The first clear example of a clock regulated gene in animals was Vasopressin, a neurotransmitter expressed in the suprachiasmatic nucleus, as well as in many other regions of the brain. Are other neurotransmitters also clock regulated? The short answer is yes. Especially in mice and Drosophila. A number of neurotransmitters including vasopressin, glutamate, GABA and PDF, have profound effects on clock controlled behavior. Essentially, without temporal control, of these factors or their receptors, behavioural clock control doesn't work. To understand regulatory mechanisms, Dopamine makes an interesting starting point for discussion. Dopamine has a number of functions, one of which is as a neurotransmitter in the reward system, chemically mediating the good feelings that come with positive stimuli. Dopamine is involved in addictive behaviour. The rate-limiting enzyme in the production of Dopamine is clock regulated. Like BMAL1, its expression is linked with the Rev-erb alpha, ROR a family. It is expressed at high levels when rev-erb alpha is low and this may explain aggressive behaviors in the rev-erb alpha mutant animals. Rhythms in any compound can be generated either by oscillations in production or destruction. The destruction of a whole host of regulated neurotransmitters- dopamine, adrenaline, noradrenaline - is mediated by enzymes called monoamine oxidase a or monoamine oxidase b. MAO A is expressed rhythmically due to the control of E boxes and its promoter. The enzymatic activity oscillates in specific regions of the mouse brain, and the oscillation is gone when the mouse Per2 gene is mutated. This rhythm in MAO A could contribute to rhythmic degradation of neurotransmitters such as dopamine, and lessen daily rhythms in mood. How else might the circadian clock be regulating behaviours? By regulating the receptors of compounds like dopamine, or regulating receptors for environmental stimuli. I showed clock-regulated olfaction in mice. Olfaction is also clock-regulated in Drosophila and in this tiny creature, as well. So olfaction is broadly regulated by the circadian clock, as are many sensory systems. We sense the world via receptors and the receptors that are regulating olfaction, vision, and blood pressure- actually the list is very, very long because this class of receptors is the largest family of genes in animal genomes. These receptors are called G protein coupled receptors, or GPCRs for short, in reference to their signaling mechanism via G proteins. A quick inspection of the list of GPCRs whose functions are known, indicates that many of these functions show a circadian rhythm. How might this occur? The most obvious mechanism would be rhythmic expression of the receptor itself, as can happen here in the case of the Dopamine D4 receptor. Another interesting mechanism has this receptor, dopamine D4, heterodimerizing with adrenergic receptors in the pineal gland to suppress melatonin synthesis in the morning hours. As a non-dimerized receptor in the evening, the adrenergic receptors are much more active once they're bound by their ligand. So that's the second mechanism for regulation of GPCRs. Finally, there's a specific mechanism for turning off the receptor, once it's been activated. This involves its phosphorylation by a specific kinase, which then allows an arrestin molecule to bind, triggering the internalization of the receptor away from the cell membrane where it can no longer pick up ligands and send signals. This kinase oscillates daily in amount, both in the nematode C elegans and the fruit fly Drosophila. GCPRs can regulate complex behaviors in at least 4 distinct ways: by controlling the amount of ligand, binding the receptor, by controlling the amount of the receptor itself, by controlling availability of the receptor and by controlling the signal transduction pathway. This suggests that it's extremely important to keep this class of molecules tightly regulated. It also implies tremendous plasticity in regulating the timing of their functions. A choreography that might be useful for adapting to different needs, for instance, to different seasons. [SOUND]
