Electrophysiological properties of pedunculopontine neurons and their postsynaptic responses following stimulation of substantia nigra reticulata

Voltage-gated ionic currents and their roles in timing coding in auditory neurons of the nucleus magnocellularis of the chick

Avian cochlear neurons of the nucleus magnocellularis (NMC) are known to encode temporal information of sound. The neuron generated only a single action potential at a stable timing even though suprathreshold currents of long duration (> 100 ms) was injected. The threshold for the action potential was -42 mV. In voltage-clamp experiments, a TTX-sensitive Na current was activated at membrane potentials more positive than -50 mV. A low voltage activated (LVA) Ca current and a high voltage activated (HVA) Ca current were observed. The LVA Ca current was activated from -65 mV and showed a voltage dependent inactivation. The HVA Ca current was activated from -40 mV and did not show any inactivation. The LVA Ca current and the HVA Ca current were sensitive to Ni2+ (0.1 mM) and Nifedipine (10-20 mM), respectively. NMC neurons showed a TEA-sensitive K current and a 4-AP-sensitive K current. With 4-AP (0.5 mM) in a bathing medium, the threshold of action potential was decreased to -49 mV and the timing of action potential generation showed a wider distribution than that of control. Ni2+ (0.1 mM) reversed effects of 4-AP on the threshold and the variability of action potential onsets. It is concluded that a 4-AP-sensitive current counteracts the LVA Ca current that facilitates Na spike generation, and sets a threshold to a higher level for generating a single action potential at a precise timing following synaptic inputs from the auditory nerve.

Quantitative morphology of physiologically identified and intracellularly labeled neurons from the guinea-pig laterodorsal tegmental nucleus in vitro

Mesopontine cholinergic neurons have been implicated in the initiation and maintenance of rapid eye movement sleep via their efferent connections to the thalamus and the medial pontine reticular formation. As a first step toward understanding how these modulatory neurons integrate synaptic input, we have investigated the dendritic architecture of laterodorsal tegmental nucleus neurons. The principal cells of the guinea-pig laterodorsal tegmental nucleus were identified electrophysiologically in a brain slice preparation, then were intracellularly injected with biocytin and reconstructed using a computer-aided tracing system. The somata were large (27 +/- 3 microns; n = 11) and gave rise to an average of 4.8 primary dendrites which, in most cases, emerged from the soma in a pattern that was radially symmetric in the plane of the slice. Primary dendrites had an average of 3.7 endings. A single axon arose from either the soma or a proximal dendrite and exited the nucleus with a medial and/or lateral trajectory. Some axons also gave rise to a local terminal plexus composed of fine fibers bearing numerous punctate swellings that ramified profusely within the neuron's dendritic field. Total dendritic area averaged about 10(5) microns2, and therefore the average contribution of the soma to the total surface area (20%) was significantly larger than the values reported for many other cell types. Dendritic diameters were non-uniform in three respects. Some processes were sparsely spiny. Most processes were varicose, with the degree of varicosity increasing substantially in secondary and tertiary dendritic segments. There was also a large degree of taper in dendritic processes; those processes with a non-negative taper had an average diameter decrease of 40 +/- 25%. Dendritic processes deviated from the criteria necessary for a Rall equivalent cylinder approximation due to non-uniformity in morphotonic path length, failure to conform to the Rall 3/2 branching rule and non-uniformity of dendritic diameter. An analysis was done to assess the impact of dendritic varicosities on the extraction of cable parameters for these cells. Voltage traces were simulated by solving the cable equation for a varicose dendrite and then membrane parameters were recovered using an equivalent cylinder model. Errors in the extracted values of specific membrane conductance and specific membrane capacitance were quite small (< or = 5%), while larger errors were seen for electrotonic length (< or = 21%) and intracellular resistivity (< or = 5%). These data indicate that the principal cells of the laterodorsal tegmental nucleus, while possessing a relatively simple dendritic structure in terms of number and branchiness of dendrites, display a heterogeneity of dendritic process types. Processes range from smooth to markedly varicose, and can be aspiny or sparsely spiny. The possibility that the dendritic varicosities function as sites of either electrical or chemical compartmentalization is discussed. The degree of error resulting from a Rall equivalent cylinder approximation in light of these varicosities indicated that a generalized cable model approach may prove more effective in estimating their cable parameters.

Cholinergic and noncholinergic tegmental pedunculopontine projection neurons in rats revealed by intracellular labeling

Morphological features of rat pedunculopontine projection neurons were investigated in in vitro preparation by using intracellular labeling with biocytin combined with choline acetyltransferase (ChAT) immunohistochemistry. These neurons were classified into two types (Type I and II), based on their electrical membrane properties: Type I had low-threshold Ca2+ spikes, and Type II had A-current. All Type I neurons (n = 17) were ChAT immunonegative (ChAT-). Type II neurons were either ChAT immunopositive (ChAT+; n = 49) or ChAT- (n = 20). In terms of topography in the tegmental pedunculopontine nucleus (PPN), Type I neurons were dispersed throughout the extent of the nucleus, whereas Type II neurons tended to be located more in the rostral and middle sections. Both Type I and II neurons consisted of small (long axis < 20 microns), medium (20-35 microns), and large (> 35 microns) cells. The small cells were round or oval; medium cells were round, triangular, or fusiform; and the large cells were primarily fusiform in shape. In terms of the soma size, there was a difference in Type I (15-38 microns) and Type II (11-50 microns) neurons, but no significant difference was found between Type II ChAT+ and ChAT- cells. Both types of neurons had three to six primary dendrites, but the dendritic field was more prominent in Type II neurons. Most of the axons originated from one of the primary dendrites, which gave off axon collaterals, some of which projected out of the nucleus. The intrinsic collaterals were thin and branched partly within the dendritic field of the parent cell. The extrinsic collaterals were thicker and could be grouped into three categories: 1) collaterals arborizing in the substantia nigra; 2) collaterals ascending mainly toward the thalamus, pretectal, and tectal area; and 3) collaterals descending toward the mesencephalic and/or pontine reticular formation. It was noted that the collaterals of both ChAT+ and ChAT-neurons were traced into the substantia nigra. There was no significant difference in antidromic latencies between Type I (m = 1.47 msec) and Type II (m = 1.36 msec) neurons following electrical stimulation of the substantia nigra.

Increased subthalamic neuronal activity after nigral dopaminergic lesion independent of disinhibition via the globus pallidus

Electrophysiological records of unit activity were used to compare the effects of excitotoxic pallidal lesions and 6-hydroxydopamine-induced damage to the midbrain dopaminergic neurons on the discharge rates and patterns of the subthalamic neurons. Removal of the pallidal input induced a slight, but statistically significant, increase (19.5%) in the discharge rate and no change in the firing pattern when compared to control animals. The rats with a dopaminergic lesion showed greater increase (105.7%) while the firing pattern activity of the subthalamic neurons became more irregular, with burst. These results indicate that the increased activity of the subthalamic neurons following a midbrain dopaminergic lesion cannot be due solely to inhibition-disinhibition involving the striato-pallido-subthalamic pathway and induced by the striatal dopaminergic depletion.

Ibotenic acid lesions of the superior colliculus produce longer lasting deficits in visual orienting behavior than aspiration lesions in the cat

We compared the effects of unilateral surgical aspiration and ibotenic acid produced lesions of the superior colliculus (SC) on visual orienting behavior in 20 cats. Four animals with aspiration lesions initially showed an hemianopia in the contralateral hemifield which recovered fully in 4.5 weeks or less. These lesions also destroyed axons in the commissure of the superior colliculus (CS). In 9 animals we produced complete loss of cells in one SC, with preservation of axons in the CSC, by injections of ibotenic acid. In these animals the contralateral hemianopia persisted for an average of 16.6 weeks, but may have persisted longer had we not intervened by either sacrificing the animal or ablating the visual cortex contralateral to the SC lesion. The cortical lesion produced an immediate hemianopia in the contralateral hemifield and a recovery in the previously hemianopic ('collicular') hemifield. In the remaining 7 animals with attempted ibotenic acid lesions, 5 had incomplete lesions and 2 others sustained major damage to the SC as well as the CSC. These 7 animals recovered visual orienting on an average of 3.0 weeks postoperatively. We conclude that unilateral loss of collicular cell function and the presence of fibers coursing through the commissure of the superior colliculus are both necessary for the prolonged deficit in visual orienting behavior. We suggest that competition between the two hemifields may play a role in the hemianopia caused by collicular manipulations and that the cholinergic pathway from the pedunculopontine nucleus to the contralateral SC via the CSC may be involved.

GABAergic neurons in the rat pontomesencephalic tegmentum: codistribution with cholinergic and other tegmental neurons projecting to the posterior lateral hypothalamus

The present study was undertaken to determine the frequency and distribution of GABAergic neurons within the rat pontomesencephalic tegmentum and the relationship of GABAergic cells to cholinergic and other tegmental neurons projecting to the hypothalamus. In sections immunostained for glutamic acid decarboxylase (GAD), large numbers of small GAD-positive neurons (approximately 50,000 cells) were distributed through the tegmentum and associated with a high density of GAD-positive varicosities surrounding both GAD-positive and GAD-negative cells. Through the reticular formation, ventral tegmentum, raphe nuclei, and dorsal tegmentum, GAD-positive cells were codistributed with larger cells, which included neurons immunostained on adjacent sections for glutamate, tyrosine hydroxylase (TH), serotonin, or choline acetyltransferase (ChAT). In sections dual-immunostained for GAD and ChAT, GABAergic neurons were seen to be intermingled with less numerous cholinergic cells (approximately 2,600 GAD+ to approximately 1,400 ChAT+ cells in the laterodorsal tegmental nucleus, LDTg). Retrograde transport of cholera toxin (CT) was examined from the posterior lateral hypothalamus, where a major population of cortically projecting neurons are located. A small number of GABAergic cells were retrogradely labeled, representing a small percentage of all the GABAergic neurons (approximately 1%) and of all the hypothalamically projecting neurons (approximately 6%) in the tegmentum. The double-labeled GAD+/CT+ cells were commonly found ipsilaterally within 1) the deep mesencephalic reticular field, codistributed with putative glutamatergic projection neurons; 2) the ventral tegmental area, substantia nigra compacta, and retrorubral field, codistributed with dopaminergic projection neurons; 3) dorsal raphe, codistributed with serotonergic projection neurons; and 4) laterodorsal and pedunculopontine tegmental nuclei, codistributed with and in similar proportion to cholinergic projection cells (20-30% in LDTg). Acting as both projection and local neurons, the pontomesencephalic GABAergic cells would have the capacity to modulate the influence of the "ascending reticular activating system" and its chemically specific constituents upon cortical activation.

Mesopontine cholinergic control over generalized non-convulsive seizures in a genetic model of absence epilepsy in the rat

Pharmacological data have shown that the cholinergic transmission participates in the control of spike-and-wave discharges in rats with genetic absence epilepsy. The corticothalamic circuitry which generates spontaneous spike-and-wave discharges, the electroencephalographic expression of absence seizures, receives important cholinergic inputs from two distinct sources: (i) the nucleus basalis projecting mainly to the cortex and (ii) the pedunculopontine and laterodorsal tegmental nuclei providing cholinergic afferents to the thalamus. In the present study, the involvement of the cholinergic mesopontothalamic projections in the control of spike-and-wave discharges was investigated. Activation of cell bodies in the pedunculopontine and laterodorsal tegmental nuclei, by local microinjections of non-toxic doses of kainate (20 pmol/side) or picrotoxin (66 pmol/side), suppressed spike-and-wave discharges. Similar effects were produced by direct cholinergic activation of the ventrolateral part of the thalamus: intrathalamic microinjections of carbachol (0.7-2.8 pmol/side), a cholinergic receptor agonist, resulted in a dose-dependent suppression of spike-and-wave discharges. This suppression was partially reversed by a simultaneous microinjection of an equimolar dose of scopolamine, a muscarinic receptor antagonist. Electrolytic or neuroexcitotoxic lesions of the pedunculopontine and laterodorsal tegmental nuclei did not modify spike-and-wave discharges. These results suggest that the cholinergic mesopontine projection to the thalamus exerts a phasic inhibitory control of generalized non-convulsive epileptic seizures.

An ultrastructural study of cholinergic and non-cholinergic neurons in the laterodorsal and pedunculopontine tegmental nuclei in the rat

Synaptic connectivity and other ultrastructural features of cholinergic and non-cholinergic neurons in the laterodorsal and pedunculopontine tegmental nuclei were investigated with electron microscopy combined with pre-embedding immunohistochemistry for choline acetyltransferase. Quantitative morphometric analyses were conducted on selected immunopositive as well as immunonegative neurons. The ultrastructure of immunoreactive neurons in the laterodorsal and pedunculopontine tegmental nuclei was similar. In both nuclei, immunoreactive neurons were among the larger neurons, and somatic areas of immunopositive neurons in single thin sections were larger than those of immunonegative neurons by an average of 40%. Immunopositive somata varied in shape, appearing polygonal, fusiform or oval. Regardless of immunoreactivity, however, neurons in the pedunculopontine nucleus tended to have more irregular shapes than those in the laterodorsal tegmental nucleus. Immunoreactive neurons in both the nuclei had abundant cytoplasmic organelles and a large, clear nucleus with a few infoldings. Usually, about a quarter of the surface of an immunopositive soma was covered with astrocytic processes, and some immunopositive somata were directly apposed to an astrocyte. Immunoreactive dendrites and, less frequently, axon terminals were seen in close apposition to endothelial cells of blood capillaries or pericytes. Immunoreactive somata and dendrites in the laterodorsal and pedunculopontine tegmental nuclei received many synapses, mainly from unlabelled axon terminals. The mean number (4.7 +/- 1.8) of synapses received by immunolabelled somata in single thin sections was greater, by about 70%, than those received by unlabelled somata. The presynaptic axon terminals synapsing with immunoreactive somata commonly contained small, round and clear vesicles, and 20% of them contained a few dense-cored vesicles as well. Immunoreactive dendrites, in addition, received synapses from unlabelled axon terminals containing flat and clear vesicles, which accounted for 15% of the synapses with immunoreactive dendrites. Many immunopositive axon terminals were present in both the tegmental nuclei. They contained clear round vesicles, and usually synapsed with unlabelled dendrites. A few immunolabelled axons, however, appeared to synapse with immunopositive somata and dendrites. Immunoreactive fibres were also present in both the tegmental nuclei. They were either thinly myelinated or unmyelinated. In conclusion, the ultrastructural morphology of cholinergic neurons in the laterodorsal and pedunculopontine tegmental nuclei is similar, and these neurons represent a distinct population of neurons in both nuclei in that they are larger and receive more synaptic contacts than non-cholinergic neurons. Cholinergic neurons, however, appear to receive synapses from cholinergic axon terminals only rarely, despite the abundance of cholinergic terminals in the tegmental nuclei.(ABSTRACT TRUNCATED AT 400 WORDS)

The pedunculopontine nucleus--auditory input, arousal and pathophysiology

This review describes the role of the pedunculopontine nucleus (PPN) in various functions, including sleep-wake mechanisms, arousal, locomotion and in several pathological conditions. Special emphasis is placed on the auditory input to the PPN and the possible role of this nucleus in the manifestation of the P1 middle latency auditory evoked response. The importance of these considerations is evident because the PPN is part of the cholinergic arm of the reticular activating system. As such, the auditory input to this region may modulate the level of arousal of the CNS and, consequently, abnormalities in the processing of this input can be expected to have serious consequences on the level of excitability of the CNS. The involvement of the PPN in such disorders as schizophrenia, anxiety disorder and narcolepsy is discussed.

Electrophysiology of the neurons in the area of the enkephalinergic magnocellular dorsal nucleus of the guinea-pig hypothalamus, studied by intracellular and whole-cell recordings

The electrophysiological characteristics of 103 hypothalamic neurons in the area of the guinea-pig enkephalinergic magnocellular dorsal nucleus were studied in a thick slice preparation with sharp microelectrodes (63 neurons) and patch pipettes for whole-cell recordings (40 neurons). Of the sampled cells, 79.6% displayed tetrodotoxin-resistant, calcium-dependent slow-depolarizing potentials when the membrane potential was hyperpolarized to approximately -70 mV (type I neurons). Half of them showed robust slow depolarizing potentials, generating bursts of fast action potentials. In the remaining neurons, the slow-depolarizing potentials did not cause burst-firing action potentials but triggered single action potentials. The other class of neurons (20.4% of the sample: type II neurons) did not exhibit calcium-dependent slow-depolarizing potentials. Resting potential, input resistance and the membrane time constant did not distinguish among the two classes of neurons. Current-voltage relationships were heterogeneous. A transient outward rectification was observed in the two classes. This was not totally blocked by 2 mM 4-aminopyridine but was abolished when using perfusion with cobalt instead of calcium. Input resistance and the time constant were higher when measured in the whole-cell mode but the other electrical parameters and the sampling of the recorded neurons were strikingly similar between the two methods of recording. Intracellular staining of 22 neurons retrogradely labelled from the lateral septum allowed confirmation of their location within the magnocellular dorsal nucleus. The study indicates that the electrical properties of these neurons did not differ from those of neurons found throughout the area explored. It also indicates the presence of distinct electrophysiological types of cells in the magnocellular dorsal nucleus, although the nucleus is composed of a single type of enkephalinergic neuron. It provides a basis for the study of the regulation of activity of the neurons at the origin of an enkephalinergic tractus which is involved in neuroendocrine, psychoneuroendocrine and immune processes.

Ultrastructural characterization of neurons recorded intracellularly in vivo and injected with lucifer yellow: advantages of immunogold-silver vs. immunoperoxidase labeling

Immunoperoxidase labeling of lucifer yellow provides a sensitive method for morphological characterization of neurons recorded intracellularly in vitro or in vivo. However, the reaction product is often so dense that it obscures ultrastructural details necessary for the analysis of synaptic contacts onto individually filled neurons. In the present study, we describe a silver intensification procedure using 1 nm gold labeling of lucifer yellow as an optimal means for immunocytochemically identifying single physiologically characterized neurons at the ultrastructural level. Single neurons in the frontal cortex of anesthetized rats were impaled in vivo and filled with lucifer yellow. The brains were then perfused with an acrolein fixative. Single vibratome sections through the recording site were reacted with a rabbit antibody directed against lucifer yellow followed by goat anti-rabbit 1 nm gold-labeled IgG and silver intensified. For comparison, additional sections were processed for immunoperoxidase detection of lucifer yellow. Labeled sections were processed for light microscopy or embedded in plastic for electron microscopy. The immunogold-silver label as well as peroxidase reaction product of lucifer yellow was readily detected in cell bodies, proximal and distal dendrites, and spines. However, in contrast to immunoperoxidase, the immunogold-silver reaction did not obscure subcellular organelles. Most importantly, the synaptic junctions formed by afferents to the filled neuron were more easily identifiable following the immunogold-silver procedure. This clear visualization of postsynaptic densities is essential for examining synaptic circuitry between afferents and physiologically characterized neurons.

Neuronal activity in the peribrachial area: relationship to behavioral state control

Extensive studies have ascribed a role to the brainstem cholinergic system in the generation of rapid eye movement (REM) sleep and ponto-geniculo-occipital (PGO) waves. Much of this work stems from systemic and central cholinergic drug administration studies. The brainstem cholinergic system is also implicated in cortical activation via basal forebrain, thalamic, and hypothalamic relay neurons. This cholinergic ascending reticular activating hypothesis has also been suggested by in vivo experiments under anesthetics and by in vitro studies using cholinergic agonists in thalamic and hypothalamic slices. During the last ten years, brainstem cholinergic neurons have been discovered to be in the peribrachial area (PBL). With the discovery of PBL cholinergic neurons, many studies were devoted to the examination of PBL neuronal activity and their connectivity. This article reviews PBL neuronal activity in behaving animals and the anatomical features of these neurons in relation to behavioral state control. The role of the PBL in the generation of REM sleep, PGO waves, and the ascending reticular activating system (ARAS) has been evaluated at the cellular and neurochemical level. Based on recent literature, tentative mechanisms of REM sleep generation, PGO waves generation, and the cortical activation process are also outlined.

Glutamatergic and cholinergic inputs from the pedunculopontine tegmental nucleus to dopamine neurons in the substantia nigra pars compacta

Postsynaptic responses of dopamine (DA) neurons in the substantia nigra pars compacta (SNc) to stimulation of the pedunculopontine tegmental nuclei (PPN) were studied in in vitro slice preparations in the rat. The recorded neurons were intracellularly injected with biocytin and also identified as DA neurons by an immunocytochemical technique. PPN stimulation induced in DA neurons monosynaptic excitatory postsynaptic potentials (EPSPs) that consisted of early transient and slow components. An application of anti-glutamatergic agents (1 mM kynurenic acid and/or 30 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX)) in the bathing media partially suppressed the EPSPs, indicating that PPN inputs to SNc DA neurons are glutamatergic and non-glutamatergic. Anti-glutamatergic resistant EPSPs were suppressed by applications of anti-cholinergic agents such as atropine, mecamylamine, and pirenzepine. These data indicate a convergence of glutamatergic and cholinergic excitatory inputs from the PPN to SNc DA neurons and that both nicotinic and muscarinic receptors are involved in the cholinergic transmission.

Auditory input to the pedunculopontine nucleus: II. Unit responses

The pedunculopontine nucleus (PPN) has been implicated in sleep-wake control, arousal responses, and motor functions. The PPN also has been implicated in the generation of the P1 middle-latency auditory-evoked potential. The present study was undertaken to determine the nature of the responsiveness of single neurons in and around the PPN following auditory stimulation. Somatosensory responsiveness also was tested in some cells. These results demonstrate a) the presence of a significant proportion of PPN neurons that respond to auditory click stimuli; b) two populations of neurons showing either low threshold/short latency/low habituation or high threshold/longer latency/high habituation; c) the responses of longer latency neurons precede the onset and peak of depth- and vertex-recorded middle-latency auditory-evoked potentials; d) thresholds of longer latency neurons similar to the threshold for wave A in the intact cat, the P13 potential in the intact rat, or the startle reflex; and e) convergent somatosensory and auditory responses at a similar latency in a number of PPN neurons. These findings suggest that neurons in and around the PPN may participate in auditory and somatosensory information processing related to arousal, and may contribute to the manifestation of the P1 auditory middle-latency evoked potential.

Lesions of the pedunculopontine tegmental nucleus abolish catalepsy and locomotor depression induced by morphine

The cataleptic and locomotor depressant effects of morphine are mediated by a series of neural structures, all of which project to the pedunculopontine tegmental nucleus (PPTg). To test the idea that the PPTg is also involved in mediating morphine's effect on these behaviours, we examined catalepsy and spontaneous motor activity following bilateral lesions of the PPTg (0.5 microliters of 0.1 M N-methyl-D-aspartate infused over 10 min). We also examined the effect of PPTg lesions on motor functioning by observing sensorimotor responses, limb use, muscle tone and locomotion. PPTg lesions completely abolished the catalepsy and decreased activity that normally follows morphine administration. In addition lesioned animals exhibited increased muscle tone, and impairments in limb use and righting reflexes. Although the deficits were subtle, these results confirm that damage to the PPTg is associated with motor abnormalities.

Prevention of Ca(2+)-mediated action potentials in GABAergic local circuit neurones of rat thalamus by a transient K+ current

1. Neurones enzymatically dissociated from the rat dorsal lateral geniculate nucleus (LGN) were identified as GABAergic local circuit interneurones and geniculocortical relay cells, based upon quantitative analysis of soma profiles, immunohistochemical detection of GABA or glutamic acid decarboxylase, and basic electrogenic behaviour. 2. During whole-cell current-clamp recording, isolated LGN neurones generated firing patterns resembling those in intact tissue, with the most striking difference relating to the presence in relay cells of a Ca2+ action potential with a low threshold of activation, capable of triggering fast spikes, and the absence of a regenerative Ca2+ response with a low threshold of activation in local circuit cells. 3. Whole-cell voltage-clamp experiments demonstrated that both classes of LGN neurones possess at least two voltage-dependent membrane currents which operate in a range of membrane potentials negative to the threshold for generation of Na(+)-K(+)-mediated spikes: the T-type Ca2+ current (IT) and an A-type K+ current (IA). Taking into account the differences in membrane surface area, the average size of IT was similar in the two types of neurones, and interneurones possessed a slightly larger A-conductance. 4. In local circuit neurones, the ranges of steady-state inactivation and activation of IT and IA were largely overlapping (VH = 81.1 vs. -82.8 mV), both currents activated at around -70 mV, and they rapidly increased in amplitude with further depolarization. In relay cells, the inactivation curve of IT was negatively shifted along the voltage axis by about 20 mV compared with that of IA (Vh = -86.1 vs. -69.2 mV), and the activation threshold for IT (at -80 mV) was 20 mV more negative than that for IA. In interneurones, the activation range of IT was shifted to values more positive than that in relay cells (Vh = -54.9 vs. -64.5 mV), whereas the activation range of IA was more negative (Vh = -25.2 vs. -14.5 mV). 5. Under whole-cell voltage-clamp conditions that allowed the combined activation of Ca2+ and K+ currents, depolarizing voltage steps from -110 mV evoked inward currents resembling IT in relay cells and small outward currents indicative of IA in local circuit neurones. After blockade of IA with 4-aminopyridine (4-AP), the same pulse protocol produced IT in both types of neurones. Under current clamp, 4-AP unmasked a regenerative membrane depolarization with a low threshold of activation capable of triggering fast spikes in local circuit neurones.(ABSTRACT TRUNCATED AT 400 WORDS)

Pedunculopontine nucleus in the squirrel monkey: cholinergic and glutamatergic projections to the substantia nigra

The distribution and chemospecificity of the pedunculonigral neurons have been studied in squirrel monkeys (Saimiri sciureus) with cholera toxin subunit B (CTb) and fluorogold (FG) as retrograde tracers combined with immunohistochemistry for choline acetyltransferase (ChAT), glutamate, and the calcium binding protein calbindin D-28k. The injection of either CTb or FG into the substantia nigra produces prominent retrograde cell labeling in the mesopontine tegmentum. Labeled neurons are particularly numerous at the level of the decussation of the superior cerebellar peduncle, where they abound principally in the pars dissipata of the pedunculopontine nucleus (PPN). A significant proportion of retrogradely labeled neurons in the PPN display ChAT immunoreactivity. Within the entire PPN, approximately 25% of the retrogradely labeled neurons express ChAT immunoreactivity, but proportions of doubly labeled neurons are about 35%, 25%, and 15% in the rostral, middle, and caudal thirds of the PPN, respectively. These doubly labeled neurons are scattered among numerous retrogradely labeled neurons that are ChAT-negative and whose number increases along the rostrocaudal extent of the PPN. Several retrogradely labeled neurons in the PPN display glutamate immunoreactivity, but very few express calbindin. This study provides the first direct evidence for the involvement of cholinergic and glutamatergic neurons in the pedunculonigral projection in primates. Furthermore, the fact that some neurons of the PPN display both ChAT and glutamate immunoreactivity indicates that single neurons in the mesopontine tegmentum may exert a two-fold effect upon dopaminergic neurons of the substantia nigra. This dual cholinergic and glutamatergic pedunculonigral projection may play a crucial role in the functional organization of primate basal ganglia.

Pedunculopontine nucleus in the squirrel monkey: distribution of cholinergic and monoaminergic neurons in the mesopontine tegmentum with evidence for the presence of glutamate in cholinergic neurons

The topographical relationships between cholinergic neurons, identified by their immunoreactivity for choline acetyltransferase (ChAT) or their staining for beta-nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase, and dopaminergic, serotoninergic, noradrenergic, and glutamatergic neurons that occur in the mesopontine tegmentum, were studied in the squirrel monkey (Saimiri sciureus). The ChAT-positive neurons in the pedunculopontine nucleus (PPN) form two distinct subpopulations, one that corresponds to PPN pars compacta (PPNc) and the other to PPN pars dissipata (PPNd). The ChAT-positive neurons in PPNc are clustered along the dorsolateral border of the superior cerebellar peduncle (SP) at trochlear nucleus levels, whereas those in PPNd are scattered along the SP from midmesencephalic to midpontine levels. At levels caudal to the trochlear nucleus, ChAT-positive neurons corresponding to the laterodorsal tegmental nucleus (LDT) lie within the periaqueductal gray and extend caudally as far as locus coeruleus levels. All ChAT-positive neurons in PPN and LDT stain for NADPH-diaphorase; the majority of large neurons in PPN and LDT are cholinergic, but some large neurons devoid of NADPH-diaphorase also occur in these nuclei. Cholinergic neurons in the mesopontine tegmentum form clusters that are largely segregated from raphe serotonin-immunoreactive neurons, as well as from nigral dopaminergic and coeruleal noradrenergic neurons, as revealed by tyrosine hydroxylase immunohistochemistry. Nevertheless, dendrites of cholinergic and noradrenergic neurons are closely intermingled, suggesting the possibility of dendrodendritic contacts. In addition, numerous large and medium-sized glutamate-immunoreactive neurons are intermingled among cholinergic neurons in PPN. Furthermore, at trochlear nucleus levels, about 40% of cholinergic neurons display glutamate immunoreactivity, whereas other neurons express glutamate or ChAT immunoreactivity only. This study demonstrates that 1) cholinergic neurons remain largely segregated from monoaminergic neurons throughout the mesopontine tegmentum and 2) PPN contains cholinergic and glutamatergic neurons as well as neurons coexpressing ChAT and glutamate in primates.

Analysis of synaptic inputs and targets of physiologically characterized neurons in rat frontal cortex: combined in vivo intracellular recording and immunolabeling

Ultrastructural immunocytochemical identification of transmitters in afferent terminals and targets of individual physiologically characterized neurons is essential for understanding the complex circuitry within the mammalian neocortex. For this type of analysis, we examined the utility of combining in vivo intracellular recording and biocytin injections with silver intensified 1 nm immunogold labeling of GABA and the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH). These transmitters are found to local neurons and afferents known to prominently modulate the activity of pyramidal neurons in the neocortex. Individual neurons were physiologically characterized and filled with biocytin in the frontal cortex of anesthetized rats. The brains were then preserved by vascular perfusion with aldehydes. Single vibratome sections through the recording site were reacted (1) for immunoperoxidase detection of biocytin and (2) for immunogold labeling of GABA or TH. Dually labeled sections were processed for light microscopy or embedded in plastic for electron microscopy. The dense peroxidase product for biocytin was detected in pyramidal neurons. These were located in superficial as well as deep cortical laminae, and were readily distinguished from immunogold silver labeling. GABA labeled terminals formed symmetric synapses with larger biocytin filled dendrites, whereas the TH labeled terminals contacted distal dendrites and spines. Peroxidase labeling for biocytin also was seen in a few axon terminals forming synapses with unlabeled and with GABA immunoreactive dendrites. These results suggest that single pyramidal neurons of the rat frontal cortex receive dual input from both GABA and catecholamine terminals. Additionally, this study demonstrates the usefulness of silver enhancement of 1 nm colloidal gold prior to plastic embedding for electron microscopic detection of neurotransmitters within afferents and targets of neurons physiologically characterized in vivo.

Brainstem control of phasic muscle activity during REM sleep: a review and hypothesis

For the generation of phasic muscle activity during rapid eye movement (REM) sleep, strong motor excitation to overcome both tonic and phasic inhibition is needed at the motoneuron level. Descending pathways originating in the rostral pons (cholinoceptive (nucleus reticularis pontis oralis-->nucleus reticularis gigantocellularis, peri-locus coeruleus pars alpha-->nucleus reticularis magnocellularis) and cholinergic (pedunculopontine tegmental nucleus-->nucleus reticularis paramedianus) pathways) are involved in motor inhibition during REM sleep. Since the origins of excitatory drives related to phasic muscle activity during REM sleep are also in the brainstem, the occurrence of phasic muscle activity can be said to be determined by brainstem activity. We review the basic and clinical studies on phasic muscle activity during REM sleep and propose the possibility that it can be a beneficial parameter for assessing brainstem activity, especially in relation to its maturation during early stage of life.

D1 dopamine receptor agonist-induced Fos-like immunoreactivity occurs in basal forebrain and mesopontine tegmentum cholinergic neurons and striatal neurons immunoreactive for neuropeptide Y

The present study sought to determine whether the D1 dopamine receptor agonist CY 208-243 increases Fos-like immunoreactivity in neurochemically distinct populations of interneurons in the 6-hydroxydopamine-denervated striatum. In vivo microdialysis studies indicate that cholinergic interneurons in the striatum are excited by the administration of CY 208-243 and that this effect is potentiated by dopaminergic deafferentation. Since Fos is considered to be a marker of neuronal activity, we examined the overlap between CY-208-243-induced Fos-like immunoreactivity and striatal cholinergic interneurons labelled with the cholinergic marker enzyme choline acetyltransferase. Unexpectedly, cholinergic interneurons in the striatum were not found to express Fos-like immunoreactivity. However, D1 agonist-induced Fos-like immunoreactivity was found in neurons immunoreactive for neuropeptide Y. Consequently, the failure of cholinergic neurons in the striatum to express D1 agonist-induced Fos-like immunoreactivity does not appear to be a general property of striatal interneurons. Indeed, CY 208-243 increased Fos-like immunoreactivity in choline-acetyltransferase-positive neurons in the basal forebrain and lateral dorsal tegmental nucleus. In the case of cholinergic basal forebrain neurons, administration of CY 208-243 has been shown to enhance the release of acetylcholine from their terminals located in the frontal cortex. Thus, unlike cholinergic interneurons in the striatum, the expression of Fos-like immunoreactivity in cholinergic basal forebrain neurons is correlated with an increase in transmitter release. Choline acetyltransferase immunoreactivity was markedly reduced in cholinergic basal forebrain neurons ipsilateral to the 6-hydroxydopamine lesion. This decrease in choline acetyltransferase immunoreactivity was confined to a pocket of cortically projecting neurons located in the posterior part of the horizontal limb of the diagonal band which included the medial preoptic nucleus. Interestingly, D1 agonist-induced Fos-like immunoreactivity was located predominantly in those cholinergic neurons which displayed depressed choline acetyltransferase immunoreactivity. Since Fos is often induced as a consequence of increased activity, it is tempting to speculate that those neurons which stained weakly for choline acetyltransferase had been excited by the D1 agonist administration. Accordingly, the destruction of mesencephalic dopaminergic neurons with 6-hydroxydopamine may have deprived these cholinergic neurons of an excitatory D1 receptor-mediated drive, resulting in a reduction of choline acetyltransferase immunoreactivity. These results suggest that the degeneration of nigrostriatal dopamine neurons may contribute to the loss of cholinergic basal forebrain function in Parkinson's disease.

Serotonergic and cholinergic inhibition of mesopontine cholinergic neurons controlling REM sleep: an in vitro electrophysiological study

Intracellular recordings were obtained from neurons of the laterodorsal tegmental and pedunculopontine tegmental nuclei in a brain-slice preparation. The action of exogenously applied 5-hydroxytryptamine and acetylcholine was studied on NADPH-diaphorase-labeled cells which contain nitric oxide synthase and are presumed to be cholinergic. Our results indicated that these cells were hyperpolarized by both 5-hydroxytryptamine and acetylcholine; the ionic mechanism of this inhibition was investigated using current and voltage clamp methods. Cells voltage-clamped at resting membrane potential exhibited a net outward current and an increased membrane conductance during 5-hydroxytryptamine and acetylcholine mediated inhibition. The membrane hyperpolarization and outward current generated by this paradigm reversed near the expected K equilibrium potential and was blocked by low concentrations of extracellular Ba. The 5-hydroxytryptamine- and acetylcholine-dependent currents showed inward rectification and the reversal potential shifted in the depolarizing direction by about 15 mV for a doubling of extracellular K, indicating that both 5-hydroxytryptamine and acetylcholine activate inwardly rectifying, potassium-selective conductances. The 5-hydroxytryptamine-evoked hyperpolarization was antagonized by spiperone and mimicked by (+)8-hydroxy-2-(Di-N-propylamino)-tetralin suggesting the presence of a 5-hydroxytryptamine1A receptor while the acetylcholine-evoked hyperpolarization was blocked by atropine and only high concentrations of pirenzepine, suggesting a muscarinic M2 receptor. The outward currents evoked by 5-hydroxytryptamine and acetylcholine were not additive, suggesting that both receptors are coupled to an overlapping pool of K channels as has been observed in several systems in which receptors are coupled to effectors by G-proteins. These results indicate that the dominant actions of 5-hydroxytryptamine and acetylcholine relate to the inhibition of mesopontine cholinergic neurons via activation of an overlapping pool of inwardly rectifying K channels. Cholinergic neurons of these nuclei are thought to play an instrumental role in the induction and maintenance of rapid eye movement sleep. It has been previously hypothesized that acetylcholine would be excitatory and that 5-hydroxytryptamine would be inhibitory to these cells in the context of rapid eye movement sleep. [McCarley R. and Massaquoi S. (1986) Am. J. Physiol. 251, R1011-R1029; McCarley R. W. et al. (1975) Science 189, 58-60]. Our results are consistent with the proposed inhibitory action of 5-hydroxytryptamine but indicate recurrent input to cholinergic neurons would be inhibitory. Accordingly, models of the neural substrate underlying rapid eye movement sleep production need to be changed to reflect this inhibitory action of acetylcholine on cholinergic neurons.

Lesions of the pedunculopontine tegmental nucleus block drug-induced reinforcement but not amphetamine-induced locomotion

It has been proposed that the positive reinforcing and motor stimulating effects of drugs involve the activation of a common neural substrate. Reinforcing effects of food, drugs and brain stimulation are blocked by lesions of the pedunculopontine tegmental nucleus (PPTg), which is a component of the mesencephalic locomotor region. This has suggested that the PPTg may be involved in both positive reinforcement and forward locomotion. In four separate experiments, rats were prepared with NMDA (0.5 microliters of 0.1 M solution) or sham lesions of the PPTg. Animals in the first two experiments were tested for the development of a conditioned place preference (CPP) to morphine (2 mg/kg x 3 pairings) or amphetamine (1.5 mg/kg x 3 pairings). Ten days later, spontaneous motor activity (SMA) was assessed in these animals following a subcutaneous injection of saline or amphetamine (1.5 mg/kg). In two further experiments, drug-naive lesioned and control animals were tested for SMA only (saline or 1.5 mg/kg amphetamine in Experiment 3, and saline, 0.5 mg/kg, or 3 mg/kg amphetamine in Experiment 4). Lesions of the PPTg blocked the development of a CPP to both morphine and amphetamine. In contrast, lesions had no effect on saline or amphetamine-stimulated SMA. The PPTg, therefore, appears to be involved in the reinforcing effects of amphetamine and morphine, but is not necessary for the expression of amphetamine-induced activity.

Projections of the dorsal raphe nucleus to the brainstem: PHA-L analysis in the rat

Early studies that used older tracing techniques reported exceedingly few projections from the dorsal raphe nucleus (DR) to the brainstem. The present report examined DR projections to the brainstem by use of the anterograde anatomical tracer Phaseolus vulgaris leucoagglutinin (PHA-L). DR fibers were found to terminate relatively substantially in several structures of the midbrain, pons, and medulla. The following pontine and midbrain nuclei receive moderate to dense projections from the DR: pontomesencephalic central gray, mesencephalic reticular formation, pedunculopontine tegmental nucleus, medial and lateral parabrachial nuclei, nucleus pontis oralis, nucleus pontis caudalis, locus coeruleus, laterodorsal tegmental nucleus, and raphe nuclei, including the central linear nucleus, median raphe nucleus, and raphe pontis. The following nuclei of the medulla receive moderately dense projections from the DR: nucleus gigantocellularis, nucleus raphe magnus, nucleus raphe obscurus, facial nucleus, nucleus gigantocellularis-pars alpha, and the rostral ventrolateral medullary area. DR fibers project lightly to nucleus cuneiformis, nucleus prepositus hypoglossi, nucleus paragigantocellularis, nucleus reticularis ventralis, and hypoglossal nucleus. Some differences were observed in projections from rostral and caudal parts of the DR. The major difference was that fibers from the rostral DR distribute more widely and heavily than do those from the caudal DR to structures of the medulla, including raphe magnus and obscurus, nucleus gigantocellularis-pars alpha, nucleus paragigantocellularis, facial nucleus, and the rostral ventrolateral medullary area. A role for the dorsal raphe nucleus in several brainstem controlled functions is discussed, including REM sleep and its events, nociception, and sensory motor control.

Electrophysiological properties of cat reticular thalamic neurones in vivo

1. The electrophysiological properties of neurones of the reticular thalamic (RE) nucleus were studied in acutely prepared cats under urethane anaesthesia. 2. Two main types of neuronal firing were recorded. At the resting membrane potential (-60 to -65 mV) tonic repetitive firing was elicited when the cell was activated synaptically or by current injection. From membrane potentials more negative than -75 mV, synaptic or direct stimulation generated a burst of action potentials. 3. The burst of RE cells consisted of a discharge of four to eight spikes riding on a slowly growing and decaying depolarization. The discharge rate during the burst showed a characteristic increase, followed by a decrease in frequency. 4. The burst response behaved as a graded phenomenon, as its magnitude was modulated by changing the intensity of the synaptic volley or the intensity of the injected current. 5. Spike-like small potentials presumably of dendritic origin occurred spontaneously and were triggered by synaptic or direct stimulation. They were all-or-none, voltage-dependent events. We postulate that these spikes originate in several hot spots in the dendritic arbor, with no reciprocal refractoriness and may generate multi-component depolarizations at the somatic level. 6. Excitatory postsynaptic potentials (EPSPs) evoked by internal capsule stimulation consisted of two components, the late one being blocked by hyperpolarization. Such compound EPSPs were followed by a period of decreased excitability during which a second response was diminished in amplitude. 7. A series of depolarizing waves at the frequency range of spindle oscillations was triggered by internal capsule stimulation. The individual depolarizing waves constituting the spindle oscillation gradually decreased in amplitude when decreasing the intensity of the stimulation. 8. These results, showing that RE cells are endowed with an excitable dendritic tree and a graded bursting behaviour, support the proposed role of RE nucleus as the generator and synchronizer of spindle rhythmicity.

Calbindin D-28k and choline acetyltransferase are expressed by different neuronal populations in pedunculopontine nucleus but not in nucleus basalis in squirrel monkeys

Single- and double-immunostaining procedures were used to study the distribution of the acetylcholine synthesizing enzyme choline acetyltransferase (ChAT) and the calcium binding protein calbindin D-28k in the nucleus basalis of Meynert (nbM) and in the pedunculopontine nucleus (PPN) of the squirrel monkey (Saimiri sciureus). As expected from previous studies in other primates, including humans, the nbM in the squirrel monkey is enriched with large ChAT-immunoreactive neurons that form clusters in the substantia innominata. Some ChAT-positive neurons are also scattered more dorsally within the internal and external medullary laminae of the pallidal complex. A smaller number of calbindin-immunoreactive cells occur in the same locations and their mean cross-sectional somatic area (424 microns 2) is not significantly different from that of the ChAT-immunoreactive cells (450 microns 2). Furthermore, 60% of the ChAT-immunopositive cells in the nbM display calbindin immunoreactivity. Most of these double-immunoreactive neurons occur in the typical clusters of the nbM, whereas the large neurons scattered in between the clusters display ChAT immunoreactivity only. In the PPN, ChAT-positive neurons are scattered around and partly within the superior cerebellar peduncle and also form a dense cluster in the lateral portion of the mesopontine tegmentum. Calbindin-immunoreactive cells also abound around the superior cerebellar peduncle, but they are more sparsely distributed and cover a larger sector of the tegmentum than the ChAT-positive neurons. These calbindin-immunoreactive cells are significantly smaller (200 microns 2) than the ChAT-immunoreactive cells (471 microns 2) and no double-immunostained neurons are present in the PPN.(ABSTRACT TRUNCATED AT 250 WORDS)

Afferent connections of the laterodorsal and the pedunculopontine tegmental nuclei in the rat: a retro- and antero-grade transport and immunohistochemical study

Increasingly strong evidence suggests that cholinergic neurons in the mesopontine tegmentum play important roles in the control of wakefulness and sleep. To understand better how the activity of these neurons is regulated, the potential afferent connections of the laterodorsal (LDT) and pedunculopontine tegmental nuclei (PPT) were investigated in the rat. This was accomplished by using retrograde and anterograde axonal transport methods and NADPH-diaphorase histochemistry. Immunohistochemistry was also used to identify the transmitter content of some of the retrogradely identified afferents. Following injections of the retrograde tracer wheatgerm agglutinin-conjugated horseradish peroxidase (WGA-HRP) into either the LDT or the PPT, labelled neurons were seen in a number of limbic forebrain structures. The medial prefrontal cortex and lateral habenula contained more retrogradely labelled neurons from the LDT, whereas in the bed nucleus of the stria terminalis and central nucleus of the amygdala, more cells were labelled from the PPT. Moderate numbers of neurons were seen in the magnocellular regions of the basal forebrain, and many labelled neurons were observed in the lateral hypothalamus, the zona incerta, and the midbrain central gray from both the LDT and the PPT. Accessory oculomotor nuclei in the midbrain as well as eye movement-related structures in the lower brainstem contained some neurons labelled from the LDT, and fewer neurons from the PPT. A few labelled neurons were seen in somatosensory and other sensory relay nuclei in the brainstem and the spinal cord. Retrograde labelling was seen in a number of extrapyramidal structures, including the globus pallidus, entopenduncular and subthalamic nuclei, and substantia nigra following PPT injections; with LDT injections, labelling was similar in density in the substantia nigra but virtually absent in the entopeduncular and subthalamic nuclei. Data with the fluorescent retrograde tracer fluorogold combined with immunofluorescence indicated that many neurons in the zona incerta-lateral hypothalamic region that were retrogradely labelled from the LDT contained alpha-melanocyte-stimulating hormone. Numerous neurons were labelled throughout the reticular formation of the brainstem following either LDT or PPT injections. Many neurons retrogradely labelled in the LDT and PPT, the dorsal and median raphe nuclei, and the locus ceruleus contained choline acetyltransferase, serotonin, and tyrosine hydroxylase, respectively. The anterograde tracers WGA-HRP and phaseolus vulgaris leucoagglutinin were used to confirm some of the projections indicated by the retrograde labelling data; anterograde labelling was seen in the LDT and PPT following injections of one of these tracers into the medial prefrontal cortex, lateral hypothalamus, and the contralateral LDT.(ABSTRACT TRUNCATED AT 400 WORDS)

Serotonin hyperpolarizes cholinergic low-threshold burst neurons in the rat laterodorsal tegmental nucleus in vitro

Serotonergic suppression of cholinergic neuronal activity implicated in the regulation of rapid eye movement sleep and its associated phenomenon, pontogeniculooccipital waves, has long been postulated, but no direct proof has been available. In this study, intracellular and whole-cell patch-clamp recording techniques were combined with enzyme histochemistry to examine the intrinsic electrophysiological properties and response to serotonin (5-HT) of identified cholinergic rat laterodorsal tegmental nucleus neurons in vitro. Sixty-five percent of the recorded neurons demonstrated a prominent low-threshold burst, and of these, 83% were cholinergic. In current-clamp recordings 64% of the bursting cholinergic neurons tested responded to the application of 5-HT with a membrane hyperpolarization and decrease in input resistance. This effect was mimicked by application of the selective 5-HT type 1 receptor agonist carboxamidotryptamine maleate. Whole-cell patch-clamp recordings revealed that the hyperpolarizing response was mediated by an inwardly rectifying K+ current. Application of 5-HT decreased excitability and markedly modulated the discharge pattern of cholinergic bursting neurons: during a 5-HT-induced hyperpolarization these neurons exhibited no rebound burst after hyperpolarizing current input and a burst in response to depolarizing current input. In the absence of 5-HT, the relatively depolarized cholinergic bursting neurons responded to an identical hyperpolarizing current input with a burst and did not produce a burst after depolarizing current input. These data provide a cellular and molecular basis for the hypothesis that 5-HT modulates rapid eye movement sleep phenomenology by altering the firing pattern of bursting cholinergic neurons.

Substantia nigra reticulata neurons during sleep-waking states: relation with ponto-geniculo-occipital waves

We have previously hypothesized that the spike bursts of brainstem peribrachial (PB) neurons, leading to ponto-geniculo-occipital (PGO) waves in thalamocortical systems, are triggered by phasic hyperpolarizations of sufficient magnitude or by excitatory inputs reaching a steadily hyperpolarized membrane. We have proposed that the source of these hyperpolarizing actions are substantia nigra pars reticulata (SNr) cells that project to, and exert inhibitory effects upon, PB neurons. Here we tested this hypothesis by recording antidromically identified SNr-PB cells in chronically implanted, naturally sleeping cats. A subpopulation of SNr-PB cells exhibited tonically increased firing preceding by 70-200 ms the thalamic PGO wave. These data support the hypothesis that an enhancement in SNr-cells' discharges may lead to hyperpolarization of PB neurons, with the consequence of spike bursts in one class of PGO-related PB-thalamic neurons.

Anatomical interconnections of the pedunculopontine tegmental nucleus and the nucleus prepositus hypoglossi in the cat

The Pedunculopontine tegmental nucleus (PPT) is a mesopontine structure containing predominantly cholinergic neurons, and physiological data indicate its neurons transfer eye-movement gated ponto-geniculo-occipital (PGO) waves to thalamus during the rapid eye movement phase of sleep. The present study, using anterograde and retrograde tracing of wheat germ agglutinin-conjugated horseradish peroxidase, found that the medullary nucleus Prepositus hypoglossi (PH), whose neurons are known to have eye-movement-related information, projects densely to PPT, and PPT has reciprocal projections to PH. The PH-PPT projection has some topographic organization, with rostral PH to rostral PPT and caudal PH to caudal PPT projections dominating. The PH-PPT projection may furnish the anatomical substrate for input of eye movement-related information into the rostral PGO wave system.

Substantia nigra pars reticulata projects to the reticular thalamic nucleus of the cat: a morphological and electrophysiological study

The projections of substantia nigra pars reticulata (SNr) toward the reticular (RE) thalamic complex of cat were studied morphologically and electrophysiologically. Numerous SNr cells were retrogradely labeled following injections of horseradish peroxidase conjugated to wheat-germ agglutinin (WGA-HRP) in the rostral and rostrolateral part of the RE thalamic nucleus. Iontophoretic injections of Phaseolus vulgaris leucoagglutinin (PHA-L) in the SNr confirmed that this retrograde labeling was not a consequence of tracer diffusion in neighboring structures, but reflected a genuine SNr projection to the RE thalamic nucleus. Indeed, widely branched and varicose PHA-L-positive fibers were found in the rostral and rostrolateral pole of the RE thalamic nucleus following PHA-L injections in the SNr. Furthermore, in agreement with previous data indicating that nigrothalamic cells are GABAergic, electrical stimulation of the SNr evoked a short-latency inhibitory effect acting on both spontaneous and cortically-evoked discharges of most RE thalamic neurons. These results are discussed in light of the possible role of the SNr and RE thalamic complex in attentional processes.

Brainstem genesis of reserpine-induced ponto-geniculo-occipital waves: an electrophysiological and morphological investigation

Several experimental results indicate that the peribrachial (PB) cholinergic area of the pedunculopontine nucleus is the final relay for the transfer of brainstem-generated pontogeniculo-occipital (PGO) waves to the thalamus. However, the mechanisms underlying the PGO-related activity of PB neurons remain unknown. In order to study these mechanisms, single unit recordings in the PB area were performed in reserpinized cats. Because PGO waves are closely related to rapid eye movements, our microelectrode explorations were also aimed to some structures of the preoculomotor network, namely, the superior colliculus (SC) and parts of the central tegmental field (FTC). We have found several classes of PGO-on cells in the PB area, most of them descharging 80 ms or less before the peak of PGO waves. These cell-classes comprised high-frequency bursting cells, slow-frequency bursting cells, and neurons discharging single spikes or doublets. Intracellular recordings showed that PGO-on single spikes arise from conventional excitatory postsynaptic potentials. Among PGO-related cells in structures outside the PB limits, it was found that most SC cells discharge during or after the PGO, whereas FTC cells increase their discharge rate several hundreds of ms before PGO waves, thus indicating that PGO waves are elaborated long before the activation of PB neurons. Massive retrograde labeling was found in FTC following horseradish peroxidase injections into the PB area. We suggest that long-lead FTC neurons provide an excitatory input to PGO-on PB neurons.