Functional Roles of Dopamine D₂and D₃Autoreceptors on Nigrostriatal Neurons Analyzed by Antisense KnockdownIn Vivo

Abstract

Two different 19-mer antisense oligodeoxynucleotides complementary to the initial coding regions of dopamine D2 or D3 receptor mRNA were infused unilaterally into the substantia nigra of rats for 3-6 d to suppress synthesis of D2 and/or D3 receptors on substantia nigra dopaminergic neurons, thereby producing specific reductions of D2 and/or D3 receptors. Autoradiographic receptor binding revealed that D2 and D3 antisense oligodeoxynucleotides specifically and significantly reduced D2 or D3 binding in the ipsilateral substantia nigra, respectively, without affecting dopamine receptor binding in the neostriatum. Either D2 or D3 antisense oligodeoxynucleotides greatly attenuated the ability of apomorphine to inhibit dopaminergic neurons in vivo, an effect that was potentiated by simultaneous administration of D2 and D3 antisenses. Despite these effects, neither the rate nor the pattern of spontaneous activity of antisense-treated nigrostriatal neurons differed from those in the control groups. The proportion of antidromic responses consisting of full spikes from antisense-treated rats was significantly greater, and the mean antidromic threshold was significantly lower than in controls, indicating that autoreceptor knockdown increased both somatodendritic and terminal excitability. These data demonstrate that selective reduction of specific dopamine receptor subtypes by antisense infusion can be effected in vivo, and that nigrostriatal neurons express both D2 and D3 autoreceptors at their somatodendritic and axon terminal regions. Although the somatodendritic and terminal autoreceptors modulate dendritic and terminal excitability, respectively, the interaction of endogenously released dopamine with somatodendritic autoreceptors does not appear to exert a significant effect on spontaneous activity in anesthetized rats.

Do silent dopaminergic neurons exist in rat substantia nigra in vivo?

“…24 However, when a higher stimulus intensity (>500 µA) was used, dopaminergic neurons were inhibited, 24 consistent with the majority of reports on the effects of striatal stimulation on identified dopaminergic neurons. 15,16,29,37,38,42,50,52 Since the stimulus parameters used in the present study were 1.0 mA with a duration of 500 µs, the stimulus would be expected to inhibit, not excite, dopaminergic neurons, and is thus unlikely to account for the failure to detect any silent dopaminergic neurons.…”

“…to inhibit the spontaneous activity of dopaminergic neurons by activating somatodendritic D 2 -like autoreceptors. 2,29,35,52 Supplementary doses of apomorphine were administered periodically to maintain the complete inhibition of spontaneous activity throughout the subsequent recording period, which typically lasted about 1 h. In some experiments, apomorphine was continuously infused through a femoral catheter at 20 µg/kg/min to maintain the inhibition. During the period of effective inhibition, recording electrodes were advanced at 2 µm/s to search for dopaminergic neurons identified by antidromic responses elicited by either striatal or MFB stimulation at 1.0 mA, a value chosen on the basis of previous experiments (e.g., Refs 48, 49 and 53) to be sufficient to activate antidromically a large proportion of dopaminergic neurons.…”

“…6,8,9 In addition, it is conceivable that D 2 -like receptor antagonists affect the extracellular field potential generated by action potentials in dopaminergic neurons. Several lines of evidence, including increases in firing rate, [6][7][8][9][10]26 alterations in firing pattern 56 and changes in somatodendritic excitability 52 of dopaminergic neurons after blockade of somatodendritic autoreceptors, suggest that under normal conditions in vivo these autoreceptors are tonically activated. This leads to the opening of potassium channels, which not only hyperpolarizes the somatodendritic region, but also decreases its membrane resistance and increases the electrotonic length of the neuron.…”

Dopamine D4 Receptor Expression in Rat Kidney: Evidence for Pre- and Postjunctional Localization

“…These recep- tors mediate vasodilatation indirectly by decreasing the noradrenergic vasoconstrictor tone (Lokhandwala 1988;Vega et al 1990;Rump and Schollmeyer 1993) and are markedly reduced in a model of denervation similar to that used in the present study (Barili et al 1997b). In the central nervous system, both dopamine D2 and D3 receptors can represent autoreceptors located presynaptically (Tepper et al 1997). Recently, Svingos and co-workers reported that, in the rat nucleus accumbens shell, most of the D4 receptors are located in the presynaptic endings (Sanada et al 1997).…”

Dopamine D4 receptors: significance for molecular psychiatry at the millennium

“…52,53 Degeneration of DA afferents projecting from substantia nigra to CPu further indicated that D 4 autoreceptors are unlikely to be found with presynaptic D 2 /D 3 autoreceptors or ionotropic glutamate NMDA, AMPA and kainate heteroceptors at nigrostriatal projection terminals. 46,49,50,54 Immunological assays Development of D 4 -selective antibodies provides another approach to selective localization of D 4 receptor proteins in brain tissue. A specific monoclonal antibody raised against the extracellular amino-terminal of the human D 4 receptor labeled cerebral cortex and substantia nigra.…”