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

Abstract

While the neural control of glucoregulatory responses to insulin-induced hypoglycemia is beginning to be elucidated, brain sites responsible for behavioral responses to hypoglycemia are relatively poorly understood. To help elucidate central control mechanisms associated with hypoglycemia unawareness, we first evaluated the effect of recurrent hypoglycemia on a simple behavioral measure, the robust feeding response to hypoglycemia, in rats. First, food intake was significantly, and similarly, increased above baseline saline-induced intake (1.1 Ϯ 0.2 g; n ϭ 8) in rats experiencing a first (4.4 Ϯ 0.3; n ϭ 8) or third daily episode of recurrent insulin-induced hypoglycemia (IIH, 3.7 Ϯ 0.3 g; n ϭ 9; P Ͻ 0.05). Because food intake was not impaired as a result of prior IIH, we next developed an alternative animal model of hypoglycemiainduced behavioral arousal using a conditioned place preference (CPP) model. We found that hypoglycemia severely blunted previously acquired CPP in rats and that recurrent hypoglycemia prevented this blunting. Pretreatment with a brain penetrant, selective orexin receptor-1 antagonist, SB-334867A, blocked hypoglycemia-induced blunting of CPP. Recurrently hypoglycemic rats also showed decreased preproorexin expression in the perifornical hypothalamus (50%) but not in the adjacent lateral hypothalamus. Pretreatment with sertraline, previously shown to prevent hypoglycemia-associated glucoregulatory failure, did not prevent blunting of hypoglycemia-induced CPP prevention by recurrent hypoglycemia. This work describes the first behavioral model of hypoglycemia unawareness and suggests a role for orexin neurons in mediating behavioral responses to hypoglycemia. diabetes; perifornical hypothalamus; serotonin; awareness HYPOGLYCEMIA UNAWARENESS AND concomitant failure to activate behavioral defenses against hypoglycemia (such as feeding) contribute to the problem of hypoglycemia-associated autonomic failure and the vicious cycle of insulin-induced hypoglycemia (IIH) in Type 1 diabetes mellitus and advanced Type 2 diabetes mellitus patients undergoing intensive insulin therapy for tight glycemic control (8, 9). The neurogenic symptoms generated by adrenergic and cholinergic activation (such as palpitations and diaphoresis) and neuroglycopenic symptoms generated by central glucopenia per se (such as diminished cognitive function) result in awareness of hypoglycemia and stimulate corrective behavioral responses (such as feeding or glucose administration). However, the central neural substrates for awareness of the hypoglycemic state and the neuronal mechanisms underlying the blunting of this awareness following repeated bouts of IIH are not well understood. More importantly, no animal model of blunted awareness following repeated bouts of IIH exists.In this series of studies, we attempted to develop a behavioral measure by which hypoglycemia unawareness could be modeled in animals. We first evaluated whether or not the robust feeding response to IIH is blunted by antecedent hypoglycemic episodes, a...

Ventromedial hypothalamus glucose‐inhibited neurones: A role in glucose and energy homeostasis?

“…The following day, they observed that the rats no longer preferred the side associated with the food treat after experiencing hypoglycaemia in that location. This indicated that the rats were “aware” of hypoglycaemia and found it aversive . This effect was blocked by orexin 1 receptor antagonism, indicating that it was mediated by orexin neurones.…”

“…They also contribute to the CRR via projections to the rostral ventrolateral medulla, leading to activation of the sympathetic splanchnic nerve and epinephrine release from the adrenal medulla . An elegant series of studies showed that orexin neurones also play a role in hypoglycaemia awareness . Hypoglycaemia awareness refers to the symptoms of hypoglycaemia (ie, sweating, anxiety) that alert an individual to falling glucose and prompt corrective behavioural responses.…”

“…Although the hormonal CRR and feeding responses to hypoglycaemia are easily measured in rodents, the lack of an animal model for hypoglycaemia awareness has significantly hampered the field. Otlivanchik et al utilised the standard conditioned place preference (CPP) test to train rats to prefer one side of a two‐sided chamber which was associated with a food treat. They then injected sufficient insulin to induce hypoglycaemia when the rats were confined on the preferred side.…”

Hypoglycemia unawareness prevention: Targeting glucagon production

“…In other words, can neuronostatin activate glucagon production and release from alpha cells already compromised by antecedent hypoglycemia? Future studies should examine this potential protective approach of neuronostatin co-infusion following a severe hypoglycemic episode now that animal models of hypoglycemia unawareness have been developed (8, 14). …”

10 lessons learned by a misguided physician

“…It was my good fortune to lure Nicole to my lab for a 3 year stint, leaving behind her husband at Washington State University to finish his veterinary neurology residency. Over this stimulating 3 year period, we had many discussions and amiable arguments that greatly advanced my understanding of this research area and launched me into this field which would provide me with a great deal of fun, intellectual stimulation and an additional source of long-time NIH funding [25, 35–39]. …”