A Network of (Autonomic) Clock Outputs

May 12, 2006Chronobiology international

A network of body clock signals controlling automatic functions

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Abstract

At least four subpopulations of neurons in the suprachiasmatic nuclei (SCN) are identified based on their timing of hormone release during the light-dark cycle.

The circadian clock in the SCN consists of thousands of oscillator neurons influenced by specific clock genes.
Anatomical connections and neurotransmitters, including vasopressin, GABA, and glutamate, help regulate hormone release patterns.
Different waves of neurotransmitter release control the activity of target neurons in the medial hypothalamus surrounding the SCN.
Subpopulations of SCN neurons do not necessarily align with the phenotypic characteristics of the neurons themselves.
Recent studies found similar distinct subpopulations of SCN neurons based on rhythmicity patterns in individual cells.
The SCN may contain specialized neurons that target specific organs such as the liver, pineal, and adrenal glands.

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The circadian clock in the suprachiasmatic nuclei (SCN) is composed of thousands of oscillator neurons, each of which is dependent on the cell-autonomous action of a defined set of circadian clock genes. A major question is still how these individual oscillators are organized into a biological clock producing a coherent output that is able to time all the different daily changes in behavior and physiology. We investigated which anatomical connections and neurotransmitters are used by the biological clock to control the daily release pattern of a number of hormones. The picture that emerged shows projections contacting target neurons in the medial hypothalamus surrounding the SCN. The activity of these pre-autonomic and neuro-endocrine target neurons is controlled by differentially timed waves of, among others, vasopressin, GABA, and glutamate release from SCN terminals. Together our data indicate that, with regard to the timing of their main release period within the light-dark (LD) cycle, at least 4 subpopulations of SCN neurons should be discerned. The different subgroups do not necessarily follow the phenotypic differences among SCN neurons. Thus, different subgroups can be found within neuron populations containing the same neurotransmitter. Remarkably, a similar distinction of 4 differentially timed subpopulations of SCN neurons was recently also discovered in experiments determining the temporal patterns of rhythmicity in individual SCN neurons by way of the electrophysiology or clock gene expression. Moreover, the specialization of the SCN may go as far as a single body structure; i.e., the SCN seems to contain neurons that specifically target the liver, pineal, and adrenal.

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A Network of (Autonomic) Clock Outputs

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