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Dissecting neural circuitry in non-human primates (NHP) is crucial to identify potential neuromodulation anatomical targets for the treatment of pharmacoresistant neuropsychiatric diseases by electrical neuromodulation. How targets of deep brain stimulation (DBS) and cortical targets of transcranial magnetic stimulation (TMS) compare and might complement one another is an important question. Combining optogenetics and tractography may enable anatomo-functional characterization of large brain cortico-subcortical neural pathways. For the proof-of-concept this approach was used in the NHP brain to characterize the motor cortico-subthalamic pathway (m_CSP) which might be involved in DBS action mechanism in Parkinson’s disease (PD). Rabies-G-pseudotyped and Rabies-G-VSVg-pseudotyped EIAV lentiviral vectors encoding the opsin ChR2 gene were stereotaxically injected into the subthalamic nucleus (STN) and were retrogradely transported to the layer of the motor cortex projecting to STN. A precise anatomical mapping of this pathway was then performed using histology-guided high angular resolution MRI tractography guiding accurately cortical photostimulation of m_CSP origins. Photoexcitation of m_CSP axon terminals or m_CSP cortical origins modified the spikes distribution for photosensitive STN neurons firing rate in non-equivalent ways. Optogenetic tractography might help design preclinical neuromodulation studies in NHP models of neuropsychiatric disease choosing the most appropriate target for the tested hypothesis.

Optogenetic neuromodulation techniques, which have emerged during the last 15 years, have considerably enhanced our ability to probe the functioning of neural circuits by allowing the excitation and inhibition of genetically-defined neuronal populations using light. Having gained tremendous popularity in the field of fundamental neuroscience, these techniques are now opening new therapeutic avenues. Optogenetic neuromodulation is a method of choice for studying the physiopathology of neurological and neuropsychiatric disorders in a range of animal models, and could accelerate the discovery of new therapeutic strategies. New therapeutic protocols employing optogenetic neuromodulation may also emerge in the near future, offering promising alternative approaches for disorders which lack appropriate treatments, such as pharmacoresistant epilepsy and inherited retinal degeneration.

Serotonin is implicated in mood and affective disorders. However, growing evidence suggests that a core endogenous role is to promote flexible adaptation to changes in the causal structure of the environment, through behavioral inhibition and enhanced plasticity. We used long-term photometric recordings in mice to study a population of dorsal raphe serotonin neurons, whose activity we could link to normal reversal learning using pharmacogenetics. We found that these neurons are activated by both positive and negative prediction errors, and thus report signals similar to those proposed to promote learning in conditions of uncertainty. Furthermore, by comparing the cue responses of serotonin and dopamine neurons, we found differences in learning rates that could explain the importance of serotonin in inhibiting perseverative responding. Our findings show how the activity patterns of serotonin neurons support a role in cognitive flexibility, and suggest a revised model of dopamine–serotonin opponency with potential clinical implications. DOI: http://dx.doi.org/10.7554/eLife.20552.001, eLife digest Serotonin is a molecule that plays various roles in the human body. In the brain, it is involved in regulating mood and emotions. Growing evidence suggests that serotonin also helps animals – including humans – adapt their behavior to changes in their environment. To allow for such behavioral flexibility, serotonin might promote changes in the underlying brain structures and activity. In a type of learning known as ‘reversal learning’, for instance, it is necessary to adapt to a sudden change in a previously familiar environment. For example, if there were a road closure on a person’s way to work, they might want to learn to stop following their usual route and learn a new and better one. Previous research has shown that when serotonin signaling is reduced, people persevere. That is, they will keep following the old route even if it is no longer the best choice. How this process works is still largely unknown. To start unraveling these mechanisms, Matias et al. trained mice in a reversal learning task while manipulating and recording the activity of the neurons that produce serotonin. The results showed that when the activity in serotonin neurons was experimentally blocked, the mice tended to keep looking for a reward that was no longer available. Then, by recording the activity of serotonin neurons, Matias et al. found that it was the surprise of discovering a change in a previously familiar environment that activates serotonin neurons. It did not matter whether the change was better or worse than expected. The findings suggest that together with dopamine, another molecule involved in learning from rewards, serotonin could play an important role during reversal learning. One next step will be to determine if serotonin mainly stops perseverance in its tracks, or whether it works by helping to unlearn the old behavior, or a combination of both. In the future, this could further our understanding of depression, which can be viewed as a disorder characterized by patients being unable to adapt to adverse situations, leaving them trapped to repeat behaviors and thoughts that are not beneficial. Future studies could also build on these findings to guide the development of new treatments for depression that involve serotonin. DOI: http://dx.doi.org/10.7554/eLife.20552.002

Sensorimotor integration is crucial to perception and motor control. How and where this process takes place in the brain is still largely unknown. Here we analyze the cerebellar contribution to sensorimotor integration in the whisker system of mice. We identify an area in the cerebellum where cortical sensory and motor inputs converge at the cellular level. Optogenetic stimulation of this area affects thalamic and motor cortex activity, alters parameters of ongoing movements and thereby modifies qualitatively and quantitatively touch events against surrounding objects. These results shed light on the cerebellum as an active component of sensorimotor circuits and show the importance of sensorimotor cortico-cerebellar loops in the fine control of voluntary movements.

Climbing fibers, the projections from the inferior olive to the cerebellar cortex, carry sensorimotor error and clock signals that trigger motor learning by controlling cerebellar Purkinje cell synaptic plasticity and discharge. Purkinje cells target the deep cerebellar nuclei, which are the output of the cerebellum and include an inhibitory GABAergic projection to the inferior olive. This pathway identifies a potential closed loop in the olivo-cortico-nuclear network. Therefore, sets of Purkinje cells may phasically control their own climbing fiber afferents. Here, using in vitro and in vivo recordings, we describe a genetically modified mouse model that allows the specific optogenetic control of Purkinje cell discharge. Tetrode recordings in the cerebellar nuclei demonstrate that focal stimulations of Purkinje cells strongly inhibit spatially restricted sets of cerebellar nuclear neurons. Strikingly, such stimulations trigger delayed climbing-fiber input signals in the stimulated Purkinje cells. Therefore, our results demonstrate that Purkinje cells phasically control the discharge of their own olivary afferents and thus might participate in the regulation of cerebellar motor learning.

The cerebellar cortex coordinates movements and maintains balance by modifying motor commands as a function of sensory-motor context, which is encoded by mossy fiber (MF) activity. MFs exhibit a wide range of activity, from brief precisely timed high-frequency bursts, which encode discrete variables such as whisker stimulation, to low-frequency sustained rate-coded modulation, which encodes continuous variables such as head velocity. While high-frequency MF inputs have been shown to activate granule cells (GCs) effectively, much less is known about sustained low-frequency signaling through the GC layer, which is impeded by a hyperpolarized resting potential and strong GABAA-mediated tonic inhibition of GCs. Here we have exploited the intrinsic MF network of unipolar brush cells to activate GCs with sustained low-frequency asynchronous MF inputs in rat cerebellar slices. We find that low-frequency MF input modulates the intrinsic firing of Purkinje cells, and that this signal transmission through the GC layer requires synaptic activation of Mg2+-block-resistant NMDA receptors (NMDARs) that are likely to contain the GluN2C subunit. Slow NMDAR conductances sum temporally to contribute approximately half the MF-GC synaptic charge at hyperpolarized potentials. Simulations of synaptic integration in GCs show that the NMDAR and slow spillover-activated AMPA receptor (AMPAR) components depolarize GCs to a similar extent. Moreover, their combined depolarizing effect enables the fast quantal AMPAR component to trigger action potentials at low MF input frequencies. Our results suggest that the weak Mg2+block of GluN2C-containing NMDARs enables transmission of low-frequency MF signals through the input layer of the cerebellar cortex.

SummaryTonic motor control involves oscillatory synchronization of activity at low frequency (5–30 Hz) throughout the sensorimotor system, including cerebellar areas. We investigated the mechanisms underpinning cerebellar oscillations. We found that Golgi interneurons, which gate information transfer in the cerebellar cortex input layer, are extensively coupled through electrical synapses. When depolarized in vitro, these neurons displayed low-frequency oscillatory synchronization, imposing rhythmic inhibition onto granule cells. Combining experiments and modeling, we show that electrical transmission of the spike afterhyperpolarization is the essential component for oscillatory population synchronization. Rhythmic firing arises in spite of strong heterogeneities, is frequency tuned by the mean excitatory input to Golgi cells, and displays pronounced resonance when the modeled network is driven by oscillating inputs. In vivo, unitary Golgi cell activity was found to synchronize with low-frequency LFP oscillations occurring during quiet waking. These results suggest a major role for Golgi cells in coordinating cerebellar sensorimotor integration during oscillatory interactions.

International audience; Organization of behavior requires rapid coordination of brainstem and forebrain activity. The exact mechanisms of effective communication between these regions are presently unclear. The intralaminar thalamic nuclei (IL) probably serves as a central hub in this circuit by connecting the critical brainstem and forebrain areas. We found that GABAergic and glycinergic fibers ascending from the pontine reticular formation (PRF) of the brainstem evoked fast and reliable inhibition in the IL via large, multisynaptic terminals. This inhibition was fine-tuned through heterogeneous GABAergic and glycinergic receptor ratios expressed at individual synapses. Optogenetic activation of PRF axons in the IL of freely moving mice led to behavioral arrest and transient interruption of awake cortical activity. An afferent system with comparable morphological features was also found in the human IL. These data reveal an evolutionarily conserved ascending system that gates forebrain activity through fast and powerful synaptic inhibition of the IL.

The inhibition of sensory responsivity is considered a core serotonin function, yet this hypothesis lacks direct support due to methodological obstacles. We adapted an optogenetic approach to induce acute, robust and specific firing of dorsal raphe serotonergic neurons. In vitro, the responsiveness of individual dorsal raphe serotonergic neurons to trains of light pulses varied with frequency and intensity as well as between cells, and the photostimulation protocol was therefore adjusted to maximize their overall output rate. In vivo, the photoactivation of dorsal raphe serotonergic neurons gave rise to a prominent light-evoked field response that displayed some sensitivity to a 5-HT1A agonist, consistent with autoreceptor inhibition of raphe neurons. In behaving mice, the photostimulation of dorsal raphe serotonergic neurons produced a rapid and reversible decrease in the animals' responses to plantar stimulation, providing a new level of evidence that serotonin gates sensory-driven responses. ERC 250334, 5-HT Optogenetic MSCA 220098 info:eu-repo/semantics/publishedVersion

The basal ganglia, and the striatum in particular, have been implicated in the generation of contraversive movements. The striatum projects to downstream basal ganglia nuclei through two main circuits, originating in striatonigral and striatopallidal neurons, and different models postulate that the two pathways can work in opposition or synergistically. Here we show striatonigral and striatopallidal neurons are concurrently active during spontaneous contraversive movements. Furthermore, we show that unilateral optogenetic inhibition of either or both projection pathways disrupts contraversive movements. Consistently, simultaneous activation of both neuron types produces contraversive movements. Still, we also show that imbalanced activity between the pathways can result in opposing movements being driven by each projection pathway. These data show that balanced activity in both striatal projection pathways is critical for the generation of contraversive movements and highlights that imbalanced activity between the two projection pathways can result in opposing motor output., The striatum is required for evoking contraversive movements from each brain hemisphere, but it is unclear how. Here, Tecuapetla et al. use optogenetics to inhibit direct and indirect downstream striatal projection pathways, and show that activity in both pathways is necessary for contraversive movements.

The olfactory bulb is densely innervated by serotonergic fibers. A study now shows that serotonin activates periglomerular interneurons, which release GABA to reduce transmitter release from olfactory sensory neurons.

Inhibitory synapses display a great diversity through varying combinations of presynaptic GABA and glycine release and postsynaptic expression of GABA and glycine receptor subtypes. We hypothesized that increased flexibility offered by this dual transmitter system might serve to tune the inhibitory phenotype to the properties of afferent excitatory synaptic inputs in individual cells. Vestibulocerebellar unipolar brush cells (UBC) receive a single glutamatergic synapse from a mossy fiber (MF), which makes them an ideal model to study excitatory–inhibitory interactions. We examined the functional phenotypes of mixed inhibitory synapses formed by Golgi interneurons onto UBCs in rat slices. We show that glycinergic IPSCs are present in all cells. An additional GABAergic component of large amplitude is only detected in a subpopulation of UBCs. This GABAergic phenotype is strictly anti-correlated with the expression of type II, but not type I, metabotropic glutamate receptors (mGluRs) at the MF synapse. Immunohistochemical stainings and agonist applications show that global UBC expression of glycine and GABAAreceptors matches the pharmacological profile of IPSCs. Paired recordings of Golgi cells and UBCs confirm the postsynaptic origin of the inhibitory phenotype, including the slow kinetics of glycinergic components. These results strongly suggest the presence of a functional coregulation of excitatory and inhibitory phenotypes at the single-cell level. We propose that slow glycinergic IPSCs may provide an inhibitory tone, setting the gain of the MF to UBC relay, whereas large and fast GABAergic IPSCs may in addition control spike timing in mGluRII-negative UBCs.

Fundamental questions that neuroscientists have previously approached with classical biochemical and electrophysiological techniques can now be addressed using optogenetics. The term optogenetics reflects the key program of this emerging field, namely, combining optical and genetic techniques. With the already impressively successful application of light-driven actuator proteins such as microbial opsins to interact with intact neural circuits, optogenetics rose to a key technology over the past few years. While spearheaded by tools to control membrane voltage, the more general concept of optogenetics includes the use of a variety of genetically encoded probes for physiological parameters ranging from membrane voltage and calcium concentration to metabolism. Here, we provide a comprehensive overview of the state of the art in this rapidly growing discipline and attempt to sketch some of its future prospects and challenges.