Search PubMed⌕ Search

Biomedical subjects

J T Buchanan

Publications and source records attributed to J T Buchanan.

At least 37 records · Page 2Linked to original sources
PubMed1993

Electrophysiological properties of identified classes of lamprey spinal neurons.

1. As part of a continuing analysis of the mechanisms of the central pattern generator underlying fictive swimming in lamprey, a systematic survey of electrophysiological properties of lamprey neurons was made in the in vitro spinal cord preparation with the use of intracellular current-clamp recordings. A total of 70 neurons was included in the study, representing 6 classes of spinal neurons. The classes were myotomal motoneurons, three classes of interneurons involved in fictive swimming [lateral interneurons, nerve cells with contralateral and caudal projecting axons (CC interneurons), and excitatory interneurons], and two classes of interneurons involved in sensory processes (edge cells and giant interneurons). The recordings were done in quiescent preparations. 2. There was little or no significant difference among the cell classes with regard to resting potential, threshold potential, action-potential amplitude, or action-potential duration. 3. The voltage versus current relationships for the cells were fairly linear near resting potential, although most cells showed a slight tendency to rectify with depolarization above resting potential. This tendency was strongest among edge cells and lateral interneurons and weakest among motoneurons and CC interneurons. The input resistances, membrane time constants, and rheo-bases for the cell classes showed significant differences among some classes. For example, CC interneurons and excitatory interneurons had significantly higher input resistances than the other cell classes. 4. The late afterhyperpolarization following the action potential tended to be larger in amplitude with an earlier peak and a longer duration in edge cells and giant interneurons than in the other cell classes. 5. All cells responded to depolarizing current injections by firing action potentials, and almost all cells fired action potentials throughout the 400-ms current pulse. The cells exhibited adaptation resulting in increasing interspike intervals during the current pulse. The adaptation, however, was insufficient to terminate firing before the end of the current pulse. The relationship between frequency of firing and input current was generally monotonic with a tendency to saturate at higher current levels. 6. The general conclusion from this study is that the spinal neurons that partake in fictive swimming (motoneurons, lateral interneurons, CC interneurons, and excitatory interneurons) are similar in their resting and action-potential mechanisms. Their most prominent differences are in size-related properties. The sensory-related interneurons, especially the edge cells and to some extent the giant interneurons, exhibited more pronounced differences in their resting and action-potential properties when compared with the other cell classes.

Afferent Pathways↗
PubMed1993

The effects of neurotransmitters on the integrative properties of spinal neurons in the lamprey.

1. The integrative behavior of lamprey central neurons was analyzed by white noise frequency domain methods and simulated with a minimal, non-linear neuronal model consisting of two voltage-dependent processes: (i) a depolarizing inwardly directed conductance carrying calcium and monovalent ions and (ii) a repolarizing outwardly directly conductance representing a generalized potassium conductance. In addition to normal properties, the effects of neurotransmitters were interpreted with the model. Specifically, N-methyl-D-aspartate (NMDA)-induced properties were simulated under conditions where the intrinsic voltage dependence of the potassium channels was constrained by properties of lamprey neurons. However, the NMDA channel kinetics were fixed by the single-channel properties of other neurons. The effects of focally applied neurotransmitters on the membrane properties of intact spinal cord neurons were quantitatively described with a reduced neuronal model that was also used to simulate transmitter-induced responses. In addition, transmitters were also released synaptically by KCl depolarization of projecting neurons. 2. Both synaptically released transmitters and focally applied putative excitatory or inhibitory transmitters directly applied to the spinal cord generally resulted in a decrease in the magnitude of the impedance function that was modeled by a decrease in membrane resistance (shunting effect). 3. Local application of the inhibitory neurotransmitters glycine or gamma-aminobutyric acid (GABA) led to small voltage responses when recorded near the resting potential. However, large decreases in the magnitude of the impedance function were observed in both current-clamp or voltage-clamp recording modes. 4. The excitatory amino acids quisqualate, kainate and glutamate evoked depolarizations in current clamp that activated intrinsic voltage-dependent conductances and obscured the direct effects of the transmitters. Under voltage-clamp conditions these transmitters caused a small decrease in the impedance magnitude that could be modeled by a shunt. 5. In contrast to the other excitatory amino acids, NMDA elicited large increases, rather than decreases, in both the magnitude and the phase lag of the impedance function. These changes were modeled by a negative conductance (a voltage-dependent conductance that produces an inward current). 6. The reduced neuron model provides an experimentally based description of the highly oscillatory and non-linear responses observed during NMDA activation of the spinal neurons involved in the pattern generation of locomotion. Simulations of sustained oscillatory behaviors consistent with experimental observations were carried out to illustrate the NMDA-induced integrative properties of central neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Amino Acids↗
PubMed1992

Apamin reduces the late afterhyperpolarization of lamprey spinal neurons, with little effect on fictive swimming.

The role of the late afterhyperpolarization (late AHP) in the firing properties of lamprey spinal neurons was tested by bath application of apamin, a selective blocker of the sk calcium-dependent potassium current. Intracellular recordings of identified motoneurons and interneurons were made with micropipette electrodes in the isolated lamprey spinal cord. Apamin reversibly reduced the amplitude of the late afterhyperpolarization without affecting other aspects of the action potential or the resting potential. The firing frequencies of the neurons were enhanced by apamin over a range of depolarizing current pulse injections. The effect of apamin was also tested on fictive swimming, which was induced in the isolated spinal cord by bath application of an excitatory amino acid (D-glutamate or N-methyl-D,L-aspartate). A concentration of apamin (10 microM) sufficient to substantially reduce the late AHP had no significant effect on the ventral root burst rate, intensity, or phase lag during fictive swimming.

Animals↗
PubMed1992

Synaptic potentials and transfer functions of lamprey spinal neurons.

1. Electrotonic and chemical synaptic potentials were measured as a function of frequency of presynaptic action potentials. Over the frequency range from 0.02 to 10 Hz, the electrotonic synaptic potential was constant, while the chemical synaptic potential decreased in magnitude. Above 10 Hz, both synaptic events decreased in magnitude consistent with filtering by the dendritic structures. 2. Electrotonic synaptic transfer functions from 0.5 to 100 Hz were measured for the I1 reticulospinal Müller axon to spinal neuron electrotonic synaptic junction of the lamprey spinal cord using paired recordings from the pre-synaptic terminals and the post-synaptic neurons. In addition to this two-point synaptic transfer function, individual single point impedance functions of both the post-synaptic soma and the pre-synaptic axon terminal were measured. 3. The measured functions were interpreted with a computational model based on a three dimensional reconstruction of a Lucifer yellow filled motoneuron. Simulations of the model for a synaptic location of the I1 synapse were consistent with the measured synaptic transfer functions. 4. Synaptic potentials were simulated for inputs on dendrites near the I1 axon as well as distal dendritic regions. The high frequency filtering increased as the synaptic location was moved from the soma to the periphery, but the potential response on distal dendrites was larger than would have been predicted from the end of the equivalent cylinder of a Rall model that was used to fit soma impedance functions. 5. Electrotonic post-synaptic potentials were enhanced by the activation of a TTX-sensitive negative conductance.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1992

Neural network simulations of coupled locomotor oscillators in the lamprey spinal cord.

The segmental locomotor network in the lamprey spinal cord was simulated on a computer using a connectionist-type neural network. The cells of the network were identical except for their excitatory levels and their synaptic connections. The synaptic connections used were based on previous experimental work. It was demonstrated that the connectivity of the circuit is capable of generating oscillatory activity with the appropriate phase relations among the cells. Intersegmental coordination was explored by coupling two identical segmental networks using only the cells of the network. Each of the possible couplings of a bilateral pair of cells in one oscillator with a bilateral pair of cells in the other oscillator produced stable phase locking of the two oscillators. The degree of phase difference was dependent upon synaptic weight, and the operating range of synaptic weights varied among the pairs of connections. The coupling was tested using several criteria from experimental work on the lamprey spinal cord. Coupling schemes involving several pairs of connecting cells were found which 1) achieved steady-state phase locking within a single cycle, 2) exhibited constant phase differences over a wide range of cycle periods, and 3) maintained stable phase locking in spite of large differences in the intrinsic frequencies of the two oscillators. It is concluded that the synaptic connectivity plays a large role in producing oscillations in this network and that it is not necessary to postulate a separate set of coordinating neurons between oscillators in order to achieve appropriate phase coupling.

Animals↗
PubMed1991

5-Hydroxytryptamine depresses reticulospinal excitatory postsynaptic potentials in motoneurons of the lamprey.

Application of 5-hydroxytryptamine (5-HT) to the lamprey spinal cord in vitro reversibly depressed the chemical component of excitatory post-synaptic potentials recorded intracellularly in motoneurons and evoked by stimulation of single reticulospinal Müller cells. The depression could be produced either by local application of small volumes of 10 mM 5-HT to the surface of the spinal cord or by bath-application of 1 or 10 microM 5-HT. No effect on the input resistance of the postsynaptic cells or their sensitivity to glutamate, the suspected transmitter at this synapse, could be detected, suggesting the possibility of a presynaptic action of 5-HT at this synapse in the lamprey.

2-Amino-5-phosphonovalerate↗
PubMed1989

Effects of 5-hydroxytryptamine on the afterhyperpolarization, spike frequency regulation, and oscillatory membrane properties in lamprey spinal cord neurons.

1. Local application of 5-hydroxytryptamine (5-HT) in the area in which a dense 5-HT plexus is located in the lamprey spinal cord leads to a marked depression of the late phase of the afterhyperpolarization (AHP) following the action potential. This effect was observed in motoneurons, premotor interneurons, and giant interneurons, whereas no effect was observed in the sensory dorsal cells and edge cells. 2. The late 5-HT sensitive phase of the AHP was increased in amplitude when calcium entry was enhanced during the prolongation of action potentials caused by tetraethylammonium (TEA). Conversely, a blockade of Ca2+ entry by manganese reduced the AHP amplitude, suggesting that a calcium-dependent current, most likely carried by potassium, underlies the late phase of the AHP in these cells, as is the case in many other types of neurons. 3. The late phase of the AHP could be depressed by 5-HT although no effects were exerted on either the resting input resistance or on the shape of the action potential in 54% of the cells. The membrane conductance increase associated with the late phase of the AHP was markedly attenuated by 5-HT application. 4. In voltage-clamp experiments, Na+ currents and most K+ currents were blocked by tetrodotoxin (TTX) and TEA, respectively. Under these conditions, voltage steps elicited a slow outward current, most likely representing a Ca2+-activated K+ current, which was depressed by 5-HT application. 5. 5-HT does not appear to reduce AHP amplitude by blocking the calcium entry occurring during the action potential. No evidence was obtained for an involvement of second messengers such as adenosine-3':5'-cyclic monophosphate (cAMP), guanosine-3':5'-cyclic monophosphate (cGMP), diacyglycerol, or arachidonic acid. The effect of 5-HT on the late AHP may be due to a direct action on the calcium-dependent potassium channels or on the intracellular handling of Ca2+ ions. 6. The amplitude reduction of the AHP has a profound influence on the spike frequency regulation of any given cell; the frequency of spikes evoked by a given excitatory stimulus is therefore markedly increased by application of 5-HT. 5-HT thus increases the "gain" of the input-output relation of interneurons and motoneurons responsible for generating the locomotor rhythm. In addition, 5-HT causes a prolongation of the depolarized plateau of the N-methyl-D-aspartate (NMDA) receptor-induced membrane potential oscillations, as expected from the 5-HT-induced effects on the Ca2+-activated K+ channels that contribute to the repolarization.

Action Potentials↗
PubMed1989

Identification of excitatory interneurons contributing to generation of locomotion in lamprey: structure, pharmacology, and function.

1. In the in vitro preparation of the lamprey spinal cord, paired intracellular recordings of membrane potential were used to identify interneurons producing excitatory postsynaptic potentials (EPSPs) on myotomal motoneurons. 2. Seventy-nine interneurons (8.4% of all neuron-motoneuron pairs tested) elicited unitary EPSPs that followed one-for-one at short, constant latencies and were therefore considered monosynaptic according to conventional criteria. Evidence was obtained for selectivity and divergence of excitatory interneuron (EIN) outputs and for convergence of EIN input to motoneurons. 3. The neurotransmitter released by EINs may be an excitatory amino acid such as glutamate, because the EPSPs were depressed by antagonists of excitatory amino acids. 4. Intracellular dye injection revealed that EINs have small cell bodies (average 11 x 27 microns), transversely oriented dendrites, and thin (less than 3 microns) slowly conducting axons (0.7 m/s) that project caudally and ipsilaterally. One EIN exhibited a system of thin multi-branching axon collaterals with periodic swellings. Ultrastructurally, these swellings contained clear spherical vesicles, and they apposed postsynaptic membrane specializations. 5. During fictive locomotion, the membrane-potential oscillations of EINs were greater in amplitude than, but similar in shape and timing to, those of their postsynaptic motoneurons. EINs fired action potentials during fictive locomotion and contributed to the depolarization of motoneurons. 6. These interneurons are proposed to be a source of excitation to motoneurons and interneurons in the lamprey spinal cord, participating in motor activity including locomotion.

Action Potentials↗
PubMed1989

Analyses of dental pulp in restored teeth.

Restored teeth were extracted from test animals at four time intervals (1 hour, 1 day, 1 week, and 3 months) following amalgam insertion. Extracted teeth were frozen in liquid nitrogen, cryo-fractured so as to expose the pulps and then freeze-dried. Pulps were analyzed for mercury content by energy dispersive spectrometry (EDS) and atomic absorption spectrophotometry (AAS). Mercury levels appeared below the detection limits of EDS but could be detected by AAS which showed the highest readings seven days after amalgam insertion.

Animals↗
PubMed1988

Simulation of the segmental burst generating network for locomotion in lamprey.

Recently a segmental network of inhibitory and excitatory interneurones, which are active during locomotion, has been described in the lamprey, a lower vertebrate. The interactions between the different neurones were established by paired intracellular recordings. A computer simulation of the segmental network has been performed, which shows that with the established neuronal connectivity rhythmic alternating burst activity can be generated within the upper part of the normal physiological range of locomotion. Three neurones of each kind were used (altogether 18 neurones). As shown previously the lower frequency range used in locomotion most likely depends on an activation of voltage-dependent N-methyl-D-aspartate (NMDA) receptors, which could, however, not be simulated with the present neuronal models.

Animals↗
PubMed1988

Immunohistochemical studies of cholecystokininlike peptides and their relation to 5-HT, CGRP, and bombesin immunoreactivities in the brainstem and spinal cord of lampreys.

The distribution of cholecystokinin (CCK)-like immunoreactivity in the brainstem and spinal cord of lampreys was studied by using CCK antisera with different properties. In the spinal cord, three separate systems reacted with CCK antisera: (1) A ventral and lateral fiber system descending from a group of neurons in the posterior reticular nucleus of the rhombencephalon was labeled by both a C-terminal-directed CCK antiserum and a monoclonal CCK antibody. (2) A dorsal root-dorsal column system of fibers originating from cell bodies in the dorsal root ganglia was labeled only by the C-terminal CCK antiserum. This CCK immunoreactivity could be abolished by preabsorption with calcitonin-gene-related peptide (CGRP), suggesting that it was due to cross-reactivity with a CGRP-like peptide. This system also contained 5-hydroxytryptamine (5-HT)-, bombesin-, and CGRP-like immunoreactivities. (3) An intraspinal system of 5-HT neurons was labeled with an antiserum to the midportion of CCK-33 but not by the other CCK antisera. The CCK labeling of this system was difficult to reduce by preabsorption with CCK peptide and thus appeared to be nonspecific. Groups of cell bodies in the middle reticular nucleus of the rhombencephalon, the reticular nucleus of the mesencephalon, and the hypothalamus were labeled by both the C-terminal and the monoclonal CCK antisera. The gut contained two types of CCK-like immunoreactivity, one of which appeared to be due to cross-reactivity with CGRP. A biochemical analysis showed that the content of CCK was low in the spinal cord compared to the brain, and these results agreed with the immunohistochemical findings.

Animals↗
PubMed1988

A new class of small inhibitory interneurones in the lamprey spinal cord.

Paired intracellular recordings of interneurones and motoneurones have been performed in the lamprey spinal cord in vitro. One new type of small interneurone (10-15 microns in diameter) which produces monosynaptic inhibitory postsynaptic potentials (IPSPs) in motoneurones is described. The IPSP is strychnine-sensitive and thus presumably glycinergic. This type of interneurone undergoes rhythmical membrane potential oscillations during fictive locomotion and intracellular stimulation can have a profound effect on the burst generation occurring during fictive locomotion.

Animals↗
PubMed1987

Newly identified 'glutamate interneurons' and their role in locomotion in the lamprey spinal cord.

A new class of excitatory premotor interneurons that are important in the generation of locomotion in the lamprey has now been described. In the isolated spinal cord, these neurons act simultaneously with their postsynaptic motoneurons during fictive swimming. They are small and numerous, and they monosynaptically excite both motoneurons and inhibitory premotor interneurons. The excitatory postsynaptic potentials are depressed by an antagonist of excitatory amino acids. These interneurons receive reticulospinal input from the brain stem and polysynaptic input form skin afferents. A model of the network underlying locomotion based on the synaptic interactions of these neurons can now be proposed for the lamprey.

Animals↗
PubMed1987

Survey of neuropeptide-like immunoreactivity in the lamprey spinal cord.

The distribution of neural elements immunoreactive to several peptides with a possible messenger role was studied in the lamprey spinal cord by using an indirect immunofluorescence method. Different patterns of immunoreactive (IR) fibers were detected by antisera raised against cholecystokinin, metorphamide, bombesin, galanin, corticotropin-releasing factor, somatostatin, peptide YY, neurotensin, calcitonin and FMRFamide. Somatostatin-IR cell bodies were located around the central canal. All immunoreactivity could be abolished by preabsorption with the corresponding peptide. The results suggest that the lamprey spinal cord has several fiber systems containing peptides sharing immunogenic properties with mammalian neuropeptides.

Animals↗
PubMed1987

Reticulospinal neurones activate excitatory amino acid receptors.

Paired intracellular recordings were used to study the monosynaptic excitatory postsynaptic potentials (EPSP) in lamprey motoneurones evoked by stimulation of single reticulospinal Müller and Mauthner cells. The chemical component of the synaptic potentials was depressed by both application of the non-selective excitatory amino acid antagonists kynurenic acid and cis-2,3-piperidine dicarboxylate. The N-methyl-D-aspartate (NMDA) antagonists Mg2+ and 2-amino-5-phosphonovalerate caused a selective depression of a late component of the EPSP. Thus, fast-conducting reticulospinal neurones appear to release an excitatory amino acid acting at both NMDA and non-NMDA receptors.

2-Amino-5-phosphonovalerate↗
PubMed1986

Transplants of embryonic brainstem containing the locus coeruleus into spinal cord enhance the hindlimb flexion reflex in adult rats.

Cell suspensions of embryonic brainstem containing the locus coeruleus were injected intervertebrally into the lumbar spinal cord of adult rats whose descending catecholamine (CA) fibers had been lesioned with intracisternal injection of 6-hydroxydopamine. Up to 1100 CA cells were found 2 and 4 months later, and these cells grew processes which produced histologically detectable reinnervation of the lumbar gray matter on the injected side of the cord. To assess the functional activity of the transplanted CA cells, the force of the hindlimb flexion reflex was measured in acute spinal rats. This reflex has been shown previously to be strongly enhanced by catecholamines. The flexion reflexes were significantly stronger in the transplanted rats than in the controls. Further, the flexion reflexes were significantly reduced by phenoxybenzamine, an alpha-adrenergic blocker, in the transplanted rats while the reflexes of controls were not significantly changed. These results demonstrate that cell suspension transplants of embryonic brainstem containing the locus coeruleus into the adult rat spinal cord survive, grow reinnervating catecholamine processes, and can affect the functional activity of the spinal cord.

Animals↗
PubMed1986

A spinal projection of 5-hydroxytryptamine neurons in the lamprey brainstem; evidence from combined retrograde tracing and immunohistochemistry.

To investigate whether there is a descending contribution to the spinal 5-hydroxytryptamine (5-HT) innervation in the lamprey, a primitive vertebrate, a study using retrograde transport of the fluorescent tracer Fast blue combined with 5-HT immunohistochemistry was conducted. Two to 4 weeks after an injection of Fast blue into the rostral spinal cord, retrogradely labelled cells were seen throughout the brainstem. Two groups of these cells, one within the posterior reticular nucleus of the rhombencephalon and another rostral to the trigeminal motor nucleus, were labelled after incubation with 5-HT antiserum. These findings suggest that in addition to the well-described local intraspinal 5-HT system, there is also, as in higher vertebrates, a descending 5-HT projection from the brainstem which extends at least 20 segments into the spinal cord.

Animals↗