Neuroendocrine Regulation of Frog Adrenocortical Cells by Neurotensin

Developmental dynamics of neurotensin binding sites in the human hypothalamus during the first postnatal year

The aim of the present study was to determine a detailed mapping of neurotensin (NT) in the human hypothalamus, during the first postnatal year using an in vitro quantitative autoradiography technique and the selective radioligand monoiodo-Tyr3-NT. Ten human postmortem hypothalami obtained from control neonates and infants (aged from 2 h to 1 year of postnatal age) were used. The biochemical kinetics of the binding in all obtained in this study revealed that the binding affinity constants were of high affinity (in the nanomolar range) and did not differ significantly between all cases investigated. Furthermore, competition experiments show insensitivity to levocabastine and were in favor of the presence of the high affinity site of NT receptor. Autoradiographic distribution showed that NT binding sites were widely distributed throughout the rostrocaudal extent of the hypothalamus. However, the distribution of NT binding sites was not homogenous and regional variations exist. In general, the highest densities were mainly present in the anterior hypothalamic level, particularly in the preoptic area. High NT binding site densities are also present at the mediobasal hypothalamic level, particularly in the paraventricular, parafornical, and dorsomedial nuclei. At the posterior level, low to very low densities could be observed in all the mammillary complex subdivisions, as well as the posterior hypothalamic area. Although this topographical distribution is almost identical during the postnatal period analyzed, age-related variations exist in discrete structures of the hypothalamus. The densities were higher in neonates/less aged infants than older infants in preoptic area (medial and lateral parts). The developmental profile is characterized by a progressive decrease from the neonate period to 1 year of postnatal age with a tendency to reach adult levels. On the other hand, the low levels of NT binding sites observed in posterior hypothalamus did not vary during the first postnatal year. They contrast in that with the very high levels we reported previously in adult. In conclusion, the present study demonstrates the occurrence of high NT binding sites density in various structures in many regions in the human neonate/infant hypothalamus, involved in the control of neuroendocrine and/or neurovegetative functions.

An autoradiographic study of neurotensin receptors in the human hypothalamus

The aim of the present investigation was to determine a detailed mapping of neurotensin (NT) in the human hypothalamus, the brain region involved in neuroendocrine control. For this, we investigated the presence and the distribution of neurotensin binding sites in the human hypothalamus, using an in vitro quantitative autoradiography technique and the selective radioligand monoiodo-Tyr3-neurotensin (2000Ci/mM). This study was performed on nine adult human postmortem hypothalami. We first determined the biochemical kinetics of the binding and found that binding affinity constants were of high affinity and do not differ significantly between all cases investigated. Our analysis of the autoradiographic distribution shows that NT binding sites are widely distributed throughout the rostrocaudal extent of the hypothalamus. However, the distribution of NT binding sites is not homogenous and regional variations exist. In general, the highest densities are mainly present in the anterior hypothalamic level, particularly in the preoptic region and the anterior boarding limit (i.e. the diagonal band of Broca). Important NT binding site densities are also present at the mediobasal hypothalamic level, particularly in the paraventricular, parafornical and dorsomedial nuclei. At the posterior level, relatively moderate densities could be observed in the mammillary complex subdivisions, apart from the supramammillary nucleus and the posterior hypothalamic area. In conclusion, the present study demonstrates the occurrence of high concentrations of NT binding sites in various structures in many regions in the human adult hypothalamus, involved in the control of neuroendocrine and/or neurovegetative functions.

Involvement of NTS2 receptors in stress-induced analgesia

Stress activates multiple neural systems that suppress pain sensation. This adaptive phenomenon referred as stress-induced analgesia (SIA) is mediated by the activation of endogenous pain inhibitory systems. Both opioid and non-opioid forms of SIA have been elicited in rodents according to stressor parameters and duration. There is accumulating evidence that the endogenous neurotensin (NT) system plays an important role in SIA. Especially, NT-deficient mice were shown to exhibit reduced SIA following water avoidance or restraint stress. Since central NT produces naloxone-insensitive analgesic effects by acting on spinal and supraspinal NTS2 receptors, we hypothesized that NT might mediate non-opioid SIA through NTS2 activation. Here, we evaluated the influence of an opioid-independent severe stress produced by a cold-water swim for 3 min at 15 degrees C on rodent offspring's pain perception. Our results demonstrated that mice lacking NTS2 exhibit significantly reduced SIA following cold-water swim stress. Indeed, NTS2 knockout mice submitted to both acute (plantar test) and tonic (formalin test) pain stimuli show a greater sensitivity to pain in comparison to wild-type littermates. Accordingly, pretreatment with the NT receptor antagonist SR142948A results in a hyperalgesic response to stress induced by cold-water swim. Endogenous NT regulates hypothalamic-pituitary-adrenal axis activity in stress condition by increasing corticosterone plasma levels. Accordingly, the plasma levels of corticosterone measured by radioimmunoassay are significantly reduced in non-stressed and stressed NTS2-deficient mice in comparison with wild-type mice. To further investigate the site of action of NT in mediating SIA, we microinjected NTS2 agonists in lumbar spinal cord and quantified post-stress sensitivity to pain in rats using the plantar test. Exogenously administered NTS2 analogs, JMV-431, beta-lactotensin and NT69L markedly enhance the magnitude and duration of stress antinociception in both 25- and 60-day-old rats. In sum, by using genetic and pharmacological approaches, we demonstrated here that NTS2 receptors mediate non-opioid SIA. Our results also revealed that the release of endogenous NT in response to stress requires the presence of NTS2 to stimulate corticotropin-releasing factor (CRF)-induced elevation of plasma corticosterone, and that NTS2 receptors localized at the lumbar spinal cord participate to the disinhibition of descending pain control pathways. Therefore, these data highlight the significance of NTS2 as a novel target for the treatment of pain and stress-related disorders.

Molecular evolution of neuropeptide receptors with regard to maintaining high affinity to their authentic ligands

Recently, we cloned many of the bullfrog neuropeptide G protein-coupled receptors (GPCRs), including receptors for vasotocin (VT), mesotocin, gonadotropin-releasing hormone (GnRH), neurotensin, apelin, and metastin. Bullfrog GPCRs usually have high affinity for bullfrog ligands but relatively low affinity for mammalian ligands. Reciprocally, synthetic agonists and antagonists developed based upon mammalian ligands display lower affinity at bullfrog receptors. Studies using chimeric or domain-swapped receptors indicate that the motifs responsible for differential ligand selectivity usually reside within transmembrane domain 6 (TMD6)-extracellular loop 3 (ECL3)-transmembrane domain 7 (TMD7). Triple mutation of mammalian V1aR (Phe(6.51) to Tyr, Ile(6.53) to Thr, and Pro(7.33) to Thr) increases VT affinity but greatly reduces arginine vasopressin affinity. This binding profile is similar to that of bullfrog VT1R. Changing just three amino acids in the bullfrog GnRH receptor-1 (i.e. Ser-Gln-Ser in the ECL3) to those found in the type-I mammalian GnRH receptor (i.e. Ser-Glu-Pro) reverses GnRH selectivity. In conclusion, specific receptor motifs that govern ligand selectivity can be determined by comparative molecular analyses of GPCRs and their ligands. Such analysis provides clues for understanding how GPCRs maintain high affinity to their authentic ligands.