Modulatory effects of the GABAergic basal ganglia neurons on the PPN and the muscle tone inhibitory system in cats

Pedunculopontine tegmental nucleus (PPN) contributes to the control muscle tone by modulating the activities of pontomedullary reticulospinal systems during wakefulness and rapid eye movement (REM) sleep. The PPN receives GABAergic projection from the substantia nigra pars reticulata (SNr), an output nucleus of the basal ganglia. Here we examined how GABAergic SNr-PPN projection controls the activity of the pontomedullary reticulospinal tract that constitutes muscle tone inhibitory system. Intracellular recording was made from 121 motoneurons in the lumbosacral segments in decerebrate cats (n=14). Short train pulses of stimuli (3 pulses with 5 ms intervals, 10-40 mA) applied to the PPN, where cholinergic neurons were densely distributed, evoked eye movements toward to the contralateral direction and bilaterally suppressed extensor muscle activities. The identical PPN stimulation induced IPSPs, which had a peak latency of 40-50 ms with a duration of 40-50 ms, in extensor and flexor motoneurons. The late-latency IPSPs were mediated by chloride ions. Microinjection of atropine sulfate (20 mM, 0.25 ml) into the pontine reticular formation (PRF) reduced the amplitude of the IPSPs. Although conditioning stimuli applied to the SNr (40-60 mA and 100 Hz) alone did not induce any postsynaptic effects on motoneurons, it reduced the amplitude of the PPN-induced IPSPs. Subsequent injection of bicuculline (5 mM, 0.25 ml) into the PPN blocked the SNr effects. Microinjections of NMDA (5 mM, 0.25 ml) and muscimol (5 mM, 0.25 ml) into the SNr reduced and increased the amplitude of the PPN-induced IPSPs, respectively. These results suggest that GABAergic basal ganglia output controls postural muscle tone by modulating the activity of cholinergic PPN neurons which activate the muscle tone inhibitory system. The SNr-PPN projection may contribute to not only control of muscle tone during movements in wakefulness but also modulation of muscular atonia of REM sleep. Dysfunction of the SNr-PPN projection may therefore be involved in sleep disturbances in basal ganglia disorders.     

Neural regulation of slow-wave frequency in the murine gastric antrum

Gastric peristaltic contractions are driven by electrical slow waves modulated by neural and humoral inputs. Excitatory neural input comes primarily from cholinergic motor neurons, but ACh causes depolarization and chronotropic effects that might disrupt the normal proximal-to-distal spread of gastric slow waves. We used intracellular electrical recording techniques to study cholinergic responses in stomach tissues from wild-type and W/W(V) mice. Electrical field stimulation (5 Hz) enhanced slow-wave frequency. These effects were abolished by atropine and the muscarinic M(3)-receptor antagonist 4-diphenylacetoxy-N-methylpiperidine methiodide. ACh released from nerves did not depolarize antral muscles. At higher rates of stimulation (10 Hz), chronotropic effects were mediated by ACh and a noncholinergic transmitter and blocked by muscarinic antagonists and neurokinin (NK(1) and NK(2))-receptor antagonists. Neostigmine enhanced slow-wave frequency, suggesting that the frequency of antral pacemakers is kept low by efficient metabolism of ACh. Neostigmine had no effect on slow-wave frequency in muscles of W/W(v) mice, which lack intramuscular interstitial cells of Cajal (ICC-IM). These muscles also showed no significant chronotropic response to 5-Hz electrical field stimulation or the cholinergic agonist carbachol. The data suggest that the chronotropic effects of cholinergic nerve stimulation occur via ICC-IM in the murine stomach. The capacity of gastric muscles to metabolize ACh released from enteric motor neurons contributes to the maintenance of the proximal-to-distal slow-wave frequency gradient in the murine stomach. ICC-IM play a critical role in neural regulation of gastric motility, and ICC-IM become the dominant pacemaker cells during sustained cholinergic drive.     

Characterization of pancreas-projecting rat dorsal motor nucleus of vagus neurons

The electrophysiological and morphological properties of rat dorsal motor nucleus of the vagus (DMV) neurons innervating the pancreas were examined by using whole cell patch clamp recordings from brain stem slices and postfixation morphological reconstructions of Neurobiotin-filled neurons. Recordings were made from 178 DMV neurons whose projections had been identified by previous apposition of the fluorescent neuronal tracer DiI to the body of the pancreas. DMV neurons projecting to the pancreas had an input resistance of 434 +/- 14 M omega, an action potential duration of 3 +/- 0.1 ms, and an afterhyperpolarization of 18 +/- 0.4 mV amplitude and 108 +/- 7 ms time constant of decay; these electrophysiological properties resembled those of gastric-projecting neurons but were significantly different from those of intestinal-projecting neurons. Interestingly, 14 of 178 pancreas-projecting neurons showed the presence of a slowly developing afterhyperpolarization whose presence was not reported in DMV neurons projecting to any other gastrointestinal area. The morphological characteristics of pancreas-projecting neurons (soma area 274 +/- 12 microm2; soma diameter of 25 +/- 0.7 microm; soma form factor 0.74 +/- 0.01; segments 9.7 +/- 0.41), however, were similar to those of intestinal- but differed from those of gastric-projecting neurons. In summary, these results suggest that pancreas-projecting rat DMV neurons are heterogeneous with respect to some electrophysiological and morphological properties. These differences might underlie functional differences in the vagal modulation of pancreatic functions.     

Change in muscarinic modulation of transmitter release in the rat urinary bladder after spinal cord injury

Muscarinic facilitation of 14C-ACh release from post-ganglionic parasympathetic nerve terminals was studied in bladder strips prepared from spinal intact (SI) and spinal cord transected (SCT) rats. The spinal cord was transected at the lower thoracic spinal segments 3 weeks prior to the experiments. Using non-facilitatory stimulation (2 Hz) the release of ACh in spinal intact rats did not change in the presence of a non-specific muscarinic antagonist, atropine (100 nM), an M(1) specific antagonist (pirenzepine, 50 nM) or an M(1)-M(3) specific antagonist (4-DAMP, 5 nM). However, during a facilitatory stimulation paradigm (10 Hz or 40 Hz, 100 shocks) atropine and pirenzepine, but not 4-DAMP inhibited the release of ACh in bladders from spinal intact rats, indicating an M(1) receptor-mediated facilitation. In spinal cord transected rats, 2 Hz stimulation-induced release was significantly inhibited by atropine or 4-DAMP but not by pirenzepine indicating that a pre-junctional facilitatory mechanism mediated via M(3) muscarinic receptors could be induced by a non-facilitatory stimulation paradigm after spinal injury. In bladders of spinal cord transected rats, 10 Hz stimulation-evoked release of ACh was also inhibited by atropine and 4-DAMP (5 nM) but not by pirenzepine (50 nM). These results indicate that pre-junctional muscarinic receptors at cholinergic nerve endings in the bladder change after chronic spinal cord injury. It appears that low affinity M(1) muscarinic receptors are replaced by high affinity M(3) receptors. This change in modulation of ACh release may partly explain the bladder hyperactivity after chronic spinal cord injury.     

Inhibition of Muscarinic-Coupled Phosphoinositide Hydrolysis by N-Methyl-d-Aspartate Is Dependent on Depolarization via Channel Activation

The intent of this work was to elucidate the mechanism by which N-methyl-D-aspartate (NMDA) receptor agonists inhibit a second messenger system, namely, the stimulation of phosphoinositide (PI) hydrolysis activated by muscarinic cholinergic receptor agonists. NMDA inhibited cholinergic stimulation of PI hydrolysis in a dose- and time-dependent manner. NMDA exerts this effect indirectly through channel activation, because both MK-801 and N-[1-(2-thienyl)cyclohexyl]piperidine (TCP) prevented this action. Prevention of the NMDA effect by removal of sodium, but not calcium, from the incubation buffer suggested that depolarization may be the responsible mechanism. Depolarization alone proved sufficient to inhibit cholinergic activation of PI hydrolysis, because both veratridine and an elevated extracellular potassium level inhibited cholinergic stimulation of PI hydrolysis. The effect of NMDA appeared to require sodium flux through NMDA channels rather than through voltage-dependent sodium channels, because tetrodotoxin failed to inhibit the effect of NMDA. In correlative electrophysiologic experiments, NMDA profoundly inhibited evoked excitatory postsynaptic potentials and population action potentials of CA1 neurons, an effect almost certainly due to depolarization. The dose and time course of the electrophysiologic effects correlated well with the biochemical effects. Taken together, the data support the assertion that NMDA receptor activation inhibits PI hydrolysis by depolarization mediated by sodium flux through NMDA channels.     

低频电刺激大鼠脚桥核对丘脑腹外侧核神经元自发放电的影响及其机制

本文旨在观察低频电刺激脚桥核(pedunculopontine nucleus,PPN)对帕金森病(Parkinson’s disease,PD)模型大鼠丘脑腹外侧核(ventrolateral thalamic nucleus,VL)神经元自发放电活动的影响,以探讨低频电刺激PPN改善PD症状的作用机制。通过纹状体内注射6-羟多巴胺制备PD大鼠模型。采用在体细胞外记录、电刺激及微电泳方法,观察低频电刺激PPN、微电泳乙酰胆碱(acetylcholine,ACh)及其M型受体阻断剂阿托品(atropine,ATR)、γ-氨基丁酸(γ-aminobutyric acid,GABA)及其A型受体阻断剂荷包牡丹碱(bicuculline,BIC)对大鼠VL神经元放电频率的影响。结果显示,低频电刺激PPN可使正常大鼠和PD大鼠VL神经元自发放电频率增加。微电泳ACh对VL神经元具有兴奋和抑制两种作用,而微电泳ATR则主要抑制VL神经元,即使对被ACh抑制的神经元也产生抑制作用。微电泳GABA抑制VL神经元,而微电泳BIC则兴奋VL神经元。另外,在微电泳ACh的过程中微电泳GABA,被ACh兴奋或抑制的VL神经元放电频...     

The functional role of the pedunculopontine nucleus in the regulation of the electrical activity of entopeduncular neurons in the rat

The purpose of this study was to investigate the influence of the pedunculopontine nucleus (PPN) on the electrical activity of entopeduncular nucleus (EP) in the rat and to analyze the influence of the subthalamic nucleus (STN) on the PPN-evoked responses of EP cells. Most of the EP neurons recorded (65.1%) were identified electrophysiologically as output cells projecting to the lateral habenula while only a minority (3.8%) were output cells to the PPN. Stimulation of the PPN in intact rats caused a short-latency (2.5 +/- 2.0, S.D. ms) activation in 22.6% and suppression of activity in 8.5% of EP neurons recorded. The mean impulse rate of EP neurons in intact rats was 27.0 +/- 5.5, S.D. imp./s and the overall mean interspike interval 36.8 +/- 7.1, S.D. ms. In rats where the PPN had been destroyed 10-12 days before recording by a local microinjection of kainic acid only a few EP neurons were still responsive to stimulation of the PPN showing suppression of activity. In these rats the kainate lesion slowed the impulse spontaneous activity to 14.3 +/- 6.3, S.D. imp./s and markedly altered the distribution of interspike intervals in 62.5% of the EP neurons recorded. The overall mean interspike interval in this group of deregulated neurons was 68.2 +/- 20.1, S.D. ms. A small kainate lesion of the STN placed 4-5 days before recording, on the other hand, did not affect the spontaneous activity of EP cells but increased the percentage of cells which were activated (43.6%) by stimulating the PPN. The present data demonstrate a predominant activatory influence of the PPN on EP cells and suggest that destruction of the STN may affect the responsiveness of entopeduncular cells to stimulation of the PPN possibly through the removal of a tonic inhibitory STN influence on the EP.     

Muscarinic potassium channels augment dynamic and static heart rate responses to vagal stimulation

Vagal control of heart rate (HR) is mediated by direct and indirect actions of ACh. Direct action of ACh activates the muscarinic K(+) (K(ACh)) channels, whereas indirect action inhibits adenylyl cyclase. The role of the K(ACh) channels in the overall picture of vagal HR control remains to be elucidated. We examined the role of the K(ACh) channels in the transfer characteristics of the HR response to vagal stimulation. In nine anesthetized sinoaortic-denerved and vagotomized rabbits, the vagal nerve was stimulated with a binary white-noise signal (0-10 Hz) for examination of the dynamic characteristic and in a step-wise manner (5, 10, 15, and 20 Hz/min) for examination of the static characteristic. The dynamic transfer function from vagal stimulation to HR approximated a first-order, low-pass filter with a lag time. Tertiapin, a selective K(ACh) channel blocker (30 nmol/kg iv), significantly decreased the dynamic gain from 5.0 +/- 1.2 to 2.0 +/- 0.6 (mean +/- SD) beats.min(-1).Hz(-1) (P < 0.01) and the corner frequency from 0.25 +/- 0.03 to 0.06 +/- 0.01 Hz (P < 0.01) without changing the lag time (0.37 +/- 0.04 vs. 0.39 +/- 0.05 s). Moreover, tertiapin significantly attenuated the vagal stimulation-induced HR decrease by 46 +/- 21, 58 +/- 18, 65 +/- 15, and 68 +/- 11% at stimulus frequencies of 5, 10, 15, and 20 Hz, respectively. We conclude that K(ACh) channels contribute to a rapid HR change and to a larger decrease in the steady-state HR in response to more potent tonic vagal stimulation.     

A primary response with an initial negative wave

In the anterosuperior and posterosuperior portions of the ectosylvian gyrus of dogs small foci were found in which clicks evoked responses with parameters close to those of primary responses (PRs), but with an initial negative wave. An analysis of PRs with an initial wave of different polarity was carried out by studying the reproducibility of the response during an increase in the frequency of stimulation and during the action of various drugs. PRs with an initial negative ("negative" focus) and positive (auditory area AI) waves were found to be reproduced when the frequency of stimulation was 20–30 Hz, whereas the negative phase of the classical PR disappeared when the stimulation frequency reached 10–15 Hz. The polarity of the response in these foci was unchanged after injection of a lethal dose of nembutal, but the negative phase of the classical PR disappeared during moderately deep anesthesia. Strychnine, on the other hand, considerably increased the amplitude of the negative wave in the positive-negative complex, but the initial negative potential was only very slightly and temporarily increased, and it was lost in the subsequent strychnine spike. GABA inhibited both the PR with initial negative wave and also the negative phase of the classical PR. The results suggest that PRs with different polarities of their initial wave differ in origin. The results of experiments with GABA indicate that PRs with an initial negative wave arise through excitation of apical dendrites.A. A. Bogomolets' Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 2, No. 5, pp. 488–496, September–October, 1970.     

蟾蜍脊神经节神经元对外周重复刺激的反应

本工作用细胞内记录技术,研究并分析了蟾蜍离体脊神经节神经元对重复刺激其外周突(坐骨神经)的反应。所记录的66个神经元的传导速度,刺激阈值和静息膜电位分別为5.3—20.0m/s,0.02—0.10mA 和-50—-80mV。随着重复刺激频率的增加,脊神经节神经元的细胞内动作电位进行性地出现潜伏期动摇或延迟、振幅降低、后超极化减弱和时程延长。与此同时,锋电位分解成 S、NM 和 M 三种亚波成分,并进而出现脱失。S、NM 和 M 成分对刺激频率的跟随能力为 S相似文献   

铃蟾肽介导的豚鼠肠系膜下神经节非胆碱能迟慢兴奋性突触后电位

应用细胞内记录技术,对铃蟾肽(bombesin,BOM)在豚鼠离体肠系膜下神经节(inferior mesenteric ganglion,IMG)非胆碱能兴奋性突触传递中的作用进行了研究。重复电刺激突触前结肠神经,有74．3％(52／70)IMG细胞可诱发迟慢兴奋性突触后电位(ls-EPSP)。在可引出ls-EPSP的细胞中,22％(4／18)细胞同时对BOM和SP敏感。用BOM持续灌流IMG,可明显抑制对BOM敏感细胞的ls-EPSP,对BOM不敏感细胞的ls-EPSP则无影响,且BOM受体与SP受体间无交叉脱敏。BOM受体阻断剂tyr^4[D-phe^12]bombesin能明显可逆性地抑制BOM敏感细胞的ls-EPSP和去极化,但对BOM不敏感细胞则无影响。研究结果提示,BOM可能是介导豚鼠IMG细胞ls-EPSP的一种递质。     

Effects of various kinds of stimulation on ornithine decarboxylase activity in superior cervical sympathetic and nodose ganglia of rats

Levels of cyclic nucleotides and ornithine decarboxylase (ODC) activity were examined following the application of various kinds of stimuli to superior cervical sympathetic ganglia (SCG), nodose ganglia, and vagus nerve fibers excised from the rat. The level of cyclic GMP in the SCG rose rapidly to about 4.5- to 7.5-fold the unstimulated control with 10 min of incubation after applications of preganglionic electrical stimulation (10 Hz), acetylcholine (ACh; 1 mM), or high extracellular K+ ( [K+]0, 70 mM). The cyclic GMP level in nodose ganglia was increased less than in the SCG by either ACh or high [K+]0 but was not affected by ACh in vagus fibers. Cyclic AMP in the SCG was also increased about 4- to 5.5-fold over the control within 10 min with the addition of ACh, norepinephrine (NE; 0.05 mM), or high [K+]0. Although NE caused a small increase in cyclic AMP, neither ACh nor high [K+]0 produced any appreciable change in nodose ganglia or vagus fibers. The ODC activity in the SCG was increased by preganglionic stimulation of 3- to 4-hr duration but not by a shorter period. A similar change in ODC activity was caused by the addition of oxotremorine (1 mM), isoproterenol (0.1 mM), NE, cyclic AMP (1 mM), or dibutyryl cyclic GMP (1 mM). The effect was exaggerated by the further addition of 3-isobutyl-1-methylxanthine (IBMX), a phosphodiesterase inhibitor. The increase in ODC activity caused by ACh was abolished by a muscarinic cholinergic antagonist, atropine (0.01 mM), and following axotomy for a week, but not by a nicotinic antagonist or by denervation in the SCG. A similar increase in ganglionic ODC activity by NE was inhibited by an adrenergic blocker, propranolol (0.01 mM), and following axtotomy for a week, but not by denervation. Cholinergic or adrenergic stimulation did not cause an increase in ODC activity in nodose ganglia or vagus fibers. These results suggest that the stimulation-induced increase in ODC activity occurs in postganglionic neurons rather than in satellite glial cells and is mediated by muscarinic cholinergic or adrenergic receptors. The process appears to involve cyclic nucleotide-mediated protein biosynthesis in the SCG.     

Cobalt injections into the pedunculopontine nuclei attenuate the reflex diaphragmatic responses to muscle contraction in rats.

Previous studies have suggested that neurons in the pedunculopontine nucleus (PPN) are activated during static muscle contraction. Furthermore, activation of the PPN, via electrical stimulation or chemical disinhibition, is associated with increases in respiratory activity observed via diaphragmatic electromyogram recordings. The present experiments address the potential for PPN involvement in the regulation of the reflex diaphragmatic responses to muscle contraction in chloralose-urethane anesthetized rats. Diaphragmatic responses to unilateral static hindlimb muscle contraction, evoked via electrical stimulation of the tibial nerve, were recorded before and subsequent to bilateral microinjections of a synaptic blockade agent (CoCl2) into the PPN. The peak reflex increases in respiratory frequency (9.0 +/- 1.0 breaths/min) and minute integrated diaphragmatic electromyogram activity (14.6 +/- 3.3 units/min) were attenuated after microinjection of CoCl2 into the PPN (2.6 +/- 0.9 breaths/min and 4.6 +/- 2.1 units/min, respectively). Consistent diaphragmatic responses were observed in the subset of animals that were barodenervated. Control experiments suggest no effects of PPN synaptic blockade on the cardiovascular responses to muscle contraction. The results are discussed in terms of a potential role for the PPN in modulation of the reflex respiratory adjustments that accompany muscular activity.     

Background and reflex activity of guinea pig caudal mesenteric ganglion neurons

The background activity of the guinea pig caudal mesenteric ganglion (CMG) neurons and their reflex reactions to colonic distension were studied on isolated combined preparations including the CMG and a colon segment connected with the lumbar colonic nerves. In the control, 62% of the neurons under study generated background activity, which consisted of irregular or regular “fast” excitatory postsynaptic potentials (fEPSP) and action potentials (AP). In 27% of the CMG neurons called “pacemaker-like neurons” (PLN), the background activity was represented by highly regular AP never observed in the CMG completely isolated from the distal colon. Reflex responses evoked by colonic distension were recorded from 76% of the units studied. The distension evoked fEPSP and AP in “silent” neurons and increased the background activity. Both the background activity and reflex responses were shown to be due to nicotinic cholinergic transmission. In some neurons, reflex responses (regular AP) were generated as superimposed on a slow depolarization; the latter was insensitive to nicotinic antagonists and either sensitive or insensitive to muscarinic antagonists. It has been concluded that CMG neurons receive nicotinic, muscarinic, and, probably, peptidergic afferent inputs from the distal colon. Although there are no true pacemaker neurons in CMG, some neurons generate pacemaker-like activity of a synaptic origin.     

Stimulation characteristics that determine arteriolar dilation in skeletal muscle

To determine the skeletal muscle stimulation parameters that are most important in establishing vasodilation in the microvasculature, I tested whether arteriolar diameter during 2 min of repetitive, short-duration, tetanic skeletal muscle contractions increased with changes in stimulus frequency, stimulation train duration, and contraction frequency. To test this, the diameter of transverse arterioles approximately perpendicular to small bundles of cremaster muscle fibers in situ of anesthetized Golden Syrian hamsters was used as a bioassay system. Arteriolar diameter was measured before and during different stimulation patterns that consisted of a contraction frequency [6, 12, or 24 contractions per minute (cpm)], a stimulation train duration (250, 500, or 750 ms) and a stimulus frequency (4, 8, 10, 15, 20, 30, 40, 60, and 80 Hz). The magnitude of the dilation significantly increased with stimulus frequency but not in a simple linear manner. The average rate of increase was 0.32 +/- 0.02 microm/Hz from 4 to 20 Hz and 0.09 +/- 0.02 microm/Hz from 30 to 80 Hz. The magnitude of the dilation increased significantly with the contraction frequency where the dilation at 6 cpm was significantly smaller than the dilation at 24 cpm across all stimulus frequencies. Changing the train duration from 250 to 750 ms did not significantly affect the magnitude of the dilation. These observations suggest that stimulation parameters are important in determining the magnitude of the microvascular dilation and that the magnitude of the dilation was dependent on both the contraction frequency and stimulus frequency but was independent of train duration.     

Stimulation of ciliary beat frequency by autonomic agonists: in vivo

beta 2-Adrenergic bronchodilator and muscarinic cholinergic bronchoconstrictor agonists both stimulate ciliary activity in vitro. To test the hypothesis that increases in autonomic activity would result in increases in ciliary beat frequency (CBF) in vivo, a correlation analysis heterodyne laser light-scattering system was developed and validated to measure the stimulating effects of sympathomimetic and parasympathomimetic agonists on tracheal CBF in intact, anesthetized beagles. The mean baseline CBF from 42 studies of 274 measurements in 9 (5 male and 4 female) adult beagles was 6.6 +/- 1.1 Hz. The stimulating effects of a beta 2-adrenergic agonist, fenoterol, and a muscarinic cholinergic agonist, methacholine, on CBF were studied on four and eight beagles, respectively. The studies were randomized and blinded. Aerosolized 10(-5) M fenoterol stimulated the CBF from the base line of 6.8 +/- 2.5 to 32.0 +/- 17.9 Hz in four dogs. Aerosolized methacholine stimulated the CBF from the base line of 5.8 +/- 0.7 to 9.4 +/- 3.0 Hz for 10(-8) M, and to 12.6 +/- 3.1 Hz for 10(-6) M in eight dogs. These are the first data obtained in intact animals that demonstrate CBF in the lower respiratory tract is regulated by autonomic agonists.     

In vivo electrophysiological responses of pedunculopontine neurons to static muscle contraction

The pedunculopontine nucleus (PPN) has previously been implicated in central command regulation of the cardiorespiratory adjustments that accompany exercise. The current study was executed to begin to address the potential role of the PPN in the regulation of cardiorespiratory adjustments evoked by muscle contraction. Extracellular single-unit recording was employed to document the responses of PPN neurons during static muscle contraction. Sixty-four percent (20/31) of neurons sampled from the PPN responded to static muscle contraction with increases in firing rate. Furthermore, muscle contraction-responsive neurons in the PPN were unresponsive to brief periods of hypotension but were markedly activated during chemical disinhibition of the caudal hypothalamus. A separate sample of PPN neurons was found to be moderately activated during systemic hypoxia. Chemical disinhibition of the PPN was found to markedly increase respiratory drive. These findings suggest that the PPN may be involved in modulating respiratory adjustments that accompany muscle contraction and that PPN neurons may have the capacity to synthesize muscle reflex and central command influences.     

The Drosophila larval visual system: high-resolution analysis of a simple visual neuropil

The task of the visual system is to translate light into neuronal encoded information. This translation of photons into neuronal signals is achieved by photoreceptor neurons (PRs), specialized sensory neurons, located in the eye. Upon perception of light the PRs will send a signal to target neurons, which represent a first station of visual processing. Increasing complexity of visual processing stems from the number of distinct PR subtypes and their various types of target neurons that are contacted. The visual system of the fruit fly larva represents a simple visual system (larval optic neuropil, LON) that consists of 12 PRs falling into two classes: blue-senstive PRs expressing Rhodopsin 5 (Rh5) and green-sensitive PRs expressing Rhodopsin 6 (Rh6). These afferents contact a small number of target neurons, including optic lobe pioneers (OLPs) and lateral clock neurons (LNs). We combine the use of genetic markers to label both PR subtypes and the distinct, identifiable sets of target neurons with a serial EM reconstruction to generate a high-resolution map of the larval optic neuropil. We find that the larval optic neuropil shows a clear bipartite organization consisting of one domain innervated by PRs and one devoid of PR axons. The topology of PR projections, in particular the relationship between Rh5 and Rh6 afferents, is maintained from the nerve entering the brain to the axon terminals. The target neurons can be subdivided according to neurotransmitter or neuropeptide they use as well as the location within the brain. We further track the larval optic neuropil through development from first larval instar to its location in the adult brain as the accessory medulla.     

Cholinergic facilitation of trace eyeblink conditioning in aging rabbits

The hippocampus is importantly involved in learning and memory, and is severely impacted by aging. In in vitro hippocampal slices, both the post-burst afterhyperpolarization (AHP) and spike-frequency accommodation are reduced in hippocampal pyramidal neurons after hippocampally-dependent trace eyeblink conditioning, indications of increased cellular excitability. The AHP results from the activation of outward potassium currents, including sI(AHP) and muscarine-sensitive I(M). The AHP is significantly increased in aging hippocampal neurons, potentially contributing to age-associated learning deficits. Compounds which reduce the AHP and spike-frequency accommodation could facilitate learning in normal aging or in age-associated dementias such as Alzheimer's disease. The cholinesterase inhibitor metrifonate enhances trace eyeblink conditioning by aging rabbits and reduces the AHP and accommodation in hippocampal CA1 neurons in a dose-dependent manner. These reductions are mediated by muscarinic cholinergic transmission as they are blocked by atropine. Hippocampal neurons from metrifonate treated but behaviorally naive rabbits were more excitable and not desensitized to the effects of metrifonate since the AHP and accommodation were further reduced when metrifonate was bath applied to the neurons. These observations suggest that the facilitating effect of chronic metrifonate on acquisition of hippocampally dependent tasks is mediated at least partially by increasing the baseline excitability of CA1 pyramidal neurons. The issue of whether learning can be facilitated with muscarinic cholinergic agonists, in addition to cholinesterase inhibitors, was addressed by training aging rabbits during intravenous treatment with the M1 agonist CI1017. A dose-dependent enhancement of acquisition was observed, with rabbits receiving 1.0 or 5.0 mg/ml CI1017 showing comparably improved learning rates as those receiving 0.5 mg/ml or vehicle. Sympathetic side effects, mainly excess salivation, were seen with the 5.0 mg/ml dose. Post-training evaluations suggested that the effective doses of CI1017 were enhancing responsivity to the tone conditioned stimulus. These studies suggest that muscarinic cholinergic neurotransmission is importantly involved in associative learning; that learning in aging animals may be facilitated by enhancing cholinergic transmission; and that the facilitation may be mediated through actions on hippocampal neurons.     

Pontine reticular origin of cholinergic excitatory afferents to the locus coeruleus controlling the gain of vestibulospinal and cervicospinal reflexes in decerebrate cats

1. Previous experiments had shown that the medullary inhibitory reticulospinal (mRS) neurons act 180 degrees out-of-phase with respect to the excitatory vestibulospinal (VS) neurons during the vestibular and the neck reflexes involving the limb extensor motoneurons. This finding suggested that the higher the firing rate of the medullary inhibitory RS neurons in the animal at rest, the greater the disinhibition which affects the limb extensor motoneurons during side-down roll tilt of the animal or side-up neck rotation, thus leading to an increased gain of response of limb extensors to sinusoidal stimulation of labyrinth and neck receptors. The gain of these postural reflexes would then represent a sensitive test to evaluate the background discharge of the inhibitory reticulospinal system of the medulla. 2. The discharge of the inhibitory mRS neurons is under the tonic excitatory control of cholinergic pontine reticular formation (pRF) neurons which are also self-excitatory, while these cholinergic pontine neurons are in turn inhibited by the norepinephrine (NE)-containing locus coeruleus (LC) neurons, which are also self-inhibitory due to mechanisms of recurrent and/or lateral inhibition. The present experiments were performed to find out whether cholinergic and cholinoceptive pontine reticular neurons, which are under the inhibitory control of the LC neurons, also send axons to the LC on which they may exert an excitatory influence. This excitatory effect would then counteract the self-inhibitory influence mediated by the NE, which acts on the alpha 2-adrenoceptors distributed on the somatodendritic membrane of the LC neurons. 3. In precollicular decerebrate cats, local injection into the dorsal aspect of the pontine tegmentum of 0.25 microliter of a solution of the muscarinic blocker atropine sulphate at the concentration of 6 micrograms/microliter of sterile saline did neither modify the postural activity in the ipsilateral limbs nor the response gain of the ipsilateral forelimb extensor triceps brachii to sinusoidal stimulation of labyrinth receptors (roll tilt of the animal at 0.15 Hz, +/- 10 degrees). These negative results were attributed to the fact that in these preparations the activity of the cholinergic and cholinoceptive pRF neurons and the related inhibitory mRS neurons is very low, due to the tonic discharge of the NE-containing LC neurons, which exert a prominent inhibitory influence on the underlying reticular structures.(ABSTRACT TRUNCATED AT 400 WORDS)