Antagonism of morphine-induced catalepsy by L-prolyl-L-leucyl-glycinamide

Liposomal-formulated curcumin [Lipocurc™] targeting HDAC (histone deacetylase) prevents apoptosis and improves motor deficits in Park 7 (DJ-1)-knockout rat model of Parkinson's disease: implications for epigenetics-based nanotechnology-driven drug platform

BACKGROUND

Converging evidence suggests dysregulation of epigenetics in terms of histone-mediated acetylation/deacetylation imbalance in Parkinson's disease (PD). Targeting histone deacetylase (HDAC) in neuronal survival and neuroprotection may be beneficial in the treatment and prevention of neurodegenerative disorders. Few pharmacological studies use the transgenic model of PD to characterize the neuroprotection actions of a lead compound known to target HDAC in the brain.

METHODS

In our study, we investigated neuroprotective effects of liposomal-formulated curcumin: Lipocurc™ targeting HDAC inhibitor in the DJ-1(Park 7)-gene knockout rat model of PD. Group I (DJ-1-KO-Lipocurc™) received Lipocurc™ 20 mg/kg iv 3× weekly for 8 weeks; Group II: DJ-1 KO controls (DJ-1 KO-PBS) received i.v. phosphate-buffered saline (PBS). Group III: DJ-1-Wild Type (DJ-1 WT-PBS) received PBS. We monitored various components of motor behavior, rotarod, dyskinesia, and open-field behaviors, both at baseline and at regular intervals. Toward the end of the 8 weeks, we measured neuronal apoptosis and dopamine (DA) neuron-specific tyrosine hydroxylase levels by immunohistochemistry methods at post-mortem.

RESULTS

We found that DJ-KO Group I and Group II, as compared with DJ-1 WT group, exhibited moderate degree of motor impairment on the rotarod test. Lipocurc™ treatment improved the motor behavior motor impairment to a greater extent than the PBS treatment. There was marked apoptosis in the DJ-1 WT group. Lipocurc™ significantly blocked neuronal apoptosis: the apoptotic index of DJ-1-KO-Lipocurc™ group was markedly reduced compared with the DJ-KO-PBS group (3.3 vs 25.0, p<0.001). We found preliminary evidence Lipocurc™ stimulated DA neurons in the substantia nigra. The ratio of immature to mature DA neurons in substantia nigra was statistically higher in the DJ-1-KO-Lipocurc™ group (p<0.025).

CONCLUSIONS

We demonstrated for the first time Lipocurc™'s anti-apoptotic and neurotrophic effects in theDJ-1-KO rat model of PD. Our promising findings warrant randomized controlled trial of Lipocurc™ in translating the novel nanotechnology-based epigenetics-driven drug discovery platform toward efficacious therapeutics in PD.

Alpha-phenyl-N-tert-butylnitrone prevents oxidative stress in a haloperidol-induced animal model of tardive dyskinesia: investigating the behavioural and biochemical changes

Haloperidol (HP) is a widely prescribed antipsychotic drug used for the treatment of mental disorders. However, while providing therapeutic benefits, this drug also causes serious extrapyramidal side effects, such as tardive dyskinesia (TD). Upon chronic administration, HP causes behavioural supersensitivity to dopamine D2 receptor agonists, as well as the development of vacuous chewing movements (VCMs), in an animal model of human TD. Currently, a prevailing hypothesis to account for these behavioural abnormalities implicates oxidative stress. This study was undertaken to examine whether the free radical trapping agent, α-phenyl-N-tert-butylnitrone (PBN), can prevent the development of behavioural supersensitivity to dopamine D2 receptor agonists and the development of VCMs. Additionally, the study examined whether increased synthesis of apoptosis inducing factor (AIF) can result from HP-induced oxidative stress. Male Sprague-Dawley rats were treated with HP in conjunction with PBN, or its vehicle, for 4weeks. After a 24-hour washout period, behavioural observations were recorded along with the estimation of lipid peroxidation and antioxidant enzyme activities. The free radical trapping agent, PBN, prevented the development of behavioural supersensitivity, reduced lipid peroxidation and prevented the reduction of antioxidant enzyme activities. AIF concentrations at the mRNA and protein levels remained unchanged; therefore increased AIF gene expression is unlikely to be involved in HP-induced oxidative stress. The findings of the present study suggest the involvement of striatal free radicals in the development of behavioural supersensitivity, and free radical trapping agents, such as PBN, as possible options for the treatment of extrapyramidal side effects in humans.

Pro-Leu-glycinamide and its peptidomimetic, PAOPA, attenuate haloperidol induced vacuous chewing movements in rat: A model of human tardive dyskinesia

In the present experimental paradigm, we examine the effect of L-prolyl-L-leucyl-glycinamide (PLG) co-administration with haloperidol on vacuous chewing movements (VCM) in rats-a model of tardive dyskinesia (TD) in humans. We examined the dose dependent induction of VCM through both injected and orally administered PLG (MIF-1). Our results show significant levels of VCM attenuation (P<0.05) in rats treated with 10mg/kg of PLG. Doses of 1 and 100mg/kg were ineffective. Reductions were present in both orally treated and injected rats. We also examined the therapeutic effect of a peptidomimetic of PLG-PAOPA. PAOPA was able to produce similar behavioral effects to PLG at a dose, which was 100-fold lower than the effective dose of PLG. These results suggest that PLG may play a role in D2 receptor expression and function, as well as providing a therapy for neuroleptic induced TD.

Modulation of agonist stimulated adenylyl cyclase and GTPase activity by L-pro-L-leu-glycinamide and its peptidomimetic analogue in rat striatal membranes

L-prolyl-L-leucyl-glycinamide (PLG), also known as melanocyte-stimulating hormone release inhibiting factor (MIF-1), is an endogenous brain tripeptide. Previous studies have shown that PLG, and its peptidomimetic analogues, render dopamine D2 receptors more responsive to agonists by maintaining the high-affinity binding state of the receptors. In the present study, we examined the effect PLG and its analogue 3(R)-[(2(S)-pyrrolidylcarbonyl)amino]-2-oxo-1-pyrrolidineacetam ide (PAOPA) on dopamine-stimulated adenylyl cyclase and NPA-stimulated GTPase activity in rat striatal membranes. Dopamine-stimulated adenylyl cyclase activity was inhibited by both PLG and PAOPA in a dose-dependent manner, whereas R(-)-propylnorapomorphine (NPA)-stimulated low Km GTPase activity was significantly increased by 1 microM PLG or 1 nM PAOPA. These results suggest that PLG and PAOPA maintain the high affinity state of the D2 receptor by increasing GTP hydrolysis through stimulation of agonist-induced GTPase activity.

Modulatory effects of PLG and its peptidomimetics on haloperidol-induced catalepsy in rats

A behavioral model of dopaminergic function in the rat was used to examine the anticataleptic effects of L-prolyl-L-leucyl-glycinamide (PLG) and peptidomimetic analogs of PLG. Administration of 1 mg/kg PLG intraperitoneally significantly attenuated haloperidol (1 mg/kg)-induced catalepsy (as measured by the standard horizontal bar test), whereas doses of 0.1 and 10 mg/kg PLG did not. Eight synthetic PLG peptidomimetics (Calpha, alpha-dialkylated glycyl residues with lactam bridge constraint [1-4] and without [5-8]) were tested in the same manner (at a dose of 1 microg/kg) and categorized according to their activity, i.e. very active (5), moderately active (2, 3, 4, and 6), and inactive (1, 7, and 8). The catalepsy-reversal action of the diethylglycine-substituted peptidomimetic 5 was examined further and found to exhibit a U-shaped dose-response effect with an optimal dose of 1 microg/kg. The similarity between the effects of PLG and the synthetic peptidomimetics suggests a common mechanism of action. Finally, the synthetic peptidomimetics examined here, particularly peptidomimetic 5, were more effective than PLG in attenuating haloperidol-induced catalepsy.

Seborrhea and persistent tardive dyskinesia

The following communication concerns two schizophrenic patients with Tardive dyskinesia (TD) in whom fluctuations in the severity of the dyskinesias were accompanied by changes in the severity of the seborrheic skin lesions. Since seborrheic dermatitis may be associated with increased plasma melanocyte-stimulating hormone (MSH) level, these observations suggest an association between the severity of TD and increased pituitary MSH release. In addition, TD may be associated with hypothalamic-pituitary dysfunction of MSH autoregulation.

Drug effects on active immobility responses: what they tell us about neurotransmitter systems and motor functions

The literature reviewed indicates that active immobility can be promoted by systemic injections of various neurotransmitter systems, as follows: (1) Dopaminergic blockade of both D1 and D2 receptor subtypes. (2) Cholinergic agonism of both muscarinic and nicotinic receptors. (3) Noradrenergic agonism of both alpha-1 and alpha-2 receptors (but these agonists may interfere with haloperidol- and reserpine-induced catalepsy). (4) GABA agonism. (5) Histamine agonism, particularly at the H1 receptor. (6) Opiate agonism, including action of many endogenous opiate peptides, particularly those affecting mu and delta receptors. (7) Agonism by certain other peptides (neurotensin, cholecystokinin). Among the major interactions of neurotransmitter systems that regulate immobility, are the following: (1) Cholinergic-dopaminergic (cholinolytics disrupt catalepsy of dopaminergic blockade and dopaminergic agonists tend to disrupt cholinomimetic catalepsy). (2) Opiate-induced catalepsy is antagonized by the dopamine agonist, apomorphine, but is enhanced by amphetamine. It is also antagonized by certain alpha-2 adrenergic agonists, while it does not seem to be antagonized by anticholinergics. (3) Numerous other interactions have been reported, involving opiates and MSH, serotonin and dopamine mimetics, serotonin and ketamine, GABA and neuroleptics, neurotensin and anticholinergics and histamine. The significance of the multiple neurotransmitter systems is unknown. One possible explanation is that the various neurotransmitter systems participate in mediating the sensory inputs that are involved in triggering immobility and regulate the higher-order limbic and basal ganglia processing reactions that engage a final motor output pathway from the brainstem. The brain is assumed to contain two sets of systems, each with its own, or possibly overlapping, set of neurotransmitter systems, that promote either active immobility or locomotion. The systems reciprocally inhibit each other. Another view, not mutually exclusive, is that output from the locomotor-promoting system provides a negative feedback, via the active immobility pathways, to act as a "brake" on movement, while at the same time maintaining the muscular tonus that is characteristic of active immobility.

Interaction of L-prolyl-L-leucyl glycinamide with dopamine D2 receptor: evidence for modulation of agonist affinity states in bovine striatal membranes

The role of the hypothalamic tripeptide L-prolyl-L-leucyl-glycinamide (PLG) in modulating the agonist binding to bovine striatal dopamine D2 receptor was investigated using a selective high-affinity agonist, n-propylnorapomorphine (NPA). PLG caused an enhancement in [3H]NPA binding in striatal membranes in a dose-dependent manner, the maximum effect being observed at 10(-7)-10(-6) M concentration of the tripeptide. The Scatchard analysis of [3H]NPA binding to membranes preincubated with 10(-6) M PLG revealed a significant increase in the affinity of the agonist binding sites. In contrast, there was no effect of PLG on the binding pattern of the antagonist [3H]spiroperidol. The antagonist versus agonist competition curves analyzed for agonist high- and low-affinity states of the receptor displayed an increase in the population and affinity of the high-affinity form of the receptor with PLG treatment. The low-affinity sites concomitantly decreased with relatively small change in the affinity for the agonists. Almost similar results were obtained when either NPA or apomorphine was used in the competition experiments. A partial antagonistic effect of PLG on 5'-guanylylimidodiphosphate [Gpp(NH)p]-induced inhibition of high-affinity agonist binding was also observed, as the ratio of high- to low-affinity forms of the receptor was significantly higher in the PLG-treated membranes compared to the controls. Direct [3H]NPA binding experiments demonstrated that PLG attenuated the Gpp(NH)p-induced inhibition of agonist binding by increasing the EC50 of the nucleotide (concentration that inhibits 50% of the specific binding). No effect of PLG on high-affinity [3H]NPA binding, however, could be observed when the striatal membranes were preincubated with Gpp(NH)p.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological activities of the MIF-1 analogues Pro-Leu-Gly, Tyr-Pro-Leu-Gly and pareptide

The pharmacological activities of the related free acid analogues of MIF-1, Pro-Leu-Gly (PLG) and Tyr-Pro-Leu-Gly (YPLG), were investigated because of the possibility that they may be formed during the digestion of milk and wheat proteins in vivo. The amino acid sequences -Tyr-Pro-Leu-Gly- and -Pro-Leu-Gly- are present in proteins from these foods. Chronic administration of either PLG (0.25 mg/kg, SC, BID) or the control substance, pareptide (0.25 mg/kg, SC, BID), antagonized the development of tolerance to the cataleptic effects of haloperidol in mice. The effect of YPLG (0.25 mg/kg, SC, BID) on the development of this tolerance was borderline and not statistically significant. Nanomolar concentrations of PLG, YPLG, and pareptide each increased the in vitro binding of 3H-apomorphine to rat striatal receptors. In this in vitro system, bell-shaped dose response curves were observed for each peptide. The effects of these peptides on tolerance development and apomorphine binding are similar to those previously reported for MIF-1 and demonstrate that amidation at the carboxyl terminus is not required for biological activity.

Effects of Pro-Leu-Gly-NH2 and cyclo (Leu-Gly) on the binding of 3H-quinuclidinyl benzilate to striatal cholinergic muscarinic receptors

The effects of Pro-Leu-Gly-NH2 (melanotropin release inhibiting factor, MIF) and its analog, cyclo (Leu-Gly) on the mouse and rat striatal cholinergic muscarinic receptors labeled with 3H-quinuclidinyl benzilate (QNB) were investigated. 3H-QNB bound to the rat striatal muscarinic receptors at a single high affinity site with receptor density (Bmax value) of 1200 fmol per mg protein and an apparent dissociation constant (Kd value) of 53.5 pM. At 140 pM concentration of 3H-QNB, the specific binding to the receptors was 724 fmol per mg protein. MIF in a concentration range of 10(-9) to 10(-4) M did not alter the binding of 3H-QNB but at 10(-3) M decreased the binding by 25%. Cyclo (Leu-Gly), on the other hand, in the concentration range of 10(-9) to 10(-3) M had no effect on the binding of 3H-QNB. A single injection of MIF (3 or 10 mg/kg IP) to rats did not alter the Bmax or the Kd value of 3H-QNB to bind to the striatal membranes. 3H-QNB bound to the mouse striatal muscarinic receptors at a single high affinity site with a Bmax value of 991 fmol/per mg protein and a Kd value of 21 pM. Neither acute administration of MIF (3 or 10 mg/kg IP) nor chronic treatment of the peptide (2, 8 or 32 mg/kg IP, daily for 5 days) to mice could influence the binding of 3H-QNB to the striatal muscarinic receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Pro-Leu-GlyNH2 affects dopamine and noradrenaline utilization in rat limbic-forebrain nuclei

The effects of Pro-Leu-GlyNH2 (PLG), administered i.c.v. in doses of 3.5, 35, 350 and 3500 pmol, were studied on the alpha-MPT-induced disappearance of catecholamines in microdissected rat brain nuclei. PLG, dose-dependently, increased dopamine disappearance in the nucleus caudatus and globus pallidus, whereas a decrease in dopamine disappearance was observed in the nucleus dorsomedialis. Noradrenaline disappearance was decreased in the medial septal nucleus, anterior hypothalamic area and lateral amygdala. A tendency towards an increase in noradrenaline disappearance was observed in the nucl. supraopticus. These data show that PLG has a central site of action. The effects of PLG on dopamine disappearance are comparable to those previously found with vasopressin, while the effects of PLG on noradrenaline utilization show a striking similarity with those previously obtained with oxytocin.

Pro-Leu-Gly-NH2 and pareptide inhibit development of tolerance to haloperidol catalepsy in the mouse

Single doses of MIF-1 (0.03-2.0 mg/kg, SC) and chronic pretreatments with MIF-1 (0.03-2.0 mg/kg, SC, BID, 3 1/2 days) or pareptide (0.25 mg/kg, SC, BID, 3 1/2 days) did not affect the acute cataleptic response to haloperidol in the mouse. Chronic pretreatment with haloperidol (8.0 mg/kg, IP, BID, 3 days) decreased the duration of catalepsy in mice given smaller challenge dose of haloperidol (2.0 or 3.0 mg/kg, IP) 15 hours after the last pretreatment injections. Administration of either MIF-1 or pareptide to mice also chronically pretreated with haloperidol antagonized the development of tolerance.

Comparative effects of prolyl-leucyl-glycinamide and naloxone on morphine-induced inhibition of gastrointestinal transit

The effects of prolyl-leucyl-glycinamide (MIF) and naloxone on the gastrointestinal transit in mice were investigated using the charcoal meal test. MIF administered intraperitoneally (IP) (1-10 mg/kg) or intracerebroventricularly (ICV) (10 micrograms/mouse) had no effect on the transit. Administration of morphine by subcutaneous (SC) route significantly inhibited the gastrointestinal transit. The morphine-induced inhibition of the transit was not affected by MIF whether given by IP or ICV route. Administration of the opiate antagonist naloxone (1 mg/kg, IP or 10 micrograms/mouse, ICV) had no effect on the gastrointestinal transit, but it significantly antagonized the inhibition produced by morphine. Some earlier studies have indicated narcotic antagonistic effect of MIF. However, in the present study, evidence for such an action of MIF was not obtained. It is suggested that MIF does not appear to have narcotic antagonistic activity and further supports an earlier study from this laboratory that MIF may not interact with opiate receptors.

CNS dopamine receptors: effect of prolyl-leucyl-glycinamide and solubilization

In order to elucidate the mechanism of interaction of a peptide L-prolyl leucyl-glycinamide (PLG) with dopamine receptors, we have studied the action of PLG on dopamine receptors in various brain regions. The results support the hypothesis that specific PLG binding sites exist in the central nervous system and these binding sites (receptors) have a modulatory effect on the sensitivity of dopamine receptors. It is also suggested that PLG and its active analogues warrant further vigorous and systematic clinical trials to establish their therapeutic efficacy in Parkinson's disease, neuroleptic drug induced tardive dyskinesia and related extrapyramidal motor disorders. Studies carried out on solubilized dopamine receptors and adenylate cyclase suggest that dopamine receptors sites coupled to neurolic drug action and adenylate cyclase linked receptor sites might be closely interrelated. The preliminary results on lymphocyte dopamine binding sites suggest an increase in binding in schizophrenic patients, however, receptor criteria (stereospecific binding, saturation, etc.) could not be met for these binding sites (see Rotstein et al., 1983, for details).

Neuropeptides related to neurohypophyseal hormones interfere with apomorphine-induced behavioral changes

The interaction between peptides related to neurohypophyseal hormones and brain dopaminergic systems was studied by investigating in rats the effect of these peptides on behavioral changes induced by graded doses of the specific dopamine agonist apomorphine. Low doses of this drug induce hypoactivity of the animals, while higher doses result in hyperactivity and stereotyped sniffing. Desglycinamide9[Arg8]vasopressin (DG-AVP), prolyl-leucyl-glycinamide (PLG) and oxytocin did not interfere with the behavioral responses induced by the higher doses of apomorphine. Peptide treatment made the rats more sensitive to apomorphine with respect to the drug induced hypoactivity. PLG and especially DG-AVP were more effective than oxytocin. It is concluded that these peptides may have a selective action on distinct dopaminergic receptor systems in the brain, that are presumably located presynaptically in the nucleus accumbens area.

Potentiation of apomorphine-induced stereotypies by naloxone and L-prolyl-L-leucyl-glycinamide

We compared the influences of pretreatment with the narcotic antagonist drug naloxone and the neuropeptide L-prolyl-L-leucyl-glycinamide (PLG) upon apomorphine-induced stereotypic climbing activity in mice and apomorphine-induced contralateral rotational behavior in rats with unilateral 6-hydroxydopamine lesions of the substantia nigra. Naloxone produced dose-related potentiation in the mouse climbing model, while PLG was without effect. On the other hand, PLG produced dose-related potentiation in the rat rotational paradigm, while naloxone was without appreciable influence. These findings show an asymmetrical potentiation of apomorphine by naloxone and PLG in these two standard experimental models of striatal dopaminergic activity.

Behavioral effects of melanocyte stimulating hormone release-inhibiting factor-1 (MIF-1)

Consideration of the isolation, structure, localization, and behavioral effects of melanocyte stimulating hormone release inhibiting factor (MIF-1) is followed by a review of its opiate antagonistic and clinical effects. Evidence pertaining to various hypotheses offered in explanation of these behavioral effects is examined and evaluated. It is concluded that MIF-1 affects behavior in many instances with possible antagonistic effects as well as clinical possibilities.

The effect of melanotropin release inhibiting factor, its metabolites and analogs on [3H]spiroperidol and [3H]apomorphine binding sites

The effects of melanotrophin release inhibiting factor (Pro-Leu-Gly-NH2, MIF), its possible metabolites, Pro-Leu-OH, Leu-Gly-NH2, Leu-Gly-OH and an analogue, cyclo(Leu-Gly), on [3H]spiroperidol binding sites in the striatum and on [3H]apomorphine binding sites in the striatum and hypothalamus of male Sprague-Dawley rats were determined. [3H]Spiroperidol binding to dopamine receptors in striatal membranes was unaffected by any of the above peptides in concentration up to 0.1 mM. The binding of [3H]apomorphine was enhanced by MIF, Pro-Leu-OH and cyclo(Leu-Gly) in both striatal and hypothalamic membranes in submicromolar concentrations. Leu-Gly-NH2 and Leu-Gly-OH did not affect [3H]apomorphine binding to dopamine receptors in striatum of hypothalamus. The enhancement in binding of [3H]apomorphine by MIF and cyclo(Leu-Gly) was not related to the changes in the number of binding sites but to an increase in the affinity to the receptors. The results indicate that MIF and some of its related peptides do not affect dopamine receptor binding sites labeled by the neuroleptic [3H]spiroperidol but facilitate the transmission in those sites labeled by [3H]apomorphine. Since [3H]apomorphine and [3H]spiroperidol predominantly label pre- and post-synaptic dopamine receptors, it is concluded that MIF and its active analogs interact with presynaptic dopamine receptors.

Cyclo(Leu-Gly) attenuates the striatal dopaminergic supersensitivity induced by chronic morphine: agonist binding to D2 dopamine receptors correlates with stereotypic behavior

We have previously shown that MIF and its structural analog, cyclo-(Leu-Gly), block analgesic tolerance and some signs of physical dependence following chronic opiate administration. The mechanism of action of these peptides has not been clearly elucidated. The data presented here suggests that chronic opiate administration causes a behavioral supersensitivity to dopamine (DA) agonists which is highly correlated with an increase in D2-Hi receptor affinity for DA agonists, but not antagonists. Both the behavioral and receptor changes are blocked by prior administration of cyclo(Leu-Gly). This suggests that the ability of cyclo(Leu-Gly) to block the development of opiate addictive states may involve DA synaptic elements.

Similar antagonism of morphine analgesia by MIF-1 and naloxone in Carassius auratus

Prolyl-leucyl-glycinamide (MIF-1), the C-terminal tripeptide of oxytocin, and naloxone were administered intracranially (IC) to goldfish (Carassius auratus) in doses of 0.001, 0.01, 0.1, 1.0 and 10.0 mg/kg and compared to a diluent control group for their ability to reduce the effects of morphine (30 mg/kg IC) in an assay measuring analgesia to electric shock. Threshold levels of pain were determined by the voltage necessary to produce an agitated swimming response (ASR). Both MIF-1 and naloxone were found to significantly reduce the analgesic effects of morphine when compared to the diluent control group. Similar dose-response curves in an apparent sine-wave pattern were noted with both MIF-1 and naloxone when comparisons were made both at 20 minutes after administration of morphine and over the entire 150 minutes of the experiment. The results support the evidence that MIF-1 can act as an opiate antagonist.

Effect of peptides on the development of tolerance to buprenorphine, a mixed opiate agonist-antagonist analgesic

The subcutaneous administration of buprenorphine to male Sprague-Dawley rats produced a dose-dependent analgesia and hyperthermia in a dose range of 0.25-2 mg/kg. The subcutaneous administration of buprenorphine (0.5 mg/kg) twice a day for 4 days resulted in the development of tolerance to its analgesic and hyperthermic actions. Daily administration of melanotropin release inhibiting factor (Pro-Leu-Gly-NH2) or cyclo (Leu-Gly) (2 mg/kg s.c.) for 4 days, inhibited the development of tolerance to buprenorphine, as evidenced by greater analgesic and hyperthermic responses to buprenorphine in peptide-treated than in vehicle-treated buprenorphine-tolerant rats. The repeated injections of peptides did not alter the analgesic or the hyperthermic response to buprenorphine in non-tolerant rats. These studies suggest the possible role of hypothalamic peptides in blocking the development of tolerance to the pharmacological effects of buprenorphine, a potent analgesic agent.

H-Pro-[3H]Leu-Gly-NH2: uptake and metabolism in rat brain

The uptake and metabolism of H-Pro-[3H]Leu-Gly-NH2 ([3H]PLG) in rat brain was investigated by reverse-phase paired-ion high pressure liquid chromatography. Following in vitro incubation of [3H]PLG with rat brain subcellular preparations, the microsomal-cytosol fraction was about twice as active in degrading PLG as the crude mitochondrial-synaptosomal fraction. For both enzyme preparations the pH optimum was found at pH 7-7.5. The major labeled metabolite was [3H]leucine, whereas 3H]labeled Leu-Gly-NH2 as the only labeled peptide intermediate was found in trace amounts. After intravenous injection of [3H]PLG the uptake of unmetabolized peptide in the brain appeared to be very low: 0.008% and 0.001% of the administered dose/g tissue at 2 and 5 min after injection respectively, while at longer survival times intact peptide was below the detection limit. Compared with the intravenous route of administration, intracerebroventricular injection of [3H]PLG yielded much higher brain concentrations of unmetabolized PLG. Following both routes of administration, the metabolite profile was in agreement with that obtained after in vitro incubation. However, the in vivo experiments also showed considerable incorporation of [3H]leucine liberated from [3H]PLG into proteins. Both the in vitro and in vivo results indicate that the initial cleavage of PLG in rat brain occurs at the NH2-terminus and that the dipeptide intermediate H-Leu-Gly-NH2 is subsequently hydrolyzed to its constituent amino acids very rapidly.

Neuroleptic drug-induced dopamine receptor supersensitivity: antagonism by L-prolyl-L-leucyl-glycinamide

An animal model of tardive dyskinesia was used to evaluate the potential antidyskinetic properties of the neuropeptide L-prolyl-L-leucyl-glycinamide (PLG). In rats, PLG administered concurrently with the neuroleptic drug haloperidol or chlorpromazine antagonized the enhancement of specific [3H]spiroperidol binding in the striatum that is associated with long-term neuroleptic treatment. The results are discussed in relation to a possible functional coupling of the putative PLG receptor with neuroleptic-dopamine receptor complex and clinical implications for tardive dyskinesia.

Effect of L-prolyl-L-leucyl-glycinamide (PLG) on neuroleptic-induced catalepsy and dopamine/neuroleptic receptor bindings

The mechanism of action subserving the potential anti-Parkinsonian properties of L-prolyl-L-leucyl-glycinamide (PLG) was investigated in behavioural and neurochemical models of dopaminergic function in the rat. Acute administration of PLG (20 and 40 mg kg-1 SC) failed to alter appreciably the intensity of the cataleptic response elicited by haloperidol (3 mg kg-1 IP). By contrast, chronic PLG treatment (20, 40 and 80 mg kg-1 SC twice daily for five days) significantly attenuated haloperidol-induced catalepsy. The effect of PLG on in vitro dopamine/neuroleptic receptor binding in rat striatum as differentially labelled by apomorphine and spiroperidol was also examined. PLG selectively enhanced the affinity of the specific binding of agonist [3H] apomorphine to dopamine receptors in the striatum, but had no effect on [3H] spiroperidol binding. The behavioural and biochemical results obtained in the present study raise the possibility that PLG may facilitate nigro-striatal dopaminergic neurotransmission through interacting with a unique PLG receptor functionally coupled to the dopamine receptor-adenylate cyclase complex.

Effect of L-prolyl-L-leucyl-glycinamide (MIF-I) on some neutrotransmitter-receptor interactions

Some neurotransmitter-receptor interactions have been studied in an attempt to determine how L-prolyl-L-leucyl-glycinamide (MIF-I) exerts its antiparkinson effect. MIF-I affected neither the contractile responses of isolated mouse vas deferens and guinea pig ileum to noradrenaline, acetylcholine, substance P and histamine, nor the inhibitory effects of dopamine and GABA on the rat vas deferens and guinea pig ileum. MIF-I, as well as L-leucine and Pro-Leu, antagonized the contractile response of the ileum to 5-hydroxytryptamine (5-HT). Behavioural tests were used to examine the action of MIF-I on CNS transmitter-receptor interactions. MIF-I did not modify the circling produced by either dopamine agonists in nigro-striatal lesioned rats of 5-HT agonists in rats with a lesion of the medial raphe nucleus. MIF-I affected neither 5-hydroxytryptophan-induced head twitches in mice, which is a measure of 5-HT receptor stimulation, nor striatally-evoked head turning in the rat, which is a model for brain GABA function. It is concluded that MIF-I, at the doses used, does not directly modify the function of any of the CNS transmitter examined. Other possibilities to explain its antiparkinson action are discussed.

Effects of dopaminergic and cholinergic drugs. naloxone and l-prolyl-leucyl-glycinamide on LSD-induced catalepsy

In an attempt to elucidate the mechanism of d-lysergic acid diethylamide (LSD) induced catalepsy, the effects of cholinergic and dopaminergic agents, naloxone and L-propyl-leucyl-glycinamide (PLG) were studied in rats. The dose-dependent (50--500 micrograms kg-1 s.c.) and time-related cataleptic response elicited by LSD was preceded by a phase of hyperexcitability. The non-hallucinogenic analogue, 2-bromo-LSD (BOL), was without effect. Both apomorphine, the dopamine agonist, and L-DOPA antagonized LSD-induced catalepsy whereas the dopamine depleting agent alpha-methyl-p-tyrosine (alpha-MPT) slightly prolonged the cataleptic effect. Cholinergic muscarinic receptor stimulation with pilocarpine antagonized LSD-induced catalepsy. The muscarinic antagonists, atropine and scopolamine, intensified the hyperexcitable phase and potentiated the cataleptic effects of LSD. Nicotine slightly potentiated LSD action but mecamylamine antagonized it. While pre-treatment with naloxone, the narcotic antagonist and PLG prolonged the cataleptic response, post-treatment with naloxone effectively attenuated LSD-induced catalepsy. The behavioural data are interpreted to suggest that LSD-induced catalepsy may be mediated through diminished dopaminergic and cholinergic neuronal activity and under enkephalinergic modulation. The neuroanatomical foci and exact mechanism of action remain to be delineated.

Degradation of melanotropin inhibiting factor by brain

Degradation of melanotropin inhibiting factor (MIF) was measured by fluorometry, using pareptide as an internal standard, following the separation of the dansyl derivatives of MIF and its metabolites by HPLc. MIF was not split by carboxypeptidases A and B, prolidase, or pyroglutamate aminopeptidase. It was hydrolyzed by leucine aminopeptidase, aminopeptidase M, and carboxypeptidase Y. Rat brain hydrolyzed 159 nmol of MIF per mg of protein per h; the activity was linear with enzyme concentration. Hydrolysis start from the N-terminal end, as shown by the appearance of proline as the first metabolite of the MIF degradation, followed by leucine, glycinamide, leucylglycine, and glycine. Activity in the rat brain regions was in the order striatum, medulla oblongata > cortex, hippocampus, midbrain > hypothalamus, cerebellum, and pituitary. The enzyme was mostly in the supernatant, with significant amounts in the myelin and synaptosomal fractions. MIF aminopeptidase could be separated from carboxypeptidase by centrifugation at 30,000 x g for 20 min and precipitation with 45--75% (NH4)2SO4. It showed pH optima in the alkaline range (8.25 and 8.75) and was inhibited by EDTA, EGTA, SQ 14,225, puromycin, bacitracin, and bestatin.

Separation of alkylaminonaphthylenesulfonyl peptides and amin acids by high-performance liquid chromatography. Methods for measuring melanotropin inhibiting factor breakdown

N,N-Dimethyl diethyl, dipropyl, dibutyl, and N-monoisopropylaminoaphthylenesulfonyl derivatives of melanotropin inhibiting factor (MIF) and its metabolites were prepared, and their chromatographic behavior was investigated with thin-layer chromatography (TLC) and high-performance liquid chromatography (HPLC), using five solvent systems on polyamide layers and ten solvent systems on muBondapak C18 and muBondapak phenyl columns. A mixture of MIF and its metabolites derivatized with Dns chloride was adequately resolved by two-dimensional chromatography on polyamide layer with solvent systems, formic acid-water (3:97) and benzene-acetic acid (9:1). Bns-MIF and its metabolites were separated with muBondapak C18 column with the solvent system acetonitrile-0.01 M sodium sulphate buffer, pH 7 (50:50). They were separated into five groups: Gly and Bns acid; Pro-Leu, Leu-Gly and Leu; Pro; Gly-NH2; and MIF. The alkylaminonaphthylenesulfonyl derivates had strong fluorescence, which permitted their detection at the level of 10(-11) to 10(-9) mol. Dns-MIF and its derivatives had the lowest detectable amounts. HPLC with the aid of the Dns derivation is reliable and fast, and is the preferable method for study of neuropeptide breakdown.

Opiate receptor antagonism by L-prolyl-L-leucyl-glycinamide, MIF-I

Chronic administration of 1-prolyl-l-leucyl-glycinamide (MIF-I) to mice slightly reduced morphine's antinociceptive activity in the hot-plate test and modified the biphasic motor activity response to morphine. MIF-I antagonized the initial depression of activity and potentiated the increased motor activity phase. Chronic treatment of rats with MIF-I prevented morphine's antinociceptive activity in the tail flick tests. MIF-I partly antagonized the inhibition by morphine of the coaxially stimulated guinea-pig ileum preparation. The inhibition of the ileum produced by ethylketocyclazocine was weakly antagonized by MIF-I. In contrast, MIF-I had no effect on the inhibition of the stimulated mouse vas deferens produced by Leu-enkephalin. The findings show that MIF-I weakly and selectively inhibits mu-type opiate receptors which suggests that MIF-I could be an endogenous inhibitor of opiate receptors.

MIF-I's differential actions as an opiate antagonist

The effects of MIF-I (Pro-Leu-Gly-NH2) were examined in three experimental conditions in which the opiate antagonist naloxone is active. MIF-I was found to block the analgesic effects of enkephalins and also morphine in the tail-flick test but not in the vas deferens assay. Unlike naloxone, MIF-I did not seem to reduce food intake in VMH-lesioned rats. The results suggest the possibility that MIF-I may represent a class of naturally occurring opiate antagonists with varying activities in independent situations.