Endogenous amylin contributes to birth of microglial cells in arcuate nucleus of hypothalamus and area postrema during fetal development.

Amylin acts in the area postrema (AP) and arcuate nucleus (ARC) to control food intake. Amylin also increases axonal fiber outgrowth from the AP→nucleus tractus solitarius and from ARC→hypothalamic paraventricular nucleus. More recently, exogenous amylin infusion for 4 wk was shown to increase neurogenesis in adult rats in the AP. Furthermore, amylin has been shown to enhance leptin signaling in the ARC and ventromedial nucleus of the hypothalamus (VMN). Thus, we hypothesized that endogenous amylin could be a critical factor in regulating cell birth in the ARC and AP and that amylin could also be involved in the birth of leptin-sensitive neurons. Amylin+/- dams were injected with BrdU at embryonic day 12 and at postnatalday 2; BrdU+ cells were quantified in wild-type (WT) and amylin knockout (KO) mice. The number of BrdU+HuC/D+ neurons was similar in ARC and AP, but the number of BrdU+Iba1+ microglia was significantly decreased in both nuclei. Five-week-old WT and KO littermates were injected with leptin to test whether amylin is involved in the birth of leptin-sensitive neurons. Although there was no difference in the number of BrdU+c-Fos+ neurons in the ARC and dorsomedial nucleus, an increase in BrdU+c-Fos+ neurons was seen in VMN and lateral hypothalamus (LH) in amylin KO mice. In conclusion, these data suggest that during fetal development, endogenous amylin favors the birth of microglial cells in the ARC and AP and that it decreases the birth of leptin-sensitive neurons in the VMN and LH.

Development of the area postrema: an immunohistochemical study in the macaque.

The organization and chemical development of the area postrema (AP) in the macaque monkey was studied by immunohistochemistry imaged with conventional and confocal microscopy from day 40 of gestation to adulthood. The thin ependyma of the adult was found to develop from a thick continuous structure beginning in the second trimester. It was later invaded by tyrosine hydroxylase immunoreactive (TH+) and dopamine beta-hydroxylase immunoreactive (DBH+) cells and fibers, suggesting a possible route for release of neurotransmitter directly into ventricular cerebrospinal fluid. Other TH+ and/or DBH+ fibers were found in close approximation to blood vessels. Prominent vascularity of the parenchyma of AP was present late in the first trimester (fetal day (Fd)57 in the macaque) and increased further until birth. By contrast, the underlying solitary nucleus was hypervascular at Fd57, but its vascularity rapidly declined by late in the second trimester. TH+ neurons first appeared late in the first trimester, and DBH+ neurons appeared in the second trimester; these findings are consistent with the view that catecholaminergic cells in AP are the earliest members of the A2 noradrenergic group. Catecholaminergic cells or fibers in AP contained little labeling for synaptic vesicular proteins, suggesting that the release of neurotransmitter there may not involve a synaptic mechanism. Synapses were first observed in mid-second trimester, and most were associated with GABA+ fibers.

Development of vagal afferent projections circumflex to the obex in the embryonic chick brainstem visualized with voltage-sensitive dye recording.

Using voltage-sensitive dye recording, we surveyed neural responses related to the vagus nerve in the embryonic chick brainstem. In our previous studies, we identified four vagus nerve-related response areas in the brainstem. On the stimulated side, they included (1) the nucleus of the tractus solitarius (NTS: the primary sensory nucleus) and (2) the dorsal motor nucleus of the vagus nerve (DMNV), whereas on the contralateral side, they corresponded to (3) the parabrachial nucleus (PBN: the second/higher-ordered nucleus) and (4) the medullary non-NTS region. In the present study, in addition to these areas, we identified another response area circumflex to the obex. The intensity of the optical signal in the response area was much smaller than that in the NTS/DMNV, and the spatio-temporal pattern could be discerned after signal averaging. The conduction rate to the response area was slower than that to the other four areas. Ontogenetically, the response area was distributed on the stimulated side at the 6-day embryonic stage, and it spread into the contralateral side in 7- and 8-day embryonic stages. These distribution patterns were consistent with projection patterns of vagal afferent fibers stained with a fluorescent tracer, suggesting that the response area included a primary sensory nucleus. In comparison with the functional development of the other four response areas, we traced the functional organization of vagus nerve-related nuclei in the embryonic brainstem.