Cellular excitability and the regulation of functional neuronal identity: from gene expression to neuromodulation.
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Animals↗
Biomedical subjects
David J Schulz
Publications and source records attributed to David J Schulz.
Variable channel expression in identified single and electrically coupled neurons in different animals.
It is often assumed that all neurons of the same cell type have identical intrinsic properties, both within an animal and between animals. We exploited the large size and small number of unambiguously identifiable neurons in the crab stomatogastric ganglion to test this assumption at the level of channel mRNA expression and membrane currents (measured in voltage-clamp experiments). In lateral pyloric (LP) neurons, we saw strong correlations between measured current and the abundance of Shal and BK-KCa mRNAs (encoding the Shal-family voltage-gated potassium channel and large-conductance calcium-activated potassium channel, respectively). We also saw two- to fourfold interanimal variability for three potassium currents and their mRNA expression. Measurements of channel expression in the two electrically coupled pyloric dilator (PD) neurons showed significant interanimal variability, but copy numbers for IH (encoding the hyperpolarization-activated, inward-current channel) and Shal mRNA in the two PD neurons from the same crab were similar, suggesting that the regulation of some currents may be shared in electrically coupled neurons.
Action Potentials↗
Division of labor in the honey bee (Apis mellifera): the role of tyramine beta-hydroxylase.
The biogenic amine octopamine (OA) is involved in the regulation of honey bee behavioral development; brain levels are higher in foragers than bees working in the hive, especially in the antennal lobes, and treatment causes precocious foraging. We measured brain mRNA and protein activity of tyramine beta-hydroxylase (T betah), an enzyme vital for OA synthesis, in order to begin testing the hypothesis that this enzyme is responsible for the rising levels of OA during honey bee behavioral development. Brain OA levels were greater in forager bees than in bees engaged in brood care, as in previous studies, but T betah activity was not correlated with bee behavior. T betah mRNA levels, however, did closely track OA levels during behavioral development, and T betah mRNA was localized to previously identified octopaminergic neurons in the bee brain. Our results show that the transcription of this neurotransmitter synthetic enzyme is associated with regulation of social behavior in honey bees, but other factors may be involved.
Amino Acid Sequence↗
Plasticity and stability in neuronal output via changes in intrinsic excitability: it's what's inside that counts.
The nervous system faces an extremely difficult task. It must be flexible, both during development and in adult life, so that it can respond to a variety of environmental demands and produce adaptive behavior. At the same time the nervous system must be stable, so that the neural circuits that produce behavior function throughout the lifetime of the animal and that changes produced by learning endure. We are only beginning to understand how neural networks strike a balance between altering individual neurons in the name of plasticity, while maintaining long-term stability in neural system function. The balance of this plasticity and stability in neural networks undoubtedly plays a critical role in the normal functioning of the nervous system. While mechanisms of synaptic plasticity have garnered extensive study over the past three decades, it is only recently that more attention has been turned to plasticity of intrinsic excitability as a key player in neural network function. This review will focus on this emerging area of research that undoubtedly will contribute a great deal to our understanding of the functionality of the nervous system.
Animals↗
Invertebrate central pattern generation moves along.
Central pattern generators (CPGs) are circuits that generate organized and repetitive motor patterns, such as those underlying feeding, locomotion and respiration. We summarize recent work on invertebrate CPGs which has provided new insights into how rhythmic motor patterns are produced and how they are controlled by higher-order command and modulatory interneurons.
Animals↗
Octopamine modulates the axons of modulatory projection neurons.
Octopamine increases the cycle frequency of the pyloric rhythm in the crab Cancer borealis by acting at multiple sites within the stomatogastric nervous system. The junction between the stomatogastric nerve (stn) and the superior esophageal nerve (son) shows synaptic structures. When applied only to the stn-son junction, octopamine induced action potentials in the axons of the modulatory commissural neuron 5 (MCN5) that project from the commissural ganglia to the stomatogastric ganglion (STG). The activation of the MCN5 neurons was correlated with an increase in the pyloric rhythm frequency. Additionally, octopamine had direct effects on the STG, including the activation of the pyloric dilator and pyloric neurons, an increase in the pyloric frequency, and a change in the phase relationships of the pyloric neurons. Thus, the same modulator can influence the pyloric rhythm by acting at multiple sites, including the axons of identified modulatory neurons that project to the STG. These data demonstrate that axonal propagation may be influenced locally by neuromodulators acting on axonal receptors, therefore altering the conduction of information from different command and integrating centers.
Action Potentials↗
Biogenic amines in the antennal lobes and the initiation and maintenance of foraging behavior in honey bees.
Previous findings showed that high levels of octopamine and serotonin in the antennal lobes of adult worker honey bees are associated with foraging behavior, and octopamine treatment induces precocious foraging. To better characterize the relationship between amines and foraging behavior in honey bees, we performed a detailed correlative analysis of amine levels in the antennal lobes as a function of various aspects of foraging behavior. Flight activity was measured under controlled conditions in a large outdoor flight cage. Levels of octopamine in the antennal lobes were found to be elevated immediately subsequent to the onset of foraging, but they did not change as a consequence of preforaging orientation flight activity, diurnal pauses in foraging, or different amounts of foraging experience, suggesting that octopamine helps to trigger and maintain the foraging behavioral state. In contrast, levels of serotonin and dopamine did not show changes that would implicate them as either causal agents of foraging, or as neurochemical systems affected by the act of foraging. Serotonin treatment had no effect on the likelihood of foraging. These results provide further support for the hypothesis that an increase in octopamine levels in the antennal lobes plays a causal role in the initiation and maintenance of the behavioral state of foraging, and thus is involved in the regulation of division of labor in honey bees.
5-Hydroxytryptophan↗
Juvenile hormone and octopamine in the regulation of division of labor in honey bee colonies.
Forager honey bees have high circulating levels of juvenile hormone (JH) and high brain levels of octopamine, especially in the antennal lobes, and treatment with either of these compounds induces foraging. Experiments were performed to determine whether octopamine acts more proximally than JH to affect the initiation of foraging behavior. Bees treated with octopamine became foragers more rapidly than bees treated with the JH analog methoprene. Bees treated with methoprene showed an increase in antennal lobe levels of octopamine, especially after 12 days. Bees with no circulating JH (corpora allata glands removed) treated with octopamine became foragers in similar numbers to bees with intact corpora allata. These results suggest that JH affects the initiation of foraging at least in part by increasing brain levels of octopamine, but octopamine can act independently of JH. Effects of JH that are not related to octopamine also are possible, as bees treated with both octopamine and methoprene were more likely to become foragers than bees treated with only octopamine or methoprene.
Animals↗
A role for octopamine in honey bee division of labor.
Efficient division of labor is one of the main reasons for the success of the social insects. In honey bees the division of labor is principally achieved by workers changing tasks as they age. Typically, young adult bees perform a series of tasks within the colony before ultimately making the transition to foraging outside the hive for resources. This lifelong behavioral development is a well-characterized example of naturally occurring behavioral plasticity, but its neural bases are not well understood. Two techniques were used to assess the role of biogenic amines in the transition from in-hive work to foraging, which is the most dramatic and obvious transition in honey bee behavioral development. First, associations between amines and tasks were determined by measuring the levels of amines in dissected regions of individual bee brains using HPLC analysis. Second, colonies were orally treated with biogenic amines and effects on the onset of foraging were observed. Octopamine concentration in the antennal lobes of the bee brain was most reliably associated with task: high in foragers and low in nurses regardless of age. In contrast, octopamine in the mushroom bodies, a neighboring neuropil, was associated with age and not behavior, indicating independent modulation of octopamine in these two brain regions. Treating colonies with octopamine resulted in an earlier onset of foraging in young bees. In addition, octopamine levels were not elevated by non-foraging flight, but were already high on return from the first successful foraging trip and subsequently remained high, showing no further change with foraging experience. This observation suggests that octopamine becomes elevated in the antennal lobes in anticipation of foraging and is involved in the release and maintenance of the foraging state. Foraging itself, however, does not modulate octopamine levels. Behaviorally related changes in octopamine are modulated by juvenile hormone, which has also been implicated in the control of honey bee division of labor. Treatment with the juvenile hormone analog methoprene elevated octopamine and octopamine treatment 'rescued' the delay in behavioral development caused by experimentally depleting juvenile hormone in bees. Although the pathways linking juvenile hormone and octopamine are presently unknown, it is clear that octopamine acts 'downstream' of juvenile hormone to influence behavior and that juvenile hormone modulates brain octopamine levels. A working hypothesis is that octopamine acts as an activator of foraging by modulating responsiveness to foraging-related stimuli. This is supported by the finding that octopamine treatment increased the response of bees to brood pheromone, a stimulator of foraging activity. Establishing a role for octopamine in honey bee behavioral development is a first step in understanding the neural bases of this example of naturally occurring, socially mediated, behavioral plasticity. The next level of analysis will be to determine precisely where and how octopamine acts in the nervous system to coordinate this complex social behavior.
Animals↗