Biomedical subjects

S Kasparov

Publications and source records attributed to S Kasparov.

27 records · Page 2Linked to original sources

PubMed1999

Differential effects of apamin on neuronal excitability in the nucleus tractus solitarii of rats studied in vitro.

We demonstrated previously that microinjection of the calcium-dependent potassium channel antagonist, apamin, into the nucleus tractus solitarius (NTS) in vivo potentiated the baroreceptor reflex mediated bradycardia but attenuated the cardiopulmonary reflex. The latter result was surprising since, intuitively, potassium channel blockade would be expected to increase neuronal excitability leading to reflex potentiation. The aim of this in vitro study was to investigate possible neuronal mechanisms to explain our in vivo observations. Transverse brainstem slices of rat were cut at the level of area postrema and recordings were made from 36 NTS neurones in whole-cell mode. The neurones were classified into three groups, based on their response to apamin (10 nM). Each group had a similar resting membrane potential (RMP; -55 +/- 1 mV; n = 36) and input resistance (404 +/- 20 M omega; n = 36). (1) In 15/36 neurones, apamin decreased the number of spikes evoked during injection of positive current by 37 +/- 6%; this was associated with a concomitant fall in input resistance of 12 +/- 2% (P < 0.05). Stimulation of the ipsilateral tractus solitarius evoked EPSP-IPSP complexes in nine of the 12 neurones tested; the inhibitory components were increased in amplitude, at a holding potential of -46 mV, from -1.7 +/- 0.4 to -3.2 +/- 0.6 mV (P < 0.01) in the presence of apamin, while the EPSPs were unaffected. All three of these effects were bicuculline (10 microM) sensitive. (2) In 8/36 neurones, apamin increased the number of spikes evoked during injection of positive current by 27 +/- 8%, but affected neither RMP nor input resistance. Only one of five neurones tested demonstrated a synaptically evoked EPSP-IPSP complex. The remaining four neurones displayed a single evoked EPSP, the amplitudes of which were unaffected by apamin. (3) In the remaining neurones (13/36), apamin affected neither responsiveness to positive current injection, RMP, nor input resistance. Six of 12 neurones demonstrated synaptically evoked EPSP-IPSP complexes; at a holding potential of -46 mV, apamin increased the IPSP component from -2.6 +/- 1 to -3.6 +/- 0.8 mV (P < 0.05), while the EPSPs were unaffected. In conclusion, apamin can both increase and decrease NTS neuronal excitability: the former reflecting blockade of channels on the recorded neurone; the latter may possibly result from an increase in GABA release by interneurones impinging onto the recorded neurone. The possibility of a differential distribution of apamin-sensitive channels in sub-populations of NTS neurones subserving different reflexes is discussed.

Journal Article↗

PubMed1999

Reflex response and convergence of pharyngoesophageal and peripheral chemoreceptors in the nucleus of the solitary tract.

The pharynx is a common conduit for the passage of both ingested material and respiratory gases and may receive a dual control from medullary structures regulating deglutition and respiration. We sought both to compare the pattern of reflex response following stimulation of pharyngoesophageal and peripheral chemoreceptors and to assess whether these afferents converge in the nucleus of the solitary tract. In an arterially perfused working heart-brainstem preparation of mature rat, pharyngoesophageal receptors were stimulated by distension of the pharyngeal-oesophageal junction, whereas chemoreceptors were activated by sodium cyanide solution. In peripheral studies we recorded electromyographic activity from genioglossus, mylohyoideus and the lower thoracic oesophagus as well as hypoglossal, laryngeal and phrenic motor discharge. Sub-glottal pressure was also measured at constant airflow. In central studies, nucleus of the solitary tract neurons were recorded with blind whole-cell techniques. In peripheral studies spontaneous irregular electromyographic discharges (cycle length 99+/-26 s) occurred sequentially in genioglossus and mylohyoideus muscles (during the inter-phrenic nerve activity interval) and subsequently the oesophagus; these were accompanied by post-inspiratory discharges in both hypoglossal and laryngeal motor nerves and an atropine-sensitive bradycardia (-39+/-5 beats/min). Components of the reflex response elicited following stimulation of both pharyngoesophageal receptors and chemoreceptors were qualitatively similar and included: (i) expiratory-related increases in laryngeal pressure; (ii) sequential electromyographic discharge in genioglossus, mylohyoideus muscles and oesophagus; (iii) post-inspiratory burst discharge in hypoglossal, recurrent and superior laryngeal motor nerves; and (iv) an atropine-sensitive bradycardia (-38 to -95 beats/min). The chemoreceptor reflex-evoked responses were abolished after sinoaortic denervation. Of 135 whole-cell recordings of nucleus of the solitary tract neurons, 31 received a synaptic input from pharyngoesophageal receptors (22 excitatory and nine inhibitory). Cells excited by pharyngoesophageal receptor stimulation were either "spontaneously" bursting, which occurred during the inter-phrenic nerve activity interval (cycle length 79+/-22 s; n=9), or non-bursting (n=13). Of the 22 nucleus of the solitary tract neurons excited by pharyngoesophageal receptor stimulation, 77% received a convergent excitatory synaptic input from chemoreceptors (eight bursting and nine non-bursting neurons). Thus, stimulation of pharyngoesophageal receptors and chemoreceptors evoked common reflex response components including activation of hypoglossal, laryngeal adductor, cardiac vagal motoneurons and swallowing. Moreover, some excitatory pharyngoesophageal and chemoreceptors inputs typically converged on nucleus of the solitary tract neurons. The possibility that this convergence manifests a defensive reflex reaction is discussed.

Animals↗

PubMed1998

Angiotensin II receptors within the nucleus of the solitary tract mediate the developmental attenuation of the baroreceptor vagal reflex in pre-weaned rats.

This study tested the hypothesis that baroreceptor vagal reflex (BVR) attenuation in developing rats, which occurs between postnatal ages (P) of 10 to 20 days old, is due to a central action of angiotensin II (Ang II). In urethane or halothane anaesthetised mature (P > 45) or pre-weaned rats (P14-18), BVR sensitivity was estimated as the ratio between the fall in heart rate and the increase in arterial pressure induced by i.v. phenylephrine. An Ang II AT1 receptor antagonist, losartan, was administered intra-venously (i.v.) or microinjected into brainstem structures. In pre-weaned rats BVR sensitivity was increased significantly by losartan (5 mg/kg; urethane anaesthesia: p < 0.01; halothane anaesthesia: p < 0.05) while a larger dose (10 mg/kg) was ineffective in mature animals. In pre-weaned rats, microinjection of losartan (500 pmol) into the nucleus tractus solitarii (NTS) but neither area postrema nor subjacent nuclei, reversibly increased the sensitivity of BVR (+89 +/- 19%; p < 0.01, n = 12). Microinjection of losartan (500 or 1500 pmol) into the NTS of mature rats did not change the BVR. An AT2-antagonist, PD123-319 did not restore the BVR sensitivity in pre-weaned rats. Thus, AT1 receptors located within the NTS play a pivotal role in the developmental attenuation of the BVR in pre-weaned rats.

Age Factors↗

PubMed1997

Presynaptic action of the neurosteroid pregnenolone sulfate on inhibitory transmitter release in cultured hippocampal neurons.

The effects of the neurosteroid pregnenolone sulfate (PS) were studied in 3- to 9-week-old hippocampal cultures from neonatal rats. Cells were voltage clamped using CsCl filled electrodes, while action potentials and excitatory glutamatergic currents were abolished by superfusing with a combination of tetrodotoxin, 6-cyano-7-nitroquinoxaline (CNQX) and 2-amino-5-phosphonopentanoic acid (AP-5). Under these conditions spontaneous GABAergic inhibitory postsynaptic currents (sIPSCs) were seen as inward currents at a holding potential of -70 mV. Their amplitude distributions were skewed without clearly detectable peaks. PS at 1-50 microM concentrations decreased the frequency of sIPSCs, with 1 microM being the most effective concentration. The effect appeared after 10-15 min of steroid application and the magnitude of the reduction increased during the early wash period. No recovery of sIPSC frequency was found after 30 min of washing with steroid-free medium. sIPSC amplitudes were not significantly changed at the time the effect of PS on sIPSC frequency was observed. The slow onset of this effect and its duration suggest a novel presynaptic action of the neurosteroid PS on GABAergic inhibition in the mammalian brain.

Animals↗

PubMed1997

Changes in baroreceptor vagal reflex performance in the developing rat.

Ontogenesis of both vagal control of heart rate and the baroreceptor vagal reflex were evaluated in rats at postnatal ages (P) of 5/6, 10, 15, 20, 25 and >>42 days anaesthetised with urethane (1.5 g/kg). Between P5/6 and P25 heart rate rose from 372 +/- 12 to 448 +/- 20 beats per minute and mean arterial pressure increased from 33.9 +/- 3.1 to 74.59 +/- 3.25 mm Hg (mean +/- SEM, n = 7 and 11 respectively). Cardiac vagal tone was absent at P10 but significant at P20 (P < 0.05) as revealed with atropine (0.5-1 mg/kg i.v.). Baroreceptor cardiac reflex sensitivity, tested with phenylephrine (10-50 microg/kg i.v.), was attenuated significantly in P10-20 rats compared with P5/6, P25 and mature animals. In P14-17 rats stimulation of neurones in either the solitary tract or ambiguual nuclei, by microinjection of L-glutamate (100-200 pmol), evoked an atropine-sensitive bradycardia indicating a functional integrity of central and peripheral efferent pathways mediating the baroreceptor reflex. Thus, the baroreceptor vagal reflex is functional in P5/6 rats but becomes attenuated between P10-P20, which is coincident with the maturational rise in arterial pressure.

Aging↗

PubMed1994

Thyrotropin-releasing hormone enhances excitatory postsynaptic potentials in neocortical neurons of the rat in vitro.

Several lines of evidence suggest a modulatory effect of thyrotropin-releasing hormone (TRH) on synaptic transmission in the mammalian neocortex. In the present study, the effects of this tripeptide on intracellularly recorded neocortical pyramidal neurons were investigated using rat in vitro brain slice preparations. TRH (5 microM and 50 microM) added to the perfusion medium concentration-dependently increased the excitability of pyramidal neurons, reflected by the number of spikes evoked by a depolarizing current pulse and by the augmentation of the time integral of glutamatergic excitatory postsynaptic potentials (EPSPs). TRH increased preferentially the time integrals of the late components of EPSPs (1-EPSPs) and increased their voltage-dependence. The early components of the EPSPs (e-EPSPs) were changed to much lesser extent. Iontophoretically applied D-2-amino-5-phosphonovalerate (D-APV) antagonized the TRH-induced increase of the 1-EPSPs. TRH also markedly enhanced the depolarizing responses evoked by iontophoretically applied N-methyl-D-aspartate (NMDA), while the depolarizing responses evoked by (R,S)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA) and L-glutamate were not significantly affected. The depolarizing inward rectification present in all neurons studied was augmented by the higher concentration of TRH. The effects of TRH were incited after about 5 min and were long-lasting. In most neurons the effects of TRH on neuronal excitability did not completely recover during the 45 min washout period. The present data suggest that some of the non-hormonal actions of TRH in the neocortex may be due to an enhancement of glutamatergic synaptic transmission.(ABSTRACT TRUNCATED AT 250 WORDS)

2-Amino-5-phosphonovalerate↗

PubMed1993

Acamprosate (calciumacetylhomotaurinate) decreases postsynaptic potentials in the rat neocortex: possible involvement of excitatory amino acid receptors.

Acamprosate (calciumacetylhomotaurinate) is used therapeutically against relapse in weaned alcoholics. In the present study, the mechanism of action was investigated by making intracellular in vitro and extracellular in vivo recordings from rat neocortical neurons. Acamprosate (0.1-1 mM) added to the perfusion fluid in vitro reduced excitatory and inhibitory postsynaptic potentials and the depolarizing responses evoked by iontophoretic application of the excitatory amino acids, L-glutamate, L-aspartate, L-homocysteate and N-methyl-D-aspartate, but did not alter the responses to gamma-aminobutyric acid. Acamprosate decreased electrical excitability without apparently changing membrane potential, input resistance, afterhyperpolarization, or threshold and amplitude of the action potential. In vivo iontophoretic application of acamprosate reduced the extracellularly recorded unit activity elicited by iontophoretically applied L-glutamate, whereas spontaneous discharges remained unaffected. These data suggest that acamprosate reduces the postsynaptic efficacy of excitatory amino acid neurotransmitters and lowers neuronal excitability in the neocortex of the rat.

Acamprosate↗

PubMedDate not supplied

Viral vectors as tools for studies of central cardiovascular control.

During the last few years physiological genomics has been the most rapidly developing area of physiology. Given the current ease of obtaining information about nucleotide sequences found in genomes and the vast amount of readily available clones, one of the most pertinent tasks is to find out about the roles of the individual genes and their families under normal and pathological conditions. Viral gene delivery into the brain is a powerful tool, which can be used to address a wide range of questions posed by physiological genomics including central nervous mechanisms regulating the cardio-vascular system. In this paper, we will give a short overview of current data obtained in this field using viral vectors and then look critically at the technology of viral gene transfer.

Animals↗

PubMedDate not supplied

[Calcium-independent inhibition of the spontaneous release of GABA by baclofen in the rat hippocampus].

The mechanism of calcium-independent spontaneous GABA release in a long-living (3-10 weeks) rat hippocampal neuron culture was studied. It was found that Cd2+ (100 microM), a Ca2+ channel blocker, reversibly decreased the frequency and amplitude of GABAergic spontaneous miniature inhibitory postsynaptic currents (smIPSCs). Besides, Cd2+ decreased the currents evoked by muscimol application onto the neurons, which is evidence of an additional postsynaptic effect. The GABAB receptor agonist baclofen (0.1-50 microns) produced a concentration-dependent decrease in the smIPSC frequency, while not affecting the current amplitude. The baclofen effect was blocked by the pertussis toxin. The baclofen efficacy both in the presence of Cd2+ (presumably compete blocking of Ca2+ channels) and in the absence of this agent were similar and could be completely accounted for by the loss of an equivalent smIPSC fraction. Thus, the presynaptic baclofen-induced inhibition of the spontaneous GABA release can be entirely independent of the calcium channel modulation (the latter playing a decisive role in a mediator release induced by the action potential). Therefore, the smIPSC measurements (widely used in the past decade for the study of presynaptic drug activity) may inadequately reflect the drug effect in the intact brain.

Animals↗