The cavum septi pellucidi in euploid and aneuploid fetuses.

OBJECTIVE: To examine whether the cavum septi pellucidi (CSP) is larger in second- and third-trimester fetuses with chromosomal abnormalities than in euploid fetuses. METHODS: This was a retrospective study utilizing stored two-dimensional images of second- and third-trimester fetuses between 18 and 40 weeks' gestation from three centers in Germany. The width of the CSP was measured by placing the calipers on the inner portion of its lateral borders. Two operators, both of whom were blinded to the fetal karyotype and to the measurements obtained by the other, measured the CSP width. The normal range in euploid fetuses was computed based on the biparietal diameter (BPD) by applying univariate regression analysis. The CSP width in euploid and aneuploid fetuses was transformed into Z-scores and compared using Student's t-test. Univariate regression analysis was used to determine the dependency of Z-score on head biometry. RESULTS: The study population consisted of 406 singleton pregnancies, 267 with euploid fetuses, 81 with trisomy 21, 50 with trisomy 18 and eight with trisomy 13. In the euploid group, the mean CSP width was 4.5 (range, 1.8-7.4) mm. Regression analysis showed a significant dependency of CSP width on BPD (CSP width = 0.658 + (0.064 × BPD), r = 0.781, P < 0.0001; both parameters in mm). The mean CSP width increased from 3.2 to 7.1 mm for BPD values of 40 to 100 mm, respectively. In the groups of fetuses with trisomy 21, 18 and 13, mean CSP width was 5.7 (range, 2.8-10.5), 7.9 (range, 3.5-12.8) and 5.8 (range, 4.0-9.0) mm, respectively. In 42.0% of the fetuses with trisomy 21, CSP width was above the 95(th) centile. In trisomy 18 and 13, CSP width was above the 95(th) centile in 92.0% and 37.5% of cases, respectively. CONCLUSION: A large CSP width should prompt a detailed ultrasound examination to further assess the risk for chromosomal abnormalities.

Intercellular Transport-metabolic interactions in the nervous tissue developing under conditions of afferentation deficit (transplants in the anterior eye chamber).

Intercellular transport-metabolic interactions were investigated at the ultrastructural level in the grafts of the embryonic rat hippocampus and septum developing for 3 to 12 months in the anterior eye chamber of adult rats. The signs of highly increased transport from the blood capillaries ingrowing from the host's iris into the grafts (multiple pinocytotic vesicles in the endothelium and pericytes) were observed. The glial cells, which were accumulated at the graft surface, had pinocytotic invaginations and microvilli, which indicated their possible participation in the active transport of metabolites from the surrounding intraocular fluid. An increased level of direct communications, manifested in pinocytoses and large gap junctions between apposed nerve and glial cells was also present within the grafts. Moreover, microphagocytosis, as internalization of surface membrane fragments with adjacent cytoplasm of the neighbouring structure (including dendrites and axons), was often observed in the grafted tissue. It is suggested that the observed communications between neuronal and glial cells may participate in both trophic and functional interactions. An increase in the level of nonsynaptic interactions in the grafted nervous tissue may be regarded as a manifestation of the compensatory adaptation to the absence of normal tissue surrounding afferent connections and efferent targets.

Distribution and synaptology of dopaminergic fibers in the mature and developing lateral septum of the rat.

The dopamine innervation of the adult and developing lateral septum of the rat was investigated with light and electron microscope immunocytochemistry using anti-dopamine antibodies. Light microscopic analysis showed that the pattern of innervation of the lateral septum exhibited a marked reorganization during the first 2 postnatal weeks, when it acquired features comparable to the adult. Ultrastructural analysis suggested that there may be two different dopamine inputs in the lateral septum. The first develops earlier in life and, through symmetrical axodendritic synapses, affects remote parts of neurons and may cause inhibition. The second develops later and, through asymmetrical axosomatic synapses, affects neuronal somata and may cause excitation. These findings may explain the reported contradictory results concerning the physiological role of dopamine in neurons of the lateral septum.

Gonadal steroids target AMPA glutamate receptor-containing neurons in the rat hypothalamus, septum and amygdala: a morphological and biochemical study.

Interactions between glutamate and gonadal steroids are involved in the regulation of limbic and hypothalamic functions. We hypothesized that hormonal signals affect excitatory neurotransmission by regulating the expression of glutamate receptors (GluR) in limbic and hypothalamic regions. To test this hypothesis, first, the coexpression of dl-alpha-amino-3-hydroxy-5-methyl-4-isoxazone-propionate (AMPA) GluR1, GluR2/3, and androgen receptors or estrogen receptors was revealed in the same cells of septal, amygdaloid, and hypothalamic areas by double immunocytochemistry. The highest incidence of co-localization was detected in hypothalamic regions. To demonstrate a regulatory role of testosterone or estradiol on AMPA receptor expression, the hormonal milieu of male and female rats was manipulated by gonadectomy and hormonal treatment. GluR1 and GluR2/3 expression was assessed by Western blots. Statistical analysis demonstrated that testosterone and estradiol have a stimulatory influence on the expression of AMPA receptors in the hypothalamus. The regulatory effect of estradiol on AMPA receptors was found to be site and gender specific: after estradiol treatment, samples taken from the hypothalamus contained increased levels of GluR1 and GluR2/3, whereas in the septum, bed nucleus and amygdala, no changes could be detected. Furthermore, the increase in hypothalamic GluR 2/3 levels was two times higher in females, compared with males, whereas the changes in hypothalamic GluR 1 levels showed no sex differences. Our results support the hypothesis that the interaction between gonadal steroids and glutamate involves hormone regulation of GluR. This mechanism seems to be gender and site specific, suggesting that excitatory neurotransmission and related physiological mechanisms also may be distinctly different in males and females.

Unreported anatomical variation of septum pellucidum.

During gross anatomy dissections of the brain, a developmental abnormality of the septum pellucidum was found in a 31-year-old male cadaver. Other parts of the central nervous system in this cadaver were normal in every aspect. Histological samples were taken from the neighboring areas of this abnormality, and they were examined under light microscope and scanning electron microscope. In this abnormality of the septum pellucidum, the two laminae of the septum pellucidum were fused together and there was a hole located 1 cm anterior to its apex. The maximum diameter of the hole was 0.5 cm in the sagittal plane and 0.6 cm in the vertical plane. In the light microscopic and scanning electron microscopic examinations, the free margin of this foramen was regular, and the surrounding tissue was intact and histologically unique to the septum pellucidum. Ependymal cells were present at the free margin of the foramen. Cavum vergae, cavum septum pellucidum, and agenesis of the septum pellucidum are described in the literature. These three abnormalities are seen in cadavers usually with histories of schizophrenia and other psychiatric or neurologic disorders. In a retrospective study, the cadaver with this abnormality had a history of schizophrenia and no history or signs of any kind of brain or head operation. As far as we could ascertain, the abnormality described here has not been reported previously.

Perturbation of target-directed neurite outgrowth in embryonic CNS co-cultures grown in the presence of ethanol.

Studies were conducted to determine the influence of ethanol on target-directed fiber outgrowth in culture, using embryonic chick spinal cord-muscle, and fetal rat septal-hippocampal co-cultured explants. Process extension from the spinal cord and septal explants in control cultures was selectively oriented toward the appropriate target tissue. Ethanol in the culture medium (500 mg/dl) eliminated this target-oriented outgrowth in both systems, although the overall extent of neurite outgrowth was not affected. In an effort to further characterize the source of this disruption, target explants were grown alone, with and without ethanol, and the target-conditioned culture media was subsequently harvested and placed on newly plated spinal cord or septal explants, to determine whether ethanol decreased the target production of soluble substances. To determine whether deposition of substrate-bound materials by the target tissue was affected by ethanol, spinal cord or septal explants were plated in wells which had previously been occupied by the appropriate target tissue. These studies revealed that ethanol significantly inhibited production of soluble and substrate-bound materials by muscle explants, but not by hippocampal explants. It was concluded that the ethanol-induced loss of target-directed neurite outgrowth in the spinal cord explants could be accounted for primarily by the attenuated production of neurotropic/neurotropic substances by the muscle tissue. The loss of target-directionality in the septal explants appeared to be due to other factors, possibly related to ethanol-induced compromise of the capacity of the septal neurons to respond appropriately to target-derived neurotrophic/neurotropic substances. The implications of these results for the fetal alcohol syndrome are considered.

Lamina-specific synaptic connections of hippocampal neurons in vitro.

By using slice cultures as a model, we demonstrate here that different target selectivities exist among the various afferent fibers to the hippocampus. As in intact animals, septohippocampal cholinergic fibers, provided by a slice culture of septum, innervate a co-cultured slice of hippocampus diffusely, that is, without forming distinct layers of termination. As in vivo, the septal cholinergic fibers establish synapses with a variety of target cells. Conversely, fibers from an entorhinal slice co-cultured to a hippocampal slice display their normal laminar specificity. They preferentially terminate in the outer molecular layer of the fascia dentata, thereby selectively contacting peripheral dendrites of the granule cells. This preferential termination on peripheral dendritic segments is remarkable, since these fibers do not have to compete with commissural fibers, hypothalamic fibers, and septal afferents for dendritic space under these culture conditions. Moreover, in triplet cultures in which first two hippocampal slices were co-cultured and then, with a delay of 5 days, an entorhinal slice was added, the fibers from the entorhinal slice and those from the hippocampal culture terminated in their appropriate layers in the hippocampal target culture. However, in this approach the normal sequence of ingrowth of these two afferents was reversed. In normal ontogenetic development, entorhinal afferents arrive in the hippocampus before the commissural fibers. The results show that there are different degrees of target selectivity of hippocampal afferents and that the characteristic lamination of certain afferent fibers in the hippocampus is not determined by their sequential ingrowth during development.

The neurotrophic effects of ebiratide, an analog of ACTH4-9, on cultured septal cells and aged rats.

The neurotrophic effects of ebiratide, an ACTH4-9 analog, have been examined using both fetal rat septal cultures and aged rats. The 5-day treatment with ebiratide (10-100 pmol/ml) partially prevented neuronal degeneration that occurred in the cultures in which cells were sparsely plated. Ebiratide (10 pmol/ ml) increased choline acetyltransferase (ChAT) and acetylcholinesterase (AChE) activities up to 1.5 and 1.2 times the respective control values in the sub-confluent cultures. AChE cytochemistry of the cultures has shown that ebiratide increased the stained area per cell. Ebiratide subcutaneously administered by constant infusion (10 nmol/body/hr) for 4 weeks elevated ChAT activities in the septum (35% over control), neocortex (79%) and hippocampus (89%) of aged rats. Thus, the present study indicates that ebiratide shares neurotrophic properties which may prove beneficial in the therapy for CNS degenerative disorders, especially Alzheimer's disease.

Fine structure of rat septohippocampal neurons. III. Recovery of choline acetyltransferase immunoreactivity after fimbria-fornix transection.

Most cholinergic projection neurons in the medial septal nucleus (MS) lose their capability to synthesize choline acetyltransferase (ChAT) after axotomy by bilateral fimbria-fornix transection. We have recently shown that identified septohippocampal neurons survive axotomy up to 10 weeks and display fine-structural characteristics of cells in control rats. However, the fate and functional role of these neurons remained unclear. Here we describe observations made in rats which survived axotomy for 6 months. Adult Sprague-Dawley rats were subjected to bilateral transection of the fimbria-fornix system. In some animals septohippocampal projection neurons were labeled by the retrograde fluorescent tracer Fluoro-Gold (FG) prior to axotomy. After varying survival times following fimbria-fornix transection, the animals were fixed and sections of the septal region immunostained for ChAT. Three weeks postlesion, the number of ChAT-positive cells in the MS was reduced to 19% of control, suggesting a severe neuronal loss. However, 10 weeks and 6 months after axotomy this value increased to 28% and 54%, respectively. Fine-structural analysis of ChAT-positive neurons after 6 months survival revealed all characteristics of vital cells including normal input synapses. The majority of these cells could be identified as former septohippocampal projection neurons by the presence of FG. We conclude that many neurons in the MS have the capacity to restore their transmitter synthesis in a long-lasting process following axotomy.

Immunocytochemical and in vitro autoradiographic evidence for a direct somatostatinergic modulation of the enkephalinergic hypothalamoseptal tract of the guinea-pig.

The present study was undertaken to determine whether the enkephalinergic hypothalamoseptal tract originating in the magnocellular dorsal nucleus in the guinea-pig brain is under the influence of somatostatin. In the first step, double immunocytochemical labeling of enkephalinergic cells and somatostatinergic fibers was combined at the light and electron microscopic levels in the magnocellular dorsal nucleus. As a second step, an in vitro radioautography was used to determine whether somatostatin receptors are present in the same area. A close relationship between somatostatin nerve endings and enkephalin perikarya was observed at both the light and electron microscopic levels. Contracts were more numerous in the ventral part of the magnocellular dorsal nucleus. Whenever synaptic images were clearly observable, they appeared symmetrical. In the same area, a moderate concentration of G-protein-coupled somatostatin binding sites was also visualized. These results suggest that somatostatin has a regulator role on the enkephalinergic hypothalamoseptal tract, directly at the level of the magnocellular dorsal nucleus.

Origin of noradrenergic projections to GnRH perikarya-containing areas in the medial septum-diagonal band and preoptic area.

The purpose of the present study was to identify the sites of origin of the noradrenergic fibers that project to areas containing gonadotropin-releasing hormone (GnRH) perikarya since norepinephrine (NE) is known to influence the activity of GnRH neurons. Fluorescent retrograde tracers were used in combination with immunohistochemistry for dopamine-beta-hydroxylase (DBH) and GnRH. Small volumes of either Fluoro-gold (FG) or Fluoro-Ruby (FR) were pressure injected into areas that contain the largest number of GnRH cell bodies, i.e., the medical septum-diagonal band complex or preoptic area. Retrogradely labeled neurons were observed ipsilaterally in the following noradrenergic cell groups: A2 (in the nucleus tractus solitarii), A1 (in the ventrolateral medulla) and locus coeruleus. Approximately 8% of all DBH-positive neurons within the A2-cell group were retrogradely labeled, while 12% of DBH-ir neurons in the A1-group were double-labeled. Only a few retrogradely labeled DBH-ir neurons were observed in the locus coeruleus (< 1%). Double-labeled neurons were not organized into discrete cell groups, but were dispersed among other NE-neurons within the A2- and A1-cell groups. The highest concentrations of double-labeled neurons were located in the central one-third of both the A2 and A1 cell groups. The results suggest that most noradrenergic terminals in the region of the GnRH perikarya in the medial septum-diagonal band/rostral preoptic area originate from ipsilateral neurons in areas A1 and A2.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonergic innervation of the mature and developing lateral septum of the rat: a light and electron microscopic immunocytochemical analysis.

The serotonergic innervation of the adult and developing lateral septum of the rat was studied with immunocytochemical techniques at the light and electron microscopic levels. A few, relatively thick serotonergic fibres are found in the lateral septum at the time of birth, but they are restricted to its medial part. They subsequently extend towards the lateral ventricle, increase in number and attain their final distribution pattern by the end of the first postnatal week. Thereafter they become finer, with regularly spaced varicosities, show a higher density, and generally exhibit features, density, and pattern of innervation comparable to the adult at the end of the third postnatal week. In the dorsal portion of the lateral septum, serotonergic fibres form characteristic pericellular basket-like arrangements around cell somata and their primary dendrites. These baskets are first observed at P7, and subsequently increase both in number and in terms of the number of serotonergic terminals which they comprise. The present findings suggest that the development of serotonergic innervation of the lateral septum parallels the neuronal differentiation in this area. Ultrastructural analysis has shown that the vast majority (congruent to 95%) of serotonin varicosities make symmetrical synapses with somata, dendritic shafts and spines. These varicosities in new-born animals are in close association with neuronal elements, without any intervening neuroglial processes, but towards the end of the first postnatal week they exhibit well-defined synaptic specializations. The mean diameter of serotonergic varicosities making synapses does not change substantially with age. Serotonin-receptive neurons have several morphological features in common with the type I cells described in a previous Golgi study of the lateral septum [Alonso and Frotscher (1989) J. comp. Neurol. 286, 472-487]. Some speculations on the chemical identity of the serotonin-receptive cells have been put forward in the present study but double-labelling studies will certainly shed more light on the organization of the serotonergic innervation of the lateral septum.

Neurotropism of nerve growth factor for adult rat septal cholinergic axons in vivo.

Nerve growth factor (NGF) can induce sprouting of axotomized adult rat medial septum cholinergic neurons and promote their regeneration into septohippocampal nerve grafts and hippocampal formation. This study investigated the potential neurotropic (chemotactic/attracting) action of NGF in the adult rat cholinergic septohippocampal regeneration model. (i) Some animals received sciatic nerve grafts between the disconnected septum and hippocampal formations on each side. A 4-week infusion with NGF into the rostral portion of the lateral ventricle induced sprouting of cholinergic fibers in the dorsolateral septum with a gradient toward the lateral ventricle. However, the number of cholinergic axons entering the nerve bridge was only one-third that observed in vehicle-infused animals, suggesting that NGF had diverted many of the regrowing axons away from the nerve toward the ventricle. (ii) In animals implanted with nerves for 2 weeks and concurrently infused with NGF into the fornix, proximal to the lesion and grafts, cholinergic sprouting occurred in the mediodorsal septum, i.e., was oriented toward the infused fornix. Essentially no fibers had entered the nerve bridge, suggesting that all regrowing fibers had remained near the NGF source. (iii) When animals with a unilateral fimbria-fornix transection (but no nerve graft) were infused with NGF into the lateral ventricle on the opposite side, cholinergic sprouting was oriented toward the midline of the septum. (iv) Infusion of low doses of NGF directly into the (lesioned) septum induced a sprouting response localized around the infusion site. (v) No sprouting occurred when intraventricular NGF infusion was applied to normal (nonlesioned) animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Cross-species septohippocampal transplants: ultrastructure of Thy-1.2-labeled donor fibers into the dentate gyrus.

A naturally occurring species-specific membrane marker was used to identify unambiguously transplanted septal cells and their fibers which have grown into host tissue. Cell suspensions of the septum/basal forebrain region of C57Bl/6 mouse embryos were transplanted into the dentate gyrus of Sprague-Dawley rats that had received a fornix lesion. The membranes of the mouse contained Thy-1.2, while the membranes of the rat contained Thy-1.1. An antibody to Thy-1.2 clearly identified the donor tissue and did not react with the Thy-1.1 of the host's membranes. The ultrastructure of the immunoreactively labeled tissue confirmed previous biochemical findings on the distribution of Thy-1 and showed Thy-1.2 immunoreactivity on axons and dendrites, microtubules, some mitochondrial membranes, and the surface membranes of cell bodies. Within the transplant, a few glial profiles showed immunoreactive fibrils, but most glial profiles within the transplant and all glial profiles outside the transplant were not immunoreactive. Astrocyte fibers enclosed the outgrowing labeled fibers to form fascicles, but did not penetrate the fascicle. There was no other distinctive association of astrocytic profiles with immunoreactive fibers. Dendrites grew for long distances into the host's molecular layer. Many immunoreactive dendritic profiles formed synapses with unlabeled terminal profiles from the host. The host synapses on the long dendrites of the transplanted neurons may form an important source of input for the initiation of physiological activity in the new circuits established by the transplant. A few labeled (donor) synaptic terminals were observed in the molecular layer, but Thy-1.2-labeled dendritic profiles were much more prominent than labeled axonal profiles.

Membrane resealing in cultured rat septal neurons after neurite transection: evidence for enhancement by Ca(2+)-triggered protease activity and cytoskeletal disassembly.

Neurites of cultured septal neurons were transected with a laser under sterile conditions, and the subsequent membrane resealing was assayed using a dye exclusion method. In agreement with findings in other preparations, Ca2+ enhanced resealing: in normal culture medium the percentage of lesioned neurons that resealed within 20-30 min after transection increased with increasing bath [Ca2+] over the range 10(-7) to 2 x 10(-3) M; about 75% of cells resealed in 2 mM Ca2+. Mn2+ and Sr2+ also enhanced resealing, but Mg2+ inhibited it. The percentage of resealing neurons was sensitive to agents known to alter the stability of cytoskeletal components. Agents that tend to disassemble microtubules and/or neurofilaments (e.g., colchicine, low-ionic-strength media) strongly promoted resealing, whereas treatments that tend to stabilize microtubules (taxol, Mg2+) inhibited resealing. Addition of exogenous proteases (papain, trypsin, or dispase) enhanced resealing, whereas inhibitors of cysteine proteases (including a specific inhibitor of calpain, a Ca-activated neutral protease) strongly inhibited resealing. Calmodulin inhibitors inhibited resealing, consistent with reports that calmodulin facilitates calpain-mediated proteolysis of fodrin, a component of the cortical cytoskeleton. Based on these results, we hypothesize that one of the major mechanisms involved in resealing is activation of endogenous proteases by Ca2+ entry into the injured neurite. The resulting changes in the cellular cytoskeleton might promote fusion and resealing of the cut ends of the plasma membrane by enhancing membrane mobility and/or by removing structures that normally prevent membrane-membrane contact.

Collateral sprouting of serotonergic fibers in the cingulate cortex and the septum following cortical-hippocampal lesions.

The parietal cortex and dorsal hippocampus of adult rats were unilaterally ablated. One and 3 months after this operation, changes in serotonergic fiber distribution in the forebrain were studied immunohistochemically. At 1 month, increased numbers of serotonergic fibers were seen in the cingulate cortex and the medial and lateral septal nuclei of the lesioned side. This increase continued to 3 months in the ipsilateral cingulate cortex. In the present study, two different processes of serotonergic fiber collateral sprouting were noted. Plastic changes in serotonergic fibers in the ipsilateral septal nuclei and cingulate cortex may be attributable to the collateral sprouting underlying reactive synaptogenesis and the pruning effect, respectively.

Axonal growth and glial migration from co-cultured hippocampal and septal slices into fibrin-fibronectin-containing matrix of peripheral regeneration chambers: a light and electron microscope study.

In order to investigate whether a fibrin-fibronectin-containing matrix of a peripheral regeneration chamber could promote the growth of central nervous system neurons, hippocampal and septal slices were co-cultured in the presence of this acellular substrate. In introducing the peripheral matrix into a 2-mm-long tube between hippocampal and septal slices, a spatio-temporal sequence of cell migration and axonal growth was described by light and electron microscopy. Axons were able to elongate directly into the flocculent material constituting the matrix and a possible neurite-promoting activity was implicated in this process as axonal growth was not detected in direct contact with rat plasma coagulated with calcium, or chicken plasma coagulated with thrombin, used as control matrices. However, in the 3 different substrates tested, astrocytes were able to migrate and dilated astroglial processes containing intermediate filaments were detected. Axonal processes were observed growing on the glial cell surface. GFAP-positive phagocytic cells, that could be of the same origin as astrocytes, were involved in matrix removing. Neuronal growth and glial migration arose from hippocampal and septum slices and acetylcholinesterase-containing fibers were seen in the bridging structure suggesting that cholinergic axons were able to progress to the hippocampal slice. This technique appeared to provide a model in which axonal growth and cell migration can be studied 'in vitro' in a 3-dimensional environment.

Quantitation of synaptic density in the septal nuclei of young and aged Fischer 344 rats.

Synaptic density in the medial and lateral septal nuclei was examined in 3 and 24-28 months of age Fischer 344 rats. The lateral nucleus had a higher synaptic density than the medial region in both age groups. There were no statistically significant differences in synapse density in either region as a function of age, but the data suggested a subpopulation of aged animals which did show an age-related decline in synaptic density in the lateral, but not medial area of the septum. These data indicate that sample size may be an important variable in assessing possible age-related differences in synaptic density, since a broad range of values, some significantly below the range of young animals, exists in the aged brain.

Hippocampal neurotrophic factors influence the perikaryal size of septal acetylcholinesterase-containing neurons in culture.

The septal neurons were cultured under the following conditions: (1) treated with 7S nerve growth factor (NGF) (50 ng/ml); (2) grown with hippocampal cell-conditioned medium supernatant; (3) cocultivated with hippocampal cells; (4) cocultivated with cerebellar cells; (5) no treatment. Acetylcholinesterase histochemistry was used to identify cholinergic cells after pretreatment with diisopropylfluorophosphate. The mean values of the perikaryal major axis and minor axis at day 14 of culture were significantly larger in septal cells cocultivated with hippocampal cells than in septal cells grown under other conditions. NGF-treated septal cells showed a smaller, but significant, increase in the mean value of the major axis of these neurons.

Effects of inhibition of GABA catabolism by aminooxyacetic acid on enkephalinergic neurons--immunocytochemical and ultrastructural study in the enkephalinergic hypothalamoseptal tract of the guinea-pig.

The immunocytochemical and ultrastructural features within [Met]enkephalin neurons of the guinea-pig hypothalamoseptal tract were investigated under chronic inhibition of GABAergic catabolism. This was achieved by raising the brain GABA concentration with aminooxyacetic acid which inhibits GABA-transaminase, the enzyme responsible for the catabolism of GABA. Guinea-pigs were injected intraperitoneally with 10 or 20 mg/kg per day aminooxyacetic acid for two, four or eight days and killed 16 h post-dose. Repeated injections of aminooxyacetic acid produced a great increase in immunoreactivity for GABA in nerve endings surrounding enkephalinergic perikarya in the magnocellular dorsal nucleus of the guinea-pig. Extensive immunocytochemical studies stressed the increase and redistribution of the immunoreaction for [Met]enkephalin in the perikarya of the magnocellular dorsal nucleus under such GABAergic activation. Quantitative and statistical analyses showed that administration of aminooxyacetic acid for eight days significantly increased the intensity of labelling within stimulated perikarya (P less than 0.001). A concomitant accumulation of immunopositive large granules in the enkephalinergic boutons of the lateral septum was observed. In the same way, ultrastructural changes in enkephalinergic cell bodies were analysed and reflected disturbances in the biosynthetic and digestive activities of enkephalinergic perikarya. We postulate that chronic inhibition of the GABAergic catabolism leads to modification in the metabolism of enkephalinergic neurons and to an inhibitory action of GABA on the [Met]enkephalin release from nerve endings. This study give morphological support to the complex functional interactions between GABA and opioid peptide transmitter system.