In temporal lobe epilepsy, seizures initiate in or near the hippocampus, which frequently displays loss of neurons, including inhibitory interneurons. It is unclear whether surviving interneurons function normally, are impaired, or develop compensatory mechanisms. We evaluated GABAergic interneurons in the hilus of the dentate gyrus of epileptic pilocarpine-treated GIN mice, specifically a subpopulation of somatostatin interneurons that expresses enhanced green fluorescence protein (GFP). GFP-immunocytochemistry and stereological analyses revealed substantial loss of GFP-positive hilar neurons (GPHNs) but increased GFP-positive axon length per dentate gyrus in epileptic mice. Individual biocytin-labeled GPHNs in hippocampal slices from epileptic mice also had larger somata, more axon in the molecular layer, and longer dendrites than controls. Dual whole-cell patch recording was used to test for monosynaptic connections from hilar GPHNs to granule cells. Unitary IPSCs (uIPSCs) recorded in control and epileptic mice had similar average rise times, amplitudes, charge transfers, and decay times. However, the probability of finding monosynaptically connected pairs and evoking uIPSCs was 2.6 times higher in epileptic mice compared to controls. Together, these findings suggest that surviving hilar somatostatin interneurons enlarge, extend dendrites, sprout axon collaterals in the molecular layer, and form new synapses with granule cells. These epilepsyrelated changes in cellular morphology and connectivity may be mechanisms for surviving hilar interneurons to inhibit more granule cells and compensate for the loss of vulnerable interneurons.

Arterial hyper-responsiveness to 5-hydroxytryptamine (5-HT) is a hallmark of hypertension, and plasma levels of free 5-HT are elevated in hypertension. We hypothesized that chronic administration of 5-HT would cause blood pressure to 1) rise in normotensive rats and 2) rise significantly more in hypertensive rats. The deoxycorticosterone acetate (DOCA)-salt hypertensive and sham normotensive rat were used. Animals were implanted with minipumps that delivered 5-HT (or vehicle) at a rate of 25 g/kg/min for 7 days. Free plasma 5-HT was elevated significantly by this protocol. Within 48 h, mean arterial blood pressure measured telemetrically decreased in sham (106 Ϯ 2 to 83 Ϯ 2 mm Hg) and in DOCA-salt hypertensive (166 Ϯ 9 to 112 Ϯ 3 mm Hg) rats; vehicle did not change blood pressure in either group. Ganglionic blockade (hexamethonium) reduced blood pressure to a greater magnitude in DOCA vehicle-administered rats (peak fall arterial pressure, 91 Ϯ 14 mm Hg) compared with DOCA 5-HT-administered rats (40 Ϯ 6 mm Hg). Maximal acetylcholine-induced (NO-dependent) relaxation in phenylephrine-contracted aortic strips was greater in 5-HTadministered (69.2 Ϯ 9.1% relaxation) versus vehicle-administered (39.7 Ϯ 14.2%) DOCA rats; aortic endothelial cell nitric oxide synthase expression was higher in the 5-HT-versus vehicle-administered DOCA-salt rats. In normotensive and DOCAsalt hypertensive rats, the nitric oxide synthase (NOS) inhibitor N -nitro-L-arginine (0.5 g/l in water) prevented the fall in blood pressure to 5-HT. We conclude that chronic exogenous 5-HT reduces blood pressure in normotensive and hypertensive rats through mechanisms critically dependent on NOS.Serotonin [5-hydroxytryptamine (5-HT)] was discovered and characterized over 60 years ago by the Italian scientist Erspamer (Erspamer and Asero, 1952) and by Irving Page (Rapport et al., 1948; Page and McCubbin, 1953a,b). In the periphery, 5-HT is made primarily in the enterochromaffin cells of the intestine. The circulatory system is exposed to 5-HT through aggregation of platelets (which take up and store a millimolar concentration of 5-HT), release from adrenergic nerves that have taken up 5-HT, and through direct exposure to 5-HT that is free in the blood. In many tissues, 5-HT is taken up and concentrated by the serotonin transporter (SERT) and is rapidly metabolized to an inactive metabolite, 5-hydroxyindole acetic acid (5-HIAA), by intracellular monoamine oxidase.5-HT was originally described as a substance derived from serum (sero) that increased the tone of smooth muscle (tonin). Because of the close association of the platelet with the blood vessel, there has been a long-standing question as to the role of 5-HT in controlling vascular tone and modifying blood pressure under normotensive and hypertensive conditions. Several findings suggest that 5-HT contributes to systemic hypertension. These include the following findings: 1) plasma levels (free) of 5-HT are elevated in experimental and human models of hypertension (Fetkovska et al., 1990;Carr...

We hypothesized that the 5-hydroxytryptamine (5-HT; serotonin) system is present and functional in veins. In vena cava (VC), the presence of the 5-HT synthesis rate-limiting enzyme tryptophan hydroxylase-1 mRNA and accumulation of the 5-HT synthesis intermediate 5-hydroxytryptophan after incubation with tryptophan supported the ability of veins to synthesize 5-HT. The presence of 5-HT and its metabolite 5-hydroxyindole acetic acid was measured by high-performance liquid chromatography in VC and jugular vein (JV), and it was compared with similarly sized arteries aorta (RA) and carotid (CA), respectively. In rats treated with the monoamine oxidase-A (MAO-A) inhibitor pargyline to prevent 5-HT metabolism, basal 5-HT levels were higher in veins than in arteries. 5-HT uptake was observed after exposure to exogenous 5-HT in all vessels. The presence of MAO-A and the 5-HT transporter (SERT) in VC was observed by immunohistochemistry and Western analysis. However, 5-HT uptake was not inhibited by the SERT inhibitors fluoxetine and/or fluvoxamine in VC and JV, as opposed to the inhibition in RA and CA. Moreover, studies performed in VC from mutant rats lacking SERT showed no differences in 5-HT uptake compared with VC from wild type. These data suggest the SERT is not functional under physiological conditions in veins. The differences in 5-HT handling between veins and arteries may represent alternative avenues for targeting the 5-HT system in the peripheral circulation for controlling vascular tone.5-Hydroxytryptamine (5-HT; serotonin) was first described as a substance that causes contraction of smooth muscle (Rapport et al., 1948;Erspamer and Asero, 1952). The function of 5-HT as a neurotransmitter is well established, as drugs that affect 5-HT concentration [e.g., Prozac (fluoxetine hydrochloride)] are widely used to treat conditions such as depression, anxiety, and obesity. However, its role in the cardiovascular system is far from being elucidated. For the last decade, accumulating evidence supports the involvement of 5-HT in the control of pulmonary circulation under normal and hypertensive conditions. However, a role for 5-HT in systemic vasculature is a matter of debate (for review, see Watts, 2005).In the periphery, platelets represent a large 5-HT storage site, and they may function as a buffer, keeping the free circulating 5-HT in low levels (Nilsson et al., 1985;Vanhoutte, 1991;Brenner et al., 2007). Indeed, platelet 5-HT uptake is decreased with age and in hypertension accompanied by an increase in free 5-HT circulating levels (Amstein et al., 1991;Brenner et al., 2007).5-HT is abundantly synthesized in the enterochromaffin cells of the intestine, representing more than 95% of total body 5-HT. 5-HT is also synthesized in the raphe nuclei of the brain, pineal gland, and in endothelial cells lining the lung. Potential sites of 5-HT synthesis in the systemic vasculature have not yet been identified. 5-HT is synthesized from the essential amino acid tryptophan in a two-step pathway. The hydroxylation o...

The handling of serotonin [5-hydroxytryptamine (5-HT)] depends on the serotonin transporter (SERT). A SERT knockout (KO) rat is a useful model to test the hypothesis that SERT is the primary mechanism for arterial 5-HT uptake and to investigate the impact of SERT removal on blood pressure. Wild-type (WT) and KO rats were used to measure 5-HT content (plasma, raphe, aorta, carotid, and mesenteric artery), aortic isometric contraction, and blood pressure. HPLC supported the lack of circulating 5-HT in plasma (ng/ml plasma, WT, 310 +/- 96; and KO, 1.0 +/- 0.5; P < 0.05). Immunohistochemistry and Western blot analyses validated the presence of the SERT protein in the WT rats and a lesser expression in the KO rat. The aorta isolated from KO rats had a normal contraction to phenylephrine and norepinephrine and a normal relaxation to the endothelium-dependent agonist acetylcholine compared with the aorta from WT. In contrast, the potency of 5-HT was increased in the aorta from KO rats compared with WT rats [-log EC(50) (M); WT, 5.71 +/- 0.08; and KO, 6.7 +/- 0.18] and maximum contraction was reduced [%phenylephrine (10 muM) contraction, WT, 113 +/- 6%; and KO, 52 +/- 12%]. 5-HT uptake was reduced but not abolished in arteries of the KO compared with the WT rats. Diurnal mean arterial blood pressure, heart rate, and locomotor activity level of the KO rats were similar to the WT rats. These data suggest that there are other mechanisms of 5-HT uptake in the arteries of the rat and that although the absence of circulating 5-HT and/or SERT function sensitizes arteries to 5-HT, SERT dysfunction does not impair normal blood pressure.

The corticospinal tract is unique to mammals and the corpus callosum is unique to placental mammals (eutherians). The emergence of these structures is thought to underpin the evolutionary acquisition of complex motor and cognitive skills. Corticospinal motor neurons (CSMN) and callosal projection neurons (CPN) are the archetypal projection neurons of the corticospinal tract and corpus callosum, respectively. Although a number of conserved transcriptional regulators of CSMN and CPN development have been identified in vertebrates, none are unique to mammals and most are coexpressed across multiple projection neuron subtypes. Here, we discover 17 CSMN-enriched microRNAs (miRNAs), 15 of which map to a single genomic cluster that is exclusive to eutherians. One of these, miR-409-3p, promotes CSMN subtype identity in part via repression of LMO4, a key transcriptional regulator of CPN development. In vivo, miR-409-3p is sufficient to convert deep-layer CPN into CSMN. This is a demonstration of an evolutionarily acquired miRNA in eutherians that refines cortical projection neuron subtype development. Our findings implicate miRNAs in the eutherians’ increase in neuronal subtype and projection diversity, the anatomic underpinnings of their complex behavior.