Elevated B-type natriuretic peptide (BNP) regulates cGMP-phosphodiesterase activity. Its elevation is regarded as an early compensatory response to cardiac failure where it can facilitate sympathovagal balance and cardiorenal homeostasis. However, recent reports suggest a paradoxical proadrenergic action of BNP. Because phosphodiesterase activity is altered in cardiovascular disease, we tested the hypothesis that BNP might lose its efficacy by minimizing the action of cGMP on downstream pathways coupled to neurotransmission. BNP decreased norepinephrine release from atrial preparations in response to field stimulation and also significantly reduced the heart rate responses to sympathetic nerve stimulation in vitro. Using electrophysiological recording and fluorescence imaging, BNP also reduced the depolarization evoked calcium current and intracellular calcium transient in isolated cardiac sympathetic neurons. Pharmacological manipulations suggested that the reduction in the calcium transient was regulated by a cGMP/protein kinase G pathway. Fluorescence resonance energy transfer measurements for cAMP, and an immunoassay for cGMP, showed that BNP increased cGMP, but not cAMP. In addition, overexpression of phosphodiesterase 2A after adenoviral gene transfer markedly decreased BNP stimulation of cGMP and abrogated the BNP responses to the calcium current, intracellular calcium transient, and neurotransmitter release. These effects were reversed on inhibition of phosphodiesterase 2A. Moreover, phosphodiesterase 2A activity was significantly elevated in stellate neurons from the prohypertensive rat compared with the normotensive control. Our data suggest that abnormally high levels of phosphodiesterase 2A may provide a brake against the inhibitory action of BNP on sympathetic transmission.

Hypertension is associated with cardiac noradrenergic hyperactivity, although it is not clear whether this precedes or follows the development of hypertension itself. We hypothesized that Ca ²⁺ homeostasis in postganglionic sympathetic neurons is impaired in spontaneously hypertensive rats (SHRs) and may occur before the development of hypertension. The depolarization-induced rise in intracellular free calcium concentration ([Ca ²⁺ ] _i ; measured using fura-2-acetoxymethyl ester) was significantly larger in cultured sympathetic neurons from prehypertensive SHRs than in age matched normotensive Wistar-Kyoto rats. The decay of the [Ca ²⁺ ] _i transient was also faster in SHRs. The endoplasmic reticulum Ca ²⁺ content and caffeine-induced [Ca ²⁺ ] _i amplitude were significantly greater in the young SHRs. Lower protein levels of phospholamban and more copies of ryanodine receptor mRNA were also observed in the young SHRs. Depleting the endoplasmic reticulum Ca ²⁺ store did not alter the difference of the evoked [Ca ²⁺ ] _i transient and decay time between young SHRs and Wistar-Kyoto rats. However, removing mitochondrial Ca ²⁺ buffering abolished these differences. A lower mitochondrial membrane potential was also observed in young SHR sympathetic neurons. This resulted in impaired mitochondrial Ca ²⁺ uptake and release, which might partly be responsible for the increased [Ca ²⁺ ] _i transient and faster decay in SHR sympathetic neurons. This Ca ²⁺ phenotype seen in early development in cardiac stellate and superior cervical ganglion neurons may contribute to the sympathetic hyperresponsiveness that precedes the onset of hypertension.

Neuropathic pain is a chronic pain condition caused by damage or dysfunction of the central or peripheral nervous system. Electroacupuncture (EA) has an antinociceptive effect on neuropathic pain, which is partially due to inhibiting astrocyte activation in the spinal cord. We found that an intrathecal injection of 8-cyclopentyl-1,3-dipropylxanthine (DPCPX), a selective adenosine A1 receptor antagonist, reversed the antinociceptive effects of EA in a chronic constriction injury-induced neuropathic pain model. The expression of GFAP in L4-L6 spinal cord was significantly upgraded, while DPCPX suppressed the effect of the EA-mediating inhibition of astrocyte activation, as well as wiping out the EA-induced suppression of cytokine content (TNF-α). These results indicated that the adenosine A1 receptor is involved in EA actions during neuropathic pain through suppressing astrocyte activation as well as TNF-α upregulation of EA, giving enlightenment to the mechanisms of acupuncture analgesia and development of therapeutic targets for neuropathic pain.

Hypertension is associated with the early onset of cardiac sympathetic hyperresponsiveness and enhanced intracellular Ca ²⁺ concentration [Ca ²⁺ ] _i in sympathetic neurons from both prehypertensive and hypertensive, spontaneously hypertensive rats (SHRs). Oxidative stress is a hallmark of hypertension, therefore, we tested the hypothesis that the inhibitory action of the nitric oxide-cGMP pathway on [Ca ²⁺ ] _i transients is impaired in cardiac sympathetic neurons from the SHR. Stellate ganglia were isolated from young prehypertensive SHRs and age-matched normotensive Wistar-Kyoto rats. [Ca ²⁺ ] _i was measured by ratiometric fluorescence imaging. Neurons from the prehypertensive SHR ganglia had a significantly higher depolarization evoked [Ca ²⁺ ] _i transient that was also associated with decreased expression of neuronal nitric oxide synthase (nNOS), β1 subunit of soluble guanylate cyclase and cGMP when compared with the Wistar-Kyoto rat ganglia. Soluble guanylate cyclase inhibition or nNOS inhibition increased [Ca ²⁺ ] _i in the Wistar-Kyoto rats but had no effect in SHR neurons. A nitric oxide donor decreased [Ca ²⁺ ] _i in both sets of neurons, although this was markedly less in the SHR. A novel noradrenergic cell specific vector (Ad.PRSx8-nNOS/Cherry) or its control vector (Ad.PRSx8-Cherry) was expressed in sympathetic neurons. In the SHR, Ad.PRSx8-nNOS/Cherry-treated neurons had a significantly reduced peak [Ca ²⁺ ] _i transient that was associated with increased tissue levels of nNOS protein and cGMP concentration compared with gene transfer of Ad.PRSx8-Cherry alone. nNOS inhibition significantly increased [Ca ²⁺ ] _i after Ad.PRSx8-nNOS/Cherry expression. We conclude that artificial upregulation of stellate sympathetic nNOS via targeted gene transfer can directly attenuate intracellular Ca ²⁺ and may provide a novel method for decreasing enhanced cardiac sympathetic neurotransmission.