Local regulation of skin blood flow during cooling involving presynaptic P2 purinoceptors in rats

-1-Propenylcysteine, a sulfur compound in aged garlic extract, alleviates cold-induced reduction in peripheral blood flow in rat via activation of the AMPK/eNOS/NO pathway

Aged garlic extract (AGE) has been shown to improve peripheral circulatory disturbances in both clinical trials and experimental animal models. To investigate the effect of S-1-propenylcysteine (S1PC), a characteristic sulfur compound in AGE, on cold-induced reduction in tail blood flow of rat, Wistar rats were individually placed in a restraint cage and given the treatment with cold water (15˚C) after the oral administration of AGE or its constituents S1PC, S-allylcysteine (SAC) and S-allylmercaptocysteine (SAMC). After the cold-treatment the tail blood flow of rats was measured at the indicated times. The pretreatment with AGE (2 g/kg BW) and S1PC (6.5 mg/kg BW) significantly alleviated the reduction of rat tail blood flow induced by cold treatment. The effect of S1PC was dose-dependent and maximal at the dose of 6.5 mg/kg BW, whereas SAC and SAMC were ineffective. To gain insight into the mechanism of S1PC action, the concentration of nitrogen oxide metabolites (NOx) in the plasma and the levels of phosphorylated endothelial nitric oxide synthase (eNOS) and 5'-AMP-activated protein kinase (AMPK) in the aorta were measured. The pretreatment with S1PC significantly increased the plasma concentration of NOx as well as the level of phosphorylated form of AMPK and eNOS in the aorta after cold-treatment. The present findings suggest that S1PC is a major constituent responsible for the effect of AGE to alleviate the cold-induced reduction of peripheral blood flow in rat by acting on the AMPK/eNOS/NO pathway in the aorta.

Differential contribution of calcium channels to α1-adrenoceptor-mediated contraction is responsible for diverse responses to cooling between rat tail and iliac arteries

Our previous studies have shown that α₁-adrenoceptors, in addition to α₂-adrenoceptors, are involved in enhanced contraction of cutaneous blood vessels during cooling. The present study aimed to elucidate the mechanism underlying it. In tail and iliac arteries isolated from rats, isometric contraction was measured using a myograph and the phosphorylation level of myosin phosphatase target subunit 1 (MYPT1) was quantified by western blotting. The phenylephrine-induced contraction was enhanced by cooling to 24 °C in tail arteries, but was suppressed in iliac arteries. Endothelium denudation or treatment with iberiotoxin enhanced the phenylephrine-induced contraction in tail arteries at 37 °C; however, neither affected the contraction at 24 °C. The phenylephrine-induced contraction at 37 °C was largely suppressed by nifedipine in iliac arteries, but only slightly in tail arteries. The Rho kinase inhibitor H-1152 largely suppressed the phenylephrine-induced contraction at 24 °C, but only slightly at 37 °C, in both arteries. The phosphorylation level of MYPT1 at Thr855 in tail arteries was increased by the cooling. Taken together, these results suggest the following mechanism in regard to cooling-induced enhancement of α₁-adrenoceptor-mediated contraction in tail arteries: Cooling enhances the contraction of tail arteries via α₁-adrenoceptor stimulation by reducing endothelium-dependent, large-conductance Ca²⁺-activated K⁺ channel-mediated relaxation and by inducing Rho kinase-mediated Ca²⁺ sensitization, although the latter occurs even in iliac arteries. A smaller contribution of voltage-dependent Ca²⁺ channels, which are largely suppressed by cooling, to α₁-adrenoceptor-mediated contraction in tail arteries seems to be more crucially involved in the appearance of the enhanced contractile response to cooling.

Attenuation of the anti-contractile effect of cooling in the rat aorta by perivascular adipose tissue

In addition to providing mechanical support for blood vessels, the perivascular adipose tissue (PVAT) secretes a number of vasoactive substances and exerts an anticontractile effect. The main objective of this study was to find out whether the anticontractile effect of cooling in the rat aorta is affected by PVAT. Our hypothesis was that PVAT would enhance the anticontractile effect of cooling in the rat aorta. Aorta segments, with or without PVAT, were used in this investigation. Cumulative concentration-response curves were established for phenylephrine at 37°C or 24°C. Phenylephrine (10^-9 M - 10^-5 M) induced concentration-dependent contractions of aorta segments with or without PVAT at 37°C. The maximum response, but not pD₂ value, was reduced in aorta segments with PVAT. Cooling the tissues to 24 °C resulted in a significant reduction in the maximum response in aorta segments without PVAT with no change in pD₂ values. However, the anticontractile effect of cooling was attenuated in the presence of PVAT with no significant (p > 0.05) change in either the maximum response or pD₂ value. L-NAME potentiated PE-induced contractions and this was greater in aorta segments without PVAT at both temperatures. The expression of eNOS protein and basal tissue level of nitric oxide (NO) were greater in aorta segments with PVAT at both temperatures. However, PE significantly increased tissue levels of NO only in aorta segments without PVAT. We concluded that PVAT-induced loss of anticontractile effect of cooling against PE-induced contractions could be due to impaired generation of NO in aorta segments with PVAT.

Evidence for a functional vasoconstrictor role for ATP in the human cutaneous microvasculature

NEW FINDINGS

What is the central question of this study? In young adults, about half of the cold-related reduction in skin blood flow during cold exposure is mediated by noradrenaline, while the remainder is attributable to other substances co-released with noradrenaline that have yet to be identified. What is the main finding and its importance? Purinergic receptor blockade blunted the vasoconstriction response to whole-body cooling and to intradermal administration of tyramine. These results indicate that ATP is necessary to vasoconstrict blood vessels in the skin adequately and prevent heat loss in a cold environment. Noradrenaline is responsible for eliciting ∼60% of the reflex cutaneous vasoconstriction (VC) response in young adults, while the remainder is attributable to one or more unidentified co-released sympathetic adrenergic neurotransmitter(s). Inconsistent evidence has placed neuropeptide Y in this role; however, other putative cotransmitters have yet to be tested. We hypothesize that ATP contributes to the reflex cutaneous VC response. Two protocols were conducted in young adults (n = 10); both involved the placement of three microdialysis probes in forearm skin and whole-body cooling (skin temperature = 30.5°C). In protocol 1, the following solutions were infused: (i) lactated Ringer solution (control); (ii) 10 mm l-NAME; and (iii) purinergic receptor blockade with 1 mm suramin plus l-NAME. In protocol 2, the following solutions were infused: (i) lactated Ringer solution; (ii) suramin plus l-NAME; and (iii) suramin plus l-NAME plus adrenoreceptor blockade with 5 mm yohimbine plus 1 mm propranolol. Laser Doppler flux (LDF) was measured over each microdialysis site, and cutaneous vascular conductance (CVC) was calculated (CVC = LDF/MAP) and expressed as percentage changes from baseline (%ΔCVC_BASELINE ). l-NAME was used to block the vasodilatory influence of ATP and unmask the P₂ X-mediated VC response to exogenous ATP infusion (-21 ± 6%ΔCVC_BASELINE ). During cooling, the VC response (control, -39 ± 8%ΔCVC_BASELINE ) was attenuated at the suramin site (-21 ± 4%ΔCVC_BASELINE ) and further blunted with combined adrenoreceptor blockade (-9 ± 3%ΔCVC_BASELINE ; P < 0.05). Compared with the control site (-22 ± 5%ΔCVC_BASELINE ), suramin inhibited pharmacologically induced VC to tyramine (-12 ± 6%ΔCVC_BASELINE ; P < 0.05), which displaces adrenergic neurotransmitters from axon terminals. These data indicate that ATP contributes to the cutaneous VC response in humans.

G-protein coupled estrogen receptor-mediated non-genomic facilitatory effect of estrogen on cooling-induced reduction of skin blood flow in mice

An enhanced vasoconstrictor activity of cutaneous arteries participates in the reduction of skin blood flow induced by cooling stimulation. Raynaud's phenomenon, which is characterized by intense cooling-induced constriction of cutaneous arteries, is more common in women during the period from menarche to menopause. We thus investigated the effect of 17β-estradiol (E2) on cooling-induced reduction of plantar skin blood flow (PSBF) in mouse in vivo. Ovariectomized female ddY mice, anaesthetized with pentobarbital, were treated with tetrodotoxin for eliminating the sympathetic nerve tone and artificially ventilated. The PSBF was measured by laser Doppler flowmetry. Cooling air temperature around the foot from 25 to 20, 15, or 10°C decreased the PSBF in a temperature-dependent manner, which was suppressed by the specific α_2C-adrenoceptor antagonist MK-912. When E2 was intravenously administered as a bolus followed by a constant infusion for 10min just before the cooling stimulation, the cooling-induced reduction of PSBF was facilitated by E2 in a dose-dependent manner. The facilitatory effect of E2 was not induced after the treatment with MK-912. Similar facilitatory effect was induced by an intravenous application of G-1, an agonist of G protein-coupled estrogen receptor (GPER, also termed GPR30). Moreover, the facilitatory effect of E2 was abolished by the GPER antagonist G15. These results suggest that acute administration of E2 leads to the facilitation of cooling-induced, α_2C-adrenoceptor-mediated reduction of skin blood flow via the activation of the non-genomic estrogen receptor GPER.

Sympathetic control of reflex cutaneous vasoconstriction in human aging

This Synthesis highlights a series of recent studies that has systematically interrogated age-related deficits in cold-induced skin vasoconstriction. In response to cold stress, a reflex increase in sympathetic nervous system activity mediates reductions in skin blood flow. Reflex vasoconstriction during cold exposure is markedly impaired in aged skin, contributing to the relative inability of healthy older adults to maintain core temperature during mild cold stress in the absence of appropriate behavioral thermoregulation. This compromised reflex cutaneous vasoconstriction in healthy aging can occur as a result of functional deficits at multiple points along the efferent sympathetic reflex axis, including blunted sympathetic outflow directed to the skin vasculature, reduced presynaptic neurotransmitter synthesis and/or release, and altered end-organ responsiveness at several loci, in addition to potential alterations in afferent thermoreceptor function. Arguments have been made that the relative inability of aged skin to appropriately constrict is due to the aging cutaneous arterioles themselves, whereas other data point to the neural circuitry controlling those vessels. The argument presented herein provides strong evidence for impaired efferent sympathetic control of the peripheral cutaneous vasculature during whole body cold exposure as the primary mechanism responsible for attenuated vasoconstriction.

Regulation of the microvascular circulation in the leg muscles, pancreas and small intestine in rats

To study the microvascular circulation, we examined the proportion of open and functioning capillaries in the leg muscles, pancreas and small intestine of anesthetized rats. Fluorescein isothiocyanate (FITC)-labeled Lycopersicon esculentum lectin was injected into the heart and allowed to circulate for 3 min to label open and functioning capillaries. Specimens were removed, frozen, sectioned and double-immunostained. Using one section, open and functioning capillaries were detected by immunostaining for this lectin bound to endothelial cells, while all capillaries were visualized by immunostaining for platelet endothelial cell adhesion molecule-1 (PECAM-1 or CD31). These capillaries were semi-automatically detected and counted by fluorescence microscopy. The percentages of open and functioning capillaries were as follows: the soleus muscle, 93.0 ± 5.5%; superficial zone of the gastrocnemius muscle, 90.8 ± 6.2%; deep zone of the gastrocnemius muscle, 95.6 ± 4.0%; the plantaris muscle, 94.1 ± 2.7%; the pancreas, 86.3 ± 11.7%; and the small intestine, 91.1 ± 4.9% (n = 8, each). There was no significant difference among these data by the Kruskal–Wallis test. This study clearly demonstrated that the proportions of open and functioning capillaries are high and similar among the leg muscles, pancreas and small intestine in spite of their structural and functional differences. This finding agrees with previous studies and supports the notion that the microvascular circulation is mainly controlled by changing of the blood flow in each capillary rather than changing the proportion of open and functioning capillaries.

Purinergic transmission in blood vessels

There are nineteen different receptor proteins for adenosine, adenine and uridine nucleotides, and nucleotide sugars, belonging to three families of G protein-coupled adenosine and P2Y receptors, and ionotropic P2X receptors. The majority are functionally expressed in blood vessels, as purinergic receptors in perivascular nerves, smooth muscle and endothelial cells, and roles in regulation of vascular contractility, immune function and growth have been identified. The endogenous ligands for purine receptors, ATP, ADP, UTP, UDP and adenosine, can be released from different cell types within the vasculature, as well as from circulating blood cells, including erythrocytes and platelets. Many purine receptors can be activated by two or more of the endogenous ligands. Further complexity arises because of interconversion between ligands, notably adenosine formation from the metabolism of ATP, leading to complex integrated responses through activation of different subtypes of purine receptors. The enzymes responsible for this conversion, ectonucleotidases, are present on the surface of smooth muscle and endothelial cells, and may be coreleased with neurotransmitters from nerves. What selectivity there is for the actions of purines/pyrimidines comes from differential expression of their receptors within the vasculature. P2X1 receptors mediate the vasocontractile actions of ATP released as a neurotransmitter with noradrenaline (NA) from sympathetic perivascular nerves, and are located on the vascular smooth muscle adjacent to the nerve varicosities, the sites of neurotransmitter release. The relative contribution of ATP and NA as functional cotransmitters varies with species, type and size of blood vessel, neuronal firing pattern, the tone/pressure of the blood vessel, and in ageing and disease. ATP is also a neurotransmitter in non-adrenergic non-cholinergic perivascular nerves and mediates vasorelaxation via smooth muscle P2Y-like receptors. ATP and adenosine can act as neuromodulators, with the most robust evidence being for prejunctional inhibition of neurotransmission via A1 adenosine receptors, but also prejunctional excitation and inhibition of neurotransmission via P2X and P2Y receptors, respectively. P2Y2, P2Y4 and P2Y6 receptors expressed on the vascular smooth muscle are coupled to vasocontraction, and may have a role in pathophysiological conditions, when purines are released from damaged cells, or when there is damage to the protective barrier that is the endothelium. Adenosine is released during hypoxia to increase blood flow via vasodilator A2A and A2B receptors expressed on the endothelium and smooth muscle. ATP is released from endothelial cells during hypoxia and shear stress and can act at P2Y and P2X4 receptors expressed on the endothelium to increase local blood flow. Activation of endothelial purine receptors leads to the release of nitric oxide, hyperpolarising factors and prostacyclin, which inhibits platelet aggregation and thus ensures patent blood flow. Vascular purine receptors also regulate endothelial and smooth muscle growth, and inflammation, and thus are involved in the underlying processes of a number of cardiovascular diseases.

TRPA1 is essential for the vascular response to environmental cold exposure

The cold-induced vascular response, consisting of vasoconstriction followed by vasodilatation, is critical for protecting the cutaneous tissues against cold injury. Whilst this physiological reflex response is historic knowledge, the mechanisms involved are unclear. Here by using a murine model of local environmental cold exposure, we show that TRPA1 acts as a primary vascular cold sensor, as determined through TRPA1 pharmacological antagonism or gene deletion. The initial cold-induced vasoconstriction is mediated via TRPA1-dependent superoxide production that stimulates α_2C-adrenoceptors and Rho-kinase-mediated MLC phosphorylation, downstream of TRPA1 activation. The subsequent restorative blood flow component is also dependent on TRPA1 activation being mediated by sensory nerve-derived dilator neuropeptides CGRP and substance P, and also nNOS-derived NO. The results allow a new understanding of the importance of TRPA1 in cold exposure and provide impetus for further research into developing therapeutic agents aimed at the local protection of the skin in disease and adverse climates.

Blood flow in the skin of mammals changes in response to cold, but the mechanisms driving this response are unclear. Aubdool et al. show that the non-selective cation channel, TRPA1, is a vascular cold sensor and required for the vascular protective response to local cold exposure.

Microvascular circulation at cool, normal and warm temperatures in rat leg muscles examined by histochemistry using Lycopersicon esculentum lectin

Local cooling and/or warming of the body are widely used for therapy. For safer and more effective therapy, microvascular hemodynamics needs to be clarified. To examine blood circulation in rat leg muscles at 20, 30, 37 and 40°C, fluorescein isothiocyanate (FITC)-labeled Lycopersicon esculentum lectin was injected into the cardiac ventricle. Endothelial cells of open and functioning blood vessels were labeled by this lectin for 3 min and detected by immunostaining for lectin. The percentage of open and functioning capillaries of leg muscles by the avidin-biotin method was 89.8±3.3% at 37°C, while capillaries were unclear or unstained at 20 and 30°C, probably due to a decrease of blood flow. The results using the tyramide-dinitrophenol method were 58.6±15.0% at 20°C, 68.5±12.3% at 30°C, 83.8±5.7% at 37°C and 83.3±7.8% at 40°C. The value at 20°C was significantly different from those at 37 and 40°C. The results by the tyramide-biotin method were 85.5±5.3% at 20°C, 87.3±9.7% at 30°C, 94.7±3.6% at 37°C and 92.5±2.1% at 40°C. Based on these results, it was concluded that the blood flow of each capillary considerably decreased at 20 and 30°C and probably increased at 40°C, whereas the proportion of open and functioning capillaries was essentially unchanged.

Chemosensory information processing between keratinocytes and trigeminal neurons

Trigeminal fibers terminate within the facial mucosa and skin and transmit tactile, proprioceptive, chemical, and nociceptive sensations. Trigeminal sensations can arise from the direct stimulation of intraepithelial free nerve endings or indirectly through information transmission from adjacent cells at the peripheral innervation area. For mechanical and thermal cues, communication processes between skin cells and somatosensory neurons have already been suggested. High concentrations of most odors typically provoke trigeminal sensations in vivo but surprisingly fail to activate trigeminal neuron monocultures. This fact favors the hypothesis that epithelial cells may participate in chemodetection and subsequently transmit signals to neighboring trigeminal fibers. Keratinocytes, the major cell type of the epidermis, express various receptors that enable reactions to multiple environmental stimuli. Here, using a co-culture approach, we show for the first time that exposure to the odorant chemicals induces a chemical communication between human HaCaT keratinocytes and mouse trigeminal neurons. Moreover, a supernatant analysis of stimulated keratinocytes and subsequent blocking experiments with pyrodoxalphosphate-6-azophenyl-2',4'-disulfonate revealed that ATP serves as the mediating transmitter molecule released from skin cells after odor stimulation. We show that the ATP release resulting from Javanol® stimulation of keratinocytes was mediated by pannexins. Consequently, keratinocytes act as chemosensors linking the environment and the trigeminal system via ATP signaling.

Effects of some anesthetic agents on skin microcirculation evaluated by laser Doppler perfusion imaging in mice

Background

Anesthetic agents alter microcirculation, influencing tissue oxygenation and delivery of vital substrates. Laser Doppler perfusion imaging is a widespread technique in the field of microvascular research that can evaluate noninvasively and in real time the effects of environmental conditions, physical manipulations, diseases and treatments on peripheral perfusion. This study aims to evaluate laser Doppler perfusion imaging as a means to detect changes in skin microcirculation induced by some popular anesthetic agents in a murine model. Twenty-four age- and gender-matched healthy CD1 mice were examined by laser Doppler perfusion imaging. The skin microcirculatory response was measured at the level of plantar surfaces during isoflurane anesthesia with or without subsequent dexmedetomidine or acepromazine. At the end of the procedure, dexmedetomidine was reversed by atipamezole administration.

Results

In all mice, skin blood flow under isoflurane anesthesia did not show significant differences over time (P = 0.1). The serial perfusion pattern and values following acepromazine or dexmedetomidine administration differed significantly (P < 0.05).

Conclusions

We standardized a reliable laser Doppler perfusion imaging protocol to non-invasively assess changes in skin microcirculation induced by anesthesia in mice, considering the advantages and drawbacks of this technique and its translational value.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Imaging of temperature dependent hemodynamics in the rat sciatic nerve by functional photoacoustic microscopy

Background

Vascular hemodynamics is central to the regulation of neuro-metabolism and plays important roles in peripheral nerves diseases and their prevention. However, at present there are only a few techniques capable of directly measuring peripheral nerve vascular hemodynamics.

Method

Here, we investigate the use of dark-field functional photoacoustic microscopy (fPAM) for intrinsic visualizing of the relative hemodynamics of the rat sciatic nerve in response to localized temperature modulation (i.e., cooling and rewarming).

Results and conclusion

Our main results show that the relative functional total hemoglobin concentration (HbT) is more significantly correlated with localized temperature changes than the hemoglobin oxygen saturation (SO₂) changes in the sciatic nerve. Our study also indicates that the relative HbT changes are better markers of neuronal activation than SO₂ during nerve temperature changes. Our results show that fPAM is a promising candidate for in vivo imaging of peripheral nerve hemodynamics without the use of contrast agents. Additionally, this technique may shed light on the neuroprotective effect of hypothermia on peripheral nerves by visualizing their intrinsic hemodynamics.

Immobilization-induced hypersensitivity associated with spinal cord sensitization during cast immobilization and after cast removal in rats

This study examined mechanical and thermal hypersensitivity in the rat hind paw during cast immobilization of the hind limbs for 4 or 8 weeks and following cast removal. Blood flow, skin temperature, and volume of the rat hind paw were assessed in order to determine peripheral circulation of the hind limbs. Sensitization was analyzed by measuring the expression of the calcitonin gene-related peptide (CGRP) in the spinal dorsal horn following cast immobilization. Two weeks post immobilization, mechanical and thermal sensitivities increased significantly in all rats; however, peripheral circulation was not affected by immobilization. Cast immobilization for 8 weeks induced more serious hypersensitivity compared to cast immobilization for 4 weeks. Moreover, CGRP expression in the deeper lamina layer of the spinal dorsal horn increased in the rats immobilized for 8 weeks but not in those immobilized for 4 weeks. These findings suggest that immobilization-induced hypersensitivity develops during the immobilization period without affecting peripheral circulation. Our results also highlight the possibility that prolonged immobilization induces central sensitization in the spinal cord.

Impact of 10 sessions of whole body cryostimulation on cutaneous microcirculation measured by laser Doppler flowmetry

The aim of the present study was to evaluate the basic and evoked blood flow in the skin microcirculation of the hand, one day and ten days after a series of 10 whole body cryostimulation sessions, in healthy individuals.

The study group included 32 volunteers – 16 women and 16 men. The volunteers underwent 10 sessions of cryotherapy in a cryogenic chamber. The variables were recorded before the series of 10 whole body cryostimulation sessions (first measurement), one day after the last session (second measurement) and ten days later (third measurement). Rest flow, post-occlusive hyperaemic reaction, reaction to temperature and arterio–venous reflex index were evaluated by laser Doppler flowmetry.

The values recorded for rest flow, a post-occlusive hyperaemic reaction, a reaction to temperature and arterio – venous reflex index were significantly higher both in the second and third measurement compared to the initial one. Differences were recorded both in men and women. The values of frequency in the range of 0,01 Hz to 2 Hz (heart frequency dependent) were significantly lower after whole-body cryostimulation in both men and women. In the range of myogenic frequency significantly higher values were recorded in the second and third measurement compared to the first one.

Recorded data suggest improved response of the cutaneous microcirculation to applied stimuli in both women and men. Positive effects of cryostimulation persist in the tested group for 10 consecutive days.

Pharmacological modulation of the micturition pattern in normal and cyclophosphamide pre-treated conscious rats

In the current study, we wanted to assess the influence of muscarinic receptors, nitric oxide and purinoceptors on the micturition pattern of conscious normal and cyclophosphamide (CYP) pre-treated rats. The micturition parameters were assessed using a metabolic cage. Rats were pre-treated with either saline or CYP, to induce cystitis, followed by treatment with either the muscarinic M1/M3/M5 receptor antagonist 4-diphenylacetoxy-N-methylpiperidine (4-DAMP), the nitric oxide synthase blocker N(ω)-nitro-L-arginine methyl (L-NAME), the P2 purinoceptor antagonist pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (PPADS) or a combination of 4-DAMP with PPADS or L-NAME. Voiding volumes per micturition event were significantly lower in CYP pre-treated than in saline pre-treated rats. Neither 4-DAMP nor L-NAME had any effect in the normal rats, whereas PPADS reduced the micturition volume per event. In CYP pre-treated rats, 4-DAMP and L-NAME significantly increased voiding volumes per event and micturition frequency, respectively. 4-DAMP dose-dependently reduced the differences in micturition activity between saline and CYP pre-treated rats. We show that cystitis changes the urodynamics in conscious rats and that this change seems to depend on the production of NO and on altered muscarinic receptor effects. The altered muscarinic receptor responses are likely to per se involve NO-mediated mechanisms.

Local response to cold in rat tail after spinal cord transection

Subjects with severe chronic spinal cord injury (SCI) are prone to hypothermia when they are exposed to relatively low environmental temperatures that are normally well tolerated by healthy individuals. This impaired thermoregulation is presumably due to disconnection of territories below the SCI from supraspinal thermoregulatory centers. However, it is not known how these territories respond to low temperatures. Using a complete transection at T11 in rats, we examined the responses of the tail to cold (6–9°C) by measuring changes in tail blood flow and skin temperature weekly for 8 wk after SCI. Despite no significant change in baseline mean flow or temperature in the tail, the transection effectively removed the sympathetically mediated supraspinal control of the tail vasculature, since the amplitude of the pulse flow was markedly increased and the natural variations of the mean flow were almost abolished. As expected, the cold challenge before SCI caused a marked drop in mean flow, pulse amplitude, and temperature of the tail. Surprisingly, the drops in mean blood flow and temperature were observed after SCI, although the decrease in flow was slower and the pulse amplitude was not reduced. The results suggest that the cutaneous vasculature of the tail is sensitive to cold and will constrict, despite disconnection from supraspinal centers. This local effect is slow but may be sufficient to maintain some level of thermoregulation to cold. Without this vascular reaction, the effects of SCI on temperature regulation to cold would probably be much worse.

Purinergic cotransmission

Adenosine 5'-triphosphate (ATP) is a cotransmitter with classical transmitters in most nerves in the peripheral and central nervous systems, although the proportions vary between tissues and species and in different developmental and pathophysiological circumstances. There was early evidence that ATP was released together with acetylcholine (ACh) from motor nerves supplying skeletal muscle, although it was considered at the time as a molecule involved in the vesicular uptake and storage of ACh. Later it was shown that in the developing neuromuscular junction, released ATP acted on P2X receptor ion channels as a genuine cotransmitter with ACh. Adenosine triphosphate was shown to be released from sympathetic nerves supplying the guinea-pig taenia coli in 1971. Soon after, the possibility was raised that ATP was coreleased with noradrenaline from sympathetic nerves to guinea-pig seminal vesicle, cat nictitating membrane and guinea-pig vas deferens. Sympathetic purinergic cotransmission has also been demonstrated in many blood vessels. Parasympathetic nerves supplying the urinary bladder use ACh and ATP as cotransmitters; ATP acts through P2X ionotropic receptors, whereas the slower component of the response is mediated by the metabotropic muscarinic receptor. Adenosine triphosphate and glutamate appear to be cotransmitters in primary afferent sensory neurons. Adenosine triphosphate, calcitonin gene-related peptide and substance P coexist in some sensory-motor nerves. A subpopulation of intramural enteric nerves provides non-adrenergic, non-cholinergic inhibitory innervation of gut smooth muscle. Three cotransmitters are involved, namely ATP, nitric oxide and vasoactive intestinal polypeptide. In recent years, studies have shown that ATP is released with ACh, noradrenaline, glutamate, gamma-aminobutyric acid, 5-hyroxytryptamine and dopamine in different subpopulations of neurons in the central nervous system.

Bretylium abolishes neurotransmitter release without necessarily abolishing the nerve terminal action potential in sympathetic terminals

BACKGROUND AND PURPOSE

The antidysrhythmic bretylium is useful experimentally because it selectively abolishes neurotransmitter release from sympathetic peripheral nerve terminals. Its mechanism of action seemed settled, but recent results from optical monitoring of single terminals now suggests a new interpretation.

EXPERIMENTAL APPROACH

Orthograde transport of a dextran-conjugated Ca(2+) indicator to monitor Ca(2+) in nerve terminals of mouse isolated vas deferens with a confocal microscope. In some experiments, local neurotransmitter release was detected by monitoring neuroeffector Ca(2+) transients (NCTs) in adjacent smooth muscles, a local measure of purinergic transmission. Sympathetic terminals were identified with catecholamine fluorescence (UV excitation) or post-experiment immunohistochemistry.

KEY RESULTS

Bretylium (10 microM) abolished NCTs at 60/61 junctions over the course of 2 h, indicating effective abolition of neurotransmitter release. However, bretylium did not abolish the field stimulus-induced Ca(2+) transient in most nerve terminals, but did increase both action potential delay (by 2+/-0.4 ms) and absolute refractory period (by 4+/-2 ms). Immunohistochemistry demonstrated that 85-96% of terminals orthogradely filled with a dextran-conjugated fluorescent probe contained Neuropeptide Y (NPY). A formaldehyde-glutaraldehyde-induced catecholamine fluorescence (FAGLU) technique was modified to allow sympathetic terminals to be identified with a Ca(2+) indicator present. Most terminals contained catecholamines (based on FAGLU) or secrete ATP (as NCTs in adjacent smooth muscle cells are abolished).

CONCLUSIONS AND IMPLICATIONS

Bretylium can inhibit neurotransmitter release downstream of Ca(2+) influx without abolishing the nerve terminal action potential. Bretylium-induced increases in the absolute refractory period permit living sympathetic terminals to be identified.

Role of alpha2C-adrenoceptors in the reduction of skin blood flow induced by local cooling in mice

BACKGROUND AND PURPOSE

The reduction of skin blood flow induced by local cooling results from a reflex increase in sympathetic output and an enhanced vasoconstrictor activity of cutaneous vessels. The present study investigated the latter local response in vivo in tetrodotoxin-treated mice, in which the sympathetic nerve tone was abolished.

EXPERIMENTAL APPROACH

Male ddY mice, anaesthetized with pentobarbitone, were treated with tetrodotoxin and artificially ventilated. The plantar skin blood flow (PSBF) was measured by laser Doppler flowmetry.

KEY RESULTS

Cooling the air temperature around the left foot from 25 to 10 degrees C decreased the PSBF of the left foot. Bunazosin, an alpha (1)-adrenoceptor antagonist, RS79948, an alpha (2)-adrenoceptor antagonist, and MK-912, an alpha (2C)-adrenoceptor antagonist, all significantly inhibited the cooling-induced reduction of PSBF; the inhibition by bunazosin was relatively small compared with that by RS79948 and MK-912. The response was not affected by guanethidine or bretylium, but was diminished in adrenalectomized mice. An intra-arterial injection of clonidine, an alpha (2)-adrenoceptor agonist, to the left iliac artery of adrenalectomized mice caused a transient decrease in PSBF, which was significantly augmented at 10 degrees C. MK-912 suppressed only the augmented portion at 10 degrees C. Y-27632, H-1152 and fasudil, Rho kinase inhibitors, also inhibited the cooling-induced reduction of PSBF. RS79948 caused no further reduction of the cooling-induced response after the inhibition by Y-27632.

CONCLUSIONS AND IMPLICATIONS

Local cooling-induced reduction of skin blood flow in mice primarily results from increased reactivity of alpha (2C)-adrenoceptors to circulating catecholamines, in which the Rho/Rho kinase pathway is involved.

Physiology and pathophysiology of purinergic neurotransmission

This review is focused on purinergic neurotransmission, i.e., ATP released from nerves as a transmitter or cotransmitter to act as an extracellular signaling molecule on both pre- and postjunctional membranes at neuroeffector junctions and synapses, as well as acting as a trophic factor during development and regeneration. Emphasis is placed on the physiology and pathophysiology of ATP, but extracellular roles of its breakdown product, adenosine, are also considered because of their intimate interactions. The early history of the involvement of ATP in autonomic and skeletal neuromuscular transmission and in activities in the central nervous system and ganglia is reviewed. Brief background information is given about the identification of receptor subtypes for purines and pyrimidines and about ATP storage, release, and ectoenzymatic breakdown. Evidence that ATP is a cotransmitter in most, if not all, peripheral and central neurons is presented, as well as full accounts of neurotransmission and neuromodulation in autonomic and sensory ganglia and in the brain and spinal cord. There is coverage of neuron-glia interactions and of purinergic neuroeffector transmission to nonmuscular cells. To establish the primitive and widespread nature of purinergic neurotransmission, both the ontogeny and phylogeny of purinergic signaling are considered. Finally, the pathophysiology of purinergic neurotransmission in both peripheral and central nervous systems is reviewed, and speculations are made about future developments.

Excitatory P2-receptors at sympathetic axon terminals: role in temperature control of cutaneous blood flow

The mechanisms underlying the reduction in cutaneous blood flow in response to cooling are only partially understood. A study published in this issue of the British Journal of Pharmacology now provides evidence for the involvement of excitatory P2-receptors located at sympathetic axon terminals in the cooling-induced vasoconstriction in the skin. Cooling appears to cause the release of adenine nucleotides followed by the activation of excitatory presynaptic P2-receptors at noradrenergic axon terminals. Activation of these excitatory P2-receptors induces the release of noradrenaline, which subsequently causes constriction of blood vessels in the skin by action on smooth muscle alpha(1)- and alpha(2)-adrenoceptors. The commentary discusses the implication of the results and remaining questions.