We investigated the ability of GM1 to induce phosphorylation of the tyrosine kinase receptor for neurotrophins, Trk, in rat brain, and activation of possible down-stream signaling cascades. GM1 increased phosphorylated Trk (pTrk) in slices of striatum, hippocampus and frontal cortex in a concentrationand time-dependent manner, and enhanced the activity of Trk kinase resulting in receptor autophosphorylation. The ability of GM1 to induce pTrk was shared by other gangliosides, and was blocked by the selective Trk kinase inhibitors K252a and AG879. GM1 induced phosphorylation of TrkA > TrkC > TrkB in a region-specific distribution. Adding GM1 to brain slices activated extracellular-regulated protein kinases (Erks) in all three brain regions studied. In striatum, GM1 elicited activation of Erk2 > Erk1 in a time-and concentration-dependent manner. The GM1 effect on Erk2 was mimicked by other gangliosides, and was blocked by the Trk kinase inhibitors K252a and AG879. Pertussis toxin, as well as Src protein tyrosine kinase and protein kinase C inhibitors, did not prevent the GM1-induced activation of Erk2, apparently excluding the participation of Gi and Gq/11 protein-coupled receptors. Intracerebroventricular administration of GM1 induced a transient phosphorylation of TrkA and Erk1/2 in the striatum and hippocampus complementing the in situ studies. These observations support a role for GM1 in modulating Trk and Erk phosphorylation and activity in brain.

Aromatic L-amino acid decarboxylase (AAAD) activity is enhanced in the striatum of control and MPTP-treated mice after administration of a single dose of the dopamine receptor antagonists haloperidol, sulpiride, and SCH 23390. MPTP-treated mice appear more sensitive to the antagonists, i.e., respond earlier and to lower doses of antagonists than control mice. The rise of AAAD activity induced by the antagonists is prevented by pretreatment with cycloheximide. The apparent Km values for L-3,4-dihydroxyphenylalanine (L-DOPA) and pyridoxal 5-phosphate appear unchanged after treatment with the antagonists. Increased AAAD activity was observed also after subchronic administration of dopamine receptor antagonists or treatment with reserpine. A single dose of a selective dopamine receptor agonists had no effect on AAAD activity. In contrast, administration of L-DOPA, quinpirole, or SKF 23390 for 7 days lowers AAAD activity in the striatum. We conclude that AAAD is modulated in striatum via dopaminergic receptors.

No abstract

J. Neurochem. (2008) 104, 1466–1477. Abstract The ganglioside GM1 promotes neuronal growth, differentiation, survival, phenotypic expression, and function restoration, by apparently interacting with neurotrophic factors and/or their receptors. In brain, GM1 activates the Trk receptors for neurotrophins and the Raf/MEK/ERK cascade in situ and in vivo. We have expanded these studies and explored whether GM1 recruits the phosphatidylinositol 3 (PI3)‐kinase pathway in brain also. Incubating striatal slices with GM1 increased the activity of PI3‐kinase in phosphotyrosine immunoprecipitates in a time‐ and concentration‐dependent manner, and the response was blocked by the PI3‐kinase inhibitors wortmannin and LY294002. PI3‐kinase activation following GM1 was rapid and short lasting with an EC50 of 5 μmol/L. There was a temporally parallel activation of the downstream PI3‐kinase target Akt, which was prevented by PI3‐kinase inhibition. PI3‐kinase activity was found increased in Trk and Gab1 immunoprecipitates, and co‐immunoprecipitation studies demonstrated the association of Trk and Gab1 after GM1 treatment. Enhanced PI3‐kinase activity associated with Trk or Gab1 immunoprecipitates was blocked by the Trk inhibitor K252a. GM1 did not appear to transactivate Trk and did not alter the efflux of neurotrophins in striatal slices. Our findings suggest that GM1 induces activation of PI3‐kinase that is, in part, mediated through Trk and Gab1.

A single dose of nicotine increased methionine‐enkephalin (Met‐Enk) immunoreactivity in the striatum of mice in a time‐dependent manner. Met‐Enk content reached a maximum by ∼1 h after nicotine and returned to control values by 6 h. The response to nicotine was blocked by pretreating animals with the nicotinic receptor antagonist mecamylamine. In contrast, pretreating mice with the muscarinic receptor antagonist atropine or the dopamine receptor antagonist haloperidol did not block the response. A single dose of nicotine also increased mRNA for the precursor peptide preproenkephalin (PPE). The increase of PPE mRNA preceded that of Met‐Enk and reached a maximum by ∼30 min after nicotine. PPE mRNA levels returned to near normal by ∼3 h and increased again by 6 h after nicotine. Daily administration of nicotine for 14 days increased Met‐Enk content and PPE mRNA in the striatum of mice as well. Taken together, our results suggest that nicotinic receptors modulate Met‐Enk content and PPE mRNA in the mouse striatum.