Pituitary adenylate cyclase activating polypeptide (PACAP) decreases neuronal somatostatin immunoreactivity in cultured guinea-pig parasympathetic cardiac ganglia

Postganglionic parasympathetic neurons in guinea-pig cardiac ganglia exhibit choline acetyltransferase (ChAT)-immunoreactivity, and a large fraction (60%) of the ChAT-positive cardiac neurons co-express somatostatin-immunoreactivity. This co-expression remained when the cardiac ganglia explants were maintained in culture for 72 h (40% somatostatin-immunoreactive). The guinea-pig cardiac ganglia neurons express the high affinity pituitary adenylate cyclase activating polypeptide (PACAP)-selective PAC1 receptor, and treatment of the ganglia explants with 20 nM PACAP27 for 72 h to evaluate PACAP regulation of somatostatin expression revealed a dramatic 85% decrease in the number of somatostatin-IR neurons (6% somatostatin-IR neurons) compared with untreated control explant preparations. The decrease in percentage of somatostatin-IR neurons by PACAP27 was time- and concentration-dependent, and selective for PACAP27; PACAP38 and vasoactive intestinal polypeptide were less effective. PACAP6-38, a PACAP antagonist, eliminated the PACAP27-induced change in somatostatin positive neurons. The PACAP-mediated decrease in somatostatin-IR neurons was eliminated in calcium-deficient solutions and by the addition of nifedipine, indicating a requirement for calcium influx through L-type calcium channels. The addition of either the calmodulin inhibitor N-(4-aminobutyl)-1-naphthalenesulfonamide or the MEK inhibitor PD98059, also eliminated the PACAP27-induced decrease in somatostatin-IR cells. The PACAP27-mediated effect on somatostatin expression was not affected by inhibitors of protein kinase A or phospholipase C, but was reduced by the adenylyl cyclase inhibitor SQ22356, suggesting cAMP involvement. Semiquantitative and quantitative reverse transcription PCR prosomatostatin transcript measurements showed that cardiac ganglia prosomatostatin mRNA levels were not diminished by chronic PACAP27 exposure despite the dramatic decrement in somatostatin-expressing neurons. Neuronal peptide-IR content represents a balance between production and secretion. These results suggested that one of the primary effects of PACAP exposure may be enhanced levels of neuropeptide release that exceeded production levels, resulting in somatostatin depletion and a decrement in the number of identifiable somatostatin-expressing cardiac neurons.

PACAP peptides modulate guinea pig cardiac neuron membrane excitability and neuropeptide expression

Morphological studies identified PACAP-immunoreactive nerve fibers in dense pericellular arrangements around virtually every cholinergic parasympathetic neuron of guinea pig cardiac ganglia; all postganglionic cardiac neurons expressed membrane-associated PAC1 receptor protein. Characterization of the alternative splice variants established predominant expression of the PAC1(very short) receptor transcript containing neither HIP nor HOP exons. PACAP depolarized cardiac neurons and increased membrane excitability; the excitability resulted from neither altered action potential properties nor inhibition of IM. Treatment of cardiac ganglia explants with PACAP significantly reduced the numbers of cholinergic neurons coexpressing somatostatin immunoreactivity, which did not appear to be correlated with prosomatostatin mRNA expression. The PACAP-mediated decrease in somatostatin immunoreactive neurons required calcium influx through L-type calcium channels and activation of adenylyl cyclase, whereas activation of phospholipase C or protein kinase A was not required. These observations indicate that PACAP through the PAC1 receptors elicits complex actions on guinea pig parasympathetic cardiac ganglia neurons, including modulation of membrane ion conductances and modulation of neuropeptide expression.

Localization of messenger ribonucleic acid for adrenomedullin and adrenomedullin receptor in the human placenta in normal pregnancies and pregnancies complicated by oligohydramnios

OBJECTIVE

The purpose of this study was to identify the placental expression of adrenomedullin and adrenomedullin receptor messenger ribonucleic acid and compare them between placentas from pregnancies associated with oligohydramnios as a result of uteroplacental insufficiency and placentas from normal pregnancies.

STUDY DESIGN

Total ribonucleic acid was extracted from the amnion, chorion, cotyledon, umbilical vein, and umbilical artery in 5 normal placentas and 3 placentas from pregnancies complicated by oligohydramnios. A cell line known to express messenger ribonucleic acid of adrenomedullin and its receptor was used to optimize the polymerase chain reaction and served as a positive control preparation in all experiments. Semiquantitative reverse transcriptase-polymerase chain reaction results for adrenomedullin and adrenomedullin receptor were compared between tissues as densitometric ratios of adrenomedullin or adrenomedullin receptor messenger ribonucleic acid to beta(2)-microglobulin messenger ribonucleic acid. Results were analyzed with a Kruskal-Wallis 1-way analysis of variance. Immunohistochemical staining with an antibody to human adrenomedullin was used to localize adrenomedullin in all tissue types.

RESULTS

Messenger ribonucleic acid sequences for adrenomedullin and adrenomedullin receptor genes were identified in all tested placental tissue components. Within the normal placentas the expressions of adrenomedullin and adrenomedullin receptor messenger ribonucleic acid sequences did not differ statistically between the tissue components. Within placentas from patients with oligohydramnios the expressions of adrenomedullin and adrenomedullin receptor messenger ribonucleic acid did not differ statistically between the tissue components. When normal placentas were compared with placentas from pregnancies complicated by oligohydramnios, however, a 5-fold increase in adrenomedullin messenger ribonucleic acid and a 3-fold increase in adrenomedullin receptor messenger ribonucleic acid were seen in placentas from patients with oligohydramnios. Adrenomedullin immunoreactivity was present in all tissues studied.

CONCLUSION

The expression of messenger ribonucleic acid for both adrenomedullin and its receptor in these tissue components implies that placental tissues function in both synthesis and action of adrenomedullin. The increased adrenomedullin messenger ribonucleic acid expression in the umbilical artery and the elevated adrenomedullin receptor messenger ribonucleic acid expression in the cotyledons of placentas from patients with oligohydramnios may represent a local fetoplacental physiologic adaptive response to vascular compromise.