Brain nociceptive imaging in rats using 18f-fluorodeoxyglucose small-animal positron emission tomography

doi:10.1016/j.neuroscience.2008.07.013

Neuroscience

Volume 155, Issue 4, 9 September 2008, Pages 1221-1226

https://doi.org/10.1016/j.neuroscience.2008.07.013 Get rights and content

Abstract

Preclinical exploration of pain processing in the brain as well as evaluating pain-relief drugs in small animals embodies the potential biophysical effects in humans. However, it is difficult to measure nociception-related cerebral metabolic changes in vivo, especially in unanesthetized animals. The present study used ¹⁸F-fluorodeoxyglucose small-animal positron emission tomography to produce cerebral metabolic maps associated with formalin-induced nociception. Anesthesia was not applied during the uptake period so as to reduce possible confounding effects on pain processing in the brain. The formalin stimulation at the hind paw of rats resulted in significant metabolic increases in the bilateral cingulate cortex, motor cortex, primary somatosensory cortex, secondary somatosensory cortex, insular cortex, visual cortex, caudate putamen, hippocampus, periaqueductal gray, amygdala, thalamus, and hypothalamus. Among the measured areas, clear lateralization was only evident in the primary somatosensory cortex and hypothalamus. In addition, pretreatment with lidocaine (4 mg/kg, i.v.) and morphine (10 mg/kg, i.v.) significantly suppressed formalin-induced cerebral metabolic increases in these areas. The present protocol allowed identification of the brain areas involved in pain processing, and should be useful in further evaluations of the effects of new drugs and preclinical therapies for pain.

Section snippets

Subjects

Nineteen adult male Wistar rats (8–10 weeks old; weighing approximately 250–300 g; National Laboratory Animal Center, Taipai, Taiwan, Republic of China) were used in the present study. The animals were housed in a well-controlled environment with a 12-h light/dark cycle and constant humidity and temperature. Rats were housed in plastic cages at three animals per cage with free access to food and water. All experimental procedures were approved by the Institutional Animal Care and Use Committee,

Formalin-induced nociceptive maps

The present study used ¹⁸F-FDG microPET to elucidate the nociception-induced glucose metabolic changes in the brains of conscious rats. In order to further improve the accuracy with which anatomical locations were determined, microPET and MRI images were coregistered and fused with a digital atlas of the rat brain (Paxinos and Watson, 1998). This method allowed regions of interest to be selected based on clear spatial references (Fig. 1). Averaged formalin-induced metabolic maps overlaid on the

Discussion

The present study clearly revealed formalin-induced nociceptive maps, with strong activations evident in the Cg, M, S1, S2, IC, VC, CPu, HIP, PAG, Amyg, Th, and HT. Although functional imaging techniques have been used previously to examine pain-related responses in rodent brains, such as a blood-flow-based autoradiographic method (Morrow et al., 1998) and fMRI (Tuor et al 2000, Shah et al 2005, Shih et al 2008a, Shih et al 2008b), the method employed in the current study provides new

Conclusion

The present study established an ¹⁸F-FDG microPET protocol that allowed both serial and longitudinal measurements of nociception-induced metabolic changes in the brains of conscious rats. The formalin-induced nociception maps revealed several brain regions that are possibly involved in various aspects of pain processing. The whole-brain formalin-induced nociceptive responses following lidocaine and morphine pretreatment were also examined. This microPET imaging protocol should be useful in

Acknowledgments

The authors acknowledge technical support from the Functional and Micro-Magnetic Resonance Imaging Center and Molecular-Genetic Imaging Core supported by the National Research Program for Genomic Medicine, National Science Council, Taiwan, PR China (NSC96-3112-B-001-009 and NSC96-3112-B-001-004).

References (38)

A.M. Aloisi et al.
‘Mirror pain’ in the formalin test: behavioral and 2-deoxyglucose studies
Pain
(1993)
A.M. Aloisi et al.
Sex-dependent effects of formalin and restraint on c-Fos expression in the septum and hippocampus of the rat
Neuroscience
(1997)
U. Bingel et al.
Subcortical structures involved in pain processing: evidence from single-trial fMRI
Pain
(2002)
E.H. Chudler et al.
The role of the basal ganglia in nociception and pain
Pain
(1995)
K.B. Franklin et al.
Tryptophan-morphine interactions and postoperative pain
Pharmacol Biochem Behav
(1990)
X. Lamas et al.
Effects of morphine in postaddict humans: a meta-analysis
Drug Alcohol Depend
(1994)
K.L. Malisza et al.
Functional MRI involving painful stimulation of the ankle and the effect of physiotherapy joint mobilization
Magn Reson Imaging
(2003)
A. Matsumura et al.
Assessment of microPET performance in analyzing the rat brain under different types of anesthesia: comparison between quantitative data obtained with microPET and ex vivo autoradiography
Neuroimage
(2003)
T.J. Morrow et al.
Regional changes in forebrain activation during the early and late phase of formalin nociception: analysis using cerebral blood flow in the rat
Pain
(1998)
V. Neugebauer
The amygdala: different pains, different mechanisms
Pain
(2007)

F. Pinto-Ribeiro et al.

Influence of arthritis on descending modulation of nociception from the paraventricular nucleus of the hypothalamus

Brain Res

(2008)

Y.Y. Shih et al.

ISPMER: Integrated system for combined PET, MRI, and electrophysiological recording in somatosensory studies in rats

Nucl Instrum Methods A

(2007)

A. Tjolsen et al.

The formalin test: an evaluation of the method

Pain

(1992)

U.I. Tuor et al.

Functional magnetic resonance imaging in rats subjected to intense electrical and noxious chemical stimulation of the forepaw

Pain

(2000)

U.I. Tuor et al.

Functional magnetic resonance imaging of tonic pain and vasopressor effects in rats

Magn Reson Imaging

(2002)

T.L. Yaksh et al.

Sites of action of opiates in production of analgesia

Prog Brain Res

(1988)

S. Abdi et al.

The anti-allodynic effects of amitriptyline, gabapentin, and lidocaine in a rat model of neuropathic pain

Anesth Analg

(1998)

M.T. Alkire et al.

Does the amygdala mediate anesthetic-induced amnesia?Basolateral amygdala lesions block sevoflurane-induced amnesia

Anesthesiology

(2005)

E.H. Chudler et al.

Nociceptive responses in the neostriatum and globus pallidus of the anesthetized rat

J Neurophysiol

(1993)

Cited by (28)

Real-time monitoring of cortical brain activity in response to acute pain using wide-area Ca<sup>2+</sup> imaging
2024, Biochemical and Biophysical Research Communications
Previous human and rodent studies indicated that nociceptive stimuli activate many brain regions that is involved in the somatosensory and emotional sensation. Although these studies have identified several important brain regions involved in pain perception, it has been a challenge to observe neural activity directly and simultaneously in these multiple brain regions during pain perception. Using a transgenic mouse expressing G-CaMP7 in majority of astrocytes and a subpopulation of excitatory neurons, we recorded the brain activity in the mouse cerebral cortex during acute pain stimulation. Both of hind paw pinch and intraplantar administration of formalin caused strong transient increase of the fluorescence in several cortical regions, including primary somatosensory, motor and retrosplenial cortex. This increase of the fluorescence intensity was attenuated by the pretreatment with morphine. The present study provides important insight into the cortico-cortical network during pain perception.
Cortical Modulation of Pain: Comments on "Exacerbation of tonic but not phasic pain by entorhinal cortex lesions"
2014, Neuroscience Letters
Thalamus and pain
2013, Acta Anaesthesiologica Taiwanica
Citation Excerpt :
In this regard, F–18 fluorodeoxyglucose PET imaging reveals the cumulative metabolic activity of brain cells. During the tracer uptake period, the animal can remain conscious and, thus, researchers can avoid the interference of anesthesia.48 MEMRI can also be performed on conscious behaving animals if manganese ions are delivered through a chronically implanted cannula.
The thalamus is a key relay station for the transmission of nociceptive information to the cerebral cortex. We review the input–output connection, functional imaging, direct neuronal recording, stimulation, and lesioning studies on the involvement of thalamus in acute and chronic pain functions. Based on its specific reciprocal connection with the cerebral cortex, strong nociceptive responsiveness, and the severe chronic pain when it is damaged, the thalamus may hold the key to pain consciousness and the key to understanding spontaneous and evoked pain in chronic pain conditions. A work plan is proposed for future study.
CNR1 gene deletion affects the density of endomorphin-2 binding sites in the mouse brain in a hemisphere-specific manner
2013, European Journal of Pharmacology
Citation Excerpt :
In addition, long-term opioid dependence which decreases the cerebral blood flow (CBF) in humans also reverses the right-greater-than-left CBF asymmetry observed in the healthy subjects (Pezawas et al., 2002). Furthermore, under nociceptive pain states induced by formalin, a clear lateralization in the primary somatosensory cortex is observed that can be suppressed by morphine pre-treatment, directly linking the opioid system to this effect (Shih et al., 2008). With regard to the cannabinoid system, a type of CNR1 gene variant is directly linked to a higher activity in the left caudate putamen and thalamus in patients with major depression (Domschke et al., 2008).
Endomorphin-1 (EM-1) and endomorphin-2 (EM-2) are two endogenous tetrapeptides with very high affinities for the μ-opioid receptor. Until recently, the precise neuroanatomical localization of the binding sites for these peptides was unknown. However, the recent synthesis of tritiated forms of these molecules has permitted these binding sites to be analysed with a very high degree of neuroanatomical specificity. Preliminary studies demonstrated a superior binding profile for EM-2, with less non-specific binding than EM-1. As the endogenous cannabinoid and opioid systems interact at several levels, we investigated how deletion of the CNR1 gene, which encodes the cannabinoid receptor 1 (CB₁R) protein, affects the brain distribution of EM-2 binding sites. Our results revealed no differences in the average density of EM-2 binding sites in CB₁ receptor knockout (CB₁R KO) and WT mice. However, when both hemispheres were analysed separately, we detected specific alterations in the distribution of EM-2 binding sites in the right hemisphere of CB₁R KO mice. While, the density of EM-2 binding sites in CB₁R KO mice was higher in the CA3 hippocampal field and in the pontine tegmental nuclei, it was lower in the superior colliculus and ventral tegmental area than in WT controls. No differences were observed in the left hemisphere for any of the regions analysed. For the first time these findings demonstrate a lateralization effect on cerebral opioid binding sites that may be mediated by the central cannabinoid system.
Rodent functional and anatomical imaging of pain
2012, Neuroscience Letters
Human brain imaging has provided much information about pain processing and pain modulation, but brain imaging in rodents can provide information not attainable in human studies. First, the short lifespan of rats and mice, as well as the ability to have homogenous genetics and environments, allows for longitudinal studies of the effects of chronic pain on the brain. Second, brain imaging in animals allows for the testing of central actions of novel pharmacological and nonpharmacological analgesics before they can be tested in humans. The two most commonly used brain imaging methods in rodents are magnetic resonance imaging (MRI) and positron emission tomography (PET). MRI provides better spatial and temporal resolution than PET, but PET allows for the imaging of neurotransmitters and non-neuronal cells, such as astrocytes, in addition to functional imaging. One problem with rodent brain imaging involves methods for keeping the subject still in the scanner. Both anesthetic agents and restraint techniques have potential confounds. Some PET methods allow for tracer uptake before the animal is anesthetized, but imaging a moving animal also has potential confounds. Despite the challenges associated with the various techniques, the 31 studies using either functional MRI or PET to image pain processing in rodents have yielded surprisingly consistent results, with brain regions commonly activated in human pain imaging studies (somatosensory cortex, cingulate cortex, thalamus) also being activated in the majority of these studies. Pharmacological imaging in rodents shows overlapping activation patterns with pain and opiate analgesics, similar to what is found in humans. Despite the many structural imaging studies in human chronic pain patients, only one study has been performed in rodents, but that study confirmed human findings of decreased cortical thickness associated with chronic pain. Future directions in rodent pain imaging include miniaturized PET for the freely moving animal, as well as new MRI techniques that enable ongoing chronic pain imaging.
FMRI of pain processing in the brain: A within-animal comparative study of BOLD vs. CBV and noxious electrical vs. noxious mechanical stimulation in rat
2012, NeuroImage
Citation Excerpt :
CBV fMRI study with an alternative anesthesia (e.g., α-chloralose) could clarify this issue. In pons, midbrain and thalamus, bilateral activations were observed during NES and NMS, which are consistent with the nociception patterns induced by unilateral hindpaw formalin stimulation observed by PET (Shih et al., 2008) and by unilateral sciatic nerve ligation observed by 2DG autoradiography (Mao et al., 1993). However, the bilateral brain activations in those studies have been attributed to the bilateral activations in the spinal cord (Mao et al., 1993); i.e., unilateral hindpaw formalin injection (Porro et al., 2004) and unilateral sciatic nerve injury (Mao et al., 1992) have been presumed to induce bilateral spinal cord activations.
This study aims to identify fMRI signatures of nociceptive processing in whole brain of anesthetized rats during noxious electrical stimulation (NES) and noxious mechanical stimulation (NMS) of paw. Activation patterns for NES were mapped with blood oxygen level dependent (BOLD) and cerebral blood volume (CBV) fMRI, respectively, to investigate the spatially-dependent hemodynamic responses during nociception processing. A systematic evaluation of fMRI responses to varying frequencies of electrical stimulus was carried out to optimize the NES protocol. Both BOLD and CBV fMRI showed widespread activations, but with different spatial characteristics. While BOLD and CBV showed well-localized activations in ipsilateral dorsal column nucleus, contralateral primary somatosensory cortex (S1), and bilateral caudate putamen (CPu), CBV fMRI showed additional bilateral activations in the regions of pons, midbrain and thalamus compared to BOLD fMRI. CBV fMRI that offers higher sensitivity compared to BOLD was then used to compare the nociception processing during NES and NMS in the same animal. The activations in most regions were similar. In the medulla, however, NES induced a robust activation in the ipsilateral dorsal column nucleus while NMS showed no activation. This study demonstrates that (1) the hemodynamic response to nociception is spatial-dependent; (2) the widespread activations during nociception in CBV fMRI are similar to what have been observed in ¹⁴C-2-deoxyglucose (2DG) autoradiography and PET; (3) the bilateral activations in the brain originate from the divergence of neural responses at supraspinal level; and (4) the similarity of activation patterns suggests that nociceptive processing in rats is similar during NES and NMS.

View all citing articles on Scopus

¹: Equal contribution with first author.

View full text

Pain mechanismBrain nociceptive imaging in rats using 18f-fluorodeoxyglucose small-animal positron emission tomography

Abstract

Section snippets

Subjects

Formalin-induced nociceptive maps

Discussion

Conclusion

Acknowledgments

Pain

Neuroscience

Pain

Pain

Pharmacol Biochem Behav

Drug Alcohol Depend

Magn Reson Imaging

Neuroimage

Pain

Pain

Brain Res

Nucl Instrum Methods A

Pain

Pain

Magn Reson Imaging

Prog Brain Res

The anti-allodynic effects of amitriptyline, gabapentin, and lidocaine in a rat model of neuropathic pain

Anesth Analg

Does the amygdala mediate anesthetic-induced amnesia?Basolateral amygdala lesions block sevoflurane-induced amnesia

Anesthesiology

Nociceptive responses in the neostriatum and globus pallidus of the anesthetized rat

J Neurophysiol

Pain mechanism
Brain nociceptive imaging in rats using ¹⁸f-fluorodeoxyglucose small-animal positron emission tomography