5-hydroxytryptamine (5-HT) was originally discovered as a vasoconstrictor. 5-HT lowers blood pressure when administered peripherally to both normotensive and hypertensive male rats. Because the serotonin transporter (SERT) can function bidirectionally, we must consider whether 5-HT can be transported from the bloodstream to the central nervous system (CNS) in facilitating the fall in blood pressure. The blood-brain barrier (BBB) is a highly selective barrier that restricts movement of substances from the bloodstream to the CNS and vice-versa, but the rat BBB has not been investigated in terms of SERT expression. This requires us to determine whether the BBB of the rat, the species in which we first observed a fall in blood pressure to infused 5-HT, expresses SERT. We hypothesized that SERT is present in the BBB of the male rat. To test this hypothesis, over 500 blood vessels were sampled from coronal slices of six male rat brains. Immunofluorescence of these coronal slices was used to determine if SERT and RecA-1 (an endothelial cell marker) colocalized to the BBB. Blood vessels were considered to be capillaries if they were between 1.5 and 23 μm (intraluminal diameter). SERT was identified in the largest pial vessels of the BBB (mean ± SEM= 228.70 ± 18.71 μm, N=9) and the smallest capillaries (mean ± SEM= 2.75 ± 0.12 μm, N=369). SERT was not identified in the endothelium of blood vessels ranging from 20 to 135 μm (N=45). The expression of SERT in the rat BBB means that 5-HT entry into the CNS must be considered a potential mechanism when investigating 5-HT-induced fall in blood pressure.

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