Seasonal Plasticity in GABA\u3csup\u3eA\u3c/sup\u3e Signaling is Necessary for Restoring Phase Synchrony in the Master Circadian Clock Network

Annual changes in the environment threaten survival, and numerous biological processes in mammals adjust to this challenge via seasonal encoding by the suprachiasmatic nucleus (SCN). To tune behavior according to day length, SCN neurons display unified rhythms with synchronous phasing when days are short, but will divide into two sub-clusters when days are long. The transition between SCN states is critical for maintaining behavioral responses to seasonal change, but the mechanisms regulating this form of neuroplasticity remain unclear. Here we identify that a switch in chloride transport and GABAA signaling is critical for maintaining state plasticity in the SCN network. Further, we reveal that blocking excitatory GABAA signaling locks the SCN into its long day state. Collectively, these data demonstrate that plasticity in GABAA signaling dictates how clock neurons interact to maintain environmental encoding. Further, this work highlights factors that may influence susceptibility to seasonal disorders in humans

Long Days Enhance Recognition Memory and Increase Insulin-like Growth Factor 2 in the Hippocampus

Light improves cognitive function in humans; however, the neurobiological mechanisms underlying positive effects of light remain unclear. One obstacle is that most rodent models have employed lighting conditions that cause cognitive deficits rather than improvements. Here we have developed a mouse model where light improves cognitive function, which provides insight into mechanisms underlying positive effects of light. To increase light exposure without eliminating daily rhythms, we exposed mice to either a standard photoperiod or a long day photoperiod. Long days enhanced long-term recognition memory, and this effect was abolished by loss of the photopigment melanopsin. Further, long days markedly altered hippocampal clock function and elevated transcription of Insulin-like Growth Factor2 (Igf2). Up-regulation of Igf2 occurred in tandem with suppression of its transcriptional repressor Wilm’s tumor1. Consistent with molecular de-repression of Igf2, IGF2 expression was increased in the hippocampus before and after memory training. Lastly, long days occluded IGF2-induced improvements in recognition memory. Collectively, these results suggest that light changes hippocampal clock function to alter memory, highlighting novel mechanisms that may contribute to the positive effects of light. Furthermore, this study provides insight into how the circadian clock can regulate hippocampus-dependent learning by controlling molecular processes required for memory consolidation

Reduced VIP Expression Affects Circadian Clock Function in VIP-IRES-CRE Mice (JAX 010908)

Circadian rhythms are programmed by the suprachiasmatic nucleus (SCN), which relies on neuropeptide signaling to maintain daily timekeeping. Vasoactive intestinal polypeptide (VIP) is critical for SCN function, but the precise role of VIP neurons in SCN circuits is not fully established. To interrogate their contribution to SCN circuits, VIP neurons can be manipulated specifically using the DNA-editing enzyme Cre recombinase. Although the Cre transgene is assumed to be inert by itself, we find that VIP expression is reduced in both heterozygous and homozygous adult VIP-IRES-Cre mice (JAX 010908). Compared with wild-type mice, homozygous VIP-Cre mice display faster reentrainment and shorter free-running period but do not become arrhythmic in constant darkness. Consistent with this phenotype, homozygous VIP-Cre mice display intact SCN PER2::LUC rhythms, albeit with altered period and network organization. We present evidence that the ability to sustain molecular rhythms in the VIP-Cre SCN is not due to residual VIP signaling; rather, arginine vasopressin signaling helps to sustain SCN function at both intracellular and intercellular levels in this model. This work establishes that the VIP-IRES-Cre transgene interferes with VIP expression but that loss of VIP can be mitigated by other neuropeptide signals to help sustain SCN function. Our findings have implications for studies employing this transgenic model and provide novel insight into neuropeptide signals that sustain daily timekeeping in the master clock

The function of Drosophila larval class IV dendritic arborization sensory neurons in the larval-pupal transition is separable from their function in mechanical nociception responses.

The sensory and physiological inputs which govern the larval-pupal transition in Drosophila, and the neuronal circuity that integrates them, are complex. Previous work from our laboratory identified a dosage-sensitive genetic interaction between the genes encoding the Rho-GEF Trio and the zinc-finger transcription factor Sequoia that interfered with the larval-pupal transition. Specifically, we reported heterozygous mutations in sequoia (seq) dominantly exacerbated the trio mutant phenotype, and this seq-enhanced trio mutant genotype blocked the transition of third instar larvae from foragers to wanderers, a requisite behavioral transition prior to pupation. In this work, we use the GAL4-UAS system to rescue this phenotype by tissue-specific trio expression. We find that expressing trio in the class IV dendritic arborization (da) sensory neurons rescues the larval-pupal transition, demonstrating the reliance of the larval-pupal transition on the integrity of these sensory neurons. As nociceptive responses also rely on the functionality of the class IV da neurons, we test mechanical nociceptive responses in our mutant and rescued larvae and find that mechanical nociception is separable from the ability to undergo the larval-pupal transition. This demonstrates for the first time that the roles of the class IV da neurons in governing two critical larval behaviors, the larval-pupal transition and mechanical nociception, are functionally separable from each other

Representative ddaC morphologies in control, mutant and rescued third instar larvae.

<p>(A) ddaC in wildtype larvae (<i>GAL4-ppk1</i>.<i>9</i>, <i>UAS-mCD8GFP / +; trio</i>^<i>M89</i> <i>/ +</i>, image shown is 300.8 μm x 650.5μm). (B) ddaC in simple <i>trio</i> mutant larvae (<i>GAL4-ppk1</i>.<i>9</i>, <i>UAS-mCD8GFP</i> / +; <i>trio</i>^<i>M89</i> <i>/ trio</i>^<i>P1</i>, image shown is 302.8 μm x 749.8μm). (C) ddaC in <i>seq</i>-enhanced <i>trio</i> mutants (<i>GAL4-ppk1</i>.<i>9</i>, <i>UAS-mCD8GFP</i> / <i>seq</i>^{<i>9</i>.<i>17</i>}; <i>trio</i>^<i>M89</i> <i>/ trio</i>^<i>P1</i>, image shown is 300.4 μm x 601.7μm). (D) ddaC in <i>ppk1</i>.<i>9-trio</i> rescued <i>seq</i>-enhanced <i>trio</i> mutant larvae (<i>GAL4-ppk1</i>.<i>9</i>, <i>UAS-mCD8GFP</i> / <i>seq</i>^{<i>9</i>.<i>17</i>}<i>; trio</i>^<i>M89</i> <i>/ trio</i>^<i>P1</i>, <i>UAS-trio</i>.<i>B</i>, image shown is 351.1 μm x 751.2μm). The dorsal midline runs horizontally through the approximate center of each image. In (A) representative examples of iso-neuronal avoidance defects are indicated with yellow triangles while representative examples of hetero-neuronal tiling defects are indicated with red triangles. Scale bars show 100 μm.</p

Mechanical nociception in control, mutant and rescued third instar larvae.

<p>Mechanical nociception in control, mutant and rescued third instar larvae.</p

Tissue-specific expression of <i>trio</i> rescues pupation in the <i>seq</i>-enhanced <i>trio</i> mutant background.

<p>The percent of expected pupae was determined for the following genotypes: <i>trio</i>-mutant, <i>trio</i>-mutant background <i>(trio</i>^<i>M89</i> <i>/ trio</i>^<i>P1</i><i>); seq</i>-enhanced, <i>seq</i>-enhanced <i>trio</i> mutants (<i>seq</i>^{<i>9</i>.<i>17</i>} <i>/ +; trio</i>^<i>M89</i> <i>/ trio</i>^<i>P1</i><i>);</i> R-pan neural, rescue by pan-neural <i>trio</i> expression in the <i>seq</i>-enhanced <i>trio</i> mutant background (<i>GAL4-elav</i>.<i>L</i> / <i>seq</i>^{<i>9</i>.<i>17</i>}<i>; trio</i>^<i>M89</i> <i>/ trio</i>^<i>P1</i>, <i>UAS-trio</i>.<i>B</i>); R-class IV da, rescue by class IV da <i>trio</i> expression in the <i>seq</i>-enhanced <i>trio</i> mutant background (<i>GAL4-ppk1</i>.<i>9</i>, <i>UAS-mCD8GFP</i> / <i>seq</i>^{<i>9</i>.<i>17</i>}<i>; trio</i>^<i>M89</i> <i>/ trio</i>^<i>P1</i>, <i>UAS-trio</i>.<i>B)</i>. N = 15 for all genotypes. ANOVA p-value <0.0001; *** indicates pair-wise statistical significance at p<0.001 as compared to *seq*-enhanced. R-pan neural and R-class IV da are also statistically different from each other at p<0.001. Error bars represent the standard error of the means.</p

Quantification of ddaC dendrite morphologies in control, mutant and rescued third instar larvae.

<p>Genotypes in all panels: W, wildtype larvae, N = 9; B, <i>trio</i>-mutant background, N = 8; E, <i>seq</i>-enhanced <i>trio</i> mutants, N = 9; R, class IV da <i>trio</i> expression in the <i>seq</i>-enhanced <i>trio</i> mutant background, N = 10. Error bars represent the standard error of the means. (A) Dendritic density (dendrite length in μm/μm² surface). ANOVA p-value = 0.0005; E and R are not statistically different from each other, but both are statistically different from both W and B at p<0.05. (B) Dendritic branch frequency (dendritic branches/dendrite length in μm). ANOVA p-value = 0.0022; E is statistically different than W, B and R at p<0.05. (C) Iso-neuronal avoidance defects (iso-neuronal avoidance defects/dendrite length in μm). ANOVA p-value = 0.086. (D) Hetero-neuronal tiling defects (hetero-neuronal tiling defects/μm of path length of interface between dendritic fields). ANOVA p-value = 0.197.</p