Dual mechanisms contribute to enhanced voltage dependence of an electric fish potassium channel.

The voltage dependence of different voltage-gated potassium channels, described by the voltage at which half of the channels are open (V1/2), varies over a range of 80 mV and is influenced by factors such as the number of positive gating charges and the identity of the hydrophobic amino acids in the channel's voltage sensor (S4). Here we explore by experimental manipulations and molecular dynamics simulation the contributions of two derived features of an electric fish potassium channel (Kv1.7a) that is among the most voltage-sensitive Shaker family potassium channels known. These are a patch of four contiguous negatively charged glutamates in the S3-S4 extracellular loop and a glutamate in the S3b helix. We find that these negative charges affect V1/2 by separate, complementary mechanisms. In the closed state, the S3-S4 linker negative patch reduces the membrane surface charge biasing the channel to enter the open state while, upon opening, the negative amino acid in the S3b helix faces the second (R2) gating charge of the voltage sensor electrostatically biasing the channel to remain in the open state. This work highlights two evolutionary novelties that illustrate the potential influence of negatively charged amino acids in extracellular loops and adjacent helices to voltage dependence.

Electrostatic Tuning of a Potassium Channel in Electric Fish.

Molecular variation contributes to the evolution of adaptive phenotypes, though it is often difficult to understand precisely how. The adaptively significant electric organ discharge behavior of weakly electric fish is the direct result of biophysical membrane properties set by ion channels. Here, we describe a voltage-gated potassium-channel gene in African electric fishes that is under positive selection and highly expressed in the electric organ. The channel produced by this gene shortens electric organ action potentials by activating quickly and at hyperpolarized membrane potentials. The source of these properties is a derived patch of negatively charged amino acids in an extracellular loop near the voltage sensor. We demonstrate that this negative patch acts by contributing to the global surface charge rather than by local interactions with specific amino acids in the channel's extracellular face. We suggest a more widespread role for this loop in the evolutionary tuning of voltage-dependent channels.

Differential Dopamine Regulation of Ca(2+) Signaling and Its Timing Dependence in the Nucleus Accumbens.

Dopamine action in the nucleus accumbens (NAc) is thought to drive appetitive behavior and Pavlovian reward learning. However, it remains controversial how dopamine achieves these behavioral effects by regulating medium spiny projection neurons (MSNs) of the NAc, especially on a behaviorally relevant timescale. Metabotropic glutamate receptor (mGluR)-induced Ca(2+) signaling dependent on the Ca(2+)- releasing messenger inositol 1,4,5-triphosphate (IP3) plays a critical role in controlling neuronal excitability and synaptic plasticity. Here, we show that transient dopamine application facilitates mGluR/IP3-induced Ca(2+) signals within a time window of ∼2-10 s in a subpopulation of MSNs in the NAc core. Dopamine facilitation of IP3-induced Ca(2+) signaling is mediated by D1 dopamine receptors. In dopamine-insensitive MSNs, activation of A2A adenosine receptors causes enhancement of IP3-evoked Ca(2+) signals, which is reversed by D2 dopamine receptor activation. These results show that dopamine differentially regulates Ca(2+) signaling on the order of seconds in two distinct MSN subpopulations.

"Brain sex differentiation" in teleosts: Emerging concepts with potential biomarkers.

"Brain sex differentiation" in teleosts is a contentious topic of research as most of the earlier reports tend to suggest that gonadal sex differentiation drives brain sex differentiation. However, identification of sex-specific marker genes in the developing brain of teleosts signifies brain-gonadal interaction during early sexual development in lower vertebrates. In this context, the influence of gonadotropin-releasing hormone (GnRH)-gonadotropin (GTH) axis on gonadal sex differentiation, if any requires in depth analysis. Presence of seabream (sb) GnRH immunoreactivity (ir-) in the brain of XY Nile tilapia was found as early as 5days post hatch (dph) followed by qualitative reduction in the preoptic area-hypothalamus region. In contrast, in the XX female brain a steady ir- of sbGnRH was evident from 15dph. Earlier studies using sea bass already implied the importance of hypothalamic gonadotropic axis completion during sex differentiation period. Such biphasic pattern of localization was also seen in pituitary GTHs using heterologous antisera in tilapia. However, more recent analysis in the same species could not detect any sexually dimorphic pattern using homologous antisera for pituitary GTHs. Detailed studies on the development of hypothalamo-hypophyseal-gonadal axis in teleosts focusing on hypothalamic monoamines (MA) and MA-related enzymes demonstrated sex-specific differential expression of tryptophan hydroxylase (Tph) in the early stages of developing male and female brains of tilapia and catfish. The changes in Tph expression was in agreement with the levels of serotonin (5-HT) and 5-hydroxytryptophan in the preoptic area-hypothalamus. Considering the stimulatory influence of 5-HT on GnRH and GTH release, it is possible to propose a network association between these correlates during early development, which may bring about brain sex dimorphism in males. A recent study from our laboratory during female brain sex development demonstrated high expression of tyrosine hydroxylase in correlation with catecholamine levels, brain aromatase and its related transcription factors such as fushi tarazu factor 1, Ftz-f1 and fork head box protein L2, foxl2. Taken together, gender differences in the levels of various transcripts provide new perspectives on brain sex differentiation in lower vertebrates. Sexually dimorphic or differentially expressing genes may play an essential role at the level of brain in response to gonadal differentiation, which might consequentially or causatively respond to gonadal sex.

Spatiotemporal integration of developmental cues in neural development.

Nervous system development relies on the generation of neurons, their differentiation and establishment of synaptic connections. These events exhibit remarkable plasticity and are regulated by many developmental cues. Here, we review the mechanisms of three classes of these cues: morphogenetic proteins, electrical activity, and the environment. We focus on second messenger dynamics and their role as integrators of the action of diverse cues, enabling plasticity in the process of neural development.

Dynamic regulation of neurotransmitter specification: relevance to nervous system homeostasis.

During nervous system development the neurotransmitter identity changes and coexpression of several neurotransmitters is a rather generalized feature of developing neurons. In the mature nervous system, different physiological and pathological circumstances recreate this phenomenon. The rules of neurotransmitter respecification are multiple. Among them, the goal of assuring balanced excitability appears as an important driving force for the modifications in neurotransmitter phenotype expression. The functional consequences of these dynamic revisions in neurotransmitter identity span a varied range, from fine-tuning the developing neural circuit to modifications in addictive and locomotor behaviors. Current challenges include determining the mechanisms underlying neurotransmitter phenotype respecification and how they intersect with genetic programs of neuronal specialization. This article is part of the Special Issue entitled 'Homeostatic Synaptic Plasticity'.

Inositol 1,4,5-triphosphate drives glutamatergic and cholinergic inhibition selectively in spiny projection neurons in the striatum.

The striatum is critically involved in the selection of appropriate actions in a constantly changing environment. The spiking activity of striatal spiny projection neurons (SPNs), driven by extrinsic glutamatergic inputs, is shaped by local GABAergic and cholinergic networks. For example, it is well established that different types of GABAergic interneurons, activated by extrinsic glutamatergic and local cholinergic inputs, mediate powerful feedforward inhibition of SPN activity. In this study, using mouse striatal slices, we show that glutamatergic and cholinergic inputs exert direct inhibitory regulation of SPN activity via activation of metabotropic glutamate receptors (mGluRs) and muscarinic acetylcholine receptors. While pressure ejection of the group I mGluR (mGluR1/5) agonist DHPG [(S)-3,5-dihydroxyphenylglycine] equally engages both mGluR1 and mGluR5 subtypes, the mGluR-dependent component of IPSCs elicited by intrastriatal electrical stimulation is almost exclusively mediated by the mGluR1 subtype. Ca(2+) release from intracellular stores specifically through inositol 1,4,5-triphospahte receptors (IP(3)Rs) and not ryanodine receptors (RyRs) mediates this form of inhibition by gating two types of Ca(2+)-activated K(+) channels (i.e., small-conductance SK channels and large-conductance BK channels). Conversely, spike-evoked Ca(2+) influx triggers Ca(2+) release solely through RyRs to generate SK-dependent slow afterhyperpolarizations, demonstrating functional segregation of IP(3)Rs and RyRs. Finally, IP(3)-induced Ca(2+) release is uniquely observed in SPNs and not in different types of interneurons in the striatum. These results demonstrate that IP(3)-mediated activation of SK and BK channels provides a robust mechanism for glutamatergic and cholinergic inputs to selectively suppress striatal output neuron activity.

Interplay between electrical activity and bone morphogenetic protein signaling regulates spinal neuron differentiation.

A gradient of bone morphogenetic proteins (BMPs) along the dorsoventral axis of the spinal cord is necessary for the specification of dorsal neurons. Concurrently, a gradient of calcium-mediated electrical activity is present in the developing spinal cord but in an opposing ventrodorsal direction. Whether BMPs and electrical activity interact in embryonic spinal neurons remains unknown. We show that BMP decreases electrical activity by enhancing p38 MAPK-mediated negative modulation of voltage-gated sodium channels. In turn, electrical activity affects the phosphorylation status and nuclear level of activated Smads, the canonical components of BMP signaling. This interaction between calcium spike activity and BMP signaling regulates the specification of the dorsal commissural spinal neuron phenotype. The present study identifies an unexpected interplay between BMPs and electrical activity that is critical for decoding the morphogen gradient during spinal neuron differentiation.

Electrical activity as a developmental regulator in the formation of spinal cord circuits.

Spinal cord development is a complex process involving generation of the appropriate number of cells, acquisition of distinctive phenotypes and establishment of functional connections that enable execution of critical functions such as sensation and locomotion. Here we review the basic cellular events occurring during spinal cord development, highlighting studies that demonstrate the roles of electrical activity in this process. We conclude that the participation of different forms of electrical activity is evident from the beginning of spinal cord development and intermingles with other developmental cues and programs to implement dynamic and integrated control of spinal cord function.