Neurokinin-1 receptor localisation in guinea pig autonomic ganglia

We have used multiple-labelling immunohistochemistry and confocal microscopy to determine the distribution of immunoreactivity to the tachykinin neurokinin-1 (NK₁) receptors in guinea pig sympathetic ganglia. Although nerve fibres containing immunoreactivity to substance P were common in all ganglia except the superior cervical ganglia, most neurons expressing NK₁ receptor immunoreactivity were not closely surrounded by pericellular baskets of substance P-immunoreactive boutons. Conversely, many neurons surrounded by baskets of substance P-immunoreactive boutons lacked NK₁ immunoreactivity. In the coeliac and inferior mesenteric ganglia, NK₁ receptor expression was restricted almost entirely to noradrenergic neurons that contained somatostatin immunoreactivity and projected to the enteric plexuses. In the lumbar chain and paracervical ganglia, NK₁ immunoreactivity was expressed by nonnoradrenergic vasodilator neurons containing immunoreactivity to vasoactive intestinal peptide. Taken together, our results show that sympathetic neurons in different functional pathways express NK₁ receptor immunoreactivity. However, the neurons that could respond to endogenously released substance P through NK₁ receptors may be distant from presynaptic release sites. These observations suggest that, in sympathetic ganglia, substance P may modulate ganglionic transmission through heterosynaptic actions on NK₁ receptors.