Glutamate evokes firing through activation of kainate receptors in chick accessory lobe neurons

Mechanistic Hypotheses for Proprioceptive Sensing Within the Avian Lumbosacral Spinal Cord

Animals need to accurately sense changes in their body position to perform complex movements. It is increasingly clear that the vertebrate central nervous system contains a variety of cells capable of detecting body motion, in addition to the comparatively well-understood mechanosensory cells of the vestibular system and the peripheral proprioceptors. One such intriguing system is the lower spinal cord and column in birds, also known as the avian lumbosacral organ (LSO), which is thought to act as a set of balance sensors that allow birds to detect body movements separately from head movements detected by the vestibular system. Here, we take what is known about proprioceptive, mechanosensory spinal neurons in other vertebrates to explore hypotheses for how the LSO might sense mechanical information related to movement. Although the LSO is found only in birds, recent immunohistochemical studies of the avian LSO have hinted at similarities between cells in the LSO and the known spinal proprioceptors in other vertebrates. In addition to describing possible connections between avian spinal anatomy and recent findings on spinal proprioception as well as sensory and sensorimotor spinal networks, we also present some new data that suggest a role for sensory afferent peptides in LSO function. Thus, this perspective articulates a set of testable ideas on mechanisms of LSO function grounded in the emerging spinal proprioception scientific literature.

3D Anatomy of the Quail Lumbosacral Spinal Canal-Implications for Putative Mechanosensory Function

Birds are diverse and agile vertebrates capable of aerial, terrestrial, aquatic, and arboreal locomotion. Evidence suggests that birds possess a novel balance sensing organ in the lumbosacral spinal canal, a structure referred to as the "lumbosacral organ" (LSO), which may contribute to their locomotor agility and evolutionary success. The mechanosensing mechanism of this organ remains unclear. Here we quantify the 3D anatomy of the lumbosacral region of the common quail, focusing on establishing the geometric and biomechanical properties relevant to potential mechanosensing functions. We combine digital and classic dissection to create a 3D anatomical model of the quail LSO and estimate the capacity for displacement and deformation of the soft tissues. We observe a hammock-like network of denticulate ligaments supporting the lumbosacral spinal cord, with a close association between the accessory lobes and ligamentous intersections. The relatively dense glycogen body has the potential to apply loads sufficient to pre-stress denticulate ligaments, enabling external accelerations to excite tuned oscillations in the LSO soft tissue, leading to strain-based mechanosensing in the accessory lobe neurons. Considering these anatomical features together, the structure of the LSO is reminiscent of a mass-spring-based accelerometer.

The Balance Hypothesis for the Avian Lumbosacral Organ and an Exploration of Its Morphological Variation

Birds (Aves) exhibit exceptional and diverse locomotor behaviors, including the exquisite ability to balance on two feet. How birds so precisely control their movements may be partly explained by a set of intriguing modifications in their lower spine. These modifications are collectively known as the lumbosacral organ (LSO) and are found in the fused lumbosacral vertebrae called the synsacrum. They include a set of transverse canal-like recesses in the synsacrum that align with lateral lobes of the spinal cord, as well as a dorsal groove in the spinal cord that houses an egg-shaped glycogen body. Based on compelling but primarily observational data, the most recent functional hypotheses for the LSO consider it to be a secondary balance organ, in which the transverse canals are analogous to the semicircular canals of the inner ear. If correct, this hypothesis would reshape our understanding of avian locomotion, yet the LSO has been largely overlooked in the recent literature. Here, we review the current evidence for this hypothesis and then explore a possible relationship between the LSO and balance-intensive locomotor ecologies. Our comparative morphological dataset consists of micro-computed tomography (μ-CT) scans of synsacra from ecologically diverse species. We find that birds that perch tend to have more prominent transverse canals, suggesting that the LSO is useful for balance-intensive behaviors. We then identify the crucial outstanding questions about LSO structure and function. The LSO may be a key innovation that allows independent but coordinated motion of the head and the body, and a full understanding of its function and evolution will require multiple interdisciplinary research efforts.

Modulatory effect of glutamate GluR2 receptor on the caudal neurosecretory Dahlgren cells of the olive flounder, Paralichthys olivaceus

A neuromodulatory role for glutamate has been reported for magnocellular neuroendocrine cells in mammalian hypothalamus. We examined the potential role of glutamate as a local intercellular messenger in the neuroendocrine Dahlgren cell population of the caudal neurosecretory system (CNSS) in the euryhaline flounder Paralichthys olivaceus. In pharmacological experiments in vitro, glutamate (Glu) caused an increase in electrical activity of Dahlgren cells, recruitment of previously silent cells, together with a greater proportion of cells showing phasic (irregular) activity. The glutamate substrate, glutamine (Gln), led to increased firing frequency, cell recruitment and enhanced bursting activity. The glutamate effect was not blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist MK-801, or the GluR1/GluR3 (AMPA) receptor antagonist IEm1795-2HBr, but was blocked by the broad-spectrum α-amino-3-hydroxy- 5- methyl-4-isoxazo-lepropionic acid (AMPA) receptor antagonist ZK200775. Our transcriptome sequencing study revealed three AMPA receptor (GluR1, GluR2 and GluR3) in the olive flounder CNSS. Quantitative RT-PCR revealed that GluR2 receptor mRNA expression was significant increased following dose-dependent superfusion with glutamate in the CNSS. GluR1 and GluR3 receptor mRNA expression were decreased following superfusion with glutamate. L-type Ca²⁺ channel mRNA expression had a significant dose-dependent decrease following superfusion with glutamate, compared to the control. In the salinity challenge experiment, acute transfer from SW to FW, GluR2 receptor mRNA expression was significantly higher than the control at 2 h. These findings suggest that GluR2 is one of the mechanisms which can medicate glutamate action within the CNSS, enhancing electrical activity and hence secretory output.

Activation of muscarinic acetylcholine receptors elevates intracellular Ca(2+) concentrations in accessory lobe neurons of the chick

Accessory lobes are protrusions located at the lateral sides of the spinal cord of chicks and it has been proposed that they play a role as a sensory organ for equilibrium during walking. We have reported that functional neurons exist in the accessory lobe. As there is histological evidence that synaptic terminals of cholinergic nerves exist near the somata of accessory lobe neurons, we examined the effects of acetylcholine on changes in intracellular Ca2+ concentrations ([Ca2+]i), as an index of cellular activities. Acetylcholine (0.1-100 µM) caused a transient rise in the [Ca2+]i. Acetylcholine-evoked [Ca2+]i rises were observed in the absence of extracellular Ca2+, and they were abolished in the presence of cyclopiazonic acid, an inhibitor of Ca2+-ATPase of intracellular Ca2+ stores or atropine, a muscarinic receptor antagonist. mRNAs coding M3 and M5 isoforms of the muscarinic receptors were detected in accessory lobes by the RT-PCR. These results indicate that chick accessory lobe neurons express functional muscarinic acetylcholine receptors, and that acetylcholine stimulates Ca2+ mobilization from intracellular Ca2+ stores, which elevates the [Ca2+]i in the somata of accessory lobe neurons, through activation of these receptors. Cholinergic synaptic transmission to the accessory lobe neurons may regulate some cellular functions through muscarinic receptors.

Voltage-gated Ca2+ channels in accessory lobe neurons of the chick

Birds have ten pairs of protrusions, "accessory lobes", on the lateral sides of the lumbosacral spinal cord. It has been proposed that accessory lobes act as a sensory organ of equilibrium and neurons in accessory lobes transmit sensory information to the motor center. We have reported that cells in chick accessory lobes express functional voltage-gated Na(+) and K(+) channels and generate action potentials. In this study, we examined properties of voltage-gated Ca(2+) channels (VGCCs). The amplitude of voltage-gated Ca(2+) channel currents carried by Ca(2+) and Ba(2+) increased gradually during 10 min rather than showing the usual run-down. The current-voltage relationship of Ba(2+) currents was consistent with that of the high-voltage-activated Ca(2+) channel. The proportion of Ba(2+) currents inhibited by ω-conotoxin GVIA was larger than 80%, indicating that the major subtype is N type. Amplitudes of tail currents of Ca(2+) currents evoked by repetitive pulses at 50 Hz are stable for 1 s. If the major subtype of VGCCs at synaptic terminals is also N type, this property may contribute to the establishment of stable synaptic connections between accessory lobe neurons, which are reported to fire at frequencies higher than 15 Hz, and postsynaptic neurons in the spinal cord.