Effect of ferric citrate on serum phosphate and fibroblast growth factor 23 among patients with nondialysis-dependent chronic kidney disease: path analyses.

BACKGROUND: Among patients with nondialysis-dependent chronic kidney disease (NDD-CKD) and iron-deficiency anemia (IDA), ferric citrate increases hemoglobin and iron parameters and reduces serum phosphate and fibroblast growth factor 23 (FGF23), a key phosphate-regulating hormone. We conducted post hoc analyses of a phase 3 trial to explore associations between iron replacement, serum phosphate changes and FGF23 regulation. METHODS: We employed multivariable regression and longitudinal mixed-effects models to identify and confirm, respectively, whether baseline demographic and laboratory variables were associated with ferric citrate-induced changes in serum phosphate or FGF23 concentrations. We employed path analyses to determine whether changes in FGF23 concentrations were mediated via changes in serum phosphate and/or transferrin saturation (TSAT). RESULTS: We analyzed a total of 117 and 115 ferric citrate-treated and placebo-treated patients, respectively. At 16 weeks, ferric citrate significantly reduced serum phosphate versus placebo (P = 0.006) only among patients with elevated baseline serum phosphate (≥4.5 mg/dL) and did not reduce serum phosphate among patients with baseline serum phosphate within the population reference range. Ferric citrate reduced intact FGF23 and C-terminal FGF23 partially via changes in TSAT (for C-terminal FGF23) and serum phosphate (for intact FGF23) and partially via unknown/unmeasured mechanisms. CONCLUSIONS: Ferric citrate reduced serum FGF23 concentrations (partially via effects on serum phosphate and iron balance) and did not reduce serum phosphate among patients with baseline serum phosphate concentrations within the population reference range.

GABA(B) receptor-mediated modulation of hypocretin/orexin neurones in mouse hypothalamus.

Hypocretin/orexin (Hcrt) is a critical neurotransmitter for the maintenance of wakefulness and has been implicated in several other functions, including energy metabolism and reward. Using whole-cell patch-clamp recordings from transgenic mice in which enhanced green fluorescent protein was linked to the Hcrt promoter, we investigated GABAergic control of the Hcrt neurones in hypothalamic slices. Bath application of GABA or muscimol caused an early hyperpolarization mediated by Cl(-) and a late depolarization mediated by the efflux of bicarbonate. These GABA(A) receptor-mediated responses were blocked by picrotoxin and bicuculline. Under the GABA(A) blockade condition, GABA produced consistent hyperpolarization, decreased firing rate and input resistance. The selective GABA(B) agonist (R)-baclofen caused a similar response with an EC(50) of 7.1 mum. The effects of (R)-baclofen were blocked by the GABA(B) antagonist CGP 52432 but persisted in the presence of tetrodotoxin, suggesting direct postsynaptic effects. The existence of GABA(B) modulation was supported by GABA(B(1)) subunit immunoreactivity on Hcrt cells colabelled with antisera to the Hcrt-2 peptide. Furthermore, GABA(B) receptor activation inhibited the presynaptic release of both glutamate and GABA. (R)-Baclofen depressed the amplitude of evoked excitatory postsynaptic currents (EPSCs) and inhibitory synaptic currents (IPSCs), and also decreased the frequency of both spontaneous and miniature EPSCs and IPSCs with a modest effect on their amplitudes. These data suggest that GABA(B) receptors modulate Hcrt neuronal activity via both pre- and postsynaptic mechanisms, which may underlie the promotion of non-rapid eye movement sleep and have implications for the use of GABA(B) agonists in the treatment of substance addiction through direct interaction with the Hcrt system.

Immunoreactive TRPV-2 (VRL-1), a capsaicin receptor homolog, in the spinal cord of the rat.

The vanilloid receptor-like 1 protein (VRL-1, also called TRPV2) is a member of the TRPV family of proteins and is a homolog of the capsaicin/vanilloid receptor (VR1, or TRPV1). Although VRL-1 does not bind capsaicin, like VR1 it is activated by noxious heat (>52 degrees C). Unlike VR1, however, VRL-1 is primarily expressed by medium- and large-diameter primary afferents, which suggests that nociceptive processing is but one of the functions to which VRL-1 contributes. To provide information on the diverse spinal circuits that are engaged by these VRL-1-expressing primary afferents, we completed a detailed immunocytochemical map of VRL-1 in rat spinal cord, including light and electron microscopic analysis, and generated a more comprehensive neurochemical characterization of VRL-1-expressing primary afferents. Consistent with previous reports, we found that VRL-1 and VR1 are expressed in different dorsal root ganglion (DRG) cell bodies. Almost all VRL-1-expressing cells labeled for N52 (a marker of myelinated afferents), consistent with VRL-1 expression in Adelta and Abeta fibers. EM analysis of the DRG and dorsal roots confirmed this and revealed two categories of neurons based on the intensity of immunolabeling. The densest VRL-1 immunoreactivity in the spinal cord was found in lamina I, inner lamina II, and laminae III/IV. This is consistent with the expression of VRL-1 by myelinated nociceptors that target laminae I and IIi and in nonnociceptive Abeta fibers that target laminae III/IV. Dorsal rhizotomy reduced, but did not eliminate, the immunostaining in all dorsal horn laminae, which indicates that VRL-1 expression derives from both DRG cells and from neurons intrinsic to the brain or spinal cord. Spinal hemisection reduced immunostaining of the ipsilateral dorsal columns in segments rostral to the lesion and in the dorsal column nuclei, presumably from the loss of ascending Abeta afferents, but there was no change caudal to the lesion. Thus, supraspinal sources of dorsal horn VRL-1 immunoreactivity are likely not significant. Although we never observed VRL-1 immunostaining in cell bodies in the superficial dorsal horn, there was extensive labeling of motoneurons and ventral root efferents-in particular, in an extremely densely labeled population at the lumbosacral junction. Finally, many ependymal cells surrounding the central canal were intensely labeled. These results emphasize that VRL-1, in contrast to VR1, is present in a diverse population of neurons and undoubtedly contributes to numerous functions in addition to nociceptive processing.