Single-cell RNA-seq uncovers shared and distinct axes of variation in dorsal LGN neurons in mice, non-human primates and humans

ABSTRACTAbundant anatomical and physiological evidence supports the presence of at least three distinct types of relay glutamatergic neurons in the primate dorsal lateral geniculate nucleus (dLGN) of the thalamus, the brain region that conveys visual information from the retina to the primary visual cortex. Relay neuron diversity has also been described in the mouse dLGN (also known as LGd). Different types of relay neurons in mice, humans and macaques have distinct morphologies, distinct connectivity patterns, and convey different aspects of visual information to the cortex. To investigate the molecular underpinnings of these cell types, and how these relate to other cellular properties and differences in dLGN between human, macaque, and mice, we profiled gene expression in single nuclei and cells using RNA-sequencing. These efforts identified four distinct types of relay neurons in the primate dLGN, magnocellular neurons, parvocellular neurons, and two cell types expressing canonical marker genes for koniocellular neurons. Surprisingly, despite extensive documented morphological and physiological differences between magno- and parvocellular neurons, we identified few genes with significant differential expression between transcriptomic cell types corresponding to these two neuronal populations. We also detected strong donor-specific gene expression signatures in both macaque and human relay neurons. Likewise, the dominant feature of relay neurons of the adult mouse dLGN is high transcriptomic similarity, with an axis of heterogeneity that aligns with core vs. shell portions of mouse dLGN. Together, these data show that transcriptomic differences between principal cell types in the mature mammalian dLGN are subtle relative to striking differences in morphology and cortical projection targets. Finally, we align cellular expression profiles across species and find homologous types of relay neurons in macaque and human, and distinct relay neurons in mouse.

Endogenous oxytocin inhibits hypothalamic stress responsive neurons following acute hypernatremia

Significant prior evidence indicates that centrally acting oxytocin robustly modulates stress responsiveness and anxiety-like behavior, although the neural mechanisms behind these effects are not completely understood. A plausible neural basis for oxytocin mediated stress reduction is via inhibition of corticotropin-releasing hormone (CRH) neurons in the paraventricular nucleus of the hypothalamus (PVN) that regulate activation of the hypothalamic-pituitary-adrenal (HPA) axis. Previously, we have shown that following subcutaneous injection of 2.0 M NaCl, oxytocin (OT) synthesizing neurons are activated in the rat PVN, an oxytocin receptor (Oxtr) dependent inhibitory tone develops on a subset of parvocellular neurons, and stress-mediated increases in plasma corticosterone levels are blunted. Here, we utilized transgenic male CRH-reporter mice to selectively target PVN CRH neurons for whole-cell recordings. These experiments reveal that acute salt loading produces tonic inhibition of PVN CRH neurons through a mechanism that is largely independent of synaptic activity. Further studies reveal that CRH neurons within the PVN synthesize mRNA for Oxtr(s). Salt induced Oxtr-dependent inhibitory tone was eliminated in individual PVN CRH neurons filled with GDP-β-S, and was also largely absent in PVN CRH neurons extracted form CRH-Oxtr KO mice. Additional electrophysiological studies suggest that reduced excitability of PVN CRH neurons in salt loaded animals is associated with increased activation of an inwardly rectifying potassium channel. Collectively, these data reveal a likely cellular mechanism by which endogenous oxytocin signaling reduces the excitability of PVN CRH neurons to curb stress responsiveness during times of high plasma osmolality.

Abstract P202: Impaired Central, Renal, and Blood Pressure Responses to Alterations in Fluid and Electrolyte Homeostasis in Aged Sprague-Dawley Rats

Aim: Hypertension is strongly correlated with increased age in human subjects. These studies tested the hypothesis that impaired natriuretic responses to acute and chronic challenges to fluid and electrolyte homeostasis contribute to age-dependent hypertension. Methods: Two-month old (young) and 6-month old (aged) male Sprague-Dawley rats underwent an acute IV volume expansion (VE; 5% BW) and MAP, HR, urine output, and PVN neuronal activation (c-Fos expression) were assessed (n=4 per group). In a separate study, naïve 2 and 6-month old male Sprague-Dawley rats were maintained on a normal (0.6% NaCl) or high salt (4% NaCl) diet, and on day 21, MAP and NCC activity (peak natriuresis to IV HCTZ, 2mg/kg) were assessed (n=4 per group). Results: Renal excretion of sodium and water after acute VE was impaired in aged rats (total % sodium load excreted; young 78±6 vs aged 60±7: total % water load excreted; young 96±7 vs aged 66±5, P<0.05). PVN neuronal activation was attenuated in response to acute VE in aged rats in all parvocellular regions excluding the lateral parvocellular subnucleus (PVN neuronal activation [c-fos positive cells]; medial parvocellular young 59±4 vs aged 42±7, P<0.05). CV parameters did not change in response to acute VE in either group. Chronic HS diet evoked an increase in MAP in aged rats but not young rats (MAP [mmHg]; young NS 124±2 vs young HS 126±3 vs aged HS 138±3, P<0.05). Chronic HS diet caused a decrease in NCC activity in young rats and, in contrast, an increase in aged rats (peak ΔUNaV to HCTZ [μeq/min]; young NS 9.2±0.5 vs young HS 7.1±0.3 vs aged HS 16±1, P<0.05). Conclusion: Activation of sympathoinhibitory PVN parvocellular neurons is a well-characterized response to acute VE. Our data suggest there are attenuated acute PVN sympathoinhibitory responses to alterations in fluid and electrolyte balance in aged rats. In aged animals, HS intake increased NCC-mediated sodium reabsorption and promoted the development of sodium-dependent hypertension. We speculate that this is driven by blunted HS-evoked sympathoinhibition, likely due to reduced PVN neuronal activation.