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John P Horn

Publications and source records attributed to John P Horn.

8 recordsLinked to original sources
PubMed2006

Different subcellular distributions of the vesicular monoamine transporter, VMAT2, in subclasses of sympathetic neurons.

A subpopulation of neurons in the rat superior cervical ganglion (SCG) was found to lack immunostaining for VMAT2, an isoform of the vesicular monoamine transporter that loads catecholamines into vesicles for release at the synapse. Double labeling with neuropeptide Y (NPY), a marker for vasomotor neurons, revealed selective cellular colocalization of NPY together with intense perinuclear staining for VMAT2. This implied that VMAT2-negative neurons were likely to have secretomotor and pilomotor phenotypes. We tested this by identifying peripheral noradrenergic axons by their expression of immunoreactivity for tyrosine hydroxylase (TH) and determining whether they also expressed NPY and VMAT2. This analysis revealed the presence of VMAT2-positive, non-vasomotor sympathetic axons in the submandibular gland and at the base of piloerector hairs. Together the results confirm earlier indications that virtually all sympathetic neurons in the rat SCG express VMAT2 and they show for the first time that functional subclasses of cells can be distinguished by different somatic levels of immunoreactivity for VMAT2.

Animals↗
PubMed2006

Excitatory muscarinic modulation strengthens virtual nicotinic synapses on sympathetic neurons and thereby enhances synaptic gain.

Acetylcholine excites many neuronal types by binding to postsynaptic m1-muscarinic receptors that signal to ion channels through the G(q/11) protein. To investigate the functional significance of this metabotropic pathway in sympathetic ganglia, we studied how muscarinic excitation modulated the integration of virtual nicotinic excitatory postsynaptic potentials (EPSPs) created in dissociated bullfrog B-type sympathetic neurons with the dynamic-clamp technique. Muscarine (1 muM) strengthened the impact of virtual synapses by reducing the artificial nicotinic conductance required to reach the postsynaptic firing threshold from 20.9 +/- 5.4 to 13.1 +/- 3.1 nS. Consequently, postganglionic action potential output increased by 4-215% when driven by different patterns of virtual presynaptic activity that were chosen to reflect the range of physiological firing rates and convergence levels seen in amphibian and mammalian sympathetic ganglia. In addition to inhibiting the M-type K(+) conductance, muscarine activated a leak conductance in three of 37 cells. When this leak conductance was reproduced with the dynamic clamp, it also acted to strengthen virtual nicotinic synapses and enhance postganglionic spike output. Combining pharmacological M-conductance suppression with virtual leak activation, at resting potentials between -50 and -55 mV, produced synergistic strengthening of nicotinic synapses and an increase in the integrated postganglionic spike output. Together, these results reveal how muscarinic activation of a branched metabotropic pathway can enhance integration of fast EPSPs by modulating their effective strength. The results also support the hypothesis that muscarinic synapses permit faster and more accurate feedback control of autonomic behaviors by generating gain through synaptic amplification in sympathetic ganglia.

Animals↗
PubMed2006

D2 autoreceptors chronically enhance dopamine neuron pacemaker activity.

Activation of D2 autoreceptors on midbrain dopamine neurons has been shown previously to acutely open K+ channels to inhibit intrinsically generated pacemaker activity. Here we report that D2 autoreceptors act chronically to produce an opposite action: to increase the speed and regularity of repetitive action potential firing. Voltage-, current-, and dynamic-clamp experiments, using conventional whole-cell and perforated patch-clamp recording, with cultured rat midbrain dopamine neurons show that a change in the number of functional A-type K+ channels alters firing rate and susceptibility to irregularity produced by other channels. cAMP and protein kinase A mediate the long-term action of D2 receptors in a manner that counters the short-term effect of this signaling pathway on K+ channel gating. We conclude that D2 autoreceptors, in addition to mediating acute negative feedback, are responsible for long-term enhancement of the rate and fidelity of dopamine neuron pacemaker activity.

Action Potentials↗
PubMed2005

Rostro-caudal variations in neuronal size reflect the topography of cellular phenotypes in the rat superior cervical sympathetic ganglion.

The mammalian superior cervical ganglion (SCG) contains a complex mixture of neuronal phenotypes that selectively innervate different peripheral targets. The present study examined the rostro-caudal topography of sympathetic phenotypes in the rat SCG by analyzing the relation between cell position, size, and the expression of immunoreactivity for neuropeptide Y (NPY), calretinin, and calcitonin gene-related peptide (CGRP). We observed that 64% of SCG neurons expressed NPY and had an average diameter of approximately 24 microm throughout the ganglion. Previous studies indicate that most of these cells are vasoconstrictor in function. By contrast, the size of NPY-negative neurons varied from approximately 25 microm in the rostral ganglion near the internal carotid nerve to approximately 30 microm in the caudal ganglion between the external carotid nerve and cervical sympathetic trunk. Many of the large NPY-negative neurons in the caudal ganglion were surrounded by dense axonal baskets that were immunoreactive for calretinin and therefore are likely to be secretomotor neurons projecting to salivary glands. Consistent with earlier reports, the rostral ganglion contained low numbers of presumptive pupillomotor neurons, based on their expression of NPY and contact with fibers containing CGRP. The present results indicate that neuronal size may provide a useful aid to cellular identification, especially in the caudal ganglion, and they provide further evidence of a topographic organization within the mammalian SCG.

Analysis of Variance↗
PubMed2005

Physiological classification of sympathetic neurons in the rat superior cervical ganglion.

A new scheme is presented for identifying three sympathetic phenotypes in the rat superior cervical ganglion using electrophysiology and neuropeptide Y expression. Postganglionic compound action potentials recorded from the external and internal carotid nerves each contained two peaks, 1 and 2, with distinct preganglionic stimulus thresholds. Peak 2 in the external carotid response contained subpeaks 2a and 2b having a similar stimulus threshold. Neurons corresponding to peaks 1, 2a, and 2b were identified intracellularly by antidromic stimulation, graded preganglionic stimulation, injection with neurobiotin and immunostaining. Seventeen of 53 neurons studied this way had a low threshold for preganglionic stimulation of firing that corresponded to activation of extracellular peak 1. All low-threshold neurons were neuropeptide Y (NPY)-negative. The other 36 neurons had a high presynaptic stimulus threshold that corresponded to activation of extracellular peak 2, and 12 of these cells contained NPY. Together with other known features of ganglionic organization, the results indicate that low-threshold NPY-negative neurons are secretomotor cells projecting to salivary glands, that high-threshold NPY-negative neurons are pilomotor cells responsible for extracellular peak 2a, and that high-threshold, NPY-positive neurons are vasoconstrictor cells responsible for peak 2b. Secreto-, pilo-, and vasomotor neurons identified in this way had distinct axonal conduction velocities (0.52, 0.20, and 0.10 m/s) and diameters (33, 29, and 25 microm) but were indistinguishable in terms of preganglionic conduction velocities (0.30-0.34 m/s) and number of primary dendrites (8.4-8.6). The cell classification scheme presented here will allow future comparison of ganglionic integration in different sympathetic modalities.

Action Potentials↗
PubMed2004

Estimating use-dependent synaptic gain in autonomic ganglia by computational simulation and dynamic-clamp analysis.

Biological gain mechanisms regulate the sensitivity and dynamics of signaling pathways at the systemic, cellular, and molecular levels. In the sympathetic nervous system, gain in sensory-motor feedback loops is essential for homeostatic regulation of blood pressure and body temperature. This study shows how synaptic convergence and plasticity can interact to generate synaptic gain in autonomic ganglia and thereby enhance homeostatic control. Using a conductance-based computational model of an idealized sympathetic neuron, we simulated the postganglionic response to noisy patterns of presynaptic activity and found that a threefold amplification in postsynaptic spike output can arise in ganglia, depending on the number and strength of nicotinic synapses, the presynaptic firing rate, the extent of presynaptic facilitation, and the expression of muscarinic and peptidergic excitation. The simulations also showed that postsynaptic refractory periods serve to limit synaptic gain and alter postsynaptic spike timing. Synaptic gain was measured by stimulating dissociated bullfrog sympathetic neurons with 1-10 virtual synapses using a dynamic clamp. As in simulations, the threshold synaptic conductance for nicotinic excitation of firing was typically 10-15 nS, and synaptic gain increased with higher levels of nicotinic convergence. Unlike the model, gain in neurons sometimes declined during stimulation. This postsynaptic effect was partially blocked by 10 microM Cd2+, which inhibits voltage-dependent calcium currents. These results support a general model in which the circuit variations observed in parasympathetic and sympathetic ganglia, as well as other neural relays, can enable functional subsets of neurons to behave either as 1:1 relays, variable amplifiers, or switches.

Animals↗
PubMed2003

Implementation of a fast 16-Bit dynamic clamp using LabVIEW-RT.

The dynamic-clamp method provides a powerful electrophysiological tool for creating virtual ionic conductances in living cells and studying their influence on membrane potential. Here we describe G-clamp, a new way to implement a dynamic clamp using the real-time version of the Lab-VIEW programming environment together with a Windows host, an embedded microprocessor that runs a real-time operating system and a multifunction data-acquisition board. The software includes descriptions of a fast voltage-dependent sodium conductance, delayed rectifier, M-type and A-type potassium conductances, and a leak conductance. The system can also read synaptic conductance waveforms from preassembled data files. These virtual conductances can be reliably implemented at speeds < or =43 kHz while simultaneously saving two channels of data with 16-bit precision. G-clamp also includes utilities for measuring current-voltage relations, synaptic strength, and synaptic gain. Taking an approach built on a commercially available software/hardware platform has resulted in a system that is easy to assemble and upgrade. In addition, the graphical programming structure of LabVIEW should make it relatively easy for others to adapt G-clamp for new experimental applications.

Analog-Digital Conversion↗