Probes for CFOS

Trauma can cause dysfunctional fear regulation leading some people to develop disorders such as post-traumatic stress disorder (PTSD). The amygdala regulates fear, whereas PACAP (pituitary adenylate activating peptide) and PAC1 receptors are linked to PTSD symptom severity at genetic/epigenetic levels, with a strong link in females with PTSD. We discovered a PACAPergic projection from the basomedial amygdala (BMA) to the medial intercalated cells (mICCs) in adult mice. In vivo optogenetic stimulation of this pathway increased cfos expression in mICCs, decreased fear recall and increased fear extinction. Selective deletion of PAC1 receptors from the mICCs in females reduced fear acquisition, but enhanced fear generalization and reduced fear extinction in males. Optogenetic stimulation of the BMA-mICCs PACAPergic pathway produced excitatory postsynaptic currents (EPSCs) in mICC neurons, which were enhanced by the PAC1 receptor antagonist, PACAP 6-38. Our findings show that mICCs modulate contextual fear in a dynamic and sex-dependent manner via a microcircuit containing the BMA and mICCs, and in a manner that was dependent on behavioral state.SIGNIFICANCE STATEMENTTraumatic stress can affect different aspects of fear behaviors including fear learning, generalization of learned fear to novel contexts, how the fear of the original context is recalled, and how fear is reduced over time. While the amygdala has been studied for its role in regulation of different aspects of fear, the molecular circuitry of this structure is quite complex. In addition, aspects of fear can be modulated differently in males and females. Our findings show that a specific circuitry containing the neuropeptide PACAP and its receptor, PAC1, regulates various aspects of fear including acquisition, generalization, recall and extinction in a sexually dimorphic manner, characterizing a novel pathway that modulates traumatic fear.

Transient Receptor Potential Melastatin 3 (TRPM3) is a heat-activated ion channel in primary sensory neurons of the dorsal root ganglia (DRG). Pharmacological and genetic studies implicated TRPM3 in various pain modalities, but TRPM3 inhibitors were not validated in TRPM3-/- mice. Here we tested two inhibitors of TRPM3 in male and female wild type and TRPM3-/- mice in nerve injury-induced neuropathic pain. We found that intraperitoneal injection of either isosakuranetin, or primidone reduced heat hypersensitivity induced by chronic constriction injury (CCI) of the sciatic nerve, in wild type, but not in TRPM3-/- mice. Primidone was also effective when injected locally in the hind paw, or intrathecally. Consistently, intrathecal injection of the TRPM3 agonist CIM0216 reduced paw withdrawal latency to radiant heat in wild type, but not in TRPM3-/- mice. Intraperitoneal injection of 2 mg/kg, but not 0.5 mg/kg isosakuranetin, inhibited cold and mechanical hypersensitivity in CCI, both in wild-type and TRPM3-/- mice, indicating a dose dependent off target effect. Primidone had no effect on cold sensitivity, and only a marginal effect on mechanical hypersensitivity. Genetic deletion or inhibitors of TRPM3 reduced the increase in the levels of the early genes cFos and pERK in the spinal cord and DRG in CCI mice, suggesting spontaneous activity of the channel. Intraperitoneal isosakuranetin also inhibited spontaneous pain related behavior in CCI in the conditioned place preference assay, and this effect was eliminated in TRPM3-/- mice. Overall our data indicate a role of TRPM3 in heat hypersensitivity and in spontaneous pain after nerve injury.SIGNIFICANCE STATEMENT:Neuropathic pain is a major unsolved medical problem. The heat-activated TRPM3 ion channel is a potential target for novel pain medications, but it is not clear what pain modalities it plays roles in. Here we used a combination of genetic and pharmacological tools to assess the role of this channel in spontaneous pain, heat-, cold- and mechanical hypersensitivity in a nerve injury model of neuropathic pain in mice. Our findings indicate a role for TRPM3 in heat hyperalgesia, and spontaneous pain, but not in cold, and mechanical hypersensitivity. We also find that not only TRPM3 located in the peripheral nerve termini, but also TRPM3 in the spinal cord, or proximal segments of DRG neurons is important for heat hypersensitivity.

Previous research has shown that a 5-day course of morphine enhances nociceptive sensitivity and allodynia when given 10 days after chronic constriction injury (CCI) as measured by the Von Frey test, a test where the hindpaw is poked with force (measured in grams) calibrated filament. This increased sensitivity to touch suggests that post trauma morphine makes second order sensory dorsal horn neurons more excitable. Therefore, it is hypothesized that during morphine enhanced allodynia, dorsal horn neurons will be more excitable to nociceptive stimulus. It was found that morphine enhanced allodynia causes more excitable neurons across a larger spatial range of the spinal cord, both rostral-caudal and dorsal-ventral along the dorsal horn. Mechanisms for the increased excitability of the dorsal horn are proposed and explored. These findings add to a robust literature which has detailed the paradoxical pain amplifying effects of morphine. Further, this study predicts that hyperexcitability of pain and touch pathways may occur as a clinically unintended side effect of morphine when administered to treat ongoing neuropathic pain. Lay Summary When working on the body, opioids take effect on various cells- including cells of the nervous system known as Glia. These glial cells produce various inflammatory responses that are typically known to decrease inflammation in the body. However, prior literature has revealed that following an activated state, these cells enter a “primed” form. If a second immunological activation occurs during this primed state, glial cells have been shown to release proteins that actually potentiate the state of pain. This study examines this hypothesis through the context of peripheral nerve injury (modeling injury, surgery, or trauma), followed by an administration of morphine. Using advanced imaging techniques, we are able to visualize exactly what modulators are inducing this potentiated pain response, as well as where in the nervous system these cells are found. Morphine has a potent effect on various aspects of the nervous system which may be alleviated by other treatment routes that focus specifically on the cell type and location that release the molecules that initiate the healing process. By unveiling the details of this mechanism, we can better understand how to treat patients following injury or surgery without inducing a higher pain response that can be caused by a short-term administration of morphine.  To see the complete thesis, please visit https://scholar.colorado.edu/concern/undergraduate_honors_theses/0k225c53t.