Creation of the 5-hydroxytryptamine receptor 7 knockout rat as a tool for cardiovascular research

β-arrestin biased signaling is not involved in the hypotensive actions of 5-HT7 receptor stimulation: use of Serodolin

The 5-hydroxytryptamine 7 receptor (5-HT7) is necessary for 5-HT to cause a concentration-dependent vascular relaxation and hypotension. 5-HT7 is recognized as having biased signaling, transduced through either Gs or β -arrestin. It is unknown whether 5-HT7 signals in a biased manner to cause vasorelaxation/hypotension. We used the recently described β-arrestin selective 5-HT7 receptor agonist serodolin to test the hypothesis that 5-HT7 activation does not cause vascular relaxation or hypotension via the β -arrestin pathway. Isolated abdominal aorta (no functional 5-HT7) and vena cava (functional 5-HT7) from male Sprague Dawley rats were used in isometric contractility studies. Serodolin (1 nM - 10 μM) did not change baseline tone of isolated tissues and did not relax the endothelin-1 (ET-1)-contracted vena cava or aorta. In the aorta, serodolin acted as a 5-HT2A receptor antagonist, evidenced by a rightward shift in 5-HT-induced concentration response curve [pEC50 5-HT [M]: Veh = 5.2 +/- 0.15; Ser (100 nM) = 4.49 +/- 0.08; p < 0.05]. In the vena cava, serodolin acted as a 5-HT7 receptor antagonist, shifting the concentration response curve to 5-HT left and upward (%10 μM NE contraction; Veh = 3.2 +/- 1.7; Ser (10 nM) = 58 +/- 11; p < 0.05) and blocking relaxation of pre-contracted tissue to the 5-HT1A/7 agonist 5-carboxamidotryptamine. In anesthetized rats, 5-HT or serodolin was infused at 5, 25 and 75 μg/kg/min, iv. Though 5-HT caused concentration-dependent depressor responses, serodolin caused an insignificant small depressor responses at all three infusion rates. With the final dose of serodolin on board, 5-HT was unable to reduce blood pressure. Collectively the data indicate that serodolin functions as a 5-HT7 antagonist with additional 5-HT2A blocking properties. 5-HT7 activation does not cause vascular relaxation or hypotension via the β -arrestin pathway.

Development of a 5-HT7 receptor antibody for the rat: the good, the bad, and the ugly

Our laboratory has a vested interest in measuring the location and expression of the 5-hydroxytryptamine (5-HT, serotonin) 7 (5-HT7) receptor in the rat. Determining tissue-specific receptor expression would aid in validating understood and potentially new tissues that support the 5-HT7 receptor-mediated fall in blood pressure, an event we are committed to understand. We contracted with 7TM Antibodies to develop deliberately and rigorously a rat 5-HT7 (r5-HT7) receptor specific antibody. Three antigens, two targeting the third internal loop and one the C terminus, were used in three rabbits to generate antibodies. As a positive control, HEK293(T or AD) cells were transfected with a plasmid for the r5-HT7 receptor also expressing a C terminus 3xFLAG tag. Naïve rat tissues were also used in Western and immunohistochemical analyses. Nine antibodies (3 from three different rabbits) detected a ~ 75 kDa protein absent in homogenates of vector control HEK293T cells. Only antibodies that recognized the C terminus of the 5-HT7 receptor [ERPERSEFVLQNSDH(Abu)GKKGHDT; antibodies 3, 6, and 9] positively and concentration-dependently identified the r5-HT7 receptor expressed in Westerns of transfected HEK293T cells. These same C terminus antibodies also successfully detected the r5-HT7 receptor in immunocytochemical test of the transfected HEK293AD cells, colocalizing with the detected FLAG sequence. In naive tissue, antibody 6 performed the best, identifying specific bands in the brain cortex in Western analysis. These same antibodies produced a more diverse band profile in the vena cava, identifying 6 major proteins. In immunohistochemical experiments, the same C-terminus antibodies, with antibody 3 performing the best, detected the 5-HT7 receptor in rat veins. This deliberate work has given rise to at least three antibodies that can be used with good confidence in r5-HT7 transfected cells, two antibodies that can be used in immunohistochemical analyses of rat tissues and in Westerns of rat brain; we are less confident of the use of these same antibodies in rat veins.

5-HT7 receptors mediate dilation of rat cremaster muscle arterioles in vivo

OBJECTIVE

Serotonin (5-HT) infusion in vivo causes hypotension and a fall in total peripheral resistance. However, the vascular segment and the receptors that mediate this response remain in question. We hypothesized that 5-HT7 receptors mediate arteriolar dilation to 5-HT in skeletal muscle microcirculation.

METHODS

Cremaster muscles of isoflurane-anesthetized male Sprague-Dawley rats were prepared for in vivo microscopy of third- and fourth-order arterioles and superfused with physiological salt solution at 34°C. Quantitative real-time PCR (RT-PCR) was applied to pooled samples of first- to third-order cremaster arterioles (2-4 rats/sample) to evaluate 5-HT7 receptor expression.

RESULTS

Topical 5-HT (1-10 nmols) or the 5-HT1/7 receptor agonist, 5-carboxamidotryptamine (10-30 nM), dilated third- and fourth-order arterioles, responses that were abolished by 1 μM SB269970, a selective 5-HT7 receptor antagonist. In contrast, dilation induced by the muscarinic agonist, methacholine (100 nmols) was not inhibited by SB269970. Serotonin (10 nmols) failed to dilate cremaster arterioles in 5-HT7 receptor knockout rats whereas arterioles in wild-type litter mates dilated to 1 nmol 5-HT, a response blocked by 1 μM SB269970. Quantitative RT-PCR revealed that cremaster arterioles expressed mRNA for 5-HT7 receptors.

CONCLUSIONS

5-HT7 receptors mediate dilation of small arterioles in skeletal muscle and likely contribute to 5-HT-induced hypotension, in vivo.

Serotonin 5-HT7 receptor overexpression in the raphe nuclei area produces antidepressive effect and affects brain serotonin system in male mice

Heterodimerization between 5-HT₇ and 5-HT_1A receptors seems to play an important role in the mechanism of depression and antidepressant drug action. It was suggested that the shift of the ratio between 5-HT_1A /5-HT₇ hetero- and 5-HT_1A /5-HT_1A homodimers in presynaptic neurons toward 5-HT_1A /5-HT_1A homodimers is one of the reasons of depression. Consequently, the artificial elevation of 5-HT₇ receptor number in presynaptic terminals might restore physiological homo-/heterodimer ratio resulting in antidepressive effect. Here we showed that adeno-associated virus (AAV)-based 5-HT₇ receptor overexpression in the midbrain raphe nuclei area produced antidepressive effect in male mice of both C57Bl/6J and genetically predisposed to depressive-like behavior ASC (antidepressant sensitive cataleptics) strains. These changes were accompanied by the elevation of 5-HT₇ receptor mRNA level in the frontal cortex of C57Bl/6J and its reduction in the hippocampus of ASC mice. The presence of engineered 5-HT₇ receptor in the midbrain of both mouse strains was further demonstrated. Importantly that 5-HT₇ receptor overexpression resulted in the reduction of 5-HT_1A receptor level in the membrane protein fraction from the midbrain samples of C57Bl/6J, but not ASC, mice. 5-HT₇ receptor overexpression caused an increase of 5-HIAA/5-HT ratio in the midbrain and the frontal cortex of C57Bl/6J and in all investigated brain structures of ASC mice. Thus, 5-HT₇ receptor overexpression in the raphe nuclei area affects brain 5-HT system and causes antidepressive effect both in C57Bl/6J and in "depressive" ASC male mice. Obtained results indicate the involvement of 5-HT₇ receptor in the mechanisms underlying depressive behavior.

5-HT7 receptors expressed in the mouse parafacial region are not required for respiratory chemosensitivity

Abstract

A brainstem homeostatic system senses CO₂/H⁺ to regulate ventilation, blood gases and acid–base balance. Neurons of the retrotrapezoid nucleus (RTN) and medullary raphe are both implicated in this mechanism as respiratory chemosensors, but recent pharmacological work suggested that the CO₂/H⁺ sensitivity of RTN neurons is mediated indirectly, by raphe‐derived serotonin acting on 5‐HT7 receptors. To investigate this further, we characterized Htr7 transcript expression in phenotypically identified RTN neurons using multiplex single cell qRT‐PCR and RNAscope. Although present in multiple neurons in the parafacial region of the ventrolateral medulla, Htr7 expression was undetectable in most RTN neurons (Nmb⁺/Phox2b⁺) concentrated in the densely packed cell group ventrolateral to the facial nucleus. Where detected, Htr7 expression was modest and often associated with RTN neurons that extend dorsolaterally to partially encircle the facial nucleus. These dorsolateral Nmb⁺/Htr7⁺ neurons tended to express Nmb at high levels and the intrinsic RTN proton detectors Gpr4 and Kcnk5 at low levels. In mouse brainstem slices, CO₂‐stimulated firing in RTN neurons was mostly unaffected by a 5‐HT7 receptor antagonist, SB269970 (n = 11/13). At the whole animal level, microinjection of SB269970 into the RTN of conscious mice blocked respiratory stimulation by co‐injected LP‐44, a 5‐HT7 receptor agonist, but had no effect on CO₂‐stimulated breathing in those same mice. We conclude that Htr7 is expressed by a minor subset of RTN neurons with a molecular profile distinct from the established chemoreceptors and that 5‐HT7 receptors have negligible effects on CO₂‐evoked firing activity in RTN neurons or on CO₂‐stimulated breathing in mice.

Key points

Neurons of the retrotrapezoid nucleus (RTN) are intrinsic CO₂/H⁺ chemosensors and serve as an integrative excitatory hub for control of breathing.

Serotonin can activate RTN neurons, in part via 5‐HT7 receptors, and those effects have been implicated in conferring an indirect CO₂ sensitivity.

Multiple single cell molecular approaches revealed low levels of 5‐HT7 receptor transcript expression restricted to a limited population of RTN neurons.

Pharmacological experiments showed that 5‐HT7 receptors in RTN are not required for CO₂/H⁺‐stimulation of RTN neuronal activity or CO₂‐stimulated breathing.

These data do not support a role for 5‐HT7 receptors in respiratory chemosensitivity mediated by RTN neurons.

Recent Advances in the Production of Genome-Edited Rats

The rat is an important animal model for understanding gene function and developing human disease models. Knocking out a gene function in rats was difficult until recently, when a series of genome editing (GE) technologies, including zinc-finger nucleases (ZFNs), transcription activator-like effector nucleases (TALENs), and the type II bacterial clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated Cas9 (CRISPR/Cas9) systems were successfully applied for gene modification (as exemplified by gene-specific knockout and knock-in) in the endogenous target genes of various organisms including rats. Owing to its simple application for gene modification and its ease of use, the CRISPR/Cas9 system is now commonly used worldwide. The most important aspect of this process is the selection of the method used to deliver GE components to rat embryos. In earlier stages, the microinjection (MI) of GE components into the cytoplasm and/or nuclei of a zygote was frequently employed. However, this method is associated with the use of an expensive manipulator system, the skills required to operate it, and the egg transfer (ET) of MI-treated embryos to recipient females for further development. In vitro electroporation (EP) of zygotes is next recognized as a simple and rapid method to introduce GE components to produce GE animals. Furthermore, in vitro transduction of rat embryos with adeno-associated viruses is potentially effective for obtaining GE rats. However, these two approaches also require ET. The use of gene-engineered embryonic stem cells or spermatogonial stem cells appears to be of interest to obtain GE rats; however, the procedure itself is difficult and laborious. Genome-editing via oviductal nucleic acids delivery (GONAD) (or improved GONAD (i-GONAD)) is a novel method allowing for the in situ production of GE zygotes existing within the oviductal lumen. This can be performed by the simple intraoviductal injection of GE components and subsequent in vivo EP toward the injected oviducts and does not require ET. In this review, we describe the development of various approaches for producing GE rats together with an assessment of their technical advantages and limitations, and present new GE-related technologies and current achievements using those rats in relation to human diseases.

5-HT7 Receptor Restrains 5-HT-induced 5-HT2A Mediated Contraction in the Isolated Abdominal Vena Cava

ABSTRACT

Although discovered as a vasoconstrictor, 5-hydroxytryptamine (5-HT, serotonin) infused into man and rodent reduces blood pressure. This occurs primarily through activation of 5-HT7 receptors and, at least in part, venodilation. Vascular mechanisms by which this could occur include direct receptor activation leading to vasodilation and/or suppression of contractile 5-HT receptor activation. This study tests the hypothesis that the 5-HT7 receptor restrains activation of the 5-HT2A receptor. A subhypothesis is whether agonist-induced activation-independent of constitutive activity-of the 5-HT7 receptor is necessary for this restraint. The isolated abdominal aorta and vena cava from the normal male Sprague-Dawley rat was our model. Studies used real-time PCR and a pharmacological approach in the isolated tissue bath for measurement of isometric tone. Although 5-HT2A receptor mRNA expression in both aorta and vena cava was significantly larger than that of the 5-HT7 receptor mRNA, the 5-HT7/5-HT2A receptor mRNA ratio was greater in the vena cava (0.30) than in the aorta (0.067). 5-HT7 receptor antagonism by SB266970 and DR 4458 increased maximum contraction to 5-HT in the isolated vein by over 50% versus control. The 5-HT2A receptor agonists TCB-2 and NBOH were more potent in the aorta compared with 5-HT but less efficacious, serving as partial agonists. By contrast, these same three agonists caused no contraction in the vena cava isolated from the same rats up to 10 μM agonist. Antagonism of the 5-HT7 receptor by SB269970 did not increase either the potency or efficacy of TCB-2 or NBOH. These data support that the 5-HT7 receptor itself needs to be stimulated to reduce contraction and suggest there is little constitutive activity of the 5-HT7 receptor in the isolate abdominal vena cava.

Reduction in Hindquarter Vascular Resistance Supports 5-HT7 Receptor Mediated Hypotension

The 5-HT₇ receptor is the primary mediator of both the acute (<hours) and chronic (day-week) decreases in mean arterial pressure (MAP) during low dose 5-HT infusion in rats. Previous data show the hypotensive response during chronic 5-HT infusion is due to a decrease total peripheral resistance (TPR) and specifically splanchnic vascular resistance. We hypothesized that changes in vascular resistance in both the splanchnic and skeletal muscle vascular beds are critical to the cardiovascular effects mediated by the 5-HT₇ receptor. Systemic and regional hemodynamic data were collected in conscious and anesthetized male rats using radiotelemetry, vascular catheters and transit-time flowmetry. Reversible antagonism of the 5-HT₇ receptor was achieved with the selective antagonist SB269970 (33 μg/kg, iv). From the very beginning and throughout the duration (up to 5 days) of a low dose (25 μg/kg) infusion of 5-HT, TPR, and MAP were decreased while cardiac output (CO) was increased. In a separate group of rats, the contribution of the 5-HT₇ receptor to the regional hemodynamic response was tested during 5-HT-induced hypertension. The decrease in MAP after 24 h of 5-HT (saline 83 ± 3 vs. 5-HT 72 ± 3 mmHg) was associated with a significant decrease in skeletal muscle vascular resistance (saline 6 ± 0.2 vs. 5-HT 4 ± 0.4 mmHg/min/mL) while splanchnic vascular resistance was similar in 5-HT and saline-treated rats. When SB269970 was administered acutely, MAP and skeletal muscle vascular resistance rapidly increased, whereas splanchnic resistance was unaffected. Our work suggests the most prominent regional hemodynamic response to 5-HT₇ receptor activation paralleling the fall in MAP is a decrease in skeletal muscle vascular resistance.

5-HT does not lower blood pressure in the 5-HT7 knockout rat

The fall in mean arterial pressure (MAP) after 24 h of 5-HT infusion is associated with a dilation of the portal vein (PV) and abdominal inferior vena cava (Ab IVC); all events were blocked by the selective 5-HT₇ receptor antagonist SB269970. Few studies have investigated the contribution of the 5-HT₇ receptor in long-term cardiovascular control, and this requires an understanding of the chronic activation of the receptor. Using the newly created 5-HT₇ receptor knockout (KO) rat, we presently test the hypothesis that continuous activation of the 5-HT₇ receptor by 5-HT is necessary for the chronic (1 wk) depressor response and splanchnic venodilation. We also address if the 5-HT₇ receptor contributes to endogenous cardiovascular regulation. Conscious MAP (radiotelemeter), splanchnic vessel diameter (ultrasound), and cardiac function (echocardiogram) were measured in ambulatory rats during multiday 5-HT infusion (25 μg·kg^-1·min^-1 via minipump) and after pump removal. 5-HT infusion reduced MAP and caused splanchnic venodilation of wild-type (WT) but not KO rats at any time point. The efficacy of 5-HT-induced contraction was elevated in the isolated abdominal inferior vena cava from the KO compared with WT rats, supporting loss of a relaxant receptor. Similarly, the efficacy of 5-HT causing an acute pressor response to higher doses of 5-HT in vivo was also increased in the KO vs. WT rat. Our work supports a novel mechanism for the cardiovascular effects of 5-HT, activation of 5-HT₇ receptors mediating venodilation in the splanchnic circulation, which could prove useful in the treatment of cardiovascular disease.