Expression and regulation of neuronal messenger molecules : focus on the NO-cGMP pathway and galanin in autonomic and sensory neurons

Abstract

The autonomic nervous system consists of the sympathetic, parasympathetic and enteric divisions. The sympathetic pre- and paravertebral neurons are in principle of noradrenergic phenotype. However, originally pharmacological experiments indicated that sympathetic fibers projecting to the sweat glands are cholinergic. Using immunohistochemistry, we show that in the rat stellate ganglion there is a subpopulation of cholinergic neurons and that they also are vasoactive intestinal polypeptide-positive. The anterior pelvic ganglia (APGs), in the guinea pig, harbored both sympathetic and parasympathetic neurons. Moreover, the sympathetic noradrenaline cells were concentrated in the cranial part of the APGs, whereas the parasympathetic cholinergic neurons were located caudally. The majority of these choline acetyltransferase (ChAT)- positive neurons were also nitric oxide synthase- (NOS) positive. Preganglionic neurons in the spinal cord synthesize acetylcholine (ACh), and in addition many of these neurons also expressed NOS, the enzyme responsible for NO formation. To further characterize the targets for NO, stimulation with sodium nitroprusside (SNP), an NO donor, was performed.

Our results show that the vast majority of the cGMP-positive neurons in the APGs are of noradrenergic phenotype. This suggests that NO may act as an intercellular messenger molecule, involved in intraganglionic communication in the APGs. In contrast, in sympathetic pre- and paravertebral ganglia mainly satellite cells were cGMP-positive after SNP stimulation, suggesting different targets for NO in various autonomic ganglia. Using the same strategy, adrenal glands were examined. After stimulation with a high dose of SNP (3 mM) virtually all chromaffin cells were cGMP- immunoreactive (IR) both in rat, guinea pig and mouse. In vitro studies in the rat showed that cGMP formation in chromaffin cells was dose dependent - at a low dose noradrenaline, but not adrenaline cells were cGMP-IR. In dorsal root ganglia (DRGs) NOS was upregulated following sciatic nerve transection, and here cGMP increased in satellite cells after axotomy, but not after inflammation.

Galanin is a peptide often expressed at low levels in the peripheral nervous system. We have generated a galanin over-expressing (Ga1OE) mouse under the platelet-derived growth factor-B promoter. Galanin was expressed in most SCG and DRG neurons, and in the periphery both sympathetic and sensory fibers were strongly galanin-positive. Functionally, over-expression of galanin resulted in decreased plasma extravasation and in increased pain sensitivity in the formalin test.

There is evidence that nerve growth factor (NGF) is involved in the regulation of galanin expression. In a mouse over-expressing NGF under the keratin promoter, regulation of galanin was examined in DRGs normally and after sciatic nerve transection. In addition, sympathetic neurons were analyzed. In the DRGs and trigeminal ganglia sympathetic pericellular fiber baskets were seen, and many of these expressed galanin- like immunoreactivity (LI). One week following axotomy of the sciatic nerve, galanin in the pericellular baskets was down regulated, but the sympathetic fibers (identified by tyrosine hydroxylase-LI) were still present in the DRGs without any obvious decrease in density. However, after three weeks the TH-positive baskets were reduced. There was no major difference in percentage of galanin-positive neuron profiles (NPs) comparing NGFOE mice and wild type ones. In the lumbar sympathetic trunk galanin mRNA-positive neurons were abundant, whereas neuropeptide tyrosine (NPY) mRNA was only expressed in a low percentage of cells. Following sciatic nerve transection, galanin-positive NPs decreased in the sympathetic trunk, and NPY-expressing NPs increased. Normally only a few NPY-positive pericellular baskets were seen, and following axotomy no increase of NPY-LI was seen in pericellular baskets, suggesting that only a subpopulation of sympathetic neurons projects to the DRGs.