Ventromedial Thalamic Nucleus Research Articles

Localization of enkephalinergic neurons in the central nervous system of the salmon ( Salmo salar L.) by in situ hybridization and immunocytochemistry

The distribution of neurons expressing preproenkephalin (PPE) mRNA in the brain of the salmon was investigated by means of non-radioactive in situ hybridization, and directly compared with the distribution of enkephalin-immunoreactive (ENKir) neurons. This approach, utilized here for the first time in a non-mammalian vertebrate for the identification of neurons containing opioid peptides, permitted a detailed analysis of the distribution of putative enkephalinergic neurons in the salmon brain. Several cell groups containing neurons that express PPE mRNA also contain ENKir neurons. Such cell groups are located in the ventral telencephalic area, the nucleus of the rostral mesencephalic tegmentum and another nucleus immediately dorsal to it, the torus semicircularis, the valvula cerebelli and the corpus cerebelli. These cell groups consistently contain larger numbers of PPE mRNA expressing cells than ENKir ones. Some cell groups express PPE mRNA, but do not contain ENKir neurons. These cell groups are located in the dorsal telencephalic area, the inferior lobes of the hypothalamus, the pretectal area, the magnocellular superficial pretectal nucleus, the optic tectum, the oculomotor nucleus, the trochlear nucleus, the magnocellular vestibular nucleus, the secondary gustatory nucleus, the superior and medial reticular nuclei, the motor nucleus of the vagus and the ventral horn of the spinal cord. Moreover, some cell groups contain ENKir neurons, but no PPE mRNA expressing neurons. These cell groups are located in the ventromedial thalamic nucleus, the lateral tuberal nucleus, the nucleus of the lateral recess and the nucleus of the posterior recess. The majority of these periventricular ENKir neurons were of the cerebrospinal fluid-contacting type. ENKir neurons were also located in the dorsal lateral tegmental nucleus and in area B9. The results also permitted a tentative identification of enkephalinergic neurons afferent to the optic tectum, that have previously not been identified with immunocytochemistry, located in the dorsal telencephalic area, as well as enkephalinergic neurons intrinsic to the tectum that may contribute to the laminar arrangement of ENKir fibers in the optic tectum.

The thalamic reticular nucleus does not send commissural projection to the contralateral parafascicular nucleus in the rat

The reticular nucleus of the thalamus (NRT) projects to virtually all thalamic nuclei ipsilaterally. In addition, recent studies suggest that NRT sends contralateral projections through an intrathalamic commissural fiber system to several thalamic nuclei, including the NRT itself. In the present study we used retrograde cell labeling, multi-unit anterograde labeling and immunohistochemical methods to study both ipsi- and contralateral NRT projection to the parafascicular nucleus (Pf) in the rat. Injections of the fluorescent tracers true blue or fluorogold in Pf led to massive retrograde cell labeling in rostral and dorsal portions of the ipsilateral NRT, whereas the same sectors of the contralateral NRT were devoid of labeling. Some retrogradely labeled cells were nevertheless present on the contralateral side in the borderline region between NRT and the zona incerta (ZI). Retrograde cell labeling experiments with cholera toxin B subunit (CTb) combined to immunohistochemistry for parvalbumin (PV) and calbindin D-28k (CB) indicated that the few retrogradely labeled cells encountered at the border between NRT and ZI displayed immunoreactivity for CB but not for PV. Since PV and CB label neurons belonging to NRT and ZI, respectively, it is concluded that these contralateral retrogradely labeled cells belong to ZI and not to NRT. Multi-unit cell anterograde labeling experiments with biocytin showed that NRT cells that project to Pf arborize extensively only on the ipsilateral side. The same approach, however, has revealed NRT cells projecting to both ipsi- and contralateral ventromedial thalamic nuclei. The axon of these NRT neurons arborizes more profusely ipsilaterally than contralaterally. These results reveal that the NRT projection to Pf in rodents is strictly unilateral. These findings are at variance with the emerging concept that NRT exerts a prominent bilateral influence upon most thalamic nuclei.

Two channels in the cerebellothalamocortical system

Two channels of the cerebellothalamocortical system were investigated in cats by using cerebellar-evoked synaptic responses and cortical-evoked antidromic invasion of single thalamic cells. One channel arises in interpositus and dentate cerebellar nuclei and mainly projects through ventroanterior-ventrolateral (VA-VL) thalamic nuclei to cortical motor areas 4 and 6; the other channel arises in cerebellar fastigial nuclei and projects through ventromedial (VM) thalamic nuclei to more widespread cortical areas. The antidromic response latencies of VM neurons to stimuli applied to cortical areas 4 and 6 were longer (medians 2.8 and 3.0 msec, respectively) than the antidromic response latencies of VA-VL neurons to stimulation of the same cortical areas (1.8 and 2.3 msec). This was a statistically significant difference, and it matched the longer latencies of fastigial-evoked synaptic responses of VM cells (2.9 msec) compared to the response latencies of VA-VL cells elicited by stimulation of interpositus or dentate nuclei (1.7 and 2.4 msec). These differences among thalamic nuclei relaying cerebellocortical impulses were corroborated by dissimilar effects exerted on the electroencephalogram (EEG) during high-frequency (300 Hz) pulse trains applied to different deep cerebellar nuclei. The distribution of activated EEG patterns over the cortex depended on the stimulated site. Fastigial stimulation elicited the blockage of slow EEG rhythms and the appearance of fast oscillations (20-40 Hz) over widespread cortical areas in the proreus, pericruciate, and suprasylvian gyri. At variance, the activating influence of interpositus or dentate nuclei was restricted to the motor cortex. It is proposed that, besides their role in controlling the postural axial and proximal musculature, fastigial nuclei are part of diffusely activating systems.

Afferent projections to the hypothalamus in ranid frogs.

Hypothalamic afferents were examined following applications of wheat germ agglutinin-horseradish peroxidase in the infundibulum of bullfrogs (Rana catesbeiana). Neuronal populations were generally labelled bilaterally, with an ipsilateral predominance. In the telencephalon, most labelled cells were present in the medial pallium, ventral lateral pallium, lateral amygdala, and lateral septal nucleus. A moderate number of cells were found in the medial amygdala and fewer still in the caudal striatum, olfactory tubercle/diagonal band nucleus, medial septal nucleus, and median septum. In the diencephalon, labelled neurons were primarily found in the anterior and central thalamic nuclei, anterior preoptic area, and suprachiasmatic nucleus. Fewer cells were present in the ventral habenular nucleus, anterior division of the lateral thalamic nucleus, posterior thalamic nucleus, ventromedial thalamic nucleus, magnocellular preoptic nucleus, posterior entopeduncular nucleus, and along the medial forebrain bundle. In the midbrain, a few labelled cells were found in the nucleus of the medial longitudinal fasciculus, suprapeduncular nucleus, and posteroventral tegmental nucleus. The secondary isthmal nucleus contained a large number of labelled neurons and moderate numbers of labelled cells were present in the grey medial to nucleus isthmi and in the isthmal raphe. A few lightly labelled cells were also seen in the parabrachial nucleus. Finally, after the largest applications, a few cells were seen in the obex region ventrolateral to the solitary tract.

The forebrain of gnathostomes: in search of a morphotype.

A morphotype of the forebrain of gnathostomes, i.e. those characters that must have been present in the forebrain of ancestral gnathostomes, was generated by using out-group analysis to identify the shared primitive characters present in the forebrains of extant gnathostomes. The nature of morphotypes and the steps in generating a morphotype are described. Because hypotheses of phylogenetic relationships profoundly affect the resulting morphotype, current hypotheses of gnathostome interrelationships are reviewed, and particular attention is paid to the problematic relationships of lobe-finned fishes. Ontogenetic studies provide the most common basis for how neural characters are grouped, and a review of the developmental literature indicates that gnathostome forebrains are segmented, with the diencephalon arising from a rostral parencephalic neuromere, which subsequently forms anterior and posterior divisions, and a more caudal synencephalic neuromere. Unfortunately, there is no agreement concerning the number of segments that form the secondary prosencephalon (telencephalon and hypothalamus). For this reason, the characters of the secondary prosencephalon must be analyzed in a topological manner. An out-group analysis of the characters of the diencephalon of extant gnathostomes reveals that the diencephalon of ancestral gnathostomes must have arisen from three segments: an anterior parencephalic segment, which gave rise to intermediate, ventrolateral and ventromedial thalamic nuclei; a posterior parencephalic segment, which gave rise to dorsal and ventral habenular nuclei, anterior, dorsal posterior, dorsal central, and, possibly, lateral posterior thalamic nuclei, and posterior tubercular nuclei; a synencephalic segment, which gave rise to pretectal nuclei, accessory optic nuclei and the nucleus of the medial longitudinal fascicle. The pretectal and posterior tubercular regions of ray-finned fishes appear to be highly derived, due to extensive cellular proliferations that give rise to numerous nuclei. The secondary prosencephalon of ancestral gnathostomes was probably divided rostrally into inverted and evaginated cerebral hemispheres, with paired olfactory bulbs arising directly from the hemispheres, and caudally into preoptic and hypothalamic areas. The cerebral hemispheres likely comprised a dorsally situated pallium divided into medial, dorsal and lateral pallial formations, as well as an intercalated pallial nucleus situated ventrolateral to the lateral pallium, and a ventrally situated subpallium divided medially into septal nuclei and a medial amygdalar nucleus and laterally into a corpus striatum. Both pallial and subpallial centers of ancestral gnathostomes probably received ascending thalamic and posterior tubercular inputs, with telencephalic efferent pathways terminating primarily in the hypothalamus, posterior tubercle and midbrain tegmentum. An out-group analysis further indicates that some taxa in each gnathostome radiation exhibit highly derived telencephalic characters due to the independent expansion of one or more pallial formations.

The Forebrain of Gnathostomes: In Search of a Morphotype; pp. 289–303

A morphotype of the forebrain of gnathostomes, i.e. those characters that must have been present in the forebrain of ancestral gnathostomes, was generated by using out-group analysis to identify the shared primitive characters present in the forebrains of extant gnathostomes. The nature of morphotypes and the steps in generating a morphotype are described. Because hypotheses of phylogenetic relationships profoundly affect the resulting morphotype, current hypotheses of gnathostome interrelationships are reviewed, and particular attention is paid to the problematic relationships of lobe-finned fishes. Ontogenetic studies provide the most common basis for how neural characters are grouped, and a review of the developmental literature indicates that gnathostome forebrains are segmented, with the diencephalon arising from a rostral parencephalic neuromere, which subsequently forms anterior and posterior divisions, and a more caudal synencephalic neuromere. Unfortunately, there is no agreement concerning the number of segments that form the secondary prosencephalon (telencephalon and hypothalamus). For this reason, the characters of the secondary prosencephalon must be analyzed in a topological manner. An out-group analysis of the characters of the diencephalon of extant gnathostomes reveals that the diencephalon of ancestral gnathostomes must have arisen from three segments: an anterior parencephalic segment, which gave rise to intermediate, ventrolateral and ventromedial thalamic nuclei; a posterior parencephalic segment, which gave rise to dorsal and ventral habenular nuclei, anterior, dorsal posterior, dorsal central, and, possibly, lateral posterior thalamic nuclei, and posterior tubercular nuclei; a synencephalic segment, which gave rise to pretectal nuclei, accessory optic nuclei and the nucleus of the medial longitudinal fascicle. The pretectal and posterior tubercular regions of ray-finned fishes appear to be highly derived, due to extensive cellular proliferations that give rise to numerous nuclei. The secondary prosencephalon of ancestral gnathostomes was probably divided rostrally into inverted and evaginated cerebral hemispheres, with paired olfactory bulbs arising directly from the hemispheres, and caudally into preoptic and hypothalamic areas. The cerebral hemispheres likely comprised a dorsally situated pallium divided into medial, dorsal and lateral pallial formations, as well as an intercalated pallial nucleus situated ventrolateral to the lateral pallium, and a ventrally situated subpallium divided medially into septal nuclei and a medial amygdalar nucleus and laterally into a corpus striatum. Both pallial and subpallial centers of ancestral gnathostomes probably received ascending thalamic and posterior tubercular inputs, with telencephalic efferent pathways terminating primarily in the hypothalamus, posterior tubercle and midbrain tegmentum. An out-group analysis further indicates that some taxa in each gnathostome radiation exhibit highly derived telencephalic characters due to the independent expansion of one or more pallial formations.

The Forebrain of Gnathostomes: In Search of a Morphotype; pp. 304–318

A morphotype of the forebrain of gnathostomes, i.e. those characters that must have been present in the forebrain of ancestral gnathostomes, was generated by using out-group analysis to identify the shared primitive characters present in the forebrains of extant gnathostomes. The nature of morphotypes and the steps in generating a morphotype are described. Because hypotheses of phylogenetic relationships profoundly affect the resulting morphotype, current hypotheses of gnathostome interrelationships are reviewed, and particular attention is paid to the problematic relationships of lobe-finned fishes. Ontogenetic studies provide the most common basis for how neural characters are grouped, and a review of the developmental literature indicates that gnathostome forebrains are segmented, with the diencephalon arising from a rostral parencephalic neuromere, which subsequently forms anterior and posterior divisions, and a more caudal synencephalic neuromere. Unfortunately, there is no agreement concerning the number of segments that form the secondary prosencephalon (telencephalon and hypothalamus). For this reason, the characters of the secondary prosencephalon must be analyzed in a topological manner. An out-group analysis of the characters of the diencephalon of extant gnathostomes reveals that the diencephalon of ancestral gnathostomes must have arisen from three segments: an anterior parencephalic segment, which gave rise to intermediate, ventrolateral and ventromedial thalamic nuclei; a posterior parencephalic segment, which gave rise to dorsal and ventral habenular nuclei, anterior, dorsal posterior, dorsal central, and, possibly, lateral posterior thalamic nuclei, and posterior tubercular nuclei; a synencephalic segment, which gave rise to pretectal nuclei, accessory optic nuclei and the nucleus of the medial longitudinal fascicle. The pretectal and posterior tubercular regions of ray-finned fishes appear to be highly derived, due to extensive cellular proliferations that give rise to numerous nuclei. The secondary prosencephalon of ancestral gnathostomes was probably divided rostrally into inverted and evaginated cerebral hemispheres, with paired olfactory bulbs arising directly from the hemispheres, and caudally into preoptic and hypothalamic areas. The cerebral hemispheres likely comprised a dorsally situated pallium divided into medial, dorsal and lateral pallial formations, as well as an intercalated pallial nucleus situated ventrolateral to the lateral pallium, and a ventrally situated subpallium divided medially into septal nuclei and a medial amygdalar nucleus and laterally into a corpus striatum. Both pallial and subpallial centers of ancestral gnathostomes probably received ascending thalamic and posterior tubercular inputs, with telencephalic efferent pathways terminating primarily in the hypothalamus, posterior tubercle and midbrain tegmentum. An out-group analysis further indicates that some taxa in each gnathostome radiation exhibit highly derived telencephalic characters due to the independent expansion of one or more pallial formations.

Expression of GFAP in the striatum and its projection areas in response to striatal quinolinic acid lesion in rats.

Injection of quinolinic acid (QA) into the rat striatum is known to produce neuropathological and neurochemical features of Huntington's disease (HD). In the present study the astrocytic response two weeks following intrastriatal injection of either QA (240 nmol) or solvent (1 microliter) was examined using immunohistochemistry for GFAP. Striatal QA lesion resulted in a marked GFAP expression within the whole striatum which is due to neuronal degeneration. Furthermore, the reticular part of substantia nigra, globus pallidus and ventromedial thalamic nucleus revealed increases in GFAP expression without neuronal damage. As this remote astrocytic response is located only in striatal projection areas it seems to reflect transsynaptic changes in response to the lesion.

Neostriatal modulation of motor cortex excitability

The influence of the basal ganglia motor loop on motor cortex function was examined by pharmacologically altering neostriatal activity while monitoring the electrical stimulation thresholds for eliciting movements of the ipsilateral and contralateral motor cortex in ketamine anesthetized rats. Repeated unilateral intraneostriatal infusions (1–3) of the glutamate agonist, kainic acid (0.1 μl, 75 ng), or glutamate (0.3 μl, 1.65 μg) reliably increased ipsilateral but not contralateral cortical thresholds. Single infusions of kainic acid (0.3 μl, 150 or 225 ng) elevated ipsilateral cortical thresholds for 30–45 min; with glutamate (0.3 μl, 1.65 μg), the change lasted less than 10 min. Antidromically identified striatonigral projection neurons ( n = 8) located approximately 500 μM from the infusion cannula, showed either increased firing ( n = 4) for less than 10 min following glutamate infusion or no change from their non-firing state ( n = 4). Non-antidromically activated neurons ( n = 3) were all excited by the infusion, although an interval of inhibition preceded or followed the excitation in two cases. Infusions (0.3 μl) of inhibitory agents (GABA, 31 and 310 ng; muscimol 34.2 ng; and DNQX 34.2 ng) did not alter cortical threshold, nor did saline vehicle. Lesion of the ventrolateral but not ventromedial thalamic nucleus prevented the modulation of cortical thresholds following intraneostriatal infusion of 225 ng kainic acid. Thus the neostriatal alteration of cortical thresholds indicates a modulation of cortical excitability via thalamic projections and not the outcome of competing descending cortical and neostriatal influences converging on motorneurons. These results suggest that tonic feedforward modulation of the motor cortex and the pyramidal tract by the basal ganglia can be inhibitory.

Cerebello-thalamo-cerebral projection from the dentate nucleus onto the frontal eye field in the cat.

To elucidate the role of the cerebellar dentate nucleus in oculomotor system, we investigated the dentato-cerebral projection onto the frontal cortex around the fundus of the presylvian sulcus corresponding to a subregion of the frontal eye field (FEF) in the cat. Stimulation of the dentate nucleus induced laminar field potentials consisting of mainly a surface positive-depth negative wave and a subsequent depth positive wave with or without a surface negative wave in the contralateral FEF. Thalamic relay neurones, identified by collision test, were mainly distributed in the dorsal part of the ventromedial (VM) nucleus. Injection of wheat germ agglutinin conjugated horseradish peroxidase into the thalamic VM nucleus anterogradely labelled afferent terminals in layers I and III of the FEF and retrogradely labelled somata of cerebello-thalamic neurones in the contralateral dentate nucleus. Both electrophysiological and morphological results consistently revealed cerebello-thalamo-cerebral projection onto the FEF which may be implicated in eye-limb coordination control.

Spinothalamic projections in amphibians as revealed with anterograde tracing techniques

Direct spinothalamic pathways were demonstrated in anurans ( Rana ridibunda, Xenopus laevis) and in the ribbed newt, Pleurodeles waltl. With the powerful anterograde tracers Phaseolus vulgaris leucoagglutinin and biotinylated dextran amine, rather extensive spinothalamic projections were found, including the ventromedial thalamic nucleus, the dorsal thalamus and several posterior diencephalic nuclei (anurans), and the neuropil lateral to the pars ventralis thalami as well as to the anteroventral and posterodorsal zones ( P. waltl), respectively.

Quantitative autoradiographic studies of the effects of bilateral olfactory bulbectomy in the rat brain: Central- and peripheral-type benzodiazepine receptors

We investigated the discrete regional effects of bilateral olfactory bulbectomy on central- and peripheral-type benzodiazepine receptors in rat brains at weekly intervals until one month after bulb ablation. Persistent increases in [ 3H]flunitrazepam binding to central benzodiazepine receptors were observed in the cingulum (27%) and in the frontal (15%) and parietal (14%) cortices. Progressive increases in central benzodiazepine receptors, reaching statistical significance four weeks after olfactory bulbectomy, were observed in the ventromedial thalamic nucleus (35%), the lateral hypothalamic region (22%), the basolateral amygdaloid nucleus (23%) and substantia nigra (25%). Persistent major increases (between four- and six-fold) in [ 3H]PK-11195 [ 3H]1-(2-chlorophenyl)- N-methyl- N-(1-methylpropyl)-3-isoquinoline carboxamide binding to peripheral-type benzodiazepine receptors were observed in all anterior olfactory nuclei. Similarly, throughout the time period studied, [ 3H]PK-11195 binding densities were increased two- to three-fold in the piriform cortex and lateral olfactory tract. These observations confirm the usefulness of [ 3H]PK-11195 binding as a marker of neuronal insult in the brain. Moreover, the persistent regional increases in [ 3H]flunitrazepam binding to central-type benzodiazepine receptors suggest that GABAergic transmission is altered following olfactory bulb ablation.

The distribution of GABA-immunoreactive neurons in the brain of the silver eel (Anguilla anguilla L.).

The distribution of GABA-immunoreactivity was studied in the brain of the silver eel (Anguilla anguilla) by means of antibodies directed against GABA. Immunoreactive neuronal somata were distributed throughout the brain. Positive perikarya were detected in the internal cellular layer of the olfactory bulb, and in all divisions of the telencephalon, the highest density being observed along the midline. Numerous GABA-reactive cell bodies were found in the diencephalon, particularly in the preoptic and tuberal regions of the hypothalamus, and the dorsolateral, dorsomedial and ventromedial thalamic nuclei. In the optic tectum, the majority of GABA-positive cell bodies were located in the periventricular layer. A number of immunolabeled cell bodies were observed in different tegmental structures, notably the torus semicircularis. In the cerebellum, the Purkinje cells were either very intensely or very weakly immunoreactive. In the rhombencephalon, reactive cell bodies were observed in the eminentia granularis, the valvula cerebellaris, the octavolateral nucleus, the lobus vagus and in the vagal and glossopharyngeal motor nuclei. Intensely immunoreactive axons and terminals were observed in the external granular layer and internal cellular layer of the olfactory bulb. In the telencephalon, the highest density of reactive fibres and boutons was found in the fields of the medial wall. Many immunolabeled fibres were seen in the medial and lateral forebrain bundles. In the diencephalon, intense labelling of fibres and terminals were observed in the nuclei situated close to the midline. In the optic tectum the highest density of reactive fibres was seen in the sfgs, the layer to which the retina projects massively. Finally, in the rhombencephalon the strongest labelling of neurites was observed in the nuclei of the raphé, the nucleus octavocellularis magnocellularis and the nuclei of the IXth and Xth cranial nerves. The GABAergic system of the eel, which is well developed, appears to be generally comparable to that described in tetrapod vertebrates.

Immunocytochemistry and in situ hybridization of neuropeptide Y in the hypothalamus of Xenopus laevis in relation to background adaptation

The amphibian Xenopus laevis is able to adapt to a dark background by releasing melanophores-timulating hormone from the pars intermedia of the pituitary gland. The inhibition of melanophore-stimulating hormone release is accomplished by neuropeptide Y-containing axons innervating the pars intermedia. To determine the production site of neuropeptide Y involved in this inhibitory control, the distribution of neuropeptide Y in the brain has been investigated by immunocytochemistry and in situ hybridization. Immunoreactive cell bodies were visualized in, among others, the ventromedial and posterior thalamic nuclei, and the suprachiasmatic and ventral infundibular hypothalamic nuclei. A positive hybridization signal with a Xenopus-specific probe for preproneuropeptide Y-RNA was found in the diencephalic ventromedial thalamic nucleus and in the suprachiasmatic nucleus. With both immunocytochemistry and in situ hybridization, suprachiasmatic neurons appeared to be stained only in animals adapted to a white background; animals adapted to a black background showed no staining. Quantitative image analysis revealed that this effect of background adaptation is specific for suprachiasmatic neurons because no effect could be demonstrated of the background light condition on the ventral infundibular nucleus (immunocytochemistry) or the ventromedial thalamic nucleus ( in situ hybridization). These results indicate that neurons in the suprachiasmatic nucleus enable the adaptation of X. laevis to a white background, by producing and releasing neuropeptide Y that inhibits the release of melanophore-stimulating hormone from the melanotrope cells in the pars intermedia of the pituitary gland.

Immunocytochemical Localization of Serotonin and Gonadotropin-Releasing Hormone in the Brain and Pituitary Gland of the Atlantic Croaker Micropogonias undulatus

The distribution of serotonin-immunoreactive (5-HT-ir) and gonadotropin-releasing hormone-immunoreactive (GnRH-ir) elements in the brain and pituitary gland of the Atlantic croaker ( Micropogonias undulatus) was studied using immunoperoxidase and immunofluorescence techniques. 5-HT- and GnRH-ir perikarya were observed in the ventrolateral region of the olfactory bulbs and in the preoptic-anterior hypothalamic area bordering the third ventricle as well as in regions distal to the ependyma of the third ventricle. The immunoreactive elements of the two systems appear to overlap in these regions. Cerebrospinal fluid-contacting 5-HT-ir neurons were found in the periventricular hypothalamus, particularly in the preoptic area and in the nucleus posterior periventricularis, nucleus ventromedialis thalami, nucleus recessus lateralis, and nucleus recessus posterioris. Immunoreactivity to 5-HT in the pituitary was strongest in the proximal pars distalis (PPD) and in a few scattered cells bordering the pars intermedia. Dendritic processes of 5-HT neurons could be seen contacting the adjoining hypophysial cells in the PPD. GnRH-ir fibers were seen entering the pituitary stalk and the immunoreaction was concentrated in the ventral PPD. Several large 5-HT-ir perikarya were located in the raphe region forming a column in the mediosagittal plane. The results suggest that the general organization of the 5-HT system in the brain of the Atlantic croaker is similar to that described in other vertebrates. The demonstration that 5-HT and GnRH systems are in close proximity, particularly in the olfactory bulbs and in the preoptic-anterior hypothalamic area of the croaker brain, suggests that the two systems may interact in these regions of the teleostean brain to control several reproductive functions including spawning behavior and gonadotropin release.

Distribution of corticotropin-releasing factor immunoreactive neurons in the brain of the tigerfrog, Rana tigrina

The distribution of immunoreactive (ir) neurons containing corticotropin-releasing factor (CRF) is described in the brain of the tigerfrog, Rana tigrina. The olfactory bulb, medial pallium, nucleus of the diagonal band of Broca and medial area of the amygdala of the telencephalon showed ir-CRF perikarya. The anterior and ventromedial thalamic nuclei, and the magnocellular nucleus preopticus (NPO) revealed several ir cells; a few NPO neurons were cerebrospinal fluid contacting in nature. The nucleus hypothalamicus ventromedialis contained a few cells, but the nucleus infundibularis ventralis of the infundibulum revealed several diffusely distributed perikarya. Individual ir-CRF perikarya were visualized in the optic tectum and interpeduncular nucleus. Extensive fiber terminals were present in the median eminence, but no fibers were discerned either in the neural lobe or in the pituitary gland.

Characterization of mu opioid receptor binding during amygdala kindling in rats and effects of chronic naloxone pretreatment: an autoradiographic study

Using in vitro autoradiography, mu receptor binding in rat brain was characterized at different amygdala kindling stages and in amygdaloid kindled animals pretreated chronically with naloxone. Male Sprague-Dawley rats implanted with bipolar electrodes in the right amygdala received one of the following pretreatments s.c. for 14 days via osmotic minipumps: normal saline solution, 0.5 μl/h, or naloxone HC1, 75 μg/h. Two days after treatments were accomplished animals were stimulated daily. Our data showed different patterns of mu receptor binding during the normal kindling process: during stage II–III, pronounced binding increase was detected in cingulate, temporal and entorhinal cortices, anterior amygdala, caudate putamen, thalamic nuclei, ventrolateral and dorsolateral portions of central gray, substantia nigra pars compacta and pars reticulata. Twenty-four hours after the last stage V kindled seizure, enhanced binding was observed in cingulate and frontoparietal cortices, anterior amygdala, caudate putamen and ventromedial thalamic nucleus. Twenty-eight days after the last stage V kindled seizure, binding augmentation was noticed in cingulate and frontoparietal cortices, whereas decreased binding was detected in amygdala complex, substantia nigra pars reticulata, piriform, perirhinal, parietal, temporal and entorhinal cortices. Mu receptor binding in kindled rats chronically pretreated with naloxone was significantly higher in several structures when compared with control and normal kindled groups. Our data indicate different regional selective patterns of mu receptor binding during amygdala kindling which may depend on epileptogenesis and long-term changes induced by this process. In addition, even higher mu receptor binding results from chronic naloxone administration prior to kindling.

The development of non-retinal afferent projections to the frog optic tectum and the substance P immunoreactivity of tectal connections

Accessibility in early development and the presence of a retinotopic map have made the amphibian optic tectum a popular system for exploration of the role of synaptic function in central map refinement. Although a great deal is known about the development of retinal innervation of the tectum, little information exists about the development of non-retinal tectal inputs. Since these other afferent systems may contribute to the synaptic drive of developing tectal cells and thereby be involved in the activity-dependent refinement of the retinotectal map, we sought to determine whether these inputs are present at the early tadpole stages when the first retinal axons refine their synaptic order within the tectal neuropil. Rhodamine-labelled latex beads, retrogradely transported from injection sites in the optic tecta, were used to identify tectal afferent projections. Projection patterns in very young tadpoles were identical to those found in juvenile frogs and heavily labelled regions included areas of the posteroventral tegmental field, the posterior tuberculum, the ventromedial thalamic nucleus, the ventral part of the ventrolateral thalamic nucleus, the suprachiasmatic nucleus and discrete regions within the central and anterior thalamic nuclei. Ipsilateral nucleus isthmi cells were also labelled, indicating the existence of an isthmo-tectal projection in even the youngest animals examined. Additionally, substance P-like immunoreactive tecto-isthmal fibers were traced from the optic tectum to the nucleus isthmi. The presence of these connections suggests that feedback from the nucleus isthmi and/or input from the other brain areas projecting to the tectum may play a role in modulating the cellular mechanisms that underlie the formation of the visual map.

Comparison of the Connectional Properties of the Two Forelimb Areas of the Rat Sensorimotor Cortex: Support for the Presence of a Premotor or Supplementary Motor Cortical Area

The existence of multiple motor cortical areas that differ in some of their properties is well known in primates, but is less clear in the rat. The present study addressed this question from the point of view of connectional properties by comparing the afferent and efferent projections of the caudal forelimb area (CFA), considered to be the equivalent of the forelimb area of the primary motor cortex (MI), and a second forelimb motor representation, the rostral forelimb area (RFA). As a result of various tracing experiments (including double labeling), it was observed that CFA and RFA had reciprocal corticocortical connections characterized by preferential, asymmetrical, laminar distribution, indicating that RFA may occupy a different hierarchical level than CFA, according to criteria previously discussed in the visual cortex of primates. Furthermore, it was found that RFA, but not CFA, exhibited dense reciprocal connections with the insular cortex. With respect to their efferent projection to the basal ganglia, it was observed that CFA projected very densely to the lateral portion of the ipsilateral caudate putamen, whereas the contralateral projection was sparse and more restricted. The ipsilateral projection originating from RFA was slightly less dense than that from CFA, but it covered a larger portion of the caudate putamen (in the medial direction); the contralateral projection from RFA to the caudate putamen was of the same density and extent as the ipsilateral projection. The reciprocal thalamocortical and corticothalamic connections of RFA and CFA differed from each other in the sense that CFA was mainly interconnected with the ventrolateral thalamic nucleus, while RFA was mainly connected with the ventromedial thalamic nucleus. Altogether, these connectional differences, compared with the pattern of organization of the motor cortical areas in primates, suggest that RFA in the rat may well be an equivalent of the premotor or supplementary motor area. In contrast to the corticocortical, corticostriatal, and thalamocortical connections, RFA and CFA showed similar efferent projections to the subthalamic nucleus, substantia nigra, red nucleus, tectum, pontine nuclei, inferior olive, and spinal cord.

Sources of presumptive glutamatergic/aspartatergic afferents to the mediodorsal nucleus of the thalamus in the rat.

The distribution of presumptive glutamatergic and/or aspartatergic neurons retrogradely labeled following injections of 3HD-aspartate into the mediodorsal nucleus of the thalamus (MD) in the rat was compared to the distribution of neurons labeled by comparable injections of the nonspecific retrograde tracer wheat germ agglutinin conjugated horseradish peroxidase (WGA-HRP). Cells retrogradely labeled by WGA-HRP were found in the prefrontal and agranular insular cortices; in forebrain structures such as the amygdaloid complex, the piriform cortex, the ventral pallidum and the reticular nucleus of the thalamus; and in several different parts of the brainstem, such as the superior colliculus, central grey, and substantia nigra, pars reticulata. Some, but not all, of these projections are presumably glutamatergic and/or aspartatergic. The projections to MD from the prefrontal and agranular insular cortices are well labeled with 3H-D-aspartate, as are projections from the anterior cortical amygdaloid nucleus. Projections from the superior colliculus to the lateral portion of MD also label with this tracer. However, other forebrain and brainstem projections to MD are not labeled with 3H-D-aspartate, and apparently do not use glutamate or aspartate as a neurotransmitter. These include the projections from the basal and accessory basal amygdaloid nuclei, as well as possibly GABAergic projections from the ventral pallidum and the substantia nigra, pars reticulata. A small fraction of the cells in the piriform cortex that project to MD label with 3H-D-aspartate, suggesting that this projection may be heterogeneous. In other experiments, presumptive GABAergic projections to MD were studied by using 3H-GABA as a retrograde tracer. Although in these cases the thalamic reticular nucleus is well labeled, the ventral pallidum and the substantia nigra, pars reticulata are only poorly labeled. Pallidal projections to the ventromedial thalamic nucleus (VM), which are likely to be GABAergic, were also studied with this technique. After injections of 3H-GABA into VM, only a few cells in the substantia nigra, pars reticulata, or entopeduncular nucleus were labeled. This result suggests 3H-GABA has limited usefulness as a transmitter-specific retrograde tracer.