Pharmacokinetics and immunological effects of exogenously administered recombinant human B lymphocyte stimulator (BLyS) in mice.

B lymphocyte stimulator (BLyS; also known as TNFSF20, BAFF, TALL-1, zTNF4, and THANK), a tumor necrosis factor ligand family member, has recently been identified as a factor that promotes expansion and differentiation of the B cell population, leading to increases in serum immunoglobulin levels. Here, pharmacokinetic parameters for BLyS administered i.v. and s.c. to mice are described, and the effects of different dosing regimens on serum and salivary immunoglobulin levels as well as splenic cell populations are reported. The pharmacokinetics of BLyS following i.v. injection are monophasic with a half-life of 160 min, a clearance of 0.22 ml/min-kg, and a volume of distribution of 53 ml/kg. Systemic administration of BLyS to mice resulted in increased serum IgG, IgA, IgM, and IgE and salivary IgA as well as splenic B cell population expansion and differentiation. The i.v. and s.c. routes of administration were pharmacologically equivalent, even though s.c. bioavailability of BLyS is only 25%. BLyS (s.c.) dramatically elevated serum IgG and IgA levels, and the duration of the responses after cessation of treatment (t(1/2) = 4.4 and 1.3 days, respectively) are similar to the half-lives of endogenous IgG and IgA in mice. The IgM response is more modest than that of IgG and IgA but lasts longer (t(1/2) = 7.0 days) than the half-life of endogenous IgM. A linear pharmacodynamic response was identified between days of dosing x log(dose), and increases in serum IgG, IgA, and IgM indicating that the response is more sensitive to the duration of dosing than to the cumulative dose. The implications of these findings for therapeutic administration of BLyS are discussed.

Biochemical evidence of an interaction of lead at the zinc allosteric sites of the NMDA receptor complex: effects of neuronal development.

Electrophysiological and biochemical studies have shown that Pb2+ inhibits the activation of the N-Methyl-D-Aspartate (NMDA) receptor complex, an excitatory amino acid receptor subtype known to play an important role in neuronal development and cognitive function. In the present study we have provided biochemical evidence that Pb2+ may inhibit NMDA receptor function via an interaction at the Zn2+ allosteric sites. Binding of [3H]-MK-801 to the NMDA receptor ion channel was used as a biochemical indicator of receptor function. The initial experiments indicated that Zn2+ inhibited [3H]-MK-801 binding via high and low affinity sites in PN14 membranes with potencies of 0.77 +/- 0.05 and 57.4 +/- 6.9 microM, respectively. Similar effects were present in neuronal membranes from adult rats, but Zn2+ was significantly less potent (IC50: 153 +/- 21; p < 0.025) in inhibiting [3H]-MK-801 via the low affinity site than in PN14 membranes. The addition of Pb2+ to Zn2+/[3H]-MK-801 displacement curves significantly altered the inhibition of [3H]-MK-801 binding by Zn2+. IN PN14 membranes, Pb2+ increased the potency of Zn2+ inhibition at the high affinity site and decreased the Zn2+ potency at the low affinity site suggestive of a competitive interaction. In adult membranes, Pb2+ did not alter the potency of Zn2+ inhibition of [3H]-MK-801 binding at either site in concentrations up to 40 microM. These findings suggest that the Zn2+ allosteric sites are more sensitive to Pb2+ effects in immature brain tissue.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhanced expression of peripheral benzodiazepine receptors in trimethyltin-exposed rat brain: a biomarker of neurotoxicity.

We have used the binding of the selective, high affinity, isoquinoline carboxamide, [3H]-PK11195, to measure the levels of Peripheral Benzodiazpine Receptors (PBR) in the brain of rats exposed to the well characterized neurotoxicant trimethyltin (TMT). The results demonstrate that autoradiograms of saggital sections of rats injected with a 8 mg/kg TMT dose, express a high level of [3H]-PK11195 binding in brain regions known to be damaged by TMT. The highest level of [3H]-PK11195 binding in the TMT-exposed rats occurred in the CA3/CA4 subfield of the hippocampus, followed by the primary olfactory cortex, the posteriomedial cortical amygdaloid nucleus, subiculum, and entorhinal cortex. These findings are consistent with the neuropathology of TMT in rats. The increase in [3H]-PK11195 binding in the brain of TMT-exposed rats was significant at 7 days after injection and remained elevated up to 42 days after exposure, the last time point measured in the study. This pattern is very similar to that observed for levels of the astrocyte intermediate filament protein, GFAP. The enhanced binding of [3H]-PK11195 in TMT-exposed rats was the result of a significant increase (p < 0.005) in the number of PBR with no change in affinity. The Bmax for [3H]-PK11195 binding in hippocampi from TMT-treated rats at 4 weeks post-injection was 606 +/- 25 (n = 4) fmoles/mg protein and 329 +/- 41 (n = 4) fmoles/mg protein for control. These findings suggest that the quantitative assessment of [3H]-PK11195 binding to PBR in the brain could represent a potential biomarker for assessing chemical-induced neurotoxicity. Since this ligand has been labeled with single photon (123I) or positron emitting (11C, 18F) radioisotopes, it can potentially be used with non-invasive imaging techniques such as Single Photon Emission Tomography (SPECT) or Positron Emission Tomography (PET) for human studies.

Neurochemical aspects of hippocampal and cortical Pb2+ neurotoxicity.

The amino acid glutamate is the neurotransmitter used by most excitatory synapses in the mammalian brain. Glutamatergic synapses in the hippocampus and cerebral cortex play an important role in synaptic plasticity. In the developing brain, the interaction of neurotoxins with presynaptic and/or postsynaptic sites on glutamatergic neurons could alter synaptic plasticity. Recent studies have shown that chronic lead (Pb2+) exposure may impair neuronal process underlying synaptic plasticity via a direct interaction with the N-Methyl-D-Aspartate (NMDA) glutamate receptor subtype. The NMDA receptor-ion channel complex regulates calcium influx and is involved in the initiation of changes in synaptic plasticity. In vitro and in vivo neurochemical studies have found that Pb2+ has a marked inhibitory effect on the activation of the NMDA receptor-ion channel complex. Evidence indicates that the inhibitory effect of Pb2+ on the NMDA receptor complex may be mediated by its interaction with a zinc regulatory site on the receptor complex. The ability of Pb2+ to inhibit NMDA receptor-ion channel function was shown to be age-dependent and brain region-specific. The age-dependent effects of Pb2+ on the NMDA receptor complex may help explain the selective toxicity of this heavy metal in the developing brain.

Chronic prenatal and postnatal Pb2+ exposure increases [3H]MK801 binding sites in adult rat forebrain.

We have measured the binding of [3H]MK801 to the N-methyl-D-aspartate (NMDA) receptor-ion channel in membrane preparations from adult rat forebrain exposed to lead (Pb2+) during gestation, lactation, and postweaning. Our results indicate a 30.9% increase in the number of [3H]MK801 binding sites in Pb2+ exposed animals relative to controls. No significant changes in the affinity constant were observed. The level of blood Pb2+ for which such changes were measured was 13.9 +/- 2.8 micrograms/dl. These results indicate that alterations in the NMDA receptor-ion channel complex are present at blood Pb2+ levels which are environmentally relevant and suggest that chronic Pb2+ exposure during development can influence the NMDA receptor complex in the adult rat brain.

Age-dependent effects of lead on [3H]MK-801 binding to the NMDA receptor-gated ionophore: in vitro and in vivo studies.

Lead (Pb2+) is a more potent inhibitor of [3H]MK-801 binding to rat neuronal membranes than Zn2+ and Mg2+. The inhibitory effects of Pb2+ appeared to be age-dependent since the IC50 was significantly lower in cortical membranes prepared from neonatal than from adult rats. The results also indicate that the hippocampus is more sensitive than the cerebral cortex since the Pb2+ IC50 was significantly lower in the hippocampus. These findings suggest developmental and regional brain heterogeneity to the inhibitory action of lead on [3H]MK-801 binding. Chronic in vivo exposure to Pb2+ resulted in the loss of [3H]MK-801 binding sites in the cerebral cortex of neonatal but not of adult rats. These lead-induced changes may influence developmental processes requiring NMDA receptor activation.

Developmental effects of vitamin B-6 restriction on the locomotor behavior of rats.

The nocturnal spontaneous locomotor behavior of vitamin B-6 restricted and control rats was measured using activity chambers. The results show a generalized hypoactivity in indices of horizontal and vertical activity in neonatal rats fed vitamin B-6 restricted diets during gestation and lactation. This pattern of locomotor behavior changed in the postweaning period to a gradual increase in indices of locomotor activity to a pronounced hyperkinesis in vitamin B-6 restricted young adult rats. This study suggests that long-term vitamin B-6 restriction results in damage and/or abnormal development of neuronal system(s) associated with locomotor behavior.