Optogenetic induction of hibernation-like state with modified human Opsin4 in mice

Interorgan rhythmicity as a feature of healthful metabolism

The finding that animals with circadian gene mutations exhibit diet-induced obesity and metabolic syndrome with hypoinsulinemia revealed a distinct role for the clock in the brain and peripheral tissues. Obesogenic diets disrupt rhythmic sleep/wake patterns, feeding behavior, and transcriptional networks, showing that metabolic signals reciprocally control the clock. Providing access to high-fat diet only during the sleep phase (light period) in mice accelerates weight gain, whereas isocaloric time-restricted feeding during the active period enhances energy expenditure due to circadian induction of adipose thermogenesis. This perspective focuses on advances and unanswered questions in understanding the interorgan circadian control of healthful metabolism.

Activation of oxytocinergic neurons enhances torpor in mice

Mus musculus enters a torpid state in response to caloric restriction in sub-thermoneutral ambient temperatures. This torpid state is characterized by an adaptive and controlled decrease in metabolic rate, heart rate, body temperature, and activity. Previous research has identified the paraventricular nucleus (PVN) within the hypothalamus, a region containing oxytocin neurons, as a location that is active during torpor onset. We hypothesized that oxytocin neurons within the PVN are part of this neural circuit and that activation of oxytocin neurons would deepen and lengthen torpor bouts. We report that activation of oxytocin neurons alone is not sufficient to induce a torpor-like state in the fed mouse, with no significant difference in body temperature or heart rate upon activation of oxytocin neurons. However, we found that activation of oxytocin neurons prior to the onset of daily torpor both deepens and lengthens the subsequent bout, with a 1.7 ± 0.4 °C lower body temperature and a 135 ± 32 min increase in length. We therefore conclude that oxytocin neurons are involved in the neural circuitry controlling daily torpor in the mouse.

Ground squirrels - experts in thermoregulatory adaptation

Ground squirrels exemplify one of the most extreme forms of mammalian hibernation and a convenient model for studying its mechanisms. Their thermoregulatory system demonstrates remarkable adaptive capabilities by maintaining optimal levels of body temperature both in active and hibernation states. Here, we review recent findings and unresolved issues regarding the neural mechanisms of body temperature control in ground squirrels.

Measuring body temperature of freely moving mice under an optogenetics-induced long-term hypothermic state

We present a protocol for inducing a hibernation-like state in free-moving mice using optogenetics. We have recently developed an optogenetic technique utilizing modified Opsin4, which is activated by weak blue light, resulting in prolonged neuronal excitation. We describe a protocol that includes detailed instructions for virus injection, implantation of optic fibers and temperature transmitters, photostimulation, and real-time recording of body temperature in mice. This method is valuable for investigating the mechanisms underlying torpor and thermoregulation in mice. For complete details on the use and execution of this protocol, please refer to Takahashi et al.¹.

Cre-dependent ACR2-expressing reporter mouse strain for efficient long-lasting inhibition of neuronal activity

Optogenetics is a powerful tool for manipulating neuronal activity by light illumination with high temporal and spatial resolution. Anion-channelrhodopsins (ACRs) are light-gated anion channels that allow researchers to efficiently inhibit neuronal activity. A blue light-sensitive ACR2 has recently been used in several in vivo studies; however, the reporter mouse strain expressing ACR2 has not yet been reported. Here, we generated a new reporter mouse strain, LSL-ACR2, in which ACR2 is expressed under the control of Cre recombinase. We crossed this strain with a noradrenergic neuron-specific driver mouse (NAT-Cre) to generate NAT-ACR2 mice. We confirmed Cre-dependent expression and function of ACR2 in the targeted neurons by immunohistochemistry and electrophysiological recordings in vitro, and confirmed physiological function using an in vivo behavioral experiment. Our results show that the LSL-ACR2 mouse strain can be applied for optogenetic inhibition of targeted neurons, particularly for long-lasting continuous inhibition, upon crossing with Cre-driver mouse strains. The LSL-ACR2 strain can be used to prepare transgenic mice with homogenous expression of ACR2 in targeted neurons with a high penetration ratio, good reproducibility, and no tissue invasion.