Background & Aims
Current treatments for chronic pain have been unable to adequately manage symptoms for this growing patient population (Attal et al., 2006; Goldberg & McGee, 2011). The peptide inhibitors, tPD5 and mPD5, of the scaffolding protein PICK1 (Protein Interacting with C-Kinase), have been assessed for efficacy in reducing pain-like behaviors in rodents under a wide variety of pain models and behavioral assay batteries (Christensen et al., 2020; Jensen et al., 2023; Jensen et al., 2021; Sørensen et al., 2022). Thought to interfere with synaptic plasticity, we hypothesized that these compounds would result in reduction of pain only under conditions of massive change to the neuronal landscape (i.e. central sensitization), while retaining acute nociceptive and mechanical sensation (Christensen et al., 2020; Murphy et al., 2012). To assess the effect of PICK1 inhibition on acute nociception, a battery of acute nociceptive assays was performed on naive mice.
Methods
C57Bl/6N naive mice n= 7-11, male and female. 10 umol/kg PICK1 inhibitor, 10 mg/kg morphine, or PBS
Tail Immersion: ? of the tail was submerged in 49.5 C water for a max. 10 seconds, three times with a 10 second interval, of which the mean was taken as the true measurement. A stirring hot plate was used with a submerged digital thermometer hanging on the side of a 0.5 L glass beaker, the temperature was maintained within 0.5 degrees in all trials.
Capsaicin Paw-Lick: Animals were administered 20 ul intraplantar capsaicin injection and recorded for 3 minutes for a paw-lick response. Intraplantar capsaicin injection was performed by two researchers, one scruffing the mouse, the other performing the injection in one hind paw.
Barnes’ Maze: A 9-day Barnes’ maze setup was performed with 4 days of training, a probe trial on day 5, followed by 3 days of reversal learning and a probe trial on day 9.
Open Field: 10 umol/kg or PBS was administered before 2.5 hr open field observation.
Results
All experiments were performed with naive C57Bl/6N mice, with male and female mice used for to investigate any sex related differences. It appears that, in line with the previous hypothesis, PICK1 inhibition does not interfere with acute nociception. No difference was seen between vehicle or PICK1 inhibitor in the tail immersion or capsaicin paw-lick assay, while 10 mg/kg morphine effectively blunted the acute nociceptive response in both cases, providing an optimal control condition.
As PICK1 is involved in plasticity of memory formation and is also known to bind dopamine receptors, it was crucial to assess naive mice for any effect on memory, locomotion, or emotional affect. No significant differences were seen in either of these measures in the Barnes’ Maze and 2.5-hour open field test. Likewise, we did not record any sex related differences within our assay battery.
Conclusions
In conclusion, these results suggest that PICK1 inhibitors could prove to be a good option for patients who have experienced severe trauma to prevent the development of chronic pain or treat patients that have experienced long-term chronic pain for a variety of reasons. Repeated, daily administration of PICK1 inhibitors did not evoke problems with spatial memory. Behavioral experiments in larger animals, with brains more like humans will be needed to make informed conclusions about potential side effects in humans, but the results of these experiments are optimistic.
References
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