Frank S. LaBella

On the specificity of naloxone as an opiate antagonist.

Abstract Since the discovery of endogenous opioid peptides in brain (68,69,97,113, 128) and the pituitary gland (26,81,105,125) there has been considerable interest in their possible roles in a variety of physiological and pharmacological processes. Many studies have used antagonism by naloxone as a criterion for implicating endogenous opiates in a process, assuming that naloxene has no pharmacological actions other than those related to blockade of opiate receptors. The doses of naloxene used are often higher than those required to antagonize the analgesic and other effects of morphine. However, multiple forms of opiate receptors are present in nervous tissue and higher concentrations of naloxene are required to antagonize effects mediated by some of these receptors (83). Although the earlier literature supports the assumption that the effects of naloxene are due to the blockade of opiate receptors (87), there are an increasing number of reports which indicate that naloxene may have pharmacological actions unrelated to opiate receptor blockade. The subsequent review serves to emphasize that antagonism by naloxene is a necessary but not sufficient criterion for invoking the mediation of a response by an endogenous opiate (61). Additional lines of evidence which serve to strengthen the conclusion that endogenous opiates mediate a process will be considered.

Morphine derivatives with diminished opiate receptor potency show enhanced central excitatory activity.

Central excitatory potency of morphine administered by cerebroventricular infusion in enhanced in derivatives substituted at the 3-position (phenolic group) and/or 6-position (alcoholic group). Morphine-3-glucuronide is several hundred times more potent than morphine in evoking dose-related hyperactive motor behavior which can progress to lethal convulsions. Excitatory potencies in decreasing order are: (1) 3-glucuronide; (2) 3-SO4; (3) 3-OAc, 6-OAc (heroin); (4) 6-OAc; (5) 3-OAc; (6) 3-OH, 6-OH (morphine); (7) 3-OCH3 (codeine); (8) 3-OCH3, 6-OCH3 (thebaine). Levorphanol, lacking a 6-OH group, is devoid of excitatory actions. In this series of substituted morphines, there is an inverse relationship between opiate receptor binding potency and central excitatory potency, but codeine and thebaine behave anomalously. These findings are compatible with the hypothesis that morphine acts upon a species of receptor which mediates behavioral and EEG excitation and is distinct from the recognized opiate receptor mediating sedation and analgesia.

Pituitary hormone releasing or inhibiting activity of metal ions present in hypothalamic extracts.

Abstract In vitro trophin-releasing and inhibiting activities of certain purified fractions from bovine hypothalamus were apparently due primarily to the presence of divalent metals. The most abundant metal was copper which at concentrations less than 1 μg/ml markedly stimulated the release of all hormones. Zinc ion which was much less potent than copper, inhibited release of prolactin but stimulated release of the other hormones. Nickel, as low as 1 μg/ml, specifically inhibited prolcatin release; at much higher levels it enhanced release of the other hormones.

Ultrastructural localization of acid phosphatase in the posterior pituitary of the dehydrated rat

SummaryAcid phosphatase (AcPase) was localized by means of electron-microscopic histochemistry and estimated biochemically in the posterior pituitary of rats deprived of water, given 2% NaCl ad libitum, or given tap water ad libitum over 6 days. Autophagic vacuoles, some of which gave a positive AcPase reaction, often contained neurosecretory granules (NSG) in nerve endings of control animals on tap water. Nerve endings of water-deprived or salt-treated rats were depleted of NSG, but frequently contained dense membranous residual bodies, some of which appeared to enclose microvesicles. Smooth endoplasmic reticulum located in axons and terminals appears to be a source of hydrolytic enzymes for neurohypophysial lysosomes. The total amount of AcPase per posterior lobe increased progressively to 40% above control levels after 6 days of water deprivation or salt administration, and this increase may reflect accelerated production of neuronal components in neurohypophysial cells whose secretory rate has been stimulated by elevated body osmolarity.

H3 receptor antagonist, thioperamide, inhibits adrenal steroidogenesis and histamine binding to adrenocortical microsomes and binds to cytochrome P450.

1 Thioperamide (TP), an imidazole and a highly potent, specific antagonist of the histamine H3 receptor, inhibited the secretion of cortisol from bovine isolated adrenocortical cells (IC50 0.20 μm) and, in the rat (5 mg kg−1) prevented both basal and stress‐induced secretion of corticosterone. 2 In adrenocortical microsomes, low affinity binding of [3H]‐histamine (KD 27.7 μm) was potently inhibited by TP (Ki 0.33 μm). 3 In adrenocortical microsomal membranes, both histamine and TP yielded type II difference absorption spectra, characteristic of the interaction between imidazole and cytochrome P450 enzymes. Dissociation constants for binding to P450, calculated from spectral data, were 15.9 μm and 1.5 mm for histamine, and 0.3 μm and 3.7 μm for TP. 4 In view of previously reported evidence for an intracellular mediator role of histamine in platelets, the present findings suggest a physiological role for histamine in the modulation of adrenal P450 monooxygenases that generate adrenocortical steroids. 5 The results suggest that direct adrenocortical inhibition by thioperamide at a non‐H3 intracellular site must be taken into account in studies designed to elucidate functional roles of H3 receptors.

GABA and baclofen potentiate the K+-evoked release of methionine-enkephalin from rat striatal slices

GABA potentiates the potassium-evoked release of methionine-enkephalin (ME) from slices of rat corpus striatum. This potentiation is observed only when a submaximal concentration (30 mM) of K+ is used to evoke release. The effect of GABA is dose-dependent between 100 and 100 micrometers. The basal release of ME is not altered by these concentrations of GABA. Before, but not muscimol, mimics the effect of GABA on the evoked release of ME. This effect is not stereoselective as both the (+)- and (-)-isomers of baclofen enhance ME release. Picrotoxin (100 micrometers) blocks the enhancement of ME release produced by both GABA and baclofen. Bicuculline methiodide (100 micrometers) does not block the effect of GABA. The effect of GABA on ME release may be mediated by an atypical GABA receptor which is activated by baclofen.

Opiate receptor binding activity of 17-Alpha estrogenic steroids☆

A large number of steroids of the estrane, androstane, pregnane and, etianic acid series, including their sulfate and glucuronide conjugates, were inactive in an opiate radioreceptor assay (> 10% displacement of 3H-naloxone at 10−4M). Only certain estrogens were active, the most potent being 17α-hydroxy-estrogens. The displacement curves for the active steroids were parallel to those of known opiate drugs. SC 22,000, a semisynthetic steroid with opiate effects, was a potent agonist in the assay. The sodium effect indicated the active estrane steroids to be of mixed agonist-antogonist character. IC50 for 17α-dihydroequilin, 17α-dihydroequilenin, and 17α-estradiol was 1.0, 1.8, and 3.0 μm, respectively. Sulfation or acetylation of the 3-OH or 17-OH abolished binding activity.

Electron microscopic histochemistry of lysosomes in neurosecretory nerve endings and pituicytes of rat posterior pituitary

SummaryElectron microscopic localization of acid phosphatase (AcPase) was carried out on posterior pituitary glands from rats. An estimated 5% of the neurosecretory nerve terminals contained structures which showed reaction product. Most of the lysosomes were small dense bodies, often with a membranous substructure. Other lysosomes were larger in size or were found within vacuoles. AcPase was also localized to lysosomes and the Golgi apparatus of pituicytes. Evidence is presented which would associate the large lipid droplets characteristic of pituicytes with AcPase-positive dense bodies. The present study indicates that hydrolytic activity by lysosomes occurs within the terminals of neurosecretory cells, and adds further support to the concept that this process represents a normal phenomenon of cells and their extensions in general.

Administered peptides inhibit the degradation of endogenous peptides. The dilemma of distinguishing direct from indirect effects

Virtually all peptides are biologically active following central administration as a consequence of both direct and indirect cellular actions. Direct effects are mainly interactions with specific membrane receptors but may include unions with other components of the receptor/effector complex. Significant indirect biological effects of exogenous peptides, including apparent secretagogue effects on endogenous peptides largely overlooked in practice, result from extensive competition with endogenous peptides for degradative enzymes (peptidases). A consequence of this competition is enhancement of tonic or intermittent activity of endogenous peptides. The pharmacological profile of any peptide reflects or includes, therefore, the spectrum of endogenous peptides that is protected from peptidase action. It is likely that certain pharmacologically active peptides, including a large number of di-, tri- and oligo-peptides, elicit responses mainly or exclusively by competing for peptidases. Therefore, reliable estimates of the relative contributions of direct and indirect actions of exogenous peptides may be difficult, if not impossible, to obtain.

Lipotropin: Localization by radioimmunoassay of endorphin precursor in pituitary and brain☆

Abstract Beta-Lipotropic Hormone (LPH) was estimated in pituitary and brain by a radioimmunoassay specific for the N-terminal, non-opiate portion of the protein, and endorphin activity by an opiate radioreceptor assay. The intermediate pituitary is most concentrated in LPH and endorphin. Gel filtration indicated the presence in the pituitary of intact LPH and N-terminal fragments but only intact LPH in brain. Endorphin activity in pituitary is associated primarily with a component of 3000 daltons and to a lesser extent with one of about 2000 daltons. Brain endorphin activity is mostly accounted for by a 2000 dalton component, enkephalins representing an apparently minor activity. In pituitary and brain LPH and endorphin are entirely associated with a 12,000 g/10 min fraction.