Comparative analysis of endogenous plant pararetroviruses in cultivated and wild Dahlia spp.

Two distinct caulimoviruses, Dahlia mosaic virus (DMV) and Dahlia common mosaic virus, and an endogenous plant pararetroviral sequence (DvEPRS) were reported in Dahlia spp. DvEPRS, previously referred to as DMV-D10, was originally identified in the US from the cultivated Dahlia variabilis, and has also been found in New Zealand, Lithuania and Egypt, as well as in wild dahlia species growing in their natural habitats in Mexico. Sequence analysis of three new EPRSs from cultivated dahlias from Lithuania [D10-LT; 7,159 nucleotide level (nt)], New Zealand (D10-NZ, 7,156 nt), and the wild species, Dahlia rupicola, from Mexico (D10-DR, 7,133 nt) is reported in this study. The three EPRSs have the structure and organization typical of a caulimovirus species and showed identities among various open reading frames (ORFs) ranging between 71 and 97 % at the nt when compared to those or the known DvEPRS from the US. Examination of a dataset of seven full-length EPRSs obtained to date from cultivated and wild Dahlia spp. provided clues into genetic diversity of these EPRSs from diverse sources of dahlia. Phylogenetic analyses, mutation frequencies, potential recombination events, selection, and fitness were evaluated as evolutionary evidences for genetic variation. Assessment of all ORFs using phylogenomic and population genetics approaches suggests a wide genetic diversity of EPRSs occurring in dahlias. Phylogenetic analyses show that the EPRSs from various sources form one clade indicating a lack of clustering by geographical origin. Grouping of various EPRSs into two host taxa (cultivated vs. wild) shows little divergence with respect to their origin. Population genetic parameters demonstrate negative selection for all ORFs, with the reverse transcriptase region more variable than other ORFs. Recombination events were found which provide evolutionary evidence for genetic diversity among dahlia-associated EPRSs. This study contributes to an increased understanding of molecular population genetics and evolutionary pathways of these reverse transcribing viral elements.

Genomic characterization of pararetroviral sequences in wild Dahlia spp. in natural habitats.

The genome structure and organization of endogenous caulimovirus sequences from dahlia (Dahlia spp), dahlia mosaic virus (DMV)-D10 from three wild species, D. coccinea (D10-DC), D. sherffii (D10-DS) and D. tenuicaulis (D10-DT), were determined and compared to those from cultivated species of dahlia, D. variabilis (DvEPRS). The complete ca. 7-kb dsDNA genomes of D10-DC, D10-DS, and D10-DT had a structure and organization typical of a caulimovirus and shared 89.3 to 96.6% amino acid sequence identity in various open reading frames (ORF) when compared to DvEPRS. The absence of the aphid transmission factor and the truncated coat protein fused with the reverse transcriptase ORF were common among these DMV-D10 isolates from wild Dahlia species.

Cellular correlates of olfactory learning in the rat piriform cortex.

This review describes research that combines cellular physiology with behavioral neuroscience, to study the cellular mechanisms underlying learning and memory in the mammalian brain. Rats were trained with an olfactory conditioning paradigm, in which they had to memorize odors in order to be rewarded with drinking water. Such training results in rule learning, which enables enhanced acquisition of odor memory. Training results in the following learning-related physiological modifications in intrinsic and synaptic properties in olfactory (piriform) cortex pyramidal neurons: 1. increased neuronal excitability, indicated by reduced afterhyperpolarization, and 2. increased synaptic transmission, indicated by reduced paired-pulse facilitation. These modifications are correlated to enhanced learning capability rather than to storage of memory for specific odors. In addition, using a different paradigm of odor-training, it is shown that NMDA and betra-adrenergic receptors are involved at different stages of long-term memory consolidation.

Long-lasting cholinergic modulation underlies rule learning in rats.

We studied the role of acetylcholine (ACh) in creating learning-related long-lasting modifications in the rat cortex. Rats were trained to discriminate positive and negative cues in pairs of odors, until they demonstrated rule learning and entered a mode of high capability for learning of additional odors. We have previously reported that pyramidal neurons in olfactory (piriform) cortex from trained rats had reduced spike afterhyperpolarization (AHP) for 3 d after rule learning. In the present study we examined the mechanism underlying this long-lasting modification. The cholinergic agonist carbachol reduced both slow AHP and firing adaptation in neurons from pseudotrained rats, but had no effect on neurons from trained rats, suggesting pre-existing cholinergic effect. Intracellular application of the calcium chelator BAPTA abolished the difference in slow AHP and in adaptation between groups, suggesting that the difference resulted from reduction in the ACh-sensitive, Ca(2+)-dependent potassium current, I(AHP). At the behavioral level, application of the muscarinic blocker scopolamine before each training session delayed rule learning but had no effect on further acquisition of odor memory. We suggest that intense ACh activity during rule learning enhances neuronal excitability in the piriform cortex by reducing I(AHP) and that the effect outlasts the stage of rule learning, so that ACh activity is not crucial for further odor learning.

Reduced synaptic facilitation between pyramidal neurons in the piriform cortex after odor learning.

Learning-related cellular modifications were studied in the rat piriform cortex after operand conditioning. Rats were trained to discriminate positive cues in pairs of odors. In one experimental paradigm, rats were trained to memorize 35-50 pairs of odors ("extensive training"). In another paradigm, training was continued only until rats acquired the rule of the task, usually after learning the first two pairs of odors ("short training"). "Pseudotrained" and "naive" rats served as controls. We have previously shown that "rule learning" of this task was accompanied by reduced spike afterhyperpolarization in pyramidal neurons in brain slices of the piriform cortex. In the present study, synaptic inputs to the same cells were examined. Pairs of electrical stimuli applied to the intrinsic fibers that interconnect layer II pyramidal neurons revealed significant reduction in paired-pulse facilitation (PPF) in this pathway even after short training. PPF in shortly trained rats was reduced to the same extent as in extensively trained rats. PPF reduction did not result from modification of membrane properties in the postsynaptic cells, change in postsynaptic inhibition, or impairment of the facilitation mechanism. Extracellular field potential recordings showed enhanced synaptic transmission in these synapses. The reduction in PPF became apparent only 3 d after task acquisition and returned to control value 5 d later. PPF evoked by stimulating the afferent fibers to the same neurons was increased 1 d after training for 2 d. We suggest that the transient enhancement in connectivity in the intrinsic pathway is related to the enhanced learning capability and not to memory for specific odors, which lasts for weeks.

Reduced after-hyperpolarization in rat piriform cortex pyramidal neurons is associated with increased learning capability during operant conditioning.

Learning-related cellular modifications were studied in the rat piriform cortex. Water-deprived rats were divided to three groups: 'trained' rats were trained in a four-arm maze to discriminate positive cues in pairs of odours, 'control' rats were 'pseudo-trained' by random water rewarding, and 'naive' rats were water-deprived only. In one experimental paradigm, the trained group was exposed to extensive training with rats learning to discriminate between 35 and 50 pairs of odours. Piriform cortex pyramidal neurons from 'trained', 'control' and 'naive' rats did not differ in their passive membrane properties and single spike characteristics. However, the after-hyperpolarizations (AHPs) that follow six-spike trains were reduced after 'extensive training' by 43% and 36% compared with 'control' and 'naive', respectively. This effect was not observed in the piriform cortex of another group of rats, in which hyperexcitability was induced by chemical kindling. In another experimental paradigm rats were trained only until they demonstrated 'rule learning', usually after discriminating between one and two pairs of odours ('mild training'). In this experiment, a smaller, yet significant, reduction (20%) in AHPs was observed. AHP reduction was apparent in most of the sampled neurons. AHP remained reduced up to 3 days after the last training session. 5 days or more after the last training session, AHP amplitude recovered to pre-training value and did not differ between 'trained' rats and the others. Accordingly, training suspension for 5 days or more resulted in slower learning of novel odours. We suggest that increased neuronal excitability, manifested as reduced AHP, is related to the ability of the cortical network to enter a 'learning mode' which creates favourable conditions for enhanced learning capability.

Ca2+ accumulations in dendrites of neocortical pyramidal neurons: an apical band and evidence for two functional compartments.

Apical dendrites constitute a prominent feature of the microcircuitry in the neocortex, yet their function is poorly understood. Using fura-2 imaging of layer 5 pyramidal neurons from slices of rat somatosensory cortex, we have investigated the Ca2+ influx into dendrites under intracellular, antidromic, synaptic, and receptor-agonist stimulation. We find three spatial patterns of Ca2+ accumulations: an apical band in the apical dendrite approximately 500 microns from the soma, an accumulation restricted to the basal dendrites, soma, and proximal apical dendrite, and a combination of both of these. We show that the apical band can be activated antidromically and synaptically and that, under blocked Na+ and K+ conductances, it generates Ca2+ spikes. Thus, the apical band may serve as a dendritic trigger zone for regenerative Ca2+ spikes or as a current amplifier for distal synaptic events. Our results suggest that the distal apical dendrite should be considered a separate functional compartment from the rest of the cell.

The effect of muscle stimulation during resistive training on performance parameters.

This study compared changes in movement velocity, force, and work from bilateral quadriceps muscle stimulation during resistive squatting exercise to identical exercise without stimulation. Both the group undergoing resistive training over 24 sessions (N = 9) and the group receiving the same treatment in conjunction with stimulation during the last 12 sessions (N = 9) showed significant improvements in measures of movement velocity, force, total work, power, sprint time, and vertical jump distance when compared to a control group receiving no treatment (N = 9). All subjects were baseline tested and tested at 3, 6, and 7 week intervals. Both experimental groups improved significantly for all measures, but the electrical stimulation group did not produce more significant changes overall than those with resistive training alone. However, when compared to control measures, the effect of electrical stimulation-augmented responses among some measures was more pronounced than the effect of resistive training alone.