RESUMO
Histamine regulates at the pre- and post-synaptic levels several functions of the mammalian Nervous System, in which three (H1, H2, and H3) out of the four G protein-coupled histamine receptors cloned so far are widely distributed. The histamine H3 receptor (H3R) was first identified as an auto-receptor controlling histamine synthesis and release, but several lines of evidence have shown the H3R to regulate as a hetero-receptor the release of a number of neuroactive substances, namely acetylcholine, 5-hydroxytryptamine (5-HT, serotonin), noradrenaline, dopamine, glutamate, y-aminobutyric acid (GABA) and the neuropeptides sustance P and calcitonin gene-related peptide (CGRP). H3R-mediated regulation of the release of these neurotransmitters and neuro-modulators, both in normal and pathological conditions, suggest that drugs acting at the receptor may have therapeutic use in a number of diseases such as sleep disorders, ischemia-induced cardiac arrhythmias, migraine, obesity, Alzheimer's disease and schizophrenia.
La histamina regula a nivel pre y postsináptico diversas funciones del Sistema Nervioso de los mamíferos, el cual expresa de manera abundante tres (H1, H2 y H3) de los cuatro receptores a histamina acoplados a proteínas G descritos a la fecha (H1-H4). El receptor a histamina H3 (H3R) se identificó inicialmente como el autorreceptor responsable del control de la liberación y la síntesis de la histamina. Posteriormente se estableció que este receptor se encuentra también en las terminales axónicas de otras neuronas del Sistema Nervioso Central y periférico, donde regula como heterorreceptor la liberación de varios transmisores. En este trabajo se revisan los efectos de la activación del H3R en la liberación de histamina, acetilcolina, 5-hidroxitriptamina (5-HT, serotonina), noradrenalina, dopamina, glutamato, ácido y-aminobutírico (GABA) y los neuropéptidos sustancia P y el péptido relacionado al gen de la calcitonina (CGRP). La regulación por el receptor H3 de la liberación de estos neurotransmisores y neuromoduladores, tanto en condiciones normales como patológicas, sugiere que los fármacos que actúen sobre dicho receptor pueden tener uso terapéutico en alteraciones diversas como los transtornos del sueño, las arritmias cardiacas causadas por isquemia, la migraña, la obesidad, la enfermedad de Alzheimer y la esquizofrenia.
RESUMO
In this study, the authors set out to determine the presence of M3 muscarinic receptors in rat striatum by examining the binding of [3H]N-methyl-scopolamine([3H]NMS) to striatal membranes and its displacement by antagonists with different affinity for M1 and M3 muscarinic receptors (pirenzepine; 4-diphenylacetoxy-N-methylpiperidine methiodide, 4-DAMP; and the p-fluoro analog of hexahydro-sila-difenidol, pFHHSiD). The specific binding of [3H]NMS to membranes from rat striatum (551 ñ 40 fmol.mg prot.-1, KD 0.11 ñ 0.01 nM) was displaced in a concentration-dependent manner by all three antagonists tested. Inhibition curves best fit to a single-site model for 4-DAMP(pKi 9.1 ñ 0.1), whereas for both pirenzepine and pFHHSiD, the best fit was to the two-site model. The pKi values for the high-affinity (8.0 ñ 0.2) and low-affinity (6.7 ñ 0.2) components for pirenzepine-mediated inhibition of [3H]NMS binding correspondend to those reported for M1 and M3 receptors, respectively. The pKi values for the high-affinity (7.7 ñ 0.1) and low-affinity (7.1 ñ 0.2) components for pFHHSiD inhibition were in good agreement with those reported for M3 and M1 receptors, respectively. Altogether, these results indicate the presence in rat striatum of both M1 and M3 muscarinic receptors. These findings might be relevant to the design and use of mucarinic antagonists in the treatment of neurological disorders such as Parkinson's disease