Retinal dopamine-containing amacrine neurons are rapidly activated by light, as shown by an increase in the rate of dopamine formation in vivo and a concomitant increase in the activity of tyrosine hydroxylase, measured in vitro with a subsaturating concentration of pteridine cofactor. Activation of tyrosine hydroxylase also occurs when isolated eyes from rats killed in the dark are exposed to a strobe light. Studies of amacrine neurons should provide basic data about the biochemical processing of visual information, as well as the physiological presynaptic regulatory mechanisms of dopamine-containing neurons.

A simple, rapid method is presented for the determination of acetylcholine (ACh) and choline (Ch) in neuronal tissue using HPLC with electrochemical detection. The method is based on the separation of ACh and Ch by reverse-phase HPLC and mixing the effluent as it emerges from the column with acetylcholinesterase and Ch oxidase, which converts endogenous Ch and Ch produced by the hydrolysis of ACh to betaine and hydrogen peroxide. Production of hydrogen peroxide is continuously monitored electrochemically. The sensitivity of the procedure is 1 pmol for Ch and 2 pmol for ACh. Specificity of the method is based on HPLC, two specific enzymatic reactions, and the detection of hydrogen peroxide.

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.