Superior Colliculus to VTA pathway controls orienting response and influences social interaction in mice

Substantia Nigra Dopamine Neuronal Responses to Habenular Stimulation and Foot Shock Are Altered by Lesions of the Rostromedial Tegmental Nucleus

Dopamine (DA) neurons of the substantia nigra (SN) and ventral tegmental area generally respond to aversive stimuli or the absence of expected rewards with transient inhibition of firing rates, which can be recapitulated with activation of the lateral habenula (LHb) and eliminated by lesioning the intermediating rostromedial tegmental nucleus (RMTg). However, a minority of DA neurons respond to aversive stimuli, such as foot shock, with a transient increase in firing rate, an outcome that rarely occurs with LHb stimulation. The degree to which individual neurons respond to these two stimulation modalities with the same response phenotype and the role of the RMTg is not known. Here, we record responses from single SN DA neurons to alternating activation of the LHb and foot shock in male rats. Lesions of the RMTg resulted in a shift away from inhibition to no response during both foot shock and LHb stimulation. Furthermore, lesions unmasked an excitatory response during LHb stimulation. The response correspondence within the same neuron between the two activation sources was no different from chance in sham controls, suggesting that external inputs rather than intrinsic DA neuronal properties are more important to response outcome. These findings contribute to a literature that shows a complex neurocircuitry underlies the regulation of DA activity and, by extension, behaviors related to learning, anhedonia, and cognition.

Psychostimulants and social behaviors

Mounting evidence from animal models and human studies indicates that psychostimulants can significantly affect social behaviors. This is not surprising considering that the neural circuits underlying the regulation and expression of social behaviors are highly overlapped with those targeted by psychostimulants, which in most cases have strong rewarding and, consequently, addictive properties. In the present work, we provide an overview regarding the effects of illicit and prescription psychostimulants, such as cocaine, amphetamine-type stimulants, methylphenidate or modafinil, upon social behaviors such as social play, maternal behavior, aggression, pair bonding and social cognition and how psychostimulants in both animals and humans alter them. Finally, we discuss why these effects can vary depending on numerous variables such as the type of drug considered, acute versus long-term use, clinical versus recreational consumption, or the presence or absence of concomitant risk factors.

Dopamine control of social novelty preference is constrained by an interpeduncular-tegmentum circuit

Animals are inherently motivated to explore social novelty cues over familiar ones, resulting in a novelty preference (NP), although the behavioral and circuit bases underlying NP are unclear. Combining calcium and neurotransmitter sensors with fiber photometry and optogenetics in mice, we find that mesolimbic dopamine (DA) neurotransmission is strongly and predominantly activated by social novelty controlling bout length of interaction during NP, a response significantly reduced by familiarity. In contrast, interpeduncular nucleus (IPN) GABAergic neurons that project to the lateral dorsal tegmentum (LDTg) were inhibited by social novelty but activated during terminations with familiar social stimuli. Inhibition of this pathway during NP increased interaction and bout length with familiar social stimuli, while activation reduced interaction and bout length with novel social stimuli via decreasing DA neurotransmission. These data indicate interest towards novel social stimuli is encoded by mesolimbic DA which is dynamically regulated by an IPN→LDTg circuit to control NP.

Disynaptic Inhibitory Cerebellar Control Over Caudal Medial Accessory Olive

The olivocerebellar system, which is critical for sensorimotor performance and learning, functions through modules with feedback loops. The main feedback to the inferior olive comes from the cerebellar nuclei (CN), which are predominantly GABAergic and contralateral. However, for the subnucleus d of the caudomedial accessory olive (cdMAO), a crucial region for oculomotor and upper body movements, the source of GABAergic input has yet to be identified. Here, we demonstrate the existence of a disynaptic inhibitory projection from the medial CN (MCN) to the cdMAO via the superior colliculus (SC) by exploiting retrograde, anterograde, and transsynaptic viral tracing at the light microscopic level as well as anterograde classical and viral tracing combined with immunocytochemistry at the electron microscopic level. Retrograde tracing in Gad2-Cre mice reveals that the cdMAO receives GABAergic input from the contralateral SC. Anterograde transsynaptic tracing uncovered that the SC neurons receiving input from the contralateral MCN provide predominantly inhibitory projections to contralateral cdMAO, ipsilateral to the MCN. Following ultrastructural analysis of the monosynaptic projection about half of the SC terminals within the contralateral cdMAO are GABAergic. The disynaptic GABAergic projection from the MCN to the ipsilateral cdMAO mirrors that of the monosynaptic excitatory projection from the MCN to the contralateral cdMAO. Thus, while completing the map of inhibitory inputs to the olivary subnuclei, we established that the MCN inhibits the cdMAO via the contralateral SC, highlighting a potential push-pull mechanism in directional gaze control that appears unique in terms of laterality and polarity among olivocerebellar modules.

Dopamine D2 receptors in pyramidal neurons in the medial prefrontal cortex regulate social behavior

Drugs acting on dopamine D2 receptors are widely used for the treatment of several neuropsychiatric disorders, including schizophrenia and depression. Social deficits are a core symptom of these disorders. Pharmacological manipulation of dopamine D2 receptors (Drd2), a Gi-coupled subtype of dopamine receptors, in the medial prefrontal cortex (mPFC) has shown that Drd2 is implicated in social behaviors. However, the type of neurons expressing Drd2 in the mPFC and the underlying circuit mechanism regulating social behaviors remain largely unknown. Here, we show that Drd2 were mainly expressed in pyramidal neurons in the mPFC and that the activation of the Gi-pathway in Drd2+ pyramidal neurons impaired social behavior in male mice. In contrast, the knockdown of D2R in pyramidal neurons in the mPFC enhanced social approach behaviors in male mice and selectively facilitated the activation of mPFC neurons projecting to the nucleus accumbens (NAc) during social interaction. Remarkably, optogenetic activation of mPFC-to-NAc-projecting neurons mimicked the effects of conditional D2R knockdown on social behaviors. Altogether, these results demonstrate a cell type-specific role for Drd2 in the mPFC in regulating social behavior, which may be mediated by the mPFC-to-NAc pathway.

Separation of Channels Subserving Approach and Avoidance/Escape at the Level of the Basal Ganglia and Related Brainstem Structures

The basal ganglia have the key function of directing our behavior in the context of events from our environment and/or our internal state. This function relies on afferents targeting the main input structures of the basal ganglia, entering bids for action selection at the level of the striatum or signals for behavioral interruption at the level of the subthalamic nucleus, with behavioral reselection facilitated by dopamine signaling. Numerous experiments have studied action selection in relation to inputs from the cerebral cortex. However, less is known about the anatomical and functional link between the basal ganglia and the brainstem. In this review, we describe how brainstem structures also project to the main input structures of the basal ganglia, namely the striatum, the subthalamic nucleus and midbrain dopaminergic neurons, in the context of approach and avoidance (including escape from threat), two fundamental, mutually exclusive behavioral choices in an animal's repertoire in which the brainstem is strongly involved. We focus on three particularly well-described loci involved in approach and avoidance, namely the superior colliculus, the parabrachial nucleus and the periaqueductal grey nucleus. We consider what is known about how these structures are related to the basal ganglia, focusing on their projections toward the striatum, dopaminergic neurons and subthalamic nucleus, and explore the functional consequences of those interactions.

Reward Modulates Visual Responses in the Superficial Superior Colliculus of Mice

The processing of sensory input is constantly adapting to behavioral demands and internal states. The drive to obtain reward, e.g., searching for water when thirsty, is a strong behavioral demand and associating the reward with its source, a certain environment or action, is paramount for survival. Here, we show that water reward increases subsequent visual activity in the superficial layers of the superior colliculus (SC), which receive direct input from the retina and belong to the earliest stages of visual processing. We trained mice of either sex to perform a visual decision task and recorded the activity of neurons in the SC using two-photon calcium imaging and high-density electrophysiological recordings. Responses to visual stimuli in around 20% of visually responsive neurons in the superficial SC were affected by reward delivered in the previous trial. Reward mostly increased visual responses independent from modulations due to pupil size changes. The modulation of visual responses by reward could not be explained by movements like licking. It was specific to responses to the following visual stimulus, independent of slow fluctuations in neural activity and independent of how often the stimulus was previously rewarded. Electrophysiological recordings confirmed these results and revealed that reward affected the early phase of the visual response around 80 ms after stimulus onset. Modulation of visual responses by reward, but not pupil size, significantly improved the performance of a population decoder to detect visual stimuli, indicating the relevance of reward modulation for the visual performance of the animal.SIGNIFICANCE STATEMENT To learn which actions lead to food, water, or safety, it is necessary to integrate the receiving of reward with sensory stimuli related to the reward. Cortical stages of sensory processing have been shown to represent stimulus-reward associations. Here, we show, however, that reward influences neurons at a much earlier stage of sensory processing, the superior colliculus (SC), receiving direct input from the retina. Visual responses were increased shortly after the animal received the water reward, which led to an improved stimulus signal in the population of these visual neurons. Reward modulation of early visual responses may thus improve perception of visual environments predictive of reward.

Monosynaptic inputs to ventral tegmental area glutamate and GABA co-transmitting neurons

A unique population of ventral tegmental area (VTA) neurons co-transmits glutamate and GABA as well as functionally signals rewarding and aversive outcomes. However, the circuit inputs to VTA VGluT2+VGaT+ neurons are unknown, limiting our understanding of the functional capabilities of these neurons. To identify the inputs to VTA VGluT2+VGaT+ neurons, we coupled monosynaptic rabies tracing with intersectional genetic targeting of VTA VGluT2+VGaT+ neurons in mice. We found that VTA VGluT2+VGaT+ neurons received diverse brain-wide inputs. The largest numbers of monosynaptic inputs to VTA VGluT2+VGaT+ neurons were from superior colliculus, lateral hypothalamus, midbrain reticular nucleus, and periaqueductal gray, whereas the densest inputs relative to brain region volume were from dorsal raphe nucleus, lateral habenula, and ventral tegmental area. Based on these and prior data, we hypothesized that lateral hypothalamus and superior colliculus inputs were glutamatergic neurons. Optical activation of glutamatergic lateral hypothalamus neurons robustly activated VTA VGluT2+VGaT+ neurons regardless of stimulation frequency and resulted in flee-like ambulatory behavior. In contrast, optical activation of glutamatergic superior colliculus neurons activated VTA VGluT2+VGaT+ neurons for a brief period of time at high stimulation frequency and resulted in head rotation and arrested ambulatory behavior (freezing). For both pathways, behaviors induced by stimulation were uncorrelated with VTA VGluT2+VGaT+ neuron activity. However, stimulation of glutamatergic lateral hypothalamus neurons, but not glutamatergic superior colliculus neurons, was associated with VTA VGluT2+VGaT+ footshock-induced activity. We interpret these results such that inputs to VTA VGluT2+VGaT+ neurons may integrate diverse signals related to the detection and processing of motivationally-salient outcomes. Further, VTA VGluT2+VGaT+ neurons may signal threat-related outcomes, possibly via input from lateral hypothalamus glutamate neurons, but not threat-induced behavioral kinematics.

Cannabidiol tempers alcohol intake and neuroendocrine and behavioural correlates in alcohol binge drinking adolescent rats. Focus on calcitonin gene-related peptide's brain levels

Alcohol binge drinking is common among adolescents and may challenge the signalling systems that process affective stimuli, including calcitonin gene-related peptide (CGRP) signalling. Here, we employed a rat model of adolescent binge drinking to evaluate reward-, social- and aversion-related behaviour, glucocorticoid output and CGRP levels in affect-related brain regions. As a potential rescue, the effect of the phytocannabinoid cannabidiol was explored. Adolescent male rats underwent the intermittent 20% alcohol two-bottle choice paradigm; at the binge day (BD) and the 24 h withdrawal day (WD), we assessed CGRP expression in medial prefrontal cortex (mPFC), nucleus accumbens (NAc), amygdala, hypothalamus and brainstem; in addition, we evaluated sucrose preference, social motivation and drive, nociceptive response, and serum corticosterone levels. Cannabidiol (40 mg/kg, i.p.) was administered before each drinking session, and its effect was measured on the above-mentioned readouts. At BD and WD, rats displayed decreased CGRP expression in mPFC, NAc and amygdala; increased CGRP levels in the brainstem; increased response to rewarding- and nociceptive stimuli and decreased social drive; reduced serum corticosterone levels. Cannabidiol reduced alcohol consumption and preference; normalised the abnormal corticolimbic CGRP expression, and the reward and aversion-related hyper-responsivity, as well as glucocorticoid levels in alcohol binge-like drinking rats. Overall, CGRP can represent both a mediator and a target of alcohol binge-like drinking and provides a further piece in the intricate puzzle of alcohol-induced behavioural and neuroendocrine sequelae. CBD shows promising effects in limiting adolescent alcohol binge drinking and rebalancing the bio-behavioural abnormalities.

Advancing social behavioral neuroscience by integrating ethology and comparative psychology methods through machine learning

Social behavior is naturally occurring in vertebrate species, which holds a strong evolutionary component and is crucial for the normal development and survival of individuals throughout life. Behavioral neuroscience has seen different influential methods for social behavioral phenotyping. The ethological research approach has extensively investigated social behavior in natural habitats, while the comparative psychology approach was developed utilizing standardized and univariate social behavioral tests. The development of advanced and precise tracking tools, together with post-tracking analysis packages, has recently enabled a novel behavioral phenotyping method, that includes the strengths of both approaches. The implementation of such methods will be beneficial for fundamental social behavioral research but will also enable an increased understanding of the influences of many different factors that can influence social behavior, such as stress exposure. Furthermore, future research will increase the number of data modalities, such as sensory, physiological, and neuronal activity data, and will thereby significantly enhance our understanding of the biological basis of social behavior and guide intervention strategies for behavioral abnormalities in psychiatric disorders.

How social information impacts action in rodents and humans: the role of the prefrontal cortex and its connections

Day-to-day choices often involve social information and can be influenced by prior social experience. When making a decision in a social context, a subject might need to: 1) recognize the other individual or individuals, 2) infer their intentions and emotions, and 3) weigh the values of all outcomes, social and non-social, prior to selecting an action. These elements of social information processing all rely, to some extent, on the medial prefrontal cortex (mPFC). Patients with neuropsychiatric disorders often have disruptions in prefrontal cortical function, likely contributing to deficits in social reasoning and decision making. To better understand these deficits, researchers have turned to rodents, which have revealed prefrontal cortical mechanisms for contending with the complex information processing demands inherent to making decisions in social contexts. Here, we first review literature regarding social decision making, and the information processing underlying it, in humans and patient populations. We then turn to research in rodents, discussing current procedures for studying social decision making, and underlying neural correlates.

The Orienting Reflex Reveals Behavioral States Set by Demanding Contexts: Role of the Superior Colliculus

Sensory stimuli can trigger an orienting reflex (response) by which animals move the head to position their sensors (e.g., eyes, pinna, whiskers). Orienting responses may be important to evaluate stimuli that call for action (e.g., approach, escape, ignore), but little is known about the dynamics of orienting responses in the context of goal-directed actions. Using mice of either sex, we found that, during a signaled avoidance action, the orienting response evoked by the conditioned stimulus (CS) consisted of a fast head movement containing rotational and translational components that varied substantially as a function of the behavioral and underlying brain states of the animal set by different task contingencies. Larger CS-evoked orienting responses were associated with high-intensity auditory stimuli, failures to produce the appropriate signaled action, and behavioral states resulting from uncertain or demanding situations and the animal's ability to cope with them. As a prototypical orienting neural circuit, we confirmed that the superior colliculus controls and codes the direction of spontaneous exploratory orienting movements. In addition, superior colliculus activity correlated with CS-evoked orienting responses, and either its optogenetic inhibition or excitation potentiated CS-evoked orienting responses, which are likely generated downstream in the medulla. CS-evoked orienting responses may be a useful probe to assess behavioral and related brain states, and state-dependent modulation of orienting responses may involve the superior colliculus.SIGNIFICANCE STATEMENT Humans and other animals produce an orienting reflex (also known as orienting response) by which they rapidly orient their head and sensors to evaluate novel or salient stimuli. Spontaneous orienting movements also occur during exploration of the environment in the absence of explicit, salient stimuli. We monitored stimulus-evoked orienting responses in mice performing signaled avoidance behaviors and found that these responses reflect the behavioral state of the animal set by contextual demands and the animal's ability to cope with them. Various experiments involving the superior colliculus revealed a well-established role in spontaneous orienting but only an influencing effect over orienting responses. Stimulus-evoked orienting responses may be a useful probe of behavioral and related brain states.

Modulation of cholinergic, GABA-ergic and glutamatergic components of superior colliculus affect REM sleep in rats

The superior colliculus (SC) is associated with visual attention, spatial navigation, decision making, escape and approach responses, some of which are important for defence and survival in rodents. SC helps in initiating and controlling saccadic eye movements and gaze during wakefulness. It is also activated during rapid eye movement (REM) sleep associated rapid eye movements (REMs). To investigate the contribution of SC in sleep-wake behaviour, we have demonstrated that manipulation of SC with scopolamine, carbachol, muscimol, picrotoxin and MK-801 decreased the amount of REM sleep. We observed that scopolamine and picrotoxin as well as muscimol decreased REM sleep frequency. MK-801 decreased percent amount of REM sleep, however, neither the frequency nor the duration/episode was affected. The cholinergic and GABA-ergic modulation of SC affecting REM sleep may be involved in REM sleep associated visuo-spatial learning and memory consolidation, which however, need to be confirmed. Furthermore, the results suggest involvement of efferent from SC in modulation of sleep-waking via the brainstem sleep regulating areas.

Valproic acid affects neurogenesis during early optic tectum development in zebrafish

The mammalian superior colliculus and its non-mammalian homolog, the optic tectum (OT), are midbrain structures that integrate multimodal sensory inputs and guide non-voluntary movements in response to prevalent stimuli. Recent studies have implicated this structure as a possible site affected in autism spectrum disorder (ASD). Interestingly, fetal exposure to valproic acid (VPA) has also been associated with an increased risk of ASD in humans and animal models. Therefore, we took the approach of determining the effects of VPA treatment on zebrafish OT development as a first step in identifying the mechanisms that allow its formation. We describe normal OT development during the first 5 days of development and show that in VPA-treated embryos, neuronal specification and neuropil formation was delayed. VPA treatment was most detrimental during the first 3 days of development and did not appear to be linked to oxidative stress. In conclusion, our work provides a foundation for research into mechanisms driving OT development, as well as the relationship between the OT, VPA, and ASD. This article has an associated First Person interview with one of the co-first authors of the paper.

cATR Tracing Approach to Identify Individual Intermediary Neurons Based on Their Input and Output: A Proof-of-Concept Study Connecting Cerebellum and Central Hubs Implicated in Developmental Disorders

Over the past decades, it has become increasingly clear that many neurodevelopmental disorders can be characterized by aberrations in the neuro-anatomical connectome of intermediary hubs. Yet, despite the advent in unidirectional transsynaptic tracing technologies, we are still lacking an efficient approach to identify individual neurons based on both their precise input and output relations, hampering our ability to elucidate the precise connectome in both the healthy and diseased condition. Here, we bridge this gap by combining anterograde transsynaptic- and retrograde (cATR) tracing in Ai14 reporter mice, using adeno-associated virus serotype 1 expressing Cre and cholera toxin subunit B as the anterograde and retrograde tracer, respectively. We have applied this innovative approach to selectively identify individual neurons in the brainstem that do not only receive input from one or more of the cerebellar nuclei (CN), but also project to the primary motor cortex (M1), the amygdala or the ventral tegmental area (VTA). Cells directly connecting CN to M1 were found mainly in the thalamus, while a large diversity of midbrain and brainstem areas connected the CN to the amygdala or VTA. Our data highlight that cATR allows for specific, yet brain-wide, identification of individual neurons that mediate information from a cerebellar nucleus to the cerebral cortex, amygdala or VTA via a disynaptic pathway. Given that the identified neurons in healthy subjects can be readily quantified, our data also form a solid foundation to make numerical comparisons with mouse mutants suffering from aberrations in their connectome due to a neurodevelopmental disorder.

Cerebellar dopamine D2 receptors regulate social behaviors

The cerebellum, a primary brain structure involved in the control of sensorimotor tasks, also contributes to higher cognitive functions including reward, emotion and social interaction. Although the regulation of these behaviors has been largely ascribed to the monoaminergic system in limbic regions, the contribution of cerebellar dopamine signaling in the modulation of these functions remains largely unknown. By combining cell-type-specific transcriptomics, histological analyses, three-dimensional imaging and patch-clamp recordings, we demonstrate that cerebellar dopamine D2 receptors (D2Rs) in mice are preferentially expressed in Purkinje cells (PCs) and regulate synaptic efficacy onto PCs. Moreover, we found that changes in D2R levels in PCs of male mice during adulthood alter sociability and preference for social novelty without affecting motor functions. Altogether, these findings demonstrate novel roles for D2R in PC function and causally link cerebellar D2R levels of expression to social behaviors.

The “Primitive Brain Dysfunction” Theory of Autism: The Superior Colliculus Role

A better understanding of the pathogenesis of autism will help clarify our conception of the complexity of normal brain development. The crucial deficit may lie in the postnatal changes that vision produces in the brainstem nuclei during early life. The superior colliculus is the primary brainstem visual center. Although difficult to examine in humans with present techniques, it is known to support behaviors essential for every vertebrate to survive, such as the ability to pay attention to relevant stimuli and to produce automatic motor responses based on sensory input. From birth to death, it acts as a brain sentinel that influences basic aspects of our behavior. It is the main brainstem hub that lies between the environment and the rest of the higher neural system, making continuous, implicit decisions about where to direct our attention. The conserved cortex-like organization of the superior colliculus in all vertebrates allows the early appearance of primitive emotionally-related behaviors essential for survival. It contains first-line specialized neurons enabling the detection and tracking of faces and movements from birth. During development, it also sends the appropriate impulses to help shape brain areas necessary for social-communicative abilities. These abilities require the analysis of numerous variables, such as the simultaneous evaluation of incoming information sustained by separate brain networks (visual, auditory and sensory-motor, social, emotional, etc.), and predictive capabilities which compare present events to previous experiences and possible responses. These critical aspects of decision-making allow us to evaluate the impact that our response or behavior may provoke in others. The purpose of this review is to show that several enigmas about the complexity of autism might be explained by disruptions of collicular and brainstem functions. The results of two separate lines of investigation: 1. the cognitive, etiologic, and pathogenic aspects of autism on one hand, and two. the functional anatomy of the colliculus on the other, are considered in order to bridge the gap between basic brain science and clinical studies and to promote future research in this unexplored area.