Bibliothek

Notizen: Abstract: The present study examines the possible involvement of nitric oxide (NO)-stimulated endogenous ADP-ribosylation in long-term potentiation (LTP). LTP was induced in hippocampal slices by stimulation of Schaffer collateral inputs to the CA1 pyramidal neurons. Basal and sodium nitroprusside (SNP), which generates NO, stimulation of endogenous ADP-ribosylation was then studied in CA1 subfields isolated from the slices. Control slices received no treatment or were tetanized in the presence of aminophosphonovaleric acid, an NMDA receptor antagonist that blocks the development of LTP. SNP-stimulated ADP-ribosylation of endogenous proteins was reduced by 40–70% in LTP slices relative to control slices. LTP was also associated with a small but significant reduction in basal ADP-ribosylation activity. The results demonstrate that the induction of LTP is associated with regulation of endogenous ADP-ribosylation and suggest a role for this type of covalent modification in some aspect of LTP.

Notizen: Cholera and pertussis toxin-mediated ADP-ribo-sylation has been used extensively to study regulation of guanine nucleotide binding proteins (G proteins) in the nervous system, but much less is known about possible endogenous ADP-ribosylation of G proteins in brain. The present study demonstrates endogenous ADP-ribosylation, in the absence of cholera and pertussis toxins, of four predominate proteins in homogenates of rat cerebral cortex. These proteins showed apparent molecular masses of 20, 42, 45, and SO kDa by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. The 42- and 45-kDa proteins comigrated precisely with the major cholera toxin-labeled bands. Furthermore, the endogenous ADP-ribosylated and cholera toxin-ADP-ribosylated bands yielded identical 32p-labeled peptide fragments by one-dimensional peptide mapping, indicating that they are probably the same proteins, presumably the α-subunits of Gs. In contrast, peptide maps of the 50-kDa protein, which migrated close to a 48-kDa cholera toxin-labeled band, demonstrated that this protein is distinct from the toxin-labeled band and from Gsα. Levels of endogenous ADP-ribosylation activity showed regional heterogeneity in brain, with a nearly threefold variation observed among the brain regions examined. Chronic administration (7 days) of corticosterone significantly increased overall levels of endogenous ADP-ribosylation, indicating that components of this system may be under hormonal control in vivo. Attempts to identify neurotransmitters or second messenger systems that regulate endogenous ADP-ribosylation activity in brain have so far been unsuccessful with one exception. Sodium nitroprusside, which increases the formation of nitric oxide, dramatically stimulated the endogenous ADP-ribosylation of a 36-kDa protein, as reported previously by others, and, under certain conditions, also stimulated ADP-ribosylation of the 42-, 45-, and 50-kDa proteins, although to a lesser extent than the 36-kDa protein. These findings demonstrate that GSα and additional, as yet unidentified, proteins are ADP-ribosylated in brain in the absence of bacterial toxins and suggest that such endogenous covalent modification may regulate the functional activity of these proteins in vivo.

Notizen: Abstract: The influence of chronic administration of antidepressants on cyclic AMP-dependent protein kinase activity was examined in rat frontal cortex. Chronic administration of imipramine, tranylcypromine, or electroconvulsive seizures decreased cyclic AMP-dependent protein kinase activity in soluble fractions by ∼25%, whereas enzyme activity was increased in the particulate fractions by ∼20%. In contrast, enzyme activity in crude homogenates was not altered. This effect appears to be specific to antidepressant jdrugs, because representatives of several other classes of psychotropic drugs—namely, haloperidol, morphine, and diazepam—failed to alter either soluble or particulate levels of cyclic AMP-dependent protein kinase activity in this brain region following chronic administration. When the total particulate fraction was subfractionated, it was found that chronic imipramine treatment significantly increased the activity of cyclic AMP-dependent protein kinase in crude nuclear fractions but not in crude synaptosomal or microsomal fractions. Taken together, the data raise the possibility that chronic antidepressant treatments may stimulate the translocation of cyclic AMP-dependent protein kinase from the cytosol to the nucleus. This effect would represent a novel action of antidepressants that could contribute to the long-term adaptive changes in brain thought to be essential for the blinical actions of these treatments.

Notizen: Abstract: Based on the established role of β-adrenergic receptor kinase (βARK) and β-arrestin in the desensitization of several G protein-coupled receptors, we investigated the effect of chronic morphine administration on βARK and β-arrestin levels in selected brain areas. Levels of βARK were measured by blot immunolabeling analysis using antibodies specific for two known forms of βARK, i.e., βARK1 and βARK2. It was found that chronic morphine treatment produced an ∼35% increase in levels of βARK1 immunoreactivity in the locus coeruleus, but not in several other brain regions studied. In contrast, chronic morphine treatment failed to alter levels of βARK2 immunoreactivity in any of the brain regions studied. Levels of β-arrestin immunoreactivity, measured using an antiserum that recognizes two major forms of this protein in brain, were also found to increase (by ∼20%) in the locus coeruleus. It is proposed that chronic morphine regulation of βARK1 and β-arrestin levels may contribute to opioid-receptor tolerance that is known to occur in this brain region.

Notizen: Abstract: In the present study, we investigated the effects of lithium on endogenous ADP-ribosylation in rat brain. It was found that addition of lithium in vitro inhibits endogenous ADP-ribosylation activity in extracts of frontal cortex at therapeutically relevant concentrations. Inhibition is observed at concentrations as low as 0.3 mM and is maximal at 1 mM when 50% inhibition is obtained. A similar degree of inhibition of endogenous ADP-ribosylation was observed for all substrate proteins identified, including Gsα, suggesting that lithium's effect may be achieved at the level of ADP-ribosyltransferases and not specific substrate proteins. In contrast to lithium, chloride salts of sodium and potassium do not alter endogenous ADP-ribosylation activity in frontal cortex. To assess the possible in vivo relevance of this in vitro action of lithium, we studied the effect of chronic lithium administration on levels of endogenous ADP-ribosylation in frontal cortex. It was found that chronic lithium treatment, in contrast to the inhibitory effect of the drug in vitro, produced a 〉35% increase in endogenous ADP-ribosylation activity. A similar degree of increase was observed for all of the substrate proteins identified. These novel findings raise the possibility that certain endogenous ADP-ribosyltransferases are among the acute targets of lithium in the brain and that adaptations in these enzymes may be part of the mechanisms underlying lithium's long-term effects on brain function.

Notizen: Abstract: Recent studies have demonstrated that chronic stress increases the firing rate and expression of tyrosine hydroxylase (TH) in neurons of the locus coeruleus (LC), the major noradrenergic nucleus in brain. The present study was undertaken to examine the influence of chronic stress and other treatments known to influence the activity of LC neurons on the cyclic AMP (cAMP) second messenger system in these neurons. Chronic (5 days) cold exposure significantly increased levels of TH immunoreactivity in the LC, as previously reported, but not in substantia nigra (SN) or ventral tegmentum (VT), two dopaminergic nuclei studied for comparison. Chronic cold exposure increased levels of cAMP-dependent protein kinase activity in soluble, but not particulate, fractions of the LC, and increased basal and GTP- and forskolin-stimulated adenylate cyclase activity in this brain region. In contrast, levels of the protein kinase and adenylate cyclase in VT, SN, and frontal cortex were not significantly influenced by cold exposure. To study further the relationship between regulation of LC firing rate, TH expression, and the cAMP system in the LC, other treatments known to influence TH were examined. Reserpine treatment, shown previously to increase levels of TH, was found to increase both LC firing rate and levels of soluble cAMP-dependent protein kinase activity in the LC. 6-Hydroxydopamine, shown previously to increase levels of TH and firing rate of LC neurons, also increased soluble levels of protein kinase activity. Other treatments known to either increase (adrenalectomy) or decrease (chronic imipramine) levels of TH in the LC were also found to increase or decrease, respectively, levels of cAMP-dependent protein kinase activity in this brain region. The results demonstrate the coordinate regulation of LC firing rate, TH expression, and the cAMP system by chronic stress, catecholamine depletion, and various drug and hormone treatments and raise the possibility that adaptations in the cAMP system in response to these treatments contribute to regulation of LC neuronal firing and TH expression.

Notizen: Abstract: A possible role for G proteins in contributing to the chronic actions of cocaine was investigated in three rat brain regions known to exhibit electrophysiological responses to chronic cocaine: the ventral tegmental area, nucleus accumbens, and locus coeruleus. It was found that chronic, but not acute, treatment of rats with cocaine produced a small (∼ 15%), but statistically significant, decrease in levels of pertussis toxin-mediated ADP-ribosylation of Giα and Goα in each of these three brain regions. The decreased ADP-ribosylation levels of the G protein subunits were shown to be associated with 20–30% decreases in levels of their immunoreactivity. In contrast, chronic cocaine had no effect on levels of G protein ADP-ribosylation or immunoreactivity in other brain regions studied for comparison. Chronic cocaine also had no effect on levels of GSα or Gβ immunoreactivity in the ventral tegmental area and nucleus accumbens. Specific decreases in Giα and Goα levels observed in response to chronic cocaine in the ventral tegmental area, nucleus accumbens, and locus coeruleus are consistent with the known electrophysiological actions of chronic cocaine on these neurons, raising the possibility that regulation of G proteins represents part of the biochemical changes that underlie chronic cocaine action in these brain regions.