More importantly, the upstream structures that drive their activity will have to be identified. It will be interesting to explore whether all VTA GABA neurons receive direct excitatory inputs from the lateral habenula, as has been demonstrated for the RMTg neurons in rats (Hong et al., 2011 and Balcita-Pedicino et al., 2011). Our data indicate that the footshock-induced DA neuron inhibition primarily
relies on GABAA receptor transmission (van Zessen et al., 2012). Neither GABAB receptors nor D2 receptors, both present on DA neurons and reported to mediate a slow IPSC (Cruz Selleckchem PD-1/PD-L1 inhibitor 2 et al., 2004 and Beckstead et al., 2004), seem to play an important role. Anatomical studies have also identified a small number of VTA GABA neurons that project to the nucleus accumbens. Our optogenetic manipulation most likely also activated these neurons. However, given their small number (Xia et al., 2011), it is unlikely that they would significantly contribute to the behavioral effects. It is also possible that their contribution may be masked when most DA neurons are inhibited, as in the experiments presented here. In fact, the observation of the behavioral effect was very similar in magnitude as well as the timing with which the aversion was induced; suggesting that the two
manipulations have a very similar effect on the circuit that mediates this behavior. While there is heterogeneity among DA neurons, inhibiting the majority either directly or indirectly causes strong aversion. Taken together, VTA GABA neurons, Selleck Vorinostat when activated optogenetically, may mimic the contextual aversion typically observed with a footshock. A brief electric shock to the paw is indeed a strongly aversive stimulus
for a mouse that initially causes a flight behavior, followed by place aversion and freezing, particularly when the setting precludes an escape (Lázaro-Muñoz et al., 2010). The freezing behavior observed in the unconditioned chamber may reflect a state of generalized anxiety. Interestingly a similar observation was made when before mice with decreased DA neuron excitability were exposed to repetitive footshocks (Zweifel et al., 2011). Since the amygdala plays a central role in aversion and freezing behavior, a decreased VTA-amygdala output could code for the aversive nature of the stimulus. As in the striatum, there may be some form of synaptic plasticity in the amygdala that can only be expressed in the absence of DA signaling (Shen et al., 2008). Most interestingly, the observation that the aversion persisted during the test session suggests a strong learning effect by DA neuron inhibition. Future studies will have to firm up this finding with more elaborate behavioral protocols and test for retention after longer intervals. It will also be important to identify the molecular learning mechanisms engaged by inhibition of DA neurons and parse the contribution of the neurons that are activated by the footshock (Brischoux et al.