Octopamine: presence in single neurons of Aplysia suggests neurotransmitter function

Octopamine has been identified and measured in individual neurons from Aplysia californica. Neither dopamine nor norepinephrine was detected in these cells. Thus, in Aplysia there may be separate populations of catecholaminergic and monophenolaminergic cells. Octopamine may have functions of its own in the central nervous system of mollusks.     

Structural asymmetry of AcrB trimer suggests a peristaltic pump mechanism

The AcrA/AcrB/TolC complex spans the inner and outer membranes of Escherichia coli and serves as its major drug-resistance pump. Driven by the proton motive force, it mediates the efflux of bile salts, detergents, organic solvents, and many structurally unrelated antibiotics. Here, we report a crystallographic structure of trimeric AcrB determined at 2.9 and 3.0 angstrom resolution in space groups that allow asymmetry of the monomers. This structure reveals three different monomer conformations representing consecutive states in a transport cycle. The structural data imply an alternating access mechanism and a novel peristaltic mode of drug transport by this type of transporter.     

Tryptophan 7-halogenase (PrnA) structure suggests a mechanism for regioselective chlorination

Chlorinated natural products include vancomycin and cryptophycin A. Their biosynthesis involves regioselective chlorination by flavin-dependent halogenases. We report the structural characterization of tryptophan 7-halogenase (PrnA), which regioselectively chlorinates tryptophan. Tryptophan and flavin adenine dinucleotide (FAD) are separated by a 10 angstrom-long tunnel and bound by distinct enzyme modules. The FAD module is conserved in halogenases and is related to flavin-dependent monooxygenases. On the basis of biochemical studies, crystal structures, and by analogy with monooxygenases, we predict that FADH2 reacts with O2 to make peroxyflavin, which is decomposed by Cl-. The resulting HOCl is guided through the tunnel to tryptophan, where it is activated to participate in electrophilic aromatic substitution.     

Identification of a peptide specific for Aplysia sensory neurons by PCR-based differential screening.

In order to identify genes specific for the sensory neurons of Aplysia, a miniaturized differential screening method based on the polymerase chain reaction and applicable to small amounts of tissue was used. One messenger RNA was isolated that is expressed in every mechanoreceptor sensory cluster of the Aplysia central nervous system. This messenger RNA encodes a peptide that seems to function as an inhibitory cotransmitter. The peptide selectively inhibits certain postsynaptic cells but not others and thereby allows the sensory neurons to achieve target-specific synaptic actions.     

Molecular cloning and expression of neuroleukin, a neurotrophic factor for spinal and sensory neurons

A novel 56,000-dalton growth factor found in mouse salivary gland was purified, molecularly cloned, and expressed in monkey COS cells. The protein is a neurotrophic factor and also, surprisingly, a lymphokine product of lectin-stimulated T cells. The factor was therefore named neuroleukin. Neuroleukin promotes the survival in culture of a subpopulation of embryonic spinal neurons that probably includes skeletal motor neurons. Neuroleukin also supports the survival of cultured sensory neurons that are insensitive to nerve growth factor, but has no effect on sympathetic or parasympathetic neurons. The amino acid sequence of neuroleukin is partly homologous to a highly conserved region of the external envelope protein of HTLV-III/LAV, the retrovirus that causes acquired immune deficiency syndrome.     

Long-term synaptic changes produced by a cellular analog of classical conditioning in Aplysia

A change in synaptic strength arising from the activation of two neuronal pathways at approximately the same time is a form of associative plasticity and may underlie classical or Pavlovian conditioning. A cellular analog of a classical conditioning protocol produces short-term associative plasticity at the connections between sensory and motor neurons in Aplysia. A similar training protocol produced long-term (24-hour) enhancement of excitatory postsynaptic potentials (EPSPs). EPSPs produced by sensory neurons in which activity was paired with a reinforcing stimulus were significantly larger than unpaired controls 24 hours after training. Thus, associative plasticity at the sensory to motor neuron connection can occur in a long-term form in addition to the short-term form. In this system, it should be possible to analyze the molecular mechanisms underlying long-term associative plasticity and classical conditioning.     

Recurrent excitation of secondary olfactory neurons: a possible mechanism for signal amplification

Secondary neurons of the olfactory bulb can be excited monosynaptically after activation of neighboring secondary neurons by antidromic and orthodromic volleys. Recurrent collaterals of secondary neurons are proposed to synapse with other secondary neurons, thus forming a direct recurrent excitatory pathway. Such a positive feedback system could strengthen the original input signal.     

Activity-dependent enhancement of presynaptic inhibition in Aplysia sensory neurons

Tail shock produces transient presynaptic inhibition and longer lasting presynaptic facilitation of the siphon sensory neurons in Aplysia. The facilitation undergoes activity-dependent enhancement that is thought to contribute to classical conditioning of the gill- and siphon-withdrawal reflex. Inhibition of the sensory neurons has now also been shown to undergo activity-dependent enhancement when action potential activity in the sensory neurons is paired with inhibitory transmitter. This effect appears to involve an amplification of the same cellular mechanisms that are involved in normal presynaptic inhibition. These results suggest that activity-dependent enhancement may be a general type of associative cellular mechanism.     

Associative induction of posttetanic and long-term potentiation in CA1 neurons of rat hippocampus

Electrical stimulation of fibers in the stratum radiatum causes an excitatory postsynaptic potential in CA1 neurons of the hippocampus. Other excitatory inputs to or direct depolarization of these CA1 neurons during stimulation of the stratum radiatum caused a subsequent increase in the excitatory postsynaptic potential. This enhancement was characterized as a brief potentiation (2 to 3 minutes, similar to posttetanic potentiation) and a long-term potentiation (presumed to be involved in learning and memory). These potentiations are probably induced by an interaction of the postsynaptic cell or other presynaptic terminals with the test presynaptic terminals.     

The contribution of single synapses to sensory representation in vivo

The extent to which synaptic activity can signal a sensory stimulus limits the information available to a neuron. We determined the contribution of individual synapses to sensory representation by recording excitatory postsynaptic currents (EPSCs) in cerebellar granule cells during a time-varying, quantifiable vestibular stimulus. Vestibular-sensitive synapses faithfully reported direction and velocity, rather than position or acceleration of whole-body motion, via bidirectional modulation of EPSC frequency. The lack of short-term synaptic dynamics ensured a highly linear relationship between velocity and charge transfer, and as few as 100 synapses provided resolution approaching psychophysical limits. This indicates that highly accurate stimulus representation can be achieved by small networks and even within single neurons.     

Membrane changes in a single photoreceptor cause associative learning in Hermissenda

Single type B photoreceptors in intact, restrained Hermissenda were impaled with a microelectrode and exposed to either paired or unpaired presentations of light and depolarizing current to simulate natural stimulus effects during conditioning with light and rotation. Paired, but not unpaired, stimulus presentations produced cumulative depolarization and increased input resistance in type B cells. These membrane changes are similar to those observed after pairings of light and rotation are administered to either intact animals or isolated nervous systems or when light is paired with electrical stimulation of the vestibular system in isolated nervous systems. One and two days after treatment, pairing- and light-specific suppression of phototactic behavior was observed in recovered animals. These findings indicate that the membrane changes of type B cells produced by pairing light with current injections cause acquisition of the learned behavior.     

Ionic mechanism of cholinergic inhibition in molluscan neurons

Acetylcholine, the inhibitory transmitter to the so-called H-neurons of molluscs, produces its effect by increasing the permeability of the subsynaptic membrane to chloride ions. The change in permeability gives rise to a net influx of this anion, which hyperpolarizes the neuron. The presence of an outward pump of chloride ions is postulated to account for the required electrochemical gradient. The participation of potassium ions in this inhibitory phenomenon was not detected.     

生态安全的灰关联评价方法探讨

将灰关联方法应用于生态安全评价,分别建立了生态安全评价的参考序列、比较序列和评判等级.以安徽省17个地级市为例,通过计算确定其生态安全等级,直观地得出各区域的生态安全状况.     

Release of coordinated behavior in crayfish by single central neurons

By stimulating and recording from the same interneuron at two separate points, we have shown that coordinated output to the postural abdominal muscles of crayfish can be produced by electrical stimulation of a single cell. Several central neurons can individually initiate one type of movement(for example, flexion),each producing a unique abdominal geometry.     

Mirror-image confusion in single neurons of the macaque inferotemporal cortex

Humans and animals confuse lateral mirror images, such as the letters "b" and "d," more often than vertical mirror images, such as the letters "b" and "p." Experiments were performed to find a neural correlate of this phenomenon. Visually responsive pattern-selective neurons in the inferotemporal cortex of macaque monkeys responded more similarly to members of a lateral mirror-image pair than to members of a vertical mirror-image pair. The phenomenon developed within 20 milliseconds of the onset of the visual response and persisted to its end. It occurred during presentation of stimuli both at the fovea and in the periphery.     

Brain stem neurons in modified pathways for motor learning in the primate vestibulo-ocular reflex

The vestibulo-ocular reflex (VOR) stabilizes retinal images by generating smooth eye movements that are equal in amplitude and opposite in direction to head turns. Whenever image motion occurs persistently during head turns, the VOR undergoes motor learning; as a result image stability is gradually restored. A group of brain stem neurons that are in the modified pathways has now been described. The neurons express changes in firing in association with motor learning in the VOR and receive monosynaptic inhibition from the flocculus of the cerebellum. The changes in firing have an appropriate magnitude and are expressed at the correct latency to account for the altered VOR. The response properties of the neurons point to their brain stem vestibular inputs for further investigation of the site of motor learning.