Stimulation of non-alpha7 nicotinic receptors partially protects dopaminergic neurons from 1-methyl-4-phenylpyridinium-induced toxicity in culture.

Previous work has shown that nicotine treatment protects against nigrostriatal degeneration in rodents, findings that may be of relevance to the decreased incidence of Parkinson's disease in cigarette smokers. In the present studies, we investigated the effect of nicotine against 1-methyl-4-phenylpyridinium-induced toxicity in dopaminergic ventral mesencephalic cultures to identify the nicotinic receptor population that may be involved. [3H]Epibatidine, a ligand that binds to receptors containing alpha2-alpha6 subunits, bound to at least two populations of sites that were up-regulated by nicotine in a time and dose dependent manner. We next examined the effect of nicotine on cultures treated with 1-methyl-4-phenylpyridinium, a neurotoxin that selectively damages nigrostriatal dopaminergic neurons. Pre-treatment with nicotine, at 10(-7)-10(-4) M, partially prevented the toxin-induced decline in dopaminergic cells. Pre-exposure to nicotine for 24 h resulted in optimal protection, suggesting that receptor up-regulation may contribute to the observed neuroprotective effect. Nicotine-mediated protection was blocked by pre-incubation with the nicotinic receptor antagonist d-tubocurarine (10(-4) M), but not the alpha7 receptor-selective antagonist alpha-bungarotoxin (10(-7) M). Our results show that nicotinic receptor activation partially protects nigral dopaminergic neurons from 1-methyl-4-phenylpyridinium-induced toxicity in culture and that this appears to occur through an interaction at non-alpha7 containing receptors.

Nitric oxide is involved in acetylcholinesterase inhibitor-induced myopathy in rats.

Excess activation of muscle nicotinic acetylcholine receptors due to genetic mutations, as seen in slow channel congenital myasthenic syndrome, or acetylcholinesterase (AChE) inhibition results in muscle cell degeneration. Our recent work showed that nitric oxide synthase (NOS) inhibitors prevent nicotine-induced muscle cell death in culture. In the present study, we examined the effects of NOS inhibition on nicotinic receptor-mediated myopathy in vivo. Rats injected with the AChE inhibitor paraoxon demonstrate a 90-fold increase in the number of dying muscle cells compared with control as evidenced histologically by centralized nuclei and the presence of degenerating profiles. Coadministration of the nonspecific NOS inhibitor nitro-L-arginine methyl ester or the neuronal NOS-specific inhibitor 7-nitroindazole dramatically reduced the presence of such degenerating profiles to approximately 20% of that seen with paraoxon alone. These results show that inhibition of NOS, as well as neuronal NOS, significantly reduces AChE inhibitor-induced muscle cell degeneration, suggesting that increased nitric oxide production mediates such myopathy.

Tacrine, a reversible acetylcholinesterase inhibitor, induces myopathy.

Tacrine, an acetylcholinesterase (AChE) inhibitor that has been used in the treatment of Alzheimer's disease, increases available acetylcholine (Ach) levels in the synaptic cleft thereby enhancing the activity of cholinergic pathways. However, excessive stimulation of nicotinic receptors at the neuromuscular junction results in muscle deterioration. We tested whether reversible AChE inhibitors such as tacrine may induce similar effects. In the present study, tacrine administration (7.5 mg/kg twice daily) to rats produces a 20 and 30-fold increase in the number of degenerating cells in leg and diaphragm muscle, respectively, as compared to control. This myopathy is significantly decreased by co-administration of tacrine with the nitric oxide (NO) synthase inhibitor L-NAME. These results show that tacrine can induce myopathy which may be mediated by increased NO production.

Nicotinic receptors and Parkinson's disease.

Accumulating evidence indicates that nicotinic receptors play a role in basal ganglia function. Furthermore, nicotine administration may be neuroprotective in animal models of nigrostriatal degeneration, while cigarette smoking is inversely correlated with Parkinson's disease. Because nicotinic receptors are decreased in Parkinson's disease, these observations may suggest that nicotinic agonists are beneficial in this disorder. We used two model systems to investigate this possibility. One involved non-human primates, which represent a good model because their neuroanatomical organization resembles that of man and nigrostriatal degeneration leads to biochemical and behavioral deficits similar to Parkinson's disease. To identify the subunits that comprise basal ganglia nicotinic receptors, we investigated alpha4, alpha6, alpha7, beta2, beta3 and beta4 transcript distribution in monkey substantia nigra. All mRNAs were expressed with a selective alteration in some transcripts after 1-methyl-4-phenyl-1,2,3,6-tetrahydropteridine (MPTP) induced nigrostriatal degeneration. As an approach to evaluate neuroprotective effects of nicotine against nigral neuron damage, we used mesencephalic neurons in culture, treated with a selective dopaminergic neurotoxin. The results show that nicotine pretreatment protected against dopaminergic nigral neural degeneration. These data suggest that nicotinic receptor ligands may be useful in Parkinson's disease therapy.

Activity-regulated cell death contributes to the formation of ON and OFF alpha ganglion cell mosaics.

At maturity, ON and OFF alpha ganglion cells in the cat retina are arrayed in regular mosaics, with adjacent cells commonly forming ON-OFF pairs. In the present study, we investigated the role of activity-mediated ganglion cell death in the formation of such cellular patterns. Because direct measures of ganglion cell mosaics are problematic in the developing retina, we examined the distributions of ON and OFF alpha cells in the postnatal cat retina by assessing the degree to which cells in closest proximity were of opposite sign (i.e., ON-OFF pairs). Computer simulations demonstrated that superimposition of two regular distributions results in a high incidence (approximately 90%) of opposite sign pairs. This is also the case for ON and OFF alpha cells in the mature retina, reflecting the high degree of regularity exhibited by this cell class. In contrast, during the first postnatal month, alpha cells displayed a much lower incidence of opposite sign pairs (approximately 60%), comparable to the superimposition of two simulated random distributions. We also show that there is a 20% loss of alpha cells in the central retina during postnatal development and that this magnitude of loss is sufficient to form regular distributions of ON and OFF cells. To assess the influence of sodium voltage-gated activity on this developmental process, intraocular injections of tetrodotoxin (TTX) were made during the postnatal period of alpha cell loss. When the TTX-treated animals reached maturity, there was a dose-related decrease in the incidence of opposite sign pairs, without any appreciable change in cell density. Moreover, the regularity index of ON and OFF cells was significantly lower than normal in the TTX-treated retinas. These findings demonstrate that a spatially selective pattern of ganglion cell loss contributes to the formation of regular ON and OFF ganglion cell distributions and that such cell loss is regulated by retinal activity.

Development and regulation of dendritic stratification in retinal ganglion cells by glutamate-mediated afferent activity.

In the mature retina, the dendrites of retinal ganglion cells (RGCs) are segregated into either ON or OFF sublaminae of the inner plexiform layer (IPL), but early in development the dendritic processes of these cells are multistratified, ramifying throughout the IPL. We examined the time course of dendritic stratification in developing beta cells, the largest class of ganglion cells in the cat retina, by retrograde labeling of fixed tissue with Dil. Dendritic stratification begins in the central and peripheral retina by embryonic day 50, about 2 weeks before birth and is not fully completed until 5 months postnatally. A clear central-to-peripheral gradient in the incidence of stratified beta cells first becomes evident shortly after birth. This stratification process was effectively halted by short-term intraocular injections (4-11 d) of the glutamate analog 2-amino-4-phosphonobutyrate (APB), which hyperpolarizes rod bipolar cells and ON cone bipolar cells, thereby preventing the release of glutamate by these interneurons. APB treatment did not alter the somal sizes or the tangential extent of the dendrites of developing beta cells, nor did it cause abnormal loss of these neurons. The organization of the inner nuclear layer, containing the APB-sensitive bipolar cells, was also not compromised by such injections. When APB treatment was discontinued there was a rapid resumption of dendritic stratification resulting in a normal incidence of stratified RGCs. Thus, short-term APB treatment causes a delay rather than a permanent arrest of the stratification process.(ABSTRACT TRUNCATED AT 250 WORDS)