RESUMEN

Una de las funciones más importantes del sistema circadiano es asegurar que las variables conductuales y fisiológicas de un organismo se ajusten apropiadamente a los eventos diarios en el ambiente, en un proceso llamado sincronización. En la naturaleza, la predominancia de la luz (sincronización luminosa) como el sincronizador principal del reloj circadiano (NSQ) es una clara adaptación a la vida terrestre. Sin embargo, otras ventajas biológicas pueden ser conferidas a un individuo si el sistema circadiano también es sensible a otras señales ambientales que proporcionen un estimado real del tiempo externo. De tal modo, la luz no es el único sincronizador que puede afectar al reloj biológico. Otros estímulos, como la temperatura, la actividad locomotora inducida por estímulos novedosos y ciertas drogas y fármacos, también son capaces de sincronizar el reloj biológico. En conjunto, estas señales han sido descritas como estímulos no luminosos. Durante el día subjetivo, tiempo en el cual el reloj biológico es sensible a estas señales, todas las manipulaciones no luminosas son capaces de generar avances de fase y de sincronizar un ritmo en corrimiento libre. Las respuestas de fase generadas por las señales no luminosas son de gran magnitud, aun de mayor magnitud que las inducidas por la luz. Asimismo, estas señales son capaces de inducir efectos residuales (after-effects) sobre la sincronización, de generar cambios en el periodo endógeno, de afectar el ángulo de fase en sincronización a un ciclo L:O y de promover el desarrollo del fenómeno de partición (splitting) del ritmo de actividad locomotora. Finalmente, la sincronización a la luz ha sido caracterizada en una amplia variedad de especies diurnas y nocturnas; en cambio, la sincronización no luminosa sólo se presenta en roedores nocturnos. Los estímulos no luminosos pueden ser categorizados como estímulos conductuales o farmacológicos. De entre los diferentes tipos de estímulos no luminosos que conforman estas categorías, destacan los tratamientos farmacológicos, los estímulos sociales, el estrés, la restricción de alimento y la comunicación entre la madre y el producto en la vida fetal y neonatal. Este último es de particular importancia para optimizar la función circadiana y sensibilizar al neonato a los ambientes externos. Con respecto a los mecanismos fisiológicos involucrados en este proceso, se ha sugerido que participan cuatro de los sistemas de neurotransmisión implícitos en el sistema circadiano: a) el sistema serotoninérgico proveniente del núcleo del rafé, b) el sistema inmureactivo a NPY proveniente de la hojuela intergeniculada (HIG), c) el sistema GABAérgico que se encuentra presente en la mayoría de las neuronas del NSQ y de la HIG (las proyecciones aferentes del núcleo del rafé y de la HIG hacen sinapsis con neuronas GABAérgicas en el NSQ) y 4) finalmente un sistema que implica señales dopaminérgicas y melatoninérgicas, las cuales se han implicado importantemente en sujetos en la vida fetal y neonatal.

En comparación con la cascada de señales intracelulares involucradas en la sincronización luminosa, las respuestas de fase inducidas por estímulos no luminosos no se asocian con la fosforilación de factores de trascripción que se unen a elementos de respuesta al AMPcíclico (CREB) o con la transcripción de genes de expresión temprana en el NSQ, eventos de señalización característicos de la sincronización luminosa. Las respuestas de fase generadas por las señales no luminosas ocurren durante el día subjetivo, tiempo en el cual la expresión espontánea de los genes reloj es alta en animales diurnos y nocturnos. Lo anterior sugiere que el reinicio de fase del reloj biológico a señales no luminosas puede ser generado por una supresión rápida en los niveles de expresión de los genes reloj. La disminución generada por los estímulos no luminosos en los niveles de expresión del RNAm de Per1 y Per2 en el NSQ sólo ocurre durante la mitad del día subjetivo, no durante la noche subjetiva, lo cual sugiere que estos genes participan de modo importante en el reinicio de fase durante el día subjetivo.

En los ratones knockout del gen Clock se modifican las respuestas del reloj biológico a las señales no luminosas. Cuando al ratón mutante Clock, se le aplican, durante el día subjetivo, paradigmas de actividad locomotora forzada, se generan respuestas de fase en dirección opuesta a las generadas por sujetos intactos. Esto sugiere que los distintos genes reloj participan en el origen de las respuestas de fase a estímulos no luminosos.

Palabras clave: Ritmos biológicos; sincronización no luminosa; reloj biológico

SUMMARY

One of the most important functions in which the circadian system participates is to assess that the behavioural and physiological variables adjust appropriately to daily events in the environment, a process referred to as entrainment. Since in the nature the food disposition and predators’ activity also are cyclical, the temporary relation between the circadian rhythm and periodic environmental signals maximizes the survival of each species in its temporary niche.

Thus, through this mechanism, the organisms adapt to their environment through circadian system which entrain the organism activities to different external signals. In nature environments the predominance of photic entrainment like primary zeitgeber of the biological clock (suprachiasmatic nucleus) is a clear adaptation to the earthly life; nevertheless other biological advantages can be conferred to an individual if the circadian system also is sensible to other environmental signals that they provide from the external time.

In such way, the light is not the only synchronizer affecting the biological clock. Other stimuli like the temperature and locomotor activity induced by novel stimuli and certain drugs are also able to entrain the biological clock. These signals have been described like non-photic stimuli.

The general effects of the non-photic signals are able to generate phase response and entrain a free running rhythm, only during the subjective day, time in which the biological clock is sensible to these signals which are able to generate phase advances. These phase response are of great magnitude, even of greater magnitude than the induced ones by a light signal.

The non-photic signals are also able to induce residual effects (after-effects) on entrainment process, thereby generating changes in the endogenous period, therefore affecting the phase angle in a cycle L:O and promoting the development of locomotor activity rhythm splitting. Furthermore, the light entrainment has been characterized in a wide variety of diurnal and nocturnal species. While, the non-photic entrainment only appears in nocturnal rodents. Being the hamster’s biological clock one of that responds to the greater number of biological non-photic signals such as the acute exposition to sexual odors, social interactions, as well as by simple injection of saline solution, all of these non-photic signals are able to induce phase advances of the locomotor activity rhythm in free running when they are applied onto the subjective day. The entrainment to a non-photic stimulus is also observed in humans.

Among the non-photic stimuli we can have the pharmacological treatments, social stimuli, stress, food restriction and communication between mother and product in the foetal and neonatal life. These later stimuli are of a particular importance to optimize the circadian function and sensitize the newborn to external environment. Thus the non-photic stimuli could be categorized like behavioural or pharmacological stimuli. These manipulations involve an increase in the locomotor activity, excitation or states able to phase resetting the circadian clock and peripheral oscillators in different species.

The non-photic stimuli can affect to the biological clock through an afferent projection from the SCN that translate the non-photic information and is able to induce phase responses. Additionally, non-photic stimuli could also affect the biological clock through the action of a peripheral oscillator, which is sensitive to this type of signals. These peripheral oscillators translate the non-photic information and it communicates with the SCN, through synaptic and no-synaptic mechanisms.

With regard to the physiological mechanisms involved on this process, there has been suggested to participate four neurotransmitter systems in the circadian system: a) the serotonergic system originating from the raphe nucleus, b) the NPY system from the leaflet intergeniculate (IGL), c) the GABAergic system, which it is present in most of the neurons of the SCN and IGL (the afferent projections of the raphe and the IGL nucleus make synapse with GABAergic neurons in the SCN) and 4) finally a neural system involving dopamine and melatonin signals, which have been importantly implicated in the brain in the foetal and neonatal live.

In comparison to the cascade of intracellular signals caused by glutamatergic stimulation associated to photic entrainment, which excites to the SCN cells, the transmitters implicated in the nonphotic entrainment typically inhibit the SCN neurons. For example the melatonin’s main action on the SCN neurons is inhibiting adenylyl cyclase and the translation of related signals driven by the AMPc, such inhibition of activity of the protein kinase depended of AMPc (PKA), which give rise to a decreased phospho- rylation of the transcription factor CREB. In this way, the phase responses induced by non-photic stimuli are not associate with the phosphorylation of the transcription factor (CREB) associated to responsive DNA-elements to binding AMPciclic or with the transcription of early expression genes in the SCN, events of metabothrophic signalling pathway of the photic entrainment.

The phase responses generated by the non-photic signals occur during the subjective day, time in which the spontaneous expression of clock genes is high in diurnal and nocturnal animals. A reason why the phase resetting of biological clock to non-photic signals can be generated by a fast suppression in the expression levels of the genes clock. The decrease of Per1 and Per2 messenger RNA’s expression levels in the SCN generated by non-photic stimuli occurs during a half of the subjective day, not during the subjective night, which suggests that these genes may participate in the phase resetting of biological clock during the subjective day.

The interactions between phase response induced by the light and those induced by non-photic stimuli have been described previously. When a photic stimulus is applied after a non-photic signal during subjective day, with the purpose of studying the interaction between photic stimuli and non-photic stimuli, the photic stimulus blocks or attenuates the phase advances generated in response to different non-photic stimuli applied, such as the forced locomotor activity, sleep privation, NPY administration, or serotonergic agonists (8-OH-DPAT) administration.

If the genes clock responds to the non-photic stimuli, then the lack of some of them will have to generate alterations in the response to non-photic signals. In the Clock mutant mice, the biological clock responses to the non-photic signals applied during the subjective day generate phase responses in opposed direction from those generated by intact subjects. This latter suggests that different genes clock participate in the generation of the phase response to a non-photic stimulus.

The non-photic entrainment of the circadian system has a biological and/or social importance in several contexts. In the early products life, the communication of circadian information from the mother is important in regulating the biological clock of the foetus or newborn before they are sensitive to light. Under circumstances where the social and work routines are altered, by changes of constant “work turn” (shift work), the biological clock receives photic and non-photic signals which generate a dysfunction and poor work efficiency. The absence of non-photic signals followed by a social abstinence can induce alterations in the mental health (depression). The sleep disorder, experimented blind subject can arise from a lost of the social entrainment, therefore a decrease in the efficiency of the clock mechanism. Thus latter alterations of the clock, it could be possible to develop new forms of pharmacological and behavioural treatments.

Key words: Biological rhythms; non-photic entrainment; biological clock