Leff GP, González HNE, Matus OME, Beceril VE, Téllez SC, Salazar JA, Antón PB

Idioma: Español
Referencias bibliográficas: 230
Paginas: 257-272
Archivo PDF: 268.38 Kb.

RESUMEN

La endomorfina-1 (EM1) y la endomorfina-2 (EM2) son dos péptidos bioactivos que poseen la más alta afinidad de unión selectiva por el receptor opioide μ en comparación con la unión de distintos ligandos agonistas a este subtipo de receptor opioide (véase resumen y texto del capítulo anterior, parte I). Estudios farmacológicos y conductuales han demostrado que la inyección de las EM1-2 en el área ventrotegmental (AVT) genera respuestas conductuales de sensibilización locomotora a la anfetamina (AMPH), además de incrementar la actividad locomotora de tipo horizontal en los roedores tratados. Estos estudios mostraron que la EM2 fue significativamente más potente que la EM1 en inducir las respuestas locomotoras detectadas, mediadas a través de la alteración de la actividad sináptica de dopamina (DA) y en el globus pallidus de los animales tratados. Asimismo, estudios fármaco-conductuales similares demostraron que otros sistemas de transmisión participan conjuntamente con el sistema dopaminérgicoen la generación de los efectos locomotores inducidos por las EM1-2, como es el caso del sistema gabaérgico (GABA). Más aún, la inyección de EM1 en la región AVT del cerebro de roedores mostró generar respuestas potentes de recompensa placentera, similares a las reportadas por distintos alcaloides opiáceos de alto potencial adictivo, posterior a su administración sistémica. Más aún, la inyección de endomorfinas en la región AVT del cerebro del roedor, mas no en el núcleo accumbens (NAc), mostró generar respuestas de recompensa paralela a la generada posteriormente a la administración de dosis bajas de morfina.
En línea con los efectos farmacológicos inducidos por las EM1- 2, estudios fármaco-conductuales demostraron que la administración ICV de la EM1 fue capaz de generar respuestas de preferencia de lugar en roedores tratados CPP, por sus siglas en inglés, conditioned place preference, en tanto que la administración de EM2 generó respuestas opuestas, esto es, respuestas de aversión al lugar. Estudios conductuales relacionados con el fenómeno de estrés mostraron que las EM1-2 son capaces de modular la actividad funcional del eje HHA (eje hipotálamo/hipófisis/glándula adrenal) a través de la activación del sistema de proyección neuronal del tracto solitario (NTS, por sus siglas en inglés), al hipotálamo y/o a través de la activación del área ventrolateral de la sustancia gris periacueductal (PAG, por sus siglas en inglés); componente importante del sistema opioide endógeno, que median respuestas analgésicas (antinociceptivas) inducidas por estímulos estresantes. Asimismo, la administración de endomorfinas (v.g., EM1) mostró generar incrementos de conductas de naturaleza ansiolítica en ratones expuestos a paradigmas experimentales de generación de conductas estresantes (v.g., laberinto elevado). Estos estudios sugieren que la generación de conductas de estrés-emocional inducidas por las endomorfinas es mediada a través de la activación del receptor opioide μ en neuronas del hipotálamo responsables de regular la secreción de factores liberadores de distintas hormonas hipofisiarias (v.g., CRH, LHRH). Más aún, resulta interesante que las endomorfinas sean capaces de inducir conductas antidepresivas o de tipo antidepresivos como se ha reportado recientemente en modelos animales de estrés y depresión. Estos estudios mostraron que las respuestas conductuales de reacción al estrés y las conductas antidepresivas mediadas por las EM1-2 están ligadas con la expresión neuronal del mensajero de RNA que codifica para el factor trófico (BDNF, por sus siglas en inglés, brain derived neurotrophic factor), en áreas del sistema limbico, y que es inducida en forma dosisdependiente por las endomorfinas, posterior a su administración ICV. Por lo tanto, estos estudios han permitido proponer que las endomorfinas cumplen un papel relevante durante el curso odesarrollo de las enfermedades mentales (v.g., esquizofrenia y depresión). En extensión a estos estudios conductuales, estudios recientes han demostrado la actividad orexigénica de las endomorfinas en forma similar a lo previamente detectado con distintos ligandos agonistas del receptor opioide μ (v.g., morfina, DAMGO; morfina-6Β-glucurónido). Si bien estos estudios mostraron que tanto las EM1-2 como diversos agonistas del receptor opioide μ exhiben potentes actividades orexigénicas en el SNC de roedores, la actividad de las EM1-2 parece depender de la actividad de la dinorfina A y su unión sobre su receptor opioide κ en neuronas hipotalámicas. Más aún, diversos estudios han mostrado que el sistema opioide endógeno (a través de la Β-endorfina) regula conductas de naturaleza sexual y apareamiento (v.g., lordosis), además de modular la secreción y/o actividad de hormonas de origen gonadal (estrógenos, progesterona).
Estudios similares en roedores hembras mostraron que la microinyección de EM1-2 en áreas específicas del sistema límbico y/ o la administración IT de ambos péptidos era capaz de generar respuestas sexuales de apareamiento, similares a las detectadas por la Β-endorfina y morficeptina en la misma especie de animal, siendo bloqueados los efectos por la administración de naloxona. Estas respuestas conductuales inducidas por las EM1-2 mostraron estar ligadas a la liberación neuronal de LHRH, como de la activación y modulación del sistema de transmisión gabaérgico. En cuanto a las funciones de memoria y aprendizaje, diferentes estudios han demostrado que la administración ICV de EM1-2 en ratones expuestos a diferentes paradigmas de aprendizaje experimental, los péptidos opioides alteran significativamente los mecanismos de procesamiento y consolidación de memoria a corto y largo plazo en los animales tratados. Estos efectos parecen depender de la modulación del sistema opioide (v.g., el receptor opioide μ) sobre los sistemas de transmisión colinérgica y dopaminérgica en el cerebro de los mamíferos. Asímismo, diversos estudios han demostrado que tanto las EM1-2 como los alcaloides opiáceos y opioides endógenos mod ulan funciones cardiovasculares y respiratorias. En este contexto, diversos estudios mostraron que la administración de EM1-2 en ratas normotensas e hipertensas produce cambios fisiológicos significativos en la presión sanguínea y la frecuencia cardiaca. Si bien no están del todo esclarecidos los mecanismos por los cuales las endomorfinas producen sus respuestas cardiovasculares, diversos estudios sugieren que la actividad de estos péptidos está en función de la actividad e interacción de los sistemas de transmisión gabaérgico y glutamatérgico, respectivamente. Más aún, otros estudios sugieren que las respuestas fisiológicas de estos péptidos dependen de la actividad del óxido nitroso (NO, por sus siglas en inglés) liberado de los vasos sanguíneos, en respuesta de la activación del receptor opioide μ. Finalmente, diversos estudios han mostrado que las EM1-2 y la activación del receptor opioide μ producen efectos inhibitorios sobre la contracción del músculo liso del tracto gastrointestinal, generados a través de una reducción sostenida en la liberación de neurotransmisores de terminales sinápticas del plexo mientérico, mismas que inervan el tejido muscular liso del tracto gastrointestinal.

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