In vitro effects of polychlorinated biphenyls and hydroxy metabolites on nitric oxide synthases in rat brain

Nitric oxide synthases (NOS) play a key role in motor activity in the cerebellum, hormonal regulation in the hypothalamus, and long-term potentiation (LTP), learning, and memory processes in the hippocampus. Developmental exposure to polychlorinated biphenyls (PCBs) has been shown to affect psychomotor functions, learning and memory processes, and to inhibit LTP. We hypothesized that PCBs may disrupt the regulation of such neurological functions by altering NOSs. We have studied the in vitro effects of several PCB congeners and some hydroxy PCBs on NOS activity in cytosolic (presumably neuronal NOS [nNOS]) and membrane (presumably endothelial NOS [eNOS]) fractions in different brain regions of young and adult rats. Among the two selected dichloro PCBs, the ortho-PCB, 2,2'-dichlorobiphenyl (DCB), inhibited both cytosolic and membrane NOS activity at low micromolar concentrations (3-10 microM) in the selected brain regions of all age groups while the non-ortho-PCB, 4,4'-DCB, did not. 2,2'-DCB inhibited cytosolic NOS to a greater extent than membrane NOS. Pentachloro-PCBs (PeCBs) and hexachloro-PCBs (HCBs) did not have a significant effect on adult cerebellar cytosolic or membrane NOS. However, mono- and dihydroxy derivatives of HCBs significantly decreased cytosolic NOS (IC50s: 16.33 +/- 0.47 and 33.65 +/- 4.33 microM, respectively) but resulted in a marginal effect on membrane NOS in the cerebellum. Among three adult rat brain regions, the hypothalamic cytosolic NOS was the most sensitive to 2,2'-DCB. Also, cytosolic NOS in the cerebellum and hypothalamus of young rats was less sensitive than in the older rats. In summary, these results indicate that only di-ortho-PCB inhibited both NOS and hydroxy substitution of one or more chlorine molecules significantly increased the potency of both ortho- and non-ortho-HCBs. The selective sensitivity of NOS to dichloro- ortho-PCB and hydroxy metabolites suggests that the inhibition of NOS could play a role in the neuroendocrine effects as well as learning and memory deficits caused by exposure to PCBs.