Project I:
Tachykinin Modulation of Epilepsy
Investigator: Dr. Saobo Lei

Recurrent uncontrolled seizures or epilepsy is a common neurological disorder that is characterized by excessive excitation and neurodegeneration of many brain regions including the hippocampus. Antiepileptic drugs, while somewhat effective, have side effects and target a limited number of underlying mechanisms. Therefore, identifying and characterizing additional mechanisms through which seizures are produced and therapeutic strategies that target these mechanisms is important scientifically and medically. The tachykinin family of neuropeptides including substance P, neurokinin A and neurokinin B are proconvulsant and appear to play important roles in seizure disorders. For example, substance P facilitates whereas substance P receptor antagonists block seizures. Seizures increase dramatically the neuronal synthesis and release of tachykinins, and the expression of tachykinin receptors. Moreover, animals with null mutations of the preprotachykinin gene are resistant to self-sustaining status epilepticus and to kainic acid-induced neuronal death. One still ill-defined mechanism responsible for the proconvulsant effects of tachykinins is the increased release of glutamate.The objective of this application is to determine the cellular and molecular mechanisms by which tachykinins exert epileptogenic activity. Our central hypothesis is that tachykinins promote epileptogenic activity by increasing glutamatergic synaptic transmission in the hippocampus. Our preliminary data demonstrated that application of tachykinin receptor agonists increased glutamate release at multiple synapses of the hippocampus via inhibition of presynaptic K⁺ channels because application of 4-aminopyridine at low concentration blocked the effects of tachykinins. Furthermore, application of substance P inhibited delayed rectifier K⁺ channel currents recorded from presynaptic neurons. Using transgenic mice and pharmacological approaches we have shown that phospholipase Cß1 and protein kinase C (PKC) were fully, but intracellular Ca²⁺ release was partially, required for the effects of tachykinins on glutamate release. Using the GABA_A receptor antagonist, picrotoxin, to induce spontaneous epileptogenic activity in hippocampal slices as an in vitro seizure model, we have demonstrated that activation of tachykinin receptors enhanced epileptogenic activity. Here, we will determine detailed cellular and molecular signaling mechanisms responsible for tachykinin-induced increases in glutamate release and epileptogenic activity with a focus on identifying the involved subtypes of K⁺ channels and PKC isoforms. This research is innovative because it will be the first study to elucidate the cellular and molecular signaling mechanisms whereby tachykinins facilitate glutamate release and epileptogenic activity. Our studies will provide a mechanistic foundation that would lead to the development of novel approaches for preventing and treating, with limited side effects, epilepsy.