Some currently neglected aspects of cholinergic function

Frank Beach Award Winner: Steroids as neuromodulators of brain circuits and behavior

Neurons communicate primarily via action potentials that transmit information on the timescale of milliseconds. Neurons also integrate information via alterations in gene transcription and protein translation that are sustained for hours to days after initiation. Positioned between these two signaling timescales are the minute-by-minute actions of neuromodulators. Over the course of minutes, the classical neuromodulators (such as serotonin, dopamine, octopamine, and norepinephrine) can alter and/or stabilize neural circuit patterning as well as behavioral states. Neuromodulators allow many flexible outputs from neural circuits and can encode information content into the firing state of neural networks. The idea that steroid molecules can operate as genuine behavioral neuromodulators - synthesized by and acting within brain circuits on a minute-by-minute timescale - has gained traction in recent years. Evidence for brain steroid synthesis at synaptic terminals has converged with evidence for the rapid actions of brain-derived steroids on neural circuits and behavior. The general principle emerging from this work is that the production of steroid hormones within brain circuits can alter their functional connectivity and shift sensory representations by enhancing their information coding. Steroids produced in the brain can therefore change the information content of neuronal networks to rapidly modulate sensory experience and sensorimotor functions.

The elusive role of L-glutamate as an echinoderm neurotransmitter: evidence for its involvement in the control of crinoid arm muscles

Although l-glutamate is the most widespread excitatory neurotransmitter in vertebrate and invertebrate nervous systems, there is only sparse evidence that it has this role in echinoderms. Following our previous finding that l-glutamate is widely distributed in the arms of the featherstar (crinoid echinoderm) Antedon mediterranea and initiates arm autotomy (defensive detachment), we now provide evidence of glutamatergic involvement in the control of the arm muscles of the same species using immunocytochemical and physiological methods. Immunofluorescence and immunoenzymatic techniques, which employed the same polyclonal antibody against l-glutamate conjugated to glutaraldehyde, revealed a high level of glutamate-like reactivity in the brachial muscles. By recording the mechanical responses of isolated arm pieces, we found that l-glutamate, l-aspartate and elevated [K(+)](o) induced rhythmic muscle contractions, while glycine, γ-aminobutyric acid, adrenaline and acetylcholine had either no, or no consistent, effect. The frequency and duration of the dominant component of the rhythmic contractions indicated that these may be responsible for the rhythmic activity of the arms that occurs during swimming and after autotomy. We conclude that it is highly likely that l-glutamate has at least a neuromodulatory role in the neural pathways controlling the brachial muscles of A. mediterranea.

The mechanisms and possible sites of acetylcholine release during chick primary sensory neuron differentiation

AIMS

In this study, we evaluated the ability of differentiating embryonic chick DRG neurons to release and respond to acetylcholine (ACh). In particular, we investigated the neuronal soma and neurites as sites of ACh release, as well as the mechanism(s) underlying this release.

MAIN METHODS

ACh release from DRG explants in the Campenot chambers was measured by a chemiluminescent assay. Real-time PCR analysis was used to evaluate the expression of ChAT, VAChT, mediatophore and muscarinic receptor subtypes in DRGs at different developmental stages.

KEY FINDINGS

We found that ACh is released both within the central and lateral compartments of the Campenot chambers, indicating that ACh might be released from both the neuronal soma and fibers. Moreover, we observed that the expression of the ChAT and mediatophore increases during sensory neuron differentiation and during the post-hatching period, whereas VAChT expression decreases throughout development. Lastly, the kinetics of the m2 and m3 transcripts appeared to change differentially compared to the m4 transcript during the same developmental period.

SIGNIFICANCE

The data obtained demonstrate that the DRG sensory neurons are able to release ACh and to respond to ACh stimulation. ACh is released both by the soma and neurite compartments. The contribution of the mediatophore to ACh release appears to be more significant than that of VAChT, suggesting that the non-vesicular release of ACh might represent the preferential mechanism of ACh release in DRG neurons and possibly in non-cholinergic systems.