J. S. Lacroix

Neuropeptide Y and Sympathetic Neurotransmissiona

The coexistence of neuropeptide Y (NPY) with noradrenaline (NA) in perivascular nerves as well as in sympathetic nerves to muscle in the heart, spleen and vas deferens suggests a role for NPY in autonomic transmission. Sympathetic nerve stimulation or reflexogenic activation in experimental animals or man are associated with NPY release as revealed by overflow mainly upon strong activation. This difference between NPY and NA secretion may be related to the partly separate subcellular storage whereby NPY seems to be exclusively present in the large dense-cored vesicles. The NPY secretion is likely to be regulated by the local biophase concentrations of NA acting on prejunctional alpha-2-adrenoceptors since alpha-2 agonists inhibit and antagonists enhance NPY overflow, respectively. Furthermore, after NA has been depleted by reserpine, the nerve stimulation-evoked release of NPY is enhanced leading to a progressive depletion of tissue content of NPY. Exogenous NPY binds to both pre- and postjunctional receptors, inhibits NA and NPY release, enhances NA-evoked vasoconstriction and induces vasoconstriction per se. The prejunctional action of NPY which is especially noticeable in the vas deferens may serve to reduce transmitter secretion upon excessive stimulation. The long-lasting vasoconstriction evoked by sympathetic stimulation in several tissues including skeletal muscle, nasal mucosa and spleen, which remains in animals pretreated with reserpine (to deplete NA) combined with preganglionic denervation (to prevent the concomitant excessive NPY release and depletion), is mimicked by NPY and highly correlated to NPY release. Under these circumstances the NPY content in the local venous effluent reaches levels at which exogenous NPY evokes vasoconstriction.

Pharmacology of noradrenaline and neuropeptide tyrosine (NPY)‐mediated sympathetic cotransmission

Summary— Pharmacological and physiological aspects for neuropeptide Y (NPY) and noradrenaline (NA) cotransmission have been studied in the peripheral sympathetic nervous control of blood vessels, heart, spleen and vas deferens. NPY coexists with NA in large dense cored vesicles and is released compared to NA mainly upon high frequency stimulation or strong reflex sympathetic activation. NPY release is inhibited via prejunctional α‐2 adrenoceptors and adenosine receptors but facilitated by angiotensin II or β‐receptor activation. NPY exerts prejunctional inhibitory actions on both NA and NPY release, enhances the vasoconstrictor effect of NA and evokes potent, long‐lasting vasoconstriction. Specific receptor mechanisms for NPY exist at both the pre‐ and postjunctional levels; a large amidated C‐terminal portion of NPY is necessary for receptor binding, inhibition of cyclic AMP formation and vasoconstrictor effects. Denervation results in supersensitivity for both NA and NPY‐evoked vasoconstriction. Reserpine pretreatment is associated with depletion of NA as well as NPY; the effect on NPY is entirely dependent on an intact nerve activity. Reserpine treatment combined with preganglionic denervation depletes NA by 99% while NPY levels are maintained intact. The characteristic appearance of the nerve stimulation evoked vasoconstrictor response with a high correlation to NPY outflow after reserpine treatment, suggests that NPY may be involved as a transmitter in a variety of vascular beds. NPY‐synthesis in ganglia seems to be regulated by nicotinic receptor activity; secondary stimulation by eg reserpine stimulates and nicotine antagonists decrease NPY‐synthesis. Many classical pharmacological agents including guanethidine, clonidine, yohimbine, angiotensin II, nicotine and desipramine influence NPY release. A complex interplay therefore seems to occur at both the pre‐ and postjunctional levels of transmission for the classical transmitter NA and the coexisting peptide NPY, creating a great diversity of chemical signalling potential.

Release of calcitonin gene-related peptide in the pig nasal mucosa by antidromic nerve stimulation and capsaicin

The overflow of calcitonin gene-related peptide like-immunoreactivity (CGRP-LI) in the nasal venous effluent upon antidromic stimulation of the maxillary division of the trigeminal nerve with 6.9 Hz for 3 min or upon capsaicin (0.3 mumol bolus injection) were analysed in the nasal mucosa of sympathectomized pentobarbital anaesthetized pigs. The overflow of CGRP-LI upon antidromic stimulation displayed a slower appearance in the venous effluent than the overflow upon bolus injection of capsaicin. The vascular effects as revealed by the arterial blood flow, the venous blood flow, the blood volume of the nasal mucosa, i.e., the filling of the capacitance vessels and the superficial mucosal blood flow as revealed by the laser-Doppler signal were also studied. Antidromic stimulation of the trigeminal nerve as well as capsaicin bolus injection induced a marked vasodilation which was parallel to the overflow of CGRP. However, capsaicin bolus injection also resulted in a marked increase in the mean arterial blood pressure which may be due to reflex activation of sympathetic fibers. In conclusion, we have demonstrated that chemical stimulation with capsaicin as well as antidromic stimulation of nasal sensory nerves in sympathectomized animals induces both vasodilation and overflow of CGRP-LI in vivo. This indicates that CGRP may contribute to the sensory regulation of the microcirculation in the nasal mucosa.

Sympathetic vascular control of the pig nasal mucosa: adrenoceptor mechanisms in blood flow and volume control

1 The adrenoceptor mechanisms influencing the total blood flow, volume and superficial blood flow in the nasal mucosa of pigs anaesthetized with pentobarbitone have been characterized by use of various agonists and antagonists. 2 Local intra‐arterial bolus injection of the selective α1‐agonist phenylephrine, the selective α2‐agonist UK 14.304, the mixed α1/α2‐agonist oxymetazoline and the mixed α/β‐agonists noradrenaline (NA) and adrenaline induced dosed‐related reduction of nasal arterial blood flow (BF), nasal mucosal volume (V, reflecting capacitance vessel function) and the laser Doppler flowmetry signal (LDF, reflecting superficial movement of blood cells). The rank order of α‐agonist potency regarding BF reduction was UK 14.304 > oxymetazoline > NA > phenylephrine = adrenaline. For the volume response the potency order was UK 14.304 > oxymetazoline = NA = adrenaline > phenylephrine while for the reduction of the LDF signal the potency was UK 14.304 = NA = adrenaline > oxymetazoline > phenylephrine. The selective β2‐agonist terbutaline caused dose‐dependent increase of BF whereas only a small augmentation of the V was obtained upon the highest dose (40 nmol) while no modification of the LDF signal was observed. 3 After pretreatment with the selective α1‐antagonist prazosin, the response to phenylephrine was abolished while the selective α2‐antagonist idazoxan attenuated the effect of UK 14.304. After pretreatment with α‐antagonists, both NA and adrenaline caused biphasic effects with constriction followed by vasodilatation for BF, but not for V or LDF. This vasodilatation was blocked by the β‐antagonist propranolol. 4 The reduction in nasal BF and V upon sympathetic nerve stimulation was attenuated both by prazosin and idazoxan. Propranolol enhanced the remaining reduction of BF but not of V in the presence of α‐antagonists. 5 It is concluded that α2‐adrenoceptor mechanisms in the pig nasal mucosa are dominating for the BF, V and LDF responses to exogenous agonists, α1‐Adrenoceptors also seem to be involved in the sympathetic control of BF, V and LDF. Activation of β2‐receptors increases mainly BF and does not influence the LDF signal.

Adrenergic and neuropeptide Y supersensitivity in denervated nasal mucosa vasculature of the pig.

The effects of sympathetic denervation for 2 weeks on vasoconstrictor reactivity to alpha-adrenoceptor agonists, neuropeptide Y (NPY) and alpha,beta-methylene adenosine triphosphate (mATP) were investigated in different vascular compartments of the nasal mucosa of pentobarbital-anesthetized pigs. Supersensitivity to the vasoconstrictor actions of noradrenaline (NA) was observed in the function of both resistance vessels (as revealed by a reduction in arterial blood flow) and capacitance vessels (reflected by a reduction in nasal mucosal volume). The NA supersensitivity was, to a large extent, of prejunctional type since inhibition of neuronal uptake by desipramine also markedly enhanced the NA response. Whereas the reduction in arterial blood flow and in mucosal volume induced by the alpha 1-agonist, phenylephrine, was not changed by denervation, the effects of the alpha 2-agonists UK 14.304 and oxymetazoline were enhanced and/or prolonged. Furthermore, the reduction in blood flow and volume induced by NPY was enhanced in both amplitude and duration. The effects of mATP on the amplitude of the volume response and the duration of the blood flow and volume changes were increased. The maximal reduction in superficial blood flow was larger, as revealed by the laser Doppler flowmetry signal, when NPY or adrenoceptor agonists were given to denervated animals. It is concluded that sympathetic denervation is associated with increased sensitivity and prolonged responses to a variety of vasoconstrictor agents in the pig nasal mucosa in vivo. However, alpha 2-adrenoceptor, NPY and mATP mechanisms seem to be influenced more by denervation than by alpha 1-adrenoceptor sensitivity.