Zsuzsanna Helyes

Release of somatostatin and its role in the mediation of the anti‐inflammatory effect induced by antidromic stimulation of sensory fibres of rat sciatic nerve

1 The effect of antidromic stimulation of the sensory fibres of the sciatic nerve on inflammatory plasma extravasation in various tissues and on cutaneous vasodilatation elicited in distant parts of the body was investigated in rats pretreated with guanethidine (8 mg kg−1, i.p.) and pipecuronium (200 μg kg−1, i.v.). 2 Antidromic sciatic nerve stimulation with C‐fibre strength (20 V, 0.5 ms) at 5 Hz for 5 min elicited neurogenic inflammation in the innervated area and inhibited by 50.3±4.67% the development of a subsequent plasma extravasation in response to similar stimulation of the contralateral sciatic nerve. Stimulation at 0.5 Hz for 1 h also evoked local plasma extravasation and inhibited the carrageenin‐induced (1%, 100 μl s.c.) cutaneous inflammation by 38.5±10.0% in the contralateral paw. Excitation at 0.1 Hz for 4 h elicited no local plasma extravasation in the stimulated hindleg but still reduced the carrageenin‐induced oedema by 52.1±9.7% in the paw on the contralateral side. 3 Plasma extravasation in the knee joint in response to carrageenin (2%, 200 μl intra‐articular injection) was diminished by 46.1±12.69% and 40.9±4.93% when the sciatic nerve was stimulated in the contralateral leg at 0.5 Hz for 1 h or 0.1 Hz for 4 h, respectively. 4 Stimulation of the peripheral stump of the left vagal nerve (20 V, 1 ms, 8 Hz, 10 min) elicited plasma extravasation in the trachea, oesophagus and mediastinal connective tissue in rats pretreated with atropine (2 mg kg−1, i.v.), guanethidine (8 mg kg−1, i.p.) and pipecuronium (200 μg kg−1, i.v.). These responses were inhibited by 37.8±5.1%, 49.7±9.9% and 37.6±4.2%, respectively by antidromic sciatic nerve excitation (5 Hz, 5 min) applied 5 min earlier. 5 Pretreatment with polyclonal somatostatin antiserum (0.5 ml/rat, i.v.) or the selective somatostatin depleting agent cysteamine (280 mg kg−1, s.c.) prevented the anti‐inflammatory effect of sciatic nerve stimulation (5 Hz, 5 min) on a subsequent neurogenic plasma extravasation of the contralateral paw skin. The inhibitory effect of antidromic sciatic nerve excitation on plasma extravasation in response to vagal nerve stimulation was also prevented by somatostatin antiserum pretreatment. 6 Cutaneous blood flow assessment by laser Doppler flowmetry indicated that antidromic vasodilatation induced by sciatic nerve stimulation was not inhibited by excitation of the sciatic nerve of the contralateral leg (1 Hz, 30 min) or by somatostatin (10 μg/rat, i.v.) injection. 7 Plasma levels of somatostatin increased more than 4 fold after stimulation of both sciatic nerves (5 Hz, 5 min) but the stimulus‐evoked increase was not observed in cysteamine (280 mg kg−1, s.c.) pretreated rats. 8 These results suggest that somatostatin released from the activated sensory nerve terminals mediates the systemic anti‐inflammatory effect evoked by stimulating the peripheral stump of the sciatic nerve.

Systemic anti-inflammatory effect induced by counter-irritation through a local release of somatostatin from nociceptors

1 Neurogenic plasma extravasation evoked by topical application of 1% vv−1 mustard oil on the skin of the acutely denervated rat hindleg (primary reaction) inhibited the development of a subsequent oil‐induced plasma extravasation induced in the skin of the contralateral hindleg by 49.3±7.06% (n=9) and in the conjunctival mucosa due to 0.1% wv−1 capsaicin instillation by 33.5±10.05% (n=6). The primary reaction also inhibited the non‐neurogenic hindpaw oedema evoked by s.c. injection of 5% wv−1 dextran into the chronically denervated hindpaw by 48.0±4.6% (n=5). 2 Capsaicin injection (100 μg ml−1 in 50 μl, s.c.) into the acutely denervated hindleg caused 56.5±4.0% (n=5) inhibition in the intensity of plasma extravasation elicited by 1% vv−1 mustard oil smearing on the contralateral side. After chronic denervation, subplantar injection of 5% wv−1 dextran elicited a non‐neurogenic inflammatory response with intensive tissue oedema without causing any systemic anti‐inflammatory effect. Bilateral adrenalectomy did not inhibit the mustard oil‐induced anti‐inflammatory effect in the contralateral hindleg. 3 Pretreating the rats with polyclonal somatostatin antiserum (0.5 ml rat−1, i.v.) or with the somatostatin depleting agent cysteamine (280 mg kg−1, s.c.) prevented the inhibitory action of mustard oil‐induced inflammation on subsequent neurogenic plasma extravasation and strongly diminished the inhibition of non‐neurogenic oedema formation evoked by dextran. 4 Exogenous somatostatin (10 μg kg−1, i.p.) caused a 30.3±8.3% (n=6) inhibition of plasma extravasation caused by mustard oil smearing on the acutely denervated hindleg and this inhibitory effect was abolished by somatostatin antiserum (0.5 ml rat−1, i.v.). The plasma level of somatostatin‐like immunoreactivity (SST‐LI) increased by 40.03±6.8% (n=6) 10 min after topical application of 1% vv−1 mustard oil on the acutely denervated hindpaws compared to the paraffin oil treated control group. Chronic denervation of the hindlegs or cysteamine (280 mg kg−1, s.c.) pretreatment prevented the mustard oil‐induced elevation of SST‐LI in plasma. 5 It is concluded that chemical excitation of the capsaicin‐sensitive sensory receptors not only induces local neurogenic plasma extravasation but also inhibits the development of a subsequent inflammatory reaction at remote sites of the body in the rat. A role for somatostatin in this systemic anti‐inflammatory effect is suggested.

Inhibition of nociceptin on sensory neuropeptide release and mast cell-mediated plasma extravasation in rats

Nociceptin (20 microg/kg i.p.) strongly inhibited cutaneous Evans blue accumulation in the chronically denervated hindpaw of the rat in response to mast cell degranulating peptide (MCDP, 0.25 microg in 100 microl) but it had no and marginal effect on plasma extravasation induced by 5-hydroxytryptamine (5-HT, 0.5 microg in 100 microl) and histamine (0.1 microg in 100 microl), respectively. Release of sensory neuropeptides such as substance P, calcitonin gene-related peptide (CGRP) and somatostatin from the rat isolated trachea in response to capsaicin (10(-8) M) or bradykinin (10(-7) M) were also attenuated by nociceptin (100 and 300 nM). It is concluded that chemically induced discharge of mediators from mast cells and from capsaicin-sensitive afferent nerve terminals are both inhibited by nociceptin that participates in the anti-inflammatory effect of the peptide.

Anti-nociceptive effect induced by somatostatin released from sensory nerve terminals and by synthetic somatostatin analogues in the rat

In rats anaesthetized with urethan and pretreated with pipecuronium bromide nocifensive reaction of blood pressure elevation evoked by intraarterial capsaicin injection was inhibited over 40 min by bilateral antidromic stimulation of the sensory fibres of the sciatic nerves. Rise in blood pressure, heart rate and respiratory frequency evoked by capsaicin were markedly diminished after smearing 1% mustard oil on the acutely denervated hindpaws indicating a release of mediators with anti-nociceptive action from cutaneous nociceptors. Intravenous injection of the putative mediator somatostatin (10 microg/kg) or its analogues RC-160 and TT-232, but not octreotide inhibited the cardiorespiratory and blood pressure responses evoked by topical cutaneous application of mustard oil or capsaicin instillation into the eye. It is concluded, that the endocrine and the anti-nociceptive effects of somatostatin are mediated through distinct receptor subtypes and therefore, TT-232, a novel heptapeptide analogue without endocrine action, is a promising analgesic compound.

Systemic anti-inflammatory effect of somatostatin released from capsaicin-sensitive vagal and sciatic sensory fibres of the rat and guinea-pig

The systemic anti-inflammatory effect induced by antidromic sensory nerve stimulation was investigated in rats and guinea-pigs. In atropine-pretreated rats, bilateral antidromic stimulation of vagal afferent fibres (8 Hz, 20 min, at C-fibre strength) inhibited plasma extravasation induced by 1% mustard oil on the acutely denervated hindlegs by 36.45+/-3.95%. Both the prevention of this inhibitory effect by cysteamine pretreatment and the stimulation-evoked rise of plasma somatostatin-like immunoreactivity in the two species suggest a mediator role of neural somatostatin. Since this response was blocked by systemic capsaicin pretreatment and slightly reduced after subdiaphragmal vagotomy, participation of thoracic capsaicin-sensitive afferents is indicated. In guinea-pigs pretreated with guanethidine and pipecuronium, antidromic sciatic nerve stimulation induced 45.46+/-5.08% inhibition on the contralateral leg and increased plasma somatostatin-like immunoreactivity. It is concluded that somatostatin released from the activated vagal capsaicin-sensitive sensory nerve terminals of the rat and somatic nerves of the guinea-pigs exerts a systemic humoral function.