Cellular electrophysiology of the suprachiasmatic nucleus
Within individual SCN neurons, molecular clocks consisting of gene transcription feedback loops generate circadian rhythms. Circadian rhythms in individual SCN neurons manifest as a circadian pattern of action potential firing with a higher frequency during the day. However, not all SCN neurons rhythmically express clock genes or action potentials. In the mammalian SCN, these rhythmic and non-rhythmic neurons are organized into anatomical compartments that differ in clock gene expression patterns, as well as in efferent and afferent neuronal connections. Our recent work has focused on determining the synaptic signaling mechanisms of phenotypically identified neurons in a defined SCN compartment, the calbindin subnucleus (CBsn) of the hamster, which is known to be important for generating circadian locomotor behavior. We recently demonstrated using electrophysiological and immunohistochemical techniques that a population of calbindin-expressing neurons believed to contribute to behavioral circadian rhythms do not have a circadian pattern of action potential firing (Jobst and Allen, Eur J Neurosci, 2002). Our results reveal that CBsn neurons represent a functionally distinct neuronal subpopulation in which rhythmic action potential output is not necessary for the generation of behavioral circadian rhythmicity. Based on these data, we proposed that intercellular communication between rhythmic SCN neurons and nonrhythmic SCN neurons is essential to produce a circadian output in the intact animal (Jobst and Allen, Eur J Neurosci, 2002).
To follow-up on his work we performed studies to provide a neuroanatomical framework for synaptic and gap junction communication, as well as neuron-to-neuron or glia-to-neuron signaling, within the CBsn. Using neuronal reconstructions of recorded neurons, we demonstrated that CB-expressing neurons have significantly more dorsally oriented dendritic arbors than neurons that do not express calbindin. Using double-label confocal microscopy, we showed that calbindin-expressing neurons contain GABA and GABAA receptor subunits and make intimate contacts with neurons in the CBsn. Transforming growth factor alpha (TGFa), a substance shown to inhibit locomotion, was identified as being present within the CBsn. In addition, neurons in this region express the epidermal growth factor receptor, the only receptor for TGFa. Lastly, we demonstrated that CB-expressing neurons are coupled to CB-expressing and neurons that do not express CB by gap junctions. These data provide a structural framework for synaptic communication, electrical coupling, and signaling via a growth factor within the CBsn of the hamster SCN (Jobst and Allen, Neuroscience, in press).
We are continuing to pursue understanding of the functions of neurons within the CBsn. Specific studies that are planned include 1) determining the circadian phase dependence of responses of CBsn neurons to optic nerve stimulation, 2) characterizing the regulation of action potential firing and synaptic transmission by vasoactive intestinal peptide (VIP) and pituitary adenylyl cyclase activating polypeptide (PACAP), 3) examining the responses of CBsn neurons to Transforming Growth Factor alpha (TGFa), and 4) determining whether gastrin-releasing peptide (GRP) or VIP immunoreactive neurons fire action potentials in a circadian manner.