Binding and effect of atrial natriuretic factor on cyclic GMP formation and alpha-MSH secretion of intermediate pituitary cells

Clozapine's multiple cellular mechanisms: What do we know after more than fifty years? A systematic review and critical assessment of translational mechanisms relevant for innovative strategies in treatment-resistant schizophrenia

Almost fifty years after its first introduction into clinical care, clozapine remains the only evidence-based pharmacological option for treatment-resistant schizophrenia (TRS), which affects approximately 30% of patients with schizophrenia. Despite the long-time experience with clozapine, the specific mechanism of action (MOA) responsible for its superior efficacy among antipsychotics is still elusive, both at the receptor and intracellular signaling level. This systematic review is aimed at critically assessing the role and specific relevance of clozapine's multimodal actions, dissecting those mechanisms that under a translational perspective could shed light on molecular targets worth to be considered for further innovative antipsychotic development. In vivo and in vitro preclinical findings, supported by innovative techniques and methods, together with pharmacogenomic and in vivo functional studies, point to multiple and possibly overlapping MOAs. To better explore this crucial issue, the specific affinity for 5-HT₂R, D1R, α_2c, and muscarinic receptors, the relatively low occupancy at dopamine D2R, the interaction with receptor dimers, as well as the potential confounder effects resulting in biased ligand action, and lastly, the role of the moiety responsible for lipophilic and alkaline features of clozapine are highlighted. Finally, the role of transcription and protein changes at the synaptic level, and the possibility that clozapine can directly impact synaptic architecture are addressed. Although clozapine's exact MOAs that contribute to its unique efficacy and some of its severe adverse effects have not been fully understood, relevant information can be gleaned from recent mechanistic understandings that may help design much needed additional therapeutic strategies for TRS.

On the blood-brain barrier to peptides: specific binding of atrial natriuretic peptide in vivo and in vitro

Using the intracarotid bolus injection technique, a saturable binding of [125I]atrial natriuretic peptide (ANP) was found in 8 blood-brain barrier (BBB)-protected rat brain regions as well as in the pineal gland, choroid plexus, neurointermediate and anterior lobes of the pituitary, i.e. structures lacking a BBB. The presence of specific ANP binding on the BBB, here shown for the first time by an in vivo approach, was evidenced concomitantly in vitro by incubation of isolated microvessels. A single-class high affinity binding without regional differences was obtained with Kd = 0.23 nM and Bmax = 120 fmol/mg protein. From that a density of 1,400 binding sites per endothelial cell was calculated, thought to be localized predominantly in the luminal membranes. In the in vivo study, the portion of the extracted peptide that, under the conditions used, may have crossed the BBB by passive diffusion amounted to less than 0.4% of the labeled ANP administered. ANP itself did not change the tightness of the BBB to the non-diffusible reference molecule [14C]inulin. In the BBB-free areas, ANP enhanced the inulin space by nearly 50%.

[The intermediate lobe of the pituitary, model of neuroendocrine communication]

The intermediate lobe of the pituitary is composed of a homogeneous population of endocrine cells, the melanotrophs, which secrete several bioactive peptides including alpha-melanocyte-stimulating hormone (alpha-MSH) and beta-endorphin. In contrast to most endocrine glands which are richly vascularized, the intermediate lobe of the pituitary contains very few blood vessels; in some species, the pars intermedia is virtually totally avascular. In contrast, pituitary melanotrophs are richly supplied by nerve fibers originating from the hypothalamus. The pars intermedia thus appears as a pure model of neuroendocrine communication, i.e. it is an archetype of the mode of transducing interface between the central nervous system and endocrine effectors. In mammalian species, different types of nerve terminals containing dopamine, norepinephrine, gamma-aminobutyric acid (GABA) and serotonin have been identified. In lower vertebrates, particularly in fish and amphibians, the pars intermedia is also innervated by peptidergic fibers which are though to take part in regulation of the secretory activity of the melanotroph. In these animals, the pars intermedia is regarded as a major center of neuroendocrine integration and an exceptional model to investigate the process of communication between the brain and the endocrine glands. The purpose of the present review is to summarize our current knowledge on the synthesis, processing and release of peptide hormones from pars intermedia cells and to survey the multiple regulatory mechanisms which are involved in the control of the activity of pituitary melanotrophs. Proopiomelanocortin, a multifunctional precursor. Pituitary melanotrophs synthetise a major precursor protein called proopiomelanocortin (POMC) which generates through proteolytic cleavage several biologically active peptides including adrenocorticotropic hormone (ACTH), endorphins and MSHs. In lower vertebrates, alpha-MSH is generally considered as the major hormone secreted by melanotrophs, in that it is involved in the process of skin colour adaptation. The post-translational processing of POMC, which yields to the mature hormones released by melanotrophs, includes a number of steps: glycosylation, phosphorylation, tissue-specific proteolytic cleavage, amidation and acetylation. Some of these posttranslational modifications can be regulated by neuroendocrine factors. For instance, in frogs, it has been shown that dopamine inhibits acetylation of alpha-MSH and thus reduces the secretion of the biologically active form of the peptide. The intermediate lobe of the pituitary: a model of neuroendocrine integration. In most vertebrate species, the intermediate lobe of the pituitary is innervated by catecholamine-containing fibers. In particular, the presence of dopaminergic nerve fibers has been observed in the pars intermedia of mammals and poikilotherms.(ABSTRACT TRUNCATED AT 400 WORDS)

Stimulation by atrial natriuretic factor of cyclic GMP production in cultured anterior and intermediate pituitary tissues: evidence for a major contribution of proliferating nonendocrine cells

Primary cultures of anterior and intermediate pituitary tissues were monitored immunocytochemically for the presence of endocrine and nonendocrine cells and simultaneously tested for their ability to produce cyclic GMP in response to atrial natriuretic factor (ANF). Cells cultured for 3 days and 6 days, in which nonendocrine (vimentin-positive) cells were found to rapidly overgrow the endocrine cells, showed a dramatic elevation in cyclic GMP production stimulated by ANF, with maximum stimulation 300-700% that seen in 1-day cultured cells. Also, ANF-induced accumulation of cyclic GMP in an enriched population of vimentin-positive cells appeared to closely match that triggered in a 3-day culture of anterior pituitary cells, emphasizing the major role played by nonendocrine cells and their ability to synthesize cyclic GMP. In contrast, in the homogeneous population of tumor corticotrophs AtT-20, there was a close relationship between cyclic GMP formation and cell density. It thus appears that contamination of primary cultures of anterior and intermediate pituitary tissues by proliferating nonendocrine cells (mainly fibroblasts), in which ANF-induced accumulation of cyclic GMP may be confused with that of the very secretory cells, leads to overestimation and masking of guanylate cyclase activity of endocrine cells.