Ionization-induced electron trapping in ultrarelativistic plasma wakes

The onset of trapping of electrons born inside a highly relativistic, 3D beam-driven plasma wake is investigated. Trapping occurs in the transition regions of a Li plasma confined by He gas. Li plasma electrons support the wake, and higher ionization potential He atoms are ionized as the beam is focused by Li ions and can be trapped. As the wake amplitude is increased, the onset of trapping is observed. Some electrons gain up to 7.6 GeV in a 30.5 cm plasma. The experimentally inferred trapping threshold is at a wake amplitude of 36 GV/m, in good agreement with an analytical model and PIC simulations.

Positron production by x rays emitted by betatron motion in a plasma wiggler

Positrons in the energy range of 3-30 MeV, produced by x rays emitted by betatron motion in a plasma wiggler of 28.5 GeV electrons from the SLAC accelerator, have been measured. The extremely high-strength plasma wiggler is an ion column induced by the electron beam as it propagates through and ionizes dense lithium vapor. X rays in the range of 1-50 MeV in a forward cone angle of 0.1 mrad collide with a 1.7 mm thick tungsten target to produce electron-positron pairs. The positron spectra are found to be strongly influenced by the plasma density and length as well as the electron bunch length. By characterizing the beam propagation in the ion column these influences are quantified and result in excellent agreement between the measured and calculated positron spectra.

Hose instability and wake generation by an intense electron beam in a self-ionized gas

The propagation of an intense relativistic electron beam through a gas that is self-ionized by the beam's space charge and wakefields is examined analytically and with 3D particle-in-cell simulations. Instability arises from the coupling between a beam and the offset plasma channel it creates when it is perturbed. The traditional electron hose instability in a preformed plasma is replaced with this slower growth instability depending on the radius of the ionization channel compared to the electron blowout radius. A new regime for hose stable plasma wakefield acceleration is suggested.

Multi-GeV energy gain in a plasma-wakefield accelerator

A plasma-wakefield accelerator has accelerated particles by over 2.7 GeV in a 10 cm long plasma module. A 28.5 GeV electron beam with 1.8 x 10(10) electrons is compressed to 20 microm longitudinally and focused to a transverse spot size of 10 microm at the entrance of a 10 cm long column of lithium vapor with density 2.8 x 10(17) atoms/cm3. The electron bunch fully ionizes the lithium vapor to create a plasma and then expels the plasma electrons. These electrons return one-half plasma period later driving a large amplitude plasma wake that in turn accelerates particles in the back of the bunch by more than 2.7 GeV.

Plasma wakefield acceleration in self-ionized gas or plasmas

Tunnel ionizing neutral gas with the self-field of a charged particle beam is explored as a possible way of creating plasma sources for a plasma wakefield accelerator [Bruhwiler et al., Phys. Plasmas (to be published)]. The optimal gas density for maximizing the plasma wakefield without preionized plasma is studied using the PIC simulation code OSIRIS [R. Hemker et al., in Proceeding of the Fifth IEEE Particle Accelerator Conference (IEEE, 1999), pp. 3672-3674]. To obtain wakefields comparable to the optimal preionized case, the gas density needs to be seven times higher than the plasma density in a typical preionized case. A physical explanation is given.

Cholinergic neurons of the dorsal pontine tegmentum projecting to the cerebellar vermal cortex of the kitten

Although the whole cerebellar cortex receives cholinergic afferents, the source of origin of this projection has been clarified only for some corticocerebellar regions. Experiments were performed in kittens to investigate whether the two major cholinergic groups of the brainstem, the pedunculopontine (PPT) and laterodorsal tegmental nuclei (LDT), contribute to the cholinergic innervation of the cerebellar cortex, in particular the vermal cortex. Tegmento-cerebellar projecting neurons were identified by injecting the retrograde tracer rhodamine-labeled latex microspheres in the lobules V to VII of the cerebellar vermis. Subsequently, some of these tegmento-cerebellar neurons were demonstrated to be cholinergic by using the immunohistochemical technique for choline acetyltransferase (ChAT). Only a small portion of the ChAT-positive tegmental neurons projected to the cerebellar vermis. However, among the whole population of the retrogradely labeled tegmental neurons about one third were cholinergic. These cholinergic tegmento-cerebellar neurons were located in the PPT, LDT, and also within the locus coeruleus (LC) complex, where noradrenergic neurons predominate. Since the LC complex sends noradrenergic afferents to the cerebellar cortex, it appears that the dorsal pontine area contributes to the tegmento-cerebellar projections not only with noradrenergic but also with cholinergic afferents. The physiological significance of this cholinergic projection to the cerebellar cortex has been discussed.

The neurochemical and behavioral effects of beta-amyloid peptide(25-35)

Beta-amyloid protein (A beta) fragments have been shown to be neurotoxic and/or enhance neuronal vulnerability when injected into the hippocampus. We investigated alterations in monoamine contents, including norepinephrine (NE), 5-HT and dopamine (DA) in the rat locus coeruleus (LC) one week following the injection of beta-amyloid peptide fragment 25-35 (beta (25-35)) into the left dorsal hippocampal areas CA1-3. A single treatment of beta (25-35) had no effect on any monoamine levels. Rats that received two treatments (separated by 7 days) revealed significant elevations in NE, 5-HT, and 5-HIAA as compared with the control group injected with ddH2O. However, these changes were observed in the LC on the contralateral side, whereas the injected side exhibited no significant change. These effects may result from an enhanced synthesis of NE by the contralateral LC neurons to compensate for the loss of tyrosine hydroxylase and accompanying recurrent inhibition in a small number of their population. In a second experiment, the influence of beta (25-35) on spatial learning was evaluated using a Morris water maze task. Rats received bilateral injections of beta (25-35) into hippocampal areas CA1-3. The results indicate that beta (25-35)-treated rats exhibited significantly longer latencies and swim distances to locate the submerged platform than did members of the control group.

Neuropathology of synthetic beta-amyloid peptide analogs in vivo

Considerable evidence exists demonstrating that beta-amyloid protein and its fragments 1-40 and 25-35 (beta (25-35)) are neurotoxic to cells in the rat hippocampus both in culture and in vivo. This neurotoxicity has been correlated to the aggregational state of the peptides. Previously we have shown that beta (25-35) produces a cavitational lesion in rat hippocampus and also reduces the enzyme or transmitter expressions in two subcortical structures whose axons project to the hippocampus: the locus coeruleus (LC) and the medial septum. In the present study, we further investigated the amino acid sequence that might be responsible for these effects. A series of synthetic peptide analogs of beta (25-35) with glycine substituted for serine, asparagine, lysine and methionine at positions 26, 27, 28 and 35, respectively, were injected at a 3 nmol dosage into the rat hippocampus once a week for 2 weeks. The damage to the hippocampus and immunohistochemistry of the LC and medial septum were examined 1 week following the second treatment. All of the synthetic peptides with glycine substitution produced damage to the hippocampal tissue. This damage was similar to that seen with beta (25-35). However, the reduction of enzyme expressions in the LC and medial septum was less from these substituted peptides than from that of beta (25-35). While beta (25-35) application resulted in a similar reduction of tyrosine hydroxylase (TH) and glutamate (Glu) immunoreactivities in the LC, only TH was significantly reduced in the substituted peptide groups. The least reduction of TH and Glu immunoreactivities in the LC was observed in rats treated with peptides in which glycine replaced either lysine or methionine. In the basal forebrain medial septum, the application of beta (25-35) resulted in a marked decrease in choline acetyltransferase (ChAT) immunoreactivity. This reduction was found to be less by each of the synthetic peptides. These results suggest that the biological activity of beta (25-35) is sensitive to changes in the primary structure of the peptide. Among the 4 amino acid residues examined, lysine and methionine at positions 28 and 35 appear to play more important roles in determining the action of beta (25-35).

Localization of glutamatergic neurons in the dorsolateral pontine tegmentum projecting to the spinal cord of the cat with a proposed role of glutamate on lumbar motoneuron activity

Glutamate is considered to be a major excitatory neurotransmitter in the central nervous system. The presence of glutamate-like immunoreactive neurons in the rodent locus coeruleus has been reported previously. In this study we used both immunohistochemical and electrophysiological techniques to answer two major questions: (1) Is there any glutamate-like immunoreactivity in the catecholaminergic coeruleospinal system of the cat? (2) What is the physiological role, if any, of glutamate in descending locus coeruleus control of spinal motoneurons? Following injections of rhodamine-labeled latex microspheres or Fast Blue into the seventh lumbar segment of the spinal cord of the cat, retrogradely labeled cells were found throughout the rostrocaudal extent of the dorsolateral pontine tegmentum. They were primarily observed in the nucleus locus coeruleus and the Kolliker-Fuse nucleus. Some labeled cells were also present in the nucleus subcoeruleus and, to a lesser extent, in the parabrachial nuclei. Data from immunohistochemical studies indicate that 86% of all dorsolateral pontine tegmentum neurons that project to the spinal cord contain glutamate-like immunoreactivity, and 77% co-contain both glutamate- and tyrosine hydroxylase-like immunoreactivity. Electrical stimulation (four pulses of 500 microseconds duration at 500 Hz; intensity = 50-200 microA) of the locus coeruleus, in decerebrate cats, consistently induced lumbar motoneuron discharges recordable ipsilaterally as ventral root responses. These motoneuronal responses were reversibly antagonized following chemical inactivation of noradrenergic locus coeruleus neurons by local infusion of the alpha 2-adrenergic agonist clonidine, suggesting the locus coeruleus neurons to be the main source of evoked ventral root responses. Additionally, the evoked ventral root responses were reversibly reduced by 34.20 +/- 4.45% (mean +/- S.E.M.) upon intraspinal injections of the non-N-methyl-D-aspartate receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, into the ventral horn of seventh lumbar spinal cord segment (three to four injections, 20 nmol in 0.2 microliter of 0.1 M Tris-buffered saline for each injection). Similar volumes of vehicle injections had no significant effect on the locus coeruleus-evoked ventral root responses. These ventral root responses were also partially blocked (62.30 +/- 11.76%) by intravenous administration of the alpha 1-adrenergic receptor antagonist prazosin (20 micrograms/kg). In the light of several anatomical reports of noradrenergic and glutamatergic terminals in close contact with spinal motoneurons, our present findings suggest that the locus coeruleus-evoked ventral root response probably involves the synaptic release of both norepinephrine and glutamate onto lumbar motoneurons.(ABSTRACT TRUNCATED AT 400 WORDS)

Anatomical evidence for the presence of glutamate or enkephalin in noradrenergic projection neurons of the locus coeruleus

This paper reviews the anatomical evidence for the presence of glutamate (GLU) in noradrenergic neurons of the nucleus locus coeruleus (LC) and adjacent nuclei in the dorsolateral pontine tegmentum (DLPT) that project to the spinal cord, cerebellum, or cerebral cortex. Additionally, the evidence for the existence of methionine-enkephalin (ENK) in noradrenergic neurons of the DLPT that project to the spinal cord of the cat is reviewed. In these studies, we have combined the retrograde transport of either Fast Blue (FB), rhodamine labeled latex microspheres (MS), or rhodamine labeled dextran and indirect immunofluorescence histochemistry to determine whether the neurons that contain tyrosine hydroxylase (TH) and project to these terminal fields also contain GLU or ENK. The neurons of the cat that project to the spinal cord, cerebellum, and neocortex were observed in the nucleus LC and Kölliker-Fuse (KF) nucleus. They were also present, to a lesser extent, in the nucleus subcoeruleus (SC) and nuclei parabrachialis medialis (PBM) and lateralis (PBL). In the rat the majority of the neurons that project to the neocortex and hippocampus were located in the nucleus LC. Our data revealed a major proportion of these neurons to be immunostained for both GLU and TH (cat, rat), or ENK and TH (cat). Functional implications of such colocalized neurochemicals within individual LC projection neurons are discussed.

Bulbospinal neurons of the cat that co-contain serotonin and methionine enkephalin

The present study utilizes a combined retrograde transport of Fast Blue (or rhodamine-labeled latex microspheres) and simultaneous immunofluorescence technique to demonstrate directly the coexistence of serotonin and methionine enkephalin in bulbospinal neurons of the cat. The bulbospinal neurons that immunostained for both serotonin and enkephalin were observed, without any distinct somatotopic organization, in the nuclei raphe pallidus, obscurus and magnus. They were also observed in the nucleus reticularis magnocellularis and the ventrolateral medulla (cell group B1/3). Among the bulbospinal neurons encountered within individual 5-HT-rich medullary nuclei, high proportions of these neurons co-containing serotonin and methionine enkephalin were evidenced in the nucleus raphe obscurus (64%) and nucleus raphe pallidus (56%), less so in cell group B1/3 (41%), nucleus raphe magnus (39%), and the nucleus reticularis magnocellularis (29%). Physiological significance of such a morphological substrate is discussed.

Hippocampal beta-amyloid reduces locus coeruleus glutamate and tyrosine hydroxylase

The effects of intrahippocampally injected beta-amyloid protein (beta-AP) on glutamate- (Glu) and tyrosine hydroxylase (TH)-like immunoreactivities in the neurons of the locus coeruleus (LC) were studied in rats. A synthetic peptide or the vehicle alone was injected into the hippocampus as controls. All injections were made once a week (two or three injections; 3 nmol in 2 microliters of distilled water). Fluorescent microspheres (either alone or with one of the peptides) were also injected into the hippocampus to identify coeruleo-hippocampal neurons. The results revealed cell loss in the hippocampus at the site near beta-AP or control peptide deposition. Furthermore, in beta-AP/microsphere injected animals, only 22.4% and 49.6% of hippocampal projection neurons contained Glu and TH, respectively, compared to 88.4% and 85.3% in the animals that received control peptide with microspheres. Our results suggest that beta-AP has an effect on noradrenergic cells whose axons project to the hippocampus. These effects may contribute to the TH cell loss in the LC of Alzheimer's brains.

Cotransmitter-mediated locus coeruleus action on motoneurons

This article reviews evidence for a direct noradrenergic projection from the dorsolateral pontine tegmentum (DLPT) to spinal motoneurons. The existence of this direct pathway was first inferred by the observation that antidromically evoked responses occur in single cells in the locus coeruleus (LC), a region within the DLPT, following electrical stimulation of the ventral horn of the lumbar spinal cord of the cat. We subsequently confirmed that there is a direct noradrenergic pathway from the LC and adjacent regions of the DLPT to the lumbar ventral horn using anatomical studies that combined retrograde tracing with immunohistochemical identification of neurotransmitters. These anatomical studies further revealed that many of the noradrenergic neurons in the LC and adjacent regions of the DLPT of the cat that send projections to the spinal cord ventral horn also contain colocalized glutamate (Glu) or enkephalin (ENK). Recent studies from our laboratory suggest that Glu and ENK may function as cotransmitters with norepinephrine (NE) in the descending pathway from the DLPT. Electrical stimulation of the LC evokes a depolarizing response in spinal motoneurons that is only partially blocked by alpha 1 adrenergic antagonists. In addition, NE mimicks only the slowly developing and not the fast component of LC-evoked depolarization. Furthermore, the depolarization evoked by LC stimulation is accompanied by a decrease in membrane resistance, whereas that evoked by NE is accompanied by an increased resistance. That Glu may be a second neurotransmitter involved in LC excitation of motoneurons is supported by our observation that the excitatory response evoked in spinal cord ventral roots by electrical stimulation of the LC is attenuated by a non-N-methyl-D-aspartate glutamatergic antagonist. ENK may participate as a cotransmitter with NE to mediate LC effects on lumbar monosynaptic reflex (MSR) amplitude. Electrical stimulation of the LC has a biphasic effect on MSR amplitude, facilitation followed by inhibition. Adrenergic antagonists block only the facilitator effect of LC stimulation on MSR amplitude, whereas the ENK antagonist naloxone reverses the inhibition. The chemical heterogeneity of the cat DLPT system and the differential responses of motoneurons to the individual cotransmitters help to explain the diversity of postsynaptic potentials that occur following LC stimuli.

Existence of glutamate in noradrenergic locus coeruleus neurons of rodents

This study distinguished three types of immunolabeled neurons in nucleus locus coeruleus (LC) of the rat and mouse: cells single labeled either for tyrosine hydroxylase-like immunoreactivity (TH-LI) or glutamate (Glu)-LI, and those double labeled for both antigens. Although the double labeled neurons tend to be located in the middle and ventral thirds of the rat LC nucleus, throughout its rostrocaudal extent, such feature was not apparent in the mouse. Quantitatively a majority of neurons cocontaining TH- and Glu-LI were commonly observed in the rat (62%) and mouse (77%) LC. Additional studies utilizing the combined retrograde and immunohistochemical labeling revealed that such a high incidence of coexistence of the TH- and Glu-LI was also represented by coeruleocortical neurons in the rat (69% and 75% of all ipsilateral and contralateral projection cells, respectively). A possible role of coeruleocortical neurons involvement in Glu- and norepinephrine-mediated target neuron dysfunction is discussed.

Immunohistochemical localization of glutamate-containing neurons in the lateral reticular nucleus projecting to the cerebellar vermis in the kitten

The lateral reticular nucleus (LRN) and afferents to the cerebellum are known to contain glutamate-like immunoreactive (Glu-LI) neurons and axons, respectively. However, such a direct link between the Glu-LI LRN neurons and the cerebellar vermal cortex has not been identified. In this study we have combined the retrograde transport of rhodamine labeled latex microspheres and immunofluorescence histochemistry to determine the locations of Glu-LI neurons of the kitten reticulocerebellar system. Following microsphere injections into the cerebellar vermis (lobules V-VII), retrogradely labeled neurons were encountered throughout the rostrocaudal extent of the LRN. More than 60% (n = 3 kittens) of these retrogradely labeled neurons were immunostained for Glu-like immunoreactivity. Our observations of the Glu-like immunoreactivity in a majority of the reticulocerebellar neurons suggest that Glu in these neurons may participate in LRN's control of target neuron activities in the cerebellar vermis of kittens.

Opioid action on spinal cord reflexes due to dorsolateral pontine tegmentum stimulation

Electrical stimulation of the dorsolateral pontine tegmentum (DLPT) produces phasic facilitatory and inhibitory actions on the lumbar spinal monosynaptic reflexes (MSRs) of both flexor and extensor muscle nerves in the decerebrate cat. Naloxone, an opioid receptor antagonist, given intravenously or intraspinally enhanced the DLPT-induced potentiation of MSRs in most of the reflexes studied. However, systemic naloxone had no significant effect on the unconditioned MSR of the spinal cord. Intraspinal microinjections of naloxone significantly attenuated the DLPT-induced inhibition of MSRs of both flexors and extensors, similar to the action of systemic injection of naloxone, indicating a direct opioid action at the spinal ventral horn level upon DLPT stimulation. Results of the present experiment further support the anatomical finding that there are pontospinal enkephalinergic pathways in the cat, and indicate that these descending pathways modulate spinal motor outflow.

Application of compiled BASIC in developing software for collection and analysis of neuronal firing frequency data

Two programs are described which use an IBM-AT compatible personal computer, equipped with an analog-to-digital converter, to collect and analyze electrophysiological data. The first program is used to determine neuron firing rates during intracellular experiments and to save these results on disk. The second program is used to perform off-line analysis of the frequency data. Both programs are written entirely in the well known BASIC language and the Microsoft QuickBASIC compiler is employed for their use. All of the necessary hardware can be purchased commercially. In this paper emphasis is placed on the strategies and limitations involved when this high-level language is applied to tasks often needed in data acquisition and analysis. Both on- and off-line collection schemes are considered. This software is available from the authors.

Effects of temperature and analyte application technique on neuron-based chemical sensing

Results are presented which enhance the field of neuron-based sensing by providing insight on the effects of operating temperature and analyte application technique (pulse versus back-mixed) on sensing properties. In these studies, serotonin sensing attributes of giant visceral neurons VV1 and VV2 from the pond snail Lymnea stagnalis were measured. Experiments using a rapid fluid-exchange system reveal a concentration-dependent increase in maximum firing frequency similar to that reported earlier for a slow well-mixed application. With a rapid application, however, the maximum firing frequency is reached more quickly, and there is less cell-to-cell variability in both the maximum response and sensitivity. Given an application technique, an increase in temperature causes an increase in sensitivity and maximum firing frequency, as well as a decrease in the time required for the response to return to baseline following removal of the analyte. To provide insights on the kinetics of the serotonin-induced response, the effects of temperature and concentration on the rates of activation, recovery and desensitization were examined in detail. In general, it was found that an increase in temperature increases the rates of activation and desensitization, while the effects on recovery were not apparent. In addition, both the rates of activation and desensitization have a direct dependence on concentration while the rate of recovery has an inverse dependence.

Immunohistochemical evidence for coexistence of methionine-enkephalin and tyrosine hydroxylase in neurons of the locus coeruleus complex projecting to the spinal cord of the cat

Previous studies have revealed the presence of pontospinal neurons with either methionine-enkephalin- or tyrosine hydroxylase-like immunoreactivity in the dorsolateral pontine tegmentum of the cat. Using a combined fast blue retrograde transport technique and simultaneous immunofluorescence histochemistry, the present study was designed to reveal the coexistence of enkephalin and tyrosine hydroxylase in cat coerulospinal neurons and to determine if and to what extent the coerulospinal pathway is heterogeneous. Fast blue-labelled neurons with tyrosine hydroxylase- and enkephalin-like immunoreactivities were found in the nucleus locus coeruleus, nucleus subcoeruleus, Kölliker-Fuse nucleus, and the medial and lateral parabrachial nuclei. Approximately 87% of tyrosine hydroxylase-like immunoreactive neurons had enkephalin-like immunoreactivity, whereas about 76% of the enkephalin-like immunoreactive neurons had tyrosine hydroxylase-like immunoreactivity. About 71% of all coerulospinal neurons exhibited both tyrosine hydroxylase- and enkephalin-like immunoreactivities. These findings indicate that coerulospinal activity may lead to spinal cord effects reflecting both norepinephrine and enkephalin activity in most cases but do not rule out each transmitter's isolated functions.

Optimization of glass microelectrode properties by response surface methodology

Glass microelectrodes filled with electrolyte solutions are standard tools for electrophysiological studies. However, for any given application, there are limitations to the properties of the microelectrode, such as impedance and shank length, that can yield satisfactory results. The trial and error approach in pulling electrodes with the desired properties can be time consuming. The use of a response surface procedure which allows the experimenter to change more than one factor at a time and therefore determine the desired puller condition more efficiently is demonstrated. Also, design improvements for the World Precision Instrument, Model PUL-1, Microelectrode puller, used in this study are suggested.

Coerulospinal cells containing neuropeptide Y in the cat

Spinally projecting neuropeptide Y (NPY)-immunoreactive cells were sought in the feline locus coeruleus (LC) nuclear complex after horseradish peroxidase (HRP) injection into the lumbar cord; HRP injection was followed by intracerebroventricular colchicine administration. Our results revealed that a significant number (approximately 20% of all descending cells from the LC complex) of spinally projecting NPY-immunoreactive neurons arise from the LC alpha, the subcoeruleus and the Kölliker-Fuse nuclei. Other nonspinally projecting NPY-containing cells were also evident in the laterodorsal tegmental nucleus and the LCd, in addition to those occurring in the aforementioned LC nuclear complex.

Locus coeruleus control of spinal motor output

Using electrophysiological techniques, we investigated the functional properties of the coeruleospinal system for regulating the somatomotor outflow at lumbar cord levels. Many of the fast-conducting, antidromically activated coeruleospinal units were shown to exhibit the alpha 2-receptor response common to noradrenergic locus coeruleus (LC) neurons. Electrically activating the coeruleospinal system potentiated the lumbar monosynaptic reflex and depolarized hindlimb flexor and extensor motoneurons via an alpha 1-receptor mechanism. The latter synaptically induced membrane depolarization was mimicked by norepinephrine applied iontophoretically to motoneurons. That LC inhibited Renshaw cell activity and induced a positive dorsal root potential at the lumbar cord also reinforced LC's action on motor excitation. We conclude that LC augments the somatomotor output, at least in part, via an alpha 1-adrenoceptor-mediated excitation of ventral horn motoneurons. Such process is being strengthened by LC's suppression of the recurrent inhibition pathway as well as by its presynaptic facilitation of afferent impulse transmission at the spinal cord level.

Pontospinal transmitters and their distribution

The dorsolateral pontine tegmentum of the cat is known to contain a large population of catecholaminergic neurons. Additionally, several studies have also shown the presence of other neurochemicals (acetylcholine, enkephalin, neuropeptide Y, serotonin, somatostatin and substance P). In this study, we have employed retrograde transport of horseradish peroxidase in combination with immunocytochemistry to determine the locations of pontospinal neurons which contain catecholamine, enkephalin, neuropeptide Y, and serotonin. Furthermore, we have combined the retrograde transport of Fast Blue and immunofluorescence histochemistry to determine whether enkephalin-containing neurons are catecholaminergic. All pontospinal neurons, irrespective of the neurochemical content, were observed in the ventral and lateral parts of the dorsolateral pontine tegmentum at coronal levels P1.8-P4.0. These neurons were located in the nuclei locus coeruleus alpha and subcoeruleus and the Kölliker-Fuse nucleus. A high concentration of these neurons was evident in the Kölliker-Fuse nucleus when compared to the nuclei locus coeruleus alpha and subcoeruleus. Quantitative data have revealed that enkephalin is contained in a large proportion of the pontospinal catecholaminergic neurons (75%). The observations suggest that catecholaminergic neurons may contain one or more putative peptide neurotransmitters.

Norepinephrine effects on spinal motoneurons

Intracellular recordings from cat spinal motoneurons in situ demonstrated that microiontophoretic application of NE with low-intensity ejection currents produces a slowly developing, small-amplitude depolarization of the cells, in contrast to early reports of NE-induced hyperpolarization. This depolarization was associated with an increase in excitability of the cells and a decrease in membrane conductance. These observations are consistent with the hypothesis that NE reduces potassium conductance in spinal motoneurons as has been proposed for facial motoneurons (VanderMaelen and Aghajanian, 1980) and thalamic neurons (McCormick and Prince, 1988). The time course of the facilitatory effects of NE on cat motoneuron excitability recorded intracellularly agreed very closely with the time course of NE-induced facilitation of glutamate-evoked excitability in rat spinal motoneurons recorded extracellularly. The similarity of the observations in rats and cats suggests that NE functions generally to enhance mammalian motoneuron responsiveness to excitatory input.

Responses of locus coeruleus neurons to labyrinth and neck stimulation

The electrical activity of a large population of locus coeruleus (LC)-complex neurons, some of which were antidromically activated by stimulation of the spinal cord at T12-L1, was recorded in precollicular decerebrate cats during labyrinth and neck stimulation. Some of these neurons showed physiological characteristics attributed to norepinephrine (NE)-containing LC neurons, i.e., (i) a slow and regular resting discharge; (ii) a typical biphasic response to compression of the paws consisting of short impulse bursts followed by a silent period, which was attributed to recurrent and/or lateral inhibition of the corresponding neurons; and (iii) a suppression of the resting discharge during episodes of postural atonia, associated with rapid eye movements (REM), induced by systemic injection of an anticholinesterase, a finding which closely resembled that occurring in intact animals during desynchronized sleep. Among the neurons tested, 80 of 141 (i.e., 56.7%) responded to the labyrinth input elicited by sinusoidal tilt about the longitudinal axis of the whole animal at the standard parameters of 0.15 Hz, +/- 10 degrees, and 73 of 99 (i.e., 73.7%) responded to the neck input elicited by rotation of the body about the longitudinal axis at the same parameters, while maintaining the head stationary. A periodic modulation of firing rate of the units was observed during the sinusoidal stimuli. In particular, most of the LC-complex units were maximally excited during side-up tilt of the animal and side-down neck rotation, the response peak occurring with an average phase lead of about +17.9 degrees and +34.2 degrees with respect to the extreme animal and neck displacements, respectively. Similar results were also obtained from the antidromically identified coeruleospinal (CS) neurons. The degree of convergence and the modalities of interaction of vestibular and neck inputs on LC-complex neurons were also investigated. In addition to the results described above, the LC-complex neurons were also tested to changing parameters of stimulation. In particular, both static and dynamic components of single unit responses were elicited by increasing frequencies of animal tilt and neck rotation. Moreover, the relative stability of the phase angle of the responses evaluated with respect to the animal position in most of the units tested at increasing frequencies of tilt allowed the conclusion to attribute these responses to the properties of macular ultricular receptors. This conclusion is supported by the results of experiments showing that LC-complex neurons displayed steady changes in their discharge rate during static tilt of the animal.(ABSTRACT TRUNCATED AT 400 WORDS)

Tonic enhancement of the sensitivity of baroreceptor reflex response by endogenous substance P in the rat

Modulation of baroreceptor reflex (BRR) by endogenous substance P (SP) in the brain was investigated in rats anesthetized with pentobarbital sodium. Intracerebroventricular administration of the undecapeptide (15 or 30 nmol) and its antagonist, (D-Pro2, D-Trp7,9)-SP (30 or 60 nmol) or SP antiserum (1:20), respectively, promoted a significant increase and decrease in the sensitivity of BRR response. Prolonging the endogenous activity of SP with the aminopeptidase blocker, bestatin (200 nmol) or with the endopeptidase-24.11 inhibitor, phosphoramidon (200 nmol) significantly augmented the same reflex. Combining the undecapeptide with either peptidase blocker, moreover, promoted additional potentiation of the BRR response. On the other hand, simultaneous administration of bestatin and (D-Pro2, D-Trp7,9)-SP produced a reduction of the augmented effect of bestatin on the sensitivity of BRR response. Bilateral microinjection of SP (600 pmol) or an antiserum against SP (1:20) into the nucleus tractus solitarius (NTS) elicited respectively an enhancement of and reduction in the BRR response. These data suggest that neurons that contain SP may participate in central cardiovascular control by tonically enhancing the sensitivity of the BRR response, possibly via an action on the NTS.

Intraoperative monitoring of ischiatic nerve function with sensory evoked potentials

Serial recordings of sensory evoked potentials (SEP) generated in response to stimulation of each tibial nerve were obtained from 23 anesthetized dogs. Five dogs were anesthetized for 3 hours to evaluate changes in serial SEP during general anesthesia. Nonsurgical and surgical manipulations were performed on one hind limb of five dogs to determine the effects of limb positioning and nerve retraction on SEP. In 13 dogs, the ischiatic nerve was exposed surgically and retracted until the SEP deteriorated and disappeared, to determine the relationship between amount of tension on the nerve and the time to complete deterioration of the SEP. Sensory evoked potential waveforms, which consisted of two to five peaks, were stable throughout the anesthetic period. The first two peaks were the most stable. Latency of the first two peaks was the easiest and most reliable parameter to evaluate. Although the peak latency in recordings from the superior hind limb was always slightly longer, SEP recordings from the inferior limb were good controls to monitor nerve function. There was considerable variation in sensitivity to nerve retraction. The technique proved to be a reliable way to monitor nerve function in normal anesthetized dogs.

Tonic suppression of baroreceptor reflex by endogenous neurotensin in the rat

We evaluated the modulatory role of endogenous neurotensin (NT) in baroreceptor reflex (BRR) response in Sprague-Dawley rats anesthetized with pentobarbital sodium. Intracerebroventricular (i.c.v.) administration of NT (15 or 30 nmol) significantly reduced the sensitivity of the BRR response. Blocking the endogenous activity of the tridecapeptide with its specific antagonist, (D-Trp11)-NT (4 or 8 nmol) or antiserum against NT (1:20); or inhibiting the aminopeptidases with bestatin (200 nmol), on the other hand, promoted a potentiation of BRR response. When administered together with bestatin (200 nmol), the suppressive effect of NT (15 nmol) on the BRR response was further enhanced, as was the augmentative action of (D-Trp11)-NT (4 nmol). Upon microinjection into the bilateral nucleus tractus solitarius (NTS), NT (600 pmol) and (D-Trp11)-NT (150 pmol) respectively elicited a reduction and enhancement of the BRR response. These results suggest that neurons that contain NT may participate in central cardiovascular regulation by tonically suppressing the BRR, possibly via an action on the NTS where baroreceptor afferents terminate.

Localization of enkephalinergic neurons in the dorsolateral pontine tegmentum projecting to the spinal cord of the cat

The dorsolateral pontine tegmentum of the cat is known to contain enkephalinergic neurons, with most of the enkephalin co-contained in the catecholaminergic neurons; however, enkephalinergic cells projecting to the spinal cord have not been identified. This study employs retrograde transport of horseradish peroxidase in combination with methionine-enkephalin or tyrosine hydroxylase immunocytochemistry to 1) determine the locations of pontospinal enkephalinergic neurons and 2) compare these with the locations of pontospinal catecholaminergic neurons. Pontospinal enkephalinergic neurons were observed in the nuclei locus coeruleus and subcoeruleus and the Kölliker-Fuse nucleus. A high concentration of these neurons was evident in the Kölliker-Fuse nucleus when compared to the nuclei locus coeruleus and subcoeruleus (P less than .01). Both the enkephalinergic and catecholaminergic neurons projecting to the spinal cord were located in the same general areas of the dorsolateral pontine tegmentum and there was no significant difference in the mean diameters of these two neuronal types (P greater than .05). Quantitative data concerning the pontospinal enkephalinergic neurons correlated well with previous data on pontospinal catecholaminergic neurons (Reddy et al., Brain Res. 491:144-149, '89). A majority of the descending neurons from the dorsolateral pontine tegmentum contain enkephalin (72-80%) and catecholamine (80-87%). The observations suggest that enkephalin is contained in many of the pontospinal catecholaminergic neurons.

Evaluation of neuron-based sensing with the neurotransmitter serotonin

Results are presented on the development of a novel biosensor which will use neurons or neuronal components as both the recognition elements and primary transducers for analyte quantitation. This concept is demonstrated and evaluated by exposing identified neurons from the visceral ganglia of the pond snail Limnea stagnalis to the model analyte serotonin. Experiments reveal a reversible, concentration-dependent increase in the rate of spontaneous action potential generation, over a concentration range of four orders of magnitude. Studies with the antagonist methysergide verify that this response is mediated through serotonin-sensitive receptors. Exposure of the neurons to serotonin causes the firing frequency to rapidly increase to a maximum and then slowly diminish to a sub-optimal level. It was found that the maximum frequency provides an indication of chemical concentration that is repeatable. Data are also presented which further advance the field of neuronal biosensing by demonstrating both the effects of cell to cell variability on response reproducibility and the effects of the desensitizing response on the operation of a neuron-based sensor in both a continuous and discontinuous mode.

Responses of locus coeruleus and subcoeruleus neurons to sinusoidal stimulation of labyrinth receptors

In precollicular decerebrate cats the electrical activity of 141 individual neurons located in the locus coeruleus-complex, i.e. in the dorsal (n = 41) and ventral parts (n = 67) as well as in the locus subcoeruleus (n = 33), was recorded during sinusoidal tilt about the longitudinal axis of the whole animal, leading to stimulation of labyrinth receptors. Some of these neurons showed physiological characteristics attributed to the norepinephrine-containing locus coeruleus neurons, namely, (i) a slow and regular resting discharge, and (ii) a typical biphasic response to fore- and hindpaw compression consisting of short impulse bursts followed by a silent period, which has been attributed to recurrent and/or lateral inhibition of the norepinephrine-containing neurons. Furthermore, 16 out of the 141 neurons were activated antidromically by stimulation of the spinal cord at T12 and L1, thus being considered coeruleospinal or subcoeruleospinal neurons. A large number of tested neurons (80 out of 141, i.e. 56.7%) responded to animal rotation at the standard frequency of 0.15 Hz and at the peak amplitude of 10 degrees. However, the proportion of responsive neurons was higher in the locus subcoeruleus (72.7%) and the dorsal locus coeruleus (61.0%) than in the ventral locus coeruleus (46.3%). A periodic modulation of firing rate of the units was observed during the sinusoidal stimulus. In particular, 45 out of the 80 units (i.e. 56.2%) were excited during side-up and depressed during side-down tilt (beta-responses), whereas 20 of 80 units (i.e. 25.0%) showed the opposite behavior (alpha-responses). In both instances, the response peak occurred with an average phase lead of about + 18 degrees, with respect to the extreme side-up or side-down position of the animal; however, the response gain (imp./s per deg) was, on average, more than two-fold higher in the former than in the latter group. The remaining 15 units (i.e. 18.7%) showed a prominent phase shift of this response peak with respect to animal position. Similar results were obtained from the subpopulation of locus coeruleus-complex neurons which fired at a low rate (less than 5.0 imp./s), as well as for the antidromically identified coeruleospinal neurons. The response gain of locus coeruleus-complex neurons, including the coeruleospinal neurons, did not change when the peak amplitude of tilt was increased from 5 degrees to 20 degrees at the fixed frequency of 0.15 Hz. This indicates that the system was relatively linear with respect to the amplitude of displacement.(ABSTRACT TRUNCATED AT 400 WORDS)

Raphé-produced excitation of spinal cord motoneurons in the cat

In all motoneurons examined, both flexor and extensor, raphé (pallidus) stimulation consistently produced an excitatory postsynaptic potential (EPSP). Upon interaction with a group Ia EPSP there was summation resulting in neuronal discharge. The raphé-induced EPSP also facilitated the initial segment-somadendritic coupling and hence the motoneuron excitability. These data support an excitatory role for raphé-spinal fibers on cat hindlimb motoneurons.

Spinally projecting noradrenergic neurons of the dorsolateral pontine tegmentum: a combined immunocytochemical and retrograde labeling study

Two to three days following injection of horseradish peroxidase (HRP) into the spinal cords of 5 cats, the animals were sacrificed and perfused, and the brainstems removed and sectioned. The sections were then processed for HRP and, immunocytochemically, for tyrosine hydroxylase (TH). The dorsolateral pontine tegmentum was divided into the locus coeruleus, subcoeruleus and Kölliker-Fuse nucleus; the mean percentage of pontospinal neurons containing TH were found to be 85.5 +/- 2.5 (S.E.M.), 79.6 +/- 5.6 and 87.1 +/- 3.1, respectively. The cell diameters of locus coeruleus cells were also measured.

The actions of two monoamines on spinal motoneurons from stimulation of the locus coeruleus in the cat

The present study investigates the role of the two putative amine transmitters (norepinephrine and serotonin) in mediating the facilitatory action following locus coeruleus (LC) stimulation on hindlimb flexor and extensor monosynaptic reflexes (MSRs) in unanesthetized, decerebrate cats. When administered sequentially, in either order, methysergide (a serotonergic blocker) and prazosin (an alpha 1-adrenergic blocker) were observed to cause subtotal, decremental changes in the potentiation of gastrocnemius-soleus and common peroneal MSRs by stimuli applied in the LC. These changes were determined to be independent of the blood pressure changes induced by the aminergic blockers. These results support the hypothesis that the facilitation of the group Ia reflex transmission in cat spinal cord by stimulation of LC is mediated in part by alpha 1-noradrenergic and serotonergic mechanisms.

Convergence and interaction of neck and macular vestibular inputs on locus coeruleus and subcoeruleus neurons

Extracellular recordings were obtained in precollicular decerebrate cats from 90 neurons located in the noradrenergic area of the dorsal pontine tegmentum, namely in the dorsal (LCd, n = 24) and the ventral part (LC alpha, n = 40) of the locus coeruleus (LC) as well as in the locus subcoeruleus (SC, n = 26). Among these units of the LC complex, 13 were coerulospinal (CS) neurons antidromically identified following stimulation of the spinal cord at T12-L1. Some of these neurons showed the main physiological characteristics of the norepinephrine (NE)-containing LC neurons, i.e., a slow and regular resting discharge and a typical biphasic response to fore- and hindpaw compression consisting of a short burst of excitation followed by a period of quiescence, due, in part at least, to recurrent and/or lateral inhibition. Unit firing rate was analyzed under separate stimulation of macular vestibular, neck, or combined receptors by using sinusoidal rotation about the longitudinal axis at 0.15 Hz, +/- 10 degrees peak amplitude. Among the 90 LC-complex neurons, 60 (66.7%) responded with a periodic modulation of their firing rate to roll tilt of the animal and 67 (74.4%) responded to neck rotation. Convergence of macular and neck inputs was found in 52/90 (57.8%) LC-complex neurons; in these units, the gain and the sensitivity of the first harmonic of the response corresponded on the average to 0.34 +/- 0.45, SD, impulses.s-1.deg-1 and 3.55 +/- 2.82, SD, %/deg for the neck responses and to 0.23 +/- 0.29, SD, impulses.s-1.deg-1 and 3.13 +/- 3.04, SD, %/deg for the macular responses. In addition to these convergent units, 8/90 (8.9%) and 15/90 (16.7%) LC-complex units responded to selective stimulation either of macular or of neck receptors only. These units displayed a significantly lower response gain and sensitivity to animal tilt and neck rotation with respect to those obtained from convergent units. Most of the convergent LC-complex units were maximally excited by the direction of stimulus orientation, the first harmonic of responses showing an average phase lead of about +31.0 degrees with respect to neck position and +17.6 degrees with respect to animal position. Two populations of convergent neurons were observed. The first group of units (43/52, i.e., 82.7%) showed reciprocal ("out of phase") responses to the two inputs; moreover, most of these units were excited during side-down neck rotation, but inhibited during side-down animal tilt.(ABSTRACT TRUNCATED AT 400 WORDS)

Responses of locus coeruleus neurons to convergent neck and vestibular inputs

The locus coeruleus (LC) complex, located in the dorsolateral pontine tegmentum, is composed of noradrenergic and self-inhibitory adrenoceptive neurons, which project to broad regions of the brain, including the spinal cord. Experiments were performed in decerebrate cats to find out whether units which had the physiological characteristics of noradrenergic neurons (i.e., a slow and regular resting discharge and a typical response to a noxious stimulus consisting of a short burst of excitation followed by a silent period), received a convergent input from both labyrinth and neck receptors. Among 90 LC-complex units, 13 of which could be identified antidromically as coeruleospinal (CS) neurons following electrical stimulation of the spinal cord at T12-L1, 52 (57.8%) responded to roll tilt of the animal and neck rotation at 0.15 Hz, +/- 10 degrees. The responses were particularly related to the extreme animal and neck displacements. Most of these convergent neurons (43/52, i.e., 82.7%) showed reciprocal ('out of phase') responses to the two inputs, while only a few units (9/52, i.e., 17.3%) showed parallel ('in phase') responses. Moreover, the majority of the 'out of phase' units showed an increased discharge during side-up animal tilt and side-down neck rotation. These predominant response patterns were just opposite to those of the vestibulospinal (VS) neurons projecting to the same segments of the spinal cord. The response characteristics of the LC-complex neurons to combined neck and vestibular inputs elicited during head rotation usually corresponded to those predicted by a vectorial summation of the individual neck and labyrinth responses, as shown for the VS neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Responses of locus coeruleus and subcoeruleus neurons to sinusoidal neck rotation in decerebrate cat

The electrical activity of 99 neurons located in the locus coeruleus-complex, namely in the dorsal (n = 26) and the ventral part of the locus coeruleus (n = 46) as well as the locus subcoeruleus (n = 27), has been recorded in precollicular decerebrate cats during sinusoidal displacement of the neck. This was achieved by rotation of the body about the longitudinal axis of the animal, while maintaining the head stationary. A proportion of these neurons showed some of the main physiological characteristics attributed to the noradrenergic locus coeruleus neurons, i.e. (i) a slow and regular resting discharge, and (ii) a typical biphasic response to fore and hindpaw compression consisting of short bursts of impulses followed by a period of quiescence, due at least in part to recurrent or lateral inhibition of the corresponding neurons. Moreover, 14 out of the 99 neurons were activated antidromically by stimulation of the spinal cord at T12 and L1, thus being considered as coeruleo- or subcoeruleospinal neurons. Among these locus coeruleus-complex neurons tested, 73 out of 99 (i.e. 73.7%) responded to neck rotation at the standard frequency of 0.15 Hz and at the peak amplitude of displacement of 10 degrees. In particular 40 of 73 units (i.e. 54.8%) were excited during side-down neck rotation and depressed during side-up rotation, while 18 of 73 units (i.e. 24.7%) showed the opposite pattern. In both instances the peak of the responses occurred with an average phase lead of +34.2 degrees for the extreme side-down or side-up neck displacement; however, the response gain (impulses/s per deg) was on the average more than two-fold higher in the former than in the latter group of units. The remaining 15 units (i.e. 20.5%) showed phase angle values which were intermediate between those of the two main populations. As to the coeruleo or subcoeruleospinal neurons, 11 of 14 units (78.6%) responded to the neck input, the majority (nine of 11 units, i.e. 81.8%) being excited during side-down neck rotation. Within the explored region, the proportion of responsive units was higher in the locus subcoeruleus (85.2%) than in the locus coeruleus, both dorsal and ventral (69.4%). Moreover, units located in the former structure showed on the average a response gain higher than that found in the latter structures. Similar results were also obtained from the population of locus subcoeruleus-complex neurons which fired at a low rate (less than or equal to 5.0 impulses/s).(ABSTRACT TRUNCATED AT 400 WORDS)

Coerulospinal influence on recurrent inhibition of spinal motonuclei innervating antagonistic hindleg muscles of the cat

The locus coeruleus's (LC's) effect on recurrent inhibition of gastrocnemius-soleus (GS) and common peroneal (CP) monosynaptic reflexes (MSRs) was demonstrated to exceed the concomitant facilitation, indicating the independency of LC's disinhibition and facilitation measures in this study. In contrast, the disinhibition effect correlated closely with the recurrently inhibited MSRs. The disinhibition phenomenon was also accompanied by progressive delay and diminution in the Renshaw cell field potential. Hence, the recovery of recurrently inhibited MSRs was probably due, in part at least, to the LC's inhibition of the related Renshaw cell activity. Furthermore, the site-specific, discordant changes in the disinhibition of GS, compared with CP MSRs, as revealed by tracking studies imply that representations of these antagonistic motonuclei may occupy different LC loci. Accordingly, the nonuniform disinhibition may be due to the activation of discrete aggregates of LC neurons which are responsible predominantly in controlling the recurrent inhibitory pathway belonging to one or the other of the antagonistic motonuclei. These findings support a differential LC inhibitory control of Renshaw cell activity, releasing the related motoneurones for the Ia synaptic transmission - a disinhibitory process that is crucial for the LC's independent control of the recurrent circuit of antagonistics extensor and flexor motoneurons.

Cholinergic brainstem sites for gain control of vestibulospinal reflexes in cats

Decerebrate cats were injected with carbachol into the locus coeruleus (LC) or with carbachol or bethanechol into the dorsal pontine reticular formation (pRF) of one side; recordings were made of the tonic contraction of forelimb extensor muscles of both sides and of their responses to sinusoidal roll tilt of the animal. Both drugs had similar effects when injected into the pRF: a decrease in the tonic contraction of limb extensors and a greatly enhanced amplitude and gain with slightly decreased phase lead in the responses to animal tilt of the forelimb extensor, triceps brachii, ipsilateral to the side of injection. Injected into the LC, carbachol produced a response opposite to the above: it increased the tonic contraction of limb extensors ipsilateral to the side of injection, but decreased the amplitude and gain of the EMG responses of limb extensor muscles to labyrinth stimulation induced by sinusoidal tilt. These findings did not depend on changes in posture since they were still observed when postural EMG activity was maintained constant by appropriate changes in static stretch of the muscle. Moreover, the magnitude of the effects increased in a dose-dependent manner. Results suggest that cholinergic activation of dorsal pRF neurons through muscarinic receptors increases the background discharge of medullary inhibitory reticulospinal (RS) system neurons, thus increasing their modulatory influence. Further, it is postulated that cholinergic activation of LC neurons would cause them to inhibit this tonic facilitatory drive by the pRF. Common to both sites of carbachol injection is the increase in phase lag of the EMG response of limb extensors to animal tilt.(ABSTRACT TRUNCATED AT 250 WORDS)

Labyrinthine influences on locus coeruleus neurons

The locus coeruleus (LC) complex, located in the dorsolateral pontine tegmentum, is composed principally of noradrenergic neurons, which project to broad regions of the CNS, including the spinal cord. Experiments were performed in precollicular decerebrate cats to ascertain whether units histologically identified within the LC complex, and having the physiological characteristics of noradrenergic neurons, would respond to sinusoidal stimulation of labyrinth receptors. Among 141 LC complex neurons, 16 of which could be activated antidromically by stimulation of the spinal cord at T12-L1, 80 (i.e. 56.7%) responded to roll tilt of the animal at 0.15 Hz, +/- 10 degrees. The responses were particularly related to the extreme animal displacements, thus being attributed to stimulation of macular utricular receptors. The proportion of responsive units, and also the average gain of the responses, were higher in the LCd and the subcoerular (subLC) area than in the LCa. Moreover in the same structures the majority of units showed a beta-pattern of response (excitation during side-up tilt), which contrasted with the predominant alpha-pattern (excitation during side-down tilt) displayed by the previously recorded vestibulospinal neurons projecting to the same segments of the spinal cord. The role that the noradrenergic coeruleospinal neurons exert in the dynamic control of posture during the vestibulospinal reflexes is discussed.

Effects of bestatin on the central cardiovascular regulatory mechanisms in the rat

We evaluated the effects of bestatin, the specific aminopeptidase-B and leucine aminopeptidase inhibitor, on the central cardiovascular regulatory mechanisms in Sprague-Dawley rats anesthetized with pentobarbital sodium (40 mg/kg, i.p.). Intracerebroventricular injection of bestatin (100 or 200 nmol/5 microliters) consistently elevated the basal systemic arterial pressure and heart rate. At the same time, this degradative enzyme blocker increased the sensitivity of the baroreceptor reflex responses as well as the efficacy of the modulatory actions of the medullary nucleus reticularis gigantocellularis on these reflexes. We speculate that enhancing the tonic activities of the endogenous neuropeptides in the brain by protecting them from their catabolic enzymes may affect the central cardiovascular regulatory machinery by modifying the operations of the baroreceptor feedback controls and their modulatory mechanisms.

Coerulospinal enhancement of repetitive firing with correlative changes in postspike afterhyperpolarization of cat spinal motoneurons

The present study was aimed at determining if inputs from the locus coeruleus (LC) have any effect on repetitive firing of ventral horn motoneurons in cats. In hindlimb flexor and extensor motoneurons stimulated intrasomatically with current below the threshold for repetitive discharges, added LC-evoked excitatory post-synaptic potentials (EPSPs) were consistently found to produce repetitive firing, suggesting a lowering in the repetitive firing threshold attributable to excitatory LC inputs. With those extensor motoneurons showing episodic, self-sustained firing, LC-EPSPs introduced during the quiescent period were capable of starting a continuous discharge with rhythmic frequencies higher than the spontaneous activity. In some of these cells, intracellularly applied hyperpolarizing current was able to stop the spontaneous discharges. Subsequently, LC stimuli were found to reinitiate repetitive discharges, thus counteracting the ongoing suppression of the motoneurons. Quantitative analysis of the single-spike afterhyperpolarization (AHP) indicated a consistent reduction in its amplitude and time course (duration, time-to-peak, half-decay time) for flexor and extensor motoneurons in response to LC conditioning stimuli. Present results suggest an excitatory LC action on the repetitive discharges of cat motoneurons accompanied by a concurrent decrease in the amplitude and time course of the single-spike AHPs.

Presynaptic facilitatory action of locus coeruleus stimulation upon hindlimb sensory impulse transmission in decerebrate cats

This study sought to delineate the presynaptic role of the locus coeruleus (LC) on hindlimb primary afferent terminals. Changes in presynaptic function in response to LC stimulation were assessed by measuring the dorsal root potential (DRP), interaction of LC- and peripherally-evoked DRPs, and intraspinal afferent terminal excitability. LC stimulation in unanesthetized, decerebrate cats produced a sequence of early and late positive DRPs succeeded by a small-sized negative DRP. Conditioning the negative DRPs elicited from individual hindlimb nerve branches with LC stimuli led to a decrease in test DRPs. Similarly, there was a predominant decrease in excitability in both large muscle and cutaneous afferent terminals. These data suggest a presynaptic role of the LC in augmenting afferent impulse transmission, presumably through inhibition of tonically active interneurons having axoaxonic contacts on primary afferents; functionally, presynaptic facilitation.

Effects of microinjection of a cholinergic agonist into the locus coeruleus on the gain of vestibulospinal reflexes in decerebrate cats

1. Experiments were performed in precollicular decerebrate cats to determine whether activation of locus coeruleus (LC) neurons elicited by local injection of the cholinergic agonist carbachol modifies the dynamic characteristics of responses of forelimb extensors to selective stimulation of labyrinth receptors resulting from roll tilt of the animal. 2. Injection of 0.1-0.4 microliter (usually 0.25 microliter) of carbachol at a concentration of 0.02-0.1 micrograms/microliter of sterile saline into the LC of one side, which slightly increased the tonic contraction of limb extensors ipsilateral to the side of the injection, greatly decreased the amplitude of the multiunit EMG response of the ipsilateral triceps brachii to animal tilt at 0.15 Hz, +/- 10 degrees. Correspondingly, the response gain of this forelimb extensor decreased. Moreover, a significant increase in phase lag of the responses was observed. These findings did not result from the increased postural activity, since they were still observed when the limb position was adjusted so that the spontaneous EMG activity remained constant throughout the experiments. 3. The changes in posture as well as in response characteristics of the forelimb extensor to labyrinth stimulation produced by carbachol injection appeared a few min after the injection and soon reached a plateau level which persisted for several hours before returning to the control levels. 4. The effects described above involved mainly, if not exclusively, the limbs ipsilateral to the side of the injection. However, the effects of local injection into the LC of one side could be reproduced on the contralateral side following injection into the LC of that side. 5. The increase in phase lag of the multiunit EMG responses of the triceps brachii to labyrinth stimulation appeared at a threshold lower than that required to decrease the response gain of this extensor muscle. These findings suggest that different neuronal populations within the LC complex, one projecting directly to the spinal cord, the other projecting indirectly through the pontine reticular formation, are involved in the control of phase angle and gain of the vestibulospinal reflexes, respectively. However, as soon as the threshold was reached the effects described above were dose-dependent. 6. Histological controls indicated that the structure responsible for the postural and reflex changes described above corresponded to the LC. In fact, postural and reflex changes opposite in sign to those described above were obtained when the same amount of carbachol was injected into the dorsal aspect of the pontine reticular formation (pRF) located immediately ventral to the LC.(ABSTRACT TRUNCATED AT 400 WORDS)

Effects of microinjection of cholinergic agonists into the pontine reticular formation on the gain of vestibulospinal reflexes in decerebrate cats

1. The question of which pontine neuronal groups and related receptors can mediate the cholinergic induction of the increased gain of vestibulospinal reflexes elicited by sinusoidal stimulation of labyrinth receptors was investigated by injecting in precollicular decerebrate cats either carbachol, which is a mixed muscarinic-nicotinic agonist, or bethanechol, which is a pure muscarinic agonist, via a cannula stereotaxically oriented in different pontine tegmental structures. 2. Injection of 0.1-0.2 microliter of carbachol solution (0.01-0.2 microgram/microliter of sterile saline) into the dorsal aspect of the pontine reticular formation (pRF), which slightly decreased the tonic contraction of limb extensors ipsilateral to the side of the injection, greatly increased the amplitude of the multiunit EMG response of the ipsilateral triceps brachii to roll tilt of the animal at 0.15 Hz, +/- 10 degrees, leading to selective stimulation of labyrinth receptors. Correspondingly, the response gain of the forelimb extensor to labyrinth stimulation increased. Moreover, a slight decrease in phase lead of the responses was observed. These findings were not attributable to decreased postural activity, since they were still observed when postural EMG activity was reflexly maintained by an increased static stretch of the muscle. No changes in the dynamic characteristics of the responses were observed in the contralateral triceps brachii. 3. The changes in posture as well as in response gain produced by the carbachol injection appeared suddenly, but partially declined to reach a plateau level which persisted for several hours before returning to the control level. Moreover, the magnitude of the effects increased in relation to the dose of the cholinergic agonist. 4. Histological controls indicated that the structure responsible for these postural and reflex changes was located in the dorsal aspect of the pontine tegmentum immediately ventral to the principal locus coeruleus (LC); this area corresponds to the peri-LC region and the surrounding pRF including the dorsal aspect of the central tegmental field. The effects were still obtained after chronic kainic acid lesioning of the gigantocellular area of the medulla. 5. An increase in gain of the vestibulospinal reflex which was as potent, dose-dependent, and site-specific as that previously observed with carbachol, appeared after injection of the pure muscarinic agonist bethanechol.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotonergic and non-serotonergic raphe neurons projecting to the feline lumbar and cervical spinal cord: a quantitative horseradish peroxidase-immunocytochemical study

A quantitative study of raphe-spinal neurons and serotonergic neurons in 3 medullary raphe nuclei in the cat indicates that more than 80% of the raphe-spinal neurons that project to the spinal cord are serotonergic raphe-spinal neurons in each of the nuclei. More than 85% of the descending raphe-spinal neurons in the two caudal nuclei, nucleus raphe pallidus and nucleus raphe obscurus, are serotonergic, whereas 75% of the raphe-spinal neurons in the more rostrally placed nucleus raphe magnus contain serotonin (5-HT). These results are discussed in relation to descending systems containing both neuropeptides and 5-HT and collateralizing to several spinal segments.

Suppression of the recurrent inhibitory pathway in lumbar cord segments during locus coeruleus stimulation in cats

The present study revealed a consistent reduction of ventral root-induced recurrent inhibition of monosynaptic reflexes in both extensor (gastrocnemius-soleus) and flexor (common peroneal) motonuclei upon delivery of preconditioning stimuli to the locus coeruleus (LC) in decerebrate cats. The magnitudes of the LC-induced decrease in recurrent inhibition, functionally disinhibition, and of recurrent inhibition were significantly correlated. Direct recordings from Renshaw cells have indicated that LC conditioning stimuli cause a decrease in the number of spikes and the duration of firing in response to single antidromic (ventral root) volleys. These results suggest that the LC depression of the recurrent inhibitory pathway is attributed, in part at least, to inhibition of Renshaw cell activity.

Membrane excitability changes in hindlimb motoneurons induced by stimulation of the locus coeruleus in cats

The present analysis describes the cellular mechanisms underlying the heightened membrane excitability of hindlimb flexor and extensor motoneurons upon stimulation of the locus coeruleus (LC) in unanesthetized, decerebrate cats. In a total of 73 cells, brief train stimuli to the LC at 50-300 microA intensity evoked one of 4 patterns of motoneuronal responses: a simple excitatory postsynaptic potential (EPSP) with weak trailing depolarization, a double-peak EPSP, an EPSP succeeded by a weak hyperpolarization, or a slow rising EPSP. As the initial dominant EPSP was a consistent finding among all cells and the ensuing potentials were variable in polarity, quantitative characterization was focused on the initial EPSP only. In all cells tested (n = 11), the LC-EPSP was accompanied by a decrease in input resistance. The excitatory LC action was further demonstrated by the consistent (n = 25 cells) motoneuron rheobase decrease when the latter was measured coincident with the summit of an LC-EPSP. Furthermore, the time course of the single-spike afterhyperpolarization became shortened during the LC conditioning stimuli (n = 16 motoneurons). Our data show that the descending LC action on motoneurons is typified by an EPSP accompanied by a net decrease in input resistance as well as a concurrent increase in motoneuron electrical excitability.