Non-opiate beta-endorphin fragments and dopamine--I. The neuroleptic-like gamma-endorphin fragments interfere with the behavioural effects elicited by small doses of apomorphine

The neurobiology of social play behavior in rats

Social play behavior is one of the earliest forms of non-mother-directed social behavior appearing in ontogeny in mammalian species. During the last century, there has been a lot of debate on the significance of social play behavior, but behavioral studies have indicated that social play behavior is a separate and relevant category of behavior. The present review provides a comprehensive survey of studies on the neurobiology of social play behavior. Evidence is presented that opioid and dopamine systems play a role in the reward aspect of social play behavior. The role of cholinergic, noradrenergic and opioid systems in attentional processes underlying the generation of social play behavior and the involvement of androgens in the sexual differentiation of social play behavior in rats is summarized. It is concluded that there is not only behavioral, but also neurobiological evidence to suggest that social play behavior represents a separate category of behavior, instead of a precursor for adult social, sexual or aggressive behavior.

Behavioral studies on the putative gamma-type endorphin receptor using different antibodies

To investigate the significance of endogenous, neuroleptic-like gamma-type endorphins and their putative receptors, polyclonal and monoclonal antibodies against gamma-type endorphins, which may bio-inactivate the ligands for the receptors, and monoclonal anti-idiotype antibodies, which presumably bind to the receptors, were injected into the nucleus accumbens of the rat brain. The desenkephalin-gamma-endorphin-induced antagonism of the hypomotility response elicited by challenge with apomorphine injected into the nucleus accumbens was used as test system. Both the anti-desenkephalin-gamma-endorphin antibodies and anti-idiotype antibodies blocked the action of exogenous desenkephalin-gamma-endorphin. Thus, the anti-idiotype antibodies may serve as receptor antagonists. Chronic treatment (injection into the nucleus accumbens) with the anti-idiotype antibodies induced sustained hypermotility, decreased habituation and impaired passive avoidance behavior. In such treated animals local treatment with apomorphine did not elicit hypomotility. It is suggested that gamma-type endorphins influence the setpoint for feedback regulation in dopaminergic neurons equipped with gamma-type endorphin receptor systems.

Subchronic treatment with the neuroleptic-like peptide desenkephalin-gamma-endorphin may decrease dopaminergic neurotransmission in the nucleus accumbens of rats

In rats, subchronic administration of desenkephalin-gamma-endorphin (DE gamma E) into the nucleus accumbens or subcutaneously for 10 days resulted in hypoactivity. Intra-accumbens administration caused a significant reduction in the nucleus accumbens tissue levels of the dopamine (DA) metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Systemic administration of DE gamma E decreased DOPAC and 5-hydroxyindoleacetic acid (5-HIAA) levels in nucleus accumbens tissue. Subchronic subcutaneous DE gamma E treatment reduced the basal release of [3H]DA from rat nucleus accumbens slices in vitro and the basal release of endogenous DA and DOPAC in vivo as assessed with on-line dialysis in the nucleus accumbens of freely moving rats. The DA agonist N,N-dipropyl-7-hydroxy-2-aminotetralin (DP-7-ATN) was equally effective in inhibiting [3H]DA release elicited by electrical stimulation from slices of subchronically DE gamma E and placebo treated rats. Administration of a small dose of apomorphine caused similar reductions of the in vivo release of DA and DOPAC in both placebo and DE gamma E treated rats. These results indicate that subchronic DE gamma E treatment may decrease dopaminergic neurotransmission in the nucleus accumbens. This effect is probably not due to alterations in the sensitivity of presynaptically located DA autoreceptors mediating DA release in vitro and in vivo.

Topography and characteristics of specific binding sites for non-opioid gamma-type endorphins in the rat brain as studied by autoradiography with [35S]Met-desenkephalin-gamma-endorphin

An in vitro autoradiographic study was performed to characterize specific rat brain binding sites for non-opioid neuroleptic-like gamma-type endorphins, using [35S]Met-des-enkephalin-gamma-endorphin ([35S]Met-DE gamma E; [35]S-beta-endorphins(5-17)) with high specific activity as radioligand. The binding sites appeared to be confined to rat forebrain regions, e.g., orbital cortex, frontal cortex, cingulate cortex, piriform cortex, nucleus accumbens, amygdala, mediodorsal nucleus of the thalamus and arcuate and periventricular nuclei of the hypothalamus. These regions are part of the mesocorticolimbic feedback circuit. Densitometric analysis of the autoradiographs revealed that the density of the binding sites was highest in the mediodorsal nucleus of the thalamus and the amygdala. Concentration-dependent displacement of [35S]Met-DE gamma E (500 pM) with DE gamma E yielded an IC50 of 0.6 nM whereas DE alpha E (beta-endorphin(6-16)) had an IC50 of 210 nM. Various endorphins, sharing the gamma-endorphin C terminus, displaced [35S]Met-DE gamma E to the same extent as non-labelled DE gamma E (at 10(-6) M) whereas non-endorphin peptides did not show displacing capacity. Possible relationships of the binding sites with opioid receptors were investigated. DAMGO (mu) and DPDPE (delta) displaced [35S]Met-DE gamma E to some extent at 10(-6) M whereas U69,593 (kappa) was inactive, suggesting that the binding sites for gamma-type endorphins may resemble mu- and delta-opioid receptors in some aspects. Similarly, relationships with dopamine receptors were investigated. Haloperidol partially displaced [35S]Met-DE gamma E whereas sulpiride, SKF38,393 and 3-PPP at 10(-6) M did not induce significant displacement. Thus, binding sites are distinct from dopamine receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of beta-casomorphins on apomorphine-induced hyperlocomotion

Derivatives of beta-casomorphin Tyr-Pro-Phe-Pro-Gly and their des-Tyr1-derivatives were investigated on the model of apomorphine-induced hyperlocomotion (1 mg/kg = 3 microM/kg, IP). D-Pip4 CM 5 (5 nM) inhibited the apomorphine hypermotility completely, while D-Phe3 CM 5 (5 nM) and D-Pro4 CM 5 (5 nM) decreased it only to about 50%. The normal exploration was nearly completely inhibited by D-Pro4 CM 5 (40 nM), by D-Pip4 CM 5 (5 nM) depressed to 20%, and by D-Phe3 CM 5 (10 nM) to 35%. The maximum inhibition of apomorphine-induced hyperlocomotion by the des-Tyr-casomorphin derivatives was about 50%. The dose-response curves were U-shaped. The exploratory activity was not significantly influenced. The mode of action and the involvement of different neurotransmitter systems in the inhibitory effect of beta-casomorphin derivatives on apomorphine hyperlocomotion are discussed.

Functional recovery after destruction of dopamine systems in the nucleus accumbens of rats. I. Behavioral and biochemical studies

Bilateral 6-hydroxydopamine (6-OHDA) lesions in the nucleus accumbens of rats induced motor hypoactivity 7 days after the lesion. Spontaneous functional recovery of this impaired behavior occurred in 3-4 weeks. Behavioral and biochemical studies suggest that the hypoactivity is due to damage of the dopamine systems in the nucleus accumbens. The 6-OHDA lesions induced a decrease in the nucleus accumbens levels of dopamine and its metabolites of about 30% both 7 and 20 days after the lesion. The in vitro uptake of [3H]dopamine in nucleus accumbens tissue of the 6-OHDA-lesioned rats was decreased to the same extent at 7, 14 and 28 days after the lesion. Scatchard analysis of [3H]haloperidol binding studies in nucleus accumbens tissue revealed a shift from one type of binding site in tissue of sham-lesioned rats to two types of binding sites in tissue of 6-OHDA-lesioned rats 29 days after the lesion. This shift was not present in nucleus accumbens tissue 8 days after a 6-OHDA lesion. The spontaneously recovered rats showed an enhanced behavioral response upon administration of the dopamine agonist apomorphine. The present data suggest that the spontaneous functional recovery of impaired motor activity is caused by the development of supersensitivity of the dopamine receptor systems in the nucleus accumbens. This supersensitivity may be the result of increased affinity of one type of binding site or an increased number of functional binding sites.

The neuropeptide, Org 5878 (desenkephalin-gamma-endorphin, DE gamma E), affects local cerebral glucose utilization in freely moving rats

The effect of the antipsychotic peptide, Org 5878 (desenkephalin-gamma-endorphin, beta-endorphin-(6-17), on local cerebral glucose utilization was studied in freely moving male Wistar rats. Org 5878 (20 micrograms/kg, i.v.) or saline were given acutely and local cerebral glucose utilization was measured in 116 brain structures. Glucose uptake was not altered by Org 5878 in most brain areas, including areas of the nigrostriatal system, the cortex and the thalamus. However, significant reductions in glucose uptake were observed in the ventral tegmental area, the diagonal band complex, the hippocampus, the amygdala, the interpeduncular nucleus, the reticular nucleus of the thalamus and the cerebellum. These results indicate that the nigrostriatal and cortico-thalamic systems remain unaffected but the activity of the mesolimbic ventral tegmental area and of major target areas of cholinergic basal forebrain structures is selectively reduced following Org 5878 administration. It is concluded that the effect of Org 5878 on local cerebral glucose utilization is distinct from and more selective than that of antipsychotics currently used in the clinic.

Involvement of opioid and dopaminergic systems in isolation-induced pinning and social grooming of young rats

Pinning, as a measure for play, and social grooming were simultaneously studied in juvenile rats. Short-term social isolation increased both behavioural responses. This increase was attenuated by the opioid antagonist naltrexone, whilst the opiate, morphine, and the opioid peptide beta-endorphin, increased the responses. Pinning was more sensitive to the effects of naltrexone, whilst beta-endorphin stimulated particularly social grooming. Small doses of the dopaminergic drug, apomorphine, decreased both pinning and grooming behaviour of the short-term isolated rats. Some of the effects were partially antagonized by the dopamine antagonist haloperidol, and the neurolepticum-like peptide, desenkephalin-gamma-endorphin (DE-gamma-E). A small dose of haloperidol and DE-gamma-E stimulated social grooming in particular, whilst a larger dose of haloperidol decreased pinning and social grooming. It is concluded that both opioid and dopaminergic systems are implicated in the increase of pinning and social grooming induced by short-term social isolation. The differences in sensitivity of pinning and social grooming for opioid and dopaminergic drugs and peptides are discussed in relation to possible differences in the neural systems underlying both social activities.

Dopamine release in the nucleus accumbens of freely moving rats determined by on-line dialysis: effects of apomorphine and the neuroleptic-like peptide desenkephalin-gamma-endorphin

This study examined the effects of apomorphine, sulpiride, desenkephalin-gamma-endorphin (DE gamma E) and a combination of DE gamma E with apomorphine on the release of dopamine (DA) and its main metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in the nucleus accumbens of freely moving rats. A fully automated on-line brain dialysis system was used. A small dose of s.c. administered apomorphine induced a decrease in the output of DA and DOPAC. Sulpiride, infused into the nucleus accumbens, induced a 2-fold increase in the output of DA, DOPAC and HVA. DE gamma E hardly modified either the basal release of DA, DOPAC and HVA or the apomorphine-induced attenuation of the release of DA and DOPAC. These results indicate a dissociation between the behavioural effects of DE gamma E and its effect on the release of DA in vivo.

The neuroleptic-like peptide desenkephalin-gamma-endorphin does not antagonize the dopamine receptor agonist-induced inhibition of the release of [3H]dopamine from rat nucleus accumbens slices in vitro

In rats, the non-opioid beta-endorphin (beta E) fragment desenkephalin-gamma-endorphin (DE gamma E, beta E6-17) antagonizes the hypomotility induced by a small dose of dopamine (DA) receptor agonists. It has been suggested that DE gamma E might act in this respect by a direct or indirect blockade of presynaptically located DA receptors in the nucleus accumbens, thereby causing an increase of DA release. Therefore in the present study the effect of DE gamma E was examined on DA receptor agonist-induced inhibition of the electrically evoked release of previously accumulated [3H]DA from rat nucleus accumbens slices in vitro. The DA receptor agonists apomorphine, LY 171555 and n,n-di-n-propyl-7-hydroxy-2-aminotetralin (DP-7-AT) inhibited in a concentration-dependent manner the electrically evoked release of [3H]DA. The selective D2 receptor antagonist (-)-sulpiride blocked the effects of apomorphine, corroborating that the DA receptor involved is of a D2 type. DE gamma E was tested at several concentrations (10(-9)-10(-6) M) and under various experimental conditions. DE gamma E, by itself, did not affect either the electrically stimulated or the basal release of [3H]DA. The inhibiting effect of DA receptor agonists was slightly reduced by DE gamma E, but this effect was present in some experiments only. It is concluded that DE gamma E does not function as an antagonist for the DA receptor mediating DA release and that the interaction observed in behavioural experiments between DA agonists and DE gamma E does not occur at the level of this receptor.

The hypomotility elicited by small doses of apomorphine seems exclusively mediated by dopaminergic systems in the nucleus accumbens

The reduction of motor activity elicited in rats by a subcutaneous injection of a small dose of apomorphine was reversed by pretreatment of the nucleus accumbens with haloperidol (10 pg), sulpiride (10 pg) or desenkephalin-gamma-endorphin (DE gamma E) (100 pg or 10 ng). These doses of the compounds did not change motor activity in placebo-treated rats. Pretreatment of the nucleus caudatus with the same neuroleptics or DE gamma E did not diminish the effect of subcutaneously administered low doses of apomorphine. A small dose of apomorphine decreased motor activity when it was injected directly into the nucleus accumbens. This effect was dose dependently antagonized by subcutaneous pretreatment with DE gamma E. It is suggested that the hypoactivity elicited by small doses of apomorphine is exclusively mediated by dopaminergic systems in the nucleus accumbens.

Desenkephalin-gamma-endorphin restores the impaired sensitivity of hypophysectomized rats to small doses of apomorphine

Hypophysectomized male rats, tested 7 days after removal of the pituitary, showed a reduced sensitivity to small doses of apomorphine. In these rats, subcutaneous (s.c.) treatment with apomorphine (25, 100 or 250 micrograms/kg) did not elicit any reduction in locomotor activity measured 5 min after injection in contrast to that observed in sham-operated control rats. The enhancement of locomotor activity and stereotyped behavior, 20 min after the s.c. administration of apomorphine, was similar in hypophysectomized rats and in control animals. Prolactin does not seem to be implicated in this altered sensitivity, since hyperprolactinaemia induced by pituitary homografts under the kidney capsule did not change the response to apomorphine in hypophysectomized rats. Chronic treatment with desenkephalin-gamma-endorphin (DE gamma E), a beta-endorphin fragment with neuroleptic-like properties, administered s.c. twice a day for 7 days at the dose of 10 micrograms/rat, restored the sensitivity to small doses of apomorphine in the hypophysectomized rats. The data suggest that removal of the pituitary leads to impaired sensitivity of presumably presynaptically located dopamine receptors mediating the effects of small doses of apomorphine without altering the sensitivity of postsynaptically located dopamine receptors that mediate the hypermotility and stereotypy induced by apomorphine. This impaired sensitivity to low doses of apomorphine could be restored by DE gamma E but not by prolactin, supporting the conclusions from previous experiments that DE gamma E and prolactin may selectively interfere with pre- and postsynaptic dopamine receptors respectively.

Beta-endorphin-(10-16) antagonizes behavioral responses elicited by melatonin following injection into the nucleus accumbens of rats

The behavioral changes induced by low doses of melatonin bilaterally injected into the nucleus accumbens of rats (decrease of locomotor activity and rearing and increase of grooming and sniffing behavior) were not affected by local pretreatment with beta-endorphin, but could be completely antagonized by alpha-type and gamma-type endorphins. Structure activity relationship studies revealed that the peptide beta-endorphin-(10-16) contains the essential information in this respect. The lowest effective dose of this peptide was 10 pg. The peptide, in contrast to gamma-type endorphins, did not interfere with the decrease of locomotor activity and rearing induced by injection of low doses of apomorphine into the nucleus accumbens. It is concluded that the described action of beta-endorphin-(10-16) resembles that of serotonin and various antidepressant drugs.

The effect of gamma-type endorphins on alpha-MSH release in the rat

Neuroleptic drugs increase the level of alpha-melanotropin (alpha-MSH) in the blood of the rat. We have investigated whether neuroleptic-like peptides, the gamma-type endorphins, also affect alpha-MSH release. A structure-activity study revealed that (des-enkephalin)-gamma-endorphin (DE gamma E, beta-LPH-(66-77), beta-endorphin-(6-17)) is able to increase plasma alpha-MSH levels after intracerebroventricular injection, while the longer gamma-type endorphins, i.e. gamma E (beta-LPH-(61-77)), beta-endorphin-(1-17)), and DT gamma E (beta-LPH-(62-77), beta-endorphin-(2-17)) were without effect in the dosage used. A dose-response study revealed a more or less bell-shaped relationship for the effect of DE gamma E on plasma alpha-MSH levels. The effect of DE gamma E could not be counteracted by apomorphine or naloxone. The observations indicate that DE gamma E increases plasma alpha-MSH levels in a way distinct from that of haloperidol and the opiate peptide beta-endorphin. On the other hand, a time-course of plasma alpha-MSH levels after DE gamma E administration resembled the one which has been seen after haloperidol injection. From experiments performed on pituitary neurointermediate lobes incubated in vitro, it seems not likely that DE gamma E acts directly on the dopamine receptors of the pituitary in affecting alpha-MSH release. In conclusion, it appears that DE gamma E affects alpha-MSH levels in plasma in a way distinct from that of the neuroleptic drug haloperidol and of the opiate-peptide beta-endorphin.

The supersensitivity of dopaminergic neurons to apomorphine in rats following chronic haloperidol

Supersensitivity to apomorphine's inhibitory effect on dopamine neurons was induced in Sprague-Dawley rats by chronically administering haloperidol. It is concluded that the increased sensitivity of dopamine neurons to apomorphine may be linked to its influence on autoreceptors. The possibility that autoreceptor supersensitivity could contribute to enhancement of antipsychotic therapy is discussed.

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.

gamma-Endorphin and N alpha-acetyl-gamma-endorphin interfere with distinct dopaminergic systems in the nucleus accumbens via opioid and non-opioid mechanisms

Low doses (10 ng) of the dopamine agonist apomorphine induced hypolocomotion when injected into the nucleus accumbens of rats. This behavioral response was antagonized by local treatment with either the opioid peptide gamma-endorphin (gamma E) or the non-opioid peptide N alpha-acetyl-gamma-endorphin (Ac gamma E) in a dose of 100 pg. High doses of apomorphine (10 micrograms) r amphetamine (2 micrograms) injected into the nucleus accumbens resulted in hyperlocomotion. This response was blocked by pretreatment with gamma E but not with Ac gamma E. This effect of gamma E could be prevented by local treatment with naloxone. Neither peptides interfered with the apomorphine-induced stereotyped sniffing when the substances were injected into the nucleus caudatus. It is concluded that gamma E and Ac gamma E differentially interact with distinct dopaminergic systems in the nucleus accumbens of the rat brain via an opioid and a non-opioid mechanism, suggesting that the peptide fragments originating from pro-opiomelanocortin may be specifically implicated in the control of dopaminergic activity in this brain area.

Clinical, biochemical, and hormonal aspects of treatment with Des-tyr1-gamma-endorphin in schizophrenia

Des-tyr1-gamma-endorphin (DT gamma E) was administered intramuscularly in a dose of 1 mg/day for 10 days to 18 neuroleptic-free schizophrenic patients in a double-blind crossover design. Six patients showed either a slight or no antipsychotic response; seven patients showed a moderate antipsychotic response; and the remaining five patients showed a marked antipsychotic response. DT gamma E led to a decrease of plasma prolactin levels in patients treated with DT gamma E in the first period of experimental treatment as compared to those treated with placebo. Neither plasma levels of growth hormone and cortisol nor cerebrospinal fluid concentrations of homovanillic acid, 5-hydroxyindoleacetic acid, and 3-methoxy-4-hydroxyphenylglycol were affected by DT gamma E. Patients suffering from a hebephrenic or paranoid type of schizophrenia and those presenting relatively fewer negative symptoms were most susceptible to treatment with DT gamma E. These data confirm and extend previous findings that DT gamma E has antipsychotic properties in a number of schizophrenic patients.

The influence of neuropeptides related to pro-opiomelanocortin on acquisition of heroin self-administration of rats

The influence of different neuropeptides related to pro-opiomelanocortin were tested on acquisition of heroin self-administration in rats. The animals were allowed to self-administer heroin intravenously on a continuous reinforcement schedule during 6 h daily sessions on 5 consecutive days. Treatment was performed subcutaneously 1 h before each daily session. It was found that the opioid peptides alpha-, gamma- and beta-endorphin hardly influenced acquisition of heroin self-administration, while the non-opioid fragments of alpha- and gamma- endorphin modulated this behavioral response. In fact, beta-endorphin (beta E) 2-9 tended to facilitate the rate of acquisition, while the gamma-type endorphins, des-Tyr1-gamma-endorphin (beta E 2-17) and des-enkephalin-gamma-endorphin (beta E 6-17), decreased heroin intake. Concerning the ACTH/MSH related peptides, a decreasing effect of heroin intake was found following treatment with (D-Phe7)-ACTH 4-10, with a high dose of the ACTH 4-9 analog Org 2766 and with gamma 2-MSH, while ACTH 1-24, ACTH 4-10 and a low dose of Org 2766 did not significantly influence self-injecting behavior. It is concluded that pro-opiomelanocortin serves as a precursor molecule for peptide fragments, which modulate the acquisition of heroin self-administration in rats.

Neuroleptic-like effects of ceruletide and cholecystokinin octapeptide: interactions with apomorphine, methylphenidate and picrotoxin

Haloperidol in low doses antagonized the apomorphine-induced cage-climbing behaviour of mice, whereas ceruletide (CER) and its analogue, Nle8-CER-(4-10) had very weak anticlimbing efficacy; Nle8-CER and diazepam were inactive. The ptosis caused by CER and cholecystokinin octapeptide (CCK-8) was antagonized by apomorphine in doses 27 times smaller than those effective against the haloperidol-induced ptosis. No such differences occurred when either methylphenidate or picrotoxin replaced apomorphine. Low-dose haloperidol was an antagonist to the antiptotic effect of apomorphine versus both CER and CCK-8. The rearing inhibiting effect of CER and haloperidol, in contrast to that of clonazepam, was very resistant to apomorphine. Methylphenidate was weakly effective against clonazepam and haloperidol but inactive versus CER. Picrotoxin was no antagonist to either rearing inhibiting agent. The results taken together suggest presynaptic sites of the dopaminergic system as important for the production of ptosis by CCK-like peptides.

In rats, the behavioral profile of CCK-8 related peptides resembles that of antipsychotic agents

The action of some CCK-8 related peptides, desulphated CCK-8 (CCK-DS), the sulphated form of CCK-8 (CCK-8-S) and ceruletide was explored in a number of test procedures with rats, in which antipsychotic agents are active. Following injection into the nucleus accumbens, all three peptides antagonized the hypolocomotion induced by low doses of apomorphine (10 ng). Ceruletide appeared to be the most potent in this respect (ED50: approximately 5 pg). The increased locomotion observed following injection of relatively high doses of apomorphine (10 micrograms) into the nucleus accumbens was antagonized by local pretreatment with CCK-8-S, but not with CCK-8-DS or ceruletide. None of these CCK-8 related peptides affected the stereotyped sniffing response elicited by treatment with apomorphine or amphetamine (10 micrograms) given into the nucleus caudatus. Passive avoidance behavior was facilitated following subcutaneous administration of 10 micrograms of CCK-8-related peptides 1 h before the retention test. The same periods given into the nucleus accumbens (0.3 pg) however attenuated passive avoidance behavior. intraventricular injection with CCK-8-DS and CCK-8-S induced a positive effect in various 'grip tests'. Given subcutaneously, the CCK-8-related peptides decreased the rate of ambulation and rearing especially in the middle of the open field. These results indicate that CCK-8 related peptides, especially CCK-8-DS and ceruletide, exhibit behavioral effects that are similar to those observed following treatment with gamma-type endorphins and that resemble the effects of antipsychotic agents. Very low doses of CCK-8 related peptides exert behavioral effects following injection into the nucleus accumbens, indicating that this brain area is extremely sensitive to the action of these peptides. It is postulated that certain peptides which are either present in neurons (like CCK-8-related peptides) or generated by brain endorphin systems (like gamma-type endorphins) control the activity of specific neurons of the mesolimbic dopaminergic pathways. This may be of relevance for the purported antipsychotic action of these peptides.

Local biotransformation of des-Tyr1-gamma-endorphin in brain studied by a push-pull technique and HPLC analysis

The biotransformation of [3H]des-Tyr1-gamma-endorphin locally in rat brain was studied by perfusion of the peptide via a push-pull cannula implanted in the nucleus caudatus. Metabolites were analyzed by high-pressure liquid chromatography. Free [3H]phenylalanine and a [3H]peptide with chromatographic properties of beta-endorphin2-13 were formed during push-pull perfusion. The method presented enables the detection of local proteolytic events in the brain and provides a novel tool for studies on neuropeptide biotransformation.

In vivo interaction of gamma-type endorphins with dopaminergic ligands in rat brain

The beta-endorphin (beta E) fragment des-Tyr1-gamma-endorphin (DT gamma E, beta E-(2-17)) has been reported to interact with neuroleptic binding in vivo but not in vitro. We have attempted to replicate the in vivo experiments and extended the work to include conditions in which des-enkephalin-gamma-endorphin (DE gamma E, beta E-(6-17)) exhibited behavioral activity. Systemically administered haloperidol significantly elevated plasma and decreased striatal [3H]spiperone. DE gamma E significantly elevated plasma [3H]apomorphine when both substances were injected directly into the nucleus accumbens. gamma-type endorphins consistently but non significantly decreased brain spiperone or apomorphine binding. It is concluded that the interaction between gamma-type endorphins and dopaminergic binding sites may be either indirect or limited to a subset of these sites.

Opposite interactions between alpha- and beta-endorphin fragments with dopamine mediated responses on the rat rectum in vitro

Dopamine causes a dose-dependent contraction of the rat rectum in vitro followed by a relaxation. This contraction can be inhibited by apomorphine and phenylephrine. This inhibition can be attenuated by the beta-endorphin (beta E) fragments 2-17 (des-Tyr1-gamma-endorphin, DT gamma E) and 6-17 (des-enkephalin-gamma-endorphin, DE gamma E). beta E 6-17 seems to be the shortest sequence with full activity in this respect since a shorter fragment (beta E 10-17) was less effective. The atypical neuroleptics oxypertine, sulpiride, and clozapine, the classic neuroleptic haloperidol and metoclopramide have a similar action to DE gamma E. The peptides and atypical neuroleptics do not affect the dopamine response per se while the classic neuroleptics haloperidol and metoclopramide enhance the dopamine response. The effects of the alpha-type endorphins are opposite to those of the gamma-type endorphins, since des-Tyr1-alpha-endorphin (DT alpha E, beta E 2-16) and des-enkephalin-alpha-endorphin (DE alpha E, beta E 6-16) enhance the phenylephrine-induced decreased responsiveness to dopamine. Structure-activity studies revealed that the active moiety of the alpha-endorphin fragments probably resides in the 6-9 region. In addition the alpha-type endorphins directly inhibit the dopamine response. It is concluded that the rat rectum may be used to analyse neuroleptic-like action. In this model alpha- and gamma-endorphin fragments may directly or indirectly influence the interaction of dopamine with the rectum. Because of the strong similarities between the effects of gamma-type endorphins and that of neuroleptics the results support the purported neuroleptic-like action of gamma-type endorphins. The influence of alpha-type endorphins and gamma-type endorphins on the apomorphine or phenylephrine induced decreased responsiveness to dopamine, although opposite, seems to be mediated by an influence on different dopamine sensitive systems.