Modulation of arginine vasopressin secretion from cultured ovine hypothalamic cells by glucocorticoids and opioid peptides

In sheep, arginine vasopressin (AVP) appears to be a more potent ACTH-releasing factor than ovine corticotrophin-releasing hormone. In order to investigate the neuroendocrine regulation of AVP secretion we have developed a novel system for maintaining fetal ovine hypothalamic neurones in serum-free culture. Hypothalamic neurones derived from fetal sheep at day 70 gestation (term = 145 days) secreted AVP under basal conditions and in response to repeated potassium-induced depolarizations, for up to 35 days in vitro. AVP secretion was time- and calcium-dependent. AVP secreted from ovine hypothalamic cells co-eluted with synthetic AVP on a Sephadex chromatography column and diluted in parallel with AVP standard in the radioimmunoassay. The addition of cortisol (150 nM) to medium bathing ovine hypothalamic cells significantly inhibited basal, and potassium-induced AVP secretion without altering the AVP content of the cell cultures. Furthermore, the opioid peptide [D-Pro10]Dynorphin(1-11) which acts via the kappa opioid receptor, significantly inhibited basal and potassium-stimulated AVP secretion, an effect which was abolished when cells were cultured in the presence of cortisol. These data show that hypothalamic AVP is a site for negative feedback regulation within the ovine hypothalamic-pituitary-adrenal axis. Furthermore, these data suggest that the kappa opioid system inhibits AVP secretion from ovine hypothalamic neurones, a response which is modulated by glucocorticoids.

Electrophysiological effects of selective opioid agonists on spontaneous and evoked neuronal activity in the dentate gyrus of the hippocampus in vivo

The mechanism by which endogenous opioid peptides regulate neuronal excitability in the dentate gyrus of the hippocampus remains unclear. We studied the neurophysiologic responses to various receptor-selective opioids, given both iontophoretically and systemically, in anesthetized rats. Single unit action potentials and field potential recordings were taken from electrophysiologically classified dentate granule cells (DGCs) or dentate interneurons (INTs). The mu receptor agonist ([D-Ala2, NMe-Phe4, Gly-ol]-Enkephalin (DAMGO)) increased the responsiveness of DGCs to perforant path stimulation, although it did not induce spontaneous activity in DGCs. We did not see this facilitation with systemic morphine sulfate (MS). However, both DAMGO and MS produced similar, primarily inhibitory, effects on INTs. The responsiveness of an individual INT tended to be related to the cell's location in the dentate gyrus, and to a lesser degree, to its baseline spontaneous discharge frequency. U-50488H, a selective kappa receptor agonist, had little effect on either DGCs or INTs. Our results suggest that mu selective opioids have a complex neuropharmacology in this region involving interaction among different types of INTs to produce an effect on the principal output cells.

Design of opioid peptides for a potential delta-receptor affinity label function: comparison with the mu-specific Tyr-D-Ala-Gly-(Me)Phe-chloromethyl ketone

To find a delta-opioid receptor preferring peptide structure containing an Asp residue in a potentially interacting position, Tyr-Pro-Phe-Asp, Tyr-D-Ala-Phe-Asp, Tyr-D-Ala-Gly-Phe-Asp, Tyr-D-Ala-Gly-Phe-Asp alpha- and beta-methyl ester and Tyr-Gly-Gly-Phe-Asp peptides were synthesized and their biological activities were analyzed in vitro in mouse vas deferens and longitudinal muscle strip of guinea pig ileum. Changing the beta-methyl ester for an alkylating chloromethyl ketone moiety in the delta-receptor-selective agonist Tyr-D-Ala-Gly-Phe-Asp-beta-methyl ester enhanced further the delta-receptor preference. The delta-receptor selective chloromethyl ketone but not the beta-methyl ester gave a very slow washout after prolonged incubation in the mouse vas deferens bioassay; however, it was still readily displaceable by naloxone. The washout pattern of mu-specific Tyr-D-Ala-Gly-(Me)Phe chloromethyl ketone did not differ in the bioassays from that of the corresponding Gly5-ol derivative. Both chloromethyl ketones gave irreversible characteristics in the receptor binding assay.

Opioid peptides selective for receptor types modulate cocaine-induced behavioral responses in mice

The effects of intracerebroventricular injection of mu-, kappa- and delta-selective opioid agonists on cocaine-induced behavior were investigated in mice using multidimensional behavioral analysis. Cocaine (3.0 mg/kg) produced a marked increase in linear locomotion, circling, rearing and/or grooming, although the mu-opioid agonist [D-Ala2, NMePhe4, Gly-ol] enkephalin (DAMGO) (0.003 and 0.01 microgram), the kappa-opioid agonist dynorphin A- (1-13) (3.0 and 12.5 micrograms) or the delta-opioid agonist [D-Pen2, L-Pen5]enkephalin (DPLPE) (0.3 and 1.0 micrograms) did not significantly affect behavioral responses. DAMGO (0.003 and 0.01 microgram) and dynorphin A- (1-13) (12.5 micrograms) inhibited the cocaine (3.0 mg/kg)-induced increase in linear locomotion, circling and/or rearing. In contrast, DPLPE (1.0 micrograms) enhanced the cocaine (3.0 mg/kg)-induced increase in circling. The effects of DAMGO (0.003 microgram), dynorphin A- (1-13) (12.5 micrograms) and DPLPE (1.0 micrograms) were fully reversed by receptor-selective opioid antagonists, such as beta-funaltrexamine (5.0 micrograms), Mr2266 (5.6 mg/kg) and naltrindole (10.0 mg/kg), respectively. These results suggest that the activation of mu- and kappa-opioid receptors inhibits cocaine-induced behavior, while that of delta-opioid receptors enhances the behavior.

Endogenous opiates: 1993

This paper is the sixteenth installment of our annual review of research concerning the opiate system. It is restricted to papers published during 1993 that concern the behavioral effects of the endogenous opiate peptides, and does not include papers dealing only with their analgesic properties. The specific topics this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; development; immunological responses; and other behaviors.