J-M Israel, S. Oliet et P. Ciofi inFront Neurosci.
A slice of life: Studying hypothalamic electrical activity in vitro
The July 20, 2016
Israel JM, Oliet SH, Ciofi P. Electrophysiology of Hypothalamic Magnocellular Neurons In vitro: A Rhythmic Drive in Organotypic Cultures and Acute Slices. Front Neurosci. 2016 Mar 31;10:109. 2016
This is a follow-up paper of our recent study concerning the pulsatile activity of hypothalamic neuroendocrine oxytocin neurons (Israel et al., 2014; see previous presentationand movie). A background follows below. Pictures from top to bottom: Jean Marc Israel, Stéphane Oliet, and Philippe Ciofi. Neurocentre Magendie
Philippe Ciofi: When the pituitary hormones were discovered in the 50ties, it was thought that they were circulating in the blood stream in constant and gradually changing levels for governing the endocrine glands and other organs of the body (like the gonads, adrenals, liver, thyroid, etc.). “Hormones give an imprint and are slow messengers”, was common sense at the time.
However, in the early 70ties, when accurate methods for measuring circulating hormones became available, to the surprise of endocrinologists, rapid and regular fluctuations were instead observed as a typical pattern of pituitary hormonal secretion. This hormonal “pulsatility” soon appeared important for biological activity, with disturbances being observed in pathology, as consequences or as leading causes.
Because the circulating profile of each pituitary hormone is primarily determined by a specific brain neuroendocrine system, a “brain pulsatility” was search for, and in 1973 eventually found in the hypothalamus, in the form of a periodic electrical activity in the neuroendocrine cells producing oxytocin. The oxytocin neurons release their neurohormone in discrete pulses, each triggering a uterine contraction at parturition and a milk-ejection latter on.
The next question was to understand whether the pulsatile activity of oxytocin neurons is autonomously produced (i.e., as an inherent property of all neuroendocrine cells) or determined by an external command (i.e., caused by a “pulse generator”). The best experimental approach for this is to decrypt the electrophysiological behavior of oxytocin neurons in simplified preparations like in cultured hypothalamic tissue or in brain slices.
The outcome of this research initiated in the late 70ties, was that the pulsatile activity was well visible and could be best studied in one preparation, the hypothalamic culture, but not in the traditional model of the electrophysiologists, the brain slice. It is therefore using the hypothalamic cultures that our 2014 study was done, which suggested that the pulsatility of the oxytocin neurons was indeed due to an external command from a neuroendocrine pulse generator.
In scientific research, controversy comes from the lack of conformity between experimental models. Therefore, in order to firmly establish our demonstration, we decided to understand why pulsatility in oxytocin neurons is difficult to observe in the brain slices. In this 2016 paper, we provide arguments suggesting that pulsatility is not observed in the brain slices simply because the neuroendocrine oxytocin cells are physically disconnected from their external drive by the experimental procedure. This is thus one limitation of the model, a situation often encountered with simplified preparations.
Several laboratories are currently undertaking similar research concerning the pulsatility of the other pituitary hormones. We hope to have contributed to the continuing awareness that biological processes are multiple and that any demonstration in Physiology needs confirmation in vivo.
Distribution of oxytocin neurons in the hypothalamus. Transverse section showing cell bodies grouped in the paraventricular (1) and supraoptic (2) nuclei, and axons (3) running in between. Newborn rat. Immunofluorescence.
Detail of the supraoptic nucleus showing neuroendocrine cells producing oxytocin (green) or vasopressin (red). Note the punctate aspect of the signal denoting the storage sites of the neurohormones. Newborn rat. Immunofluorescence.