas the higher, excitatory (cf. e.g. Hjorth et al., 1983; Montanaro et al., 1983) doses (0.3-1.0 mg/kg s.c.) markedly decreased body temperature. The find-
304
ings with (+)-3-PPP and apomorphine are thus congruent, indicating a correspondence between the dose ranges required for hypothermia vs. postsynaptic DA receptor (but not DA autoreceptor) activation, as assessed in other experimental situations (cf. above). The lack of hypothermic action of (-)-3-PPP also favours this interpretation. Taken together, the present data indicate that the thermoregulatory DA receptors are more akin to classical 'normosensitive' postsynaptic DA receptors than to highly agonist-responsive receptors such as, e.g., the DA autoreceptors, the chronically denervated or the non-innervated DA receptors (cf. above). If however, as suggested by Schwartz et al. (1978), the thermoregulatory DA receptors indeed belong to the maximally agonist-responsive category, they may represent an exception from the proposed general principle relating receptor responsiveness to the degree of intrinsic activity for DA receptor-active agents with putative agonist qualities - like e.g. (-)-3-PPP (Carlsson, 1983). Alternatively, (-)-3-PPP might have other as yet unidentified effects obscuring any putative hypothermic action. Further investigations, including studies of 3-PPP enantiomer activities on DA receptors under different synaptic conditions, are clearly needed to resolve these issues.
Acknowledgements The expert technical assistance of Gun Andersson, Maria Lindb~ck, Pia Lisj6 and Lena L6fberg is gratefully acknowledged. This study was financially supported in part by grants from AB Astra, the Swedish Board for Technical Development, 'Magnus Bergvalls Fond', 'Sv. Lakares~llskapet' and the Medical Faculty. Univ. of G6teborg. D.C. was supported by a postdoctoral fellowship from the Science and Engineering Research Council (U.K.).
References Argiolas, A., G. Mereu, G. Serra, M.R. Melis, F. Fadda and G.L. Gessa, 1982, N-n-propyl-norapomorphine: an extremely potent stimulant of dopamine autoreceptors, Brain Res. 231, 109.
Arnt, J., K.P. BOges6, A.V. Christensen, J. Hynel, J.-J. Larsen and O. Svendsen, 1983, Dopamine receptor agonistic and antagonistic effects of 3-PPP enantiomers, Psychopharmacology 81, 199. Brown, S.J., C.V. Gisolfi and F. Mora, 1982, Temperature regulation and dopaminergic systems in the brain: does the substantia nigra play a role?, Brain Res. 234, 275. Carlsson, A., 1983, Dopamine receptor agonists: Intrinsic activity vs. state of the receptor, J. Neural Transm. 57, 309. Carlsson, A. and M. Lindqvist, 1973, Effect of ethanol on the hydroxylation of tyrosine and tryptophan in rat brain in vivo, J. Pharm. Pharmacol. 25, 437. Clark, W.G., 1979, Changes in body temperature after administration of amino acids, peptides, dopamine, neuroleptics and related agents, Neurosci. Biobehav. Rev., 3, 179. Cox, B., 1979, Dopamine, in: Body Temperature Regulation, Drug Effects and Clinical Implications, eds. P. Lomax and E. Schonbaum (Marcel Dekker, New York) p. 231. Cox, B. and T.F. Lee, 1979, Effect of central injections of dopamine on core temperature and thermoregulatory behaviour in unrestrained rats, Neuropharmacology 18, 537. Eriksson, E., K. Modigh, A. Carlsson and H. Wikstr6m, 1983, Dopamine receptors involved in prolactin secretion pharmacologically characterized by means of 3-PPP enantiomers, European J. Pharmacol. 96, 29. Hjorth, S., 1983, On the mode of action of 3-(3-hydroxyphenyl)-N-n-propylpiperidine, 3-PPP, and its enantiomers, Acta Physiol. Scand. 118, Suppl. 517 (thesis), 1. Hjorth, S., A. Carlsson, D. Clark, K. Svensson, H. Wikstr6m, D. Sanchez, P. Lindberg, U. Hacksell, L.-E. Arvidsson, A. Johansson and J.L.G. Nilsson, 1983, Central dopamine receptor agonist and antagonist actions of the enantiomers of 3-PPP, Psychopharmacology 81, 89. Montanaro, N., A. Vaccheri, R. Dall'Olio and O. Gandolfi, 1983, Time course of rat motility response to apomorphine: a simple model for studying preferential blockade of brain dopamine receptors mediating sedation, Psychopharmacology 81,214. Plantj6, J.F., A.H. Mulder and J.C. Stoof, 1983, Effects of the enantiomers of 3-PPP on D-1 and D-2 receptors in the rat neostriatum in vitro, Pharm. Weekbl. Sci. Ed. 5, 265. Schwartz, J.C., J. Costentin, M.P. Martres, P. Protais and M. Baudry, 1978, Modulation of receptor mechanisms in the CNS: hyper- and hyposensitivity to catecholamines, Neuropharmacology 12, 665. WikstrOm, H., D. Sanchez, P. Lindberg, U. Hacksell, L.-E. Arvidsson, A. Johansson, S.-O. Thorberg, J.L.G. Nilsson, K. Svensson, S. Hjorth, D. Clark and A. Carlsson, 1984, Resolved 3-(3-hydroxyphenyl)-N-n-propylpiperidine and its analogues - central dopamine receptor activity, J. Med. Chem. 27, 1030.
299
Elsevier
D O P A M I N E R E C E P T O R - M E D I A T E D H Y P O T H E R M I A INDUCED IN RATS BY ( + ) - , BUT N O T BY ( - ) - 3 - P P P
STEPHAN HJORTH 1.,, ARVID CARLSSON 1, DAVID CLARK 1, KJELL SVENSSON 1 and DOMINGO SANCHEZ 2 1 Department of Pharmacology, and 20rg. Chem. Unit, Department of Pharmacology, University of GMeborg, P.O. Box 33031, S- 400 33 Gf~teborg, Sweden
Received 26 June 1984, revised MS received29 August 1984, accepted2 October 1984
S. HJORTH, A. CARLSSON, D. CLARK, K. SVENSSON and D. SANCHEZ, Dopamine receptor-mediated hypothermia induced in rats by ( + )-, but not by ( - )-3-PPP, European J. Pharmacol. 107 (1985) 299-304. The novel dopaminergic agents (+)- and (-)-3-PPP were evaluated for their effects upon thermoregulation in rats maintained at room temperature ( - 22°C). Although approximately 30 times less potent than apomorphine, ( +)-3-PPP induced a clearcut, dose-dependent and haloperidol/pimozide-reversible hypothermia. In contrast, the (-)-enantiomer per se lacked a significant effect upon rat body temperature. However, (-)-3-PPP clearly attenuated apomorphine-induced hypothermia. Simultaneous biochemical investigations confirmed the presence of central dopamine (DA) agonist and antagonist properties for (+)- and (-)-3-PPP, respectively, at the doses employed. The results are compared to the agonist and antagonist effects of the 3-PPP enantiomers in various other central DA receptor systems. Particular reference is made to the recent hypothesis by Carlsson (J. Neural Transm. 57 (1983) 309, relating agonist intrinsic activity to the DA receptor responsiveness state, in turn determined by the endogenous tone. Based on the findings with (+)- and (-)-3-PPP it is suggested that DA receptors mediating hypothermia in the rat may be more akin to 'normosensitive' postsynaptic than to highly 'agonist-responsive' autoreceptors. 3-PPP enantiomers
Dopamine receptors
Thermoregulation
1. Introduction
Dopamine (DA) appears to play an important modulatory role in the maintenance of thermoregulation (e.g. Cox, 1979). Thus, hypothermia is generally found in rats and mice treated with central DA receptor agonists (for refs. see Clark, 1979). Ample evidence also demonstrates that such hypothermia is readily antagonised by pretreatment with DA receptor blockers such as pimozide and haloperidol, confirming the dopaminergic nature of the response (Cox, 1979). Although it seems generally accepted that, in the rat, the medial preoptic hypothalamic area represents a critical target locus, DA receptors in other brain regions may also participate in the thermoregulatory events
* To whom all corresoondenceshould be addressed. 0014-2999/85/$03.30 © 1985 ElsevierSciencePublishers B.V.
Rat
(e.g. Brown et al., 1982). The relative involvement of DA auto- vs. postsynaptic receptors in body temperature control remains to be established. Recently, we have detailed the neurochemical and behavioural properties of the novel dopaminergic agent 3-(3-hydroxyphenyl)-N-n-propylpiperidine, 3-PPP and its enantiomers (for review see Hjorth, 1983). Whilst both are able to stimulate DA autoreceptors, ( + ) - and ( - ) - 3 - P P P concomitantly exert agonist and antagonist actions, respectively, on normosensitive postsynaptic DA receptors. However, not only the (+)-, but also the ( - )-enantiomer display significant agonist properties on presumed non-synaptic DA receptors, i.e. r e d u c t i o n of p l a s m a p r o l a c t i n levels in reserpine-pretreated rats (Eriksson et al,, 1983), rotation in unilaterally substantia nigra-lesioned (6-OH-DA) rats (Arnt et al., 1983) and emesis induction in dogs (Arnt et al., 1983; G. Paalzow,
300 personal communication). As pointed out by Carlsson (1983), it appears likely that the responses obtained in the different experimental models are related not only to the neuroanatomical location but also to the (basal) functional state of the receptors involved (see further Discussion section). That is, the agonist properties of (-)-3PPP and similar agents are more clearly evident in systems assumed to possess highly responsive DA receptors. By extension the hypothesis might also imply that, depending on the prior functional state, receptors may differ in their relative susceptibility to sensitisation vs. down-regulation of responsiveness. Interestingly, based on their resistance to supersensitivity induction and their easy desensitisation DA receptors mediating DA-agonist hypothermia have been suggested to be in a state of 'maximal functional sensitivity' (Schwartz et al., 1978). In view of the considerations, and in order to further characterise the overall pharmacological profiles of (+)- and (-)-3-PPP, the effects of the compounds on rat body temperature were investigated.
2. Materials and methods
2.1. Animals Male Sprague-Dawley rats (200-300 g; ALAB, Stockholm, Sweden) were used throughout the studies. The animals were kept under controlled standard environmental conditions (temp. 25°C; humidity -60%; lights on 6 a.m.-8 p.m.) for at least a week after arrival in the Department until used in the experiments. Food pellets (Ewos, Srdert~ilje, Sweden) and tap water were allowed ad lib.
2.3. Dissections and biochemical analyses Brain dissections were carried out according to Carlsson and Lindqvist (1973). The determinations of monoamines and their metabolites in the limbic (containing e.g. the olfactory tubercles, nucleus accumbens and nucleus amygdaloideus centralis) and striatal brain portions were performed by standard HPLC (EC detection) techniques or slight modifications thereof (Svensson and Lindberg, in preparation). 2.4. Drugs The drugs used in the study were (+)- and (-)-3-PPP HC1 (Wikstri3m et al., 1984), apomorphine HCI x 1/2 H20 (Sandoz, Basle, Switzerland), pimozide and haloperidol (Leo, Helsingborg, Sweden). Pimozide and haloperidol were dissolved in a minimal quantity of glacial acetic acid and made up to volume with 5.5% glucose solution, whereas the other compounds were dissolved in physiological (0.9%) saline (in the case of apomorphine a few grains of ascorbic acid were added to prevent oxidation). The injection volume was 5 ml/kg. For information on dosage, routes of administration and time schedules, see figure and table texts. 2.5. Statistics Student's t-test was employed and was preceded by an analysis of variance where appropriate; probability levels ~<5% were considered significant. 3. Results
3.1. Body temperature 2.2. Temperature measurements Body temperatures were monitored thermoelectrically (Yellow Springs Instr. Co. Inc., Ohio, U.S.A.; YSI Model 43 TK) 20-30 s after inserting the thermistor probe 6 cm into the rectum of lightly manually restrained rats. The experiments were always carried out between 8 and 12 a.m. and at ambient temperatures of 21-23°C.
Table 1 and fig. 1 show that (+)-, but not ( - )-3-PPP (6.8 mg = 27/~mol/kg s.c.), induced a clearcut hypothermia (1.2-2.1°C) in rats maintained at an ambient temperature of 21-23°C. Although not significantly different, this hypothermic response tended to be somewhat more pronounced at 60 than at 30 rain after administration, possibly indicating a contribution from a potential
301 TABLE 1 Effects of ( + )- and ( - )-3-PPP upon the rat rectal temperature. Interaction with haloperidol. Rats were given haloperidol (0.~ m g / k g i.p.) 30 rain prior to ( + ) - or ( - ) - 3 - P P P (6.8 m g / k g s.c.). Controls received appropriate vehicle injections at coi:responding time intervals. Rectal temperatures were monitored immediately before (t = 0) and 30 rain after the s.c, injections and immediately before killing (i.e. corresponding to 30, 60 and 90 min after haloperidol administration). The brains were subsequently dissected and analysed for monoamine and metabolite levels (see table 2 and Results section). Statistics: analysis of variance followed by t-test: a p < 0.001 from corresponding vehicle-treated group, and b p < 0.005 from corresponding (+)-3-PPP (only) group. Treatment
Vehicle Haloperidol(H) (+)-3-PPP H+(+)-3-PPP (-)-3-PPP H+(-)-3-PPP
Treatment interval (rain)
n
0
30
38.1+-0.11 38.35:0.07 38.2+-0.04 38.15:0.30 38.25:0.12 38.25:0.17
38.2+-0.15 38.1+0.09 4 38.3+0.12 38.2+0.11 4 37.1+0.20 a 36.1+0.25 a 4 38.2+0.24 b 38.4+0.20 b 4 38.05:0.11 38.25:0.13 4 37.95:0.17 38.25:0.15 4
60
active metabolite(s). In a separate experiment it was shown that pimozide pretreatment (0.5 mg/kg i.p~, 4 h before) blocked the hypothermic response to (+)-3-PPP (6.8 mg/kg s.c.) administration (temp. change 0-60 min; (+)-3-PPP: -1.60_+ 0.23°C, vs. pimozide/(+)-3-PPP: +0.08 _+ 0.11°C; n = 4-5, P < 0.001). Likewise, haloperidol pretreatment (0.5 mg/kg i.p., 30 rain before) completely abolished (+)-3-PPP-induced hypothermia, while alone or in combination with (-)-3-PPP it failed to influence rat body temperature (table 1). From the dose-response experiments in fig. 1 it is seen that lower doses of (+)-3-PPP were either completely ineffective (0.43-1.7 mg/kg s.c.) or produced, at best, a trend to hypothermia (3.4 mg/kg s.c.). No statistically significant temperature changes were, however, obtained over the entire time period studied (15-90 rain post-injection; data not shown). In contrast, the higher doses of (+)-3PPP (6.8 and 13.6 mg/kg s.c.) produced a clearcut °C
z~°C 0
58.5
-0.5
58.0
\\o_
:
.
\\ ~ o ~ / 9.
•
:
~°
. /
o/
zx ,//A
• 10
57.5 -1.5
57.0
\,,/
- 2.0
0.03 '
0.1 '
05'
I.'o
30 '
I0.0 ' 30.0 ' mg/kcj s.c.
Fig. 1. Dose-dependent hypothermia induced in the rat by apomorphine and (+)-3-PPP. The graph was prepared from data obtained in four separate experiments. Absolute temperature changes in individual vehicle (0.9% NaCl)-treated control animals ranged between - 0 . 3 and +0.5°C over the time period studies (data not shown; cf. also table 1). Rats were given apomorphine (0.03-1.0 m g / k g s.c.) (O ©) or ( + )3-PPP (0.43-13.6 m g / k g s.c.) (zx zx), The rectal temperatures were monitored immediately before and 30 min after drug injection. The mean absolute temperature changes (A°C) 0-30 min + S . E M . ( n = 3-8) are shown. Statistical comparisons: Student's t-test: *** P < 0.001 and ** P < 0.01 from corresponding control values.
56.5 ~)
I
I
20
I
410
I
610
I
810 min.
Fig. 2. ( - ) - 3 - P P P antagonism of apomorphine-induced hypothermia. Rats were given apomorphine (0.3 or 1.0 mg/kg) and ( - )-3-PPP (6.8 mg/kg) simultaneously s.c. on either side of the neck region. Controls received vehicle (0.9% NaCI) instead of ( - ) - 3 - P P P . The rectal temperatures were monitored immediately before and at 10 min intervals after drug injection. The mean temperatures (°C) are shown; n = 5 at each time point, standard errors omitted for clarity. O O Apomorphine 0.3/NaC1, • • apomorphine 0 . 3 / ( - )-3PPP, zx zx apomorphine 1.0/NaCI, • • apomorphine 1 . 0 / ( - ) - 3 - P P P . Statistical comparisons: Student's t-test: *** P < 0.001, **(*) P < 0.005, ** P < 0.01 and * P < 0.05 vs. corresponding apomorphine/NaCl treatment group.
302 TABLE 2 3-PPP effects on brain DA metabolism. Interaction with haloperidol. Treatment
Limbic
Striatum
DA
DOPAC
HVA (ng/g)
DA
DOPAC
HVA (ng/g)
Vehicle Haloperidol(H) (+)-3-PPP H+(+)-3-PPP (-)-3-PPP H +(-)-3-PPP
2140+_ 91 1791.+ 55 a 2171-+ 94 1684_+66 a'b 2048-+ 15 1827_+170a
370+_ 9 990.+ 31 a 223.+ 15 a
243.+41 622_+16a 108+ 5 ~ 634___43a'b 244.+16 592±95 a'b
7079.+506 6096-+305 7365.+510 5582_+318~'b 6667.+260 5967_+335
980.+ 28 3590-+ 92 a 635+ 30 a 3047_+352~'b 1463_+149a 3246±400 ~'b
769.+ 92 2242_+ 92 361+_ 44 a 2320_+ 66 a'b 1088_+94 a 2404±123 ~'b
784-+ 66 a'b
386_+ 11 794_+102a'b
n 4 4 4 4 4 4
For experimental detail see legend to table 1. Statistical comparisons (P < 0.05 or less): a significantly different from vehicle-treated. and b significantly different from corresponding 3-PPP (no haloperidol)-treated group. hypothermic response which was maximal (2.52.6°C) at about 1.5 h and was evident for approximately 4 h (13.6 m g / k g s.c.; data not shown). Apomorphine was roughly 30 times more potent than ( + ) - 3 - P P P in inducing hypothermia in the rat. Thus, while 0.03 m g / k g (s.c.) of apomorphine failed to alter body temperature, 0.1-1.0 m g / k g caused a dose-dependent hypothermia. The duration of the hypothermic action of apomorphine (1.0 m g / k g s.c.) was approximately 60-70 rain and the nadir ( - 1 . 7 ° C ) was reached about 30 min after administration. While ( - ) - 3 - P P P (0.43-6.8 m g / k g s.c.) did not significantly alter rat body temperature (table 1 and unpublished data), it clearly reduced DA agonist-induced hypothermia. Thus, the temperature decrease elicited by apomorphine (0.3 or 1.0 m g / k g , s.c.) was partly, though significantly, attenuated by concurrent treatment with ( - ) - 3 - P P P (6.8 m g / k g s.c.) (fig. 2). While the time course remained essentially unaltered, the amplitude of the apomorphine-induced hypothermic effect was blunted by ( - ) - 3 - P P P treatment. Thus, at 20-40 and 20-50 min after administration ( - ) - 3 - P P P significantly antagonised the decreases in body temperature caused by 0.3 and 1.0 m g / k g of apomorphine, respectively. The somewhat delayed ( - )-3-PPP antagonism had been seen during studies of the ability of ( - ) - 3 - P P P to counteract apomorphine-induced locomotor hyperactivity (Hjorth et al., 1983, and unpublished data). The body temperature experiments described do not differentiate between drug effects on heatgenerating vs. heat-loss systems as they were carried out only at ambient temperatures of - 22°C.
3.2. Brain monoamines and monoamine metabolites
Immediately following completion of the experiment shown in table 1, the rats were decapitated and the brain regions analysed for dopamine (DA) and its metabolites (DOPAC and HVA). As shown in table 2, ( - ) - 3 - P P P did not change the limbic but increased the striatal levels of D O P A C and HVA. On the other hand, in both regions these levels were decreased by ( + ) - 3 - P P P and increased by haloperidol. The (+)-3-PPP-induced D O P A C and HVA reductions were prevented by haloperidol. In fact, except for the slightly lower (P < 0.025) limbic D O P A C values seen after ( + )3 - P P P / h a l o p e r i d o l vs. haloperidol only, the D O P A C and HVA values in all haloperidol-treated groups were similar. The brain DA levels remained essentially unchanged by the treatments depicted. However minor (16-22%), and in four our of six cases significant, decreases occurred in the haloperidol-treated groups. Simultaneous assessments of the noradrenaline, 5-hydroxytryptamine and 5hydroxyindole acetic acid (5-HIAA) levels revealed no changes in any of the treatment groups (data not shown).
4. Discussion
The present study showed that (+)-3-PPP, similarly to the well-known DA agonist apomorphine, could induce a clearcut haloperidol/pimozide-reversible hypothermia when administered to rats maintained at room temperature ( - 2 2 ° C ) . In contrast, its enantiomeric twin, ( - ) - 3 - P P P , failed
303 to alter rat body temperature per se but significantly counteracted the apomorphine-induced hypothermic response. Consistent with these data are our observations that (-)-3-PPP was capable of antagonising the temperature-lowering effect of (-)-3-PPP (Hjorth, 1983; D.C., S.H. and K.S., unpublished results). Furthermore, confirming and extending previous findings, the biochemical results obtained in the present study (see table 2) may also be taken to illustrate the presence of dopaminergic agonist and antagonist properties of (+)- and (-)-3-PPP, respectively (Hjorth, 1983; Hjorth et al., 1983), at the doses used. While (+)-3-PPP appears to act as a full dopaminergic agonist in most experimental situations (e.g. Hjorth, 1983; cf. however, Arnt et al., 1983), the action of its (-)-counterpart is more complex. Thus, (-)-3-PPP displays either full agonist, partial agonist or weak antagonist effects depending on the test model applied. Generally, however, its DA agonistic properties seem to predominate in tests assessing presumed non-synaptic and/or 'supersensitive' DA receptor activity (cf. Introduction). Carlsson (1983) has recently proposed that the degree of previous agonist occupancy determines DA receptor responsiveness, and thereby in part the intrinsic activity of agents interacting with the receptors. That is, at DA receptors adapted to low or high agonist occupancy, compounds such as (-)-3-PPP would act as an agonist or antagonist, respectively. The (hypothalamic?) DA receptors implicated in the DA agonist-induced hypothermic response have been suggested to be in a state of 'maximal functional sensitivity' (Schwartz et al., 1978), possibly related to a minimum of endogenous tone (Cox and Lee, 1979). Given the adaptational characteristics of these receptors (easily desensitised but resistant to supersensitivity induction; Schwartz et al., 1978), they might be expected to fall into the highly agonist-responsive DA receptor category (exemplified by DA autoreceptors and non-synaptic receptors), according to Carlsson (1983). Similarly to the agonist activity displayed on the latter receptors (Arnt et al., 1983; Eriksson et al., 1983) both 3-PPP enantiomers should thus, like apomorphine, be able to elicit a clearcut hypothermic response in non-pretreated
rats. However, hypothermia was induced by (+)(>/13 /~mol = 3.4 mg/kg s.c.) but not by (-)-3PPP (3.4-27 /~mol/kg s.c.). This should be compared to the potencies for both enantiomers, e.g. in reducing plasma prolactin levels in reserpinised rats (EDs0s - 2/~mol/kg s.c.; Eriksson et al., 1983) or in producing contralateral rotation in unilaterally substantia nigra 6-OH-DA-lesioned rats (EDs0s 0.77-2.1 ~tmol/kg s.c.; Arnt et al., 1983). These responses are considered to reflect activation of presumed non-synaptic and 'denervationsupersensitive' postsynaptic, respectively, DA receptors. The doses used in the present studies thus fall well within the supramaximal range for eliciting responses from highly agonist-sensitive central DA receptors. Yet, only the (+), but not the ( - ) enantiomer induced the expected DA agonist response - hypothermia - under these conditions. It may therefore be inferred that the DA 'hypothermia' receptors do not belong to the highly DA agonist-responsive receptor category. Instead, (-)-3-PPP behaved as an antagonist, counteracting the apomorphine-induced hypothermic action. Similarly, (+)- and (-)-3-PPP act as agonist and antagonist, respectively, at 'normosensitive', low agonist-responsive (Carlsson, 1983) postsynaptic DA receptors associated with motor activation (Hjorth et al., 1983) and control of striatal ACh release (Plantj6 et al., 1983). It is also important to note the close correspondence between the (+)-3-PPP doses inducing dopaminergic behavioural (motor) activation, and those required for hypothermia (i.e. >/3.4 mg/kg s.c.). On the other hand, lower doses of the agent (0.43-1.7 mg/kg s.c.) failed to alter the body temperature of the rats (present data), despite clearly being sufficient to cause dose-dependent reductions in central DA synthesis and spontaneous locomotor activity (Hjorth et al., 1983). Similarly, a low 'DA autoreceptor' dose of apomorphine (0.03 mg/kg s.c.; cf. e.g. Argiolas et al., 1982; Montanaro et al., 1983) was without hypothermic effect. Apomorphine at a dose of 0.1 mg/kg (s.c.) induced significant but mild hypothermia, whereas the higher, excitatory (cf. e.g. Hjorth et al., 1983; Montanaro et al., 1983) doses (0.3-1.0 mg/kg s.c.) markedly decreased body temperature. The find-
304
ings with (+)-3-PPP and apomorphine are thus congruent, indicating a correspondence between the dose ranges required for hypothermia vs. postsynaptic DA receptor (but not DA autoreceptor) activation, as assessed in other experimental situations (cf. above). The lack of hypothermic action of (-)-3-PPP also favours this interpretation. Taken together, the present data indicate that the thermoregulatory DA receptors are more akin to classical 'normosensitive' postsynaptic DA receptors than to highly agonist-responsive receptors such as, e.g., the DA autoreceptors, the chronically denervated or the non-innervated DA receptors (cf. above). If however, as suggested by Schwartz et al. (1978), the thermoregulatory DA receptors indeed belong to the maximally agonist-responsive category, they may represent an exception from the proposed general principle relating receptor responsiveness to the degree of intrinsic activity for DA receptor-active agents with putative agonist qualities - like e.g. (-)-3-PPP (Carlsson, 1983). Alternatively, (-)-3-PPP might have other as yet unidentified effects obscuring any putative hypothermic action. Further investigations, including studies of 3-PPP enantiomer activities on DA receptors under different synaptic conditions, are clearly needed to resolve these issues.
Acknowledgements The expert technical assistance of Gun Andersson, Maria Lindb~ck, Pia Lisj6 and Lena L6fberg is gratefully acknowledged. This study was financially supported in part by grants from AB Astra, the Swedish Board for Technical Development, 'Magnus Bergvalls Fond', 'Sv. Lakares~llskapet' and the Medical Faculty. Univ. of G6teborg. D.C. was supported by a postdoctoral fellowship from the Science and Engineering Research Council (U.K.).
References Argiolas, A., G. Mereu, G. Serra, M.R. Melis, F. Fadda and G.L. Gessa, 1982, N-n-propyl-norapomorphine: an extremely potent stimulant of dopamine autoreceptors, Brain Res. 231, 109.
Arnt, J., K.P. BOges6, A.V. Christensen, J. Hynel, J.-J. Larsen and O. Svendsen, 1983, Dopamine receptor agonistic and antagonistic effects of 3-PPP enantiomers, Psychopharmacology 81, 199. Brown, S.J., C.V. Gisolfi and F. Mora, 1982, Temperature regulation and dopaminergic systems in the brain: does the substantia nigra play a role?, Brain Res. 234, 275. Carlsson, A., 1983, Dopamine receptor agonists: Intrinsic activity vs. state of the receptor, J. Neural Transm. 57, 309. Carlsson, A. and M. Lindqvist, 1973, Effect of ethanol on the hydroxylation of tyrosine and tryptophan in rat brain in vivo, J. Pharm. Pharmacol. 25, 437. Clark, W.G., 1979, Changes in body temperature after administration of amino acids, peptides, dopamine, neuroleptics and related agents, Neurosci. Biobehav. Rev., 3, 179. Cox, B., 1979, Dopamine, in: Body Temperature Regulation, Drug Effects and Clinical Implications, eds. P. Lomax and E. Schonbaum (Marcel Dekker, New York) p. 231. Cox, B. and T.F. Lee, 1979, Effect of central injections of dopamine on core temperature and thermoregulatory behaviour in unrestrained rats, Neuropharmacology 18, 537. Eriksson, E., K. Modigh, A. Carlsson and H. Wikstr6m, 1983, Dopamine receptors involved in prolactin secretion pharmacologically characterized by means of 3-PPP enantiomers, European J. Pharmacol. 96, 29. Hjorth, S., 1983, On the mode of action of 3-(3-hydroxyphenyl)-N-n-propylpiperidine, 3-PPP, and its enantiomers, Acta Physiol. Scand. 118, Suppl. 517 (thesis), 1. Hjorth, S., A. Carlsson, D. Clark, K. Svensson, H. Wikstr6m, D. Sanchez, P. Lindberg, U. Hacksell, L.-E. Arvidsson, A. Johansson and J.L.G. Nilsson, 1983, Central dopamine receptor agonist and antagonist actions of the enantiomers of 3-PPP, Psychopharmacology 81, 89. Montanaro, N., A. Vaccheri, R. Dall'Olio and O. Gandolfi, 1983, Time course of rat motility response to apomorphine: a simple model for studying preferential blockade of brain dopamine receptors mediating sedation, Psychopharmacology 81,214. Plantj6, J.F., A.H. Mulder and J.C. Stoof, 1983, Effects of the enantiomers of 3-PPP on D-1 and D-2 receptors in the rat neostriatum in vitro, Pharm. Weekbl. Sci. Ed. 5, 265. Schwartz, J.C., J. Costentin, M.P. Martres, P. Protais and M. Baudry, 1978, Modulation of receptor mechanisms in the CNS: hyper- and hyposensitivity to catecholamines, Neuropharmacology 12, 665. WikstrOm, H., D. Sanchez, P. Lindberg, U. Hacksell, L.-E. Arvidsson, A. Johansson, S.-O. Thorberg, J.L.G. Nilsson, K. Svensson, S. Hjorth, D. Clark and A. Carlsson, 1984, Resolved 3-(3-hydroxyphenyl)-N-n-propylpiperidine and its analogues - central dopamine receptor activity, J. Med. Chem. 27, 1030.