Glial alpha 2-receptors probably inhibit the high-affinity uptake of noradrenaline into astrocytes in the rat brain in vivo

Comparative anatomy of α2 and β adrenoceptors in the adult and developing brain of the marine teleost the red porgy (Pagrus pagrus, Sparidae): [3H]clonidine and [3H]dihydroalprenolol quantitative autoradiography and receptor subtypes immunohistochemistry

The present study aimed to determine the anatomic distribution and developmental profile of alpha(2) and beta adrenoceptors (AR) in marine teleost brain. Alpha 2 and beta adrenoceptors were studied at different developmental stages by using [(3)H]clonidine and [(3)H]dihydroalprenolol, respectively, by means of in vitro quantitative autoradiography. Furthermore, immunohistochemical localization of the receptor subtypes was performed to determine their cellular distribution. Saturation studies determined a high-affinity component of [(3)H]clonidine and [(3)H]dihydroalprenolol binding sites. High levels of both receptors were found in preglomerular complex, ventral hypothalamus, and lateral torus. Dorsal hypothalamus and isthmus included high levels of alpha(2) AR, whereas pretectum and molecular and proliferative zone of cerebellum were specifically characterized by high densities of beta AR. From the first year of life, adult levels of both AR were found in most medial telencephalic, hypothalamic, and posterior tegmental areas. Decreases in both receptors densities with age were prominent in ventral and posterior telencephalic, pretectal, ventral thalamic, hypothalamic, and tegmental brain regions. Immunohistochemical data were well correlated with autoradiography and demonstrated the presence of alpha(2A), alpha(2C), beta(1), and beta(2) AR subtype-like immunoreactivity. Both the neuronal (perikaryal or dendritic) and the glial localization of receptors was revealed. The localization and age-dependent alterations in alpha(2) and beta AR were parallel to plasticity mechanisms, such as cell proliferation in periventricular thalamus, hypothalamus, and cerebellum. In addition, the biochemical characteristics, distribution pattern, and neuronal or glial specificity of the receptors in teleost brain support a similar profile of noradrenergic transmission in vertebrate brain evolution.

Expression of the extraneuronal monoamine transporter (uptake2) in human glioma cells

Tritiated methylphenylpyridinium ([3H]MPP+), a substrate of the neuronal and extraneuronal noradrenaline transporter (uptake1 and uptake2, respectively) and of the organic cation transporter (OCT1), was used to characterize the amine transport system of the established human glioma cell line SK-MG-1. Uptake of [3H]MPP+ (25 nM) into SK-MG-1 cells increased linearly with time for up to 15 min. Selective uptake1 inhibitors (e.g. (+)oxaprotiline) or omission of Na+ or Cl-ions did not affect [3H]MPP+ uptake, whereas uptake2 inhibitors such as O-methyl-isoprenaline (OMI) or corticosterone as well as depolarizing concentrations of K+ or Ba2+ strongly reduced [3H]MPP+ uptake. Initial rates of OMI(100 microM)-sensitive [3H]MPP+ uptake were saturable, with a K(m) of about 17 microM and a maximal rate of about 50 pmol/(min x mg protein). IC50 (or Ki) values for inhibition of [3H]MPP+ uptake by substrates and inhibitors of uptake2 or OCT1 were highly significantly correlated with published IC50 values for inhibition of uptake2 but not with corresponding values for inhibition of OCT1. The results presented here clearly demonstrate that human glioma cells express an uptake2 transporter. Thus, glial cells in the human central nervous system endowed with this transporter are likely to contribute to the inactivation of neuronally released noradrenaline.