The first optic ganglion of the bee

Each visual unit (ommatidium) of the compound eye of the honey bee contains nine retinula cells, six of which end as axons in the first synaptic ganglion, the lamina, and three in the second optic ganglion, the medulla. A technique allowing light- and electron microscopy to be performed on the same silver-impregnated sections has made it possible to follow all types of retinula axons of one ommatidium to their terminals in order to study the shape of the terminal branches with their position in the cartridge. 1. The axons of retinula cells 1-6 (numbered according to Menzel and Snyder, 1974) end as three different types of short visual fibres (svf) in the lamina; the axons of retinula cells 7-9 run through the lamina to terminate in the medulla and are known as long visual fibres (lvf). Retinula cells of each type are identified by the location of their cell bodies and by the direction of their microvilli. The retinula cells 1 and 4 (group I according to Gribakin, 1967) end as svf type 1 with three tassel-like branches in stratum B of the first synaptic region. The pair of cells 3, 6 and the pair 2, 5 (group II) end in the first synaptic region in stratum A. Cells 3 and 6 have forked endings, svf type 2, whereas cells 2 and 5 have tapered endings, svf type 3. The remaining retinula cells 7, 8 and 9 have long fibres. Nos. 7 and 8 (group III) have tapered endings and are termed lvf types 1 and 2, respectively. The 9th cell is the lvf type 3 with a highly branched ending. 2. The nine axons in the bundle from one ommatidium have relative positions which do not change from the proximal retina to the monopolar cell body layer. 3. By following silver-stained retinula cells and their corresponding axons, it is possible to describe mirror-image arrangements of fibres in the axon bundles in different parts of the eye. This correlation of numbered retinula cells with specific axon types, together with the highly organized pattern in an axon bundle, allows the correlation between histological and physiological findings on polarization and colour perception.     

The first optic ganglion of the bee

Summary The synaptic relationships between and within receptor-cell axons (RCAs), first-order interneurones (L-fibres) and accessory fibres (acc) in the first optic ganglion (the lamina) of the worker bee were studied in serial sections with Golgi-EM and routine transmission electron microscopy. The ommatidium contains nine retinular (photoreceptor) cells all of which project as RCAs to a single optical cartridge in the lamina. Six of the RCAs end as short visual fibres (svf) in the lamina, while the remaining three, the so-called long visual fibres (lvf), pass the lamina and end in the second optic ganglion, the medulla. In addition to the RCAs and an unknown number of accessory fibres, the cartridge also contains four L-fibres (L 1–4). The spatial arrangement of the RCAs and L-fibres within a cartridge is constant throughout the depth of the lamina. Serial sections reveal a great number of chemical synapses interconnecting RCAs, L-and acc fibres. Double T-shaped presynaptic dense projections are surrounded and in close association with either spherical or flattened synaptic vesicles. The finding of gap junctions between and within identified RCAs and L-fibres suggest that these axons may be electronically coupled. A model for information processing in the lamina of the bee is suggested from observations of synaptic connectivity between and within fibres of one cartridge.     

Anatomical identification of spectral receptor types in the retina and lamina of the Australian orchard butterfly,Papilio aegeus aegeus D.

Summary The nine receptor cells examined in each ommatidium of the butterfly Papilio aegeus aegeus can be named according to their positional orientation across the fused rhabdom. Six of them end as short visual fibres (svf) in the second stratum of the lamina, whereas the remaining three retinula cells (lvf) pass together with the lamina fibres (L-fibres) the first optic ganglion and the outer chiasma to end in the three most distal layers of the second optic ganglion, the medulla. The organization of the retinula-cell axons within the pseudocartridge and the cartridge remains almost uniform throughout the first optic ganglion. Five L-fibres, which have their origin in the fenestrated layer (FL), join each laminar cartridge before entering the neuropil of the first optic region. Four of these L-fibres (L-1, L-2, L-3 and L-4) could be definitely located and characterized using Golgi-stained light- and electron-microscopic techniques. Whereas L-1 and L-3 show a lateral branching pattern reaching only fibres of the same cartridge, L-2 and L-4 have long collaterals interconnecting several neighbouring cartridges in a characteristic pattern. Serial sections of silver-impregnated retinula-cell axons as well as L-fibres were investigated for their synaptic connectivity patterns between and within these fibres. These cellular interactions and possible information processing are discussed.     

Circadian Rhythms of Locomotor Activity in Lycosa tarentula (Araneae, Lycosidae) and the Pathways of Ocular Entrainment

Lycosa tarentula is a ground-living spider that inhabits a burrow where it awaits the appearance of prey or conspecifics. In this study, circadian rhythms of locomotor activity were examined as well as the ocular pathway of entrainment. Thirty-three adult virgin females were examined under constant darkness (DD); all of them exhibited robust circadian rhythms of locomotor activity with a period averaging 24.1h. Fourteen of these spiders were studied afterwards under an LD 12:12 cycle; they usually entrained to in the first or second day, even when the light intensity was as low as 1 lx. During the LD cycle, locomotor activity was generally restrained to the darkness phase, although several animals showed a small amount of diurnal activity. Ten males were also examined under LD; they were also nocturnal, but were much more active than the females. Seven females were examined under constant light (LL); under this they became arrhythmic. Except for the anterior median eyes (OMAs), all the eyes were capable of entraining the locomotor activity to an LD cycle. These results demonstrate that under laboratory conditions and low light intensities locomotor activity of Lycosa tarentula is circadian and in accordance with Aschoff's 'rule'. Only OMAs are unable to entrain the rhythm; the possible localization of circadian clock is therefore discussed.     

Golgi studies of the first optic ganglion of the ant,Cataglyphis bicolor

The neurons of the first optic ganglion (the lamina) in the desert ant, Cataglyphis bicolor, have been studied with the light microscope after Golgi silver impregnation. The different types of retinal and laminal fibres and their configuration are compared with the results obtained in the bee. The first synaptic region in the visual system of the ant lies proximally to the fenestrated layer below the basement membrane and the layer containing the monopolar cell bodies. The synaptic region can be separated into three morphologically different zones: (1) The most distal layer where the short visual fibres end at two different levels. The short visual fibres and some laminal fibres (monopolar cell fibres) also show lateral elements in this region. (2) The second layer appears almost free of branches of retinal and laminal fibres. (3) The most proximal layer, which has a characteristically dense horizontal structure resulting from the lateral elements of long visual, centrifugal, monopolar and tangential fibres. Nine cell axons arising from each ommatidium leave the retina. Six of these are short visual fibres and end at two different levels in the lamina. Three different types of short visual fibres can be distinguished by their different terminal depths and lateral branching pattern. The remaining three fibres, the long visual fibres, terminate in the medulla. They can be distinguished from each other by their lateral elements in the lamina neuropile. The five morphologically different laminal fibre types (axons of the monopolar cells in the lamina) have different shapes and different arborizations at different levels. Tangential, centrifugal and incerta sedis-fibres, which originate either from cell bodies in the cell body layer at the periphery of the outer chiasma or more centrally, terminate in the synaptic region of the lamina. Consideration is given to the clearly demarkated arrangement and length of the branching pattern of retinal and laminal fibres at different levels of the synaptic region of the lamina. In addition, a hypothetical connectivity pattern is discussed.     

The retina-lamina projection in the visual system of the bee,Apis mellifera

Single Golgi impregnated visual cells and their axons were treated from the retina to the first synaptic layer (lamina) in serial electron microscopic sections. This analysis of the retina-lamina projection was undertaken in the upper dorso-median eye region which is known to be involved in the perception of polarized light. For identification of individual visual cells and their fibres a numbering system was used which relates the number of each of the nine visual cells within one retinula to the transverse axis of the rhabdom (TRA) (Fig. 1). Because of the twist of the retinula along its course to the basement membrane (Fig. 6), individual visual cells change their position relative to any eye-constant co-ordinate system. Each axon bundle originating from one 9-celled retinula performs a 180 degrees-rotation before entering the lamina (Fig. 2). The direction of rotation (clockwise or counter-clockwise), which may differ even between adjacent bundles, is related to the two mirror-image types of rhabdoms in the corresponding retinulae and is opposite to the direction of rhabdom twist. Thus, even in small groups of the in total 5500 ommatidia in the eye of the bee, two types of retinulae exist which can be characterized by the geometry of the rhabdoms as well as by the direction of rotation of the retinulae and the axon bundles (Fig. 1). Visual cell numbers 1, 2, and 9, the microvilli of which are oriented in the direction of TRA, form three long visual fibres terminating in the second synaptic layer (medulla). In cross sections of laminar pseudocartridges they appear as the smallest fibre profiles arranged in a symmetrical line of the pseudocartridge bundle (=the transverse axis of the pseudocartridge; TPA) (Fig. 4). The remaining six fibres (cell numbers 3-8) only project to the lamina (short visual fibres; svf's). Two of them (cell numbers 5 and 6), which are the largest cells in the proximal retinula and have their microvilli perpendicularly arranged to TRA (Fig. 1), give rise to the two thickest axons of the underlaying pseudocartridge. In cross sections, t he connecting line of these two axons is orthogonally oriented to TPA (Fig. 5). A model was developed, in which all long visual fibres originate from ultraviolet receptors and in which the polarization sensitivity of the basal ninth cell is enhanced by the twist of the rhabdom. Finally, this model is discussed in light of behavioral experiments revealing the ultraviolet receptors as the only cells involved in the detection of polarized light.     

Evidence for projections from medullary nuclei onto serotonergic and dopaminergic neurons in the midbrain dorsal raphe nucleus of the rat

Summary The anterograde tracer Phaseolus vulgaris-leucoagglutinin was injected into the medial nucleus of the solitary tract and into the rostral dorsomedial medulla. A sequential two-color immunoperoxidase staining was accomplished in order to demonstrate the co-distribution of presumed terminal axons with chemically distinct neurons in the dorsal raphe nucleus of the midbrain central gray, i.e., B7 serotonergic and A10dc dopaminergic neurons. Black-stained efferent fibers from the medial nucleus of the solitary tract and the rostral dorsomedial medulla intermingled with brown-stained serotonergic (5-hydroxytryptamine-immunoreactive) or dopaminergic (tyrosine hydroxylase-immunoreactive) neurons. Light microscopy revealed that the black-stained efferent axons exhibited numerous en passant and terminal varicosities that were often found in close apposition to brown-stained serotonergic and dopaminergic somata, and to proximal and distal dendrites and dendritic processes. The close association of immunoreactive elements suggests the presence of axo-somatic and axodendritic synaptic contacts of medullary fibers with serotonergic and dopaminergic neurons in the dorsal raphe nucleus. These projections could be involved in the modulation of dorsal raphe neurons, depending on the autonomic status of an animal.     

Morphology of electrophysiologically identified baroreceptor afferents and second order neurones in the brainstem of the cat

Baroreceptor afferent fibres and second order baroreceptor neurones were identified by their discharge pattern and were intracellularly injected with horseradish peroxidase. Three afferent fibres and three second order neurones were reconstructed by camera lucida drawings from serial sections of the brainstem. The afferent fibres were classified as A delta-fibres and had terminal arborizations with synaptic boutons in the dorsomedial region of the nuclei of the solitary tract (TS). The afferent fibres had additional collaterals with a medial projection to the commissural nucleus and in a direction lateral to the TS. The terminals of these collaterals could not be demonstrated. The second order neurones were located in the same dorsomedial region as the synaptic boutons of the afferent fibres. Neurones were small and spindle-shaped with two primary dendrites: one dendrite projected cranially along the medial border of the TS, and the second one projected caudally and medially into the commissural nucleus. The unmyalinated axons of these neurones could be traced over a distance of 1 mm. In only one neurone could an axon collateral be detected. The axons projected dorsally around the TS in a ventrolateral direction beyond the boundaries of the nuclei of the TS. The axon collateral projected in the medial direction into the commissural nucleus. In no case were axon terminals demonstrated.     

The olivocerebellar projection to lobules I and II.

The olivocerebellar projection to lobules I and II was studied by means of retrograde transport from implants of the crystalline WGA-HRP complex. Retrogradely labelled neurons were found in the medial and dorsal accessory olives. Judged from the distribution of labelled cells, we conclude that parasagittally the olivocerebellar terminal zones A and B (i.e., the cerebellar cortical strips receiving axons from the olivary A and B regions) extend anteriorly into lobule Ia, whereas the fused olivocerebellar terminal C1/C3 zones reach lobule IIa. The olivocerebellar terminal C2 zone extends into lobule IIIa but not into lobule IIa. In lobule I the medial border of the B zone lies about 1 mm from the midline, in lobule II the B zone extends somewhat more medially. The lateral border of this zone is 1.9-2 mm from the midline. Compared to previous results, it appears that most of the Purkinje cells in lobule I projecting to the vestibular nuclei lie medially to the olivocerebellar terminal B zone.     

The Organization of the lamina ganglionaris of the prawn,Pandalus borealis (Kröyer)

The Lamina ganglionaris (first optic neuropile) of the decapod crustacean Pandalus borealis has its optic cartridges (synaptic compartments) arranged in horizontal rows. Each optic cartridge contains seven receptor axon terminals and the branching axis fibres of five monopolar second order neurons. Four types of monopolar neurons are classified. Their cell bodies are arranged in two layers. The inner layer contains the cell bodies of exclusively one of these types, and each cartridge is invaded by two neurons of this neuron type (type M 1:a and M 1:b). The outer layer contains the cell bodies of the remaining three types (M 2, M3 and M4). One gives rise to a large radially branched axis fibre in the centre of the cartridge. The other two have wide branches which may make inter-cartridge contacts, one proximally and the other distally in the plexiform layer, which is clearly bistratified. The receptor axons terminate in two levels corresponding to these strata. Two sets of tangenital fibres form networks in the proximal and the mid-portion of the lamina. Both networks have fibres with primary branches in the vertical plane and secondary branches in the horizontal plane. The fibres of the networks are derived from axons that pass from the second optic neuropile, the medulla externa.     

Neuronal connectivity patterns in the compound eyes of Artemia salina and Daphnia magna (Crustacea: Branchiopoda)

The neuronal types and patterns in the visual system of the species Artemia salina and Daphina magna have been studied with the Golgi method and electron microscopy. The lamina contains five classes of neurons: photoreceptor axons, monopolar, centrifugal, tangential and amacrine neurons. The terminals of the receptor axons are distributed in two (A. salina) or three (D. magna) layers. The dilated terminals have an extensive and wide array of fine branches. One axon from each ommatidium bypasses the lamina and terminates in the medula in A. salina. A. salina has four types of monopolar neurons, two of which are stratified, whereas in D. magna only two types are found, one of which is bistratified. Tangential T-neurons connect the lamina with the protocerebrum. D. magna has in addition one tangential T-neuron connecting both the lamina and the medulla with the protocerebrum. In both species monopolar-type centrifugal neurons connect the medulla and the lamina, whereas that of A. salina has a wide laminar distribution. Both species also have amacrine cells in the lamina. The medulla contains, besides those shared with the lamina, transmedullary neurons (two types in A. salina), amacrine cells and neurons originating in the protocerebrum. "Cartridge"-type synaptic compartments are lacking in the investigated species, although a periodic arrangement is discernible in the distal portion of the lamina of A. salina. The receptors from three types of specialized contacts in Artemia, one of which involves a dyad. D. magna has only one-to-one synapses. Neurosecretory fibres are absent in A. salina.     

Postembryonic changes in the optic primordia and optic bud in the flesh fly Sarcophaga ruficornis fabr. (Diptera: Sarcophagidae)

Differentiation of the optic lobe anlagen begin in the brain of second instar. Each is an elongated disc of cortical cells placed on the dorsolateral border of each protocerebrum. In the late second instar the disc elongates and its two ends bend inwards which gradually separate from the central region, thus giving three imaginal discs. The protocerebral neuropile extends into these discs and medulla interna and externa are formed. The rudiments of compound eyes (cephalic complex) appear in the early laid larva. These are attached with the brain and pharyngeal wall separately. The posterior portion of cephalic complex (optic bud), after establishing a nervous association with the central optic lobe anlage (lamina ganglionaris), forms the compound eye. Ech optic bud is attached to the brain by a non-nervous stalk. The epiblast cells of the optic bud do not migrate into the brain and the lamina is formed by the proliferation of the central imaginal disc. The reorientation of the optic lobe anlagen starts in the late third instar and the medulla interna divides into two unequal lobes. In 2 day pupa the nerve fibres from the lamina travel into the optic stalk and the optic nerve is formed. The epiblast cells of the optic bud differentiate to form a peripheral epithelial layer which becomes pigmented and gets apposed to the lateral boundary of the brain. The central epiblast cells of the optic bud form several ommatidia. The optic nerve degenerates gradually and various components of the compound eye are formed by the epiblast cells. Chiasm internum is present but chiasm externum is absent.     

The distribution of aminergic neurones and their projections in the brain of the teleost,Myoxocephalus scorpius

Summary Fluorescent histochemistry was carried out on the brain of the teleost Myoxocephalus scorpius to show the distribution of monoaminergic neurones and their projections.Posterior to the obex of the fourth ventricle, at the junction of the spinal chord and medulla, there is an unpaired dorsal nucleus of catecholaminergic cells. A second group of catecholaminergic perikarya are scattered lateral to the vagal and glossopharyngeal motor nuclei. Both groups of aminergic cells contribute to a tract which crosses the fourth ventricle at the obex and runs along the lateral wall of the medulla towards the diencephalon.At the level of the isthmus there is a lateral nucleus composed of large catecholaminergic cells with prominent fluorescent axons and its possible homology with the locus coeruleus is considered. Medially, in the same region a nucleus of serotonergic neurones lies between the paired tracts of the fasciculus longitudinalis medialis.In the diencephalon there are three paraventricular nuclei, the nuclei recessus posterioris and lateralis and the paraventricular organ pars anterior. Ventral to the lateral recess there is a further nucleus less closely associated with the ependyma.The distribution of fluorescent fibres is described and the dispositions of the aminergic nuclei compared to those of other teleosts.     

溴化乙锭诱导的短暂脱髓鞘增强PRV的逆行转导效率 *

目的 改进伪狂犬病病毒(Pseudorabies virus,PRV)的注射方法,通过溴化乙锭(ethidium bromide,EB)诱导的短暂脱髓鞘提高PRV的转导效率。方法 选用18只正常成年Wistar大鼠,随机分为肌肉组、NS组和EB组(n=6)。肌肉组将2μl滴度为2×10 ⁹的PRV工具病毒注射到胫骨前肌和腓肠肌上;NS组将2μl生理盐水(normal saline,NS)注射到坐骨神经上,1周后相同位置注射2μl滴度为2×10 ⁹的PRV;EB组将2μl 0.1%的EB注射到坐骨神经上,1周后相同位置注射2μl滴度为2×10 ⁹的PRV。大鼠注射PRV 5天后,灌流取材并制作冰冻切片,观察各级神经元的感染情况。 结果 EB组大鼠L4-L5脊髓前角神经元和背根神经节(DRG)、T8脊髓中间神经元、C4脊髓中间神经元、延髓、中脑、大脑皮层均有大量神经元被PRV标记。肌肉组和NS组各级神经元仅有少量被PRV标记。结论 EB诱导坐骨神经脱髓鞘后能够显著提高PRV的逆行转导效率。     

Immunocytochemical localization of a growth-associated protein (GAP-43) in rat adrenal gland

We have localized at light and electron-microscopic level the growth-associated protein GAP-43 in adrenal gland using single and double labelling immunocytochemistry. Clusters of GAP-43-immunofluorescent chromaffin cells and many immunofluorescent fibres were observed in the medulla. GAP-43-immunoreactive fibres also formed a plexus under the capsule, crossed the cortex and ramified in the zona reticulata. Double labelled sections showed the coexpression of GAP-43 with a subpopulation of tyrosine hydroxylase-and of dopamine--hydroxylase-immunoreactive chromaffin cells. Dual colour immunofluorescence for GAP-43 and calcitonin gene-related peptide (CGRP) revealed that some of the GAP-43-immunoreactive fibres also express CGRP. Pre-embedding electron microscopy showed GAP-43 immunoreactivity associated with the plasma membranes and cytoplasm of noradrenaline-producing chromaffin cells, and with processes of nonmyelin-forming Schwann cells. Immunoreactive unmyelinated axons and terminals were also observed. The immunostained terminals made symmetrical synaptic contacts with chromaffin cells. Immunoreactive unmyelinated fibres and small terminals were present in the cortex. Our results show that GAP-43 is expressed in noradrenergic chromaffin cells and in various types of nerve fibres that innervate the adrenal. Likely origins for these fibres include preganglionic sympathetic fibres which innervate chromaffin cells, postganglionic sympathetic fibres in the cortex, and CGRP containing sensory fibres.     

Organization of the lamina ganglionaris of the optic lobe of the butterfly Pararge aegeria (Linné) (Lepidoptera : Satyridae)

The present work reports on a neuroanatomical study of the butterfly Pararge aegeria (Lepidoptera : Satyridae) focusing on the lamina ganglionaris underlying two different regions of the retina of the compound eye: the dorsal rim area and the large dorsal region. No differences between both lamina regions, concerning the structure of the cartridges and the morphology of the identified neurons, could be detected. After passing the basement membrane, the visual cell axons are organized in retinotopic bundles (pseudocartridges), in which the axons of the 9 visual cells (V1 and 5, D2, 4, 6, 8, H3 and 7, B9) are arranged in the same way as in the retina. In the pseudocartridge there are no synaptic contacts. Before entering the lamina cartridge, the bundles rotate 90 °. The cartridges are joined by the fibres of 4 monopolar cells (L1, L2, L3 and L4), which could be identified and located inside the lamina cartridges in serial EM-sections. Golgi impregnations revealed the morphology of these fibres. Thus, the regional specialization of the retina (dorsal rim area and large dorsal region) does not seem to be reflected at the level of the first visual neuropil. Additionally, the cartridges of both lamina regions were investigated qualitatively for synaptic contacts among fibres. In addition to monadic chemical synapses and multiple contact synapses with presynaptic ribbons, cell contacts are also facilitated by invaginations and bridges. These cellular interactions and their functional implications are discussed.     

Distribution of serotonergic neurons in the eyestalk and brain of the crab,Cancer antennarius

1. Serotonin-containing neurons were localized immunocytochemically in crab cerebral ganglia and their extensions in the eyestalk.2. Approximately 155 serotonergic cells were found in identifiable regions of the brain, the largest number being localized in the anterior cell cluster (40 reactive cells) and the bilateral anterior olfactory cell clusters (40 cells each).3. Serotonin immunoreactive cells were found in all three ganglionic divisions of the eyestalk. The medulla terminalis contains up to 15 reactive cells, of which only one occurs in the X-organ (origin of neurosecretory axons in the sinus gland nerve). The m. terminalis also contains three identifiable cells in the mediolateral border adjacent to the sinus gland nerve, of which one is a giant (up to 100 μm diameter), designated MT-1. The axon of MT-1 branches profusely after entering the m. terminalis neuropil.4. No serotonin immunoreactivity was apparent within the sinus gland, the sinus gland nerve or the organ of Bellonci.5. These findings are discussed in relation to the known serotonergic control of peptide hormone secretion by the eyestalk X-organ-sinus gland complex.     

Sexual dimorphism in the visual system of flies: The divided brain of male Bibionidae (Diptera)

Summary The mapping of the compound eyes onto the visual neuropils and the cell types in the lamina and the lobula complex of Bibionidae (Diptera) were studied by means of extracellular cobalt injections and Golgi impregnations. Dorsal and ventral eyes in males map into separate dorsal-and ventral neuropils up to the level of the lobula complex. The dorsal-eye lamina is unilayered, while the ventral-eye lamina in males and the lamina in females are multilayered: layers A and C are invaded by en-passant terminals of long visual fibres, layer B by the terminals of short visual fibres. Long visual fibres have a short and a long terminal in the ventral medulla with terminal specialisations in three distinct layers. Only one type of receptor ending exists in the dorsal medulla, the terminal branches of which are restricted to one layer only. Arrays of contralateral neurones are found in the medial part of the dorsal lobula, which receives input from the zone of binocular vision of the ipsilateral dorsal eye, and in the posterior dorsal lobula and lobula plate. The dorsal lobula plate contains large tangential neurones, the dendritic arborisations of which are revealed by cobalt injection into the thoracic ganglia. The divided brain of male bibionids offers the opportunity to investigate separately the nervous systems involved in sex-specific visually guided flight behaviour and in general visually guided flight control.     

Synaptic communication between somatostatinergic axons and growth hormone-releasing factor (GRF) synthesizing neurons in the arcuate nucleus of the rat

Summary Growth hormone (GH) production of the anterior pituitary gland is controlled by inhibiting and releasing hormones that are synthesized in the diencephalon. In order to elucidate the possible interrelationships between somatostatin and growth hormone-releasing factor (GRF) synthesizing neurons at the hypothalamic level, immunocytochemical double labelling studies were performed on sections containing the arcuate nucleus (ARC) of the rat. Somatostatin producing neurons were located in the dorsomedial part of the ARC, while somatostatin immunoreactive (IR) axons were found in the ventro-lateral part of the nucleus, an area containing GRF-synthesizing cells. The use of the dual antigen localization technique revealed the approach and juxtaposition of somatostatin containing axons to dendrites and cell bodies of GRF-synthesizing neurons. At the light microscopic level, several somatostatinergic axon varicosities were clustered around single GRF-synthesizing cells. Ultrastructural analysis of the ventrolateral part of the ARC showed that (i), somatostatinergic axons established synaptic connections (ii), GRF-producing neurons received axons terminals on their somata and dendrites and (iii), somatostatin-IR axons formed asymmetric synaptic specializations with both dendrites and somata of GRF-synthesizing neurons.These morphological findings indicate that the hormone preduction and release of hypophysiotrophic GRF-IR neurons can be influenced by the central somatostatin system via direct synaptic mechanisms. The data support the concent that the interaction of inhibiting and releasing hormones, which determines responses of the pituitary target cells, may take place also at the hypothalamic level.Supported by grants from the National Institute of Health (NIH NS 19266), the National Science Foundation (INT 8703030, 8602688), the Hungarian Academy of Sciences (OTKA 104) and the National Foundation of Technical Development (OKKFT Tt 286/1986)