Morphine glucuronidation increases its analgesic effect in guinea pigs

Pain management in pet guinea pigs (Cavia porcellus): a review of limitations of current knowledge and practice

OBJECTIVE

To describe recent evidence-based and peer-reviewed literature on the topic of pain assessment and pain management in guinea pigs in a systematic review. To consider impacts of current practice on guinea pig welfare and to identify areas in need of further research.

DATABASES USED

ScienceDirect, PubMed and CAB abstracts along with relevant information from books and ebooks were searched. Terms used were guinea pigs, cavies, Cavia porcellus, pain, surgery, surgical procedures, grimace, dental, bloat, analgesia, non-steroidal anti-inflammatories and opiates.

CONCLUSIONS

There are few studies available in the areas of pain assessment and pain management in guinea pigs. Grimace scales and in-clinic pain assessment scales are available for other species but have not been developed for guinea pigs. Doses of analgesic drugs are extrapolated from other species or used from anecdotal evidence in the absence of validated values. The specified dose for the only UK licenced analgesic drug for guinea pigs (meloxicam) is significantly lower than doses suggested in the reviewed literature which may have a negative impact on the welfare of this species. To improve the welfare of guinea pigs, further research into the pharmacokinetic (PK) and pharmacodynamic (PD) properties of analgesic drugs specific to this species are required. Further research into the development and validation of pain assessment methods is also indicated to promote better welfare of this species.

Protein phosphatase 2A deficiency in hippocampal CA1 inhibits priming effect of morphine on conditioned place preference in mice

Studies have shown that protein phosphorylation plays an important role in morphine abuse. However, the neurobiological mechanism of protein phosphatase 2A (PP2A) underlying the morphine-priming process is still unclear. Here we constructed T29-2-Cre; PP2Afl/fl conditional knockout mice (KO) and investigated the role of hippocampal PP2A in morphine priming. We observed that the deficit of PP2A inhibited the priming behavior of morphine and blocked the priming-induced long-term potentiation (LTP) in the hippocampus of KO mice. Moreover, the expression levels of Rack1 and the membrane GluN2B were significantly reduced in the nucleus accumbens of KO mice compared with those in the control mice, which may be attributed to the decreased HDAC4 in the hippocampus of KO mice. Consistent with it, the similar inhibited priming effects were also observed in the wild-type mice treated with sodium butyrate (NaB)-a nonspecific inhibitor of histone deacetylases-3 h after morphine administration. Taken together, our results suggest that hippocampal PP2A may be involved in morphine priming through the PP2A/HDAC4/Rack1 pathway.

Endogenous Opiates and Behavior: 2015

This paper is the thirty-eighth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2015 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia, stress and social status, tolerance and dependence, learning and memory, eating and drinking, drug abuse and alcohol, sexual activity and hormones, pregnancy, development and endocrinology, mental illness and mood, seizures and neurologic disorders, electrical-related activity and neurophysiology, general activity and locomotion, gastrointestinal, renal and hepatic functions, cardiovascular responses, respiration and thermoregulation, and immunological responses.

Intrathecal Catheterization and Drug Delivery in Guinea Pigs

Background

Intrathecal infusion of opioids in dogs, sheep, and humans produces local space-occupying masses. To develop a small-animal model, the authors examined effects of intrathecal catheterization and morphine infusion in guinea pigs.

Methods

Under isoflurane, polyethylene or polyurethane catheters were advanced from the cisterna magna to the lumbar enlargement. Drugs were delivered as a bolus through the externalized catheter or continuously by subcutaneous minipumps. Hind paw withdrawal to a thermal stimulus was assessed. Spinal histopathology was systematically assessed in a blinded fashion. To assist in determining catheter placement, ex vivo images were obtained using magnetic resonance imaging in several animals. Canine spinal tissue from previous intrathecal morphine studies was analyzed in parallel.

Results

(1) Polyethylene (n = 30) and polyurethane (n = 25) catheters were implanted in the lumbar intrathecal space. (2) Bolus intrathecal morphine produced a dose-dependent (20 to 40 μg/10 μl) increase in thermal escape latencies. (3) Absent infusion, a catheter-associated distortion of the spinal cord and a fibrotic investment were noted along the catheter tract (polyethylene > polyurethane). (4) Intrathecal morphine infusion (25 mg/ml/0.5 μl/h for 14 days) resulted in intrathecal masses (fibroblasts, interspersed collagen, lymphocytes, and macrophages) arising from meninges proximal to the catheter tip in both polyethylene- and polyurethane-catheterized animals. This closely resembles mass histopathology from intrathecal morphine canine studies.

Conclusions

Continuous intrathecal infusion of morphine leads to pericatheter masses that morphologically resemble those observed in dogs and humans. This small-animal model may be useful for studying spinal drug toxicology in general and the biology of intrathecal granuloma formation in particular.