Behavioral and radioligand binding evidence for irreversible dopamine receptor blockade by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline

N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ), an irreversible alpha adrenergic antagonist, also acts as a potent and longlasting in vivo antagonist of D-2 dopamine receptors. Rats given EEDQ 3-10 mg/kg i.p. exhibit catalepsy and greatly reduced apomorphine-induced stereotypy, behavioral effects associated with D-2 dopamine receptor blockade. These effects are apparent up to 4 days after drug administration, with scores returning to control level by day 7. In vitro receptor binding assays of striatal membrane preparations from these animals using the radioligand 3H-spiroperidol directly demonstrate that EEDQ is a potent D-2 dopamine receptor antagonist, revealing the apparent basis of the behavioral effects of EEDQ. This antagonism proceeds via a reduction in D-2 receptor Bmax, with no change in the observed KD for 3H-spiroperidol, and is resistant to extensive washing of the membrane preparation after in vivo EEDQ exposure. These observations suggest that EEDQ inhibition of D-2 receptors is irreversible. Administration of behaviorally active doses of EEDQ effect a reduction of 50-85% in D-2 receptor number. Recovery of this loss roughly parallels recovery of normal catalepsy and apomorphine stereotypy scores. These doses of EEDQ also reduce binding of 3H-flupentixol to D-1 and 3H-dopamine to D-3 type dopaminergic binding sites, putative dopamine receptors with no known behavioral correlates. Recovery of D-1 and D-3 binding also occurs with a similar timecourse. Because of the apparent covalent nature of its interaction with dopamine receptors and because of its activity after peripheral administration, EEDQ may prove useful in the study of the function and turnover of dopamine receptors.     

Chronic estradiol treatment increases ovariectomized rat striatal D-1 dopamine receptors

Striatal D-1 dopamine (DA) receptors were investigated following chronic 17 beta-estradiol (10 micrograms, b.i.d., s.c., for two weeks) to ovariectomized (OVX) female rats. This treatment initiated the day after ovariectomy has revealed that the maximal density in homogenates of striatal D-1 DA receptors (Bmax) labelled with [3H] SCH 23390 was increased (44% without and 28% with 120 mM NaCl in the assay buffer). Estradiol treatments initiated 2 or 4 weeks after ovariectomy did not induce D-1 DA receptor binding modifications. The affinity (Kd) of the ligand for the receptor remains unchanged by the steroid treatment while NaCl increased both the density and the affinity of [3H] SCH 23390 binding to striatal D-1 DA receptors. By autoradiography, the increase of striatal [3H] SCH 23390 binding to D-1 DA receptors after chronic estradiol treatment was found to be homogenously distributed in this brain region. Thus, chronic treatment with estradiol of ovariectomized rats leads to an increased density of striatal D-1 DA receptors but, this hormonal modulation of D-1 DA receptors is lost when treatment is started 2 weeks after ovariectomy or later.     

Permissive role of D-1 receptor stimulation for the expression of D-2 mediated behavioral responses: a quantitative phenomenological study in rats

The syndrome of behavioral stimulation induced in male Sprague-Dawley rats by two dopaminergic agents was studied by distinguishing specific behavioral items and quantifying them in terms of their incidence. The specific D-2 agonist LY 171555 elicited yawning, genital grooming, exploratory behavior, downward sniffing and licking but failed to induce gnawing even at high doses. On the other hand, the D-1/D-2 agonist apomorphine elicited the full stereotyped syndrome including gnawing. Depletion of endogenous dopamine (DA) by alpha-methyltyrosine (alpha-MT) prevented the ability of LY 171555 to elicit all the items of behavioral stimulation including the stereotyped ones (sniffing and licking). In contrast, the ability of apomorphine to induce stereotypies was not reduced by depletion of endogenous DA by alpha-MT pretreatment. Blockade of D-1 receptors with SCH 23390 abolished the capacity of both LY 171555 and apomorphine to elicit all the items of behavioral stimulation. In alpha-MT pretreated rats, administration of low doses of the D-1 agonist SKF 38393 (2.5 mg/kg s.c.) reinstated the ability of LY 171555 to elicit behavioral stimulation and eventually conferred the ability of inducing gnawing. The results support the hypothesis that stimulation of D-1 receptors exerts a permissive role for the expression of behavioral stimulation following D-2 receptor stimulation. Endogenous DA appears to provide sufficient D-1 input to permit full expression of yawning, genital grooming, exploratory behavior, downward sniffing and licking following D-2 stimulation; pharmacological stimulation of D-1 in addition to D-2 receptors seems however necessary for full expression of the highest rank stereotypy item, gnawing.     

Behavioral recovery after irreversible inactivation of D-1 and D-2 dopamine receptors

Irreversible inactivation of both D-1 and D-2 dopamine (DA) receptors by N-ethoxycarbonyl-2-ethoxy-1,2-dihydroquinoline (EEDQ) resulted in complete loss of stereotypy response to R-(-)-N-propylnorapomorphine (NPA; 0.1-1.0 mg/kg, s.c.) 24 hr later. Stereotyped sniffing recovered much more rapidly than oral behaviors. The D-2 antagonist sulpiride (200 mg/kg) and the putatively nonselective antagonist cis-flupenthixol (2 mg/kg), administered prior to EEDQ, prevented the loss of NPA-induced sniffing but only partially protected against loss of oral behaviors 24 hr later. Complete protection of both behaviors was seen after pretreatment with a combination of sulpiride and the selective D-1 antagonist SCH 23390 (1 mg/kg); pretreatment with the selective D-1 antagonist SCH 23390 alone, however, did not modify the rate of recovery of either behavioral response. The results suggest that either different populations of DA receptors mediate expression of these behaviors or stimulation of a small fraction of the total DA receptor pool may be sufficient to elicit sniffing but not oral responses. Furthermore, maintaining a normal complement of D-2 rather than D-1 receptors appears to be a critical determinant for the elicitation of these behaviors.     

Effects of isomers of apomorphines on dopamine receptors in striatal and limbic tissue of rat brain

The optical isomers of apomorphine (APO) and N-propylnorapomorphine (NPA) were interacted with three biochemical indices of dopamine (DA) receptors in extrapyramidal and limbic preparations of rat brain tissue. There were consistent isomeric preferences for the R(-) configuration of both DA analogs in stimulating adenylate cyclase (D-1 sites) and in competing for high affinity binding of 3H-spiroperidol (D-2 sites) and of 3H-ADTN (DA agonist binding sites) in striatal tissue, with lesser isomeric differences in the limbic tissue. The S(+) apomorphines did not inhibit stimulation of adenylate cyclase by DA. The tendency for greater activity or higher apparent affinity of R(-) apomorphines in striatum may reflect the evidently greater abundance of receptor sites in that region. There were only small regional differences in interactions of the apomorphine isomers with all three receptor sites, except for a strong preference of (-)NPA for striatal D-2 sites. These results do not parallel our recent observations indicating potent and selective antidopaminergic actions of S(+) apomorphines in the rat limbic system. They suggest caution in assuming close parallels between current biochemical and functional, especially behavioral, methods of evaluating dopamine receptors of mammalian brain.     

Two dopamine receptors: biochemistry, physiology and pharmacology

In 1979, two categories of dopamine (DA) receptors (designated as D-1 and D-2) were identified on the basis of the ability of a limited number of agonists and antagonists to discriminate between these two entities. In the past 5 years agonists and antagonists selective for each category of receptor have been identified. Using these selective drugs it has been possible to attribute the effects of DA upon physiological and biochemical processes to the stimulation of either a D-1 or a D-2 receptor. Thus, DA-induced enhancement of both hormone release from bovine parathyroid gland and firing of neurosecretory cells in the CNS of Lymnaea stagnalis has been attributed to stimulation of a D-1 receptor. Likewise, the DA-induced inhibition of the release of prolactin and alpha-MSH from the pituitary gland, as well as of acetylcholine, DA and beta-endorphin from brain, the DA-induced inhibition of chemo-sensory discharge in rabbit carotid body and the DA-induced hyperpolarization of neurosecretory cells in the CNS of Lymnaea stagnalis have been attributed to stimulation of a D-2 receptor. Independently two categories of DA receptors (designated as DA-1 and DA-2) were identified in the cardiovascular system. Stimulation of a DA-1 receptor increases the vascular cyclic AMP content and causes a relaxation of vascular smooth muscle in renal blood vessels, whereas stimulation of a DA-2 receptor inhibits the release of norepinephrine from certain postganglionic sympathetic neurons. Recent studies with the newly developed drugs discriminating between D-1 and D-2 receptors suggest however that the independently developed schemata for classification of dopamine receptors in either the central nervous and endocrine systems or the cardiovascular system are similar although maybe not completely identical.     

Interactions of novel dopaminergic ligands with D-1 and D-2 dopamine receptors

The interactions of three novel dopaminergic ligands, SKF38393, SKF82526 and SKF83742, with D-1 and D-2 dopamine (DA) receptors have been investigated using radioligand binding techniques and computer modeling procedures. Using the bovine anterior pituitary D-2 DA receptor system, SKF38393 and SKF82526 behave as agonists demonstrating biphasic agonist/3H-antagonist competition curves. For both drugs, the high affinity phase comprised 30% of the total displacement curve. Such findings are atypical as previously tested classical dopamine agonists demonstrated high and low affinity displacement phases in equal proportions. Such behavior exhibited by the SKF agonists may be related to their activity as partial agonists. In contrast, SKF83742 behaves as an antagonist exhibiting homogeneous monophasic competition curves. Similar results are obtained in the rat striatal membrane D-2 DA receptor system. Both SKF38393 and SKF82526 also demonstrate shallow biphasic displacement curves on rat striatal D-1 receptors labeled with 3H-flupentixol whereas SKF83742/3H-flupentixol curves are uniphasic. Of all the ligands, only SKF38393 clearly demonstrates higher affinity for 3H-flupentixol labeled D-1 receptors.     

Continued administration of GM1 ganglioside is required to maintain recovery from neuroleptic-induced sensorimotor deficits in MPTP-treated mice

Injection of a dose of haloperidol that has no obvious behavioral effects in normal mice, produces akinesia, catalepsy, and sensory neglect in MPTP-treated mice. Chronic GM1 ganglioside administration improves the behavioral impairments, partially restores striatal dopamine (DA) content and prevents DA D-2 receptor up-regulation. Discontinuation of GM1 ganglioside treatment results in a time-dependent decline of striatal DA content to pretreatment pathological levels, return of haloperidol-induced sensorimotor deficits and a rise of DA D-2 receptor density in the striatum. Apparently, continuous administration of GM1 ganglioside is necessary to maintain the biochemical and behavioral recovery in the MPTP-treated mouse. These observations may provide useful cues for understanding the mechanism of action of GM1 ganglioside.     

3H-dopamine binding to rat striatal D-2 and D-3 sites: Enhancement by magnesium and inhibition by guanine nucleotides and sodium

Previous studies have demonstrated high affinity ³H-dopamine binding sites on mammalian striatal membranes. These putative dopamine receptors of unknown physiological significance have been termed D-3 sites. Such studies have failed, however, to demonstrate high affinity ³H-dopamine binding to D-2 sites, which can be labeled by ³H-butyrophenones, and which represent the putative dopamine receptors most stronly implicated in the behavioral correlates of dopaminergic CNS activity. We now report that preincubation of membrane homogenates with Mg⁺⁺ and inclusion of Mg⁺⁺ (1–10mM) or other divalent metal cations during binding allows high affinity D-2 specific ³H-dopamine binding in rat striatal membranes, and that these ions also increase the Bmax of D-3 specific ³H-dopamine binding. GTP, GDP, and GppNHp can completely abolish all D-2 specific ³H-dopamine binding, while only a magnesium-dependent portion of D-3 sites appears to be GTP sensitive. These data are consistent with the hypothesis that the striatal D-2 receptor exists in two agonist affinity states whose interconversion is effected by guanine nucleotides and divalent metal cations. The GTP sensitive/magnesium dependent nature of ³H-dopamine binding to so-called D-3 sites suggests that some such sites may in fact represent a high agonist-affinity state of the D-1 adenylate cyclase stimulating dopamine receptor also found in this tissue.     

Cyclo (Leu-Gly) + haloperidol: effects on dopamine receptors and conditioned avoidance responding

Behavioral effects of cyclo (Leu-Gly) (cLG), administered either acutely or chronically, were assessed in combination with haloperidol in the rat. cLG administered chronically, produced a significant reduction in the increase in apomorphine-induced stereotypy produced by chronic haloperidol infusion. On the other hand, the same dose of cLG which reduced this induction of dopamine receptor supersensitivity due to chronic haloperidol treatment, failed to produce a change in the potency of haloperidol in blocking conditioned avoidance responding in the rat. Furthermore, degeneration-induced supersensitivity of dopamine neurons, produced by unilateral destruction of the nigrostriatal pathway, was not reduced by acute or chronic treatment with cLG as measured by apomorphine-induced rotation. These data suggest that cLG may decrease motor system side effects thought to be caused by chronic antipsychotic administration without affecting the therapeutic efficacy of the antipsychotic agent.     

Dopaminergic 3H-agonist receptors in rat brain. New evidence on localization and pharmacology

Recent methodological advances have allowed the reliable assay of specific dopaminergic 3H-agonist binding sites in rat striatum. Successful assay depends on preincubation of tissue homogenates at 37 degrees C; this results in a guanyl nucleotide-sensitive and dopamine (DA)-dependent increase in the density (Bmax) of 3H-agonist binding. Lesions of DA terminals or drugs which deplete DA levels prevent the preincubation-induced increase in binding, and this effect is completely reversible by preincubation with added DA. In contrast, kainic acid lesions irreversibly reduce 3H-agonist binding. It is concluded that the evidence supporting the existence of presynaptic "D-3" sites is artefactual and that 3H-DA binding sites are more likely related to post-synaptic receptors. 3H-DA binding involves two sites, one of which has pharmacologic properties similar to D-1 receptors, whereas the other resembles D-2 receptors. The affinity of 15 antipsychotic drugs for 3H-haloperidol binding sites was highly correlated (R = 0.94) with their inhibitory potency at a subset of 3H-DA binding sites. However, the inhibition of 3H-DA binding by antipsychotic drugs was noncompetitive. These findings can be explained by an allosteric model, whereby antagonists bind to a site different from but allosterically linked to a high-affinity 3H-DA binding site.     

Localization and pharmacology of some dopamine receptor complexes in the striatum and the pituitary gland: synaptic and son-synaptic communication

The striatum receives massive dopaminergic projections from neurons in the ventral tegmental area, the substantia nigra and the retro-rubral cell group. Dopaminergic neurons in the arcuate nucleus and periventricular hypothalamic nuclei project to the median eminence and the neuro-intermediate lobe of the pituitary gland. The anterior lobe of the pituitary gland is not innervated by dopaminergic neurons, but receives dopamine via a vascular route from the median eminence. Two categories of dopamine receptors (D-1 and D-2) can be identified on the basis of the ability of various drugs to discriminate between these two entities. Dopamine stimulates both D-1 and D-2 receptors. The affinity of dopamine for the D-2 receptor is approximately 1000 times higher than for the D-1 receptor. Dopamine is involved in synaptic as well as non-synaptic communication. Examples of non-synaptic communication via D-2 receptors are the dopamine induced inhibition of prolactin release from the anterior pituitary gland and most likely the D-2 receptor mediated inhibition of the release of acetylcholine in the striatum. Examples of synaptic communication have been found in the striatum where (with ultrastructural techniques) synaptic contacts between dopaminergic nerve terminals and elements from cells containing GABA, substance P or enkephalin have been demonstrated. It is tempting to speculate that synaptic and non-synaptic communication occurs via D-1 and D-2 receptors respectively.     

Chronic amphetamine alters D-2 but not D-1 agonist-induced behavioral responses in rats

In two experiments, using different drug doses and periods of drug administration, rats were given amphetamine (AMPH) either continuously (via slow-release pellets), or intermittently (via injections). In both experiments, only the rats pretreated with intermittent AMPH subsequently showed heightened responsivity to the selective D-2 dopamine agonist LY171555 but not to SKF38393 (a D-1 agonist). This altered response to LY171555 was still present 30 days after the AMPH withdrawal, implying that D-2 dopamine receptors at least partially mediate AMPH inverse tolerance effects. The behavioral response to the D-2 agonist was clearly different in animals receiving high versus low doses of AMPH, suggesting that different drug-state learning may have occurred during pretreatment. In a third experiment, in which rats were given repeated daily injections of either the D-1 or the D-2 agonist, only rats pretreated with the D-2 agonist and subsequently injected with the D-2 agonist clearly showed heightened responsivity. These data imply an important role of D-2 receptors in the AMPH inverse tolerance effect.     

Modification of morphine-induced change in striatal (3H)-spiroperidol binding and stereotype behavior by cyclo(Leu-Gly)

Recent reports from our laboratories have indicated that the peptide cyclo(Leu-Gly), an analog of MIF (Pro-Leu-Gly-NH₂), administered prior to chronic exposure to morphine, prevents the development of both analgesic tolerance and some signs of physical dependence. The same peptide treatment also prevented the development of morphine-induced increases in certain behavioral responses to the dopamine agonist apomorphine. The present study investigated behavioral (stereotypy) and neurochemical receptor changes (specific (³H)-spiroperidol binding) occuring in the rat striatal dopamine (DA) system following chronic morphine treatment with and without prior cyclo(Leu-Gly)administration. While chronic morphine treatment (s.c. 5 pellet implant for 3 days, each pellet contained 65 mg morphine free base) did not alter the total number of high-affinity striatal (³H)-spiroperidol binding sites (28 fmol/mg tissue), it did increase the affinity of the receptor for the ligand (K_D decreased from 40 to 24 pM). Cyclo(Leu-Gly) (8 mg/kg) prevented the morphine induced increase in dopamine receptor affinity. In parallel, cyclo(Leu-Gly) prevented the increase in apomorphine-induced stereotypy which was observed in chronic morphine treated rats. The peptide alone did not alter any of the binding characteristics. These data suggest that the ability of the peptide to block the development of physical dependence induced by morphine may involve the ability of the peptide to interfere with morphine-induced changes in dopaminergic systems.     

Pharmacological effects of a specific dopamine D-1 antagonist SCH 23390 in comparison with neuroleptics

Neuroleptics such as thioxanthenes (cis($\text{Z}$)-flupentixol and cis($\text{Z}$)-clopenthixol) and phenothiazines (fluphenazine and perphenazine), which block both dopamine (DA) D-1 and D-2 receptors and the butyrophenones (haloperidol and spiroperidol), which block D-2 receptors only, are equipotent both behaviorally and clinically. A new compound SCH 23390 which selectively blocks DA D-1 receptors, resembles many neuroleptics in its pharmacological profile: antistereotypic effects in mice, rats and dogs, cataleptogenic effect and inhibitory effect on amphetamine circling. In contrast SCH 23390 has no effect on apomorphine-induced vomiting in dogs and little effects on 6-OHDA-denervated supersensitive DA receptors, stimulated by the DA agonist 3-PPP. In a series of experiments where methylphenidate-induced stereotyped gnawing in mice was inhibited by neuroleptics, it was shown that concomitant treatment with scopolamine or diazepam attenuated the effect of butyrophenones (D-2 antagonists). The same treatment attenuated the effect of phenothiazines, to a lesser extent, and hardly attenuated the effect of thioxanthenes and SCH 23390 at all. It is concluded that DA D-1 receptors are as important as D-2 receptors for the expression of neuroleptic activity in most animal models believed to be predictive of antipsychotic and extrapyramidal side-effect potential. However, the D-1 antagonist is less sensitive than D-2 antagonists to antimuscarinic compounds and benzodiapines.     

Effects of zotepine, chlorpromazine and haloperidol on D-1, D-2, D-3 and D-4 subtypes of dopamine receptors in rat striatal and bovine caudate nucleus membranes

Inhibitory effects of zotepine (Zot) on D-1, D-2, D-3 and D-4 subtypes of dopamine (DA) receptors were investigated in crude synaptic membranes of rat striatum and bovine caudate nucleus and compared to those of chlorpromazine (CPZ) and haloperidol (HAL). From the IC₅₀-values of Zot, CPZ and HAL, the K-values of each drug are estimated as follows: 34.4, 152 and 244 nM (D-1, ³H-labeled cis-flupenthixol binding (1.0 nM) to rat membranes); 37.4, 7.1 and 2.4 nM (D-2, [³H]spiperone (Spi) binding (0.5 nM) to rat membranes in the presence of 0.1 μM ketanserin); 73.1, 15.2 and 22.4 nM (D-3, ³H-labeled N-propylapomorphine (NPA) binding (0.29 nM) to bovine membranes in the presence of 0.1 μM Spi); 9.5, 65.3 and 3.1 nM (D-4, [³H]NPA binding (0.29 nM) bovine membranes in the presence of 25 nM DA), respectively. Zot binds with higher affinity to D-4 but lower affinity to D-3 than to other subtypes. It is also presumed that Zot binds to D-1 with high affinity and D-2 and D-3 with low affinity compared to CPZ and HAL.     

Modulation of [3H]dopamine release from rabbit retina

The calcium-dependent release of [3H]dopamine ([3H]DA) elicited by field stimulation or potassium is modulated through activation of stereoselective inhibitory DA autoreceptors of the D-2 subtype that are pharmacologically different from the D-1 DA receptor subtype linked to the stimulation of adenylate cyclase (EC 4.6.1.1). The D-2 DA autoreceptors appear to be endogenously activated by DA because DA receptor antagonists such as S-sulpiride increased the stimulation-evoked release of [3H]DA. Nanomolar concentrations of norepinephrine (NE) and epinephrine (E) inhibited in a concentration-dependent manner the electrical stimulation-evoked release of [3H]DA. The inhibitory effect of these catecholamines was not modified by S-sulpiride, which, on the contrary, selectively antagonized the inhibition of [3H]DA release elicited by exogenous DA. Phentolamine or (+/-)-propranolol did not affect the release of [3H]DA from rabbit retina. The alpha antagonist phentolamine competitively antagonized the inhibitory effect of both NE and E, which suggests that these catecholamines activate alpha receptors in retina. The decrease by catecholamines of the calcium-dependent release of [3H]DA appears not to involve beta adrenoceptors because their inhibitory effect was not modified by propranolol. Under identical experimental conditions (i.e., nomifensine, 30 microM), serotonin did not modify the stimulated release of [3H]DA. In conclusion, in the rabbit retina, DA autoreceptors of the D-2 subtype appear to modulate endogenously released DA whereas inhibitory presynaptic alpha receptors might be of pharmacological importance as sites of action for retinal or blood-borne catecholamines.     

Striatal D-2 dopamine agonist binding sites fluctuate during the rat estrous cycle

Striatal D-2 dopamine (DA) antagonist and agonist binding sites were measured during the rat estrous cycle and compared to ovariectomized (OVX) rats. Dopaminergic D-2 antagonist binding sites were constant during the estrous cycle while agonist binding sites show a rapid and significant decrease of the ratio of high to low D-2 agonist binding sites from proestrus AM (PAM) to diestrus 1 (D1) and return to OVX value in diestrus 2 (DII). Thus, physiological fluctuations of hormones as occur during the estrous cycle can modulate extrahypothalamic biogenic amine activity, namely striatal DA systems which are not involved in the control of hormone secretion.     

Novel 8 alpha-ergolines with inhibitory and stimulatory effects on prolactin secretion in rats

Four derivatives of the ergot dopamine (DA) agonist lisuride (LIS), namely 6-n-propyl-lisuride (6-n-propyl-LIS), transdihydrolisuride (TDHL), 6-n-propyl-transdi-hydrolisuride (6-n-propyl-TDHL) and 2-bromolisuride (2-Br-LIS) were investigated in female rats with regard to their influence on hyperprolactinaemia induced by pretreatment with reserpine (2 mg/kg i.p., 24 h) at various intervals following their subcutaneous or oral administration (0.05 mg/kg). Two hours after administration, LIS, 6-n-propyl-LIS, and 6-n-propyl-TDHL caused a statistically significant inhibition of reserpine-induced hyperprolactinaemia of about the same extent. Eight hours after administration 6-n-propyl-LIS and 6-n-propyl-TDHL were as active as after 2 h in inhibiting prolactin (PRL) secretion whereas LIS was almost ineffective in this respect. TDHL caused a statistically significant inhibition of PRL secretion at 2 and 8 h after oral administration; this effect was less pronounced after s.c. administration. In contrast to the aforementioned derivatives 2-Br-LIS further increased the reserpine-induced hyperprolactinaemia. In normal male rats pretreatment with 2-Br-LIS (0.025-6.25 mg/kg s.c., 2 h) dose-dependently stimulated PRL secretion. The present data support the assumption of the longlasting DA agonistic action of 6-n-propyl-LIS and 6-n-propyl-TDHL and of the antidopaminergic properties of 2-Br-LIS recently derived from behavioural studies.     

Amphetamine-induced in vivo release of dopamine in the striatum is influenced by local pH

The effect of local pH on the in vivo efflux of endogenous dopamine (DA) and 3,4-dihydroxyphenylacetic acid (DOPAC) following administration of d-amphetamine (AMPH) was examined in the striatum of the anesthetized rat using two bilaterally placed push-pull cannulae. At both pH 7.3 and 6.4, the baseline efflux values for DA and DOPAC were approximately 0.2 and 25 pmoles/15 min, respectively. Subcutaneous injection of 2 mg/kg AMPH induced a 3-fold increase of DA release at pH 7.3 and a 21-fold increase of DA release at pH 6.4. In both cases, the maximum was reached at about 30 min after the drug administration. Following the administration of AMPH, the efflux of DOPAC was reduced to the same degree (20% of control values) under both pH conditions. In vitro data showed that the lower pH did not alter the recovery of DA or DOPAC. In addition, release of DA produced by local perfusion with 5 uM AMPH was also greater at the lower pH (50-fold increase over baseline) than at the physiological pH (10-fold increase over baseline). The stimulated DA release produced by local perfusion with 35 mM K+, however, was the same at both pH values. Preliminary experiments also indicated that there was a pH effect for AMPH-induced serotonin (5-HT) release but that the difference in the amount of 5-HT in the two media was not nearly as large as that obtained for DA. The markedly elevated level of extracellular DA at the lower pH might be due to a higher affinity of the DA uptake system for AMPH, thereby producing greater inhibition of DA uptake as well as enhanced DA release. The data also suggest an enhanced affinity of AMPH for 5-HT uptake sites at the lower pH.