INTRODUCTION

Insulin is an important regulator of neuronal function on single cell and network level in the central nervous system (CNS). Animal studies have shown that brain‐specific deletion of the insulin receptor in mice results in diet‐induced obesity and insulin resistance [Brüning et al.,2000]. Insulin is a hormone released in response to food intake and increasing glucose levels and is involved in the regulation of energy homeostasis [Schwartz et al.,2000; Woods et al.,1998]. In humans, it is hypothesized that reduced CNS insulin signaling contributes to the pathogenesis of common metabolic disorders, including diabetes and obesity [Porte et al.,2005]. Indeed, insulin resistance of the brain is associated with obesity and related metabolic diseases as well as cognitive impairments [Hallschmid and Schultes,2009; Tschritter et al.,2006].

To study the effect of insulin on the brain, insulin has been administered intravenously or intranasally (review in Hallschmid and Schultes [2009]). Intranasal insulin application, compared to intravenous systemic insulin administration, has the major advantage that insulin levels are specifically increased in the CNS. Thus, the unwanted side effects of systemic hyperinsulinaemia (e.g., hypoglycaemia, increased counterregulatory hormones, increased systemic blood flow) can be avoided.

Intranasal insulin reduces body weight and body fat mass in lean male subjects [Hallschmid et al.,2004] and improves memory function in humans [Benedict et al., 2007]. Furthermore, a reduced level of insulin action in the brain can play a role in neurodegenerative diseases [Craft and Watson,2004]. Reger et al. [2008] showed that intranasal insulin application improves memory function in Alzheimer's disease. In summary, insulin has various effects in the brain, an organ that has been considered to be insulin independent for a long time.

In addition, insulin may also affect cerebral blood flow (CBF) directly by masking insulin effects on neuronal activity. In the periphery, insulin acts as a vasoactive hormone and causes vasodilatation of peripheral tissues like skeletal muscle [Baron,1996]. Several studies investigated changes in CBF or brain activity measured by blood oxygenation level‐dependent (BOLD) signal during hyperinsulinaemic hypoglycaemic and euglycaemic conditions with positron emission tomography or functional magnetic resonance imaging (fMRI). During hypoglycaemia, it was consistently shown that basal CBF was increased [Kennan et al.,2005; Kerr et al.,1993; Powers et al.,1996]. However, it is unclear whether the increase in CBF during hypoglycaemic hyperinsulinaemia is caused by a systemic effect of peripheral vasodilatation, stress‐induced counterregulation, or specifically increased local CBF. In contrast, only two studies using systemically induced euglycaemic hyperinsulinaemia [Rotte et al.,2005; Seaquist et al.,2006] and one study using intranasal insulin application [Guthoff et al.,2010] have been performed by means of fMRI. Although Seaquist et al. [2006] reported a decrease of the BOLD response in the visual cortex during hyperinsulinaemia, Rotte et al. [2005] and Guthoff et al. [2010] did not find insulin‐induced changes in the visual cortex. However, these studies did not measure CBF.

To test whether insulin has a specific effect on CBF and BOLD response under euglycaemic conditions, we used an intranasal insulin application and simultaneously measured the corresponding BOLD and CBF responses by using arterial spin‐labeling MRI. Furthermore, we compared the insulin effect with caffeine. Caffeine is an adenosine antagonist and a well‐known vasoconstrictive agent reducing basal CBF [Liau et al.,2008; Perthen et al.,2008]. Therefore, caffeine ingestion is used as a standard approach to validate our experimental findings by measuring pharmacological‐induced changes in CBF.

We used visual stimulation, in addition to basal measurements, to elaborate the influence of insulin on both basal and task‐induced CBF, because these two measurements could be controlled via different neurochemical pathways. For example, Mintun et al. [2004] found that intravenously injected lactate does affect stimulus‐evoked CBF, but not baseline CBF. For visual stimulation, we used a flickering checkerboard, which is a passive perceptual stimulation that is not confounded by cognitive or emotional demands and a standard experimental approach to detect functional activation in the primary visual cortex.

We hypothesized that intranasal insulin will not change CBF, both during the basal and task‐induced state, in comparison with the predose measurement. Caffeine, however, as a vasoconstrictive agent should lead to a pronounced decrease in CBF in relation to the predose measurement.