Maria Hadjiconstantinou

Aromatic L‐Amino Acid Decarboxylase Activity of Mouse Striatum Is Modulated via Dopamine Receptors

Abstract: 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.

Treatment with GM1 Ganglioside Restores Striatal Dopamine in the 1‐Methyl‐4‐Phenyl‐1,2,3,6‐Tetrahydropyridine‐Treated Mouse

Abstract: 1‐Methyl‐4‐phenyl‐1,2,3,6‐tetrahydropyridine (MPTP), 30 mg/kg i.p. daily for 7 days, was administered to mice. This dosage regimen resulted in an ∼50% reduction of striatal dopamine (DA) level. Chronic administration of GM1 ganglioside (II3NeuAc‐GgOse Cer), beginning between 1 to 4 days after terminating MPTP dosing, resulted in partial restoration of the striatal DA level. From dose‐ and time‐response studies, it appeared that 30 mg/kg i.p. of GM1 administered daily for ∼23 days resulted in an ∼80% restoration of the DA level and complete restoration of the 3,4‐dihydroxyphenylacetic acid (DOPAC) content. This dosage of GM1 also restored the turnover rate of DA in the striatum to near normal. Discontinuing GM1 treatment resulted in a fall of DA and DOPAC levels to values found in mice treated with MPTP alone. There was no evidence for regeneration of nerve terminal amine reuptake in the GM1‐treated mice as evaluated by DA uptake into synaptosomes. Our biochemical findings in animals suggest that early GM1 ganglioside treatment of individuals with degenerative diseases of dopaminergic nigrostriatal neurons might be fruitful.

GM1 ganglioside induces phosphorylation and activation of Trk and Erk in brain.

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 concentration‐ and 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.

Epidermal growth factor enhances striatal dopaminergic parameters in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-treated mouse

Intracerebroventricular infusion of epidermal growth factor (EOF) into mice with 1‐methyl‐4‐phenyl‐1, 2, 3, 6‐tetrahydropyridine (MPTP)‐induced degeneration of dopaminergic nigrostriatal neurons partially enhanced the content of dopamine (DA) and 3, 4‐dihydroxyphenylacetic acid as well as the activity of tyrosine hydroxylase in the striatum. EGF also enhanced these parameters in control, unlesioned animals. Neurotrophic activity also was observed in embryonic mesencephalic cultures, where EGF enhanced DA uptake after a lesion with the neurotoxic metabolite of MPTP, 1‐methyl‐4‐phenylpyridinium ion. Our in vivo and in vitro studies suggest that EGF may be a neurotrophic factor for dopaminergic neurons, or may act indirectly by inducing the release of a dopaminergic trophic factor from other cells.

Nicotine abstinence in the mouse.

We present evidence that intermittent administration of nicotine, 2 mg/kg s.c., four times daily to mice for 14 days produces a somatic abstinence syndrome after discontinuing treatment. The nicotine abstinence was mild and protracted, lasting more than 92 h. The constellation of abstinence signs was characterized by rearing, jumping, shakes, abdominal constrictions, chewing, facial tremor and scratching. No autonomic symptomatology was observed. Nicotine abstinence was attenuated with a single dose of nicotine administered at 24 or 48 h into withdrawal. The nicotinic antagonist mecamylamine, 3 mg/kg, induced a small increase in the total abstinence score when given 60 min after the last nicotine injection. Nicotine-abstinent mice displayed reduced locomotor activity. Finally, mice lost weight during the nicotine treatment which was not recovered during the withdrawal. Along with the rat nicotine abstinence model, the mouse model of intermittent nicotine administration and abstinence might be useful for studying the pharmacological and biochemical mechanisms of nicotine addiction and tobacco use.

Spinal cord serotonin: A biochemical and immunohistochemical study following transection

The serotonin (5-HT) content of rat spinal cord was studied following complete cord transection, transverse hemisection and rhizotomy by high pressure liquid chromatography-electrochemical detection (HPLC-EC) chromatography and immunohistochemically with rabbit anti-5-HT antiserum. Spinal cord 5-HT decreased but did not disappear after complete cord transection when studied 5 or 10 days after lesioning. Indeed the indole content 5 or 10 days after section were similar. Below the transection 5-HT-like immunoreactive neuronal elements were present, appeared normal but were significantly reduced compared with control cord. Although neuronal fibers were present after transection, no immunoreactive neuronal cell bodies were observed. The neuronal elements remaining after transection were capable of synthesizing and metabolizing 5-HT as evidenced by elevated 5-HT and decreased 5-hydroxyindoleacetic acid (5-HIAA) after inhibition of monoamine oxidase. Complete cord transection resulted in a fall of 5-HT in the ventral roots suggesting that they contain efferent 5-HT elements that originate above the transection. Rhizotomy plus cord transection did not change cord indole content more than transection alone demonstrating that the indoles that remain in the cord after transection did not originate from peripheral afferent 5-HT neurons. Hemitransection resulted in partial loss of immunoreactive neuronal elements on the cut side, but 5-HT-like immunoreactive nerve fibers were observed crossing within the cord from the intact side by the spinal canal. Analysis of indole content in the hemitransected cord were consistent with crossing of 5-HT fibers within spinal segments. Our studies, taken together with reports by other laboratories, support the notion that significant 5-HT elements remain in the spinal cord after transection. These elements appear normal morphologically and biochemically.

GM1 ganglioside : In vivo and in vitro trophic actions on central neurotransmitter systems

Abstract: There is now substantial evidence that GM1 ganglioside is effective in partially correcting the consequences of neuroinjury in a number of in vivo and in vitro model systems. Although the molecular mechanism(s) and the substrates for the neurotrophic activity of the gangliosides are not fully understood, the published experimental work suggests that GM1 has antineurotoxic, neuroprotective, and neurorestorative effects on various central neurotransmitter systems. This review focuses attention on studies reporting that GM1 restores neuronal integrity and function in the brain of lesioned young as well as aged animals. Critical analysis of these studies can provide guidance for future ganglioside research and may point to novel approaches for treating neuroinjury and a variety of degenerative conditions, including aging.

Preproenkephalin mRNA and methionine-enkephalin content are increased in mouse striatum after treatment with nicotine

Abstract: 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.

Aromatic L-Amino Acid Decarboxylase Is Modulated by D1 Dopamine Receptors in Rat Retina

Abstract: Aromatic L‐amino acid decarboxylase (AAAD) activity of rat retina increases when animals are placed in a lighted environment from the dark. The increase of activity can be inhibited by administering the selective dopamine D1 receptor agonist SKF 38393, but not the selective D2 agonist quinpirole, or apomorphine. Conversely, in the dark, enzyme activity can be enhanced by administering the selective D1 antagonist SCH 23390 or haloperidol, but not the selective D2 antagonist (–)‐sulpiride. Furthermore, in animals exposed to room light for 3 h, the D1 agonist SKF 38393 reduced retinal AAAD activity, and this effect was prevented by prior administration of SCH 23390. In contrast, quinpirole had little or no effect when administered to animals in the light. Kinetic analysis indicated that the apparent Vmax for the enzyme increases with little change in the apparent Km for the substrate 3,4‐dihydroxyphenylalanine or the cofactor pyridoxal‐5′‐phosphate. We suggest that dopamine released in the dark tonically occupies D1 receptors and suppresses AAAD activity. When the room light is turned on, D1 receptors are vacated and selective D1 agonists can either prevent the rise of AAAD or reverse light‐enhanced AAAD activity.

Activation of dopamine-containing amacrine cells of retina: light-induced increase of acidic dopamine metabolites.

The acidic metabolites of dopamine, homovanillic acid (HVA) and 3,4-dihydroxyphenylacetic acid (DOPAC), are present in rat retina. DOPAC is the most abundant metabolite. Both metabolites increase in parallel when rats are taken from a dark to a lighted environment. Haloperidol treatment also increases the metabolites in both dark and light while apomorphine decreases both metabolites in dark and light and partially antagonizes the increase induced by haloperidol.