Orexinergic Activation of Medullary Premotor Neurons Modulates the Adrenal Sympathoexcitation to Hypothalamic Glucoprivation

Abstract

Glucoprivation activates neurons in the perifornical hypothalamus (PeH) and in the rostral ventrolateral medulla (RVLM), which results in the release of adrenaline. The current study aimed to establish 1) whether neuroglucoprivation in the PeH or in the RVLM elicits adrenaline release in vivo and 2) whether direct activation by glucoprivation or orexin release in the RVLM modulates the adrenaline release. Neuroglucoprivation in the PeH or RVLM was elicited by microinjections of 2-deoxy-D-glucose or 5-thio-D-glucose in anesthetized, euglycemic rats. Firstly, inhibition of neurons in the PeH abolished the increase in adrenal sympathetic nerve activity (ASNA) to systemic glucoprivation. Secondly, glucoprivation of neurons in the PeH increased ASNA. Thirdly, in vivo or in vitro glucoprivation did not affect the activity of RVLM adrenal premotor neurons. Finally, blockade of orexin receptors in the RVLM abolished the increase in ASNA to neuroglucoprivation in the PeH. The evoked changes in ASNA were directly correlated to levels of plasma metanephrine but not to normetanephrine. These findings suggest that orexin release modulates the activation of adrenal presympathetic neurons in the RVLM.Glucoprivation is a metabolic challenge capable of eliciting adrenaline release, an important mechanism for the restoration of normal blood glucose levels. Additionally, neuroglucoprivation produced by 2-deoxy-D-glucose (2DG) is used as an experimental tool to study glucoregulatory neurons (1-4). Previous findings suggest that adrenaline release in response to glucoprivation involves activation of neurons in the perifornical hypothalamus (PeH) and rostral ventrolateral medulla (RVLM). Systemic glucoprivation using 2DG excites RVLM sympathetic premotor neurons (5,6) and orexinergic neurons (7) in the PeH (8). Additionally, neurotropic viruses injected into the adrenal gland transsynaptically label neurons in the RVLM (9) and PeH (10). Disinhibition of perifornical neurons produces an increase in endogenous glucose production in the liver, which is mediated by the autonomic nervous system (11). However, it remains unknown whether intrinsic glucose sensitivity or projections from hypothalamic glucose-sensitive neurons (4,12,13) play an important role in the excitation of RVLM adrenal premotor neurons in response to glucoprivation; in particular, whether the responses evoked in RVLM neurons are modulated by orexinergic inputs (14,15).In this study, we hypothesized that PeH neurons respond to neuroglucoprivation and elicit adrenaline release by orexinergic activation of sympathetic premotor neurons in the RVLM. To test this hypothesis, we used a combination of in vivo and in vitro electrophysiological techniques to first examine the role played by neurons in the PeH in driving adrenal sympathetic nerve activity (ASNA). We then demonstrate for the first time that these effects are independent of any intrinsic sensitivity

Hindbrain Catecholamine Neurons Activate Orexin Neurons During Systemic Glucoprivation in Male Rats

“…This is consistent with our previous work comparing spinal and hypothalamic DSAP injections (10,35) and with results using various other experimental approaches indicating that spinally projecting catecholamine neurons mediate the adrenal medullary response to glucose deficit (60,61). Orexin neurons also project spinally (62) and recent evidence suggests an independent role for these projections in controlling blood glucose (63). Accordingly, orexin neurons that project directly to the spinal cord might contribute to control of blood glucose in PVH-or PeFLH-DSAP-injected rats during glucoprivation.…”

Orexin signaling is necessary for hypoglycemia-induced prevention of conditioned place preference

“…The fact that CPP blunting was prevented by concurrent systemic administration of a brain-penetrant orexin-1 receptor antagonist (25,42) strongly suggests that orexin neurons, which are found only in the PFH and LHA (12,43), are critical components of this response. In fact, we have shown that this same antagonist blunts the Epi response to IIH (37) in keeping with the known connections of PFH orexin neurons with brain stem sympathetic outputs (29) and polysynaptic inputs to the adrenal gland (21). Our results demonstrating a selective decrease in PFH, but not LHA preproorexin expression following RH, further potentially implicate PFH orexin neurons in the blunting of the behavioral response to RH.…”

“…This suggests a potential mechanism of hypoglycemia unawareness, i.e., diminished ability of hypoglycemia to stimulate arousal-promoting orexin neurons of the PFH, which are well described to be activated by hypoglycemia (5,6,22,34). Because PFH orexin neurons are implicated by the present findings in awareness of hypoglycemia, and by previous work in neurohumoral CRR to glucoprivation (29,37), they constitute a potentially important central neural substrate for mediating multiple components of hypoglycemia-associated autonomic failure. Orexin neurons, especially of the PFH, may contribute to behavioral responses during hypoglycemia, which are due to their activation during lowered glucose levels and which are independent of the symptoms caused by elevated Epi levels.…”

“…We propose that awareness of these aversive effects of hypoglycemia in humans and rats requires a state of heightened arousal to recognize both the peripheral effects associated with Epi release [tremor, tachycardia, diaphoresis (27)] and a central component modulated by perifornical hypothalamic (PFH) orexin/hypocretin (here referred to as "orexin") neurons. These neurons are glucosensing (4,35,56) and are involved in the autonomic responses to glucoprivation (29,37), feeding (14,43), reward-based learning, and behavioral arousal (23,24,28,40).…”

Activation of Medulla-Projecting Perifornical Neurons Modulates the Adrenal Sympathetic Response to Hypoglycemia: Involvement of Orexin Type 2 (OX2-R) Receptors

“…In particular, neurons found in the perifornical hypothalamus (PeH) and in the medulla. These neuronal groups communicate between themselves, are activated by hypoglycemia, and their activation is essential for counterregulatory hormonal and feeding responses (2)(3)(4)(5)(6). Furthermore, both groups indirectly communicate (ie, polysynaptically) with chromaffin cells in the adrenal medulla (7,8).…”