Raised intracranial pressure alters cortical vascular function and cephalic allodynia

Abstract

Raised intracranial pressure (ICP) is associated with altered cerebral haemodynamics and cephalic pain. The relationship between the algetic response and cortical neurovascular changes in raised ICP is unclear. This study aimed to evaluate this relationship and determine whether lowering ICP (using a glucagon-like peptide-1 receptor agonist) could ameliorate the algetic response. We also sought to explore the role of calcitonin gene-related peptide in cephalic pain driven by raised ICP by inhibiting calcitonin gene-related peptide signalling and quantifying changes in the algetic response. In a rat model of raised ICP, created by intracisternal kaolin injection, mechanical thresholds were measured alongside steady-state potential and cerebral blood flow responses to spreading depolarization. Nuclear magnetic resonance spectroscopy evaluated energetic substrates in animals with raised ICP ex vivo. The glucagon-like peptide-1 receptor (GLP-1R) agonist exenatide and the calcitonin gene-related peptide receptor (CGRP-R) antagonist olcegepant were injected daily, and measurements were repeated. Kaolin increased ICP [median (range) 15.96 (8.97) mmHg, n = 8] versus controls [6.02 (1.79) mmHg, n = 6, P = 0.0007]. Animals with raised ICP exhibited reduced mechanical thresholds [mean (standard deviation) hind paw baseline: 5.78 (2.81) g, Day 7: 3.34 (2.22) g, P < 0.001; periorbital baseline: 6.13 (2.07) g, Day 7: 2.35 (1.91) g, n = 12, P < 0.001]. Depolarization and repolarization durations were increased [depolarization, raised ICP: 108.81 (222.12) s, n = 11, controls: 37.54 (108.38) s, n = 9, P = 0.038; repolarization, raised ICP: 1824.26 (3499.54) s, n = 12, controls: 86.96 (140.05) s, n = 9, P < 0.0001]. Cerebral blood flow change was also reduced [85.55 (30.84)%, n = 9] compared with controls [217.64 (37.70)%, n = 8, P < 0.0001]. Substrates for cellular energetics (ADP, ATP and NAD+) were depleted in rodent brains with raised ICP (P = 0.009, P = 0.018 and P = 0.011, respectively). Exenatide significantly lowered ICP [exenatide: 9.74 (6.09) mmHg, n = 19, vehicle: 18.27 (6.67) mmHg, n = 16, P = 0.004] and rescued changes in mechanical withdrawal. Exenatide recovered characteristic spreading depolarization responses [depolarization duration, exenatide: 56.46 (25.10) s, n = 7, vehicle: 115.98 (58.80) s, n = 6, P = 0.033; repolarization duration, exenatide: 177.55 (562.88) s, n = 7, vehicle: 800.85 (1988.67) s, n = 6, P = 0.002]. In the setting of raised ICP, olcegepant prevented changes in periorbital mechanical thresholds. We conclude that raised ICP disrupted the cortical neurovascular responses, reduced algetic thresholds and depleted crucial energetic substrates. Exenatide reduced ICP, improving algetic thresholds and cortical neurovascular changes. Importantly, olcegepant alleviated the cerebral algesia, suggesting a role for calcitonin gene-related peptide in driving pain responses in elevated ICP. These studies support the rationale that reducing ICP improves cephalic pain in conditions of raised ICP. Furthermore, the data suggest that headache pain in diseases associated with raised ICP could be ameliorated therapeutically though blockade of the calcitonin gene-related peptide pathway.

Keywords: calcitonin gene-related peptide receptor antagonist; exenatide; glucagon-like peptide receptor agonist; olcegepant.

Figure 1

Methodology of the raised intracranial pressure rat model and assessments. (A) Kaolin or an equal volume of saline (control) was injected into the cisterna magna of rats percutaneously. (B) Parameters calculated from the first depolarization event following stimulation. (C) Surgical preparation of spreading depolarization assessments, which measured cerebral blood flow and steady-state potential (direct current, DC) changes during evoked spreading depolarization using a KCl-soaked pellet on the cortex. (D) Time line of mechanical withdrawal threshold assessments in animals with raised intracranial pressure versus controls, and of drug versus vehicle. AU = arbitrary units. Created in BioRender.

Figure 2

Kaolin induced ventricular dilatation, raised intracranial pressure and reduced hind paw and periorbital mechanical thresholds. (A) Coronal slices from control (saline: top) and raised intracranial pressure (ICP) animals (kaolin: bottom) demonstrating ventricular dilatation in those with raised ICP. (B) The ventricle:brain ratio was significantly higher in animals with raised ICP. (C) ICP was significantly increased in kaolin-injected animals compared with controls. (D) Hind paw mechanical withdrawal thresholds were significantly reduced in raised ICP animals at Day 7 compared with baseline. (E) Periorbital mechanical thresholds were also significantly reduced at Day 7 in raised ICP animals. Ventricle:brain ratio and ICP were analysed using Mann–Whitney U-testing; mechanical thresholds were tested using paired two-tailed t-tests. *P < 0.05, ***P < 0.001 and ****P < 0.0001.

Figure 3

Evoked spreading depolarization responses were drastically altered in animals with raised intracranial pressure. (A and B) Representative steady-state potential (direct current, DC) and cerebral blood flow (CBF) response to evoked spreading depolarization in control (A) and raised intracranial pressure (ICP) animals (B). Arrows indicate when 5 μl KCl was added to the cotton pellet every 15 min. (C) Depolarization duration was increased in raised ICP animals. (D) Repolarization duration was increased in raised ICP animals. (E) Depolarization latency was similar between control and raised ICP animals. (F) Percentage change in CBF was lower in animals with raised ICP. (G) The number of CBF peaks within the 1 h recording was significantly lower in animals with raised ICP. Depolarization and repolarization durations were compared using Mann–Whitney U-testing; all remaining parameters were tested using unpaired two-tailed t-tests. *P < 0.05, **P < 0.01 and ****P < 0.0001. AU = arbitrary units.

Figure 4

Energetic metabolites were altered in the brains of animals with raised intracranial pressure. (A) NADPH was increased in animals with raised intracranial pressure (ICP). (B–D) ADP (B), NAD⁺ (C) and ATP (D) were significantly reduced in animals with raised ICP. (E) NAD⁺ was correlated with ventricle:brain ratios, indicating that those with more ventricular dilatation had lower NAD⁺ concentrations (r = −0.7328, P = 0.039). (F) Hind paw mechanical thresholds at Day 7 also exhibited a correlation with ATP concentrations (r = −0.8428, P = 0.004). Metabolites were compared using unpaired two-tailed t-tests. *P < 0.05 and **P < 0.01.

Figure 5

The effects of GLP-1R agonist and CGRP-R antagonist on intracranial pressure (ICP) and weight in animals with raised ICP. (A) ICP was lower in animals treated with GLP-1R agonist versus vehicle. (B) The weight of animals with raised ICP was similar between those treated with GLP-1R agonist and vehicle over 7 days. (C) ICP in controls and in animals with raised ICP in the absence of any intervention, in addition to GLP-1R agonist, CGRP-R antagonist and vehicle-treated subcutaneously (s.c,) or intraperitoneally (i.p.). ICPs were compared using unpaired two-tailed t-tests. *P < 0.05, **P < 0.01 and ***P < 0.0001. CGRP-R = calcitonin gene-related peptide receptor.

Figure 6

GLP-1R agonist prevented changes in periorbital and hind paw mechanical withdrawal thresholds. (A) Hind paw thresholds were significantly reduced at Day 7 in rats with raised intracranial pressure (ICP) treated with vehicle but not GLP-1R agonist. (B and C) Association between hind paw thresholds and ICP in rats treated with GLP-1R agonist (B) or vehicle (C). (D) Periorbital thresholds were significantly reduced at Day 7 in rats treated with vehicle but not GLP-1R agonist. (E and F) Periorbital thresholds were significantly associated with ICP in rats treated with GLP-1R agonist (E) and vehicle (F). (G) Periorbital mechanical thresholds were reduced in rats with raised ICP treated with vehicle but not CGRP-R antagonist. (H) Hind paw thresholds were reduced at Day 7 animals treated with vehicle and with CGRP-R antagonist. Mechanical withdrawal thresholds were compared using paired two-tailed t-tests. **P < 0.01, ***P < 0.001 and **** = P < 0.0001. CGRP-R = calcitonin gene-related peptide receptor.

Figure 7

GLP-1R agonist restored cortical and cerebral blood flow responses to spreading depolarization in animals with raised intracranial pressure. (A and B) Representative direct current (DC) and cerebral blood flow (CBF) spreading depolarization response in animals with raised intracranial pressure (ICP) treated with vehicle (A) and GLP-1R agonist (B). Arrows indicate when 5 μl KCl was added to the cotton pellet every 15 min. (C) Depolarization duration was reduced following GLP-1R agonist treatment. (D) Repolarization duration showed a trend towards reduction in GLP-1R agonist-treated animals. (E) Depolarization latency was similar between vehicle and GLP-1R agonist-treated animals. (F) Percentage change in CBF. (G) The number of CBF peaks was higher in GLP-1R agonist-treated animals. Repolarization duration and depolarization latency were compared using Mann–Whitney U-testing; all remaining parameters were tested using unpaired two-tailed t-tests. *P < 0.05 and **P < 0.01. AU = arbitrary units.