In vitro and in vivo comparison of [³H](+)-PHNO and [³H]raclopride binding to rat striatum and lobes 9 and 10 of the cerebellum: a method to distinguish dopamine D3 from D2 receptor sites.

In vitro binding characteristics of the dopamine D3/D2 antagonist [³H]raclopride were compared to the D3/D2 agonist [³H](+)-PHNO in membrane preparations from rat striatum, cerebellum Lobules 9 and 10 (CB L9,10), and other cerebellar regions. In striatum, both radioligands labeled a single binding site. [³H](+)-PHNO showed higher affinity, though lower B(max) , compared with [³H]raclopride and was sensitive to inhibition by Gpp(NH)p. [³H](+)-PHNO showed significant specific binding to CB L9,10 membranes with higher affinity compared to striatal membranes. [³H](+)-PHNO binds to a high- and a low-affinity binding site in CB L9,10 membranes; the high-affinity site was not Gpp(NH)p-sensitive. [³H](+)-PHNO did not significantly bind cerebellum left hemisphere membranes. Very low specific binding of [³H]raclopride was found in CB L9,10. The selective dopamine D3 antagonist SB-277011 did not displace the binding of either ligand to striatal membranes but potently inhibited the binding of [³H](+)-PHNO in CB L9,10 membranes. The highly selective D2 antagonist SV-156 showed the opposite profile. In vivo experiments were consistent with and supported by in vitro results. In summary, [³H](+)-PHNO and [³H]raclopride mainly label dopamine D2 receptors in rat striatum, with [³H](+)-PHNO labeling a D2(High) population. In vitro and in vivo, [³H](+)-PHNO labels CB L9,10 dopamine D3 receptors that are apparently in a high affinity state whereas [³H]raclopride gave only very low signal in this region. The present approaches appear useful for selectively labeling dopamine D3 and D2 receptors in different rat brain regions and offer the possibility to demonstrate D3 versus D2 receptor selectivity of compounds using native rat brain tissue.

Reversal of a cholinergic-induced deficit in a rodent model of recognition memory by the selective 5-HT6 receptor antagonist, Ro 04-6790.

RATIONALE: Accumulating evidence suggests a potential role for the 5-HT(6 )receptor in cognitive function and the potential use of 5-HT(6) receptor antagonists in the treatment of learning and memory disorders. OBJECTIVES: The aim of the current study was to investigate the effect of the selective 5-HT(6) receptor antagonist, Ro 04-6790, on both the performance of normal adult rats and restoration of a pharmacological disruption of memory function produced by the non-selective muscarinic receptor antagonist, scopolamine, or the dopamine D(2) receptor antagonist, raclopride, in a rodent model of recognition memory. METHODS: Passive, perceptually based, recognition memory was assessed using a novel object discrimination task. Following habituation to an arena, rats were presented with two identical objects during trial 1 (T(1)) and a novel and familiar object during trial 2 (T(2)). The time spent exploring the two objects in each trial was measured and novel object discrimination assessed in T(2). RESULTS: In the absence of drug all rats spent an equal time exploring the two identical objects in T(1) but more time exploring the novel object in T(2). Scopolamine (but not N-methylscopolamine) and raclopride both produced a dose-dependent reduction in novel object discrimination whilst the 5-HT(6) receptor antagonist, Ro 04-6790, had no effect on discrimination when given alone but completely reversed the scopolamine- but not the raclopride-induced deficit. CONCLUSION: This study demonstrates that acute administration of Ro 04-6790 reverses a cholinergic but not a dopaminergic deficit in a rodent model of recognition memory and provides further support for a role of the 5-HT(6) receptor in the regulation of cognitive function.