Abstracts

RATIONALE: Previous studies have shown that NPY has anti-convulsant actions in hippocampus through presynaptic effects that depress excitatory synaptic events, without affecting inhibition (Klapstein and Clomers, 1993). In contrast, NPY activates two different receptor subtypes (Y1 & Y2), which mediate presynaptic inhibition of GABA release and postsynaptic activation of a K+ conductance, respectively, in the thalamus (Sun, et al., 2001). GABA mediated IPSCs are a key player in generation of absence seizures and are a target of anti-absence medications (Huguenard, 1999). We tested the hypothesis that NPY suppresses epileptiform activity by more than one mechanism in the thalamus.
METHODS: Using standard techniques, we studied the effects of local NPY perfusions on evoked whole cell post-synaptic currents from directly-visualized neurons of the thalamic ventrobasal relay nucleus (VB) and nucleus reticularis (nRt). We also assessed NPY effects on abnormal extracellular oscillatory rhythms within the thalamic network induced with bicuculline applications.
RESULTS: 1) NPY projections arising from nRt form recurrent connections onto other nRt neurons and feed-forward connections onto relay neurons. 2) NPY, via NPY1 receptors, activated a potassium current in neurons of nRt and VB, mediated by G-protein-activated, inwardly- rectifying K+ channels (GIRK). 3) NPY, via NPY2 receptors, reduced GABA release in nRt (60%, n=14) and VB neurons (25%, n= 16), an action due to inhibition of presynaptic calcium channels. 4) NPY reduced the probability of burst generation in nRt neurons, and rebound bursts in VB relay cells. The effects on nRt neurons were mediated predominantly by NPY1 receptors and membrane hyperpolarization, as these actions were mimicked by Leu31Pro34NPY, and blocked by BIBP3226 or by manually clamping the membrane potential to control level. The effects of NPY on VB neuron responses were caused by both presynaptic inhibition of GABA release that was mimicked by NPY13-36 (NPY2 agonist), and hyperpolarization, mimicked by Leu31Pro34NPY. 5) NPY decreased the number of spikes in extracellularly-recorded spindle-like bursts by 34% (N=12), and shortened the duration of paroxysmal oscillations in slices exposed to bicuculline by 50%.
CONCLUSIONS: NPY decreases thalamic network excitability through multiple actions at different sites, including 1) Y1 receptor-mediated activation of K+ channels and hyperpolarization; and 2) Y2 receptor-mediated inhibition of GABA release onto thalamocortical neurons and consequent anti-oscillatory effects. These actions, along with the frequency-dependent release and long term effects of peptides, make NPY an ideal endogenous antiepileptic neurotransmitter in thalamocortical epilepsies.
Support: NINDS Grants 06477 and 39579, and the Pimley Research and Training Funds.