Biomedical subjects

O Hardy

Publications and source records attributed to O Hardy.

At least 19 recordsLinked to original sources

PubMed2006

Thelytokous parthenogenesis and its consequences on inbreeding in an ant.

Thelytokous parthenogenesis, that is, the production of diploid daughters from unfertilized eggs, may involve various cytological mechanisms, each having a different impact on the genetic structure of populations. Here, we determined the cytological mechanism of thelytokous parthenogenesis and its impact on inbreeding in the ant Cataglyphis cursor, a species where queens use both sexual and asexual reproduction to produce, respectively, workers and new queens. It has been suggested that thelytokous parthenogenesis in C. cursor might have been selected for to face high queen mortality and, originally, to allow workers to replace the queen when she passes away. We first determined the mode of thelytokous parthenogenesis by comparing the rate of transition to homozygosity at four highly polymorphic loci to expectations under the different modes of parthenogenesis. Our data show that thelytoky is achieved through automictic parthenogenesis with central fusion. We then estimated the proportion of colonies headed by worker-produced queens in a natural population. We designed a model linking the observed homozygosity in queens to the proportion of queens produced by workers, based on the assumption that (i) parthenogenesis is automictic with central fusion and (ii) queen lineage is asexually produced, resulting in an increase of the inbreeding over generations, whereas workers are sexually produced and therefore not inbred. Our results indicate that more than 60% of the colonies should be headed by a worker-produced queen, suggesting that queen's lifespan is low in this species.

Animals↗

PubMed2000

Higher impact of female than male migration on population structure in large mammals.

We simulated large mammal populations using an individual-based stochastic model under various sex-specific migration schemes and life history parameters from the blue whale and the Asian elephant. Our model predicts that genetic structure at nuclear loci is significantly more influenced by female than by male migration. We identified requisite comigration of mother and offspring during gravidity and lactation as the primary cause of this phenomenon. In addition, our model predicts that the common assumption that geographical patterns of mitochondrial DNA (mtDNA) could be translated into female migration rates (Nmf) will cause biased estimates of maternal gene flow when extensive male migration occurs and male mtDNA haplotypes are included in the analysis.

Animals↗

PubMed2000

Evidence for glutamatergic tectotectal neurons in the cat superior colliculus: a comparison with GABAergic tectotectal neurons.

The tectotectal commissural pathway is commonly regarded as responsible for the reciprocal inhibition that takes place between the two superior colliculi (SC). Although this hypothesis has received strong support from electrophysiological studies, more recent investigations have suggested that some collicular cells, e.g. fixation neurons, may establish excitatory connections with cells in the contralateral SC through the collicular commissure. The goal of the present study was to seek immunohistochemical evidence for glutamatergic tectotectal cells in the cat SC by using a double-labelling technique. Tectotectal cells were retrogradely labelled with wheat germ agglutinin (WGA) -horseradish peroxidase (HRP) coupled to colloidal gold injected in the contralateral SC, and neurons containing glutamate or gamma-aminobutyric acid (GABA) were then identified with immunohistochemical techniques. The present study provides evidence that, in the cat SC, equal numbers of tectotectal cells are immunopositive to glutamate and GABA, suggesting that the tectotectal pathway may consist of two distinct functional components. The finding that an equal number of tectotectal cells are GABAergic and glutamatergic is somewhat surprising as electrophysiological studies have invariantly indicated that the inhibitory component of the tectotectal projection predominates. Another striking feature of the GABAergic and glutamatergic tectotectal cell populations is their identical topographic distribution in the SC. These results suggest that not only cells in the rostral fixation zone establish excitatory connections with the contralateral SC. Tectotectal projections could be potentially important to shape the spatial pattern of saccade-related activity that may occur simultaneously in the two SC during vertical and oblique orienting movements.

Animals↗

PubMed1999

Tectotectal connectivity in goldfish.

The vertebrate optic tectum is a functionally coupled bilateral structure which plays a major role in the generation of motor commands for orienting responses. However, the characteristics of the tectotectal connectivity are unknown in fish, and have been reported only to a limited extent in other vertebrates. The purpose of the present study was to determine the anatomical basis underlying the functional coupling between tecta in goldfish, and to identify both similarities and differences to those features reported in other vertebrate species. The present experiments used the bidirectional tracer biotinylated dextran amine to map the distribution of labeled cells and synaptic boutons in the contralateral tectum following injections into identified tectal sites. Fibers that interconnect both tecta coursed through the tectal commissure. The cells of origin of these fibers, the tectotectal cells, and their synaptic endings were located in the deep layers, mainly in the strata periventricular and griseum central, respectively. Corresponding sites throughout the two tecta were interconnected in a symmetrical point-to-point fashion. The tectal commissure was composed of at least two distinct bundles of axons, which differed in their dorsoventral location, fiber diameter, and projection targets. The dorsal axons were tectotectal axons, they were thinner in diameter and profusely branched, and gave off en passant and terminal boutons in the deep layers of the contralateral tectum. The ventral axons were thicker in diameter, and formed the contralateral tectofugal-descending tract. Such fibers had few axon collaterals and boutons in the contralateral tectum. Boutons adjacent to retrogradely labeled tectotectal cells were very scarce. The data are discussed in terms of the coupling between tecta generating the motor commands required for orienting movements.

Animals↗

PubMed1999

Hyper- and hypogravity alter posture in rats compensated on Earth for a vestibular asymmetry.

Head posture and neck muscle activity (EMGs) were examined in unilateral (UL) and bilateral (BL) vestibularly lesioned rats in hypergravity (1.7 g) and hypogravity (0 g) during parabolic flights. Compared with BL rats taken as control, the head and the body of UL deviated toward the lesion side at 0 g and toward the intact side at 1.7 g. Recorded in head fixed condition, left and right EMGs remained symmetrical in BL while UL rats displayed an asymmetry between left and right muscles at 1.7 g, but not at 0 g. These results demonstrate that an experimental otolithic asymmetry, compensated on Earth, can become unbalanced in altered gravity. Paradoxically, the utricular system appears to play a major role in that process.

Animals↗

PubMed1998

Influence of the tectal zone on the distribution of synaptic boutons in the brainstem of goldfish.

This study investigated whether the topographic differences in the functional properties of the tectal motor map of goldfish are related to particular patterns of connections with downstream structures. With this aim, the distribution of synaptic boutons in the mesencephalic and rhombencephalic structures was studied after discrete injections of the tracer biotinylated dextran amine were placed at separate sites along the tectal anteroposterior axis. Irrespective of the location of the injection site, the boutons were more abundant in the mesencephalon than in the rhombencephalon, and they were located chiefly ipsilaterally all throughout the brainstem. In the mesencephalon, the boutons were found in its ventrolateral reticular formation and, to a lesser extent, in the nucleus of the medial longitudinal fasciculus, the oculomotor and isthmi nuclei, and the torus semicircularis. In the mesencephalic reticular formation, the bouton location was distributed topographically with respect to the injection site. Terminals were also observed in the nucleus of the medial longitudinal fasciculus after injections into anteromedial or middle tectal zones. In the oculomotor nucleus, boutons were present exclusively in the case of the anteromedial injection. In the rhombencephalon, most boutons were found in the superior reticular formation, and their number decreased in the medial and inferior reticular formations. A topographic distribution could be observed within the superior reticular formation, although its density was attenuated compared with that observed in the mesencephalic reticular formation. The domains of synaptic endings on the ipsilateral side were different from those on the contralateral side: The ipsilateral synaptic endings were located more medially. Finally, a few boutons were also found in the vestibulocerebellar area on either the ipsilateral or the contralateral side, depending on the injection site. From these data, the authors conclude that, in goldfish, irrespective of the tectal injection site, the endings are in similar nuclei in the brainstem; however, the distribution of synaptic boutons within such nuclei can be related to the functional properties of each tectal zone.

Anatomy, Artistic↗

PubMed1997

Topographical characteristics of preposito-collicular projections in the cat as revealed by Phaseolus vulgaris-leucoagglutinin technique. A possible organisation underlying temporal-to-spatial transformations.

A neuronal pathway from the nucleus prepositus hypoglossi (PH) to the superior colliculus (SC) has been documented in previous studies using retrogradely transported tracer methods. This pathway may underlie a feedback control of gaze-related collicular activities. The present study provided a detailed description of this pathway in the cat using the Phaseolus vulgaris-leucoagglutinin technique. Two axonal trajectories exist that depend on the location of injections into the PH. As described in previous studies, injections within the caudal PH labelled axons that ran through the contralateral side and terminated in the contralateral SC (cSC). Injections in the rostral PH labelled axons ascending in the midbrain on the ipsilateral side and ending within both colliculi--mostly in the rostral SC on the ipsilateral side and over a large rostrocaudal extent on the contralateral side. A quantitative analysis of the density of synaptic terminal boutons was done in three out of six cats which revealed that, independently of axonal trajectories, the density of boutons increased from rostral to caudal in the cSC. Thus the preposito-collicular projection is weighted along the rostrocaudal axis of the cSC. From this result, a simulation was done in order to examine how excitatory sensory activities and a topographically weighted inhibitory feedback might interact within collicular networks. This simulation was able to mimic electrophysiological data obtained in the cat and in the monkey that showed that the motor error is topographically coded over the collicular map. Present results give a strong morphological support for a temporal-to-spatial transformation of feedback signals related to eye movement parameters.

Animals↗

PubMed1996

Firing properties of preposito-collicular neurones related to horizontal eye movements in the alert cat.

The projection from the nucleus prepositus hypoglossi (PH) to the superior colliculus (SC) has been proposed to provide a feedback control of collicular saccadic activities. The present study aimed to identify the functional properties of PH neurones projecting to the SC relative to eye movement parameters. Preposito-collicular neurones were identified in alert cats by antidromic invasion and collision tests following electrical stimulations of the contralateral SC. Their discharges were then correlated with the horizontal component of eye movements. Particular attention was given to the timing of discharges relative to saccade onsets. Most preposito-collicular neurones (12/14) displayed transient activities associated to eye velocity, and onsets preceded the saccade onset by 9.4-19.9 ms. The mean eye velocity sensitivity of these "early" preposito-collicular neurones (1.46 +/- 0.53 spikes/s per degree per second) was quite similar to that calculated from a sample of putative motoneurones or interneurones that have been recorded within abducens nucleus and quantified in the same conditions. The remaining two preposito-collicular neurones exhibited transient activity related to saccades, but this followed the transient putative motoneuronal discharge. These "delayed" neurones also had lower eye velocity sensitivities (0.38 sp/s per degree per second and 0.58 sp/s per degree per second, respectively) compared with early neurones. Both classes of preposito-collicular neurones also displayed a subsequent tonic activity correlated with the eye position. Taken together, these results demonstrate that preposito-collicular neurones code both eye position and eye velocity just like ocular motoneurones, but in a predictive manner. The anticipatory discharge of early neurones makes them likely candidates for the control of peak activities of saccade-related collicular neurones, particularly in the caudal colliculus. Delayed preposito-collicular neurones may also participate in the control of collicular activities, but probably in more rostral SC, where peak activities occur later during eye movements together with smaller motor error coding.

Animals↗

PubMed1993

Distribution of synaptic terminals from prepositus neurones on the collicular maps.

The distribution of synaptic terminals was quantified in the superior colliculus (SC) following injections of Phaseolus vulgaris-Leucoagglutinin in the ventral prepositus hypoglossi nucleus (PH) and adjacent reticular formation. Labelled axons distribute terminals within the intermediate and the deep layers on both sides. Within the former, their distribution reproduces the representation of the visual hemifield on the same side as the injected PH. Additionally the density of boutons gradually increases in the contralateral SC, from the projection of the area centralis towards the periphery, i.e. towards regions coding for larger saccades. Such a differential synaptic input may provide the neuronal basis for a temporal to spatial transformation of the feedback signal controlling gaze shifts. A theoretical model is proposed.

Animals↗

PubMed1992

Characteristics of saccades induced by neck torsions: a re-examination in the normal guinea pig.

Torsion of the neck relative to the fixed head results in several reflexes involving the eyes, the neck and the body. One of these reflexes, the cervico-ocular reflex, has been described as having a small gain in the normal animal. However, when the body of a guinea pig is moved relative to the fixed head with a ramp-like velocity profile, saccades are systematically elicited in the direction of body movement. We re-examined the characteristics of this reflex in the normal guinea pig and demonstrated that: (1) it occurs mainly in the range of high velocity body movements; (2) the latency of the saccades is shorter than previously suspected; (3) the saccades are triggered at specific positions relative to the starting and ending positions of rotation, revealing some degree of flexibility in the reflex. We hypothesize that these saccades of nuchal origin are under the control of the same neuronal circuit as visually triggered saccades and quick phases of vestibular nystagmus. Thus, this nuchal reflex may fundamentally subserve orienting behaviour in normal animal.

Animals↗

PubMed1992

Firing behaviour of anticompensatory neurones in the prepositus hypoglossi nucleus of alert cat.

The present results demonstrate that anticompensatory neurones could be recorded within the prepositus hypoglossi nucleus of the alert cat. These neurones, like burster-driving neurones described in paralysed cats, were characterized by a tonic increase of firing for head turning in the contraversive direction (type II) associated to bursts for each vestibular quick phase in the same direction. They were also involved in the generation of visually triggered saccades since they displayed a burst preceding contraversive saccades and a pause in the opposite direction. Thus, these neurones code eye velocity for both vestibular quick phases and visually triggered saccades in addition to head velocity. Such a firing behaviour suggests that, in addition to the classical crossed tecto-reticulo-spinal network, an ipsilateral pathway could also be involved in the generation of visually triggered rapid eye movements.

Animals↗

PubMed1991

GABA and non-GABA immunostained neurones in the nucleus prepositus and the periparabigeminal area projecting to the guinea pig superior colliculus.

We have investigated the possibility that GABAergic neurones may be involved in two ascending projections to the superior colliculus, originating in the nucleus prepositus hypoglossi and in the periparabigeminal area of the mesencephalon, respectively. The projecting neurones of both structures were identified using gold-WGA-apoHRP, a retrogradely transported tracer, injected unilaterally into the superior colliculus. GABA was detected in these neurones by means of immunocytochemical staining. The results show that 25% of the projecting neurones in the prepositus hypoglossi are indeed GABA-immunoreactive. They could exert a direct inhibitory influence on the colliculus. By contrast, only a few (7%) gold-filled-GABAergic cells were detected in the periparabigeminal area, which suggests that this region cannot participate in an important inhibitory afferent system to the colliculus.

Afferent Pathways↗

PubMed1991

Transient increase of contraversive saccade parameters following kainic acid injection in the periparabigeminal area of guinea pig.

Anatomical and electrophysiological data have shown that, in the guinea pig as well as in the cat, the nucleus prepositus hypoglossi gives rise to a disynaptic ascending projection to the superior colliculus via the peri-parabigeminal area in the mesencephalon. The functional role of this indirect pathway in the generation of eye movements has been studied by pharmacologically interfering at the mesencephalic level and by examining the induced effects on two differently elicited saccades in the alert guinea pig. A small iontophoretic injection of kainic acid induces a transient increase of the spontaneous saccadic activity in the contraversive direction leading to a pseudo-nystagmus. Both the amplitude and the mean velocity of the contraversive saccades are greater than before the injection. A similar enhancement also affects the parameters of the contraversive saccades induced by trunk rotations. These results suggest that peri-parabigeminal neurones exert an excitatory influence on their target cells in the ipsilateral superior colliculus which is responsible for contraversive rapid eye movements.

Animals↗

PubMed1991

Possible excitatory and inhibitory feedback to the superior colliculus: a combined retrograde and immunocytochemical study in the prepositus hypoglossi nucleus of the guinea pig.

We have investigated in the guinea pig the precise localization and the immunoreactivity of the neurones in the prepositus hypoglossi nucleus involved in a direct ascending projection onto the superior colliculus. The projecting neurones were characterized by a retrograde tracer (WGA-ApoHRP coupled to gold particles), injected in the intermediate and deep layers of the superior colliculus. After revealing gold particles, the sections were then treated using an antibody either against GABA or against glutamate, thus allowing identification of gold-filled-immunoreactive neurones. The retrogradely labelled cells were exclusively distributed on the contralateral side, and preferentially in the caudal two thirds of the prepositus hypoglossi nucleus, in its ventral and ventrolateral division. In addition, about 23% of these projecting neurones appear immunopositive when the sections are treated with a GABA antibody and around 27% are immunopositive to glutamate. Furthermore, these two classes of GABA-like or glutamate-like projecting neurones are intermingled within the prepositus hypoglossi nucleus. We conclude, in spite of a probable underestimation of these two populations, that the ascending projection is formed by an excitatory pathway that probably involves glutamate as well as an inhibitory pathway mediated by GABA. Thus we cannot consider this feedback as exclusively inhibitory as was suggested in theoretical models of the oculomotor system.

Animals↗

PubMed1987

Electrophysiological properties of neurons recorded intracellularly in slices of the pigeon optic tectum.

The electrical properties of pigeon's optic tectum neurons located in the non-retinorecipient region of layer II have been studied in vitro slice preparations by using intracellular recordings. As judged from the somatodendritic characteristics of cells intracellularly labeled with horseradish peroxidase recordings were obtained from pyramidal neurons, the main morphological type, as well as from ganglion cells. When stimulated with depolarizing current pulses of 300-500 ms duration, three distinct modes of firing were observed. Most neurons (Type I) responded with a continuous firing of fast action potentials whose frequency rate increased regularly when current strength was raised. Another group of cells (Type II) also exhibited sustained firing. However, in Type II cells, grouped discharges formed by 2-6 fast action potentials per group fired in rapid succession were elicited within a certain range of current intensity. Finally, another group of cells (Type III) responded at all intensities tested by a short train of fast action potentials only at the onset of the current step. At current strength close to threshold the spike undershoot of type I neurons was followed by a slow hyperpolarizing afterpotential while the spike undershoot of Type II cells was followed by a hump-like depolarization and a slow hyperpolarizing afterpotential. In Type II cells, we have also observed a pronounced increase of the hyperpolarizing afterpotential after a grouped discharge. Type III cells were characterized by a small amplitude and short duration hyperpolarizing afterpotential, barely visible in most of them. In Type I and II cells the slow hyperpolarizing afterpotential was blocked by replacing Ca2+ with Mg2+ or Cd2+ in the saline. These results support the idea that in these two types of neurons the slow hyperpolarizing afterpotential is primarily caused by a Ca2+-dependent K+ conductance. Furthermore, blocking the slow hyperpolarizing afterpotential provoked a pronounced increase of the firing frequency of Type I cells. In Type II cells blockade of the slow hyperpolarizing afterpotential had a greater effect on firing behavior: i.e. when Ca2+ was replaced with Mg2+ or Cd2+, Type II neurons exhibited repetitively fired action potentials at high frequency but were incapable of discharging repetitive grouped discharges. These observations indicate that the Ca2+-dependent K+ conductance involved in the generation of the slow hyperpolarizing afterpotential is the main modulator of the firing behavior of both types of cells.(ABSTRACT TRUNCATED AT 400 WORDS)

Action Potentials↗

PubMed1986

Synaptic organization of inhibitory circuits in the pigeon's optic tectum.

The synaptic organization of inhibitory systems in the pigeon's optic tectum was studied with intracellular recording techniques. An extrapolation procedure based on response latency was used to determine the synaptic delay of the postsynaptic potentials (PSPs) and the velocity of conduction of the associated retinal axons. Tectal cells receive mostly disynaptic, trisynaptic or polysynaptic inhibition from retinal ganglion cells. However, evidence was found which together with previous studies raised the possibility of the existence of a direct inhibitory retino-tectal path. Our present results also suggest that inhibition is transmitted from the retina to the tectal cells by way of both, feedforward and feedback pathways.

Animals↗

PubMed1986

Synaptic transmission of excitation from the retina to cells in the pigeon's optic tectum.

Intracellular recordings were used to study the synaptic excitation of optic tectum neurons in the pigeon. Electrical stimulation of both contralateral optic nerve and ipsilateral optic tract evoked in the tectal neurons EPSPs which in most cases were followed by an IPSP. An extrapolation procedure based on response latency was used to reveal that the EPSPs were mediated by way of mono-, di- and polysynaptic connections with the retinal endings. The laminar location of the recorded cells was estimated according to the field potential and the recording depth with the exception of one cell which was intracellularly stained with HRP. Monosynaptic EPSPs were recorded from cells in the retinorecipient region (sublayers IIa-f) as well as in the non-retinorecipient region (sublayers IIg-j and layer III) of the tectum, while di- and polysynaptic EPSPs were never recorded from the input layers. Tectofugal projections arise largely from layer III neurons. Thus, these results indicate that retinal excitation is transmitted to the output tectal cells by way of mono-, di- or polysynaptic pathways. The conduction velocities of most retinal fibers mediating the EPSP ranged from 4 to 22 m/s (average 12 m/s). However, in a number of retinal fibers the conduction velocities were in a faster range, up to 36 m/s.

Animals↗

PubMed1985

Morphology and laminar distribution of electrophysiologically identified cells in the pigeon's optic tectum: an intracellular study.

The responses of 65 cells to electrical stimulation of the contralateral optic nerve were intracellularly recorded in the pigeon optic tectum by using micropipettes filled with a solution of horseradish peroxidase. Nineteen of them were successfully labeled. Microscopic examination of the filled cells shows that our sample includes six pyramidal, ten ganglion, two stellate, and one bipolar horizontal cells. Thus, pyramidal and ganglion neurons constitute the most numerous types of cells in our sample. Pyramidal cells were located in layer II but mostly in its non-retinorecipient part, and they had restricted ascending dendritic trees oriented orthogonal to the tectal lamination. Ganglion cells were located in layer III with one exception, which was in sublayer IIi. These cells had non-oriented dendritic trees which ramify over considerable distances. Terminal dendritic branches from a number of pyramidal and ganglion cells extended superficially well within the region of optic fibers termination. In our study, ganglion cells constituted the efferent tectal elements. Pyramidal cells responded to optic nerve stimulation with a pure EPSP, with an EPSP-IPSP sequence, or with a pure IPSP. Ganglion cells always exhibited an IPSP either alone or preceded by an EPSP. Stellate and bipolar cells responded with a pure EPSP. The study of the laminar distribution of labeled and non-labeled cells shows from surface to depth, a gradual increase in the number of cells responding with an EPSP-IPSP or with a pure IPSP and a gradual decrease in the number of those exhibiting a pure EPSP. The analysis of the sensitivity of EPSPs and IPSPs to high frequency optic nerve stimulation shows that monosynaptic as well as polysynaptic EPSPs can be recorded from cells in the non-retinorecipient tectal region, a number of ganglion and pyramidal cells receive a direct retinal excitatory input as their dendrites pass through the region of optic endings, most IPSPs are polysynaptic, some cells located in the retinorecipient region may receive direct retinal inhibitory connections.

Animals↗