Dopamine receptors in canine caudate nucleus

Abstract

This study was done to obtain direct in vitro evidence for the possible existence of more than one type of dopaminergic binding site in homogenates of the caudate nucleus from calf brain. Five radioligands for dopaminergic sites were tested. The inhibitions of two agonist radioligands ([3H]dopamine, [3H]apomorphine) by haloperidol, chlorpro-mazine, or piflutixol were biphasic. The inhibitions of [3H-haloperidol, [3H]spiroperidol, and [3Hldihydroergocryptine binding by dopamine and (-)-norepinephrine were also bi-phasic. Thus, both 3H-labeled agonists and 3H-labeled antagonists were possibly binding to two high-affinity sites. There are several fundamental issues as yet unresolved concerning the nature of the dopaminergic receptor sites in the brain. A major question is whether or not there exists more than one type of dopamine receptor in the brain. Compelling evidence from in vio studies (1-5) points to the existence of presynaptic receptors (or autoreceptors) in addition to the classical type of postsynaptic receptor. Cools et al. have even further subdivided (from in vivo data) the pre-and postsynaptic receptors into excitatory and inhibitory classes (6). There has not been, however, any direct evidence for such multiple brain dopamine receptors from direct radioreceptor studies in vitro (7). Hitherto, only a single high-affinity site has been described for such dopamine receptor 3H-labeled ligands as [3H]dopamine (8-14), [3H]haloperidol (7-16), [3H]apo-morphine (17), and [3H]spiperone (18, 19). Recently, however, using [3H]dihydroergocryptine to tag dopamine receptors (in the presence of excess phentolamine to preclude binding to a-adrenergic sites), we obtained direct evidence for two dopaminergic binding sites (20). Encouraged by these observations with [3H]dihydroergocryptine, we reexamined in detail the binding patterns of the other four dopaminergic 3H-labeled ligands and found the same type of evidence for at least two high-affinity dopaminergic binding sites in each case. MATERIALS AND METHODS Preparation of Calf Caudate Nucleus Homogenates. The experiments were done on crude homogenates of calf caudate nucleus, prepared as previously described (16). In order to retain all the dopamine receptors, the homogenates were not purified or subfractionated. Calf brains were obtained fresh from the Canada Packers Hunnisett plant (Toronto). The caudates were removed within 2 hr after death, pooled, sliced into small cubes, and suspended in buffer at an approximate concentration of 50 mg of wet weight per ml of buffer. The buffer contained 15 mM Tris-HCI (pH 7.4), 5 mM Na2EDTA, 1.1 mM ascorbate, and 12.5 ,uM nialamide. A preliminary crude homogenate of the suspension was made using a glass homogenizer with a Teflon piston (0. 12-0.18 mm clearance). This piston, rotating The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U. S. C. §1734 solely to indicate this fact. at 500 rpm, was passed ...

Limbic and cortical information processing in the nucleus accumbens

“…Conversely, administration of a D 2 agonist attenuates and D 2 antagonist facilitates cortical inputs without affecting limbic inputs [33], indicating that (i) limbic inputs are selectively modulated by D 1 -receptor stimulation, (ii) cortical inputs are selectively modulated by D 2 -receptor stimulation and (iii) low baseline concentration of DA tonically attenuates cortical input via D 2 -receptor stimulation. This is consistent with DA-receptor-binding assays showing that D 2 receptors exhibit higher affinity for DA than do the D 1 receptors [34]. …”

Decreased Striatal Dopamine Release Underlies Increased Expression of Long-Term Synaptic Potentiation at Corticostriatal Synapses 24 h after 3-Nitropropionic-Acid-Induced Chemical Hypoxia

“…Reynolds et al (2001) and Reynolds and Wickens (2002) proposed a similar theory to explain D2 receptor dependent LTD and D1 receptor-dependent LTP at corticostriatal synapses. Our data support these theories because 1) LTP is enhanced when 3-NP reduces dopamine release in the striatum and 2) exogenous addition of dopamine or the D2 receptor agonist quinpirole eliminates the 3-NP enhanced LTP and increases LTD. More LTP is seen in the dorsomedial striatum as are more D1 receptors and thus, when dopamine is reduced it has a greater probability of binding to D1 receptors (Ariano et al, 1989; Maeno, 1982; Partridge et al, 2000; Smith et al, 2001). By contrast, normal dopamine release in control slices or addition dopamine to dopamine-depleted brain slices increases the probability of dopamine binding to D2 receptors, which increases LTD expression at corticostriatal synapses (Akopian and Walsh, 2006; Calabresi et al, 1992; Kreitzer and Malenka, 2007; Wang et al, 2006).…”

“…However, our selection of the dorsomedial striatum increased the likelihood of D1 receptor participation over D2 receptors (Ariano et al, 1989; Maeno, 1982). Wang et al (2006) found dopamine triggers D2 receptor-dependent LTD by acting presynaptically to reduce acetyl choline (ACh) release from cholinergic interneurons.…”

Aversive behavior induced by optogenetic inactivation of ventral tegmental area dopamine neurons is mediated by dopamine D2 receptors in the nucleus accumbens

“…MSNs, and D2R is predominantly expressed in the enkephalin (coded by Penk gene)-expressing MSNs; each type of MSNs constitutes the direct and indirect pathways, respectively, in the NAc (3). As the affinity for DA is much higher for the D2R (nM order) than for the D1R (μM order) (22,23), a reduction in DA levels is thought to result in the inactivation of G i -coupled D2R but to have no appreciable effect on the D1R (3, 24), thereby up-regulating the neural activity specifically in the indirect pathway. Moreover, the Fos activation was more prominently observed in Penk-or Drd2 (D2R)-expressing cells than in the Tac1-or Drd1a (D1R)-expressing cells (Fig.…”