Cortical Plasticity Induced by Spike-Triggered Microstimulation in Primate Somatosensory Cortex

Electrical stimulation of the nervous system for therapeutic purposes, such as deep brain stimulation in the treatment of Parkinson’s disease, has been used for decades. Recently, increased attention has focused on using microstimulation to restore functions as diverse as somatosensation and memory. However, how microstimulation changes the neural substrate is still not fully understood. Microstimulation may cause cortical changes that could either compete with or complement natural neural processes, and could result in neuroplastic changes rendering the region dysfunctional or even epileptic. As part of our efforts to produce neuroprosthetic devices and to further study the effects of microstimulation on the cortex, we stimulated and recorded from microelectrode arrays in the hand area of the primary somatosensory cortex (area 1) in two awake macaque monkeys. We applied a simple neuroprosthetic microstimulation protocol to a pair of electrodes in the area 1 array, using either random pulses or pulses time-locked to the recorded spiking activity of a reference neuron. This setup was replicated using a computer model of the thalamocortical system, which consisted of 1980 spiking neurons distributed among six cortical layers and two thalamic nuclei. Experimentally, we found that spike-triggered microstimulation induced cortical plasticity, as shown by increased unit-pair mutual information, while random microstimulation did not. In addition, there was an increased response to touch following spike-triggered microstimulation, along with decreased neural variability. The computer model successfully reproduced both qualitative and quantitative aspects of the experimental findings. The physiological findings of this study suggest that even simple microstimulation protocols can be used to increase somatosensory information flow.     

Calcium-induced changes in cytoskeleton and motility of cultured human keratinocytes

In normal epidermis keratinocytes migrate upward from the basal layer as they undergo terminal differentiation, yet they also have the capacity for lateral movement during wound healing. The purpose of our experiments was to investigate these two types of movement by manipulating the calcium ion concentration of the medium so that keratinocytes formed monolayers (0.1 mM calcium) or stratified sheets (2.0 mM calcium). Time-lapse video recording indicated that keratinocytes in low-calcium medium were laterally more motile than keratinocytes in normal medium. This was consistent with the ultrastructural appearance of the cells and the lack of desmosomal junctions, determined by scanning and transmission electron microscopy. During calcium-induced stratification keratinocytes moved upward from the basal layer by gliding over their neighbors and forming contacts with other suprabasal cells. Keratinocytes in low-calcium medium migrated into wounds made in the cultures, a process which was inhibited by monensin; however, stratified keratinocytes in normal medium did not enter wounds. Cytochalasin D caused rapid cell rounding and disruption of actin filaments in keratinocytes grown in low-calcium but not in normal medium, indicating more rapid treadmilling of actin and consistent with the greater motility of keratinocytes in low-calcium medium. Our results suggest that desmosome formation may place constraints on the movement of individual keratinocytes and that the actomyosin cytoskeleton is involved in lateral migration.     

The catalytic subunits of the (Na+,K+)-ATPase alpha and alpha(+) isozymes are the products of different genes

The sequences of the first 14 amino acids of the (Na+,K+)-ATPase catalytic subunits from rat kidney (alpha) and rat brain axolemma (alpha(+)) have been determined. They are: (alpha), NH2-Gly-Arg-Asp-Lys-Tyr-Glu-Pro-Ala-Ala-Val-Ser-Glu-His-Gly; (alpha(+)), NH2-Gly-Arg-Glu-Tyr-Ser-Pro-Ala-Ala-Glu-Val-Ala-Glu-Val-Gly. Although they are highly homologous, it is clear these sequences are also sufficiently different to conclude they are the products of different genes, or at least different exons of the same, differentially spliced, gene. Among mammals, the amino terminal sequence of the kidney alpha chain is essentially invariant. Thus this section of the (Na+,K+)-ATPase molecule is more highly conserved in one tissue between several species than between different tissues in the same species. This may reflect upon the difference in function of the alpha and alpha(+) isozymes of (Na+,K+)-ATPase.     

Efflux of beta-galactosidase products from Escherichia coli.

Several different strains of Escherichia coli were grown on a variety of carbon sources under various growth conditions. Lactose was added (usually at mid-log phase), and the concentrations of the products of beta-galactosidase action on this sugar (galactose, glucose, and allolactose) were determined at various times thereafter in the total culture and in the medium. It was found that with each strain, with all carbon sources, and under all of the conditions studied, a very large proportion of the products were found in the medium. Control studies were carried out which showed that these results were not artifacts of the method of separating the cells from the medium. The results also did not arise from the secretion of beta-galactosidase into the medium, from the diffusion of substrates and products into and out of the cells due to leaks in the membrane, or from faults in the method of sugar analysis. In addition, the results showed that there were very high levels of products inside the cells under the conditions used and that the efflux of the products was rapid. The efflux might be energetically advantageous to the cell as well as being a means of storing excess products until needed.     

Purinergic stimulation of K+-dependent Na+/Ca2+ exchanger isoform 4 requires dual activation by PKC and CaMKII

K⁺-dependent Na⁺/Ca²⁺-exchanger isoform 4 (NCXK4) is one of the most broadly expressed members of the NCKX (K⁺-dependent Na⁺/Ca²⁺-exchanger) family. Recent data indicate that NCKX4 plays a critical role in controlling normal Ca²⁺ signal dynamics in olfactory and other neurons. Synaptic Ca²⁺ dynamics are modulated by purinergic regulation, mediated by ATP released from synaptic vesicles or from neighbouring glial cells. Previous studies have focused on modulation of Ca²⁺ entry pathways that initiate signalling. Here we have investigated purinergic regulation of NCKX4, a powerful extrusion pathway that assists in terminating Ca²⁺ signals. NCKX4 activity was stimulated by ATP through activation of the P2Y receptor signalling pathway. Stimulation required dual activation of PKC (protein kinase C) and CaMKII (Ca²⁺/calmodulin-dependent protein kinase II). Mutating T312, a putative PKC phosphorylation site on NCKX4, partially prevented purinergic stimulation. These data illustrate how purinergic regulation can shape the dynamics of Ca²⁺ signalling by activating a signal damping and termination pathway.     

EXPERIMENTAL INITIATION OF HAUSTORIA IN AGALINIS PURPUREA (SCROPHULARIACEAE)

The initiation and early developmental stages of the haustorium were studied in Agalinis purpurea (Scrophulariaceae). Plants were grown in a 0.9% agar inorganic medium with a 0.5% sucrose supplement. Root exudate collected from Lespedeza sericea induced the initiation of haustoria, with earliest stages evident in 6-12 hr. A 30-min exposure to exudate produced an increased frequency of haustoria and a 24-hr exposure yielded haustorial frequencies equal to the number that were initiated on control plants continuously exposed to root exudate for the 5-day growth period. The early cytological features of haustorial development are described and the possible significance of haustorial initiation in host recognition is discussed.