Signal transduction modulation by lithium: cell culture, cerebral microdialysis and human studies

Considerable evidence suggests that signal transduction pathways are targets of lithium (Li) action. A number of investigators have reported that Li attenuates both adenylate cyclase (AC) activity and phosphoinositide (PI) turnover in rodents and in humans, thus "dampening" these systems. We have studied selected components of these second-messenger systems in a series of clinical and preclinical investigations. To overcome confounding effects of alterations in mood state, we examined AC activity and G-protein ribosylation in peripheral blood cells from 10 healthy volunteers, prior to and following 14 days of Li administration. Basal and postreceptor [cesium fluoride (CsF) or Gpp(NH)p] stimulated AC activity were unaffected in lymphocytes. In contrast, both basal and stimulated AC activity in platelets were significantly augmented, compatible with an attenuation of Gi function. Ribosylation of platelet Gs by cholera toxin was unchanged, whereas that of Gi by pertussis toxin (PT) was increased. Given that undissociated G protein is the preferred substrate for PT, our results suggest that Li interferes with subunit dissociation and the subsequent activation of Gi. To determine if Li has similar effects on Gi in the central nervous system, we measured extracellular (EC) cyclic adenosine monophosphate (cAMP) in rat brain by in vivo microdialysis, revealing a dose-dependent increase in cAMP by norepinephrine (NE) antagonized by propranolol. Chronic (4-week) Li doubled basal EC cAMP, while decreasing the fractional response to 100 microM NE. Thus, using in vivo microdialysis, we observed the reported reduction in NE-stimulated AC activity, but only as a function of elevated basal cAMP. Increased basal AC activity has been observed following chronic Li in both humans and rat tissues but generally has not been considered relevant. The PI generating system is another proposed major target for Li that we have studied using an in vitro cell culture model of peripheral blood cells. Chronic (6-day) exposure of neutrophil-like HL60 cells to 1 mM LiCl did not affect agonist fMet-Leu-Phe (fMLP) induced PI turnover. In contrast, Li attenuated both agonist and phorbol ester stimulated Na+/H+ exchange, suggesting reduced protein kinase C (PKC) function. Western blot analysis revealed altered levels of PKC in both membrane and cytosolic fractions. The functional consequences of these complex effects on the two major signal transduction pathways and their interactions in the intact living organism remain to be elucidated.