Pain and unpleasant stimuli carry a negative hedonic valence, indicating an intrinsic aversiveness useful for avoiding harm. There are instances, however, of unpleasant stimuli carrying positive valence – such as the pleasure from spicy food, suggesting that pain and aversion can be decoupled. The Dhaka lab has discovered a small molecule Analgesic Screen 1 (AS1) which reverses the valence of a number of nociceptive and other aversive stimuli, whereby animals prefer normally aversive stimuli. Behavioral studies with zebrafish indicate that AS1 induces preference for noxious heat, painful chemical (AITC) and normally aversive dark environments. As positive valence or reward is often mediated by the neurotransmitter dopamine, we tested for the affects of dopamine antagonism on AS1-evoked behavior and found that the effects of AS1 are reversed by a D1 dopamine receptor antagonist. We currently propose that AS1 potentiates activity in the dopamine reward system in the presence of nociceptive and other aversive stimuli via D1R activation, thereby creating “pleasure from pain.” Understanding these pathways and the mechanisms underlying AS1 action, could provide a path forward for the development of novel therapeutics to treat debilitating pain disorders.

Millions of people have vision diseases that are not yet treatable, leading to blindness. Mouse models exist for some inherited retinal diseases, and thus have helped develop vision loss therapies. However, other common retinal diseases like glaucoma lack accurate mouse models. Human pluripotent stem cells (hPSCs) are a promising technology that provide a new way to model human retinal diseases. hPSCs can be induced to become layered, 3D mini-retinas called retinal organoids. Retinal organoids mirror early neurogenesis of the human retina, and thus can be used for modeling developmental disorders. However, we and other research groups have shown that as retinal organoids mature, they lose many features of the normal human retina. In particular, the neurons that are damaged during glaucoma, called retinal ganglion cells (RGCs), are not well preserved in organoids. As the organoid matures, the RGC layer becomes disorganized and RGCs migrate through the retina. RGCs are the projection neurons of the human retina, so their axons extend through the optic nerve and carry visual information to the brain. Unlike in human development, retinal organoids are grown in isolation from their brain targets. We wondered whether we could preserve RGC organization by providing organoid RGCs with synaptic targets. We investigated this hypothesis by combining the retinal organoids with their natural targets in the brain, the lateral geniculate nucleus and the superior colliculus. It is not yet known how to make these brain regions from hPSCs, so we used newborn mouse brain and combined the retinal organoids with these brain regions into structures called “assembloids.” We are now testing whether these assembloids preserve the RGC survival and laminar organization that retinal organoids lack. Promoting RGC survival and organization will allow organoids to become better in vitro models for glaucoma and improve the outcome for patients with vision loss.

Retinal diseases tend to affect specific neuron subtypes, ranging from age-related macular degeneration, which is caused by the deterioration of photoreceptors near the central portion of the retina (macula), to glaucoma, in which abnormally high intraocular pressure leads to ganglion cell death. Unfortunately, adult mammals are not able to regenerate retinal neurons. However, zebrafish and other amphibians can completely regenerate their retinal neurons in many different models of damage, and restore retinal structure and visual function. The source of regeneration stems from the resident Müller glia cells, which normally provide neuronal support and span all three retinal layers. A critical gene for the initiation of transforming Müller glia into neurons was found to be Ascl1. This led our lab to hypothesize that the introduction and upregulation of Ascl1 in mammalian Müller glia might stimulate them to become retinal neurons after damage, as occurs in these other regenerating species. Indeed, after introducing Ascl1 into the Müller glia of mice, we found newly regenerated retinal interneurons (bipolar cells) that successfully integrated into the retinal circuitry and functionally responded to light stimulus. In addition to Ascl1, we have identified two other transcription factors, that when introduced in combination with Ascl1, stimulate the generation of two different retinal neurons (ganglion cells and amacrine cells). We are currently developing a model of glaucoma, damaging the ganglion cells with a neurotoxin, and then testing the visual acuity using Optomotry to determine whether regenerated ganglion cells will mediate a functional improvement. Ectopic expression of a proneural transcription factor to stimulate retinal regeneration provides a potential therapeutic intervention for treating blinding diseases, that even now, have few modest treatment options.