Neurotransmitter-mediated anxiogenic action of PACAP-38 in rats.

The action of PACAP-38 was studied by measuring the anxiogenic-anxiolytic behavior of rats in an elevated plus maze. PACAP-38 was administered into the lateral brain ventricle and the behavior of the animals was measured 3h later. The possible involvement of transmitters was measured by pretreating the animals with receptor blockers which alone did not influence the task, but in the doses used were effective with other neuropeptides. The receptor antagonist PACAP 6-38 (a PAC 1/VPAC2 receptor antagonist of PACAP-38 receptor), haloperidol (a non-selective dopamine receptor antagonist), phenoxybenzamine (an α1/α2ß-adrenergic receptor antagonist), propranolol(a ß-adrenergic receptor antagonist), bicuculline (a gamma-aminobutyric acid subunit A receptor antagonist), methysergide (a nonselective 5-HT2 serotonergic receptor antagonist), atropine (a nonselective muscarinic acetylcholine receptor antagonist), naloxone (a nonselective opioid receptor antagonist) and nitro-l-arginine which acts by blocking the enzyme nitric oxide synthase, thereby blocking the nitric oxide synthesis, were tested. The following parameters were measured: the time spent in open arms/the time spent in total entries. PACAP-38 decreased the ratio of time spent in open arms to the time spent in total entries, indicating anxiogenic action. The total number of entries was not altered significantly either by PACAP-38 or by the receptor blockers. The following receptor blockers diminished the action of PACAP-38: PACAP 6-38,haloperidol, methysergide, naloxone and nitro-l-arginine. Pretreatment with atropine, phenoxybenzamine, propranolol and bicuculline did not influence the action of PACAP-38 on the time spent in open arms. The results demonstrate that PACAP-38 administered into the lateral brain ventricle exerted anxiogenic action at 3 h following treatment. Pretreatment of the animals with various receptor blockers indicated that a nonselective dopaminergic receptor antagonist, 5HT2 serotonergic and opioid receptors, nitric oxide and PAC1 receptors are involved in the anxiogenic action induced by PACAP-38.

Mediators involved in the hyperthermic action of neuromedin U in rats.

Neuromedin U (NmU), first was isolated from the porcine spinal cord, has subsequently been demonstrated in a number of species, in which it is present in the periphery and also the brain. Two receptors have been identified: NmU1R is mainly present in peripheral tissues, and Nmu2R in the central nervous system. NmU, a potent endogenous anorectic, serves as a catabolic signaling molecule in the brain; it inhibits food uptake, increases locomotion, activates stress mechanism, having cardiovasscular effects and, causes hyperthermia. The mechanism of this hyperthermia is unknown. In the present experiments, the effects of NmU on the colon temperature following i.c.v administration were studied in rats. For an investigation of the possible role of receptors in mediating hyperthermia, the animals were treated simultaneously with CRF 9-41 and antalarmin, a CRH1 receptor inhibitors, astressin 2B, a CRH2 receptor antagonist, haloperidol a dopamine receptor antagonist, atropine a muscarinic cholinergic receptor antagonist, noraminophenazone a cyclooxygenase inhibitor or isatin, a prostaglandin receptor antagonist. NmU increased the colon temperature, maximal action being observed at 2-3h. CRF 9-41, antalarmin, astressin 2B haloperidol, atropine, noraminophenazone and isatin prevented the NmU-induced increase in colon temperature. The results demonstrated that, when injected into the lateral brain ventricle NmU increased the body temperature, mediated by CRHR1 and CRHR2, dopamine and muscarinic cholinergic receptors. The final pathway involves prostaglandin.

Urocortin 3 administration impairs fear motivated learning in mice is mediated by transmitters.

Urocortin 3 (Ucn 3) was found to impair passive avoidance learning in male and female mice. The possible involvement of transmitters in the action, the animals were pretreated with the following receptor antagonists in doses which alone could not influence the measurement. Haloperidol (a D2, dopamine receptor antagonist), phenoxybenzamine (a nonselective α1-adrenergic receptor antagonist), atropine (a nonselective muscarinic acetylcholine receptor antagonist), bicuculline (a γ-aminobutyric acid subunit A receptor antagonist), nitro-L-arginine (a nitric oxide synthase inhibitor), antalarmin (a CRF1 receptor antagonist) and astressin 2B (a CRF2 receptor antagonist). Haloperidol, phenoxybenzamine, bicuculline, atropine, nitro-L-arginine and astressin 2B prevented the action of Ucn 3, in both sexes, whereas antalarmin exerted no action in either male or female animals.

Anxiolytic action of neuromedin-U and neurotransmitters involved in mice.

Peptide Neuromedin-U (NmU) is widely distributed in the central nervous system and the peripheral tissues. Its physiological effects include the regulation of blood pressure, heart rate, and body temperature, and the inhibition of gastric acid secretion. The action of NmU in rats is mediated by two G-protein-coupled receptors, NmU-1R and NmU-2R. NmU-2R is present mainly in the brain, and NmU-1R mainly in the periphery. Despite the great variety of the physiological action of NmU, little is known about its possible effects in different forms of behavior, such as anxiety. In the present work, NmU-23 (the rodent form of the peptide) was tested for its effect on anxiety in elevated plus maze test in mice. For detection of the possible involvement of neurotransmitters, the mice were pretreated with receptor blockers: haloperidol (a D2, dopamine receptor antagonist), propranolol (a ß-adrenergic receptor antagonist), atropine (a nonselective muscarinic acetylcholine receptor antagonist), phenoxybenzamine (a nonselective α-adrenergic receptor antagonist) or nitro-l-arginine (a nitric oxide synthase inhibitor). The peptide and nitro-l-arginine were administered into the lateral brain ventricle, while the receptor blockers were applied intraperitoneally. An NmU-23 dose 0.5µg elicited anxiolytic action, whereas this action is faded away when the dose was increased. For further testing therefore 0.5µg i.c.v. was used. Propranolol and atropine fully blocked the NmU-induced anxiolytic action, while haloperidol, phenoxybenzamine and nitro-l-arginine were ineffective. The results suggest that ß-adrenergic and cholinergic mechanisms are involved in the anxiolytic action of NmU.

Involvement of transmitters in the anxiolytic action of urocortin 3 in mice.

Urocortin 3 (Ucn 3) was tested for anxiolytic action in mice an elevated plus maze. For detection of the possible involvement of neurotransmitters, the mice were pretreated with receptor blockers: haloperidol, phenoxybenzamine, propranolol, atropine, methysergide, bicuculline or naloxone. The peptide was administered into the lateral brain ventricle; the receptor blockers were applied intra- peritoneally. Ucn 3 alone elicited dose-dependent bell-shaped anxiolytic action. The most effective dose was 0.5 µg. In the combined testing a 0.5 µg dose was used. Haloperidol, propranolol, atropine, methysergide, and naloxone, blocked the Ucn 3-induced anxiolytic action, while phenoxybenzamine and bicuculline were ineffective. The results suggest that dopaminergic, beta-adrenergic, cholinergic, serotonergic and opiate transmissions are involved in the anxiolytic action of Ucn 3.

Neurotransmissions of antidepressant-like effects of kisspeptin-13.

Kisspeptins are G protein-coupled receptor ligands originally identified as human metastasis suppressor gene products that have the ability to suppress melanoma and breast cancer metastasis and which have recently been found to play an important role in initiating the secretion of gonadotropin-releasing hormone at puberty. In the brain, the gene is transcribed within the hippocampal dentate gyrus. Kisspeptin-13, one of the endogenous isoforms, consists of 13 amino acids. In this work, antidepressant-like effects of kisspeptin-13 were studied and the potential involvement of the adrenergic, serotonergic, cholinergic, dopaminergic and gabaergic receptors in its antidepressant-like effects was investigated in a modified forced swimming test (FST) in mice. The mice were pretreated with a nonselective α-adrenergic receptor antagonist, phenoxybenzamine, an α(1)/α(2ß)-adrenergic receptor antagonist, prazosin, an α(2)-adrenergic receptor antagonist, yohimbine, a ß-adrenergic receptor antagonist, propranolol, a mixed 5-HT(1)/5-HT(2) serotonergic receptor antagonist, methysergide, a nonselective 5-HT(2) serotonergic receptor antagonist, cyproheptadine, a nonselective muscarinic acetylcholine receptor antagonist, atropine, a D(2),D(3),D(4) dopamine receptor antagonist, haloperidol, or a γ-aminobutyric acid subunit A receptor antagonist, bicuculline. The FST revealed that kisspeptin-13 reversed the immobility, climbing and swimming times, suggesting antidepressant-like effects. Phenoxybenzamine, yohimbine and cyproheptadine prevented the effects of kisspeptin-13 on the immobility, climbing and swimming times, whereas prazosin, propranolol, methysergide, atropine, haloperidol and bicuculline did not modify the effects of kisspeptin-13. The results demonstrated that the antidepressant-like effects of kisspeptin-13 in a modified mouse FST are mediated, at least in part, by an interaction of the α(2)-adrenergic and 5-HT(2) serotonergic receptors.

Neurotransmission of the antidepressant-like effects of the growth hormone-releasing hormone antagonist MZ-4-71.

MZ-4-71 is an antagonist of growth hormone-releasing hormone (GH-RH) which suppresses the secretion of GH-RH. It has been shown that MZ-4-71 has antidepressive-like effects in a modified forced swimming test (FST) in mice, exerts anxiolytic effects in an elevated plus maze test, improves memory consolidation in passive avoidance learning, and corrects the impairment of memory consolidation caused by ß-amyloid 25-35 in mice. However, little is known about the mechanisms of action of MZ-4-71 on brain functions. The involvement of the adrenergic, serotonergic, cholinergic, dopaminergic or GABA-ergic receptors in the antidepressant-like action of MZ-4-71 (1.0 µg/2 µl, intracerebroventricular (i.c.v.)) was studied in a modified mouse forced swimming test (FST). Mice were pretreated with a non-selective α-adrenergic receptor antagonist, phenoxybenzamine, an α1/α2ß-adrenergic receptor antagonist prazosin, an α2-adrenergic receptor antagonist, yohimbine, a ß-adrenergic receptor antagonist, propranolol, a mixed 5-HT1/5-HT2 serotonergic receptor antagonist methysergide, a non-selective 5-HT2 serotonergic receptor antagonist, cyproheptadine, a non-selective muscarinic acetylcholine receptor antagonist, atropine, a D2, D3, D4 dopamine receptor antagonist, haloperidol or a γ-aminobutyric acid subunit A (GABA-A) receptor antagonist bicuculline. Phenoxybenzamine, prazosin, methysergide, cyproheptadine and atropine prevented the effects of MZ-4-71 on the immobility, the climbing and the swimming times. Yohimbine, propranolol, haloperidol and bicuculline did not change the effects of MZ-4-71. The results demonstrated that the antidepressant-like effects of MZ-4-71 in this modified mouse FST are mediated, at least in part, by the an interaction of the α1-adrenergic, 5-HT1/5-HT2 serotonergic, and muscarinic acetylcholine receptors.

Anxiolytic action of urocortin 3 fragments in mice.

In the present experiments, the anxiolytic action of Ucn 3 fragments were studied in an elevated plus maze in mice following icv administration in an attempt to identify the biologically active center of the molecule. Ucn 3, Ucn 3 (18-38), Ucn 3 (19-27), Ucn 3 (28-38), Ucn 3 (34-38), and Ucn 3 (36-38) but not Ucn 3 (34-36), demonstrated anxiolytic action by increasing the time the mice spent in open arms, and Ucn 3 and Ucn 3 (34-38) did so by increasing the number of entries into open arms. The shortest molecule which elicited anxiolytic effects was the tripeptide Ucn 3 (36-38), H-Ala-Gln-Ile-NH(2). The results indicate that the total sequence of Ucn 3 is not necessary for the anxiolytic action of Ucn 3. Establishment the smallest active sequence of Ucn 3 will allow the synthesis of analogs or mimetics for anxiolytic action.

Antidepressant-like effects of urocortin 3 fragments.

Urocortin 3 (Ucn 3) and various Ucn 3 fragments were tested with regard to antidepressive action in a modified forced swimming test following icv administration to mice. The fragment Ucn 3 (34-36) proved ineffective, whereas Ucn 3 (18-38), Ucn 3 (19-27), Ucn 3 (28-38), Ucn 3 (34-38), and Ucn 3 (36-38) demonstrated antidepressive-like action by shortening the immobility time and increasing the climbing and swimming times. The shortest molecule which elicited most of the antidepressive effects was the tripeptide Ucn 3 (36-38), H-Ala-Gln-Ile-NH(2). The results indicate that the total sequence of Ucn 3 is not necessary for the antidepressive action of Ucn 3. Furthermore, the antidepressant actions of Unc3 (19-27) and Ucn 3 (36-38) can be blocked by the CRF2 receptor antagonist Astressin 2ß. Establishment of the smallest active sequence of the molecule may allow the synthesis of analogs or mimetics for antidepressive drugs.

Effects of the LHRH antagonist Cetrorelix on affective and cognitive functions in rats.

The decapeptide LHRH antagonist, Cetrorelix, inhibits gonadotropin and sex-steroid secretion. Cetrorelix is used for IVF-ET procedures and for the treatment of benign prostatic hyperplasia, endometriosis and leiomyomas. However little is known about the effects of Cetrorelix on brain functions. Previously we have tested Cetrorelix in mice on the impairment of the consolidation of a passive avoidance behavior caused by beta-amyloid 25-35, anxiolytic action in the plus-maze, antidepressive action in a forced swimming test, tail suspension and open-field behavior following its administration into the lateral brain ventricle. In the present study we repeated and extended the experiments in rats in order to determine whether there are species differences in the action of Cetrorelix between mice and rats. The effects of Cetrorelix evaluated included the methods used in mice without tail suspension test and extended by measuring core temperature. Cetrorelix fully blocked the impairment of the consolidation of passive avoidance learning when given icv 30 min following administration of beta-amyloid 25-35. If beta-amyloid 25-35 and Cetrorelix were given simultaneously, Cetrorelix was ineffective. Cetrorelix elicited slight anxiogenic and stronger anxiolytic action in the plus-maze, depending on the dose used. In the forced swimming tests, Cetrorelix showed antidepressive-like action. In open-field behavior tests Cetrorelix displayed a U-type action on locomotion with 0.5 and 2 microg increasing locomotion, and increase rearing but and had no effect on grooming at 0.5-2 microg. Cetrorelix had no action on core temperature. Our findings demonstrate that Cetrorelix is able to correct the impairment of the memory consolidation caused by beta-amyloid 25-35. Cetrorelix elicits anxiolytic and antidepressive action, slightly increases locomotion and rearing in open field, but it does not influence the core temperature. The results obtained in rats are similar to those reported previously by us in mice. Collectively our findings confirm the effects of Cetrorelix on brain function in two species and suggest the possible merit of a clinical trial with Cetrorelix in patients with anxiety, depression and Alzheimer's disease.

Involvement of CRH receptors in urocortin-induced hyperthermia.

The actions of individual corticotropin-releasing hormone (CRH) receptor (CRHR1 and CRHR2) were studied on the hyperthermia caused by urocortin 1, urocortin 2 and urocortin 3 in rats. Urocortin 1, urocortin 2 or urocortin 3 was injected into the lateral brain ventricle in conscious rats and the colon temperature was measured at different times following injection, up to 6h. In order to study the possible role of CRH receptors, the animals were treated with a urocortins together with the urocortin receptor inhibitors CRF 9-41, antalarmin and astressin 2B to influence the action of urocortins in initiating hyperthermia. Urocortin 1 at a dose of 2microg caused an increase in colon temperature, maximal action being observed in body temperature at 3h. CRH 9-41 and antalarmin, CRHR1 receptor antagonists, prevented the urocortin-induced increase in colon temperature while astressin 2B (CRHR2 receptor antagonist) was ineffective. Urocortin 2 at a dose of 2microg showed a byphasic action in increase in colon temperature having the first peak between 30 min and 1h and the second peak at 4h following treatment. CRF (9-41) and antalarmin was ineffective while astressin 2B fully blocked the action of urocortin 2. Urocortin 3 in a dose of lmicrog increased colon temperature; the maximal effect was observed at 2h. CRF (9-41) and antalarmin was ineffective while astressin 2B fully blocked the action of urocortin 3. The results demonstrated that urocortin 1, 2 or 3 when injected into the lateral brain ventricle caused increases in body temperature is mediated by urocortin receptors. The action of urocortin 1 is mediated by CRHR1 receptor, while in the action of urocortin 2 and urocortin 3 CRHR2 receptor is involved.

The action of urocortins on body temperature in rats.

The actions of individual urocortins on colon temperature were studied in rats. Urocortin 1, urocortin 2 or urocortin 3 was injected into the lateral brain ventricle in conscious rats and the colon temperature was measured at different times following injection, for up to 6 h. In order to study the possible role of prostaglandins, the animals were treated with either a urocortin together with the pyrazolone derivative noraminophenazone to inhibit the action of cyclooxygenase in initiating hyperthermia, or with noraminophenazone 30 min following urocortin administration to act on existing hyperthermia. Noraminophenazone was administered intramuscularly in a dose of 50 mg/kg. Urocortin 1 caused a dose-related increase in colon temperature, maximal action being observed at a dose of 2 microg with the maximal increase in body temperature at 4 h. Noraminophenazone prevented the urocortin-induced increase in colon temperature and attenuated the already existing elevated body temperature. Somewhat similar action was observed with urocortin 2. However, following treatment with 0.5 or 1.0 microg urocortin 2, the action was already over at 2 h, whereas 2 microg increased the colon temperature steadily, with a maximum at 4 h. Noraminophenazone blocked or diminished the action of urocortin 2. Urocortin 3 in a dose of 1 microg was the most effective in increasing the colon temperature; the maximal effect was observed at 2 h. Noraminophenazone blocked the development of urocortin 3-induced hyperthermia, or attenuated it when the hyperthermia was already present. The results demonstrated that urocortin 1, 2 or 3 caused increases in body temperature when injected into the lateral brain ventricle, though the optimal dose and the duration of hyperthermia differed for the individual urocortins. The cyclooxygenase inhibitor blocked or diminished the action of these urocortins, indicating the involvement of prostaglandins in urocortin-induced hyperthermia.

Mediation of the behavioral, endocrine and thermoregulatory actions of ghrelin.

The action of ghrelin on telemetrically recorded motor activity and the transmission of the effects of this neuropeptide on spontaneous and exploratory motor activity and some related endocrine and homeostatic parameters were investigated. Different doses (0.5-5 microg) of ghrelin administered intracerebroventricularly caused significant increases in both square crossing and rearing activity in the "open-field" apparatus, while only the dose of 5 microg evoked a significant increase in the spontaneous locomotor activity recorded by telemetry. Ghrelin also induced significant increases in corticosterone release and core temperature. To determine the transmission of these neuroendocrine actions, the rats were pretreated with different antagonists, such as a corticotropin-releasing hormone (CRH) antagonist (alpha-helical CRH(9-41)), the nitric oxide synthase inhibitor Nomega-nitro-L-arginine-methyl ester (L-NAME), haloperidol, cyproheptadine or the cyclooxygenase inhibitor noraminophenazone (NAP). The open-field and biotelemetric observations revealed that the motor responses were diminished by pretreatment with the CRH antagonist and haloperidol. In the case of HPA (hypothalamic pituitary adrenal) activation, only cyproheptadine pretreatment proved effective; haloperidol and L-NAME did not modify the corticosterone response. NAP had only a transient, while cyproheptadine elicited a more permanent impact on the hyperthermic response evoked by ghrelin; the other antagonists proved to be ineffective. The present data suggest that both CRH release and dopaminergic transmission may be involved in the ghrelin-evoked behavioral responses. On the other hand, ghrelin appears to have an impact on the HPA response via a serotonergic pathway and on the hyperthermic response via a cyclooxygenase and a serotonergic pathway.

Involvement of neurotransmitters in urocortin-induced passive avoidance learning in mice.

The action of urocortin on one-way passive avoidance learning was tested in mice. Urocortin was administered into the lateral brain ventricle and the latency of the passive avoidance response was measured 24 h later. For study of the roles of various neurotransmitters in mediating the action of urocortin on the consolidation of memory, the animals were pretreated with different receptor antagonists. Urocortin facilitated the acquisition, consolidation and also retrieval of the passive avoidance response. The following receptor antagonists blocked the action of urocortin on consolidation: haloperidol, atropine, phenoxybenzamine, bicuculline, the CRF antagonist CRF9-41 and methysergide. Propranolol attenuated, but did not fully block the action of urocortin, while naloxone and nitro-L-arginine were ineffective. The results obtained demonstrate that urocortin is able to improve learning and memory and also retrieval processes in a passive avoidance learning in mice. D2, muscarinic cholinergic, alfa-adrenergic, CRF, serotonergic (5HT 1-2), GABA B receptors are involved in the consolidation of the passive avoidance response.

Behavioral, neuroendocrine and thermoregulatory actions of apelin-13.

As the distribution of apelinergic neurons in the brain suggests an important role of apelin-13 in the regulation of neuroendocrine processes, in the present experiments the effects of this recently identified neuropeptide on the open-field activity, the hypothalamo-pituitary-adrenal (HPA) system and the body temperature were investigated. I.c.v. administration of apelin-13 (1-10 microg) to rats caused significant increases in square crossing, rearing, plasma corticosterone release and core temperature, whereas it did not influence the spontaneous motor activity during telemetric observation. To determine the mediation of the actions of apelin, a corticotropin-releasing hormone (CRH) antagonist, the nonselective dopamine antagonist haloperidol, the selective dopamine D1 receptor antagonist SCH-23390 and the nitric oxide synthase inhibitor Nomega-nitro-L-arginine-methyl ester (L-NAME) were administered to the rats. The apelin-evoked HPA activation was diminished by preadministration of the CRH antagonist, while the dopamine antagonist and L-NAME attenuated only the square crossing and rearing induced by apelin-13. To characterize the transmission of the thermoregulatory action of apelin, animals were pretreated either with L-NAME, the CRH antagonist or with the cyclooxygenase inhibitor noraminophenazone. L-NAME and the CRH antagonist did not cause significant inhibition of the apelin-evoked increase in core temperature, while the cyclooxygenase inhibitor, applied 30 min before peptide treatment, did not prove effective in preventing the apelin-evoked thermoregulatory response, whereas when it was administered 2 h after the peptide treatment, it transiently and significantly reduced the hyperthermic response. The present data suggest that apelin-13 plays an important role in the regulation of behavioral, endocrine and homeostatic responses in the CNS, and dopamine, nitric oxide and prostaglandins seem to take part in the mediation of its effects. Since the corticosterone response could be blocked by the CRH antagonist, it is likely to be mediated through the activation of the CRH neurons.

Involvement of different receptors in pituitary adenylate cyclase activating polypeptide induced open field activity in rats.

The action of pituitary adenylate cyclase activating polypeptide (PACAP) 38 was tested in an open field 30 min, 3 h, 6 h and 24 h after icv PACAP 38 administration in rats. The effects on locomotion, rearing and grooming were measured. The possible roles of different receptors were tested in animals that had been pretreated with different receptor blockers followed by PACAP 38 administration. The locomotion, rearing and grooming activities were increased at 30 min, after PACAP 38 administration, whereas at 3 and 6 h there was no change in grooming, while the locomotion and rearing activities were sharply decreased. At 24 h after PACAP administration, there was no change in any of the parameters studied. PACAP antiserum, a PACAP antagonist (PACAP 6-38), haloperidol, phenoxybenzamine, propranolol and naloxone each prevented the changes observed at 30 min and 3 h. Atropine, nitro-l-arginine, bicuculline and methysergide were ineffective. The data demonstrate that the action of PACAP 38 on the open-field activity is regulated by D2, alpha- and beta-adrenergic and opiate receptors.

The involvement of dopamine and nitric oxide in the endocrine and behavioural action of endomorphin-1.

Previous publications have demonstrated a prominent central and corticotropin releasing hormone-mediated action of the endomorphins (EMs) on both open-field behaviour and the hypothalamo-pituitary-adrenal (HPA) axis. In the present experiments, the direct action of endomorphin-1 (EM1) on pituitary adrenocorticotropic hormone (ACTH) release, adrenal corticosterone secretion and the roles of nitric oxide (NO) and dopamine (DA) in the HPA and behavioural responses elicited by EM1 were investigated in mice. In vitro perifusion studies indicated that the action of EM1 on the HPA system appears to be confined to the hypothalamus, as EM1 did not influence the corticosterone secretion from adrenal slices and moderately attenuated the ACTH release from anterior pituitary slices. In in vivo experiments, NG-nitro-L-arginine (L-NNArg) pretreatment brought about a profound inhibition of both the endocrine and the behavioural responses. On the other hand, haloperidol completely abolished the increases in square crossing and rearing, without affecting corticosterone release. The direct action of EM1 on striatal DA release was therefore also investigated in an in vitro superfusion system. Although EM1 did not influence the basal release of tritiated DA, it significantly enhanced the transmitter release evoked by electric impulses and pretreatment with L-NNArg resulted in a considerable inhibition of the release elicited by EM1. In conclusion, our endocrine studies suggest an important role of NO in the mediation of the EM1-evoked corticosterone secretion. They also indicate that EM1 activates the HPA axis at a hypothalamic level and dopamine is not involved in this process. In contrast, the behavioural experiments reflect that the locomotor activation induced by EM1 is mediated by NO and dopamine, and the superfusion studies demonstrate that NO transmits the dopamine release enhancing effect of EM1.

Behavioral and neuroendocrine actions of the Met-enkephalin-related peptide MERF.

The effects and the mediation of the action of the proenkephalin derivative Met(5)-enkephalin-Arg(6)-Phe(7) (MERF) on the hypothalamo-pituitary-adrenal (HPA) system and open-field behavior were investigated in mice. Intracerebroventricular injection of the heptapeptide increased square crossing, rearing, and plasma corticosterone level. To characterize the receptors involved in these neuroendocrine processes, animals were pretreated either with the nonselective opioid antagonist naloxone or the kappa-antagonist nor-binaltorphimine (nor-BNI). Both antagonists dose-dependently attenuated the HPA activation elicited by MERF. Naloxone also blocked the behavioral responses, but nor-binaltorphimine did not elicit a significant inhibition. The dopamine antagonist haloperidol and a corticotropin-releasing hormone (CRH) antagonist were also preadministered to shed light on the transmission of the actions of MERF. Both the motor responses and the HPA activation were diminished by the preadministration of the CRH antagonist, while haloperidol attenuated only square crossing and rearing. To investigate the direct effect of MERF on the dopaminergic system, dopamine release of striatal slices was measured in a superfusion system. Neither the basal nor the electric impulse-evoked dopamine release was modified by MERF. The results suggest that opioid-mediation predominate in the neuroendocrine actions of MERF, and the effect of the heptapeptide on the HPA system seems to be mediated by kappa-receptors. In the behavioral responses evoked by MERF, both CRH release and the action of the dopaminergic neurons of the subcortical motor system might be involved. MERF also appears to activate the paraventricular CRH neurons, but dopaminergic transmission does not seem to play a significant role in its hypothalamic action.

The action of orexin A on passive avoidance learning. Involvement of transmitters.

The action of orexin A on one-way passive avoidance learning was studied in rats. Orexin A administered into the lateral brain ventricle in conscious rats facilitated learning, the consolidation of learning and also retrieval processes in a dose-dependent manner in a passive avoidance paradigm. The involvement of transmitters was studied by pretreating the animals with receptor antagonists, which had proved to be effective with other neuropeptides in attenuating or blocking the action of orexin A. The following receptor blockers were used: haloperidol, phenoxybenzamine, propranolol, atropine, bicuculline, naloxone and nitro-L-arginine, which can block nitric oxide synthase. In the doses used all of the receptor blockers attenuated, but none of them fully blocked the action of orexin A on the consolidation of passive avoidance learning. The results demonstrate that orexin A is able to facilitate learning, consolidation of learning and also retrieval processes in a passive avoidance paradigm. A number of transmitters could be involved in the action of consolidation, but none of them is absolutely essential.