Effects of intravenous infusion of substance P on arginine vasopressin and oxytocin secretion in normal men

Substance P receptor antagonists in psychiatry: rationale for development and therapeutic potential

Increasing evidence suggests that substance P (SP) and its receptor (neurokinin [NK]-1 receptor [NK1R]) might play an important role in the modulation of stress-related, affective and/or anxious behaviour. First, SP and NK1R are expressed in brain regions that are involved in stress, fear and affective response (e.g. amygdala, hippocampus, hypothalamus and frontal cortex). Second, the SP content in these areas changes upon application of stressful stimuli. Third, the central administration of SP produces a range of fear-related behaviours. In addition, the SP/NK1R system shows significant spatial overlap with neurotransmitters such as serotonin and noradrenaline (norepinephrine), which are known to be involved in the regulation of stress, mood and anxiety. Therefore, it was hypothesised that blockade of the NK1R might have anxiolytic as well as antidepressant effects. Preclinical studies investigating the effects of genetic or pharmacological NK1R inactivation on animal behaviour in assays relevant to depression and anxiety revealed that the behavioural changes resemble those seen with reference antidepressant or anxiolytic drugs. Furthermore, antagonism or genetic inactivation of the NK1R causes alterations in serotonin and norepinephrine neuronal transmission that are likely to contribute to the antidepressant/anxiolytic activity of NK1R antagonists but that are--at least partially--distinct from those produced by established antidepressant drugs. This underlines the conceivable unique mechanism of action of this new class of compounds. In three independent clinical trials with three different compounds (aprepitant [MK-869], L-759274 and CP-122721), an antidepressant effect of NK1R antagonists could be demonstrated. These results, however, have been challenged by recent failed studies with aprepitant. There are numerous indications from preclinical studies that, in addition to SP and NK1R, other neurokinins and/or neurokinin receptors might also be involved in the modulation of stress-related behaviour and that exclusive blockade of the NK1R might not be sufficient to produce consistent anxiolytic and antidepressant effects. One such candidate is the neurokinin-2 receptor (NK2R), and clinical trials to assess the antidepressant effects of NK2R antagonists are currently underway. Of special interest might also be substances that block more than one receptor type such as NK1/2R antagonists or NK1/2/3R antagonists. These compounds may be more efficacious in antagonising the effects of SP than compounds that only block the NK1R.

Melatonin inhibits the substance P-induced secretion of vasopressin and oxytocin from the rat hypothalamo-neurohypophysial system: in vitro studies

The aim of the present investigations was to study the influence of substance P (a member of a family of peptides known as tachykinins) on basal and K(+)-evoked vasopressin (AVP) and oxytocin (OT) release from rat hypothalamo-neurohypophysial system in vitro as well as to determine whether this effect of substance P is sensitive to melatonin. The present results show that substance P stimulates basal AVP and OT release from isolated hypothalamo-neurohypophysial system, when used at the concentrations of 10(-6) and 10(-7)M/l. At the concentration of 10(-9)M/l, however, substance P was found to stimulate the in vitro secretion of AVP, but not that of OT. Melatonin diminished basal release of AVP; it also significantly inhibited the substance P-stimulated secretion of AVP and OT. K(+)-evoked release of the neurohypophysial hormones was not further modified by either substance P or melatonin. The present results show that the stimulatory effect of substance P on basal release of AVP and OT from rat hypothalamo-neurohypophysial system in vitro is sensitive to inhibitory influence of melatonin.

Perturbations of arginine vasopressin secretion during inflammatory stress. Pathophysiologic implications

Pro-inflammatory cytokines, such as interleukin-1 beta (IL-1 beta), interleukin-6 (IL-6), and tumor necrosis factor-alpha (TNF alpha), released from inflammatory foci, can activate the hypothalamus to produce corticotrophin-releasing hormone (CRH) and arginine vasopressin (AVP). These hypothalamic peptides in synergy increase ACTH production by the pituitary gland and hence corticosteroid (CS) secretion by the adrenal cortices. CS dampens inflammation. The pituitary also produces prolactin (PRL), which is pro-inflammatory, and macrophage inhibitory factor (MIF), which by counteracting the anti-inflammatory and immunosuppressive effects of CS, is pro-inflammatory. Lewis rats develop a variety of induced-autoimmune inflammatory conditions, such as streptococcal cell wall arthritis, whereas the histocompatible F344 Fisher rats are resistant to this condition. Lewis rats have a defective hypothalamic-pituitary adrenal (HPA) response to a variety of hypothalamic stimuli, but have augmented systemic secretion of AVP. Patients with rheumatoid arthritis (RA) have deficient CS with exaggerated PRL responses to inflammatory stimuli. Within inflammatory foci, CRH is pro-inflammatory. AVP, which augments autologous mixed lymphocyte reactions, can replace the IL-2 requirement for gamma IFN production by T cells via V1a receptors, and potentiates primary antibody responses, is also pro-inflammatory. Lewis rats have significantly high plasma levels, hypothalamic content, and in vitro release of AVP in comparison to the inflammatory disease-resistant Fischer rats. Immunoneutralization of AVP attenuates inflammatory responses. In Sprague-Dawley rats, AVP potentiates PRL secretion. Preliminary studies in patients with RA have shown that the circulating levels of AVP are significantly increased, which might be a compensatory response to low CS levels or a result of elevated levels of IL-6 in these patients but could nevertheless contribute to rheumatoid inflammation. A similar observation has been made in patients with ankylosing spondylitis.

Effect of substance P on basal and thyrotropin-releasing hormone-stimulated thyrotropin release in humans

To test the possible effects of intravenous administration of substance P (SP) on basal and thyrotropin-releasing hormone (TRH)-stimulated thyrotropin (TSH) release, SP was infused alone (0.5 or 1.5 pmol/kg-1/min-1 for 60 minutes) or after TRH (20 or 400 micrograms in an intravenous bolus) in 21 normal male subjects (aged 26 to 36 years) and in 18 normal women (aged 25 to 32 years). Women were studied during follicular (day 6 to 8) and luteal (day 21 to 23) phases of following regular menstrual cycles. In addition, plasma cortisol levels during SP infusion were measured. In agreement with previous findings, significant increments in plasma cortisol levels were observed in men and women when the higher (1.5 pmol/kg-1/min-1) but not the lower (0.5 pmol/kg-1/min-1) amount of SP was administered. In contrast, in both men and women basal and TRH (20 or 400 mg)-induced TSH releases were not modified by SP at any tested amount. Results in the follicular and luteal phase were similar. These data suggest that in normal men and women plasma SP is not involved in the control of TSH release, at least not outside the blood-brain barrier.