Interaction of Topotecan,DNA Topoisomerase I Inhibitor,with Double-Stranded Polydeoxyribonucleotides. 1. Topotecan Dimerization in Solution

Behavior of topotecan, DNA topoisomerase I inhibitor, was studied in aqueous solutions by optical methods. Topotecan absorption spectra were recorded in the pH range 0.5–11.5 and its pKa were determined. Quantum chemical calculations were made for all charge states of the topotecan molecule in lactone and carboxylate form. The calculated absorption maxima agree well with the experimental data. Protonation of the topotecan D ring (pKa 3.6) was revealed. Comparison of experimental and calculated data showed topotecan structure with a proton at the oxygen atom at C16a rather than N4 to be the most preferable. Topotecan molecules were shown to form dimers at concentrations above 10^–5M. Topotecan dimerization is accompanied by an increase in the pKa of hydroxy group of the A ring from 6.5 ([TPT] = 10^–6M) to 7.1 ([TPT] = 10^–4M), which indicates participation of this group in dimer stabilization, perhaps due to intermolecular hydrogen bonding with N1 of the B ring of a neighboring molecule. Probable dimer structures were proposed. The topotecan dimerization constant was determined, K = (4.0 ± 0.7)·10³M^–1.     

DNA-Binding activity of bis-netropsin containing a cis-diaminoplatinum group between two netropsin fragments

The binding of Pt-bis-Nt and its modified analog Pt*-bis-Nt, which has two additional glycine residues in the linker between two netropsin fragments, to DNA has been studied. The elongation of the linker in the bis-netropsin molecule increases the cytotoxicity and leads to an almost complete loss of the antiherpetic activity of bis-netropsin. The study of the binding of two bis-netropsins to an oligonucleotide duplex containing an AT cluster, which is present at the origin of replication of herpes virus (OriS), revealed significant structural differences between the complexes of bis-netropsins with this DNA oligomer. It was shown by CD spectroscopy that the binding of Pt-bis-Nt in the extended conformation and in hairpin form with the parallel orientation of two bis-netropsin fragments makes a greater contribution to the interaction with the duplex than in the case of Pt*-bis-Nt. At high binding levels, Pt*-bis-Nt binds to the AT cluster in OriS predominantly in the form of associates based on the antiparallel, double-stranded, pyrrolcarboxyamide motif. The interaction of Pt-bis-Nt and Pt*-bis-Nt with a single-stranded oligonuclotide (64 nt) corresponding to the upper strand at the origin of replication of herpes virus (OriS*) was also studied. Substantial differences in the binding of bis-netropsins to OriS* and the thermostability of the resulting complexes were found by CD spectroscopy and UV melting studies.     

Neuronal response produced in the human thalamic nucleus reticularis by significant and non-significant verbal and sensory stimuli

Response recorded by microelectrode techniques during the course of 46 stereotaxic operations on dyskinesia patients was investigated in 340 units of the nucleus reticularis (rt) of the human thalamus. Differences were found between the multistage response of three types of rt neurons (A, B, and C) to verbal (or acousticcum-sensory) functionally significant stimuli (FSS) at both the stage of stimulus presentation and during the performance of goal-directed motion. Phasic activation produced by FSS presentation (as well as onset and execution of movement) in 102 out of 183 type A cells (or 55.7%) was characteristic of these cells, combined with inhibition of B type neurons in 82 out of a 139 sample (or 59.0%) produced by FSS and at the preparatory as well as the execution stage of movement. Activity of type C neurons remained unchanged. A correlation was revealed between response in A and B cells and "excitatory" trigger stimuli, but no specificity with respect to physical or semantic parameters of verbal signals. A correlation occurred during the course of movement performance with somatosensory afferents without any specific relationship to type and somatotopic aspects of movement. The time-related dynamics of A and B cell response is thought to illustrate the interaction of two neuronal subsystems within the rt participating in the performance of goal-directed motor performance triggered by speech.Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow. Institute of Neurosurgery, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 22, No. 4, pp. 441–451, June–July, 1990.     

Dynamics of neuronal interaction in the human thalamic nucleus reticularis produced by significant and non-significant verbal stimuli

Dynamics of neuronal interaction recorded by microelectrodes were examined in 90 arrays of cells of the human thalamic reticular nucleus (Rt) during stereotaxic surgical procedures. Cooperative interaction between adjacent neurons was found to occur in neuronal arrays after presentation of verbal (or sensory-cum-acoustic) functionally significant stimulus (FSS) as well as at stages of initiation and performance of goal-directed movement. Specialized dynamics were noted in the pattern of interaction between neuronal arrays of two types (A and B) with irregular background activity and 2–5 Hz bursting rhythm (types A and B respectively). This dynamic local neuronal interaction correlates with the stage of significant verbal stimulus presentation and that of performing goal-directed movements. The matching transient correlation between activity of A and B cell arrays reflects matching operation of two sequences of regulatory and control processes involved in processing of functionally significant verbal (or sensory) information and performance of goal-directed movement.Institute of Chemical Physics, Academy of Sciences of the USSR, Moscow Institute of Neurosurgery, Academy of Medical Sciences of the USSR, Moscow. Translated from Neirofiziologiya, Vol. 22, No. 4, pp. 451–459, June–July, 1990.