Neurons in ventral tegmental area tonically inhibit sympathetic outflow to brown adipose tissue: possible mediation of thermogenic signals from lateral habenula

Developmental or adult-onset deletion of neurotensin receptor-1 from dopamine neurons differentially reduces body weight

Central neurotensin signaling via neurotensin receptor-1 (NtsR1) modulates various aspects of physiology, including suppressing feeding and promoting locomotor activity that can support weight loss. However, it remains unclear when and where NtsR1 expression contributes to control of body weight vs. other effects. We previously showed that activating ventral tegmental area (VTA) dopamine (DA) neurons that express NtsR1 promotes weight loss. We therefore hypothesized that deleting NtsR1 from DA neurons would promote weight gain by increasing food intake and decreasing physical activity. In contrast, developmental deletion of NtsR1 from DA neurons (by crossing DATCre mice with NtsR1flox/flox mice) had no impact on the feeding or body weight of mice fed a chow diet, though it augmented locomotor activity. Developmental deletion of NtsR1 from DA neurons protected mice from diet-induced obesity, but not via altering feeding, physical activity, or energy expenditure. Given that NtsR1 may exert distinct roles within development vs. adulthood, we then examined the impact of adult-onset deletion of NtsR1 from VTA DA neurons. We injected adult NtsR1flox/flox mice in the VTA with adeno associated virus to Cre-dependently delete NtsR1 in the VTA (VTAR1Null mice) and compared them to mice with intact NtsR1 (Controls). Again, in contrast to our hypothesis, VTAR1Null mice gained less weight than Controls while on normal chow or high fat diets. Moreover, VTAR1Null mice exhibited blunted feeding after fasting, suggesting a role for NtsR1 in adult VTA DA neurons in coordinating energy need and intake. Altogether, these data suggest that intact expression of NtsR1 in DA neurons is necessary for appropriate regulation of body weight, but a lack of NtsR1 in the developing vs. adult DA system protects from weight gain via different mechanisms. These findings emphasize the need for temporal and site-specific resolution to fully understand the role of NtsR1 within the brain.

Central sympathetic network for thermoregulatory responses to psychological stress

In mammals, many types of psychological stressors elicit a variety of sympathoexcitatory responses paralleling the classic fight-or-flight response to a threat to survival, including increased body temperature via brown adipose tissue thermogenesis and cutaneous vasoconstriction, and increased skeletal muscle blood flow via tachycardia and visceral vasoconstriction. Although these responses are usually supportive for stress coping, aberrant sympathetic responses to stress can lead to clinical issues in psychosomatic medicine. Sympathetic stress responses are mediated mostly by sympathetic premotor drives from the rostral medullary raphe region (rMR) and partly by those from the rostral ventrolateral medulla (RVLM). Hypothalamomedullary descending pathways from the dorsomedial hypothalamus (DMH) to the rMR and RVLM mediate important, stress-driven sympathoexcitatory transmission to the premotor neurons to drive the thermal and cardiovascular responses. The DMH also likely sends an excitatory input to the paraventricular hypothalamic nucleus to stimulate stress hormone release. Neurons in the DMH receive a stress-related excitation from the dorsal peduncular cortex and dorsal tenia tecta (DP/DTT) in the ventromedial prefrontal cortex. By connecting the corticolimbic emotion circuit to the central sympathetic and somatic motor systems, the DP/DTT → DMH pathway plays as the primary mediator of the psychosomatic signaling that drives a variety of sympathetic and behavioral stress responses. These brain regions together with other stress-related regions constitute a central neural network for physiological stress responses. This network model is relevant to understanding the central mechanisms by which stress and emotions affect autonomic regulations of homeostasis and to developing new therapeutic strategies for various stress-related disorders.

Central Neural Circuits Orchestrating Thermogenesis, Sleep-Wakefulness States and General Anesthesia States

Great progress has been made in specifically identifying the central neural circuits (CNCs) of the core body temperature (Tcore), sleep-wakefulness states (SWs), and general anesthesia states (GAs), mainly utilizing optogenetic or chemogenetic manipulations. We summarize the neuronal populations and neural pathways of these three CNCs, which gives evidence for the orchestration within these three CNCs, and the integrative regulation of these three CNCs by different environmental light signals. We also outline some transient receptor potential (TRP) channels that function in the CNCs-Tcore and are modulated by some general anesthetics, which makes TRP channels possible targets for addressing the general-anesthetics-induced-hypothermia (GAIH). We suggest this review will provide new orientations for further consummating these CNCs and elucidating the central mechanisms of GAIH.

Alpha2-adrenergic receptor agonists prevent emotional hyperthermia

Emotionally significant stimuli, including potential threats from the external environment, trigger an increase in body temperature, a response known as emotional hyperthermia. Sympathetically-mediated brown adipose tissue (BAT) thermogenesis contributes substantially to this hyperthermic response. The systemic administration of α₂-adrenergic agonists is known to inhibit both febrile and shivering responses. In the present study, we investigated whether systemic administration of clonidine, a α₂-adrenoceptor agonist, attenuates the emotional hyperthermia evoked in conscious unrestrained rats suddenly confronted with a second (intruder) rat, itself confined to a small cage. Pre-implanted thermistors were used to measure BAT and body temperature in conscious, freely moving, male Sprague-Dawley rats. The rats were pre-treated with intraperitoneally administered vehicle (Ringer solution) or clonidine (1, 10 and 100 µg/kg). Clonidine, in a dose-dependent manner, reduced the intruder-elicited increases in BAT (log-dose linear regression F(1,16) = 9.52, R² = 0.37, P < 0.01) and body temperature (F(1,16) = 6.48, R² = 0.29, P < 0.05). We also investigated, in anesthetized rats, whether systemic clonidine administration inhibits BAT sympathetic nerve discharge evoked via activation of neurons in the lateral habenula (LHb) - a nucleus involved in the regulation of emotional hyperthermia. In anesthetized rats, clonidine abolished the BAT sympathetic nerve discharges elicited via bicuculline-mediated disinhibition of the LHb. These results suggest that activation of central α₂-adrenergic receptors attenuates the process of emotional hyperthermia by reduction of BAT thermogenesis.

Activating dopamine D2 receptors reduces brown adipose tissue thermogenesis induced by psychological stress and by activation of the lateral habenula

Emotional hyperthermia is the increase in body temperature that occurs as a response to an animal detecting a salient, survival-relevant stimulus. Brown adipose tissue (BAT) thermogenesis, controlled via its sympathetic innervation, contributes to this temperature increase. Here, we have used an intruder rat experimental model to determine whether quinpirole-mediated activation of dopamine D₂ receptors attenuates emotional hyperthermia in conscious rats. In anesthetized rats, we determined whether systemic quinpirole reduces BAT nerve discharge induced by activation of the medullary raphé and the lateral habenula (LHb). We measured BAT and body temperature with chronically implanted thermistors in conscious, freely moving, individually housed, male rats (resident rats). Either vehicle or quinpirole was administered, intraperitoneally, to the resident rat 30 min before introduction of a caged intruder rat. Quinpirole, in a dose-dependent manner, reduced intruder-elicited increases in BAT and body temperature. Pre-treatment with the D₂ antagonist spiperone, but not the selective D₁ antagonist SCH-23390, prevented this quinpirole-elicited decrease. In anesthetized rats, quinpirole abolished BAT sympathetic nerve discharge elicited by bicuculline-mediated activation of the LHb, but not the medullary raphé. Thus, activation of dopamine D₂ receptors reduces the BAT thermogenesis that contributes to emotional hyperthermia. We provide evidence that these dopamine D₂ receptors are located in the thermogenic pathway between the LHb and the lower brainstem pre-sympathetic control centre in the medullary raphé.