Endocrine Abstracts

The hypothalamic suprachiasmatic nucleus (SCN) is the key-structure of the control of circadian rhythms. Several observations support the view that glutamate is the primary transmitter of the retinal projection to this cell group. The glutamatergic innervation of the nucleus is not limited to this projection, it is much more extended. The aim of our investigations was (1) to examine whether are glutamatergic neurons existing in the SCN and (2) to get information about the relationship between glutamatergic axon terminals and vasoactive intestinal polypeptide (VIP)-, GABA- and arginine-vasopressin (AVP)-containing neurons. Vesicular glutamate transporter type 2 (VGluT2) was used as marker of the glutamatergic elements. Single and double label immunocytochemistry was applied and the brain sections were examined by confocal laser scanning microscopy and under the electron microscope. We detected VGluT2 immunoreactive neurons in the SCN and observed VGluT2 axon terminals in synaptic contact with GABA, VIP, AVP and with VGluT2-positive perikarya or dendrites. The morphology of the contacts indicated asymmetric type synapses. Our observations provide the first neuromorphological evidence for the view that glutamatergic neurons exist in the SCN and further they demonstrate for the first time terminations of glutamatergic boutons on prominent cell groups of the SCN. The findings are in line with the view that the intranuclear organization of the circadian clock is extremely complex.

This work was supported by the National Scientific Research Fund (OTKA T-042516 to B.H., OTKA T-049455 to K.J.), the Ministry of Health (ETT 020/2006), National Research Program (NKFP 1A002-04 8/6/04) and the Hungarian Academy of Sciences.