Cholinergic receptor activation potently controls striatal levels of DA, a neuromodulator crucial for the expression of coordinated motor activity and Pavlovian cue-reward associations (reviewed by Wise, 2004
; Sulzer, 2011
). In this report, we characterize the effects of selective CIN activation on accumbal DA levels. We find that, while having a relatively sparse distribution, CINs profoundly modulate DAergic output in NAc.
We show that selective optogenetic stimulation of CINs evokes DA release in a β2-containing nAChR-dependent manner. While electrophysiological studies have hypothesized that dopamine can be released in a manner that is not contingent upon ongoing activity in dopaminergic fibers (Ding et al., 2010), our data reveal previously unseen dynamics of this release process directly. Furthermore, we identify the convergence of different neurotransmitter systems participating in this phenomenon. Increased DA concentration during blockade of mAChRs suggests a critical role of these receptors in controlling ACh release. Consistent with recent reports demonstrating glutamate release from CIN terminals, interfering with AMPA receptor signaling weakens optically evoked DA release. More importantly, we determine that DA release can also be evoked by blue light activation of CINs in vivo.
Study of frequency-dependent relations between CIN stimulation and DA levels showed a clear paired-pulse depression, suggesting strong mechanisms of presynaptic control of release at either, or both, CIN and DA neuron terminals. Although this has been described separately at DA and ACh synapses, more detailed studies are necessary to demonstrate how interactions between these two sites of release interact into determining final DA levels. Moreover, we report that sustained optical stimulation of CINs does not mimic the nicotine-dependent high-pass filtering of electrically evoked DA release (Exley and Cragg, 2008
). Together, these results point to a crucial role of mAChR activation in limiting the effects of persistent endogenous ACh activity on nAChRs. This feedback mechanism is absent under the effect of nicotine, which promotes desensitization of nAChRs, thought to be the main mechanism underlying nicotine-evoked high-pass filtering of DA release (Rice and Cragg, 2004
; Exley and Cragg, 2008
). In support of this notion, we confirmed that β2-containing nAChRs mediate ACh-evoked release of DA, and that mAChRs play a predominant role in limiting endogenous ACh release, because ACh-evoked DA release is enhanced (albeit modestly) following blockade of mAChRs.
Glutamate modulates DA release by acting on dopaminergic terminals (Chéramy et al., 1986b
; Krebs et al., 1991
; Chéramy et al., 1998
) and because CINs mediate glutamatergic transmission (Guzman et al., 2011
; Higley et al., 2011
), we hypothesized that a fraction of the DA released by selective stimulation of CINs involves activation of glutamate receptors. Supporting this view, we found that CIN-evoked DA release relies –at least partially– on activation of AMPA receptors. This establishes even broader implications, given that glutamate released from CINs mediates not only excitation of MSNs, as previously described (Higley et al., 2011
), but also shapes accumbal DA release.
The present experiments uncover a multiplicity of regulatory mechanisms that converge to control DA release elicited by the selective activation of CINs. In behaving animals, CINs encode reward-related events (Morris et al., 2004
). While DA neurons increase or decrease their basal firing rate in response to the presentation or omission of reward, CINs respond with a brief pause independently of the outcome (Aosaki et al., 1994
; Morris et al., 2004
). This has been interpreted as the establishment of the appropriate temporal window for contingencies to be encoded, while DAergic responses are theorized to carry a learning signal about future outcomes (Morris et al., 2004
). Here, we determined that in vivo
DA release is in fact triggered by endogenous release of ACh. This allows new considerations to be taken into account for the way that CIN activity may set the stage for DA neuron activity to produce its postsynaptic effects. Reward-related activity of CINs consists of several phases (initial rise, pause, and second rise) (Morris et al., 2004
; Aosaki et al., 1994
; Shimo and Hikosaka, 2001
; Apicella et al., 1991
; Apicella, 2007
). In response to reward, the peak of the initial phase coincides with the rise in DA neuron activity. We speculate that the initial rise phase of CIN firing rate and subsequent ACh-Glu release could act as a priming event, exciting MSN neurons and boosting DA release originating from the midbrain, whereas the transition to the pause in CIN activity may allow for the hypothesized contrast enhancement of the midbrain signal (Zhang and Sulzer, 2004
; Cragg, 2006
; Nicola et al., 2004
). Moreover, activation of nAChRs promotes long-term depression of corticostriatal glutamatergic transmission via regulation of DA release (Partridge et al., 2002
), and thus our findings provide evidence of a link between CIN activity and synaptic plasticity implicated in reinforcement learning. Our results generate a novel conceptual framework with which to interpret the regulation of accumbal DA release and its role in reward-directed behaviors.