Endogenous biotin staining in a subset of spinal neuronal cell bodies: a potential confounding factor for neuroanatomical studies

Latent modulation: a basis for non-disruptive promotion of two incompatible behaviors by a single network state

Behavioral states often preferentially enhance specific classes of behavior and suppress incompatible behaviors. In the nervous system, this may involve upregulation of the efficacy of neural modules that mediate responses to one stimulus and suppression of modules that generate antagonistic or incompatible responses to another stimulus. In Aplysia, prestimulation of egestive inputs [esophageal nerve (EN)] facilitates subsequent EN-elicited egestive responses and weakens ingestive responses to ingestive inputs [Cerebral-Buccal Interneuron (CBI-2)]. However, a single state can also promote incompatible behaviors in response to different stimuli. This is the case in Aplysia, where prestimulation of CBI-2 inputs not only enhances subsequent CBI-2-elicited ingestive responses, but also strengthens EN-elicited egestive responses. We used the modularly organized feeding network of Aplysia to characterize the organizational principles that allow a single network state to promote two opposing behaviors, ingestion and egestion, without the two interfering with each other. We found that the CBI-2 prestimulation-induced state upregulates the excitability of neuron B65 which, as a member of the egestive module, increases the strength of egestive responses. Furthermore, we found that this upregulation is likely mediated by the actions of the neuropeptides FCAP (Feeding Circuit Activating Peptide) and CP2 (Cerebral Peptide 2). This increased excitability is mediated by a form of modulation that we refer to as "latent modulation" because it is established during stimulation of CBI-2, which does not activate B65. However, when B65 is recruited into EN-elicited egestive responses, the effects of the latent modulation are expressed as a higher B65 firing rate and a resultant strengthening of the egestive response.

Physiology and morphology indicate that individual spinal interneurons contribute to diverse limb movements

Overlapping neuronal networks have been shown to generate a variety of behaviors or motor patterns in invertebrates, but the evidence for this is more circumstantial in vertebrates. The turtle spinal cord can produce multiple forms of hindlimb scratching movements as well as hindlimb withdrawal, but it is still uncertain whether individual spinal cord interneurons contribute to the motor output for more than one type of limb motor pattern. In this study, individual spinal cord interneurons were recorded intracellularly in vivo in spinal immobilized turtles, and, after characterization, were filled with Neurobiotin. Interneurons that were rhythmically activated during multiple forms of ipsilateral fictive hindlimb scratching often had axon-terminal arborizations in the ventral horn of the spinal cord hindlimb enlargement. This provides some of the strongest evidence to date that interneurons involved in multiple forms of scratching contribute directly to hindlimb motor output. Moreover, most of these interneurons were also active during contralateral fictive scratching and during ipsilateral fictive hindlimb withdrawal, suggesting that they contribute to motor output for these additional behaviors as well. Such interneurons may provide the cellular basis for the contralateral contributions to ipsilateral scratching that have been demonstrated previously. Taken together, these findings suggest that diverse vertebrate limb movements are produced by spinal cord interneuronal networks that include some shared components.

Propriospinal projections to the ventral horn of the rostral and caudal hindlimb enlargement in turtles

In limbed vertebrates, the capacity to generate rhythmic motor patterns for locomotion and scratching is distributed over spinal cord segments of the limb enlargement (e.g., lumbosacral segments), but within this region, rostral segments are more rhythmogenic than caudal segments. The underlying reasons for this rostrocaudal asymmetry are not clear. One possibility is that rostral and caudal segments receive distinct sets of propriospinal projections. To test this hypothesis, I injected horseradish peroxidase (HRP) into the ventral horn unilaterally in a rostral or caudal segment of the turtle hindlimb enlargement. I quantitatively assessed the distributions of retrogradely labeled neurons in six hindlimb enlargement and pre-enlargement segments. The cross-sectional distribution did not depend on which segment was injected. Ipsilateral labeling occurred predominantly in the deep dorsal horn, the lateral part of the intermediate zone, and the dorsal two-thirds of the ventral horn, while contralateral labeling occurred mainly in the medial part of the ventral horn and the lateral part of the intermediate zone. This cross-sectional distribution is similar to what has been seen in mammals. The rostrocaudal distribution of labeled cells, however, depended on which segment was injected. Rostral injections gave rise to rostrally skewed distributions, dominated by descending propriospinal neurons. Caudal injections gave rise to caudally skewed distributions, dominated by ascending propriospinal neurons. Thus, rostral segments of the hindlimb enlargement received more propriospinal inputs from immediately rostral than immediately caudal segments, while the reverse was true for inputs to caudal segments. This anatomical asymmetry may contribute to known functional asymmetries within the enlargement.

Biotin is endogenously expressed in select regions of the rat central nervous system

The vitamin biotin is an endogenous molecule that acts as an important cofactor for several carboxylases in the citric acid cycle. Disorders of biotin metabolism produce neurological symptoms that range from ataxia to sensory loss, suggesting the presence of biotin in specific functional systems of the CNS. Although biotin has been described in some cells of nonmammalian nervous systems, the distribution of biotin in mammalian CNS is virtually unknown. We report the presence of biotin in select regions of rat CNS, as revealed with a monoclonal antibody directed against biotin and with avidin- and streptavidin-conjugated labels. Detectable levels of biotin were primarily found caudal to the diencephalon, with greatest expression in the cerebellar motor system and several brainstem auditory nuclei. Biotin was found as a somatic label in cerebellar Purkinje cells, in cell bodies and proximal dendrites of cerebellar deep nuclear neurons, and in red nuclear neurons. Biotin was detected in cells of the spiral ganglion, somata and proximal dendrites of cells in the cochlear nuclei, superior olivary nuclei, medial nucleus of the trapezoid body, and nucleus of the lateral lemniscus. Biotin was further found in pontine nuclei and fiber tracts, the substantia nigra pars reticulata, lateral mammillary nucleus, and a small number of hippocampal interneurons. Biotin was detected in glial cells of major tract systems throughout the brain but was most prominent in tracts of the hindbrain. Biotin is thus expressed in select regions of rat CNS with a distribution that correlates to the known clinical sequelae associated with biotin deficiencies.