Research reportExpression of serotonin transporter protein in developing rat brain
Introduction
Serotonin (5-HT) is one of the earliest appearing transmitter systems during development. In rats, 5-HT neurons appear on embryonic day 12 (E12) 27, 35, 43, earlier than the genesis of most other transmitter neurons. 5-HT's early ontogenic appearance and distribution in the brain regions coincides with many neuronal differentiation and growth events. It is not surprising that 5-HT has been proposed as a signaling chemical for neuronal development [for review see Ref. [23]]. 5-HT has been shown to induce neurogenesis and neuronal differentiation 10, 22, 26, 44, affect cranial neural crest migration [32], induce synaptogenesis [44], and inhibit growth cone motility [17]. If 5-HT actively plays a role in these developmental events, an active regulation of 5-HT action most likely exists.
Serotonin transporter (5-HTT) is one of the most critical regulators, through removal of extracellular 5-HT, to fine-tune 5-HT signaling. The ontogeny of 5-HTT and its close relationship with 5-HT neurons is important for understanding the regulation of 5-HT and thus 5-HT activity during development.
On another spectrum, 5-HTT is a target of many 5-HT related drugs of abuse, from cocaine and amphetamine to methamphetamine. 5-HTT is also a major site of action of many antidepressants, including tricyclics and serotonin-specific re-uptake inhibitors (SSRIs). Growing concerns about prenatal exposure to drugs of abuse or even SSRIs during fetal development may be well grounded if 5-HTT appears early during brain development [38].
Currently the temporal appearance and the topographic distribution of the 5-HTT protein have not been reported. A functional binding site for 5-HTT in mouse [8]and a distribution of mRNA [15]in rats described in the developmental stage are available but with many incongruities. We previously reported that 5-HTT is not only distributed and functions near the synaptic terminal but also, for the most part, on the axonal membrane [47]. How 5-HTT is distributed during the growth of 5-HT neurons is among the questions that remain to be answered. The appearance and distribution of the 5-HTT protein in the developing brain is reported here, and the distribution of 5-HTT protein and mRNA are compared.
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Materials and methods
Time pregnant Sprague–Dawley rats (Harlan, Indianapolis, IN) were used for the study. Animals were mated after dark, and the sperm check for indication of pregnancy was performed the next morning. The day of the positive sperm check was designated as gestation day 0 (E0). Animals were provided with normal rat chow and water ad lib. Fetuses of E12, 13, 14, 16, 18, and 20 were obtained from dams under deep ether anesthesia in the morning. At least three dams per age were used. Two embryos from
Results
No 5-HTT-im somas were seen in nigral, locus ceruleus, hypothalamus, or any forebrain structures in our prenatal samples, except for very light 5-HTT-im neurons in cortex in. Prominent 5-HTT-im cell bodies were encountered only in the raphe complex at the birthplace of 5-HT neurons. 5-HTT-im axons extended beyond raphe with very similar morphology of 5-HT-im axons. They are chronologically displayed hereafter. The feature expression is noted when they first appear in the age group. All sagittal
Discussion
The comparison of embryonic days varied depending on the definition of the sperm/plug confirmation day in the literature. Most of studies have adopted the mating period as beginning in late afternoon with sperm confirmation the next morning. The sperm/plug positive day is considered here as day 0. Comparisons with other literature were standardized for the convenience of the discussion. In addition, the embryonic day of rat is approximated one day behind mouse (i.e., rat E13 corresponds to
Acknowledgements
This study is supported by PHS RO3 MH 50602 and PHS P50AA07611-11 to F.C.Z.
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