The objective of the present study was to investigate whether oxytocinergic mechanisms may contribute to the antinociceptive effect of non-noxious, sensory stimulation. To test this hypothesis, oxytocin levels in plasma and cerebrospinal fluid (CSF) were measured in control rats as well as in rats exposed for 30 min to electro-acupuncture (2 Hz), thermal stimulation (40 degrees C) or vibration (100 Hz). All modes of stimulation induced significant elevations of oxytocin levels in plasma and/or in CSF, 30 or 90 min after the end of stimulation. Secondly, the antinociceptive effects of these treatments were investigated in the tail-flick test with and without prior administration of the oxytocin antagonist 1-deamino-2-D-Tyr-(OEt)-4-Thr-8-Orn-oxytocin (1 mg kg-1 i.p.). All three modes of stimulation caused a significant delay of the tail-flick latency to the same degree as that caused by injection of oxytocin 1 mg kg-1 i.p. (electro-acupuncture P < 0.01, thermal stimulation and vibration P < 0.05). In all cases, the delay was reversed by administration of the oxytocin antagonist (1 mg kg-1 i.p.). These findings suggest that analgesic effects induced by non-noxious sensory stimulation may, in part, be mediated through activation of oxytocinergic mechanisms.

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The aim of the present study was to investigate whether gastrin, cholecystokinin (CCK) and somatostatin secretion can be influenced by sensory stimulation and if so, whether such effects are mediated via the vagal nerves. Male rats anaesthetized with chloral hydrate were exposed to three different stimuli, i.e. to low frequency (2 Hz) electrical stimulation of muscles via needles (electro-acupuncture), to thermal stimulation at 40 degrees C or to vibration at 100 Hz. The two former stimuli activate mainly small and medium sized myelinated fibres from muscles and skin respectively, whereas vibration activates large myelinated fibres from skin, subcutaneous tissue and muscles. Experiments were also performed on animals that were vagotomized or exposed to prior treatment with atropine (0.5 mg kg-1). Blood was collected at various time intervals and plasma levels of gastrin, CCK and somatostatin were measured with radioimmunoassay (RIA). All three stimuli, i.e. electro-acupuncture, vibration and thermal stimulation caused significant elevations of gastrin (103 +/- 11-151 +/- 16 pM, 105 +/- 8-140 +/- 12 pM and 105 +/- 14-162 +/- 4 pM) and cholecystokinin (9 +/- 0.8-15 +/- 2.8 pM, 8 +/- 0.5-10 +/- 1.5 pM and 8.0 +/- 0.5-10.5 +/- 1.5). Somatostatin was raised in response to electro-acupuncture (10 +/- 1-14 +/- 3 pM). Vagotomy and atropinization abolished the release of gastrin and CCK in response to all three stimuli. CCK levels were significantly reduced following electro-acupuncture in atropinized rats. In conclusion, gastrin and cholecystokinin release is stimulated by activation of sensory afferent, originating in skin, subcutaneous tissue as well as in muscle.(ABSTRACT TRUNCATED AT 250 WORDS)

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