Axonal transport of the synaptic vesicle monoamine carrier identified by [3H]dihydrotetrabenazine binding in rat sciatic nerve

Subcellular distribution of receptor sites in human brain: differentiation between heavy and light structures of high and low density

Studies of the subcellular localization of neuroreceptors in the rat brain have shown that most of them are associated with light and low density subcellular fractions. In two human brain areas, quite different subcellular distributions were observed. After fractionation by differential centrifugation of frontal cortex homogenates, benzodiazepine and serotonin 5-HT2 receptors were mainly found in the heavy mitochondrial (M) fraction, whereas mu-opiate and muscarinic cholinergic receptors were mainly concentrated in the microsomal (P) fraction. In human putamen, the presynaptic markers of dopaminergic nerve terminals (neurotensin receptors, dopamine uptake sites and amine vesicular transporter-binding sites), benzodiazepine receptors and serotonin uptake sites were recovered both in the high and low density fractions, whereas the muscarinic, opiate and, to a lesser extent, dopamine D2 receptors were mostly concentrated in the microsomal fraction. In the cerebral cortex, after isopycnic centrifugation in sucrose gradients, neuroreceptors were found in the high density fractions where the peaks of cytochrome oxidase and that of nerve endings, as identified by amine uptake and by means of electron microscopy were also found. A single peak of benzodiazepine receptors was observed in high density (1.15-1.17 g/ml) fractions suggesting that these receptors are much more concentrated in the nerve terminals or dendrites rather than in the dendritic spines or vesicles. The fact that muscarinic and opiate receptors were recovered in the P fraction with plasma membrane constituents and also in M and L fractions, which is confirmed by a bimodal distribution in sucrose gradient, suggests that they are localized in both the nerve terminals or dendrites and in the small vesicles or dendritic spines. In the putamen, much of the specific binding to uptake sites for dopamine and serotonin was recovered in the high density fractions, but the existence of another peak at a lower density indicates the presence of microsomal uptake sites. The results indicate that differential and isopycnic fractionation methods performed on human brain samples, make it possible to separate tissue fractions enriched in nerve endings, dendrites, dendritic spines, plasma membranes or vesicles.

Axonal transport of dopamine-containing vesicles labelled in vivo with [3H]reserpine

Axonal transport of the vesicular monoamine transporter was assayed in the rat brain by in vivo binding of the specific ligand [3H]reserpine. Because of the marked localization of reserpine binding sites in dopaminergic cell bodies and nerve terminals, the dopaminergic nigrostriatal pathway was chosen for the study of the axonal transport of the monoamine carrier present in the membrane of synaptic vesicles. When labelled reserpine was injected into the substantia nigra, a delayed accumulation of radioactivity in the ipsilateral striatum was observed approximately 4 h after the injection. Similarly, injection into the right striatum was followed by a substantial accumulation of radioactivity in the ipsilateral substantia nigra, which was prevented by peripheral injection of unlabelled reserpine or tetrabenazine. This process was rapid and dependent on microtubules. In senescent rats, the amount of retrogradely transported [3H]reserpine was significantly reduced. These results demonstrate that labelled reserpine may be used to monitor in vivo fast axonal transport in central neurons.

Axonal transport of synaptic vesicles and muscarinic receptors: effect of protein synthesis inhibitors

The effect of cycloheximide, a protein synthesis inhibitor, was studied on the axonal transport of noradrenergic synaptic vesicles and presynaptic muscarinic receptors, identified by in vitro binding of [3H]dihydrotetrabenazine and [3H]quinuclidinylbenzilate, respectively, in rat sciatic nerve. Cycloheximide (1.5 mg/kg) administered subcutaneously 2 h before ligation decreased by approximately 50% the accumulation of vesicles and receptors in the proximal segment above the ligature placed on the nerve; its action was detectable after a lag period of 10 h and disappeared 96 h after administration. Double ligatures were placed on the nerve at various time intervals between the first (distal) and the second (proximal) ligature, and the accumulation of vesicles and receptors proximal to the second ligature was measured; the first ligature diminished the accumulation above the second ligature. At an interval of 96 h between the first and the second ligature, cycloheximide completely prevented the accumulation of vesicles and receptors proximal to the second ligature. The effects of double ligatures and the response to cycloheximide treatment can best be explained on the assumption that an important proportion of synaptic vesicles and presynaptic receptors is being recycled in the nerve cell bodies after retrograde transport.

Quantitative autoradiography of the rat brain vesicular monoamine transporter using the binding of [3H]dihydrotetrabenazine and 7-amino-8-[125I]iodoketanserin

The binding of [3H]dihydrotetrabenazine, a specific ligand of the monoamine transporter present on serotonin and catecholamine synaptic vesicles, was studied on rat brain sections. The characteristics of binding (Kd = 5.0 nM, k1 = 0.13 x 10(6) M-1 s-1; k-1 = 0.66 x 10(-3) s-1) were similar to those previously observed on tissue homogenates. The rostrocaudal topographical distribution of dihydrotetrabenazine binding sites was analysed by quantitative autoradiography. High labelling was observed in regions richly innervated by monoaminergic systems: dopamine in the striatum and olfactory tubercles, noradrenaline in the striatal fissure and in the paraventricular and dorsomedial hypothalamus and serotonin in the lateral septum, islands of Calleja and suprachiasmatic nucleus. Cell bodies were also labelled in the substantia nigra and ventral tegmental area (dopamine), in locus coeruleus (noradrenaline) and in raphe nucleus (serotonin). The pituitary gland (particularly the neural lobe) and the pineal gland were also labelled. Low labelling was observed in various areas of the cerebral cortex and in the cerebellum. Unilateral 6-hydroxydopamine lesion of the substantia nigra dramatically reduced [3H]dihydrotetrabenazine labelling in the ipsilateral striatum. Moreover, ketanserin has recently been shown to possess a nanomolar affinity for the vesicular monoamine transporter, and autoradiographic localization of brain monoaminergic synaptic vesicles was also obtained by means of the derivative 7-amino-8-[125I]iodoketanserin in the presence of 5-hydroxytryptamine2 and alpha 1 antagonists, although the non-specific labelling was higher than with [3H]dihydrotetrabenazine. It is concluded that [3H]dihydrotetrabenazine may represent a valuable monoaminergic marker in in vitro autoradiographic studies.