Adolescent dopamine neurons represent reward differently during action and state guided learning

The neuronal underpinning of learning cause-and-effect associations in the adolescent brain remains poorly understood. Two fundamental forms of associative learning are Pavlovian (classical) conditioning, where a stimulus is followed by an outcome, and operant (instrumental) conditioning, where outcome is contingent on action execution. Both forms of learning, when associated with a rewarding outcome, rely on midbrain dopamine neurons in the ventral tegmental area (VTA) and substantia nigra (SN). We find that in adolescent male rats, reward-guided associative learning is encoded differently by midbrain dopamine neurons in each conditioning paradigm. Whereas simultaneously recorded VTA and SN adult neurons have a similar phasic response to reward delivery during both forms of conditioning, adolescent neurons display a muted reward response during operant but a profoundly larger reward response during Pavlovian conditioning suggesting that adolescent neurons assign a different value to reward when it is not gated by action. The learning rate of adolescents and adults during both forms of conditioning was similar further supporting the notion that differences in reward response in each paradigm are due to differences in motivation and independent of state versus action value learning. Static characteristics of dopamine neurons such as dopamine cell number and size were similar in the VTA and SN but there were age differences in baseline firing rate, stimulated release and correlated spike activity suggesting that differences in reward responsiveness by adolescent dopamine neurons are not due to differences in intrinsic properties of these neurons but engagement of different networks.

Improving Well-Being and Survival in the 6-OHDA Lesion Model of Parkinson´s Disease in Mice: Step-By-Step Protocol

Background: Parkinson’s disease (PD) is the most common neurodegenerative motor disorder and primarily affects movement control but also a range on non-motor functions. With unknown cause and lack of cure, much research is dedicated to find treatment. PD is characterized by progressive degeneration of midbrain dopamine neurons, primarily those of the substantia nigra pars compacta (SNc). Due to the complexity of the disease, animal models intended to represent symptoms similar to those observed in PD patients are instrumental to advance treatment prospects for PD. Among these, the 6-hydroxydopamine (6-OHDA) lesion model, in which dopaminergic neurons are chemically destroyed, is often favoured. However, while reproducing several features of clinical PD, including motor symptoms, mice exposed to 6-OHDA often suffer systemic dysfunction causing premature death. To avoid unnecessary spill of lives of laboratory mice and to increase the reliability of data obtained, there is a need for improved experimental protocols. Here we tested the effects of three parameters; lowered dose of toxin, careful post-operative care, and shortened interval between injection and sacrifice. Results: A detailed 6-OHDA lesion protocol using lower dose of toxin than commonly seen in the literature alongside careful post-operative care and decreased time post-injection resulted in high survival rate. Successful degeneration of midbrain dopamine neurons was confirmed in the SNc using several markers, and nigrostriatal projections were lost in the lesioned hemisphere. Dopamine neurons of the ventral tegmental area (VTA) were substantially affected as well. Conclusions: We demonstrate parameters that can be improved to increase well-being and survival of mice while preserving characteristic parkinsonian features in the 6-OHDA lesion model. A step-by-step protocol for implementation in any laboratory is provided.