Brain network alterations in the inflammatory soup animal model of migraine

Abstract

Advances in our understanding of the human pain experience have shifted much of the focus of pain research from the periphery to the brain. Current hypotheses suggest that the progression of migraine depends on abnormal functioning of neurons in multiple brain regions. Accordingly, we sought to capture functional brain changes induced by the application of an inflammatory cocktail known as inflammatory soup (IS), to the dura mater across multiple brain networks. Specifically, we aimed to determine whether IS alters additional neural networks indirectly related to the primary nociceptive pathways via the spinal cord to the thalamus and cortex. IS comprises an acidic combination of bradykinin, serotonin, histamine and prostaglandin PGE2 and was introduced to basic pain research as a tool to activate and sensitize peripheral nociceptors when studying pathological pain conditions associated with allodynia and hyperalgesia. Using this model of intracranial pain, we found that dural application of IS in awake, fully conscious, rats enhanced thalamic, hypothalamic, hippocampal and somatosensory cortex responses to mechanical stimulation of the face (compared to sham synthetic interstitial fluid administration). Furthermore, resting state MRI data revealed altered functional connectivity in a number of networks previously identified in clinical chronic pain populations. These included the default mode, sensorimotor, interoceptive (Salience) and autonomic networks. The findings suggest that activation and sensitization of meningeal nociceptors by IS can enhance the extent to which the brain processes nociceptive signaling, define new level of modulation of affective and cognitive responses to pain; set new tone for hypothalamic regulation of autonomic outflow to the cranium; and change cerebellar functions.

Keywords: Inflammatory soup; Migraine; Rat; Resting state networks; fMRI.

Figure 1. Experimental Design

Top of the figure displays a schematic of the unit utilized to deliver thermal and mechanical stimulation to the face of the rat. The unit did not cause artifacts on the MRI images. The rat was immobilized as described elsewhere (Becerra et al., 2011). The Middle panel depicts the position the cannula over the dura (red) and the exit port for IS/SIF administration (green). The details of how the thermal and mechanical unit were placed are also displayed. Bottom panel indicates the timing of the experimental procedure; after administration of IS or SIF, rats were returned to their cages for 1 hour before being prepare for the scan. After loading them on the MRI cradle under anesthesia, they were left to recover from anesthesia. Functional MRI studies started 45 minutes after loading of the rat. The imaging sessions started with a structural (Str) scan, 3 fMRI scans were acquired, the first a resting state scan (RSN) followed by evoked mechanical and thermal stimulation of the face.

Figure 2. Evoked Mechanical Stimulation Brain Activation

The Figure displays some brain areas with statistically significant increased brain response to mechanical stimulation when comparing IS vs. SIF-treated rats. Only one brain area, retrosplenial cortex, displayed less activity in the IS rats compared to SIF. All areas of activation are listed in Table 1. Key: Hipp: Hippocampus, S1: somatosensory cortex-barrel field, vpm/vpl: thalamic ventro posterior medial and lateral nuclei, vm, ventro medial, Rpl: retrosplenial cortex.

Figure 3. Resting State Networks

Networks identified in the IS+SIF analysis from a comparison with published networks in healthy rats (Becerra et al., 2011). Key: DMN: Default Mode Network. SMN: sensorimotor network. IN: interoceptive (salience) network. AN: autonomic network. BGN: Basal ganglia networks. CN: cerebellar network. Numbers indicate distance from bregma (in mm). Color code corresponds to the values of z-statistic.

Figure 4. Changes in Brain Resting State Networks in IS rats

Comparing IS vs. SIF treated RSN resulted in mostly increased connectivity of brain networks with several brain structures. Key: DMN: Default Mode Network; SMN: Sensorimotor Network; SN: salience network; AN: Autonomic network; BGN: Basal ganglia networks; CN: Cerebellar network; Cg: cingulate cortex; Ins: insula; Pu: Caudate/Putamen; V2: Secondary Visual; Th: Thalamus; S2; secondary somatosensory; A: amygdala; Hipp: hippocampus; IC: inferior colliculus; M1: Primary Motor; Rpl: retrosplenial; Hypo: Hypothalamus. For specific subdivisions of some brain structures, please see text and Table 1.