Contractile properties of periosteal arterioles in the guinea-pig tibia

A Neuroskeletal Atlas: Spatial Mapping and Contextualization of Axon Subtypes Innervating the Long Bones of C3H and B6 Mice

Nerves in bone play well-established roles in pain and vasoregulation and have been associated with progression of skeletal disorders, including osteoporosis, fracture, arthritis, and tumor metastasis. However, isolation of the region-specific mechanisms underlying these relationships is limited by our lack of quantitative methods for neuroskeletal analysis and precise maps of skeletal innervation. To overcome these limitations, we developed an optimized workflow for imaging and quantitative analysis of axons in and around the bone, including validation of Baf53b-Cre in concert with R26R-tdTomato (Ai9) as a robust pan-neuronal reporter system for use in musculoskeletal tissues. In addition, we created comprehensive maps of sympathetic adrenergic and sensory peptidergic axons within and around the full length of the femur and tibia in two strains of mice (B6 and C3H). In the periosteum, these maps were related to the surrounding musculature, including entheses and myotendinous attachments to bone. Three distinct patterns of periosteal innervation (termed type I, II, III) were defined at sites that are important for bone pain, bone repair, and skeletal homeostasis. For the first time, our results establish a gradient of bone marrow axon density that increases from proximal to distal along the length of the tibia and define key regions of interest for neuroskeletal studies. Lastly, this information was related to major nerve branches and local maps of specialized mechanoreceptors. This detailed mapping and contextualization of the axonal subtypes innervating the skeleton is intended to serve as a guide during the design, implementation, and interpretation of future neuroskeletal studies and was compiled as a resource for the field as part of the NIH SPARC consortium. © 2021 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR)..

Synchrony of spontaneous Ca2+ activity in microvascular mural cells

Spontaneous rhythmic constrictions known as vasomotion are developed in several microvascular beds in vivo. Vasomotion in arterioles is considered to facilitate blood flow, while venular vasomotion would facilitate tissue metabolite drainage. Mechanisms underlying vasomotion periodically generate synchronous Ca²⁺ transients in vascular smooth muscle cells (VSMCs). In visceral organs, mural cells (pericytes and VSMCs) in arterioles, capillaries and venules exhibit synchronous spontaneous Ca²⁺ transients. Since sympathetic regulation is rather limited in the intra-organ microvessels, spontaneous activity of mural cells may play an essential role in maintaining tissue perfusion. Synchronous spontaneous Ca²⁺ transients in precapillary arterioles (PCAs)/capillaries appear to propagate to upstream arterioles to drive their vasomotion, while venules develop their own synchronous Ca²⁺ transients and associated vasomotion. Spontaneous Ca²⁺ transients of mural cells primarily arise from IP₃ and/or ryanodine receptor-mediated Ca²⁺ release from sarcoendoplasmic reticulum (SR/ER) Ca²⁺ stores. The resultant opening of Ca²⁺-activated Cl^- channels (CaCCs) causes a membrane depolarisation that triggers Ca²⁺ influx via T-type and/or L-type voltage-dependent Ca²⁺ channels (VDCCs). Mural cells are electrically coupled with each other via gap junctions, and thus allow the sequential spread of CaCC or VDCC-dependent depolarisations to develop the synchrony of Ca²⁺ transients within their network. Importantly, the synchrony of spontaneous Ca²⁺ transients also requires a certain range of the resting membrane potential that is maintained by the opening of K_v7 voltage-dependent K⁺ (K_v7) and inward rectifier K⁺ (K_ir) channels. Thus, a depolarised membrane would evoke asynchronous, 'premature' spontaneous Ca²⁺ transients, while a hyperpolarised membrane prevents any spontaneous activity.

Neural regulation of the contractility of nutrient artery in the guinea pig tibia

Nutrient arteries provide the endosteal blood supply to maintain bone remodelling and energy metabolism. Here, we investigated the distribution and function of perivascular nerves in regulating the contractility of the tibial nutrient artery. Changes in artery diameter were measured using a video tracking system, while the perivascular innervation was investigated using fluorescence immunohistochemistry. Nerve-evoked phasic constrictions of nutrient arteries were suppressed by phentolamine (1 μM), an α-adrenoceptor antagonist, guanethidine (10 μM), a blocker of sympathetic transmission, or fluoxetine (10 μM), a serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitor. In arteries pretreated with guanethidine, residual nerve-evoked constrictions were abolished by a high concentration of propranolol (10 μM) that is known to inhibit 5-HT receptors, or ketanserin (100 nM), a 5-HT₂ receptor antagonist, but not SB207216 (1 μM), an antagonist of 5-HT₃ and 5-HT₄ receptors. Bath-applied 5-HT (100 nM) induced arterial constriction that was suppressed by propranolol (10 μM) or ketanserin (100 nM). Nerve-evoked arterial constrictions were enhanced by spantide (1 μM), a substance P (SP) receptor antagonist, or L-nitro arginine (L-NA; 100 μM), an inhibitor of nitric oxide synthase (NOS). Immunohistochemistry revealed 5-HT-positive nerves running along the arteries that are distinct from perivascular sympathetic or substance P-positive primary afferent nerves. For the first time, functional serotonergic nerves are identified in the tibial nutrient artery of the guinea pig. Thus, it appears that tibial nutrient arterial calibre is regulated by the balance between sympathetic and serotonergic vasoconstrictor nerves and vasodilator afferent nerves that release substance P-stimulating endothelial nitric oxide (NO) release.