Immunohistochemical localization of tyrosine hydroxylase in the ventral nerve cord of the stick insect, Carausius morosus , including neurons innervating the salivary glands

Abstract

The distribution of tyrosine hydroxylase-like immunoreactive neurons is mapped in the ventral nerve cord of the stick insect, Carausius morosus. This study also examines the tyrosine hydroxylase- and serotonin-like immunoreactive elements in the salivary glands of Carausius morosus. Tyrosine hydroxylase is the first and rate-limiting enzyme in the pathway for the production of catecholamines; therefore, tyrosine hydroxylase-like immunoreactive neurons are likely to contain catecholamines. Approximately 225 tyrosine hydroxylase-like immunoreactive neurons are present in the ventral nerve cord. The majority of these neurons appear to be interneurons. The suboesophageal ganglion contains the only unpaired neuron and the only pair of peripherally projecting tyrosine hydroxylase-like immunoreactive neurons in the ventral nerve cord. The peripherally projecting neurons project to the salivary glands via the salivary nerve. Each neuron in this pair is termed the salivary neuron 1. The remaining tyrosine hydroxylase-like immunoreactive neurons in the ventral nerve cord are interneurons and exhibit a characteristic distribution within the thoracic and the abdominal ganglia. Serotoninlike immunoreactivity is also present in the salivary glands. Positive staining of the suboesophageal ganglion for serotoninlike immunoreactivity indicates the presence of several neuron pairs including a large pair along the ventral posterior midline that project to the salivary glands via the salivary nerve. Each neuron in this pair is termed the salivary neuron 2. Backfilling of the salivary nerve with cobalt chloride reveals the presence of only two neurons within the suboesophageal ganglion that project to the salivary glands; these neurons are the salivary neurons 1 and 2. Reverse-phase high-performance liquid chromatography coupled with electrochemical detection of ventral nerve cord and salivary gland homogenates confirms the presence of dopamine and serotonin.