Multiple neurotransmitter receptors

[3H]Bradykinin receptor binding in mammalian tissue membranes

[3H]Bradykinin binds to membranes from a variety of mammalian tissues in a saturable fashion with a dissociation constant of about 5 nM. Highest levels of binding are detected in guinea pig ileum, colon, and duodenum and in the oestrus rat uterus. The relative potencies of bradykinin derivatives in competing for these binding sites in guinea pig ileum membranes correlate with their contractile potencies in the ileum better than with contractile effects in the uterus. Thus, receptor recognition sites may differ in ileum and uterus. Monovalent and divalent cations at physiological concentrations reduce [3H]bradykinin binding. Of the monovalent cations, medium lowers binding 50% at about 80 mM. Among the divalent cations, calcium lowers binding about 50% at 5 mM, suggesting a link to the calcium conductance channel.

Differentiation of opiate receptors in the brain by the selective development of tolerance

The selective development to either the delta-opiate receptor agonist D-Ala2,D-Leu5-enkephalin (DADL) or the mu-agonist sufentanyl (SUF) has been studied in the central nervous system of rats by use of osmotic minipumps for chronic administration of the drugs. The opiate-sensitive parameters analgesia and catatonia were investigated. Chronic intracerebroventricular infusion of DADL for 7 days produced a 15-fold shift in that for catatonia. In these rats, the potency of SUF in inducing analgesia and catatonia did not differ between DADL-treated animals and saline-infused controls. Similarly, chronic infusion of SUF resulted in tolerance towards SUF for both analgesia and catatonia. In these animals, DADL displayed a similar degree of tolerance w.r.t. its ability to evoke analgesia, whilst no tolerance could be detected for DADL-induced catatonia. The data indicate that prolonged stimulation of specific opiate receptors in the brain by selective agonists may bring about the selective development of tolerance for particular receptors. The data conflict with the notion that mu-receptors specifically mediate analgesia and delta-receptors catatonia.

3H-Flupenthixol binding in post-mortem brains of schizophrenics: evidence for a selective increase in dopamine D2 receptors

The binding of 3H-cis flupenthixol (3H-FPT) to dopamine receptors in membrane preparations from control subjects and schizophrenics was studied. Using a fixed concentration of 3H-FPT, no differences were observed between controls and all schizophrenics, although 3H-FPT binding was increased in schizophrenics who apparently were drug-free at the time of death. Scatchard analysis of 3H-FPT binding revealed that in drug-treated schizophrenics both the number of binding sites (BM) and the dissociation constant (KD) were increased, whilst in drug-free schizophrenics only the BM was increased. Using domperidone to differentiate 3H-FPT binding to D1 and D2 dopamine sites, it was found that only D2 sites were increased in drug-free schizophrenics. The results are discussed with reference to previous studies on dopamine receptors in schizophrenia, and the effects of neuroleptic treatment. It is suggested that a selective increase in D2 receptors may be associated with the disease process in schizophrenia.

Distinct cholecystokinin receptors in brain and pancreas

125I-Labeled (Bolton-Hunter) cholecystokinin triacontatriapeptide (CCK-33) binds saturably and reversibly to distinct receptors in brain and pancreatic membranes. The peptide specificity of pancreatic CCK binding is the same as that for pancreatic amylase release. In brain, gastrin and pentagastrin display nanomolar affinity for binding sites, whereas in pancreas these two peptides are virtually inactive. Though these differences indicate that brain and pancreas possess distinct CCK receptors, the two tissues show some similarities. Both pancreas and brain receptors show greater sensitivity to sulfated than to desulfated COOH-terminal octapeptide of CCK and display dissociation constants of 0.3-9.5 nM. The pancreas possesses about 300 times more binding sites than does brain. CCK binding in both brain and pancreas is enhanced by divalent cations and reduced by monovalent cations. Receptor binding in both tissues is regulated in a selective fashion by guanyl nucleotides.

Differentiation of delta and mu opiate receptor localizations by light microscopic autoradiography

We have observed two discrete populations of opiate receptors that are differently localized in rat brain. Morphine-like (mu) receptors, labeled by 125I-labeled [D-Ala-2MePhe4Met(O)5-ol]enkephalin, are concentrated selectively in lamina IV of the cerebral cortex, certain thalamic nuclei, and the periaqueductal grey, while delta receptors, labeled by 125I-labeled [D-Ala2-D-Leu5]enkephalin, are more diffused, having high densities in cerebral cortex, corpus striatum, amygdala, and olfactory tubercle. Because of similarities in their localizations, we propose that mu and delta receptors are respectively the physiologic receptors for [Met]- and [Leu]enkephalin neurons. These distributions reflect the different physiological functions attributed to mu and delta receptors and thus represent discrete functions of [Met]- and [Leu]enkephalin neurons.

Brain peptides as neurotransmitters

Numerous peptides appear to be neurotransmitter candidates in the brain. Some, such as the opioid peptide enkephalins, neurotensin, and substance P, were first isolaterd from the brain. Peptides, such as cholecystokinin and vasoactive intestinal polypeptide, were known as intestinal hormones and later recognized as brain constituents. Certain hypothalamic-releasing hormones, pituitary peptides, and blood-derived peptides like angiotensin II and bradykinin, may also be central neurotransmitters. The diversity of localization of these peptides throughout the brain implies a multiplicity of potential roles.

Differential effects of potassium on the potency of different opioids in the mouse vas deferens

The effect of potassium on the potency of different opioids to inhibit electrically evoked twitches of the mouse vas deferens has been studied. In general, opioids lose activity as the concentration of potassium increases. Distinct differences, however, were observed between specific opiate receptor agonists. Least sensitive to the effect of potassium proved mu- and delta-opiate receptor agonists, showing only a 2-fold increase in the concentration required to cause a half-maximal inhibition at 14 mM K+. In contrast, the supposed dynorphin receptor/kappa-receptor agonists exhibited at this concentration up to an 8-fold loss of potency. Dynorphin1-13 was found to be intermediate with a 5-fold loss of inhibitory activity. These data suggest a modulatory action of potassium for dynorphin- and kappa-receptors.