Nucleus Rotundus Research Articles

Computation of different optical variables of looming objects in pigeon nucleus rotundus neurons.

Three types of looming-selective neurons have been found in the nucleus rotundus of pigeons, each computing a different optical variable related to image expansion of objects approaching on a direct collision course with the bird. None of these neurons respond to simulated approach toward stationary objects. A detailed analysis of these neurons' firing pattern to approaching objects of different sizes and velocities shows that one group of neurons signals relative rate of expansion tau (tau), a second group signals absolute rate of expansion rho (rho), and a third group signals yet another optical variable eta (eta). The rho parameter is required for the computation of both tau and eta, whose respective ecological functions probably provide precise 'time-to-collision' information and 'early warning' detection for large approaching objects.

Bottlebrush dendritic endings and large dendritic fields: Motion-detecting neurons in the tectofugal pathway

In avian and mammalian brains, visual information from the retina is conveyed to the telencephalon via two separate pathways: the thalamofugal and the tectofugal pathways. Recently, Karten et al. ([1997] J. Comp. Neurol. 387:449-465) examined a portion of the tectofugal pathway, the projection from the optic tectum to the nucleus rotundus thalami, in pigeons. They defined two distinct subpopulations of tectal neurons projecting from the stratum griseum centrale (SGC; tectal layer 13) to specific divisions of the rotundus. The goal of this study in chick was to verify the existence of the type I and type II SGC neurons, as defined by Karten et al., and then examine in greater detail the connectivity and morphology of these SGC neurons. Furthermore, our results suggest how the unique morphological characteristics of SGC neurons contribute to the large receptive fields (20-50 degrees) found in physiological recordings and the SGC neuronal response to extremely small (ca. 0.05 degree), fast-moving (100 degrees/second) stimuli. By injecting retrograde tracer into various divisions of the chick rotundus, we verified that, indeed, the chick did possess type I and type II SGC neurons, as well as a "new" type of SGC neuron, type III, that is not found in the pigeon. We then used intracellular cell-filling techniques to define further these three types of SGC neurons. Our examination revealed the following: Type I SGC neurons had large, circular dendritic fields (average diameter, 1,725 microns) composed of smooth dendrites and ending in spine-rich, bottlebrush endings located in retinorecipient tectal layer 5b; type II SGC neurons had elliptical dendritic fields (average 1,447 microns) and dendritic endings located never more superficially than tectal layer 8; and type III SGC neurons had large dendritic fields (average 1,800 microns) of unknown shape and bottlebrush dendritic endings located in retinorecipient tectal layer 4. We suggest that the neuronal features of the SGC neurons (i.e., bottlebrush dendritic endings and large dendritic fields) are key morphological characteristics for the detection of motion within the tectofugal pathway. Furthermore, because neurons with similar morphology have also been found in the tecta of both mammals and reptiles, we suggest that these neuronal features are fundamental components of a phylogenetically conserved system used for the "extrastriate" detection of motion in vertebrates.

Bottlebrush dendritic endings and large dendritic fields: Motion‐detecting neurons in the tectofugal pathway

In avian and mammalian brains, visual information from the retina is conveyed to the telencephalon via two separate pathways: the thalamofugal and the tectofugal pathways. Recently, Karten et al. ([1997] J. Comp. Neurol. 387:449–465) examined a portion of the tectofugal pathway, the projection from the optic tectum to the nucleus rotundus thalami, in pigeons. They defined two distinct subpopulations of tectal neurons projecting from the stratum griseum centrale (SGC; tectal layer 13) to specific divisions of the rotundus. The goal of this study in chick was to verify the existence of the type I and type II SGC neurons, as defined by Karten et al., and then examine in greater detail the connectivity and morphology of these SGC neurons. Furthermore, our results suggest how the unique morphological characteristics of SGC neurons contribute to the large receptive fields (20–50°) found in physiological recordings and the SGC neuronal response to extremely small (ca. 0.05°), fast-moving (100°/second) stimuli. By injecting retrograde tracer into various divisions of the chick rotundus, we verified that, indeed, the chick did possess type I and type II SGC neurons, as well as a “new” type of SGC neuron, type III, that is not found in the pigeon. We then used intracellular cell-filling techniques to define further these three types of SGC neurons. Our examination revealed the following: Type I SGC neurons had large, circular dendritic fields (average diameter, 1,725 μm) composed of smooth dendrites and ending in spine-rich, bottlebrush endings located in retinorecipient tectal layer 5b; type II SGC neurons had elliptical dendritic fields (average 1,447 μm) and dendritic endings located never more superficially than tectal layer 8; and type III SGC neurons had large dendritic fields (average 1,800 μm) of unknown shape and bottlebrush dendritic endings located in retinorecipient tectal layer 4. We suggest that the neuronal features of the SGC neurons (i.e., bottlebrush dendritic endings and large dendritic fields) are key morphological characteristics for the detection of motion within the tectofugal pathway. Furthermore, because neurons with similar morphology have also been found in the tecta of both mammals and reptiles, we suggest that these neuronal features are fundamental components of a phylogenetically conserved system used for the “extrastriate” detection of motion in vertebrates. J. Comp. Neurol. 396:399–414, 1998. © 1998 Wiley-Liss, Inc.

Glutamatergic Neurotransmission from the Optic Tectum to the Contralateral Nucleus Rotundus in Pigeons

Neuronal responses of 114 cells to electrical stimulation of the optic tectum were extracellularly recorded from the contralateral nucleus rotundus (nRt) in pigeons (Columba livia), and the effects of two glutamatergic antagonists CNQX and CPP, as well as those of GABA, were examined on rotundal cells. The results show that tectal stimulation evokes excitation in neurons of the contralateral nRt and that the excitatory responses are blocked by microiontophoretically applied CNQX, CPP and GABA. It is suggested that the contralateral tectorotundal transmission is excitatory in nature and mediated by both glutamatergic AMPA and NMDA receptors and that GABA is an inhibitory transmitter within the nucleus as well. We also review our previous findings in discussing transmitters participating in the tectofugal pathway.

Organisation of the tectorotundal and SP/IPS‐rotundal projections in the chick

The organisation of the neural projections from the optic tectum and pretectal nuclei complex, n. subpretectalis / n. interstitio-pretecto-subpretectalis (SP/IPS), to the nucleus rotundus (Rt) in chicks was studied by using retrograde tracing techniques. After the injection of fluorescent retrograde tracers, rhodamine-conjugated latex microspheres, fluorescein-conjugated latex microspheres, True Blue, Fluoro-Gold, or rhodamine B isothiocyanate, into different regions of Rt and its middorsal extension, the nucleus triangularis (T), the distribution of retrogradely labelled neuronal cell bodies in the tectum and pretectal nuclei was assessed. Both the ipsilateral and contralateral tectorotundal projections were found to be organised topographically in as much as different sublaminas of the stratum griseum centrale (SGC) project in an orderly manner to Rt and T. The deepest stratum of SGC overlapping into the stratum album centrale projects to T. Deep SGC projects to the dorsal Rt and superficial SGC to the ventral Rt. A band running through the centre of Rt receives input from the central sublamina of SGC, and the caudal central Rt receives input from a deeper sublamina than does the rostral central Rt. The SP/IPS projects to the ipsilateral Rt only and the projection order is dorsal SP to dorsal Rt, ventral SP to ventral Rt and middle SP to the central band of Rt. The neurones in IPS and the nucleus of the tractus tectothalamicus project to T. Thus, Rt and T receive topographically both tecto- (excitatory) and SP/IPS- (inhibitory) projections. The possible functional implications for parallel information processing in these projections are discussed. J. Comp. Neurol. 394:171–185, 1998. © 1998 Wiley-Liss, Inc.

Organisation of the tectorotundal and SP/IPS‐rotundal projections in the chick

The organisation of the neural projections from the optic tectum and pretectal nuclei complex, n. subpretectalis / n. interstitio-pretecto-subpretectalis (SP/IPS), to the nucleus rotundus (Rt) in chicks was studied by using retrograde tracing techniques. After the injection of fluorescent retrograde tracers, rhodamine-conjugated latex microspheres, fluorescein-conjugated latex microspheres, True Blue, Fluoro-Gold, or rhodamine B isothiocyanate, into different regions of Rt and its middorsal extension, the nucleus triangularis (T), the distribution of retrogradely labelled neuronal cell bodies in the tectum and pretectal nuclei was assessed. Both the ipsilateral and contralateral tectorotundal projections were found to be organised topographically in as much as different sublaminas of the stratum griseum centrale (SGC) project in an orderly manner to Rt and T. The deepest stratum of SGC overlapping into the stratum album centrale projects to T. Deep SGC projects to the dorsal Rt and superficial SGC to the ventral Rt. A band running through the centre of Rt receives input from the central sublamina of SGC, and the caudal central Rt receives input from a deeper sublamina than does the rostral central Rt. The SP/IPS projects to the ipsilateral Rt only and the projection order is dorsal SP to dorsal Rt, ventral SP to ventral Rt and middle SP to the central band of Rt. The neurones in IPS and the nucleus of the tractus tectothalamicus project to T. Thus, Rt and T receive topographically both tecto- (excitatory) and SP/IPS- (inhibitory) projections. The possible functional implications for parallel information processing in these projections are discussed.

Ascending projections from the optic tectum in the lizard Podarcis hispanica.

The ascending projections of the optic tectum, including their cells of origin, have been studied in the lizard Podarcis hispanica by means of a two-step experimental procedure. First, tracers were injected in the tectum to study the anterograde labeling in the forebrain. Second, the cells of origin of these projections have been identified by analyzing the retrograde labeling after tracer injections in the thalamus, hypothalamus, and pretectum. Three main tectal ascending pathways have been described: the dorsal tecto-thalamic tract (dtt), the medial tecto-thalamic tract (mtt), and the ventral tecto-thalamic tract (vtt). The dtt originates in radial cells of layers 5 and 7 and bipolar cells of layers 8 and 10 that project to the lateral neuropile of the dorsal lateral geniculate nucleus (GLD), to the intergeniculate leaflet (IGL), and to the ventral lateral geniculate nucleus (GLV). The mtt arises from radial neurons of layers 3 and 5 and bilaterally reaches the putative reticular thalamus and its boundary with the hypothalamus, the rostral IGL, and the area triangularis (AT). The vtt is composed of fibers from ganglion and multipolar cells of the layer 7 that project bilaterally to the nucleus of the vtt, the ventrolateral thalamic nucleus, the medial posterior thalamic nucleus (MP), the nucleus rotundus (Rot), the IGL, and the cell plate of the GLD. Therefore, the GLD receives not only direct retinal afferents but also two different tectal inputs, thus constituting a convergence point in the two visual pathways to the telencephalon. Moreover, different tectal cells specifically project to the ventrolateral thalamus and to pretectal nuclei. These results are discussed from comparative and functional viewpoints.

Golgi and Electron-Microscopic Golgi-GABA Immunostaining Study of the Avian Optic Tectum

The neuronal structures and their possible connections in the optic tectum of chicks were studied by using Golgi impregnation, in light microscopy, and Golgi-GABA immunogold staining in electron microscopy. The terminals of the optic fibers displayed different patterns and orientation in various laminae of the avian optic tectum. The shape of the branching and the terminals of the optic fibers conformed to the postsynaptic neuronal structures. In Golgi preparations, the terminals in tectal layers 4 and 5 seemed to be located rather densely, and they contact, among other things, the ‘dendritic terminal sections’ of large ganglion cells of layer 13 of the tectum. This connection could be responsible for direct optic transmission via ganglion cells to the nucleus rotundus. Additionally, the contacts established by optic terminals in these layers with the tectal neurons arranged with their dendrites parallel to the surface of the tectum, could provide the structural basis for an inhibitory system. The terminals of isthmic nuclei were found in this optic lamina, too. The optic terminals in layer 7 contacted the dendritic side branches of some radial tectal neurons.

Hyperstriatum ventrale in pigeons: effects of lesions on color-discrimination and color-reversal learning.

Previous lesion studies of color-reversal learning in pigeons show that an impairment results when (1) the tectofugal visual pathway is damaged at either the thalamic level (nucleus rotundus) or the telencephalic level (ectostriatum), or (2) the thalamofugal visual pathway is damaged at the telencephalic level (the visual Wulst). An impairment does not result, however, when the thalamic source of thalamofugal input (n. opticus principalis thalami or OPT) to the visual Wulst is damaged. These results suggest that the visual Wulst plays a role in color-reversal learning as a consequence of visual information routed from the tectofugal pathway via other visual areas in the telencephalon. One such area is the hyperstriatum ventrale (HV). In the present study, after ablation of the medial and lateral regions of HV, pigeons were trained postoperatively to discriminate between two colors presented simultaneously. After reaching criterion, the pigeons were required to perform a series of discrimination reversals in which the positive and negative stimuli were interchanged. Lesions of medial HV resulted in impaired performance of a color-discrimination task (i.e. original learning), but did not affect discrimination reversal. An impairment in color-reversal learning resulted from combined damage to lateral HV and the fronto-thalamic tract (FT), which carries ascending visual input from OPT to the visual Wulst. No deficits were observed when either lateral HV or FT were damaged alone. These findings suggest that both the thalamofugal and tectofugal pathways provide the visual Wulst with visual input relevant to color-reversal learning.

Independent effects of incubation temperature and gonadal sex on the volume and metabolic capacity of brain nuclei in the leopard gecko (Eublepharis macularius), a lizard with temperature-dependent sex determination.

The extent to which variation within and between the sexes can be assigned to genes vs. environment is problematic, because, in most vertebrates, males and females differ genetically. However, factors other than sex chromosomes and the consequent sex-typical gonadal hormone secretions may play important roles in the differentiation of the neural mechanisms underlying individual and sex differences in aggressive and sexual behavior. The leopard gecko, like many oviparous reptiles, lacks sex chromosomes. Instead, gonadal sex is determined by temperature during embryogenesis, with low and high incubation temperatures producing females and intermediate temperatures producing mixed sex ratios. In essence, this allows for the study of individual and sex differences without the confounding variable of genetically determined gender. Experiments have shown that the temperature experienced during incubation plays a critical role in establishing the adult morphological, endocrinological, and behavioral phenotype. In this experiment, the independent effects of incubation temperature and gonadal sex on the morphology and metabolic capacity of specific brain nuclei were determined. Both individual and sex differences in the volume of the preoptic area and ventromedial nucleus of the hypothalamus are determined primarily by incubation temperature, not by gonadal sex. However, incubation temperature and gonadal sex are both important in determining the metabolic capacity in the anterior hypothalamus, external amygdala, dorsal lateral nucleus of the hypothalamus, dorsal lateral nucleus of the thalamus, dorsal ventricular ridge, habenula, lateral hypothalamus, nucleus rotundus, nucleus sphericus, periventricular nucleus of the hypothalamus, preoptic area, periventricular nucleus of the preoptic area, septum, striatum, torus semicircularis, and ventromedial nucleus of the hypothalamus. This is the first demonstration in a vertebrate that factors other than gonadal sex hormones, which arise from the individual's genetic constitution, can affect the sexual differentiation of the brain.

Androgen control of vocal control region volumes in a wild migratory songbird (Junco hyemalis) is region and possibly age dependent.

Previous laboratory studies have shown that photoperiodic adult songbirds experience seasonal variations in singing frequency that correlate with plasma androgen levels, as well as changes in the brain regions that control singing (vocal control regions). The present study investigates naturally occurring seasonal changes in the sizes of these regions in a wild migratory species (dark-eyed junco, Junco hyemalis), with samples from adolescence to post-breeding fall migration. In adult males, the volumes of the vocal control regions area X and the higher vocal center (HVC) were large during the breeding season when birds were singing and androgen levels were high, and decreased in size after the breeding season when singing had stopped and androgen levels were low. HVC volume in adolescent males caught in the fall (no singing), when plasma androgen levels were low, was smaller than in breeding adults, thereby following the seasonal pattern of change in plasma androgen levels. In adolescent males, however, area X volume was the same as in breeding adults. Thus, area X size in adolescent male juncos may be testosterone independent. The seasonal pattern of robust nucleus of the archistriatum volume was similar to that of the HVC. The volumes of neither the magnocellular nucleus of the anterior neostriatum nor the nucleus rotundus, a control region, differed seasonally. Castration of breeding adult males caused both area X and HVC volumes to decrease compared to castrated controls with testosterone replacement, indicating that maintenance of these two region volumes is testosterone dependent in adults.

Some Morphological Features of a Visual Thalamic Nucleus in a Reptile: Observations on Nucleus rotundus in <i>Caiman crocodilus</i>

The morphology of nucleus rotundus, a visual thalamic nucleus, was investigated in one species of reptiles. Caiman crocodilus, using Nisst stained material in transverse, sagittal, and horizontal planes. The topographical location of nucleus rotundus and its relationship to surrounding thalamic nuclear groups are described. Nucleus rotundus in Caiman can be subdivided into three areas: (1) an outer shell; (2) an inner core; and (3) a cell poor zone located between the shell and core. Most rotundal core neurons were round, fusiform, triangular, pear-shaped, or elliptical. Core neurons were not distributed evenly throughout the nucleus but, in many instances, were arranged in clusters composed of two to ten neurons. Quantitative measurements of area, perimeter, and eccentricity (greatest width/greatest length), which served as an index of cell roundness, were made on rotundal core neuron profiles in transverse, sagittal, and horizontal planes of section. Qualitative and quantitative observations were not appreciably different regardless of the plane of orientation. Both qualitative and quantitative data suggest that relay cells located in the core of nucleus rotundus are not a homogeneous population of neurons but comprise several subtypes.

Distribution of AVT gene expressing neurons in the prosencephalon of Japanese quail and chicken

Detailed anatomical studies about the distribution of arginine-vasotocin (AVT) gene expressing neurons in the avian brain are so far non-existent. Most of the investigations are limited to discrete brain nuclei in the chicken and concern particular experimental conditions. Since no data on AVT gene expression are up to now available in Japanese quail, we report here the distribution of AVT mRNA containing neurons within the prosencephalon of this bird and a comparison with the same system of the chicken. The AVT mRNA containing perikarya were detected with a specific AVT cDNA probe directed toward the distal 3' glycopeptide encoding part of the chicken AVT gene. The specificity of the probe was tested by Northern and Southern blot analysis. The AVT gene expressing neurons showed the same pattern of distribution in both species; however, a larger number of labelled neurons was observed in the chicken by visual inspection of the sections. AVT gene expressing neurons were distributed within the hypothalamus (in the medial and ventral aspects of the rostral preoptic area, in the lateral forebrain bundle, close to the ependyma of the third ventricle and in several periventricular hypothalamic nuclei) and in some extrahypothalamic regions (around the nucleus rotundus, close to the nucleus commissurae pallii and in a region probably corresponding to the nucleus strial terminalis). AVT gene expressing neurons were observed close to the organum septi laterale of the chicken. Taken together, these results confirm the specificity of the employed probe and its suitability for further studies on the modulation of AVT gene expression in quail.

GABAergic inputs to the nucleus rotundus (pulvinar inferior) of the pigeon (columba livia).

The avian nucleus rotundus, a nucleus that appears to be homologous to the inferior/ caudal pulvinar of mammals, is the major target of an ascending retino-tecto-thalamic pathway. Further clarification of the inputs to the rotundus and their functional properties will contribute to our understanding of the fundamental role of the ascending tectal inputs to the telencephalon in all vertebrates, including mammals. We found that the rotundus contains a massive plexus of glutamic acid decarboxylase (GAD)-immunoreactive axons using antibodies against GAD. The cells within the rotundus, however, were not immunoreactive for GAD. The retrograde tracer cholera toxin B fragment was injected into the rotundus to establish the location of the afferent neurons and determine the source of the gamma-aminobutyric acid (GABA) inputs into the rotundus. In addition to the recognized bilateral inputs from layer 13 of the tectum, we found intense retrograde labeling of neurons within the ipsilateral nuclei subpretectalis (SP), subpretectalis-caudalis (SPcd), interstitio-pretecto-subpretectalis (IPS), posteroventralis thalami (PV), and reticularis superior thalami (RS). All the neurons of the SP, SPcd, IPS, and PV were intensely GAD-immunoreactive. The neurons of layer 13 of the tectum were not immunoreactive for GAD. Following the destruction of the ipsilateral SP/IPS complex, we found a major reduction in the intensity of the GAD axonal immunoreactivity within the ipsilateral rotundus, but this destruction did not diminish the intensity of the GAD-immunoreactivity within the contralateral rotundus. Our studies indicated that the source of the massive GAD-immunoreactive plexus within the rotundus was from the ipsilateral SP, SPcd, IPS, and PV nuclei. These nuclei, in turn, received ipsilateral tectal input via collaterals of the neurons of layer 13 in the course of their projections upon the rotundus. We suggest that the direct bilateral tecto-rotundal projections are excitatory, whereas the indirect ipsilateral projections from the SP/IPS and PV are mainly inhibitory, possibly acting via a GABA-A receptor.

Effects of diencephalic lesions on approach responses and color preferences in quail

The effects of stereotaxic radio-frequency lesions on artificially selected approach responses and artificially selected or imprinted red or blue preferences were studied in Japanese quail chicks. No effects were found from lesions in anterior preoptic or pretectal areas and only slight attenuation of red preferences by lesions in nucleus rotundus, opticus principalis thalami and geniculatus lateralis pars ventralis. Extensive lesions in medial diencephalic and bordering areas of telencephalon, ansa lenticularis, lateral and medial forebrain bundle diminished approach tendencies and greatly attenuated the genetically influenced red preferences. Medial diencephalic lesions confined to the dorsomedial thalamic complex and lateral hypothalamus left approach tendencies intact but similarly attenuated only red preferences. Imprinted red or blue preferences of a highly imprintable quail line were also attenuated; red preference strongly, blue preference moderately. The data indicate differential diencephalic mediation of the quail's genetically distinct color preferences and suggest diverse subtelencephalic channels for mediation of approach responses and stimulus preferences. The results also suggest common neural mediation of genetically determined and acquired stimulus preferences.

Excitatory and inhibitory neurotransmitters in the nucleus rotundus of pigeons.

Rotundal neurons in pigeons (Columba livia) were examined for the effects of glutamate and its agonists NMDA and AMPA, antagonists CPP and CNQX, as well as of GABA and its antagonist bicuculline, on visual and tectal stimulation-evoked responses. Glutamate applied by iontophoresis excited all 48 rotundal cells tested, and this excitation was blocked by CNQX but not by CPP in 98% of cases, with 2% of cells being blocked by either CNQX or CPP. Out of 21 cells excited by AMPA, 20 were also excited by NMDA, indicating that AMPA and NMDA receptors may coexist in most rotundal cells. Action potentials were evoked in 36 additional cells by electrical stimulation applied to the tectum and they were also blocked by CNQX but not CPP. Visual responses recorded from a further eight luminance units and 21 motion-sensitive units were also blocked by CNQX and not CPP. On the other hand, GABA inhibited visual responses as well as responses evoked by tectal stimulation. An inhibitory period following tectal stimulation was eliminated by bicuculline. Taken together, these results indicate that glutamate may be an excitatory transmitter acting predominantly through non-NMDA receptors (AMPA receptors) in tectorotundal transmission. Meanwhile, GABA may be an inhibitory transmitter in the pigeon nucleus rotundus.

The dorsal thalamus of jawed vertebrates: a comparative viewpoint.

In anamniotes, the dorsal thalamus comprises: (1) a caudal division, the collothalamus, which receives its predominant input from the midbrain roof and projects ipsilaterally to the telencephalon, predominantly to the striatum, and (2) a rostral division, the lemnothalamus, which predominantly receives a direct retinal (lemniscal) input and projects bilaterally to the telencephalon, predominantly to the pallium. In amniotes, collothalamic nuclei relay visual, auditory, and somatosensory-multisensory inputs from the midbrain roof to the ipsilateral telencephalon, terminating in both striatum and pallium. For example, the collothalamic visual nuclei consist of the LP-pulvinar complex in mammals and nucleus rotundus in diapsid reptiles, birds, and turtles. Among amniotes, the latter nuclei are homologous to each other as discrete nuclei, as are the collothalamic auditory and collothalamic somatosensory-multisensory nuclei. Lemnothalamic nuclei (and nuclear groups) in amniotes predominantly (and/or plesiomorphically) receive lemniscal inputs; some project to the telencephalon bilaterally, and most, in contrast to collothalamic nuclei, do not project to the striatum. In mammals, the lemnothalamic nuclei include most of those in the anterior, medial, intralaminar, and ventral nuclear groups and the dorsal lateral geniculate nucleus. In diapsid reptiles, they include the dorsomedial and dorsolateral anterior nuclei and the dorsal lateral optic nucleus; comparable nuclei are present in birds and turtles, with birds additionally having a discrete somatosensory lemniscal relay nucleus. These lemnothalamic nuclei in each amniote radiation are homologous as a field to the lemnothalamus (i.e., nucleus anterior) in anamniotes. Both divisions of the dorsal thalamus were elaborated to some degree in the common ancestral amniote stock. A further major elaboration of the lemnothalamus characterized the ancestral stock of mammals and may have been one of the key events in early mammalian evolution. Birds have independently, to a lesser degree, elaborated the lemnothalamus.

Bombesin-like immunoreactivity in the forebrain and pituitary and regulation of anterior pituitary hormone release by bombesin in goldfish.

The presence and distribution of bombesin (BBS)/gastrin-releasing peptide (GRP)-like immunoreactivity (IR) was examined in the goldfish pituitary and forebrain. BBS/GRP-like IR nerve fibres were consistently observed throughout the pituitary neurointermediate lobe (NIL); occasionally, BBS/GRP-like material was localized in cells and few fibres within the pars distalis. Within the goldfish forebrain, sparse, fine-beaded BBS/GRP-like IR fibres and few perikarya were detected in the preoptic hypothalamus. BBS/GRP-like IR material was also present in the ventro-posterior hypothalamus and the hypothalamic inferior lobes, and, in particular, within the nucleus lateral tuberis pars anterioris (NLTa) and nucleus lateral tuberis pars posterioris (NLTp), nucleus anterior tuberis (NAT), nucleus recessus lateralis (NRL), nucleus recessus posterioris (NRP), and the nucleus diffusus lobi inferioris (NDLI). Several BBS/GRP-like IR perikarya were observed in periventricular regions of the NLT, NRL, and NRP. Finally, BBS/GRP-like IR material was detected in the nucleus habenularis, the nucleus rotundus, several thalamic nuclei, and the optic tectum of the dorso-posterior diencephalon. The presence of BBS/GRP-like IR material within the preoptic hypothalamus, hypophysial stalk, and in the pituitary suggests that BBS-like peptides may regulate pituitary hormone release in fish. Perifusion of goldfish pituitary fragments with initial pulses of 0.1, 10, 100 or 1,000 nM BBS stimulated both growth hormone (GH) and gonadotropin (GtH-II) release. Repeated pulses of the same dose of BBS generally stimulated GH and GtH-II release to a similar magnitude. These studies provide initial evidence that BBS/GRP-like peptides are present within the central nervous system of teleost fish. The anatomical distribution of BBS/GRP-like IR in goldfish hypothalamic feeding center supports our previous report indicating a role for BBS in the central regulation of food intake in fish. Additionally, the presence of BBS/GRP-like IR material in the pituitary and brain areas associated with the regulation of anterior pituitary hormone secretion, as well as evidence demonstrating a direct action of BBS at the level of the goldfish pituitary to modify GH and GtH-II secretion, indicates that BBS/GRP-like peptides likely act to regulate pituitary hormone release in teleosts.

Central Melatonin Binding Sites in Rainbow Trout (Onchorhynchus mykiss)

A combination of in vitro autoradiography and membrane homogenate receptor assays has been used to localize and characterize 2-[125I]iodomelatonin binding sites in the brain of the rainbow trout (Onchorhynchus mykiss). Specific 2-[125I]iodomelatonin binding, defined as that displaced by 1 μM melatonin, increased linearly with increasing protein concentration in membrane homogenates of whole trout brain. Specific binding was both time and temperature dependent and reversible in the presence of 1 μM melatonin. Binding was saturable at between 100-150 pM 2-[125I]iodomelatonin and Scatchard analysis of saturation isotherms revealed a dissociation constant (Kd) of 15.00 ± 0.95 pM and a maximum receptor number (Bmax) of 42.35 ± 2.70 fm/mg protein (n = 16). Addition of 10-4M GTPγS (an analogue of guanosine triphosphate) to saturation isotherms apparently reduced the Bmax by 75% on average with no apparent change in the affinity of the binding. Scatchard analysis of saturation isotherms generated from whole brain membrane homogenates of trout kept on long days (15 hr light:9 hr dark) and killed either during the midlight or middark phase showed no significant differences in either the Kd or the Bmax of 2-[125I]iodomelatonin binding, although a robust rhythm in melatonin concentration was confirmed in these fish. Displacement of 2-[125I]iodomelatonin binding with increasing concentrations of competing ligands gave an order of potency of 2-iodomelatonin > melatonin ⪢ 5-HT. Localization of specific central 2-[125I]iodomelatonin binding in the rainbow trout showed high levels of binding associated with neuronal areas involved in the processing of visual signals, particularly the optic tectum and nucleus rotundus. Lower levels of binding were present over the hypothalamus but no specific binding was found over the pituitary. Thus 2-[125I]iodomelatonin binding in the rainbow trout brain fulfills the basic criteria for a receptor linked to a mediatory G-protein and is found widely localized within the trout brain.

Unilateral optic nerve transection decreases 2-[ 125I]-iodomelatonin binding in retinorecipient areas and visual pathways of chick brain

In chick brain, specific 2-[ 125I]-iodomelatonin-binding was localized primarily in the visual system, i.e., retinorecipient and relay nuclei and fiber tracts of the tectofugal, thalamofugal, circadian and accessory visual pathways. Unilateral transection of the optic nerve (ONX) significantly reduced the binding of 2-[ 125I]-iodomelatonin (75 pM) in many, but not all, primary retinal targets and visual pathways at 7 and 14 days, but not 1 day, postlesion. As measured using quantitative autoradiography, 2-[ 125I]-iodomelatonin binding was decreased by 90% in both the central portion of the lesioned optic tract and one of its targets, the nucleus of the basal optic root (nBOR). Other retinorecipient areas exhibiting substantial decreases (60%) in 2-[ 125I]-iodomelatonin-binding included the optic tectum, lateroventral and dorsolateral geniculate nuclei and tectal gray area contralateral to the lesion. These findings are consistent with the hypothesis that melatonin receptors are located presynaptically on incoming optic nerve terminals in many retinorecipient areas. This localization may account for most of the binding sites in nBOR. In other primary visual areas, however, melatonin receptors also appear to be located on postsynaptic cells and/or non-retinal afferents. ONX had no significant effect on 2-[ 125I]-iodomelatonin binding in two retinorecipient areas, the visual suprachiasmatic nucleus and the dorsolateral anterior thalamus, which are part of the circadian/oculomotor and thalamofugal pathways, respectively. An unexpected consequence of ONX was that 2-[ 125I]-iodomelatonin binding was decreased in certain secondary (nucleus rotundus, isthmi nuclei) and tertiary level (ectostriatum) nuclei along the prominent tectofugal visual pathway. Binding in the tectorecipient nucleus triangularis was not significantly altered, however. Analysis of secondary level relay nuclei in the oculomotor pathway revealed that binding after ONX was decreased in the ipsilateral Edinger-Westphal nucleus but not in the oculomotor nuclei. Selective transsynaptic changes in 2-[ 125I]-iodomelatonin binding after lesion of the visual input most likely reflect activity-dependent regulation and functional plasticity of central melatonin receptors.