Facilitation of L-Type Ca2+Channels in Dendritic Spines by Activation of β2Adrenergic Receptors

Abstract

We studied the contribution of L-type Ca²⁺channels to action potential-evoked Ca²⁺influx in dendritic spines of CA1 pyramidal neurons and the modulation of these channels by the β₂adrenergic receptor. Backpropagating action potentials (bAPs) (three at 50 Hz) were evoked by brief somatic current injections, and Ca²⁺transients were recorded in proximal basal dendrites and associated spines. The R- and T-type Ca²⁺channel blocker NiCl₂(100 μm) significantly reduced Ca²⁺transients in both spines and their parent dendrites (∼50%), suggesting that these channels are the major source of bAP-evoked Ca²⁺influx in these structures. The L-type Ca²⁺channel blockers nimodipine and nifedipine (both 10 μm) reduced spine Ca²⁺transients by ∼10%, whereas the L-type Ca²⁺channel activators FPL 64176 (2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methylester) and Bay K 8644 ((±)-1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyridine carboxylic acid methyl ester) (both 10 μm) significantly enhanced the spine Ca²⁺transients by 40-50%. Activation of β₂adrenergic receptors with salbutamol (40 μm) or formoterol (5 μm) resulted in significant enhancements of the spine (40-50%) but not dendritic Ca²⁺transients. This increase was prevented when L-type Ca²⁺channels were blocked with nimodipine (10 μm) or when cAMP-dependent protein kinase A (PKA) was inhibited with KT5720 (3 μm), Rp-cAMPS (Rp-adenosine cyclic 3′,5′-phosphorothioate) (100 μm), or PKI (100 μm). The above data suggest that L-type Ca²⁺channels are functionally present in dendritic spines of CA1 pyramidal neurons, contribute to spine Ca²⁺influx, and can be modulated by the β₂adrenergic receptor through PKA in a highly compartmentalized manner.