Transmitter status in cultured sympathetic principal neurons: plasticity, graded expression and diversity

Segregation of the classical transmitters norepinephrine and acetylcholine and the neuropeptide Y in sympathetic neurons: modulation by ciliary neurotrophic factor or prolonged growth in culture

Recent evidence has demonstrated that cotransmission from mammalian neurons is not uniquely achieved by costorage and corelease of transmitters and cotransmitters from single varicosities, but also by the concurrent release of mediators segregated in separate synapses of individual neurons. An important question to be addressed is whether neurons show defined patterns of segregation or whether this is a plastic feature. We addressed this question by exploring the segregation pattern of the classical sympathetic transmitters norepinephrine (NE) and acetylcholine (ACh) and the cotransmitter neuropeptide Y (NPY) in sympathetic ganglionic neurons cocultured with cardiac myocytes. Using antibodies against NPY and the vesicular NE and ACh transporters VMAT2 and vesicular acetylcholine transporter (VAChT), we investigated the effect of ciliary neurotrophic factor (CNTF) or long (three weeks) culture periods on the segregation of VMAT2, VAChT, and NPY to separate varicosities. We found that although ganglionic neurons showed cell body coexpression of all the markers examined after three days, VMAT2 was segregated from VAChT in 43% of the VAChT-positive varicosities. In contrast, VMAT2 was only segregated from NPY in 16.3% of the NPY-positive varicosities. Cotransmitter segregation and VAChT expression was potentiated by both CNTF and longer times in culture. We also found two types of varicosities: one was smaller and located further from neuronal somata, and the other was larger, proximal to neuronal somata and had a higher level of segregation. These data demonstrate segregation of classical transmitters in sympathetic neurons and plasticity of neurotransmitter segregation. Finally, we discuss a possible functional correlate of segregation in sympathetic neurons.

Peptidergic nerves in the eye, their source and potential pathophysiological relevance

Over the last five decades, several neuropeptides have been discovered which subsequently have been found to be highly conserved during evolution, to be widely distributed both in the central and peripheral nervous system and which act as neurotransmitters and/or neuromodulators. In the eye, the first peptide to be explored was substance P which was reported to be present in the retina but also in peripherally innervated tissues of the eye. Substance P is certainly the best characterized peptide which has been found in sensory neurons innervating the eye. Functionally, it has been shown to act trophically on corneal wound healing and to participate in the irritative response in lower mammals, a model for neurogenic inflammation, where it mediates the noncholinergic nonadrenergic contraction of the sphincter muscle. Over the last three decades, the interest has extended to investigate the presence and distribution of other neuropeptides including calcitonin gene-related peptide, vasoactive intestinal polypeptide, neuropeptide Y, pituitary adenylate cyclase-activating polypeptides, cholecystokinin, somatostatin, neuronal nitric oxide, galanin, neurokinin A or secretoneurin and important functional results have been obtained for these peptides. This review focuses on summarizing the current knowledge about neuropeptides in the eye excluding the retina and retinal pigment epithelium and to elucidate their potential functional significance.

Activin type II receptors in embryonic dorsal root ganglion neurons of the chicken

Activin induces neuropeptide expression in chicken ciliary ganglion neurons. To determine if activin might also influence neuropeptide expression in developing sensory neurons, we examined whether type II activin receptors are expressed during embryonic development of the chicken dorsal root ganglia (DRG), and also examined the effects of activin on neuropeptide expression in cultured DRG neurons. Using reverse transcription polymerase chain reaction (rtPCR), we detected mRNAs for both the activin receptors type IIA (ActRIIA) and type IIB (ActRIIB) in DRG from embryonic day 7 through posthatch day 1. With in situ hybridization, we found that morphologically identifiable neurons express mRNAs for both ActRIIA and ActRIIB. With developmental age, a subset of neurons that hybridizes more intensely with riboprobes to these receptor mRNAs becomes evident. A similar pattern of expression is observed with immunocytochemical staining using antisera against activin type II receptors. To examine whether embryonic DRG cells respond to activin we treated dissociated cultures of DRG with activin A and assessed the expression of vasoactive intestinal peptide (VIP) and calcitonin gene related peptide (CGRP) mRNAs using semiquantitative rtPCR. Activin treatment results in an increase in VIP mRNA, but does not affect CGRP mRNA levels. These observations indicate that neurons in the embryonic chicken DRG can respond to activin and suggest that activin has the potential to play a role in the development and function of DRG sensory neurons.

Age-dependent neurotransmitter plasticity of ciliary ganglion neurons

We have examined neurotransmitter plasticity in postmitotic cholinergic neurons isolated from 6.5- to 11-day-old embryonic quail ciliary ganglia. Purified neurons were labeled with DiI, transplanted into the trunk of young chick embryos, and assayed for catecholamine content and [3H]thymidine uptake 4 to 5 days later. For ciliary neurons derived from 6.5- to 8-day-old embryos, as many as 25% (average of 9% overall) expressed catecholamines in the host sympathetic ganglia, migratory stream, aortic plexuses, and adrenal medulla. In contrast, neurons from >8-day-old ganglia did not acquire or produce detectable catecholamines, indicating a limited time period over which phenotypic conversion can occur in vivo. As a control, ciliary neurons were also injected into the head mesenchyme of young embryos; no catecholamine expression was observed. Interestingly, after transplantation some DiI-labeled postmitotic ciliary neurons took up [3H]thymidine with or without phenotypic change. These results suggest that phenotypic plasticity in ciliary neurons is age-dependent, is location-dependent, and may involve resumption of DNA replication, a characteristic feature of some differentiating adrenergic sympathetic neurons. Apoptosis of a few proliferating transplanted cells may be induced independently or in association with transmitter change.

Collateral sprouting of the central terminals of cutaneous primary afferent neurons in the rat spinal cord: pattern, morphology, and influence of targets

The capacity of the central terminals of primary afferents to sprout into denervated areas of neonatal spinal cord and the morphology of any novel terminals has been investigated. In rats which had undergone sciatic nerve section on the day of birth, 12 of 18 physiologically characterized intact saphenous hair follicle afferents (HFAs) were labelled intra-axonally with horseradish peroxidase (HRP) were shown to sprout up to 2,000 microns into the deafferented sciatic terminal field. The morphology of these sprouts depended on which area of the sciatic nerve territory was invaded by the afferent sprouts. Six HFAs sprouted into areas normally innervated by glabrous skin afferents and the morphology of the collateral sprouts in this region resembled that of rapidly adapting (RA) afferents. The other six saphenous HFAs had sprouted into sciatic "hairy" skin areas and the morphology of these sprouts, although abnormal, was flame shaped. In rats whose sural, saphenous, and superficial peroneal nerves were cut at birth, 4 of 7 single HRP labelled RA afferents had central terminals that had sprouted into regions of cord normally devoted to "hairy" input. These showed clear signs of HFA morphology despite their peripheral receptive fields remaining in the glabrous skin. The results show collateral sprouting of single cutaneous sensory afferent axons into adjacent inappropriate central target regions following neonatal deafferentation. Such plasticity may provide some compensation following neonatal injury. The morphology of the sprouted terminals is appropriate to the new target area rather than to its functional class and is also independent of the peripheral receptive field location providing an example of central rather than peripheral control over afferent growth patterns.