Jesus Bertran-Gonzalez

Goal-directed actions are established through the encoding of predictive relationships between behaviour and its consequences, a process involving dopamine-dependent plasticity in the dorsal striatum. Early phases of learning entail complex associative landscapes from which useful action-outcome contingencies must be identified. Although dopamine processes are central to associative learning, the cellular mechanisms through which behaviourally relevant plasticity is selected from competing signalling events remain unknown. Here we show that D2-spiny projection neurons (D2-SPNs) are transiently recruited during the earliest stages of instrumental learning, when critical action-outcome contingencies emerge. We identify a dominant cell-to-cell transmodulation process by which D2-SPNs regulate dopamine-dependent signalling by constraining protein kinase A activity in D1-SPNs. This mechanism overrides extracellular dopamine signals and can be reproduced in vivo through calibrated optogenetic activation of cAMP signalling. Enhancing D2→D1-SPN transmodulation during uncertain reinforcement schedules durably improves action-outcome encoding, revealing a mechanism through which striatal networks promote relevant contingencies under associative ambiguity.

The phospholipid and free fatty acid (FFA) composition of neuronal membranes plays a crucial role in learning and memory, but the mechanisms through which neuronal activity affects the brain’s lipid landscape remain largely unexplored. Saturated FFAs, particularly myristic acid (C14:0), strongly increase during neuronal stimulation and memory acquisition, suggesting the involvement of phospholipase A1 (PLA1) activity in synaptic plasticity. Here, we show that genetic ablation of the DDHD2 isoform of PLA1 in mice reduced memory performance in reward-based learning and spatial memory models prior to the development of neuromuscular deficits, and markedly reduced saturated FFAs across the brain. DDHD2 was shown to bind to the key synaptic protein STXBP1. Using STXBP1/2 knockout neurosecretory cells and a haploinsufficient STXBP1 ^+/- mouse model of STXBP1 encephalopathy that is also associated with intellectual disability and motor dysfunction, we show that STXBP1 controls the targeting of DDHD2 to the plasma membrane and the generation of saturated FFAs in the brain. Our findings suggest key roles for DDHD2 and STXBP1 in the lipid metabolism underlying synaptic plasticity, learning and memory.

Multidisciplinary evidence suggests that instrumental performance is governed by two major forms of behavioural control: goal-directed and autonomous processes. Brain-state abnormalities affecting the striatum, such as ageing, often shift control towards autonomous—habit-like—behaviour, although the neural mechanisms responsible for this shift remain unknown. Here, combining instrumental conditioning with cell-specific functional mapping and manipulation in striatal neurons, we explored strategies that invigorate goal-directed action capacity in aged mice. In animals performing instrumental actions, D2- and D1-neurons of the aged striatum were engaged in a characteristically counterbalanced manner, something that related to the propensity to express autonomous behaviour. Long-lasting, cell-specific desensitisation of D2-neurons in aged transgenic mice recapitulated the uneven D2-to D1-neuron functional correspondence observed in young mice, an effect that enabled successful goal-directed action. Our findings contribute to the understanding of the neural bases of behavioural control and propose neural system interventions that enhance cognitive functioning in habit-prone brains.

Performing several actions in swift succession is often necessary to exploit known contingencies in the environment. However, in order to remain successful when contingency rules change, streamlined action sequences must be adaptable. Here, by combining analyses of behavioural microstructure with circuit-specific manipulation in mice, we report on a relationship between action timing variability and successful adaptation that relies on post-synaptic targets of primary motor cortical (M1) projections to dorsolateral striatum (DLS). Using a two-lever instrumental task, we found that mice build successful action sequences by first establishing action scaffolds, from which they dynamically elongate as task requirements extend. Specific interruption of the M1→DLS circuit altered these dynamics, prompting actions that were less variable in their timing, overall reducing opportunities for success. Our results reveal a role for M1→DLS circuitry in setting the exploration/exploitation balance that is required for adaptively guiding the timing and success of instrumental action. Based on evidence from transsynaptic tracing experiments, we propose that such function may involve additional downstream subcortical processing relating to collateralisation of descending motor pathways to multiple basal ganglia centres.

Psychostimulants such as amphetamine target dopamine neuron synapses to engender drug-induced plasticity. While dopamine neurons modulate the activity of striatal cholinergic interneurons (ChIs) with regional heterogeneity, how amphetamine affects ChI activity has not been elucidated. Here, we applied quantitative fluorescence imaging approaches to map the dose-dependent effects of a single dose of amphetamine on ChI activity at 2.5 and 24 hours after injection across the mouse striatum using the activity-dependent marker phosphorylated ribosomal protein S6 (p-rpS6). We found that amphetamine did not induce neurotoxic effects on ChIs, as their distribution and morphology were not affected. While low- or high-dose amphetamine did not affect ChI activity after 2.5 hours, ChI activity was significantly reduced in all striatal subregions after 24 hours, with a dose-dependent effect in the nucleus accumbens. Thus, our findings suggest that a single dose of amphetamine has delayed regionally heterogeneous effects on ChI activity. Significance statement Using the activity dependent marker phosphorylated ribosomal protein S6 (p-rpS6), we mapped amphetamine effects on the activity of cholinergic interneurons (ChIs) across the striatum. Amphetamine caused a delayed attenuation of ChI activity in all striatal subregions, and a dose-dependent effect in the ventral striatum/nucleus accumbens, a critical site of psychostimulant action.

Summary: The acquisition of goal-directed action requires the encoding of specific action-outcome associations involving plasticity in the posterior dorsomedial striatum (pDMS). We first investigated the relative involvement of the major inputs to the pDMS argued to be involved in this learning-related plasticity, from prelimbic prefrontal cortex (PL) and from the basolateral amygdala (BLA). Using ex vivo optogenetic stimulation of PL or BLA terminals in pDMS, we found that goal-directed learning potentiated the PL input to direct pathway spiny projection neurons (dSPNs) bilaterally but not to indirect pathway neurons (iSPNs). In contrast, learning-related plasticity was not observed in the direct BLA-pDMS pathway. Using toxicogenetics, we ablated BLA projections to either pDMS or PL and found that only the latter was necessary for goal-directed learning. Importantly, transient inactivation of the BLA during goal-directed learning prevented the PL-pDMS potentiation of dSPNs, establishing that the BLA input to the PL is necessary for the corticostriatal plasticity underlying goal-directed learning.

A learning mechanism in the striatum: An intriguing characteristic of the striatum is the random spatial distribution and high degree of intermingling between expression of dopamine receptor types 1 (D1) and 2 (D2) within striatal projection neurons (SPNs). The resulting highly entropic mosaic extends through a homogeneous space and is mostly devoid of histological boundaries. The rules established locally by D1- and D2-expressing SPNs (D1-SPNs and D2-SPNs) are thus likely critical in defining how functional territories develop throughout the striatum. Matamales et al. found that activated D2-SPNs access and modify developing behavioral programs encoded by regionally defined ensembles of transcriptionally active D1-SPNs. This process is slow because it depends on the molecular integration of additive neuro-modulatory signals. However, with time, it creates the regional functional boundaries that are necessary to identify and shape specific learning in the striatum. Science , this issue p. 549

Extinction learning allows animals to withhold voluntary actions that are no longer related to reward and so provides a major source of behavioral control. Although such learning is thought to depend on dopamine signals in the striatum, the way the circuits mediating goal-directed control are reorganized during new learning remains unknown. Here, by mapping a dopamine-dependent transcriptional activation marker in large ensembles of striatal projection neurons (SPNs) expressing dopamine receptor type 1 (D1-SPNs) or 2 (D2-SPNs) in mice, we demonstrate an extensive and dynamic D2- to D1-SPN trans-modulation across the dorsal striatum that is necessary for updating previous goal-directed learning. Our findings suggest that D2-SPNs suppress the influence of outdated D1-SPN plasticity within functionally relevant striatal territories to reshape volitional action.