Neurosecretory axo-axonic synapses in the median eminence of the turtle, Geoclemys reevesii

Distribution of serotonin in the caudal neurosecretory complex. A light and electron microscopic study

The caudal neurosecretory complex (CNc) of poecilids has previously been shown to receive serotonergic inputs. In the present study, immunohistochemical techniques were applied at the light and electron microscopic levels to characterize serotonergic terminals in the neuroendocrine nucleus. A dense plexus of varicose fibers observed in the rostral CNc neuropil was absent in the spinal cords of deafferented fish, indicating that the origin of this input was extranuclear. Ultrastructural study revealed no direct contacts between labeled structures and neuroendocrine cells. Non-synaptic terminals (varicosities) were the predominantly labeled structures in the neuropil. Synaptic terminals were observed on cellular and axonal targets in the CNc. Small cells containing 70 nm dense-core vesicles received serotonergic input on their perikarya. Labeled synapses were also found on unlabeled axon terminals which made axo-axonal synapses on neuroendocrine processes. Non-synaptic terminals may be responsible for a variety of serotonin-mediated effects in the CNc. Synaptic interactions with local catecholaminergic and afferent cholinergic inputs to the CNc are likely.

Noradrenergic afferents to median eminence: inhibitory role in rhythmic growth hormone secretion

The involvement of noradrenergic (NA) afferents to the median eminence (ME) in the regulation of growth hormone (GH) secretion was investigated in chronically cannulated, unanaesthetized male rats by using systemically administered 6-hydroxy-dopamine (6-OHDA). One day after 6-OHDA treatment (50 mg/kg, i.v.) GH peak frequency was substantially increased and examination of catecholamine (CA) fluorescence indicated disruption of CA innervation of the ME, but not other central nervous system (CNS) structures. Prolactin secretion at this time was normal and the administration of butaclamol (1.0 mg/kg, i.v.), a dopamine (DA) antagonist, was effective in increasing secretion, indicating that ME DA mechanisms were functionally intact following 6-OHDA treatment. GH secretory patterns returned to normal by 21 days following 6-OHDA treatment and this corresponded with re-emergence of a normal pattern of CA-fluorescence in the ME. In an additional study, the administration of the alpha-NA agonist clonidine (130 microgram/kg, i.v.) increased GH secretion in previously untreated animals. Administration of the alpha-NA agonist oxymetazoline (45 microgram/kg, i.v.), which does not readily pass the blood-brain barrier (BBB), suppressed GH secretion. These findings indicate that ME NA afferents are inhibitory to GH secretion and are a major determinant of the 3 h pattern of GH release in the rat. This inhibitory input is subsidiary to a NA stimulatory input to an as yet unidentified site inside the BBB.