Peptide-Secreting Dendrites: New Controls for Neuroendocrine Neurons

Abstract

Peptidergic neurons are well known to synthesize and package peptides in their perikarya, and transport the peptide-containing dense-cored vesicles along their axons for release at the axon terminals. Immunocytochemistry and electron microscopy reveal that the dendrites of peptidergic neurons also contain substantial amounts of peptide-containing dense-cored vesicles in addition to receiving the synaptic boutons afferent to the neurons. The release of peptides from dendrites and the ability of dendrites to synthesize proteins has been investigated, using the magnocellular neurosecretory system of the hypothalamus, which has already proved itself a robust model for investigation of peptidergic neurons. Wild-type and homozygous Brattleboro rats which have hyperactive magnocellular neurons were studied. The dendrites contain all the cellular machinery needed to synthesize proteins, but there is little evidence of classical Golgi-cisterns to package them. It seems unlikely that the vasopressin or oxytocin packaged in dense-cored vesicles are synthesized in the dendrites, but mRNAs encoding receptor and microtubule proteins are present in the dendrites and could be translated there to provide finer control of afferent inputs. Neurotransmitters which stimulate magnocellular neurons elicit the exocytosis of vasopressin and oxytocin from both the dendrites and cell bodies, though that from dendrites is quantitatively the greater. Oestradiol and oxytocin are also capable of stimulating dendritic peptide release. There is increasing evidence that the release of dendritic’ peptide has important local effects on the magnocellular neurons themselves and on surrounding glia. Dendritic peptide thus provides local autocrine and paracrine controls on the activity of neurosecretory neurons.