Hemicholinium-3-resistant choline uptake system linked to acetylcholine synthesis in the rat hippocampus

Nitric oxide synthase inhibition does not impair visual or spatial discrimination learning

Nitric oxide (NO) is a candidate retrograde messenger involved in synaptic plasticity, and is linked to the cholinergic system in the brain. We examined the role of NO in the acquisition of visual and spatial discriminations by daily administration of either saline or 1-nitroarginine methyl ester (L-NAME), an NO synthase inhibitor. Brains were assayed for NO synthase activity and two presynaptic cholinergic markers: hemicholinium-3 (HC-3) binding, which determines the number of sodium-dependent high-affinity choline uptake sites, and activity of choline acetyltransferase (ChAT), which is the synthetic enzyme for acetylcholine. In both behavioral tasks, the acquisition rate was not different between groups. L-NAME reduced NO synthase activity by 85% in all brain areas assayed and HC-3 binding by 38% in hippocampus and 48% in posterior cortex. ChAT activity was not different between groups in any region assayed. These data suggest that NO does not play a role in visual or spatial discrimination learning. However, NO synthase inhibition may play a role in the regulation of cholinergic activity.

Effects of supplemental choline on extracellular acetylcholine in the nucleus accumbens during normal behavior and pharmacological acetylcholine depletion

Brain microdialysis was used to determine whether systemic or local application of choline would modify the extracellular concentration of acetylcholine (ACh) in the nucleus accumbens (NAc) of freely moving rats. Supplemental choline given intraperitoneally or into the NAc of normal rats did not increase extracellular ACh. When local ACh interneurons in the NAc were treated pharmacologically to deplete the intracellular stores of ACh, then systemic choline (80 mg/kg) was an effective treatment. Specifically, 1) blockade of the high-affinity choline transporter with hemicholinium-3 (HC-3) to reduce ACh synthesis caused a decrease in extracellular ACh, but choline supplementation restored ACh toward its normal level in the NAc. 2) Local bicuculline treatment released ACh to the point of depletion, but systemic choline or locally infused choline helped maintain normal ACh levels. These results suggest that choline supplementation might be useful in preventing depletion of ACh in the nucleus accumbens during pathological conditions.

Acetylcholine synthesis is modulated by acetylcholine content of cytosolic fraction but not by that of releasable fraction

Synthesis and release of acetylcholine (ACh) in the rat hippocampal slices were examined to clarify the mechanism of modulation of ACh synthesis. Treatment with 2-(4-phenylpiperidino)cyclohexanol (AH5183, 50 microM), an inhibitor of ACh transport from cytosol to synaptic vesicles, inhibited the increase in ACh content of the membrane-bound fraction which is readily releasable, but did not affect the cytosolic ACh content. Under these conditions, the total ACh content reached a plateau value. These results indicate that ACh synthesis is modulated by cytosolic ACh content but not by the vesicular fraction.

Regional differences in extracellular choline dependency of acetylcholine synthesis in the rat brain

Acetylcholine (ACh) synthesis under conditions of restricted extracellular choline uptake was investigated in order to clarify the procurement of choline for ACh synthesis using slices of several regions of the rat brain. Extracellular choline-independent ACh synthesis was observed in the hippocampus, frontal cortex and caudate putamen, which contain cholinergic nerve terminals, whereas little or no synthesis was observed in the medial septum or basal nucleus of Meynert, which contain cholinergic cell bodies. These results indicate that cholinergic nerve terminals, but not the cell bodies, may be able to synthesize choline for ACh biosynthesis. Extracellular choline-dependent ACh synthesis was observed in all regions examined. In the presence of 10 microM choline, the highest content of newly synthesized ACh and the proportionate increase in ACh compared with the unreleasable fraction (basal level) were observed in the caudate putamen. In the frontal cortex, although the level of synthesized ACh was low, the proportionate increase in ACh was high. In contrast, in the medial septum and basal nucleus of Meynert, high levels of ACh with a low proportionate increase compared with basal levels were observed. In the presence of hemicholinium-3, extracellular choline was also taken up for ACh synthesis in all regions examined, the level being especially high in the frontal cortex and medial septum. The present results indicate that the manner of choline procurement for ACh synthesis is heterogeneous among the various regions of the rat brain.