Date of Award

8-11-2008

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Biology

First Advisor

James E. Porter

Abstract

Adrenergic receptors (ARs) represent a family of G protein-coupled receptors endogenously activated by norepinephrine and epinephrine. One area of the brain where AR activation modulates cellular activity is CA1 hippocampus, however it is not known which AR subtypes are specifically involved, nor the mechanism behind the AR-mediated changes in activity. The goal of this study was thus two-fold; (1) Identify the specific AR subtypes expressed in CA1, and (2) Identify the mechanism by which AR activation changes CA1 neuron excitability. I set forth the hypothesis that α1B-AR activation initiates phospholipase C (PLC) signaling in CA1 interneurons, causing them to depolarize and release GABA onto neighboring pyramidal neurons. Single-cell RT-PCR experiments were first used to determine which AR subtypes were transcriptionally expressed in CA1. Pyramidal cells contained mRNA for the β1- and β2-AR; interneurons, particularly somatostatin-expressing, stratum oriens interneurons, contained mRNA for the α1A- and α1B-AR. Single-cell, cell-attached electrophysiology recordings were then used to determine which AR subtypes were functionally expressed by CA1 pyramidal neurons. Using the subtype-selective β-AR antagonists atenolol, ICI 118,551, and butoxamine, I generated concentration-response curves for the selective β-AR agonist isoproterenol. Schild analysis of the data points revealed that pyramidal cells functionally express β2-ARs. Similar experimentation was conducted on CA1 interneurons. Using the subtype-selective α 1-AR antagonists 5-methylurapidil, WB-4101, L-765,314, and BMY7378, I generated concentration-response curves for the selective α1 -AR agonist phenylephrine. Schild analysis of the data points revealed that a subset of interneurons functionally express α1A-ARs. Single-cell, whole-cell electrophysiology recordings were then used to determine pre- and post-synaptic consequences of α1A-AR activation. Activation of this AR subtype initiates a sodium-dependent, PLC-independent inward current in a subpopulation of interneurons. Furthermore, α 1A-AR activation produces a decrease in pyramidal neuron activity via pre-synaptic GAGA and somatostatin release. Field recordings revealed a significant α1A-AR-mediated decrease in pyramidal neuron activity in one in vitro epilepsy model, suggesting α 1A-AR activation may represent a novel antiepileptic strategy. Together, my studies demonstrate that adrenergic effects in CA1 are predominately mediated by pyramidal β2-ARs and interneuronal α1A-ARs. Furthermore, α1A-AR activation depolarizes CA1 interneurons in a PLC-independent manner, initiating GABA and somatostatin release onto neighboring pyramidal neurons.

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