Dorsal raphe nucleus projecting retinal ganglion cells: Why Y cells?

Short-term acute bright light exposure induces a prolonged anxiogenic effect in mice via a retinal ipRGC-CeA circuit

Light modulates mood through various retina-brain pathways. We showed that mice treated with short-term acute bright light exposure displayed anxiety-related phenotypes in a prolonged manner even after the termination of the exposure. Such a postexposure anxiogenic effect depended upon melanopsin-based intrinsically photosensitive retinal ganglion cell (ipRGC) activities rather than rod/cone photoreceptor inputs. Chemogenetic manipulation of specific central nuclei demonstrated that the ipRGC-central amygdala (CeA) visual circuit played a key role in this effect. The corticosterone system was likely to be involved in this effect, as evidenced by enhanced expression of the glucocorticoid receptor (GR) protein in the CeA and the bed nucleus of the stria terminalis and by the absence of this effect in animals treated with the GR antagonist. Together, our findings reveal a non-image forming visual circuit specifically designed for "the delayed" extinction of anxiety against potential threats, thus conferring a survival advantage.

Serotonergic modulation across sensory modalities

The serotonergic system has been widely studied across animal taxa and different functional networks. This modulatory system is therefore well positioned to compare the consequences of neuromodulation for sensory processing across species and modalities at multiple levels of sensory organization. Serotonergic neurons that innervate sensory networks often bidirectionally exchange information with these networks but also receive input representative of motor events or motivational state. This convergence of information supports serotonin's capacity for contextualizing sensory information according to the animal's physiological state and external events. At the level of sensory circuitry, serotonin can have variable effects due to differential projections across specific sensory subregions, as well as differential serotonin receptor type expression within those subregions. Functionally, this infrastructure may gate or filter sensory inputs to emphasize specific stimulus features or select among different streams of information. The near-ubiquitous presence of serotonin and other neuromodulators within sensory regions, coupled with their strong effects on stimulus representation, suggests that these signaling pathways should be considered integral components of sensory systems.

Separable gain control of ongoing and evoked activity in the visual cortex by serotonergic input

Controlling gain of cortical activity is essential to modulate weights between internal ongoing communication and external sensory drive. Here, we show that serotonergic input has separable suppressive effects on the gain of ongoing and evoked visual activity. We combined optogenetic stimulation of the dorsal raphe nucleus (DRN) with wide-field calcium imaging, extracellular recordings, and iontophoresis of serotonin (5-HT) receptor antagonists in the mouse visual cortex. 5-HT1A receptors promote divisive suppression of spontaneous activity, while 5-HT2A receptors act divisively on visual response gain and largely account for normalization of population responses over a range of visual contrasts in awake and anesthetized states. Thus, 5-HT input provides balanced but distinct suppressive effects on ongoing and evoked activity components across neuronal populations. Imbalanced 5-HT1A/2A activation, either through receptor-specific drug intake, genetically predisposed irregular 5-HT receptor density, or change in sensory bombardment may enhance internal broadcasts and reduce sensory drive and vice versa.

Functional Organization of the Sympathetic Pathways Controlling the Pupil: Light-Inhibited and Light-Stimulated Pathways

Pupil dilation is mediated by a sympathetic output acting in opposition to parasympathetically mediated pupil constriction. While light stimulates the parasympathetic output, giving rise to the light reflex, it can both inhibit and stimulate the sympathetic output. Light-inhibited sympathetic pathways originate in retina-receptive neurones of the pretectum and the suprachiasmatic nucleus (SCN): by attenuating sympathetic activity, they allow unimpeded operation of the light reflex. Light stimulates the noradrenergic and serotonergic pathways. The hub of the noradrenergic pathway is the locus coeruleus (LC) containing both excitatory sympathetic premotor neurones (SympPN) projecting to preganglionic neurones in the spinal cord, and inhibitory parasympathetic premotor neurones (ParaPN) projecting to preganglionic neurones in the Edinger-Westphal nucleus (EWN). SympPN receive inputs from the SCN via the dorsomedial hypothalamus, orexinergic neurones of the latero-posterior hypothalamus, wake- and sleep-promoting neurones of the hypothalamus and brain stem, nociceptive collaterals of the spinothalamic tract, whereas ParaPN receive inputs from the amygdala, sleep/arousal network, nociceptive spinothalamic collaterals. The activity of LC neurones is regulated by inhibitory α₂-adrenoceptors. There is a species difference in the function of the preautonomic LC. In diurnal animals, the α₂-adrenoceptor agonist clonidine stimulates mainly autoreceptors on SymPN, causing miosis, whereas in nocturnal animals it stimulates postsynaptic α₂-arenoceptors in the EWN, causing mydriasis. Noxious stimulation activates SympPN in diurnal animals and ParaPN in nocturnal animals, leading to pupil dilation via sympathoexcitation and parasympathetic inhibition, respectively. These differences may be attributed to increased activity of excitatory LC neurones due to stimulation by light in diurnal animals. This may also underlie the wake-promoting effect of light in diurnal animals, in contrast to its sleep-promoting effect in nocturnal species. The hub of the serotonergic pathway is the dorsal raphe nucleus that is light-sensitive, both directly and indirectly (via an orexinergic input). The light-stimulated pathways mediate a latent mydriatic effect of light on the pupil that can be unmasked by drugs that either inhibit or stimulate SympPN in these pathways. The noradrenergic pathway has widespread connections to neural networks controlling a variety of functions, such as sleep/arousal, pain, and fear/anxiety. Many physiological and psychological variables modulate pupil function via this pathway.

Neurochemical Systems of the Retina Involved in the Control of Movement

Recent studies have revealed that the retina may exert control over deep brain function and may be importantly involved in the etiology, progression, and treatment of disorders such as Parkinson's disease (PD). While such a concept is uncharted territory and even less is known about the mechanism by which this might be achieved, this study was undertaken to determine how retinal dopamine (DA), serotonin (5-HT), and melatonin (MEL) neurotransmitter systems might be involved in the control of movement in their own right. To explore these further, intravitreal (IVIT) injections of DA, 5-HT, and MEL were made 0.5 or 3 h prior to testing horizontal and vertical movement in the open field as well as assessment on three motor tests used routinely to evaluate movement as a preclinical model of PD. The doses of DA (2 µl of 25 and 75 µg/µl), 5-HT (2 µl of 5 and 15 µg/µl), and MEL (2 µl of 5 µg/µl) were chosen because of previous work demonstrating an anatomically precise effect of these transmitters after they were injected directly into the brain. The postinjection times of testing were also chosen on the basis of previous intracerebral and IVIT work intimating the importance of the circadian cycle in determining the efficacy of such effects. 0.5 h after IVIT injection of DA at the 25 and 75 µg/µl doses, significant inhibition of motor function was observed. While IVIT injection of 10 or 30 µg of 5-HT also inhibited motor performance, this was significantly less than that seen with DA. In fact, IVIT injection increases motor performance compared to vehicle injection on some parameters. The IVIT injection of 10 µg of MEL facilitated motor function on many parameters compared to DA, 5-HT, and vehicle injection. When rats were tested 3 h after IVIT injection, the inhibition of vertical movement was also observed compared to controls. The present results illustrate that specific retinal neurotransmitter systems participate in the normal control of bodily motor function. The possible involvement of these systems in movement disorders such as PD is the subject of ongoing research.

A retinoraphe projection regulates serotonergic activity and looming-evoked defensive behaviour

Animals promote their survival by avoiding rapidly approaching objects that indicate threats. In mice, looming-evoked defensive responses are triggered by the superior colliculus (SC) which receives direct retinal inputs. However, the specific neural circuits that begin in the retina and mediate this important behaviour remain unclear. Here we identify a subset of retinal ganglion cells (RGCs) that controls mouse looming-evoked defensive responses through axonal collaterals to the dorsal raphe nucleus (DRN) and SC. Looming signals transmitted by DRN-projecting RGCs activate DRN GABAergic neurons that in turn inhibit serotoninergic neurons. Moreover, activation of DRN serotoninergic neurons reduces looming-evoked defensive behaviours. Thus, a dedicated population of RGCs signals rapidly approaching visual threats and their input to the DRN controls a serotonergic self-gating mechanism that regulates innate defensive responses. Our study provides new insights into how the DRN and SC work in concert to extract and translate visual threats into defensive behavioural responses.

Serotonin 5-HT4 receptors modulate the development of glutamatergic input to the dorsal raphe nucleus

The dorsal raphe nucleus (DRN) is a major serotonin (5-hydroxytryptamine, 5-HT)-producing region in the central nervous system. It receives glutamatergic inputs from several brain regions, which are reciprocally modulated by serotonergic signals. We investigated whether serotonin 5-HT₄ receptors (5-HT₄Rs) play a role in the development of glutamatergic control of the DRN, with an emphasis on cortical inputs. Double-label immunohistochemistry and confocal microscopy were used to quantify vesicular glutamate transporter 1 (vGluT1)-immunoreactive terminals in the DRN of mice with a null-mutation in the 5-HT₄R gene. We found no significant change in the overall density of vGluT1-positive terminals in homozygous and heterozygous mice, but heterozygous mice had a significantly higher density of vGluT1-positive terminals contacting serotonergic neurons. These results suggest that altered 5-HT₄R expression may affect the development of cortical glutamatergic control of the DRN.

A Pontine Region is a Neural Correlate of the Human Affective Processing Network

The in vivo neural activity of the pons during the perception of affective stimuli has not been studied despite the strong implications of its role in affective processing. To examine the activity of the pons during the viewing of affective stimuli, and to verify its functional and structural connectivity with other affective neural correlates, a multimodal magnetic resonance imaging methodology was employed in this study. We observed the in vivo activity of the pons when viewing affective stimuli. Furthermore, small-world connectivity indicated that the functional connectivity (FC) between the pons and the cortico-limbic affective regions was meaningful, with the coefficient λ being positively associated with self-reported emotional reactivity. The FC between the pons and the cortico-limbic-striatal areas was related to self-reported negative affect. Corroborating this finding was the observation that the tract passing through the pons and the left hippocampus was negatively related to self-reported positive affect and positively correlated with emotional reactivity. Our findings support the framework that the pons works conjunctively with the distributed cortico-limbic-striatal systems in shaping individuals' affective states and reactivity. Our work paves the path for future research on the contribution of the pons to the precipitation and maintenance of affective disorders.

ON and OFF retinal ganglion cells differentially regulate serotonergic and GABAergic activity in the dorsal raphe nucleus

The dorsal raphe nucleus (DRN), the major source of serotonergic input to the forebrain, receives excitatory input from the retina that can modulate serotonin levels and depressive-like behavior. In the Mongolian gerbil, retinal ganglion cells (RGCs) with alpha-like morphological and Y-like physiological properties innervate the DRN with ON DRN-projecting RGCs out numbering OFF DRN-projecting RGCs. The DRN neurons targeted by ON and OFF RGCs are unknown. To explore retino-raphe anatomical organization, retinal afferents labeled with Cholera toxin B were examined for association with the postsynaptic protein PSD-95. Synaptic associations between retinal afferents and DRN serotonergic and GABAergic neurons were observed. To explore retino-raphe functional organization, light-evoked c-fos expression was examined. Light significantly increased the number of DRN serotonergic and GABAergic cells expressing c-Fos. When ON RGCs were rendered silent while enhancing the firing rate of OFF RGCs, c-Fos expression was greatly increased in DRN serotonergic neurons suggesting that OFF DRN-projecting RGCs predominately activate serotonergic neurons whereas ON DRN-projecting RGCs mainly target GABAergic neurons. Direct glutamatergic retinal input to DRN 5-HT neurons contributes to the complex excitatory drive regulating these cells. Light, via the retinoraphe pathway can modify DRN 5-HT neuron activity which may play a role in modulating affective behavior.