Resumen de El sistema serotoninérgico en el paciente deprimido. Segunda parte

María Guadalupe Campos, Carlos A. Torner Aguilar, Maria del Carmen Lara Muñoz, Julia Moreno

• español

  Actualmente existe un gran número de evidencias que sustentan la importancia de las monoaminas en el trastorno depresivo. Estas se han obtenido por medio del estudio del efecto a largo plazo del tratamiento antidepresivo sobre dichas monoaminas, y en aspectos como la densidad de sus receptores en modelos animales y en pacientes deprimidos, así como mediante estudios realizados en tejidos post mortem de los mismos.

  Se comenta que un incremento agudo en la cantidad de monoaminas en las sinapsis parece inducir la desensibilización de los autorreceptores y heteroreceptores localizados en ciertas regiones del cerebro, lo que puede deberse a la alta actividad monoaminérgica central que coincide con la aparición de la respuesta terapéutica en el paciente deprimido.

  Estas evidencias proporcionan mayor sustento a la hipótesis de que el efecto antidepresivo resulta de los grandes cambios adaptativos en los receptores autorreguladores y heterorreguladores monoaminérgicos.

  Con base en las consideraciones anteriores, en la primera parte de esta revisión se analizaron los estudios post mortem relacionados con el sistema serotoninérgico. Además, se revisó la utilidad de la medición de los niveles de serotonina, de su precursor (TRP), y de sus metabolitos, en diferentes fluidos biológicos del paciente deprimido. En esta segunda parte se discuten las alteraciones del transportador y de los receptores serotoninérgicos.

  El transportador de serotonina en el paciente deprimido.

  Debido a que los estudios sobre el transportador serotoninérgico del tejido nervioso han producido resultados confusos, y a la dificultad para obtener muestras de este tejido para su estudio, a lo que se aúnan discrepancias encontradas en los resultados post mortem, se ha sugerido que las plaquetas podrían ser un modelo periférico para el estudio del transportador serotoninérgico neuronal.

  Los estudios sobre la serotonina en plaquetas han sido de utilidad en aspectos clínicos como los trastornos depresivos. Se han encontrado así disminuciones en los sitios de unión de [3H]-imipramina en plaquetas de pacientes deprimidos sin tratamiento. Este hallazgo es uno de los más reproducidos en la biología de los trastornos afectivos, y se ha propuesto como un indicador específico y como un marcador de estado de la depresión. Sin embargo, algunos autores no han reproducido los resultados, y tampoco se ha establecido una explicación racional de la variabilidad de la densidad de los sitios de unión.

  Alteraciones de los receptores serotoninérgicos en la depresión.

  Aunque hay inconsistencias entre los estudios sobre los receptores 5-HT2A plaquetarios de pacientes deprimidos, la mayoría de éstos refiere que la densidad de los receptores serotoninérgicos 5-HT2A se encuentra elevada en el paciente deprimido. Sin embargo, más bien parece ser la conducta suicida la que se correlaciona muy claramente con el incremento en la den sidad de los receptores 5-HT2A plaquetarios, situación que también se ha reportado en la mayoría de los estudios post mortem.

  Las técnicas modernas, no invasivas -que permiten obtener imágenes del cerebro humano y medir directamente algunos fenómenos, como el metabolismo de la glucosa o del oxígeno, así como substancias metabólicas o receptores-, serán junto con los avances en genética molecular, herramientas adicionales que favorecerán el entendimiento de la neurobiología de la depresión y su tratamiento.

• English

  Nowadays there are increasing number of studies to support the crucial role of monoamines in depressive disorders. Among them are studies such as long-term treatment of antidepressants whose mechanism of action regulates monoamine metabolism, monoamine receptor density and post-mortem studies.

  An acute increase in monoamine concentration at the synaptic cleft might induce desensitization of brain auto- and hetero-receptors which explains the therapeutic antidepressive response. This has been proved by monoamine depletion studies in which an antidepressant effect or a patient relapse has been observed. Likewise, the antidepressive therapeutic response occurs earlier when auto-receptors are pharmacologically blocked at nervous and somatodendritic terminals.

  In the first part of this review, post-mortem studies related with the serotoninergic system were analyzed, as well as the usefulness of measuring serotonin, triptophan, and serotonin metabolite levels in different biological fluids of depressed patients. In this second part, alterations in platelet transporter and serotonin receptors are discussed as platelet is considered a neural serotoninergic model. Platelets are capable of storing and releasing serotonin in a similar manner as serotoninergic synaptosomes. Thus, platelets and serotoninergic synaptic terminals share biochemical and morphological properties.

  Serotonin transporter in platelets of depressed patients Due to the difficulty to obtain human brain samples and disagreements in the post-mortem studies, platelets have been suggested as a peripheral model to study neural serotonin uptake. The model is supported by the fact that platelet properties are similar to those of neuronal serotoninergic synaptic terminals.

  Serotonin studies in platelets have been useful in clinical aspects such as depressive disturbances. Radioligand studies in platelets from untreated depressed patients have shown a decrease in [H]-imipramine binding sites, compared to the binding in platelets from control subjects. Since that decrease has been consistently confirmed in studies on affective subjects, it has been proposed as a specific biomarker of depressed patients. Nevertheless, some researchers have not found similar results, and no explanation of the variability in the density of [H]-imipramine binding sites has been proposed.

  Serotonin receptor changes in depressed patients The hypothesis on receptor adaptative changes proposes that there is a depletion of monoaminergic neurotransmitters in depressed subjects which induces a compensatory regulation in the number and/or function of receptors. To explore this different techniques as the following have been developed:

  • Techniques to evaluate receptor density and affinity, including post-mortem radioligand binding to serotonin receptors in brain tissue and in platelets from depressed patients.

  • Techniques to evaluate receptor regulation and sensitivity by using neuroendocrine tests described below.

  Somatodendritic 5HT 1A autoreceptor dysfunction in depressive disorders Dysfunction of presynaptic somatodendritic 5HTja autoreceptors has been found in behavioral changes related to anxiety, depression and alcoholism. Neuroendocrine tests after the administration of 5-HT1a agonists have been used as an index of 5-HTta receptor function. It seems that azapirodecanediones increase plasmatic concentrations of prolactin, somatotropin, and adrenocortico-tropin; they also seem to decrease body temperature.

  In depressed patients, the hypothermia response, following presynaptic 5-HTta receptor stimulation, and the neuroendocrine response, following hypothalamus postsynaptic 5-HTta receptor stimulation, were both diminished. These findings suggest a desensitization or down-regulation of pre- and post-synaptic 5-HTja receptors in depressed patients.

  Platelet 5-HT 2A receptors in depressed patients Density and affinity Most radioligand studies have found an increase of platelet 5-HT2a receptors either in major depression patients or in attempted suicide patients. However, Rosel et al. studied platelets from depressed patients, finding an increment in the 5-HT2a platelet receptors affinity for [H]-ketanserin, but not in the receptors density.

  Functional capacity The evaluation of receptor function and sensitivity in platelets is performed after serotonin stimulation by using neuroendocrine tests and some other functional tests, such as platelet aggregation, morphological changes, quantification of intracellular calcium, and second messengers quantification.

  Despite being widely used, neuroendocrine tests are not completely reliable because they could be influenced by factors such as: stress on the hypothalamus-hypophysis axis, the lack of stereo-selective agonists and antagonists for different subtype serotonin receptors, and the effect of the drugs on other neurotransmitter systems. Other methodological aspects, such as: population heterogeneity, small samples, lack of variable control (i.e. age, sex, doses, diet, menstrual cycle), and placebo effects, are limitations to the neuroendocrine tests related to a single neurotransmitter system (serotonin).

  Results from platelet functional studies are contradictory as well. Platelet aggregation assays in depressed patients suggested a down-regulation of 5-HT2A receptors, compared to platelets from healthy subjects. However, some other studies have found no differences. Other platelet function responses mediated by 5-HT2A receptors, such as morphology changes, intracellular calcium, and phosphatidyl inositol hydrolysis, suggest a receptor up-regulation or hypersensitivity in depressed patients.

  Despite some disagreement among the results of platelet 5-HT2A receptor studies in depressed patients, most of them have reported an increase in 5-HT2A receptors density in these patients. However, suicidal behavior is clearly correlated to such an increase. Similar results have been observed in most post-mortem studies reporting an increase of 5-HT2A receptors in the prefrontal cortex. Protein synthesis and mRNA for 5-HT2A receptors are increased in prefrontal cortex and hippocampus in adolescent and adult suicide victims. These findings suggest that changes in the brain serotonergic system are related to depressive states and suicidal behavior.

  Human brain imaging techniques as well as molecular genetics studies may be additional tools to support the understanding of the neurobiology of depressive states, and their treatment.