Ultrastructural evidence that substance P neurons form synapses with noradrenergic neurons in the A7 catecholamine cell group that modulate nociception.

Potent antinociception can be produced by electrical stimulation of spinally projecting noradrenergic neurons in the A7 catecholamine cell group and this effect is blocked by intrathecal injection of alpha2-adrenoceptor antagonists. Microinjection of substance P near A7 neurons also produces antinociception that is blocked by intrathecal injection of alpha2-adrenoceptor antagonists. These observations suggest that substance P produces antinociception by activating noradrenergic A7 neurons. However, it is not known whether this effect of substance P is produced by a direct or an indirect action on A7 neurons. Although light microscopic studies have demonstrated the existence of both substance P-containing axon terminals and neurokinin-1 receptors in the region of the A7 cell group, it is not known whether substance P terminals form synapses with noradrenergic A7 neurons. These experiments used double-labeling immunocytochemical methods and electron microscopic analysis to determine whether substance P-containing axons form synapses with noradrenergic neurons in the A7 cell group. Pre-embedding immunocytochemistry, combined with light and electron microscopic analysis, was used to provide ultrastructural evidence for synaptic connections between substance P-immunoreactive terminals labeled with immunoperoxidase and tyrosine hydroxylase-immunoreactive A7 neurons labeled with silver-enhanced immunogold. Tyrosine hydroxylase labeling was found in perikarya and dendrites in the A7 region, and substance P labeling was found in axons and synaptic terminals. Substance P-labeled terminals formed asymmetric synapses with tyrosine hydroxylase-labeled dendrites, but only a few of these were present on tyrosine hydroxylase-labeled somata. Substance P-labeled terminals also formed asymmetric synapses with unlabeled dendrites, and many unlabeled terminals formed both symmetric and asymmetric synapses with tyrosine hydroxylase-labeled dendrites. These results demonstrate that substance P neurons form a significant number of synapses with the dendrites of noradrenergic A7 neurons and support the conclusion that microinjection of substance P in the A7 cell group produces antinociception by direct activation of spinally projecting noradrenergic neurons.

Effect of inosine on pial arterioles: potentiation of adenosine-induced vasodilation.

We examined the cerebral vasoactivity of inosine and its effect on pial arteriolar vasodilation induced by adenosine. Pial circulation was observed through cranial windows implanted in rats anesthetized with halothane. No significant change in venous or arteriolar diameter was apparent when inosine (10(-6) to 10(-3) M) was superfused. In contrast, adenosine in concentrations of 10(-7) and 10(-6) M dilated pial arterioles by 9.0 +/- 1.2 and 17.7 +/- 1.7%, respectively. Addition of 10(-5) M inosine had no effect, whereas 10(-4) M inosine enhanced the vasodilation induced by 10(-7) M adenosine to 19.4 +/- 1.7% and that by 10(-6) M adenosine to 23.3 +/- 2.3%. When theophylline (5 X 10(-5) M) was perfused together with 10(-7) M adenosine and 10(-4) M inosine, the vasodilation was almost completely abolished. The present study indicates that inosine alone does not affect pial vessel diameter but potentiates the response of pial arterioles to exogenous adenosine. This potentiating action of inosine may be attributed to an increase in perivascular adenosine and may be explained by the action of inosine as an adenosine uptake inhibitor.