The Effects of SKF 83959 on Time-Based Operant Behaviors and the Underlying Neural Mechanisms

近年來與多巴胺功能相關的研究，除探討各型多巴胺受體引發細胞內訊息傳遞機制，亦開始檢測其次級受體之間如何互動及其功能作用為何。近期研究在大腦中的紋狀體發現了由第一型多巴胺受體（D1R）與第二型多巴胺受體（D2R）所共同組成的雜二聚體（heterodimer），其活化會激發一連串有別於傳統多巴胺傳導的訊息傳遞路徑，其中包括經磷脂酶C調控的磷酸肌醇之水解，以及後續的細胞內鈣離子釋放。目前，此D1-D2 雜二聚體在活體（in vivo）層級的功能不明，仍尚待釐清。因此，本研究以一個屬於D1-D2 雜二聚體致效劑的SKF 83959藥物，檢測其對大鼠於兩項與時距有關的操作式制約行為表現（FI 30-s與DRL 10-s）和自發性活動量之影響，並測量四個大腦多巴胺相關區域的特定蛋白質表現受此藥物行為作用之影響。本研究結果發現，隨著SKF 83959藥物處理的劑量增加，大鼠於FI 30-s以及DRL 10-s作業上的反應率受到了顯著的降低，然而自發性活動量並未受到藥物效果影響。此項結果顯示SKF 83959可能對獲取酬賞物相關的內在動機歷程有影響，而不是因藥物引起運動失能之效。在生化測試蛋白質表現量的結果，SKF 83959對背側紋狀體以及伏隔核中的pCaMKII、PKA、及pCREB引發了較明顯的蛋白質表現量變化，在前額葉與海馬迴則未有此效。本研究另外嘗試藉由藥理拮抗的實驗，檢測多巴胺受器拮抗劑是否能反轉SKF 83959藥物對行為表現。實驗結果大致顯示單獨施打SCH 23390以及共同施打SCH 23390與eticlopride的前處理，無法反轉SKF83959對於操作式制約行為表現的影響；然而，低劑量的eticlopride對回復SKF 83959所引發的低反應率有部份藥理反轉的效果。綜合以上結果，SKF 83959會有不等程度的影響本研究所採之兩種時距有關的操作式制約行為，其可能涉及大腦紋狀體內CaMKII/CREB的生化作用。未來研究可考慮直接操弄細胞內蛋白質的表現量，或者觀測D1-D2 雜二聚體於活體內的活動，以提供更多關於此D1-D2 雜二聚體參與個體的行為功能與其相關神經機制。As the functions and signaling mechanisms of dopamine (DA) receptor subtypes remain popular topics of research, recent studies have also begun to investigate the interactions between different subtypes of receptors. The formation of DA D1-D2 receptor complexes was discovered in the striatum, whose activation leads to a novel signaling pathway via phospholipase C-mediated phosphoinositide hydrolysis, followed by intracellular calcium release. As the in vivo functional role of this D1-D2 receptor heteromer remains to be elucidated, the present study investigated the effects of SKF 83959, a proposed D1-D2 heteromer-selective agonist, on the performance of schedule-controlled behaviors (FI 30-s and DRL 10-s), locomotor activity, and the expression of related proteins in four terminals of the mesocorticolimbic DA system, which included the prefrontal cortex (PFC), dorsal striatum (DS), nucleus accumbens (NAc), and dorsal hippocampus. The administration of SKF 83959 was found to reduce the response rates of FI 30-s and DRL 10-s in a dose-dependent manner, whereas the locomotor activity was not affected. This suggests that SKF 83959 may have affected the processes of intrinsic motivation to obtain the reinforcers, rather than motor control. In respective to protein expression, SKF 83959 induced prominent changes in the levels of pCaMKII, PKA, and pCREB in the DS and NAc relative to the PFC and hippocampus. Experiments of pharmacological antagonism were conducted in attempts to reverse the behavioral effects of SKF 83959. The results showed that the pretreatments of SCH 23390 alone and SCH 23390 combined with eticlopride did not reverse the effects of SKF 83959 on operant behaviors. However, low dose eticlopride appeared to have a partial effect in restoring the decline in operant response rates by SKF 83959. Together, the current data showed that SKF 83959 altered the time-based operant behaviors tested to different degrees, possibly via its influence on CaMKII-CREB signaling in the NAc. Future studies that manipulate the activation of intracellular proteins or quantify the levels of D1-D2 heteromer activation may provide more information regarding the in vivo activation mechanisms of D1-D2 heteromers