Analgesic Effect of Oxytocin Receptor Modulation

Background

Chronic pain is still a largely unresolved issue, causing suffering, disability and high social costs. The search for novel pharmacological targets is therefore a priority. A recent study on postpartal bleeding came to the accidental finding of a possible analgesic action of the oxytocin agonists carbetocin.

Oxytocin is a well known nonapeptide synthesized in the hypothalamus, acting as neurohormone during parturition and the milk ejection reflex. Animal studies have found that descending pathways for oxytocin synthetizing neurons project to the lamina I-II of the spinal cord, where they activate a subpopulation of glutamatergic and GABAergic interneurons. In addition to GABAergic hyperpolarisation, models of oxytocin selectively blocking A-delta and C-fibers have been published. Intrathecal administration of oxytocin prevents long-term potentiation in the dorsal horn, which is thought to be an important mechanism of enhanced central pain processing.

Antagonism to GABAergic and glycinergic neurotransmission mimics many symptoms of inflammatory and neuropathic pain. A loss of synaptic inhibition in the dorsal horn occurs in animal models of experimental pain.

Inhibitory synaptic transmission in the spinal cord dorsal horn use GABA and glycine as their principal fast neurotransmitters. Both of them open the Cl- -channels, which induce postsynaptic hyperpolarisation and impairs the propagation of excitatory potentials on dendrites of neurons. Immunofluorescence studies have revealed abundant glycinergic innervations in the dorsal horn. According to this model, inhibitory GABAergic and glycinergic interneurons in the superficial spinal dorsal horn are key components in the control of pain transmission from the periphery to the brain. The model states that a non-painful stimulation is felt as non painful as long as the synaptic GABAergic and glycinergic inhibition remains intact. A human study on GABAergic modulation of pain by benzodiazepines has been recently performed by our group and was suggestive for an analgesic action. However, these drugs cause sedation and addiction, which strongly limit their clinical usefulness. A pharmacological GABA modulation via the oxytocin receptor may be an attractive alternative, since oxytocin agonists are devoid of these side effects.

Quantitative sensory tests (QST) are used to explore the central processing of painful stimuli in healthy volunteers and patients. They are based on a multimodal and multi-tissue approach, combining different pain modalities applied to different tissues in order to gather sufficient and differentiated information about the human nociceptive system under normal and pathological conditions. QST will be our tool to characterize analgesic efficacy of carbetocin.

Objective

We will test the hypothesis that carbetocin produces analgesia in healthy volunteers, as assessed by multimodal experimental pain testing.

Methods

Intradermal capsaicin injection in the volar forearm is used to create experimental pain an hyperalgesia. The area of hyperalgesia to pinprick and brush allodynia is quantified, and pressure, heat, cold and electrical pain thresholds as well as nociceptive withdrawal reflex thresholds are assessed 30 minutes after capsaicin injection (baseline assessments). Carbetocin 0.1 mg is injected intravenously and the above measurements repeated after 10, 60 and 120 minutes. Blood samples are taken in order to investigate plasma carbetocin levels at 10, 60 and 120 minutes and genetic variants of the oxytocin receptor gene.