Inhibition by lipopolysaccharide of naloxone-induced luteinising hormone secretion is accompanied by increases in corticotropin-releasing factor immunoreactivity in hypothalamic paraventricular neurones in female rats

Acute inflammation reduces kisspeptin immunoreactivity at the arcuate nucleus and decreases responsiveness to kisspeptin independently of its anorectic effects

Severe inflammatory challenges are frequently coupled to decreased food intake and disruption of reproductive function, the latter via deregulation of different signaling pathways that impinge onto GnRH neurons. Recently, the hypothalamic Kiss1 system, a major gatekeeper of GnRH function, was suggested as potential target for transmitting immune-mediated repression of the gonadotropic axis during acute inflammation, and yet key facets of such a phenomenon remain ill defined. Using lipopolysaccharide S (LPS)-treated male rats as model of inflammation, we document herein the pattern of hypothalamic kisspeptin immunoreactivity (IR) and hormonal responses to kisspeptin during the acute inflammatory phase. LPS injections induced a dramatic but transient drop of serum LH and testosterone levels. Suppression of gonadotropic function was associated with a significant decrease in kisspeptin-IR in the arcuate nucleus (ARC) that was not observed under conditions of metabolic stress induced by 48-h fasting. In addition, absolute responses to kisspeptin-10 (Kp-10), in terms of LH and testosterone secretion, were significantly attenuated in LPS-treated males that also displayed a decrease in food intake and body weight. Yet pair-fed males did not show similar alterations in LH and testosterone secretory responses to Kp-10, whose magnitude was preserved, if not augmented, during food restriction. In summary, our data document the impact of acute inflammation on kisspeptin content at the ARC as key center for the neuroendocrine control of reproduction. Our results also suggest that suppressed gonadotropic function following inflammatory challenges might involve a reduction in absolute responsiveness to kisspeptin that is independent of the anorectic effects of inflammation.

Neonatal lipopolysaccharide exposure exacerbates stress-induced suppression of luteinizing hormone pulse frequency in adulthood

Early life exposure to immunological challenge has programming effects on the adult hypothalamo-pituitary-adrenocortical axis stress responsivity, and stress is known to suppress GnRH pulse generator activity, especially LH pulses. We investigated the effects of neonatal exposure to endotoxin on stress-induced suppression of pulsatile LH secretion and the involvement of corticotropin-releasing factor (CRF) receptor mechanisms in adult rats. Pups at 3 and 5 d of age were administered lipopolysaccharide (LPS, 50 microg/kg, ip). At 12 wk of age, they were ovariectomized and implanted with sc 17beta-estradiol capsules and i.v. cannulas. Blood samples (25 microl) were collected every 5 min for 5 h for LH measurement. After 2 h of sampling, rats were given LPS (25 microg/kg, iv). CRF and CRF-R1 and CRF-R2 receptor mRNA was determined by RT-PCR in medial preoptic area (mPOA) micropunches collected at 3 h after LPS administration. There was no difference in basal LH pulse frequency between neonatal LPS- and neonatal saline-treated controls. However, neonatal endotoxin-treated rats exhibited a significantly greater LPS stress-induced suppression of LH pulse frequency. Basal mPOA CRF-R1 expression was unchanged in neonatal LPS- and neonatal saline-treated rats. However, CRF-R1 expression was significantly increased in response to LPS stress in neonatal LPS-treated animals but not in neonatal saline-treated controls. CRF and CRF-R2 expression was unchanged in all treatment groups. These data demonstrate that exposure to bacterial endotoxin in early neonatal life programs long-term sensitization of the GnRH pulse generator to the inhibitory influence of stress in adulthood, an effect that might involve up-regulation of CRF-R1 expression in the mPOA.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 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 (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).

Lipopolysaccharide increases gamma-aminobutyric acid synthesis in medial preoptic neurones in association with inhibition of steroid-induced luteinising hormone surge in female rats

Lipopolysaccharide (LPS), an endotoxin produced by gram-negative bacteria, inhibits gonadotrophin secretion and ovulation in female mammals. The present study was undertaken to examine whether gamma-aminobutyric acid (GABA)-synthesising neurones in the medial preoptic area (mPOA) may be involved in the inhibition by LPS of progesterone-induced luteinising hormone (LH) surge in ovariectomised oestrogen-primed rats. An intravenous injection of LPS (10 microg) at noon significantly increased the content of glutamic acid decarboxylase 67 (GAD67), a GABA-synthesising enzyme, in brain tissues including the mPOA. LPS treatment also significantly increased the number of detectable GAD67-immunoreactive (ir) as well as GABA-ir cells in the mPOA, compared to control rats. The same LPS challenge completely eliminated the steroid-induced LH surge, which was normally induced in control rats in the afternoon. A local injection of muscimol (100 ng), a GABA(A) receptor agonist, into the mPOA also significantly attenuated the LH surge, mimicking the effect of LPS. These results suggest that LPS stimulates GABA synthesis in preoptic neurones, which in turn inhibit the LH surge in female rats. The results also support the hypothesis that diminution of the GABAergic suppressive activity in the mPOA permits the LH surge to be induced.