Enkephalin, a natural ligand for opiate receptors is composed of the pentapepides H-Tyr-Gly-Gly-Phe-Met-OH and H-Tyr-Gly-Gly-Phe-Leu-OH. The evidence is based on the determination of the amino acid sequence of natural enkephalin by the dansyl-Edman procedure and by mass spectrometry followed by synthesis and comparison of the natural and synthetic peptides.

Identification of two related pentapeptides from the brain with potent opiate agonist activity

Hypothalamic Integration: Organization of the Paraventricular and Supraoptic Nuclei

Nucleus locus ceruleus: new evidence of anatomical and physiological specificity

Efferent connections of the parabrachial nucleus in the rat.

[3H]Diazepam appears to bind specifically to a single, saturable, binding site located on rat brain membranes, with an affinity constant near 3 nM at pH 7.4. Specific binding constitutes more than 90% of total binding at 0 degrees and less than 10% of total binding at 37 degrees. Arrhenius plots suggest a sharp conformational change in the diazepam receptor near 18 degrees. Mitochondrial fractions from rat kidney, liver, and lung exhibit some [3H]diazepam binding that can be displaced by nonradioactive diazepam and several other benzodiazepines. However, Ro-4864, which is almost inactive in displacing [3H]diazepam from brain membranes, is extremely potent in displacing it from kidney mitochondria. Conversely, clonazepam, the most potent inhibitor of brain binding, is an extremely weak inhibitor of kidney binding. Furthermore, diazepam binding to kidney mitochondria has an affinity constantof 40 nM, about 15 times higher than that in brain. No specific diazepam binding was detected in intestine or skeletal muscle. Thus, specific [3H]diazepam binding to membranes appears to be restricted to brain, where it is unevenly distributed: the density of diazepam receptors is about five times higher in cortex (the highest density) than in pons-meddula (lowest density). Trypsin and chymotrypsin completely abolished specific [3H]diazepambinding in brain and kidney.

https://www.pnas.org/content/pnas/74/9/3805.full.pdf

Specific benzodiazepine receptors in rat brain characterized by high-affinity (3H)diazepam binding

Immunohistochemical mapping of enkephalin containing cell bodies, fibers and nerve terminals in the brain stem of the rat

An endogenous morphine-like factor in mammalian brain.

Neurotensin-containing cell bodies, fibers and nerve terminals in the brain stem of the rat: immunohistochemical mapping.

Adenosine receptors were made visible on light microscopy by autoradiography with tritiated cyclohexyladenosine. In the cerebellum, adenosine receptors were absent in Weaver mice, which lack granule cells, and were displaced in Reeler mice, which have displacements of granule cells. Thus, adenosine receptors appear to be located on the axon terminals of excitatory granule cells in the cerebellum. Removal of one eye of a rat depleted adenosine receptors in the contralateral superior colliculus, suggesting that the receptors occur on axon terminals of excitatory projections from retinal ganglion cells. The presence of adenosine receptors on excitatory axon terminals may explain synaptic inhibition by adenosine and the behavioral effects of xanthines.

https://science.sciencemag.org/content/sci/220/4600/967.full.pdf

Adenosine receptors: autoradiographic evidence for their location on axon terminals of excitatory neurons

Just as multiple forms or \"isoenzymes\" of particular enzymes exist, so there may be multiple receptors for an individual neurotransmitter. Some of these have been well known for many years; for instance, the distinction of muscarinic and nicotinic cholinergic receptors, aand P-adrenergic receptors, and , more recently, distinct 6,and P,adrenergic receptors. In different parts of the body histamine acts at distinct H, and H, receptor sites. All of these discriminations of receptor populations were based initially on pharmacological variations. During the past few years it has been possible to label neurotransmitter receptors by binding techniques (Snyder, 1978), an approach which affords more subtle molecular discriminations of receptor properties. In pharmacological experiments drugs may vary in potency because of differential bioavailability and metabolism, even in isolated organs, rather than distinct populations of receptors. Measuring receptor binding in isolated membranes overcomes potentially artifactual sources of variations in drug potency and has permitted identification of several new subclasses of neurotransmitter receptors.

Robert R. Goodman

Papers

Immunohistochemical mapping of enkephalin containing cell bodies, fibers and nerve terminals in the brain stem of the rat

An endogenous morphine-like factor in mammalian brain.

Neurotensin-containing cell bodies, fibers and nerve terminals in the brain stem of the rat: immunohistochemical mapping.

Adenosine receptors: autoradiographic evidence for their location on axon terminals of excitatory neurons

Multiple neurotransmitter receptors.