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Received for publication August 29, 2005.
Revised October 24, 2005.
Accepted for publication October 24, 2005.
The hypothalamic paraventricular nucleus (PVN) neurons that project to the spinal intermediolateral cell column and brainstem are important for the control of sympathetic outflow. Stimulation of 
1 adrenergic receptors in the PVN increases sympathetic outflow, but the cellular mechanisms remain unclear. In this study, we determined the role of 1 adrenergic receptors in the regulation of glutamatergic and GABAergic synaptic inputs to spinally projecting PVN neurons. Whole-cell and cell-attached patch-clamp recordings were performed on retrogradely labeled PVN-spinal neurons in rat brain slices. Bath application of 50-100 µM phenylephrine, an 1 adrenergic receptor agonist, significantly increased the frequency of spontaneous excitatory postsynaptic currents (EPSCs) in a concentration-dependent manner. This effect was blocked by the 1 adrenergic receptor antagonists prazosin or corynanthine. Phenylephrine also significantly increased the frequency of miniature inhibitory postsynaptic currents (mIPSCs) but not the amplitude and decay constant of mEPSCs. Furthermore, activation of 1 adrenergic receptors with phenylephrine or cirazoline significantly decreased the frequency of spontaneous EPSCs and mEPSCs, and this effect also was blocked by corynanthine. Additionally, 50 µM phenylephrine significantly increased the firing rate of 13 labeled PVN neurons from 3.16 ± 0.42 to 5.83± 0.65 Hz. However, phenylephrine failed to increase the firing of most labeled PVN neurons in the presence of GABAA and ionotropic glutamate receptor antagonists. Thus, these data suggest that activation of 1 adrenergic receptors increases the excitability of PVN presympathetic neurons primarily through augmentation of glutamatergic tone and attenuation of GABAergic inputs.
Key words:
GABA, glutamate, hypothalamus, presynaptic modulation, sympathetic nervous system, synaptic transmission
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