Skip to main page content
Access keys NCBI Homepage MyNCBI Homepage Main Content Main Navigation
, 10, 62

Sustained Pain-Related Depression of Behavior: Effects of Intraplantar Formalin and Complete Freund's Adjuvant on Intracranial Self-Stimulation (ICSS) and Endogenous Kappa Opioid Biomarkers in Rats

Affiliations

Sustained Pain-Related Depression of Behavior: Effects of Intraplantar Formalin and Complete Freund's Adjuvant on Intracranial Self-Stimulation (ICSS) and Endogenous Kappa Opioid Biomarkers in Rats

Michael D Leitl et al. Mol Pain.

Abstract

Background: Intraplantar administration of complete Freund's adjuvant (CFA) and formalin are two noxious stimuli commonly used to produce sustained pain-related behaviors in rodents for research on neurobiology and treatment of pain. One clinically relevant manifestation of pain is depression of behavior and mood. This study compared effects of intraplantar CFA and formalin on depression of positively reinforced operant behavior in an assay of intracranial self-stimulation (ICSS) in rats. Effects of CFA and formalin on other physiological and behavioral measures, and opioid effects on formalin-induced depression of ICSS, were also examined.

Results: There were four main findings. First, consistent with previous studies, both CFA and formalin produced similar paw swelling and mechanical hypersensitivity. Second, CFA produced weak and transient depression of ICSS, whereas formalin produced a more robust and sustained depression of ICSS that lasted at least 14 days. Third, formalin-induced depression of ICSS was reversed by morphine doses that did not significantly alter ICSS in saline-treated rats, suggesting that formalin effects on ICSS can be interpreted as an example of pain-related and analgesic-reversible depression of behavior. Finally, formalin-induced depression of ICSS was not associated with changes in central biomarkers for activation of endogenous kappa opioid systems, which have been implicated in depressive-like states in rodents, nor was it blocked by the kappa antagonist norbinaltorphimine.

Conclusions: These results suggest differential efficacy of sustained pain stimuli to depress brain reward function in rats as assessed with ICSS. Formalin-induced depression of ICSS does not appear to engage brain kappa opioid systems.

Figures

Figure 1
Figure 1
Effects of complete Freund’s adjuvant (CFA), formalin, or respective controls on paw width, body weight and mechanical sensitivity. The abscissae for all panels is hours or days following bilateral 100 ul injection of CFA (gray bars, Panels A, C, E), formalin (filled bars, Panels B, D, F), or respective saline controls (open bars, all panels). Bars above “BL” show baseline data before injection. Ordinates (Panels A, B): paw width in mm. Ordinates (Panels C, D): body weight in grams. Ordinates (Panels E, F): paw withdrawal threshold from von Frey filaments in grams (log scale). Dollar signs ($) indicate a significant within-group difference from the respective baseline, and asterisks (*) indicate a significant between-group difference at a given time point, as determined by a significant two-way Repeated Measures ANOVA followed by the Holm-Sidak post hoc test (p < 0.05). All points show mean ± SEM from 8 rats. See Additional file 1 for detailed statistical results.
Figure 2
Figure 2
Effects of complete Freund’s adjuvant (CFA), formalin, or respective controls on intracranial self-stimulation (ICSS). Panels A and B: Abscissae show hours or days following bilateral 100 ul injection of CFA (gray bars, Panel A), formalin (filled bars, Panel B), or respective saline controls (open bars, both panels). Ordinates show ICSS rate expressed as total stimulations per component relative to pre-injection baseline. Dollar signs ($) indicate a significant within-group difference from the respective baseline, and asterisks (*) indicate a significant between-group difference at that time point. Panels C and D show full frequency-rate ICSS curves for selected time points from A and B. Abscissae show frequency of electrical brain stimulation (Log Hz). Ordinates show ICSS rate expressed as percent maximum control rate (%MCR). Filled points indicate statistical significance of treatment effects relative to the pre-injection baseline. All statistical analyses were performed using two-way Repeated Measures ANOVA followed by the Holm-Sidak post hoc test (p < 0.05). All points show mean ± SEM from 8 rats. See Additional file 1 for detailed statistical results.
Figure 3
Figure 3
Effects of the mu opioid agonist morphine (0.32-3.2 mg/kg) on ICSS eight days after bilateral intraplantar saline or 5% formalin. Panels A and B: Abscissae show frequency of electrical brain stimulation (Log Hz) in rats that received intraplantar saline (A) or formalin (B). Ordinates show ICSS rate expressed as percent maximum control rate (%MCR). “BL” shows the baseline frequency-rate curve determined before intraplantar treatment, “0.0” shows the frequency-rate curve determined on Day 8 after intraplantar treatment but before morphine treatment. Filled points in panel B show significant morphine effects relative to “0.0.” Panel C: The abscissa shows morphine dose in mg/kg in rats that received intraplantar saline (open bars) or formalin (filled bars). The ordinate shows ICSS rate expressed as total stimulations per component relative to baseline. Dollar signs ($) indicate a significant within-group difference from “0.0”, and asterisks (*) indicate a significant between-group difference at a given morphine dose. All statistical analyses were performed using a two-way Repeated Measures ANOVA followed by the Holm-Sidak post-hoc test (p < 0.05). All data show mean ± SEM from 8 rats per treatment. See Additional file 1 for detailed statistical results.
Figure 4
Figure 4
Formalin induced-depression of intracranial self-stimulation (ICSS) is dose and time related. Abscissa: Days after varying doses of intraplantar formalin or saline administration. Ordinate: ICSS rate expressed as percent baseline stimulations per component. Statistical analysis was performed using two-way Repeated Measures ANOVA followed by the Holm-Sidak post hoc test (p < 0.05). Dollar signs ($) indicate a significant within-group difference from the pre-injection baseline, and filled points indicate a significant between-group difference at that time point relative to saline treatment. All data show mean ± SEM from 6 rats per treatment. See Additional file 1 for detailed statistical results.
Figure 5
Figure 5
Role of the endogenous kappa opioid system in pain-related depression of ICSS. Panels A and B: Transcript levels of PDYN (A) or KOR (B) mRNA as measured by qRT-PCR in brain regions implicated in mood disorders (CPu: caudate/putamen, NAc: nucleus acumbens core, NAcSh: nucleus acumbens shell, PFC: prefrontal cortex, VTA: ventral tegmental area). Abscissae: Brain area evaluated. Ordinates: Transcript levels expressed as “Fold-Induction” relative to saline controls. “ND” in Panel B signifies “Not Determined” due to low transcript levels below the level of detection in some rats. Panel C-D: Effects of the kappa antagonist norBNI (32 mg/kg) on ICSS in rats treated with intraplantar saline (C) or formalin (D). Abscissae show frequency of electrical brain stimulation (Log Hz). Ordinates show ICSS rate expressed as percent maximum control rate (%MCR). “BL” shows the frequency-rate curve determined on Day 7 after intraplantar saline or formalin and immediately before norBNI treatment. ICSS was then redetermined 24 hr after norBNI. Summary data are shown in Panel E, where the abscissa shows norBNI dose in mg/kg in rats that received intraplantar saline (open bars) or formalin (filled bars), and the ordinate shows ICSS rate expressed as total stimulations per component relative to the pre-formalin baseline. The asterisk (*) indicates a significant between-group difference at a given dose. All data show mean ± SEM from 6 rats. See Additional file 1 for detailed statistical results.

Similar articles

See all similar articles

Cited by 25 PubMed Central articles

See all "Cited by" articles

References

    1. Negus SS, Vanderah TW, Brandt MR, Bilsky EJ, Becerra L, Borsook D. Preclinical assessment of candidate analgesic drugs: recent advances and future challenges. J Pharmacol Exp Ther. 2006;319:507–514. doi: 10.1124/jpet.106.106377. - DOI - PubMed
    1. Mogil JS, Davis KD, Derbyshire SW. The necessity of animal models in pain research. Pain. 2010;151:12–17. doi: 10.1016/j.pain.2010.07.015. - DOI - PubMed
    1. Whiteside GT, Pomonis JD, Kennedy JD. An industry perspective on the role and utility of animal models of pain in drug discovery. Neurosci Lett. 2013;557:65–72. doi: 10.1016/j.neulet.2013.08.033. - DOI - PubMed
    1. Brannen GE, Coffey DS, Wilson JD, Andre J, Rochefort H, Danzo BJ, Eller BC, Mercado-Pichardo E, Rosado-Garcia A, Samuels LT. Serum-mediated immunoresponses to spermatozoa. Investig Urol. 1975;50:319–323.
    1. Stein C, Millan MJ, Herz A. Unilateral inflammation of the hindpaw in rats as a model of prolonged noxious stimulation: alterations in behavior and nociceptive thresholds. Pharmacol Biochem Behav. 1988;31:445–451. doi: 10.1016/0091-3057(88)90372-3. - DOI - PubMed

Publication types

MeSH terms

Feedback