Serotonin Modulates Differently the Functional Properties of Damaged and Intact Motoneurons in the Frog Spinal Cord.

When studying a preparation of the isolated spinal cord segment of an adult frog, damaged and intact lumbar motoneurons were found to differ significantly in the membrane potential, input resistance and the action potential properties (amplitude, duration, fast and medium phases of the afterhyperpolarization, and the frequency of spikes). Serotonin (5-HT) reduced the amplitude of afterpolarization and increased the frequency of the spikes of the intact neurons, while in the damaged motoneurons, 5-HT increased the amplitude of afterpolarization and had no effect on the frequency of discharges.

Presynaptic serotonin 5-HT1B/D receptor-mediated inhibition of glycinergic transmission to the frog spinal motoneurons.

Endogenous monoamine 5-hydroxytryptamine (5-HT, serotonin) is a phylogenetically ancient neurotransmitter present in vertebrates. The functions of 5-HT in central nervous system are intensively studied; however, the presynaptic effects of 5-HT in frog spinal motoneurons are practically unexplored. We have previously shown that 5-HT decreases the frequency of glycinergic miniature inhibitory postsynaptic potentials (mIPSPs), but does not affect the frequency of GABAergic mIPSPs and increases the frequency of glutamatergic postsynaptic potentials. In the present study, using pharmacological methods and intracellular recordings in motoneurons from an adult frog's isolated spinal cord, we aimed to identify the 5-HT receptor subtype responsible for inhibiting the release of glycine. Аn agonist of 5-HT1A and 5-HT7 receptors, 8-OH-DPAT, and a selective agonist of 5-HT2 receptors, α-Ме-5-НТ, did not show any significant effect on inhibitory transmission, indicating that 5-HT1A, 5-HT2, and 5-HT7 receptors are not involved in the modulation of glycine release in the adult frog spinal cord. An agonist of 5-HT1B/D receptors sumatriptan decreased the frequency (but not the amplitude) of glycinergic mIPSPs similar to 5-HT. An antagonist of 5-HT1,2 receptors, methysergide, abolished the effect of sumatriptan. Together our results suggest that 5-HT inhibits the release of glycine by activation of 5-HT1B/D receptors.

Distribution of calcium-binding proteins, parvalbumin and calbindin, in the midbrain auditory center (MLd) of a pigeon.

Immunohistochemical distribution of calcium-binding proteins, parvalbumin (PV) and calbindin (CB), has been studied in the mesencephalic auditory center (MLd) of pigeon (Columba livia). In the central region of the MLd (core, ICC), an overlap in distribution of the PVand CB-immunopositive (ip) neurons and neuropil has been observed, with different patterns in the central and peripheral parts. In the peripheral region of the MLd (belt, ICS, and ICX), both neurons and neuropil contained only CB. A selective CB chemospecificity of the belt, ICS, and ICX is an evolutionary conserved feature characteristic of all avian species. Interspecies differences in the distribution of PV and CB immunoreactivity in the ICC are a result of adaptive functional specialization, which provides specific processing of different aspects of the auditory information.

[PRESYNAPTIC SEROTONERGIC MODULATION OF SPONTANEOUS AND MINIATURE SYNAPTIC ACTIVITY IN THE FROG LUMBAR MOTONEURONS].

In this study we investigated the effect of 5-HT on the spontaneous and miniature synaptic activity in lumbar motoneurons from isolated frog spinal cord using intracellular recording. 5-HT increased the frequency of the spontaneous and miniature postsynaptic potentials (mPSPs). The effect of 5-HT on different subpopulations of mPSPs was multidirectional: it increased the frequency of glutamatergic excitatoty mPSP by 18 % and decreased the frequency of glycinergic inhibitory mPSP by 28 %, but had no effect on the frequency of GABAergic inhibitory mPSP. The amplitude and kinetic parameters of any subpopulation of mPSPs did not change. The data obtained show that 5-HT regulates the probability of glutamate and glycine release from presynaptic terminals ending at the frog spinal motoneurons. 5-HT shifts the ba- lance between synaptic excitation and inhibition in the spinal neural network toward excitation. Thus, 5-HT participates in control of motor output and provides its facilitation.

[SYNAPTIC AND ELECTROTONIC CONTACTS ON THE PRIMARY AFFERENT AXONS OF THE SPINAL CORD IN THE LAMPREY LAMPETRA FLUVIATILIS].

Distribution of GABA and glycine immunoreactivity was studied in synapses on primary afferent axons of the spinal cord in the lamprey Lampetrafluviatilis by double labeling technique. Approximately 25 % of synapses on afferent axons revealed immunoreactivity to GABA and more than 70 % were im- munoreactive to both neurotransmitters. As in other vertebrates, axo-axonal contacts represented three-component synaptic complexes, the so-called triades, where the immunoreactive terminal was in sy- naptic contact both with the afferent axon and the dendrite contacting with this afferent. Contact zones with membrane specializations of gap junction type were found between adjacent afferents that suggests the presence of electrotonic interaction between afferents which serves, evidently, for synchronization of afferent flow and presents a structural base for a mechanism of fast interneuronal communication of func- tionally uniform neurons which is an important element in organization of coordinated locomotor acts. Besides, our studies provide evidence that afferent-afferent interaction may be mediated not only electro- tonically but also with the aid of chemical synapses. This fact gives grounds to suppose that depolariza- tion of primary afferents, produced by glutamate, occurs not only through autoreception mechanism but also by its direct action on the membrane of an afferent axon.

[THE INHIBITORY EFFECT OF GLYCINE AND GABA ON MONOSYNAPTIC EPSPs IN THE FROG SPTNAL MOTONFUCTYNMR].

In this research we have studied the inhibitory effect of glycine and GABA on the monosynaptic EPSPs induced by microstimulation of presynaptic fibers close to the frog lumbar motoneurones. Monosynaptic EPSPs had two components mediated by AMPA/KA (78%) and NMDA (22%) receptors. Both inhibitory mediators considerably decreased EPSP's decay time by 33.4 +/- 4.0% (n = 18) for glycine and by 40.2 +/- 3.6% (n = 18) for GABA. The decrease of EPSP's decay time was reduced after blockade ofNMDA receptors by AP5. The time characteristics were recovered in the normal saline. Glycine and GABA inhibited NMDA component of EPSPs to a greater extent than AMPA/KA component. It was confirmed by our previous data about the predominant inhibitory effect of glycine and GABA on responses induced by NMDA (as compared with AMPA and KA) application to motoneurone membrane.

[AMYGDALA: NEUROANATOMY AND NEUROPHYSIOLOGY OF FEAR].

This work describes neuroanatomical and neurophysiological mechanisms of Pavlovian fear conditioning, focusing on contributions of the amygdala, a subcortical nuclear group, to control of conditioned fear responses. The mechanisms of synaptic plasticity at projections to the amygdala and within amygdala were shown to mediate the formation and retention of fear memory. This work reviews current data on anatomical organization of the amygdala, as well as its afferent and efferent projections, in respect to the role of the amygdala in auditory fear conditioning during which acoustic signals serve as the conditioned stimulus.

The evolution of the centrifugal visual system of vertebrates. A cladistic analysis and new hypotheses.

In a recent review of the available data concerning the centrifugal visual system (CVS) of vertebrates [Repérant, J., Ward, R., Miceli, D., Rio, J.P., Médina, M., Kenigfest, N.B., Vesselkin, N.P., 2006. The centrifugal visual system of vertebrates: a comparative analysis of its functional anatomical organization, Brain Res. Rev. 52, 1-57], we have shown that this feature of the visual system is not a particularity of birds, but is a permanent component of the vertebrate central nervous system which nevertheless shows considerable morphological and functional variation from one taxonomic group to another. Given these findings, the primary objective of the present article is an attempt to specify the evolutionary significance of this phylogenetic diversity. We begin by drawing up an inventory of this variation under several headings: the intracerebral location of the retinopetal neurons; the mode of intra-retinal arborizations of the centrifugal fibres and the nature of their targets; their neurochemical properties; and the afferent supplies of these neurons. We subsequently discuss these variations, particularly that of the intracerebral location of the retinopetal neurons during development and in adult forms, using the neuromeric terminology and in the framework of cladistic analysis, and seek to interpret them in a phylogenetic context. From this analysis, it becomes evident that the CVS is not a homogeneous entity formed by neurons with a common embryological origin, but rather a collection of at least eight distinct subsystems arising in very different regions of the neuraxis. These are the olfacto-retinal, dorsal thalamo-retinal, ventral thalamo-retinal, pretecto-retinal, tecto-retinal, tegmento-mesencephalo-retinal, dorsal isthmo-retinal and ventral isthmo-retinal systems. The olfacto-retinal system, which is probably absent in Agnatha, appears to be a pleisiomorphic characteristic of all Gnathostomata, while on the other hand the tegmento-mesencephalo-retinal system appears to be present only in Agnatha. Our cladistic analysis also shows that the remaining six subsystems are polyphyletic in origin and have arisen independently on several occasions in different radiations of Gnathostoma. In conclusion, we suggest that, in the course of the palaeontological history of vertebrates, these different retinopetal pathways have been selected on the basis of widely different environmental pressures which remain to be identified.

The centrifugal visual system of vertebrates: a comparative analysis of its functional anatomical organization.

The present review is a detailed survey of our present knowledge of the centrifugal visual system (CVS) of vertebrates. Over the last 20 years, the use of experimental hodological and immunocytochemical techniques has led to a considerable augmentation of this knowledge. Contrary to long-held belief, the CVS is not a unique property of birds but a constant component of the central nervous system which appears to exist in all vertebrate groups. However, it does not form a single homogeneous entity but shows a high degree of variation from one group to the next. Thus, depending on the group in question, the somata of retinopetal neurons can be located in the septo-preoptic terminal nerve complex, the ventral or dorsal thalamus, the pretectum, the optic tectum, the mesencephalic tegmentum, the dorsal isthmus, the raphé, or other rhombencephalic areas. The centrifugal visual fibers are unmyelinated or myelinated, and their number varies by a factor of 1000 (10 or fewer in man, 10,000 or more in the chicken). They generally form divergent terminals in the retina and rarely convergent ones. Their retinal targets also vary, being primarily amacrine cells with various morphological and neurochemical properties, occasionally interplexiform cells and displaced retinal ganglion cells, and more rarely orthotopic ganglion cells and bipolar cells. The neurochemical signature of the centrifugal visual neurons also varies both between and within groups: thus, several neuroactive substances used by these neurons have been identified; GABA, glutamate, aspartate, acetylcholine, serotonin, dopamine, histamine, nitric oxide, GnRH, FMRF-amide-like peptides, Substance P, NPY and met-enkephalin. In some cases, the retinopetal neurons form part of a feedback loop, relaying information from a primary visual center back to the retina, while in other, cases they do not. The evolutionary significance of this variation remains to be elucidated, and, while many attempts have been made to explain the functional role of the CVS, opinions vary as to the manner in which retinal activity is modified by this system.

Comparative study of spinal motoneuron axon collaterals.

Numerous spinal motoneurons in mammals possess recurrent axon collaterals included in a feedback loop for controlling motoneuron activity. For nonmammalian vertebrates, the data concerning the existence of collaterals and their intraspinal branching are fragmentary and contradictory. We focused on axonal branching of motoneurons in lampreys, frogs, turtles and young rats, using light microscopic analysis of HRP- or neurobiotin-labeled motoneurons. In lampreys, only a restricted portion of spinal motoneurons, related to the dorsal fins, showed recurrent collaterals. In frogs, a great complexity and high total length of collateral branches as well as a great number of axon swellings were found. In turtles, axon collateralization of spinal motoneurons was much more restricted, and present in particular in lumbar motoneurons innervating proximal hindlimb muscles. Young rat spinal motoneurons have rather abundant recurrent axon collaterals. It is likely that the presence of axon collaterals from spinal motoneurons is related to the level of complexity of locomotion.

Dual effect of GABA on descending monosynaptic excitatory postsynaptic potential in frog lumbar motoneurons.

Monosynaptic excitatory postsynaptic potentials (EPSPs) evoked by stimulating ipsilateral ventrolateral column (VLC) in the thoracic section were recorded in lumbar motoneurons within the isolated spinal cord of the frog Rana ridibunda. Bath application of the selective GABAB receptor agonist (-)-baclofen (0.05 mM) caused a reduction in the peak amplitude of VLC EPSP. Baclofen did not cause any consistent change in the membrane potential or in the EPSP waveform within frog motoneurones. The selective GABA(B) receptor antagonist saclofen (0.1 mM) completely blocked the effect of (-)-baclofen on VLC EPSP. A decrease in VLC EPSP peak amplitude was also observed during GABA (0.5 mM) application. Unlike (-)-baclofen, inhibition of VLC EPSP induced by GABA was accompanied by a shortening of the EPSP time course and a reduction in membrane input resistance within lumbar motoneurons. The decrease in VLC EPSP peak amplitude induced by (-)-baclofen and GABA was accompanied by an increase in the paired-pulse facilitation. These data provide evidence for a dual pre- and postsynaptic GABAergic inhibition of the VLC monosynaptic EPSP in lumbar motoneurons within the frog spinal cord.

Ultrastructural study of glutamate- and GABA-immunoreactive terminals contacting the primary afferent fibers in frog spinal cord. A double postembedding immunocytochemical study.

The in vitro HRP application to the dorsal root of the frog spinal cord produced an intensive staining of primary afferent fibers. A double postembedding GABA and glutamate immunocytochemical study revealed GABA- or glutamate-immunopositive presynaptic boutons establishing axo-axonic synapses onto HRP-stained primary afferent fibers in the spinal cord intermedial zone.

Dual innervation of the lamprey retina by GABAergic and glutamatergic retinopetal fibers. A quantitative EM immunogold study.

A quantitative electron microscopic analysis of glutamate and GABA immunocytochemistry, using the postembedding immunogold technique, was undertaken in the lamprey retina to determine the proportion of glutamate-immunoreactive (GLU-ir) centrifugal visual terminals which were identified by anterograde axonal transport after an iontophoretic deposit of HRP in the sectioned optic nerve. Single immunogold labeling carried out with two different GABA and GLU antibodies showed that about 45% of the retinopetal axon terminals were GABAergic, and that two types of GLU-ir terminals (GLU-ir/1 and GLU-ir/2) were observed in the inner plexiform layer. The former type showed a high density of gold particles (Neosystem: 19.38+/-0.74; Sigma: 106.26+/-5.70) which statistically differed from the GLU-ir/2 (Neosystem: 3.23+/-0.31; Sigma: 31.73+/-5.61). Subcellular estimates showed that gold particles were concentrated over the vesicular pool in the GLU-ir/1 terminals and over mitochondria in the second terminal type. Consecutive sections alternately processed for GABA and GLU revealed that the GABA-immunonegative terminals corresponded to the GLU-ir/1, whereas the GABA-ir terminals only contained few GLU-ir particles, mostly concentrated in mitochondria. The occurrence of GABAergic and glutamatergic retinopetal terminals in the adult lamprey retina is discussed in functional terms of their differential inhibitory and excitatory effects on ganglion cell activity and the possible role of the centrifugal visual system in visually-guided behavior.

Evolutionary significance of different neurochemical organisation of the internal and external regions of auditory centres in the reptilian brain: an immunocytochemical and reduced NADPH-diaphorase histochemical study in turtles.

An immunocytochemical and histochemical study was undertaken of the torus semicircularis and nucleus reuniens, the mesencephalic and diencephalic auditory centres, in two chelonian species, Testudo horsfieldi and Emys orbicularis. The nucleus centralis of the torus semicircularis receives few 5-HT-, TH-, substance P-, and menkephalin-immunoreactive fibres and terminals, in marked contrast to the external nucleus laminaris of the torus semicircularis, in which 5-HT-, TH-, substance P-, and menkephalin-immunoreactive elements and cell bodies show a laminar distribution. Dense NPY-positive terminal-like profiles and cell bodies were observed in both the nuclei centralis and laminaris, and many NADPH-d-positive cell bodies were observed in the cell layers of the latter. In the nucleus reuniens, the distribution of 5-HT-, TH-, substance P-, and menkephalin-immunolabelling resembles that seen in the torus semicircularis, but at a lower density. The dorsorostral regions of the nucleus reuniens, as in the nucleus centralis, is insignificantly labelled, in contrast to the ventrocaudal regions in which labelled elements abound. NPY-positive elements are uniformly distributed throughout the nucleus, but no labelled cell bodies were observed. NADPH-d-positive fibres and terminals were observed in both dorsal and ventral regions of the nucleus reuniens, but the few labelled cell bodies to be observed were located in the peripheral regions of the nucleus. These findings are discussed in terms of the evolution of the core-and-belt organisation of sensory nuclei observed in other vertebrate species.

Axo-axonic GABA-immunopositive synapses on the primary afferent fibers in frogs.

In three frog species Rana esculenta, Rana temporaria and Xenopus laevis, the contacts established by gamma-aminobutyric acid and glutamate decarboxylase immunoreactive (-ir) terminals upon primary afferent fibers were studied using confocal and electron microscopy. For confocal microscopy, the primary afferent fibers were labeled through the dorsal root with Dextran-Texas Red, whereas gamma-aminobutyric acid and glutamate decarboxylase immunoreactivity were revealed with fluorescein isothiocyanate. Appositions of gamma-aminobutyric acid and glutamate decarboxylase immunoreactive profiles onto primary afferent fibers were observed and were considered as putative axo-axonic contacts of GABAergic terminals upon primary afferents. The latter was confirmed by the ultrastructural finding of axo-axonic synapses from gamma-aminobutyric acid immunopositive boutons upon the HRP-labeled primary afferent fibers in postembedding immunoelectron microscopic study. Such synapses may represent the morphological basis of GABAergic presynaptic inhibition of primary afferent fibers.

Pretectal connections in turtles with special reference to the visual thalamic centers: a hodological and gamma-aminobutyric acid-immunohistochemical study.

Projections of the pretectal region to forebrain and midbrain structures were examined in two species of turtles (Testudo horsfieldi and Emys orbicularis) by axonal tracing and immunocytochemical methods. Two ascending gamma-aminobutyric acid (GABA)ergic pathways to thalamic visual centers were revealed: a weak projection from the retinorecipient nucleus lentiformis mesencephali to the ipsilateral nucleus geniculatus lateralis pars dorsalis and a considerably stronger projection from the nonretinorecipient nucleus pretectalis ventralis to the nucleus rotundus. The latter is primarily ipsilateral, with a weak contralateral component. The interstitial nucleus of the tectothalamic tract is also involved in reciprocal projections of the pretectum and nucleus rotundus. In addition, the pretectal nuclei project reciprocally to the optic tectum and possibly to the telencephalic isocortical homologues. Comparison of these findings with previous work on other species reveals striking similarities between the pretectorotundal pathway in turtles and birds and in the pretectogeniculate pathway in turtles, birds, and mammals.