Opposite alterations of NPFF1 and NPFF2 neuropeptide FF receptor density in the triple MOR/DOR/KOR-opioid receptor knockout mouse brains

Anxiogenic and Stressor Effects of the Hypothalamic Neuropeptide RFRP-3 Are Overcome by the NPFFR Antagonist GJ14

RFamide-related peptide-3 (RFRP-3) is a recently discovered neuropeptide that has been proposed to play a role in the stress response. We aimed to elucidate the role of RFRP-3 and its receptor, neuropeptide FF (NPFF1R), in modulation of stress and anxiety responses. To achieve this, we characterized a new NPFF1R antagonist because our results showed that the only commercially available putative antagonist, RF9, is in fact an agonist at both NPFF1R and the kisspeptin receptor (KISS1R). We report here the identification and pharmacological characterization of GJ14, a true NPFFR antagonist. In in vivo tests of hypothalamic-pituitary-adrenal (HPA) axis function, GJ14 completely blocked RFRP-3-induced corticosterone release and neuronal activation in CRH neurons. Furthermore, chronic infusion of GJ14 led to anxiolytic-like behavior, whereas RFRP-3 infusion had anxiogenic effects. Mice receiving chronic RFRP-3 infusion also had higher basal circulating corticosterone levels. These results indicate a stimulatory action of RFRP-3 on the HPA axis, consistent with the dense expression of NPFF1R in the vicinity of CRH neurons. Importantly, coinfusion of RFRP-3 and GJ14 completely reversed the anxiogenic and HPA axis-stimulatory effects of RFRP-3. Here we have established the role of RFRP-3 as a regulator of stress and anxiety. We also show that GJ14 can reverse the effects of RFRP-3 both in vitro and in vivo. Infusion of GJ14 causes anxiolysis, revealing a novel potential target for treating anxiety disorders.

Involvement of Mammalian RF-Amide Peptides and Their Receptors in the Modulation of Nociception in Rodents

Mammalian RF-amide peptides, which all share a conserved carboxyl-terminal Arg-Phe-NH2 sequence, constitute a family of five groups of neuropeptides that are encoded by five different genes. They act through five G-protein-coupled receptors and each group of peptide binds to and activates mostly one receptor: RF-amide related peptide group binds to NPFFR1, neuropeptide FF group to NPFFR2, pyroglutamylated RF-amide peptide group to QRFPR, prolactin-releasing peptide group to prolactin-releasing peptide receptor, and kisspeptin group to Kiss1R. These peptides and their receptors have been involved in the modulation of several functions including reproduction, feeding, and cardiovascular regulation. Data from the literature now provide emerging evidence that all RF-amide peptides and their receptors are also involved in the modulation of nociception. This review will present the current knowledge on the involvement in rodents of the different mammalian RF-amide peptides and their receptors in the modulation of nociception in basal and chronic pain conditions as well as their modulatory effects on the analgesic effects of opiates.

Development, sex steroid regulation, and phenotypic characterization of RFamide-related peptide (Rfrp) gene expression and RFamide receptors in the mouse hypothalamus

Arginine-phenylalanine-amide (RFamide)-related peptide 3 (RFRP-3, encoded by the Rfrp gene) is the mammalian ortholog of gonadotropin-inhibiting hormone and can inhibit GnRH neuronal activity and LH release. However, the development and regulation of the RFRP-3 system in both sexes is poorly understood. Using in situ hybridization, we examined changes in Rfrp-expressing neurons in mice of both sexes during development and under different adulthood hormonal milieus. We found no sex differences in Rfrp expression or cell number in adult mice. Interestingly, we identified two interspersed subpopulations of Rfrp cells (high Rfrp-expressing, HE; low Rfrp-expressing, LE), which have unique developmental and steroidal regulation characteristics. The number of LE cells robustly decreases during postnatal development, whereas HE cell number increases significantly before puberty. Using Bax knockout mice, we determined that the dramatic developmental decrease in LE Rfrp cells is not due primarily to BAX-dependent apoptosis. In adults, we found that estradiol and testosterone moderately repress Rfrp expression in both HE and LE cells, whereas the nonaromatizable androgen dihydrotestosterone has no effect. Using double-label in situ hybridization, we determined that approximately 25% of Rfrp neurons coexpress estrogen receptor-α in each sex, whereas Rfrp cells do not readily express androgen receptor in either sex, regardless of hormonal milieu. Lastly, when we looked at RFRP-3 receptors, we detected some coexpression of Gpr147 but no coexpression of Gpr74 in GnRH neurons of both intact and gonadectomized males and females. Thus, RFRP-3 may exert its effects on reproduction either directly, via Gpr147 in a subset of GnRH neurons, and/or indirectly, via upstream regulators of GnRH.

FMRFamide-related peptides: anti-opiate transmitters acting in apoptosis

Members of the FMRFamide-related peptide (FaRP) family are neurotransmitters, hormone-like substances and tumor suppressor peptides. In mammals, FaRPs are considered as anti-opiate peptides due to their ability to inhibit opioid signaling. Some FaRPs are asserted to attenuate opiate tolerance. A recently developed chimeric FaRP (Met-enkephalin-FMRFa) mimics the analgesic effects of opiates without the development of opiate-dependence, displaying a future therapeutical potential in pain reduction. In this review we support the notion, that opiates and representative members of the FaRP family show overlapping effects on apoptosis. Binding of FaRPs to opioid receptors or to their own receptors (G-protein linked membrane receptors and acid-sensing ion channels) evokes or suppresses cell death, in a cell- and receptor-type manner. With the dramatically increasing incidence of opiate abuse and addiction, understanding of opioid-induced cell death, and in this context FaRPs will deserve growing attention.

Facilitation of spinal morphine analgesia in normal and morphine tolerant animals by neuropeptide SF and related peptides

Neuropeptide FF and related synthetic amidated peptides have been shown to elicit sustained anti-nociceptive responses and potently augment spinal anti-nociceptive actions of spinal morphine in tests of thermal and mechanical nociception. Recent studies have described the occurrence of another octapeptide, neuropeptide SF (NPSF) in the spinal cord and the cerebrospinal fluid and demonstrated its affinity for the NPFF receptors. This study examined the effects of NPSF and two putative precursor peptides, EFW-NPSF and NPAF, on the spinal actions of morphine in normal and opioid tolerant rats using the tailflick and pawpressure tests. In normal rats, NPSF demonstrated weak intrinsic activity but sub-effective doses of the peptide significantly increased the magnitude and duration of spinal morphine anti-nociception in both tests. A low-dose of NPSF also augmented the spinal actions of a delta receptor agonist, deltorphin. The morphine-potentiating effect of NPSF was shared by EFW-NPSF and the octadecapeptide NPAF. In animal rendered tolerant by continuous intrathecal infusion of morphine for 6 days, low dose NPSF itself elicited a significant anti-nociceptive response and potently increased morphine-induced response in both tests. In animals made tolerant by repeated injections of intrathecal morphine, administration of NPSF, EFW-NPSF, and NPAF with morphine reversed the loss of the anti-nociceptive effect and restored the agonist potency. The results demonstrate that in normal animals NPSF and related peptides exert strong potentiating effect on morphine anti-nociception at the spinal level and in tolerant animals these agents can reverse the loss of morphine potency.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.