Mechanistic studies of opioid-induced exacerbation of chronic pain responses

Painful (noxious) stimuli, detected by sensory nerve fibres, warn us of potential and actual tissue damage. The sensation of pain is fundamental to our survival, but sometimes we need to override this protective system to escape a dangerous situation. To make this possible, an inhibitory class of molecules (opioids) act as natural internal painkillers. These chemical messengers are produced in pain-related regions of the brain and spinal cord, and act at specific protein receptors on neuronal cells to reduce pain. Opioid drugs, such as morphine, act at these same receptors and are often prescribed to treat pain. Like much of the spinal cord and brain, the pain pathways and the opioid receptor system are not 'hard-wired', and exposure to painful stimuli or drugs can change the way these pathways function and alter the experience of pain in the longer term.

Both clinical and experimental studies in rodents show that use of opioid drugs, like morphine, before pain circuits are activated actually worsens the experience of pain later. Although this may appear of theoretical interest only, many people are prescribed opioids for long durations, especially females. Understanding how opioid treatment changes the processing of subsequent painful inputs by the central nervous system will create valuable new knowledge, which will impact upon life-long health. This is especially important as the incidence of diseases that have pain as the major symptom, like osteoarthritis, increases with age and negatively influences everyday activities.

Using well-understood rodent models of chronic pain, we will investigate how prior and continued morphine treatment changes the way the body responds to a painful injury, focusing on key pain processing areas - including the spinal cord. We will use a novel experimental approach to simultaneously record activity from populations of neurones across the spinal cord to understand how prior opioid treatment affects the processing of painful inputs. We will also use this information to produce a map of altered spinal cord activity, identifying regions to focus on for subsequent anatomical investigations.

We will then use a cutting-edge microscopy approach to study these spinal pain circuits, providing evidence of whether prior opioid treatment strengthens connections between pain-sensing nerve fibres from the body and the spinal neurones that receive their input. A strengthening of these connections would be experienced as greater pain sensation, and this new microscopy approach allows measurement of a large number of identified connections much faster than other techniques. As continued activation of the opioid receptors is known to reduce their inhibitory function, we will also use this microscopy method to quantify changes in the number and location of receptors within sensory fibre terminals. We will measure opioid molecules naturally present in the body to identify whether changes in levels also contribute to these effects.

The final experiments will probe the contribution of a molecule called BDNF in these effects of morphine. BDNF is known to play important roles in altering the excitability of the brain and spinal cord, and we will establish whether selectively removing this molecule in the spinal cord reverses morphine-induced facilitation of pain behaviour in a model of chronic pain. These experiments will close the loop in our understanding of the processes by which morphine can facilitate painful responses, and will provide a target for reversing these events in the future.

Greater knowledge of how opioid drugs alter the response of the body to future painful injuries is important to support changes in policy focused upon prescribing behaviour. Furthermore, this knowledge will also provide new ideas for approaches aimed at reversing opioid drug-induced changes in pain circuitry, reducing the number of people living with the consequences of prior opioid treatment.

We will investigate the mechanisms by which prior and continued morphine treatment alters behavioural and neuronal responses to future painful injuries in female rats. Outcomes will contribute to the MRC priority to improve human health.

Rats will be treated with morphine for 7 days prior to induction of a model of chronic pain (osteoarthritis or neuropathic pain), behavioural pain responses will be measured until day 14, and then a subset of rats will be used to map changes in peripheral hindpaw-evoked neuronal activity across the spinal dorsal horn (DH), compared to control groups, using a novel multi-electrode array method. This activity map will be used to guide Stochastic Optical Reconstruction Microscopy (STORM) studies, which will measure changes in the abundance of AMPA-type glutamate receptors (AMPA-Rs) at identified sensory fibre terminals, to provide a nanoscale measure of altered synaptic strength associated with morphine-induced facilitation of chronic pain responses. Levels of endogenous opioids and mu-opioid receptor (MOR) desensitisation will be quantified in the DH and brain regions central to descending DH neuromodulation, to probe their potential contributions to morphine-induced exacerbation of chronic pain behaviour. This will be complemented by STORM analysis of MOR internalisation in the spinal terminals of the sensory afferent fibres.
In vitro evidence implicates a role of spinal BDNF in morphine-induced changes in nociceptive processing. The effects of sequestering spinal BDNF on morphine treatment-induced facilitation of chronic pain behaviour will be determined, and STORM analysis will determine whether this treatment reverses changes in the abundance of AMPA-Rs and/or MOR internalisation in the DH.
Advances in understanding of the role of spinal BDNF will provide the foundational knowledge required for future therapies aimed at normalising spinal pain processing and preventing morphine-induced exacerbation of chronic pain response.