Salamone J D, Wisniecki A, Carlson B B, Correa M
Department of Psychology, University of Connecticut, Storrs, CT 06269-1020, USA.
Neuroscience. 2001;105(4):863-70. doi: 10.1016/s0306-4522(01)00249-4.
It has been suggested that dopamine in nucleus accumbens is involved in the process of enabling organisms to overcome work-related response costs. One way of controlling work costs with operant schedules is to use fixed ratio schedules with different ratio requirements. In the present study, the effects of nucleus accumbens dopamine depletions were investigated using six schedules: fixed ratio 5, 20, 50, 100, 200, and 300. In the first three schedules the food reinforcement consisted of one 45 mg food pellet per ratio completed. In the remaining schedules the food reinforcement per ratio completed was increased to two pellets for fixed ratio 100, four pellets for fixed ratio 200, and six pellets for fixed ratio 300. All rats were trained extensively prior to surgery, and rats were able to maintain high levels of responding on all schedules up to the fixed ratio 300. After training, rats were injected with either ascorbate vehicle or 6-hydroxydopamine into the nucleus accumbens. Rats were tested post-surgically on each of the schedules, with 3 days of testing per schedule. Rats with nucleus accumbens dopamine depletions exhibited behavioral deficits that were highly dependent upon the ratio value. There were small and transient effects of dopamine depletion on fixed ratio 5 lever pressing, but as the ratio value got larger the impairment became greater. On the fixed ratio 20 and 50 schedules, response rates were partially reduced in dopamine-depleted rats. Responding on the fixed ratio 200 and 300 schedules was severely impaired, and on the last day of fixed ratio 300 testing no dopamine-depleted rats obtained a single reinforcer. These data are consistent with previous reports that accumbens dopamine depletions enhance 'ratio strain', making rats more sensitive to high ratio values. The induction of ratio strain by dopamine depletions does not appear to be related to a loss of appetite, and seems to be relatively independent of the baseline rate of responding and the overall density of food reinforcement across the session. We conclude that dopamine in nucleus accumbens may be important for enabling rats to overcome behavioral constraints such as work-related response costs, and may be critical for the behavioral organization and conditioning processes that enable animals to emit large numbers of responses in the absence of primary reinforcement.
有人提出，伏隔核中的多巴胺参与使生物体克服与工作相关的反应成本的过程。使用操作性程序控制工作成本的一种方法是采用具有不同比率要求的固定比率程序。在本研究中，使用六种程序研究了伏隔核多巴胺耗竭的影响：固定比率5、20、50、100、200和300。在前三种程序中，每完成一个比率的食物强化由一粒45毫克的食物颗粒组成。在其余程序中，每完成一个比率的食物强化增加到：固定比率100时为两粒颗粒，固定比率200时为四粒颗粒，固定比率300时为六粒颗粒。所有大鼠在手术前都经过广泛训练，并且大鼠能够在高达固定比率300的所有程序上保持高水平的反应。训练后，将大鼠注射抗坏血酸载体或6-羟基多巴胺到伏隔核中。术后对大鼠在每个程序上进行测试，每个程序测试3天。伏隔核多巴胺耗竭的大鼠表现出高度依赖于比率值的行为缺陷。多巴胺耗竭对固定比率5的杠杆按压有微小且短暂的影响，但随着比率值变大，损伤变得更大。在固定比率20和50程序中，多巴胺耗竭的大鼠的反应率部分降低。在固定比率200和300程序上的反应严重受损，并且在固定比率300测试的最后一天，没有多巴胺耗竭的大鼠获得一次强化。这些数据与先前的报道一致，即伏隔核多巴胺耗竭会增强“比率应激”，使大鼠对高比率值更敏感。多巴胺耗竭引起的比率应激诱导似乎与食欲丧失无关，并且似乎相对独立于反应的基线率和整个实验过程中食物强化的总体密度。我们得出结论，伏隔核中的多巴胺对于使大鼠克服行为限制（如与工作相关的反应成本）可能很重要，并且对于使动物在没有初级强化的情况下发出大量反应的行为组织和条件作用过程可能至关重要。
