Noradrenergic Activity in the Human Brain: A Mechanism Supporting the Defense Against Hypoglycemia
J Clin Endocrinol Metab, June
Noradrenergic Activity in the Human Brain: A Mechanism Supporting the Defense Against Hypoglycemia
Renata Belfort-DeAguiar 2
Jean-Dominique Gallezot 1
Janice J. Hwang 2
Ahmed Elshafie 2
Catherine W. Yeckel 0
Owen Chan 4
Richard E. Carson 1
Yu-Shin Ding 3
Robert S. Sherwin 2
0 Yale School of Public Health, Yale School of Medicine , New Haven, Connecticut 06510 , USA
1 PET Center, Department of Diagnostic Radiology, Yale University , New Haven, Connecticut, 06519 , USA
2 Department of Internal Medicine, Section of Endocrinology, Yale University School of Medicine , New Haven, Connecticut 06519 , USA
3 Department of Radiology, New York University Medical Center , New York, New York 10016 , USA
4 Department of Internal Medicine, Division of Endocrinology, Metabolism and Diabetes, University of Utah , Salt Lake City, Utah 84112 , USA
Context: Hypoglycemia, one of the major factors limiting optimal glycemic control in insulintreated patients with diabetes, elicits a brain response to restore normoglycemia by activating counterregulation. Animal data indicate that local release of norepinephrine (NE) in the hypothalamus is important for triggering hypoglycemia-induced counterregulatory (CR) hormonal responses. Objective: To examine the potential role of brain noradrenergic (NA) activation in humans during hypoglycemia. Design: A hyperinsulinemic-hypoglycemic clamp was performed in conjunction with positron emission tomographic imaging. Participants: Nine lean healthy volunteers were studied during the hyperinsulinemic-hypoglycemic clamp. Design: Participants received intravenous injections of (S,S)-[11C]O-methylreboxetine ([11C]MRB), a highly selective NE transporter (NET) ligand, at baseline and during hypoglycemia. Results: Hypoglycemia increased plasma epinephrine, glucagon, cortisol, and growth hormone and decreased [11C]MRB binding potential (BPND) by 24% 6 12% in the raphe nucleus (P , 0.01). In contrast, changes in [11C]MRB BPND in the hypothalamus positively correlated with increments in epinephrine and glucagon levels and negatively correlated with glucose infusion rate (all P , 0.05). Furthermore, in rat hypothalamus studies, hypoglycemia induced NET translocation from the cytosol to the plasma membrane. Conclusions: Insulin-induced hypoglycemia initiated a complex brain NA response in humans. Raphe nuclei, a region involved in regulating autonomic output, motor activity, and hunger, had increased NA activity, whereas the hypothalamus showed a NET-binding pattern that was associated with the individual's CR response magnitude. These findings suggest that NA output most likely is important for modulating brain responses to hypoglycemia in humans. (J Clin Endocrinol Metab 103: 2244-2252, 2018)
Abbreviations: [11C]MRB, (S,S)-[11C]O-methylreboxetine; BPND, binding potential; CNS,
central nervous system; CR, counterregulatory; GIR, glucose infusion rate; IV, intravenous;
MRB, methylreboxetine; NA, noradrenergic; NE, norepinephrine; NET, norepinephrine
transporter; PET, positron emission tomography; ROI, region of interest; VMH,
Hinsulin-treated patients with diabetes (
). The body
ypoglycemia is a common complication observed in
responds to hypoglycemia by promoting a
counterregulatory (CR) response, which involves the activation
of glucose-sensing cells in peripheral tissues and the
central nervous system (CNS) (
). In particular, the CNS
plays a central role in coordinating this CR response to
hypoglycemia by stimulating the release of CR hormones
(glucagon, epinephrine, cortisol, and growth hormone)
and initiating symptom responses, including activation of
the sympathetic autonomic nervous system (palpitations,
tremors, sweating) and induction of neuroglucopenic
symptoms (hunger, dizziness, confusion) (
). Together, these
integrated responses lead to an increase in endogenous
glucose production, a decrease in peripheral glucose
utilization, hypoglycemia awareness, and stimulation of
food-seeking behavior and food intake.
Animal studies have provided evidence that systemic
hypoglycemia triggers changes in specific glucose-sensing
neurons in the CNS that stimulate the release of CR
hormones and promote endogenous glucose production,
which serves as a primary defense mechanism (
particular, local glucoprivation of glucose sensing
neurons in the hypothalamus can elicit the CR response (
). Hypothalamic activity is modulated by changes in
local neurotransmitter release, including g-aminobutyric
acid, glutamate, serotonin, and norepinephrine (NE) (
With regard to the latter, noradrenergic (NA) activity in
the hypothalamus is increased by both systemic (
) hypoglycemia and appears to be particularly
important in mounting a CR response. Moreover,
activity of hypothalamic tyrosine hydroxylase, the
ratelimiting enzyme of NE synthesis, increases in response
to both hypoglycemia and neuroglycopenia (11). This
provides additional evidence that NA neurons play an
important role in regulating glucose metabolism. In
keeping with this view, local injection of NE into the
hypothalamus has been reported to increase blood
glucose levels and promote food intake in rodents (
However, these observations have been limited to
animals. Thus, the extent to which central activation of the
CNS NA system promotes the CR response to
hypoglycemia in humans remains to be determined.
Of note, the cell bodies for the central NA system are
grouped in nuclei in the medulla and pons (
) and project
axons to most CNS regions, including the cortex,
amygdala, hypothalamus, hippocampus, thalamus, brain stem
(including the raphe nuclei), cerebellum, and spinal cord
). When these neurons are activated, they release NE
from the terminal axon into the synaptic cleft, which will
bind to postsynaptic adrenergic receptors and complete
NA neurotransmission. The NE released is cleared from
the synaptic cleft by the presynaptic NE transporter (NET),
which represents the major mechanism for terminating NE
). By regulating NE concentration in the
synapse, NET modulates NA neurotransmission (
serves as an indicator of NA system activity.
Selective ligands for assessing NET binding are now
available for use in positron emission tomography (PET)
experiments to evaluate NET in vivo in the human brain
). In particular, (S,S)-[(11)C]O-methylreboxetine
([11C]MRB), a highly selective NET ligand, has been used
for human brain studies. MRB (an analog of reboxetine
that has been approved in Europe as an antidepressant)
has a high affinity for NET (
), and the radiotracer
[11C]MRB is capable of helping estimate NET availability
through determination of its binding potential (BPND).
Therefore, the current study was designed to evaluate the
CNS NA system in the human brain during hypoglycemia
by using a hypoglycemic-hyperinsulinemic clamp
technique in conjunction with PET imaging after a bolus
injection of the radioligand [11C]MRB. We postulated
that hypoglycemia would activate NA neurons, thereby
stimulating NE release in the synaptic cleft and in turn
competing with [11C]MRB binding to NET, eventually
lowering specific binding of [11C]MRB in NET-rich
regions of the brain. In addition, we also probed the
properties of the hypothalamic NET-specific response
in a rat model. NET protein levels in hypothalamic tissue
(cytosolic and membrane fractions) were determined
under both baseline and hypoglycemia conditions.
Materials and Methods
Nine healthy volunteers participated in this study [five men
and four women; age, 34 6 4 years; hemoglobin A1c, 5.1% 6
0.2% (32 6 0.2 mmol/mol); body mass index, 23.4 6 1.0 kg/
m2]. The volunteers came to the Hospital Research Unit at the
Yale New Haven Hospital in New Haven, Connecticut, for a
screening visit, including a medical history and physical
examination performed by one of the study physicians and a urine
toxicology screen. Participants with a history of any major
medical or psychiatric disease were excluded. Participants who
qualified were invited to participate in the PET–hypoglycemic
clamp study. The Human Investigation Committee and
Radiation Safety Committee at Yale University approved the study.
All participants provided written informed consent before
participating in the study. This research study is registered at
Participants arrived at the PET center at 7 AM after an 8-hour
overnight fast. An intra-arterial line was placed in the wrist of
one arm for blood draws (tracer kinetics, glucose, and
hormones) and two intravenous (IV) access sites were obtained
in the contralateral arm: one for a bolus infusion of the
radiotracer and a second for infusion of insulin and dextrose. The
participants were scanned twice: at baseline and during the
hypoglycemic-hyperinsulinemic clamp (2 mU/kg per min)
(Fig. 1). Before the baseline scan, the participants received an IV
bolus injection of [11C]MRB (639 6 126 MBq) with injected
mass of 1.97 6 0.78 mg and specific activity of 292 6 121 MBq/
nmol. PET acquisition of the brain was obtained for the
subsequent 2 hours, starting at the beginning of the [11C]MRB
bolus, with a High Resolution Research Tomograph (Siemens/
CTI, Knoxville, TN) PET scanner (with an intrinsic resolution of
~3 mm full width half maximum). The second PET scan was
started ~2 hours after the baseline scan was completed. A
primed-continuous IV infusion of insulin was given at the start
of the 6-minute PET transmission scan, which was followed by
the second [11C]MRB bolus (644 6 85 MBq; injected mass,
1.88 6 0.85 mg; and specific activity = 372 6 318 MBq/nmol),
administered 259 6 32 minutes after the first bolus injection.
The insulin infusion continued throughout the total duration
of the PET scan (~120 minutes). Plasma glucose levels were
allowed to drop freely to 60 mg/dL, at which time a variable
IV dextrose infusion was started, and glucose levels were
kept at ~55 mg/dL by adjusting the glucose infusion rate (GIR)
based on the plasma glucose levels. Plasma glucose was
measured every 5 minutes throughout the study. Insulin and CR
hormones (glucagon, catecholamines, cortisol, and growth
hormone) were obtained throughout the
A 3-Tesla MRI (Siemens) examination of the brain was
performed in each participant for anatomical coregistration
with the PET imaging studies (
The synthetic procedures for [11C]MRB have been described
). During the PET scans, participants wore a
rigid optical tracking tool, attached to the head with a swim cap,
to record head motion with an infrared detector (Vicra, NDI
Systems, Waterloo, ON, Canada). A 6-minute transmission
scan was acquired after initiation of head-motion recording and
just before [11C]MRB injection. List mode data were acquired
for 120 minutes and reconstructed with all corrections
(attenuation, normalization, scatter, randoms, dead-time, and
motion) by using the MOLAR algorithm (22), and a second step of
motion correction was applied, as previously described . The
BPND values, an index of the number of available binding sites
[for review, see Innis et al. (
)], were calculated by using the
multilinear reference tissue model (
), with optimizations to
reduce noise in parametric images (
) with a reference region
(the caudate nucleus) that has lowest levels of NET
MRI-based region-of-interest definition
The regions of interest [ROIs; predetermined NET-rich
areas of the brain (
), including the locus ceruleus, raphe
nuclei, thalamus, and hypothalamus] were defined in
template space (29) and were applied to the PET images by using
the participant’s MR image as an intermediate step, as
previously described (
). The paracentral cortical lobe,
thalamus, and caudate ROIs were taken from the automated
anatomical labeling template (30). The locus ceruleus and
raphe nuclei ROIs were drawn on average PET images
resliced in template space in previous studies using tracers
with high uptake in each of these structures (
hypothalamus ROI was manually drawn on the template
The plasma glucose concentration was determined by the
glucose oxidase method (YSI Inc., Yellow Springs, OH). Plasma
insulin and glucagon (Millipore, St. Charles, MO), growth
hormone (MP Biomedical, Irvine, CA), and cortisol (Diagnostic
Products Corp., Los Angeles, CA) were measured by
doubleantibody radioimmunoassay, and plasma catecholamines were
measured by high-performance liquid chromatography (ESA,
Statistical analyses were performed by using SPSS software,
version 19.0 (IBM, Armonk, NY). All values represent the
mean 6 standard error of the mean. A paired t test was
performed to compare hormone levels and the [11C]MRB BPND
from the preselected ROIs at baseline and during the
hypoglycemic clamp. Pearson correlations were performed between
the BPND from these brain areas and hormones levels.
Methods for animal studies
Studies were performed in male Sprague-Dawley rats
(weight ~280 to 300 g; Charles River Labs, Raleigh, NC) that
were individually housed in the Yale Animal Resources Center
in temperature- (22°C to 23°C) and humidity-controlled rooms.
The animals had free access to rat chow (Harlan Teklad,
Indianapolis, IN) and water. The animals were acclimatized
to handling and a 12-hour light cycle (lights on between
0700 hours and 1900 hours) for 1 week before experimental
manipulation. Principles of laboratory animal care were
followed, and experimental protocols were approved by
the Institutional Animal Care & Use Committee at Yale
Hypoglycemia was induced in the animals with a single
intraperitoneal injection of regular human insulin (10 U/kg;
Eli Lily, Indianapolis, IN). Blood glucose was monitored
every 30 minutes from a tail nick. One hour after insulin
administration, when plasma glucose levels had reached
~30 to 40 mg/dL, the animals were euthanized with an
overdose of sodium pentobarbital; the brains were rapidly
harvested and frozen on dry ice. A second group of animals
received a saline injection and were euthanized under
similar conditions. Four animals were studied in each set of
The brains were coronally sectioned on a cryostat, and
frozen micropunches were taken through the ventromedial
hypothalamus (VMH). The membrane and cytosolic
fractions were separated by using a membrane protein extraction
kit (ProteoExtract Native Membrane Protein Extraction Kit,
Calbiochem®, Merck KGaA, Darmstadt, Germany), and
the membrane fraction was confirmed by the presence of
cadherin, a protein that is expressed exclusively in membranes.
Ten micrograms of protein was loaded onto a 4% to 20%
gradient gel and the samples were run at 85 V for 1 to 1.5 hours
at room temperature. The protein was subsequently
transferred onto a nitrocellulose membrane overnight at 25 V at
4°C. The next day, the membranes were blocked with 5% milk
in tris-buffered saline–Tween 20 for 1 hour at room
temperature. After blocking, the membrane was incubated in the
primary antibody against the NET (Millipore AB2234)
overnight at a 1:1000 dilution or b-actin (1:5000). After
labeling of the NET, the membranes were stripped and labeled
with the b-actin antibody. After several washes, the
membrane was incubated in the secondary anti-rabbit antibody
(1:5000) or anti-mouse (1:20,000) for 1 hour. After washing,
chemiluminescent reagent was applied to the membrane for
2 minutes before the membrane was exposed to film. Relative
optical density was quantified by using Scion Image software
(Scion Corp., Frederick, MD) and expressed as a ratio against
the loading control.
Human study participants had normal fasting plasma
glucose (88 6 2 mg/dL) and hemoglobin A1c (5.1% 6
0.2%) levels. Before the start of the
hypoglycemichyperinsulinemic clamp study, plasma insulin levels
were 10 6 2 mU/mL. After initiation of the insulin
infusion, glucose levels declined into the hypoglycemic
range; mean glucose levels during the last 30 minutes of
the PET-hypoglycemic clamp were 57 6 1 mg/dL (Fig.
1b). The GIR required to keep glucose within the target
range averaged 3.5 6 0.6 mg/kg/min (Fig. 1c). As shown
in Table 1, all hormones, except for NE,
significantly increased during hypoglycemia (P , 0.05). Plasma
epinephrine at the end of the hypoglycemic clamp
(120-minute time point) rose nearly ninefold (P , 0.005).
The average injected dose of [11C]MRB per scan was
642 6 104 MBq (0.58 to 3.47 mg). [11C]MRB BPND was
measured in NET-rich areas of the brain, including locus
ceruleus, raphe nuclei, thalamus, hypothalamus, and
paracentral lobule (Fig. 2a). During hypoglycemia, in
comparison with the baseline scan, [11C]MRB BPND
decreased by 24% 6 12% in the raphe nuclei (P , 0.01).
No statistically significant changes in MRB binding were
observed in the other NET-rich areas measured with PET
imaging (Fig. 2b).
The hypothalamus was the only NET-rich brain region
in which changes in [11C]MRB BPND positively
correlated with changes in epinephrine (r = 0.723; P = 0.028)
and glucagon levels (r = 0.670; P = 0.048), and inversely
correlated with GIR (r = 20.805; P = 0.009) (Fig. 3). In
contrast, the changes in [11C]MRB BPND in the raphe
nuclei during the two PET scans (hypoglycemic-clamp
To further explore these correlations
between the changes in NET-binding in
the hypothalamus and CR responses, we
determined that the changes in [11C]
MRB BPND in this region, in contrast
to the raphe, were centered around zero
for the average CR response, with no
absolute changes in [11C]MRB BPND
during hypoglycemia and baseline. On
the basis of these findings, we divided
the study participants into two groups
according to their changes in [11C]MRB
BPND in the hypothalamus
(hypoglycemia minus baseline scan). Figure 4 shows
that participants (n = 5) with a decrease
in NET binding (increased NA activity)
had a low peripheral CR hormonal
response (epinephrine and glucagon) and a
correspondingly elevated GIR. In
contrast, when NET binding increased (low
NA activity), the participants (n = 4)
showed more robust CR hormonal
responses and a correspondingly low
requirement for exogenous glucose (P ,
0.05 for the comparison of the groups by
increase vs decrease in [11C]MRB BPND
in the hypothalamus). There were no
statistically significant differences in age,
body mass index, hemoglobin A1c,
glucose, and insulin levels between the two
groups (all P = not significant), indicating
that NA activity in the hypothalamus
during insulin-induced hypoglycemia was
not regulated directly by blood glucose
levels but by the magnitude of the CR
scan BPND minus baseline scan BPND) did not correlate
with GIR or with increments in the CR hormones glucagon,
epinephrine, growth hormone, and cortisol; although the
binding response in the raphe nuclei negatively correlated
with changes in NE levels (hypoglycemia at 90 to 120
minutes minus baseline levels, r = 20.678; P = 0.045).
Cytosolic and plasma membrane NET
protein levels in the VMH-projecting
NA neurons of normal
hypoglycemianaive rats were compared under
baseline (euglycemic) conditions and after an
acute bout of hypoglycemia. Under
euglycemic conditions, there was no
significant difference between NET
protein levels in the cytosolic and
membrane fractions. However, after the induction of
hypoglycemia, NET protein levels in the membrane
fraction were significantly higher than those in the
cytosolic fraction (Fig. 5), suggesting that hypoglycemia
may have induced translocation of NET from the
cytosolic pool to the plasma membrane.
The current study demonstrates that the PET radioligand
[11C]MRB has the ability to identify the NET-rich areas
of the human brain (
)—the locus ceruleus, raphe
nuclei, thalamus, hypothalamus, and paracentral lobule—
both at baseline and during hypoglycemia. Of
particular interest, these human experiments demonstrate
that NET binding in the raphe nuclei was reduced by
24% during hypoglycemia, thus providing evidence that
NA system activation in this specific CNS region likely
contributes to the in vivo CR response to hypoglycemia in
humans. It is also noteworthy that it was not the raphe
nuclei but instead the NET-binding changes in the
hypothalamus that was associated with the magnitude of
both the peripheral CR hormonal and metabolic
responses to hypoglycemia. Moreover, individuals with
poor CR responses showed the greatest NA activation in
To evaluate the CNS NA system response in vivo
during hypoglycemia in humans, we used [11C]MRB,
a highly selective NET-ligand radiotracer for PET
imaging. NET is a transmembrane transporter that plays
an important role in regulating NA
). This radiotracer is capable of imaging NET
availability in living systems (
). In our study, [11C]
MRB-PET imaging not only identified the NET-rich
areas of the brain but also provided a means to
measure acute changes in NET occupancy. This tracer thus
holds promise as a clinical tool to evaluate the CNS NA
system in humans.
Unexpectedly, we identified the raphe nuclei as being
responsive to hypoglycemia. The raphe nuclei are
located in the brainstem and receive inputs from the
locus coeruleus via NA neurons. The raphe nuclei consist
predominantly of serotonergic neurons that project to the
entire CNS and serve as the main source of serotonin for
the brain (
). These neurons are involved in
modulating pain sensation, motor activity, sleep-wake cycles,
mood disorders, circadian rhythm, and food intake
). Although not widely studied, insulin-induced
hypoglycemia in cats increases activity in NA neurons
in the locus ceruleus (
) and decreases
serotoninergic neuronal activity in medullary (
), but not in
), raphe nuclei. In these animal studies,
insulin-induced hypoglycemia promoted a CR
hormonal response, which was accompanied by a decrease
in neuronal firing in the medullary raphe nuclei and a
decrease in muscle tone (
). In humans, fluoxetine, a
selective serotonin reuptake inhibitor increases
autonomic as well as motor CR responses in healthy
participants and in patients with type 1 diabetes mellitus
). Although MRB PET imaging cannot determine
the effect of hypoglycemia within distinct raphe nuclei,
these findings suggest that increased NA activity in
raphe nuclei may promote increased autonomic output,
decreased motor activity, increased stress response, and
hunger, all of which may be important for recognizing
hypoglycemia in humans.
The significant decrement observed in [11C]MRB
BPND in the raphe nuclei during hypoglycemia could be
due to reduced NET availability in the presynaptic
membrane of the NA neuron and/or increased competition
caused by endogenously released NE. The [11C]MRB
tracer binds to NET and at baseline [11C]MRB BPND
corresponds to NET availability in the presynaptic
membrane of the NA neuron. On the basis of a prior
study showing that increased NET occupancy results in
decreased [11C]MRB-binding (
), we postulate that
during hypoglycemia, activated NA neurons release NE
in the synapse, thereby effectively competing with the
[11C]MRB ligand for NET binding and in turn decreasing
[11C]MRB BPND. The rodent studies demonstrating
greater membrane NET levels compared with cytosolic
levels following the induction of hypoglycemia
supports the latter conclusion. During hypoglycemia,
activated NA neurons released NE in the synapse. NET
is then translocated to the plasma membrane to take
up and repackage NE for release. This increase in
NET trafficking is consistent with the observed
decrease in [11C]MRB BPND in our human studies.
However, further studies will be needed to precisely
define how hypoglycemia affects NET and [11C]MRB
BPND. Furthermore, we also cannot exclude a direct
effect of insulin on NET. Long-term insulin treatment
has been reported to decrease NET messenger RNA
levels in the locus ceruleus of the rat (
). Thus, human
studies using the euglycemic-hyperinsulinemic clamp
technique will be required to differentiate the effects
of hypoglycemia and hyperinsulinemia per se on the
central NA system.
As expected, hypoglycemia induced the release of CR
hormones in the periphery. The individual changes in
NET binding in the hypothalamus during hypoglycemia
positively correlated with hormonal changes as well as
negatively correlated with GIR (Fig. 3). These results
highlight that NET binding in the hypothalamus appears
titrated, rather than a simple uniform directional
response (as observed in the raphe nuclei). When
participants successfully mounted a large hormonal CR
response to hypoglycemia, [11C]MRB BPND in the
hypothalamus was unchanged or increased (consistent with
low local NA activation). In contrast, participants who
appeared to have an unsuccessful CR glucose response
(low CR hormones and high GIR) presented with
decreased NET binding (i.e., elevated NA activation)
(Figs. 3 and 4). These results raise the question of whether
the NA system could be a mechanism involved in
regulating hypothalamic activity in response to the magnitude
of the CR hormonal response.
The hypothalamus has been shown in animal studies
to be the central coordinator of the CR response to
), which may, at least in part, be mediated
by local release of NE (
) from NA neurons.
However, in our human studies, [11C]MRB BPND in the
hypothalamus was not statistically different when we
compared the hypoglycemic-clamp and baseline scans,
because of CR responders and nonresponders. Animal
studies have demonstrated that systemic insulin-induced
hypoglycemia increases extracellular NE in the VMH and
paraventricular nucleus, but not in the lateral
hypothalamus nucleus (
). [11C]MRB PET imaging determines
NET binding by anatomical brain regions. Therefore, we
cannot exclude that distinct changes in NA neuronal
activity within specific hypothalamic nuclei may have
occurred. In addition, although the [11C]MRB tracer can
accurately determine changes in NET occupancy under
different conditions (
), the low signal-to-noise ratio
observed with this tracer (
) may have limited our ability
in identifying minor changes in NET binding. However,
despite that, we were able to correctly localize the
NETrich regions of the brain and that hypoglycemia alters NA
activity in the raphe nuclei. Another potential limitation
of this tracer is that it requires ~2 hours of imaging
acquisition, limiting our ability to evaluate the time course
of NET-binding throughout the hypoglycemic clamps,
which could have helped further establish the interplay
between the CNS NA system and CR.
In summary, [11C]MRB, a selective NET-ligand
radiotracer for PET imaging, was shown to be an effective
tool for measuring the complex brain NA system activity
induced by acute hypoglycemia in humans. We showed
that humans experiencing hypoglycemia had increased
NA activation in the raphe nuclei, a region that has the
capacity to influence the individual’s autonomic and
symptomatic responses to hypoglycemia. In contrast,
hypothalamic NET-binding changes under hypoglycemia
revealed a pattern across a wide range of individual CR
responses. The largest trigger for NA activation in healthy
humans was observed in those with poor CR responses.
Taken together, these findings suggest that although the
CNS NA system is activated, the hypothalamus may titrate
its response in healthy humans. Thus, a hypoglycemic
clamp combined with a PET imaging approach using [11C]
MRB may provide insights into the potential role of
the CNS NA system in the development of hypoglycemia
unawareness in insulin-treated patients with diabetes.
We thank Christian Schmidt, Ralph Jacob, Mikhail Smolgovsky,
Irene Chernyak, and Codruta Todeasa for their technical
assistance, the staff of the Yale PET Center and the Hospital Research
Unit at the Yale New Haven Hospital, and the volunteers who
participated in this study.
Financial Support: This work was supported by in part by
National Institute of Diabetes and Digestive and Kidney
Diseases (grants R01 DK20495 and T32 DK 07058; R.S.S.), the
Diabetes Research Center (grant P30 DK045735; R.S.S.), and
the Yale Center for Clinical Investigation supported by the
Clinical and Translational Science Award (grant UL1 TR001863
from the National Center for Advancing Translational Science;
Clinical Trial Information: ClinicalTrials.gov no.
NCT02056249 (registered 3 February 2014).
Correspondence and Reprint Requests: Renata
BelfortDeAguiar, MD, PhD, 300 Cedar Street, TAC S135, New Haven,
Connecticut 06520. E-mail: .
Disclosure Summary: The authors have nothing to
Belfort-DeAguiar et al
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