Effect of cyclo(Leu-Gly) on the supersensitivity of dopamine receptors in spontaneously hypertensive rats

Spontaneously hypertensive rats exhibited dopamine receptor supersensitivity as evidenced by a greater hypothermic response to apomorphine in comparision with normotensive Wistar-Kyoto rats. A single injection of cyclo(Leu-Gly) given prior to apomorphine administration did no alter apomorphine induced hypothermia in either the normotensive or the hypertensive rats. Chronic administration of cyclo(Leu-Gly) for 7 days did not affect apomorphine response in normotensive rats, but blocked the exaggerated response to apomorphine in the hypertensive rats. These studies suggest that cyclo(Leu-Gly) interacts with the dopamine receptors and that the central dopamine receptors may play a role in the pathophysiology of hypertension.     

Enhanced response to apomorphine in rats treated with multiple injections of human beta endorphin and its blockade by Pro-Leu-Gly-NH2 and cyclo (Leu-Gly)

Repeated intraventricular injections of human β-endorphin to rats every eight hours for three days resulted in the development of super-sensitivity of brain dopamine receptors as evidenced by an enhanced locomotor activity response to apomorphine, a dopamine receptor agonist. Daily subcutaneous injections of Pro-Leu-Gly-NH₂ or its cyclic analog, cyclo (Leu-Gly) blocked the enhanced apomorphine-induced stimulation of locomotor activity in β-endorphin treated rats. Thus, the development of brain dopamine receptor supersensitivity induced by chronic β-endorphin administration may be regulated by the hypothalamic peptides.     

Enhanced striatal 3H-spiroperidol binding induced by chronic haloperidol treatment inhibited by peptides administered during the withdrawal phase

Chronic intragastric administration of haloperidol (1.5 mg/kg/day) for 21 days followed by a 3-day withdrawal period resulted in the development of enhanced locomotor activity response to apomorphine, and an increase in the number of binding sites for 3H-spiroperidol in the striatal membranes of the rat brain. Subcutaneous administration of Pro-Leu-Gly-NH2 or cyclo(Leu-Gly) in doses of 2 mg/kg/day given for 3-days after termination of haloperidol treatment inhibited the enhanced response to apomorphine, as well as the increases in the number of 3H-spiroperidol binding sites in the striatum. If indeed, the supersensitivity of striatal dopamine receptors is one of the mechanisms in the development of tardive dyskinesia symptoms, the present results suggest that the above peptides may be helpful in ameliorating some of the symptoms of tardive dyskinesia induced by neuroleptic drugs.     

Effects of cyclo (Leu-Gly) on neurochemical indices of striatal dopaminergic supersensitivity induced by prolonged haloperidol treatment

The effects of a prolonged treatment with cyclo (Leu-Gly) and/or haloperidol on biochemical parameters indicative of striatal dopamine target cell supersensitivity have been investigated in the rat. When given acutely, cyclo (Leu-Gly) (2 mg/kg sc) did not affect striatal homovanillic acid, dihydroxyphenylacetic acid and acetylcholine levels both under basal conditions or after acute haloperidol (1 mg/kg ip) treatment. When given concomitantly with haloperidol (infused by means of osmotic minipumps at a rate of 2.5 μg/h sc) for 14 days, cyclo (Leu-Gly) (2 mg/kg sc once daily) failed to prevent the fall of striatal dopamine metabolites observed 2 days following withdrawal and the tolerance to the elevation of dopamine metabolites which occurs in response to challenge with the neuroleptic during withdrawal. Prolonged treatment with cyclo (Leu-Gly) also failed to affect the tolerance to the decrease of striatal acetylcholine levels which occurs under chronic haloperidol treatment. These data suggest that the mechanism whereby cyclo (Leu-Gly) inhibits the development of neuroleptic-induced dopaminergic supersensitivity does not involve an action of the peptide on nigro-striatal dopaminergic and striatal cholinergic neurons and is probably exerted distally to both dopaminergic and cholinergic synapses.     

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.     

Inability of cyclo (Leu-Gly) to facilitate the development of tolerance to and physical dependence on morphine in the rat

The effect of cyclo (Leu-Gly), an analog of melanotropin release inhibition factor on the development of tolerance to and physical dependence on morphine in the rat was investigated. Administration of cyclo (Leu-Gly) (1 μg/rat/day) prior to and during morphine pellet implantation failed to facilitate the development of tolerance to the analgesic and hypothermic effects of morphine. Similarly the development of dependence on morphine was not facilitated by cyclo (Leu-Gly) as evidenced by changes in body weight and body temperature observed during abrupt withdrawal of morphine. These studies do not lend support to the previous observations that cyclo (Leu-Gly) and other related peptides facilitate the development of tolerance to and physical dependence on morphine.     

Dopaminergic supersensitivity in striatum and olfactory tubercle following chronic administration of haloperidol of clozapine

Mice were maintained on diets containing haloperidol or clozapine for 8–10 days. Two days after these drug-containing diets were withdrawn the effects of apomorphine were determined on locomotor activity and on the retardation of dopamine depletion produced by synthesis inhibition with α-methyltyrosine. After either neuroleptic the effects of apomorphine were enhanced when compared with mice maintained on a control diet, suggesting the development of supersensitive dopamine receptors.     

Antagonism of kappa opioid mediated effects in the rat by cyclo(Leu-Gly)

The effect of cyclo(Leu-Gly) on U-50,488H- induced pharmacological actions was determined in male Sprague-Dawley rats. Intraperitoneal (i.p.) administration of U-50,488H to rats produced analgesia (tail-flick) and increased urinary output. Cyclo (Leu-Gly) (1-4 mg/kg, s.c.) antagonized the analgesic response to U-50,488H (25 mg/kg; i.p.). A dose of 10 mg/kg (i.p.) of U-50,488H increased the spontaneous urinary output which was antagonized by cyclo (Leu-Gly) (1-4 mg/kg; s.c.). To determine whether cyclo (Leu-Gly) was acting as a kappa-opioid receptor antagonist, the effect of cyclo (Leu-Gly) on the binding of [3H]ethylketocyclazocine (EKC) to membranes of rat cerebral cortex and spinal cord was determined. The IC50 values of cyclo(Leu-Gly) in displacing [3H]EKC from its binding sites in cortex and spinal cord were 1.44 and 0.40 mM, respectively. Chronic administration of U-50,488H (25 mg/kg; i.p., b.i.d.) for 4 days induced tolerance to its analgesic effect. The latter was not affected by cyclo(Leu-Gly) (2 to 8 mg/kg; s.c.) given once a day for 4 days. It is concluded that cyclo(Leu-Gly) antagonizes acute actions of U-50,488H and that such effects of cyclo(Leu-Gly) are not mediated via a direct action on kappa-opioid receptors.     

Neurochemical evidence for denervation supersensitivity: the effect of unilateral substantia nigra lesions on apomorphine-induced increases in neostriatal acetylcholine levels

Unilateral lesions of the nigro-neostriatal dopaminergic projection were induced by injections of 6-hydroxydopamine into the zona compacta of the substantia nigra. This resulted in a reduction of neostriatal dopamine to less than 6 percent of the control side. Two months later intraperitoneal injections of apomorphine (1 mg/kg) produced contralateral turning and a significant increase in neostriatal acetylcholine levels. The increase was significantly greater in the neostriatum ipsilateral to the lesion than in the intact side. Haloperidol (1 mg/kg) produced a significant decrease in neostriatal acetylcholine but this decrease did not differ between the “denervated” and intact neostriata. The nigral 6-hydroxydopamine lesions did not by themselves affect neostriatal acetylcholine levels. The fact that apomorphine produces a greater increase in neostriatal acetylcholine after lesions of the dopaminergic nigro-neostriatal projection supports earlier behavioral data suggestive of denervation supersensitivity of neostriatal dopaminergic receptors after these lesions.     

Dopamine receptor sensitivity after repeated morphine administrations to rats.

It was studied in rats, if chronic morphine treatment induces a supersensitivity of dopamine receptors in brain. The rats were treated twice daily for 8–11 days with single doses of morphine, increasing from 10 to 20 mg/kg i.p. The experiments were carried out 16–20 hours after the last injection of morphine. After chronic morphine treatment, the potency of apomorphine in lowering the striatal dopamine turnover was increased. On the other hand, apomorphine was not more potent in inducing stereotypies (sniffing, licking, gnawing) after chronic morphine administration than in saline controls. Finally, dopamine activated the adenylate cyclase in striatal homogenates of rats after chronic morphine treatment to a similar extent as in homogenates of control rats. The results suggest that a supersensitivity of dopamine receptors in brain is not necessarily involved in symptoms of an increased dopaminergic activity after chronic morphine application.     

Modulation of apomorphine-induced rotations in unilaterally 6-hydroxydopamine lesioned rats by cholinergic agonists and antagonists

In the present study, we examined the effects of the agonists and antagonists of cholinergic receptors on central dopaminergic function using the 6-hydroxydopamine model of dopamine receptor supersensitivity. Unilateral lesioning of the substantia nigra with 6-hydroxydopamine was carried out in Wistar rats. Two weeks after surgery, the rats were tested for the presence of dopaminergic supersensitivity by their response to the dopamine receptor agonist, apomorphine. Apomorphine-induced rotations were significantly reinforced by the muscarinic receptor antagonist, atropine. In contrast to atropine, the muscarinic receptor agonist oxotremorine attenuated apomorphine's effects. Acute treatment of nicotine significantly reduced apomorphine-induced rotations. However, when increasing doses of nicotine were given for nine days, the rotations of the nicotine-dependent rats were significantly enhanced. So the fact that both muscarinic and nicotinic cholinergic activity could modulate apomorphine-induced rotations was readily apparent in these experiments.     

The effect of chronic levodopa on haloperidol induced behavioral supersensitivity in the guinea pig

The effect of chronic levodopa-carbidopa administration (200 mg/kg for 21 days) on guinea pigs rendered behaviorally supersensitive by the prior administration of haloperidol (.5 mg/kg for 21 days) was examined. Animals who showed an increased behavioral response to apomorphine after chronic haloperidol administration were treated with levodopa-carbidopa and then apomorphine - induced stereotypy was reexamined. Although the chronic levodopa control groups and the chronic haloperidol control remained supersensitive to the behavioral effect of apomorphine, the haloperidol-levodopa group's behavioral response to apomorphine returned to normal. Both chronic dopaminergic antagonist and agonist administration have been demonstrated to induce heightened apomorphine-induced stereotypy and this has been interpreted as a reflection of altered striatal dopamine receptor site sensitivity. The finding that the serial administration of a chronic dopaminergic antagonist followed by a chronic dopaminergic agonist results in a return to normal of a striatal dopamine receptor-dependent behavior suggests that these chronic treatments affect dopamine receptor sites by different mechanisms of action. Since neuroleptic induced dopaminergic supersensitivity in animals is an accepted model of tardive dyskinesia, levodopa may also reverse dopaminergic supersensitivity in patients and might be a potential therapeutic agent in tardive dyskinesia.     

Pharmacological characterization of apomorphine-induced hypothermia in the spontaneously hypertensive rat

The dopamine agonist apomorphine was more potent in eliciting hypothermia in spontaneously hypertensive rats (SHRs) than in normotensive Wistar rats (NWRs), while normotensive Wistar-Kyoto rats (WKYs) were intermediate in response. Various drug interventions were attempted in an effort to explain the greater sensitivity of SHRs to apomorphine. Haloperidol produced abolition of apomorphine-induced hypothermia in SHRs but at greater doses than required for antagonism of the drug effect in WKYs and NWRs. Chronic hydralazine treatment that reduced the high blood pressure of SHRs failed to appreciably influence the magnitude of apomorphine-induced hypothermia, compared to the response in control SHRs that received no hydralazine. These findings suggested to us that the enhanced hypothermic effect of apomorphine in SHRs was entirely dopamine receptor-mediated and that it was also independent of the high blood pressure. We also found that chronic lithium treatment that had no influence upon apomorphine-induced hypothermia in WKYs and NWRs significantly reduced the drug effect in SHRs. Based on this finding, we suggest that the greater hypothermic effect induced by apomorphine in SHRs might be due to a supersensitivity of hypothermia-mediating dopamine receptors in the hypertensive strain.     

Reduced proliferation in the adult mouse subventricular zone increases survival of olfactory bulb interneurons

Neurogenesis in the adult brain is largely restricted to the subependymal zone (SVZ) of the lateral ventricle, olfactory bulb (OB) and the dentate subgranular zone, and survival of adult-born cells in the OB is influenced by factors including sensory experience. We examined, in mice, whether survival of adult-born cells is also regulated by the rate of precursor proliferation in the SVZ. Precursor proliferation was decreased by depleting the SVZ of dopamine after lesioning dopamine neurons in the substantia nigra compacta with 6-hydroxydopamine. Subsequently, we examined the effect of reduced SVZ proliferation on the generation, migration and survival of neuroblasts and mature adult-born cells in the SVZ, rostral migratory stream (RMS) and OB. Proliferating cells in the SVZ, measured by 5-bromo-2-deoxyuridine (BrdU) injected 2 hours prior to death or by immunoreactivity against Ki67, were reduced by 47% or 36%, respectively, 7 days after dopamine depletion, and by 29% or 31% 42 days after dopamine depletion, compared to sham-treated animals. Neuroblast generation in the SVZ and their migration along the RMS were not affected, neither 7 nor 42 days after the 6-hydroxydopamine injection, since the number of doublecortin-immunoreactive neuroblasts in the SVZ and RMS, as well as the number of neuronal nuclei-immunoreactive cells in the OB, were stable compared to control. However, survival analysis 15 days after 6-hydroxydopamine and 6 days after BrdU injections showed that the number of BrdU+ cells in the SVZ was 70% higher. Also, 42 days after 6-hydroxydopamine and 30 days after BrdU injections, we found an 82% increase in co-labeled BrdU+/γ-aminobutyric acid-immunoreactive cell bodies in the granular cell layer, while double-labeled BrdU+/tyrosine hydroxylase-immunoreactive cell bodies in the glomerular layer increased by 148%. We conclude that the number of OB interneurons following reduced SVZ proliferation is maintained through an increased survival of adult-born precursor cells, neuroblasts and interneurons.     

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.     

Genotypic differences in the metabolism of brain dopamine and in the dopamine-dependent forms of behavior in mice

Metabolism of dopamine in striatum and the dopamine-dependent forms of behaviour of 6 inbred mice were studied. Interstrain significant differences were shown in the level of dopamine metabolite, homovanillic acid (HVA), as well as in locomotor activity and climbing produced by injection of dopamine agonist apomorphine. Significant negative correlation between the level of HVA in striatum and spontaneous locomotor activity was found. Different levels of HVA and significant behavioural differences were discovered in two similar strains--BALB/c and CC57BR. According to its behavioural characteristics, DBA/2 should also be considered among the most interesting.     

The effect of chronic L-DOPA administration on supersensitive pre- and postsynaptic dopaminergic receptors in rat brain

Chronic administration of haloperidol induced supersensitivity of the pre- and postsynaptic dopaminergic receptors in rat brain. The response of the presynaptic receptors was determined by an enhanced inhibitory effect of apomorphine on dopamine synthesis after gamma-butyrolactone injection. This change in the receptor function was detected both in the nigrostriatal and mesolimbic pathways. Haloperidol also increased the ³H-spiperone binding sites in striatal membranes, indicating supersensitivity of the postsynaptic receptors. Subsequent prolonged treatment with high doses of L-DOPA/carbidopa resulted in a decrease in ³H-spiperone binding sites, but had no effect on the supersensitive presynaptic receptors. It is suggested that tardive dyskinesia may be a state of both pre- and postsynaptic dopamine receptor supersensitivity and that chronic L-DOPA treatment may have a differential effect on these sites.     

7-[3-(4-[2,3-Dimethylphenyl]piperazinyl)propoxy]-2(1H)-quinolinone (OPC-4392), a presynaptic dopamine autoreceptor agonist and postsynaptic D2 receptor antagonist

7-[3-(4-[2,3-dimethylphenyl]piperazinyl)propoxy]-2(1H)-quinolinone (OPC-4392), was synthesized in our laboratories and compared with apomorphine, 3-(3-hydroxyphenyl)-N-n-propylpiperidine (3-PPP) and dopamine antagonists in a series of tests designed to characterize dopamine receptor activation and inhibition. The assertion that OPC-4392 acts as an agonist at presynaptic dopamine autoreceptors is supported by the following behavioral and biochemical observations: OPC-4392, 3-PPP and apomorphine inhibited the reserpine-induced increase in DOPA accumulation in the forebrain of mice and in the frontal cortex, limbic forebrain and striatum of rats. In addition, the gamma-butyrolactone (GBL)-induced increase in DOPA accumulation in the mouse forebrain was also inhibited by OPC-4392, 3-PPP and apomorphine. Haloperidol antagonized the inhibitory effect of OPC-4392 in both instances. The inhibitory effect of OPC-4392 on GBL-induced DOPA accumulation lasted for at least 8 hours after oral administration to mice, while that of 3-PPP and apomorphine disappeared in 4 hours after subcutaneous injection. OPC-4392 failed to increase spontaneous motor activity in reserpinized mice, enhance spontaneous ipsilateral rotation in rats with unilateral striatal kainic acid (KA) lesions, induce contralateral rotation in rats with unilateral striatal 6-hydroxydopamine (6-OHDA) lesions and inhibit 14C-acetylcholine (Ach) release stimulated by 20 mM KCl in rat striatal slices. In addition, OPC-4392 appears to block postsynaptic D2 receptors since OPC-4392, as well as dopamine antagonists, was able to inhibit stereotyped behavior and climbing behavior induced by apomorphine in mice, displace the 3H-spiroperidol binding to rat synaptosomal membranes in vitro and reverse the inhibitory effect of apomorphine on Ach release in rat striatal slices. These results suggest that OPC-4392 acts as a dopamine agonist at presynaptic autoreceptors related to dopamine synthesis and acts as dopamine antagonist at postsynaptic D2 receptors.     

Effect of methamphetamine on the locomotor activity in the 6-OHDA dorsal hippocampus lesioned rat

The present studies were carried out to examine a possible role of hippocampal dopamine in the hyperactivity induced by methamphetamine. For this purpose, 6-hydroxydopamine (6-OHDA) lesion of the dorsal hippocampus (D-HPC) was made in desmethylimipramine pretreated rats in order to specifically destroy dopamine neurons. D-HPC lesions produced a large (96%) and selective depletion of content of dopamine in the D-HPC. This lesion did not change spontaneous locomotion and rearing behavior. The 6-OHDA lesioned rat produced a blockade of the increase in locomotor activity induced by 1.0 and 2.0 mg/kg of methamphetamine. In contrast, the 6-OHDA lesion of the D-HPC failed to influence the methamphetamine-induced rearing activity. These results indicate that dopamine neurons in the D-HPC may have some role in methamphetamine-induced locomotion, but not in methamphetamine-induced rearing.     

Enhancement of reserpine-elicited dopaminergic supersensitivity by repeated treatment with apomorphine and alpha-methyl-p-tyrosine.

Pretreatment of rats with reserpine 5 mg/Kg/day for 2 days elicits an enhanced stereotyped response following injection of apomorphine or amphetamine which persists through the 17th day. Since apomorphine acts as a direct postsynaptic receptor agonist in dopaminergic neurons this effect may represent a postsynaptic supersensitivity. In an attempt to prevent the development of supersensitivity apomorphine was administered repeatedly during the reserpinization period. Contrary to expectations a further enhancement of supersensitivity was seen when animals were challenged days later with apomorphine. This may be the result of presynaptic dopamine-synthesis-inhibition following apomorphine. Apomorphine-induced enhancement of reserpine supersensitivity was not seen in animals challenged with amphetamine. Alpha-methyl-para-tyrosine, but not scopolamine, repeatedly administered during the reserpinization mimics the effect of apomorphine, supporting the concept that the potentiating effects of apomorphine are mediated presynaptically. Furthermore it is suggested that the direct presynaptic action of apomorphine, and not that mediated via cholinergic interneurons, is operant in the development of enhanced supersensitivity.