Identification of an adenylyl cyclase inhibitor for treating neuropathic and inflammatory pain

Neuropathic pain, often caused by nerve injury, is commonly observed among patients with different diseases. Because its basic mechanisms are poorly understood, effective medications are limited. Previous investigations of basic pain mechanisms and drug discovery efforts have focused mainly on early sensory neurons such as dorsal root ganglion and spinal dorsal horn neurons, and few synaptic-level studies or new drugs are designed to target the injury-related cortical plasticity that accompanies neuropathic pain. Our previous work has demonstrated that calcium-stimulated adenylyl cyclase 1 (AC1) is critical for nerve injury-induced synaptic changes in the anterior cingulate cortex. Through rational drug design and chemical screening, we have identified a lead candidate AC1 inhibitor, NB001, which is relatively selective for AC1 over other adenylate cyclase isoforms. Using a variety of behavioral tests and toxicity studies, we have found that NB001, when administered intraperitoneally or orally, has an analgesic effect in animal models of neuropathic pain, without any apparent side effects. Our study thus shows that AC1 could be a productive therapeutic target for neuropathic pain and describes a new agent for the possible treatment of neuropathic pain.     

A Computational Model of the Temperature-dependent Changes in Firing Patterns in Aplysia Neurons

We performed experiments using Aplysia neurons to identify the mechanism underlying the changes in the firing patterns in response to temperature changes. When the temperature was gradually increased from 11℃ to 31℃ the firing patterns changed sequentially from the silent state to beating, doublets, beating-chaos, bursting-chaos, square-wave bursting, and bursting-oscillation patterns. When the temperature was decreased over the same temperature range, these sequential changes in the firing patterns reappeared in reverse order. To simulate this entire range of spiking patterns we modified nonlinear differential equations that Chay and Lee made using temperature-dependent scaling factors. To refine the equations, we also analyzed the spike pattern changes in the presence of potassium channel blockers. Based on the solutions of these equations and potassium channel blocker experiments, we found that, as temperature increases, the maximum value of the potassium channel relaxation time constant, τ(n)(t) increases, but the maximum value of the probabilities of openings for activation of the potassium channels, n(t) decreases. Accordingly, the voltage-dependent potassium current is likely to play a leading role in the temperature-dependent changes in the firing patterns in Aplysia neurons.     

Evidence for synaptotagmin as an inhibitory clamp on synaptic vesicle release in Aplysia neurons.

While previous studies have demonstrated that synaptotagmin plays an essential role in evoked neurotransmitter release, it has been difficult to determine whether it acts to facilitate or inhibit release. To address this question, we used acute genetic manipulations to alter the expression of synaptotagmin in Aplysia neurons. Transient overexpression of synaptotagmin in acutely dissected cholinergic neurons and in cultured glutaminergic neurons decreased the amplitude of the excitatory postsynaptic potential (EPSP) by 32% and 26%, respectively. In contrast, treatment of cultured presynaptic neurons with synaptotagmin antisense oligonucleotides increased the amplitude of the EPSP by 50-75%. These results are consistent with a role of synaptotagmin as an inhibitor of release.     

Overexpression of an Aplysia shaker K+ channel gene modifies the electrical properties and synaptic efficacy of identified Aplysia neurons.

Although potassium channels play a variety of roles in shaping the electrical properties of neurons, little is known about how these channels are constituted in neurons. To examine the assembly and physiological function of A-type K+ channels in mature differentiated neurons, we have developed a highly efficient gene transfer method for Aplysia neurons that has allowed us to express about 10(7) copies of the cloned Aplysia Shaker (Sh) K+ channel (AK01a) in single identified cells. We find that expression of AK01a phenocopies one of the native transient K+ currents (IAdepol), suggesting that the native channel carrying IAdepol is assembled as a homooligomer of AK01a. Overexpression of AK01a has substantial effect on the action potential, shortening its duration, enhancing its hyperpolarizing afterpotential, and depressing by more than half the amount of transmitter release by the action potential from the terminals. Thus, the AK01a channel not only contributes to the firing properties within a given neuron but also can regulate the signaling between interconnected cells.     

Suppression of long-term facilitation by Rab3-effector protein interaction

Long-term facilitation (LTF) in Aplysia is achieved by the modulation of presynaptic release. However, the underlying mechanism that might be related with the regulation of synaptic vesicle release remains unknown. Since Rab3, a neuronal GTP-binding protein, is known to be a key regulator of synaptic vesicle fusion, we investigated the involvement of Rab3 in LTF. To address this issue, we examined the effect of overexpression of wild type Aplysia Rab3 (apRab3) and its mutant forms on LTF. Overexpression of either apRab3 Q80L, a constitutively active apRab3 mutant, or wild type apRab3 completely inhibited LTF. This inhibitory role of apRab3 appears to be mediated by an interaction with an effector molecule(s), possibly Rim. Expression of apRab3 Q80L, V54E double mutant, which do not bind effector molecules such as Rim or Rabphilin, had no effect on LTF. Furthermore, expression of apRab3 Q80L, F18L, D19E triple mutant, which has reduced binding activity with Rim but normally binds with Rabphilin, enhanced evoked basal synaptic release, and the increase in synaptic strength occluded LTF. In conclusion, our data suggest that apRab3 may act as a negative clamp of LTF through the interaction with effector protein(s), possibly Rim.     

Impairment of a parabolic bursting rhythm by the ectopic expression of a small conductance Ca2+-activated K+ channel in Aplysia neuron R15

The electrical properties of neurons are produced by the coordinated activity of ion channels. K+ channels play a key role in shaping action potentials and in determining neural firing patterns. Small conductance Ca2+-activated K+ (SK(Ca)) channels are involved in modulating the slow component of afterhyperpolarization (AHP). Here we examine whether rat type 2 SK(Ca) (rSK2) channels can affect the shape of the action potential and the neural firing pattern, by overexpressing rat SK2 channels in Aplysia neuron R15. Our results show that rSK2 overexpression decreased the intra-burst frequency and changed the regular bursting activity of neurons to an irregular bursting or beating pattern in R15. Furthermore, the overexpression of rSK2 channels increased AHP and reduced the duration of the action potential. Thus, our results suggest that ectopic SK(Ca) channels play an important role in regulating the firing pattern and the shape of the action potential.     

Effect of Intensity of Unconditional Stimulus on Reconsolidation of Contextual Fear Memory

Memory reconsolidation is ubiquitous across species and various memory tasks. It is a dynamic process in which memory is modified and/or updated. In experimental conditions, memory reconsolidation is usually characterized by the fact that the consolidated memory is disrupted by a combination of memory reactivation and inhibition of protein synthesis. However, under some experimental conditions, the reactivated memory is not disrupted by inhibition of protein synthesis. This so called "boundary condition" of reconsolidation may be related to memory strength. In Pavlovian fear conditioning, the intensity of unconditional stimulus (US) determines the strength of the fear memory. In this study, we examined the effect of the intensity of US on the reconsolidation of contextual fear memory. Strong contextual fear memory, which is conditioned with strong US, is not disrupted by inhibition of protein synthesis after its reactivation; however, a weak fear memory is often disrupted. This suggests that a US of strong intensity can inhibit reconsolidation of contextual fear memory.     

Erasing injury-related cortical synaptic potentiation as a new treatment for chronic pain

Synaptic plasticity in the spinal cord and the cortex is believed to be important for the amplification of painful information in chronic pain conditions. The investigation of molecular mechanism responsible for maintaining injury-related plastic changes, such as through the study of long-term potentiation in these structures, provides potential novel targets for designing new medicine for chronic pain. Recent studies using integrative neurobiological approaches demonstrate that protein kinase M zeta (PKMζ) maintains pain-induced persistent changes in the anterior cingulate cortex (ACC), and inhibiting PKMζ by ζ-pseudosubstrate inhibitory peptide produces analgesic effects in animal models of chronic pain. We propose that targeting PKMζ, or its up- or downstream signaling proteins, in the ACC may provide novel clinical treatment for chronic pain.