A hypotensive and bradycardic action of gamma 2-melanocyte-stimulating hormone (gamma 2-MSH) microinjected into the nucleus tractus solitarii of the rat.

In conscious and urethane-anesthetized rats intravenously (i.v.) administered gamma 2-melanocyte-stimulating hormone (gamma 2-MSH), a melanotropin derived from the precursor peptide pro-opiomelanocortin (POMC), has been shown to induce a pressor response combined with a tachycardia. A site of action within the hindbrain, e.g. the nucleus tractus solitarii (NTS) or the area postrema (AP), has been suggested. In order to test the postulate that gamma 2-MSH acts within these hindbrain regions, the peptide was microinjected into various parts of the NTS and into the AP of urethane-anesthetized rats and blood pressure (BP) and heart rate (HR) were measured. Injection of gamma 2-MSH (100-500 pmol) into the NTS resulted in a dose-dependent decrease in BP and HR rather than in the expected pressor and tachycardiac response which is generally found in conscious and urethane-anesthetized rats following i.v. administration of the peptide. With respect to the depressor and bradycardiac effect the melanotropin was far more potent when injected into the pars commissuralis than into the medial part of the NTS. The responses were maximal after 3-4 min and lasted for about 15 min. gamma 2-MSH had no effect when injected into the AP. It is noteworthy that also a hypotensive and bradycardic effect for gamma 2-MSH is found in pentobarbital-anesthetized rats following i.v. administration. Therefore, we conclude that in addition to a pressor and tachycardic response gamma 2-MSH can elicit an opposite effect by interaction with structures within a discrete region in the NTS, the pars commissuralis.(ABSTRACT TRUNCATED AT 250 WORDS)

Peptide-induced grooming behavior and caudate nucleus dopamine release.

We simultaneously measured the display of grooming behavior and, by monitoring the extracellular dopamine concentration via transversal microdialysis, the release of dopamine in the caudate nucleus in freely moving rats after i.c.v. administration of 1 micrograms adrenocorticotropic hormone-(1-24) (ACTH-(1-24)). During a period of 1 h after administration of the peptide, the incidence of excessive grooming behavior was increased. Concomitantly, the concentration of dopamine in the caudate nucleus dialysates was significantly increased (maximal effect 151% of basal release) whereas that of its metabolite DOPAC was unchanged. The potent alpha-melanocyte stimulating hormone (alpha-MSH) receptor agonist, [Nle4,D-Phe7]alpha-MSH, induced grooming behavior and stimulated caudate nucleus dopamine release (maximal effect 148% of basal release) whereas ACTH-(7-16)-NH2 did neither induce grooming behavior nor cause an increase in caudate nucleus dopamine release. Single-dose tolerance was observed for ACTH-induced grooming but not for ACTH-induced dopamine release. These data are in support of the proposed involvement of brain dopamine systems in grooming behavior of the rat but at the same time suggest that the effect of ACTH/MSH-like peptides on dopaminergic transmission in the caudate nucleus is proximal to the final neural pathway involved in ACTH-induced grooming behavior.

Effect of ACTH-(4-10) and gamma 2-MSH on blood pressure after intracerebroventricular and intracisternal administration.

After intracerebroventricular (i.c.v.) administration of ACTH-(4-10) or gamma 2-MSH in doses of < or = 1,500 pmol, no changes were observed in mean arterial pressure (MAP) of conscious and urethane-anesthetized rats. When gamma 2-MSH was administered intracisternally (i.c.), a significant increase in MAP of approximately 15 mm Hg was observed after the two highest doses used (500 and 1,500 pmol) in conscious rats and also, though less pronounced, in urethane-anesthetized rats. Although the pressor effect of gamma 2-MSH after intravenous (i.v.) administration to conscious rats was maximal within 25 s and MAP had returned to preinjection values < or = 60 s, the pressor response after i.c. administration was slower in onset (maximal effect after 1-2 min) and of longer duration (return to preadministration values after 5 min). ACTH-(4-10) had a slight pressor effect after i.c. administration in doses of 1,500 and 2,500 pmol in conscious rats, but had no effect in urethane-anesthetized rats. These results indicate that sustained leakage of the peptides after i.c. administration, but not after i.c.v. administration, to the periphery is the causal factor of a modest pressor response. These results do not support the suggestion that the central nervous system (CNS) is the principal target of gamma-MSH-like peptides with respect to their pressor effect.

The hemodynamic effects of gamma 2-melanocyte-stimulating hormone and related melanotropins depend on the arousal potential of the rat.

In conscious rats, i.v. administered adrenocorticotropic hormone (ACTH-(4-10)) and gamma 2-melanocyte-stimulating hormone (gamma 2-MSH) induced a dose-dependent increase in blood pressure (BP), heart rate (HR) and pulse pressure (PP). No circadian influence on these effects was observed. The structurally related peptide, alpha-melanocyte-stimulating hormone (alpha-MSH), only caused an increase in HR, which was not dose-dependent, whereas the stable ACTH-(4-9) analog, Org 2766, was without effect on these hemodynamic parameters. In rats under light urethane-induced anesthesia, which is known to maintain reflexes and sufficient sympathetic tone, gamma 2-MSH caused hemodynamic responses similar to those observed in conscious rats. In contrast, gamma 2-MSH had an opposite effect in rats under deep pentobarbital-induced anesthesia: a depressor effect combined with a slight bradycardia. A comparative study with rats of a more arousable Wistar rat substrain (Riv:TOX) and of a less excitable rat substrain (U:WU) showed that the dose-pressor response curves for ACTH-(4-10) and gamma 2-MSH were shifted to the left in the more excitable rats as compared to the in the less excitable rats. We conclude that a restricted amino acid sequence in the N-terminal part of the pro-opiomelanocortin (POMC)-molecule (gamma 2-MSH/ACTH-(4-10)-like) is responsible for the stimulating effects on the cardiovascular system and that those effects are strongly dependent on the state of arousal, i.e. sympathetic tone, of the rat. These stimulatory effects override a depressor phenomenon which can only be detected during central depression.

Effect of the dopamine D2 receptor agonist quinpirole on the in vivo release of dopamine in the caudate nucleus of hypertensive rats.

Using an in vivo microdialysis method, we found that the extracellular concentrations of dopamine and its main metabolite dihydroxyphenylacetic acid (DOPAC) were lower in the caudate nucleus of 8-week-old spontaneously hypertensive rats (SHR) than in the same area of age-matched normotensive Wistar-Kyoto rats (WKY). No differences in the extracellular concentrations of dopamine and DOPAC were found between renal and deoxycorticosterone acetate (DOCA)-salt hypertensive rats when compared to their respective controls. After subcutaneous administration of the dopamine D2 receptor agonist quinpirole (10, 33 and 100 micrograms/kg), the amount of dopamine and DOPAC in the dialysates was diminished dose dependently. The quinpirole-mediated inhibition of dopamine release was more pronounced in SHR than in WKY, whereas inhibition of the extracellular DOPAC concentration was not different. Compared to WKY, the dose-response curve for the inhibition of dopamine release by quinpirole was shifted to the left in SHR and the maximal inhibition in response to the highest dose was significantly greater. Renal and DOCA-salt hypertensive rats showed no differences in the quinpirole-induced inhibition of the extracellular concentrations of striatal dopamine and DOPAC compared to their controls. The present findings on changes in dopaminergic neurotransmission and D2 autoreceptor-mediated modulation of dopamine release in genetically hypertensive rats but not in rats with experimentally induced hypertension provide further evidence for the hypothesis that alterations in the nigrostriatal dopamine system may be involved in the initiation of the development of spontaneous hypertension.

Effects of dynorphin A(1-13) and of fragments of beta-endorphin on blood pressure and heart rate of anesthetized rats.

The effect on blood pressure and heart rate of central administration of dynorphin A(1-13) and of beta-, gamma-, and alpha-endorphin related peptides was studied in urethane-anesthetized rats. Intracerebroventricular (i.c.v., 0.1-10 micrograms) administration of beta-endorphin resulted in a dose-dependent, naltrexone-reversible hypotension and bradycardia. N-terminally modified fragments of beta-endorphin did not reduce blood pressure and heart rate. On the other hand, a dose of 10 micrograms of beta-endorphin(1-27), which lacks the four C-terminal amino acid residues of beta-endorphin, induced a fall in blood pressure and had a biphasic effect on heart rate. These responses, however, were resistant to pretreatment with naltrexone. None of the fragments of beta-endorphin smaller than beta-endorphin(1-27) affected blood pressure when administered i.c.v. in a dose of 10 micrograms. A small transient bradycardia was observed after i.c.v. administration of 10 micrograms of beta-endorphin(1-26), alpha, and gamma-endorphin. The naltrexone-reversible bradycardic response of alpha- and gamma-endorphin was not present in des-tyrosine- and des-enkephalin-alpha- and gamma-endorphin and also not in alpha-endorphin(10-16) and gamma-endorphin(10-17). Upon i.c.v. administration (0.1-50 micrograms) a dose-dependent, naltrexone-reversible decrease in blood pressure and heart rate was induced by dynorphin A(1-13). The present data indicate a hypotensive influence of beta-endorphin, beta-endorphin(1-27), and dynorphin A(1-13), whereas other fragments of beta-endorphin had little or no effect on the cardiovascular parameters investigated.

Participation of opiate receptors located in the nucleus tractus solitarii in the hypotension induced by alpha-methyldopa.

The local administration of the opiate receptor antagonist, naltrexone, into the nucleus tractus solitarii (NTS) inhibited the hypotension induced by systemically injected alpha-methyldopa in conscious rats. In addition, the local injection into the NTS of a beta-endorphin antiserum but not of antisera against [Met5]enkephalin and dynorphin A(1-13) prevented the alpha-methyldopa-induced hypotension. These results suggest a role of opiate receptors in the NTS, or in a closely located medullary site, in the centrally mediated hypotension induced by alpha-methyldopa.

Microscope adapted for the generation of various contrast types: theory and experiment.

With the aid of a polarizer, an analyzer, and two birefringent double plates placed in conjugate positions at the front focal plane of the condenser and the rear focal plane of the objective, respectively, a microscope is adapted to generate in a pure form one of four contrast types, i.e., either phase or amplitude differential or nondifferential. The theory of such an arrangement is discussed, and some practical results are shown.