Simultaneous immunogold labeling of GABAergic terminals and vasopressin-containing neurons in the rat paraventricular nucleus

?-Amino Butyric Acid Control of Arginine Vasopressin Release from the Ewe Hypothalamus In Vitro: Sensitivity to Oestradiol

The present study aims to ascertain the influence of gamma-amino butyric acid (GABA)(A or B) receptors on arginine vasopressin (AVP) release in vitro and determine whether E(2) modulates GABA-AVP interaction. Within 10 min of ewe killing, saggital midline hypothalamic slices (from the anterior preoptic area to the mediobasal hypothalamus along with the median eminence, 2-mm thick, two per ewe) were dissected, placed in oxygenated minimum essential media (MEM)-alpha at 4 degrees C and within 2 h were singly perifused at 37 degrees C with oxygenated MEM-alpha (pH 7.4; flow rate 0.15 ml/min), either with or without E(2) (24 pg/ml). After 4-h equilibration, 10-min fractions were collected for 4 h interposed with a 10-min exposure at 60 min to a specific GABA(A or B) receptor agonist or antagonist at various doses (0.1-10 mm). GABA(A) (muscimol; no E(2), n = 7 perifusion chambers, with E(2), n = 11) or GABA(B) (baclofen; no E(2), n = 8, with E(2), n = 15) agonists (10 mm) did not influence AVP concentrations. However, AVP release increased (p < 0.05) 20-30 min after exposure to 10 mm GABA(A or B) antagonists (bicuculline, no E(2), n = 7: from 4.6 +/- 0.7 to 33.0 +/- 0.4, with E(2), n = 17: from 11.9 +/- 1.4 to 32.8 +/- 6.0; CGP52432, with E(2), n = 14: from 14.0 +/- 2.6 to 28.8 +/- 3.9 pg/ml). At the end of the collection period, hypothalamic slices responded to KCl (100 mm) with AVP efflux (p < 0.05). GABA(B) but not GABA(A) antagonist-stimulated AVP release was enhanced in the presence of E(2). In summary, AVP release is under the inhibitory influence of GABA input with further potentiation by E(2) through GABA(B) receptors in vitro.

Neuropeptide FF and neuropeptide VF inhibit GABAergic neurotransmission in parvocellular neurons of the rat hypothalamic paraventricular nucleus

Neuropeptide FF (NPFF) and neuropeptide VF (NPVF) are octapeptides belonging to the RFamide family of peptides that have been implicated in a wide variety of physiological functions in the brain, including central autonomic and neuroendocrine regulation. The effects of these peptides are mediated via NPFF1 and NPFF2 receptors that are abundantly expressed in the rat brain, including the hypothalamic paraventricular nucleus (PVN), an autonomic nucleus critical for the secretion of neurohormones and the regulation of sympathetic outflow. In this study, we examined, using whole cell patch-clamp recordings in the brain slice, the effects of NPFF and NPVF on inhibitory GABAergic synaptic input to parvocellular PVN neurons. Under voltage-clamp conditions, NPFF and NPVF reversibly and in a concentration-dependent manner reduced the evoked bicuculline-sensitive inhibitory postsynaptic currents (IPSCs) in parvocellular PVN neurons by 25 and 31%, respectively. RF9, a potent and selective NPFF receptor antagonist, blocked NPFF-induced reduction of IPSCs. Recordings of miniature IPSCs in these neurons following NPFF and NPVF applications showed a reduction in frequency but not amplitude, indicating a presynaptic locus of action for these peptides. Under current-clamp conditions, NPVF and NPFF caused depolarization (6-9 mV) of neurons that persisted in the presence of TTX but was abolished in the presence of bicuculline. Collectively, these data provide evidence for a disinhibitory role of NPFF and NPVF in the hypothalamic PVN via an attenuation of GABAergic inhibitory input to parvocellular neurons of this nucleus and explain the central autonomic effects of NPFF.

Neuropeptide FF (NPFF) control of magnocellular neurosecretory cells of the rat hypothalamic paraventricular nucleus (PVN)

Neuropeptide FF (NPFF) is an octapeptide belonging to an extended family of RF amide peptides that have been implicated in a wide variety of physiological functions in the brain. NPFF and its receptors are abundantly expressed in the rat brain and spinal cord including the hypothalamic paraventricular nucleus (PVN), an autonomic nucleus critical for the secretion of neurohormones and the regulation of sympathetic outflow. In this study, we sought to examine the effects of NPFF on GABAergic inhibitory synaptic input to magnocellular neurosecretory cells (MNCs) of the PVN, which secrete the neurohormones, vasopressin and oxytocin from their terminals in the neurohypophysis. Whole cell patch clamp recordings under voltage clamp conditions were performed from PVN MNCs in the brain slice. Bicuculline-sensitive inhibitory postsynaptic currents (IPSCs) were isolated in the presence of glutamate receptor blockers. In tetrodotoxin, NPFF (5 microM) caused an increase in frequency, but not amplitude of miniature inhibitory postsynaptic currents (mIPSCs) in MNCs indicating a presynaptic locus of action for this peptide. Intracerebroventricular application of NPFF resulted in an activation of GABAergic neurons located adjacent to the PVN as revealed by immunohistochemistry for Fos protein and in situ hybridization for glutamic acid decarboxylase (GAD67) mRNA. Based on these observations we conclude that NPFF facilitates inhibitory input to MNCs of the PVN via GABAergic interneurons located in immediate vicinity of the nucleus. These findings provide a cellular and anatomic basis for the NPFF-induced inhibition of vasopressin release has been reported consequent to hypovolemia and hyperosmolar stimulation.

Adrenomedullin influences magnocellular and parvocellular neurons of paraventricular nucleus via separate mechanisms

We previously reported that adrenomedullin (AM) decreases blood pressure following microinjection into the paraventricular nucleus of the hypothalamus (PVN) of the rat. With the use of whole cell recordings in rat hypothalamic slice preparations, we characterized the effects of AM on electrophysiologically identified PVN neurons and described the membrane events underlying such actions. AM hyperpolarized magnocellular (type I) neurons in a dose-dependent manner, a response associated with an increase in the frequency and amplitude of inhibitory postsynaptic potentials. Blockade of action potentials with tetrodotoxin (TTX) abolished AM effects on membrane potential and synaptic activity in magnocellular neurons, suggesting direct actions on inhibitory interneurons. Furthermore, blockade of inhibitory synaptic transmission with the GABA(A) receptor antagonist bicuculline methiodide also abolished AM effects on membrane potential in magnocellular neurons. In contrast, parvocellular (type II) neurons depolarized following AM receptor activation. AM effects on parvocellular neurons were dose dependent and were maintained in the presence of TTX, indicating direct effects on this population of neurons. Voltage-clamp recordings from parvocellular neurons showed AM enhances a nonselective cationic conductance, suggesting a potential mechanism through which AM influences membrane potential. These observations show clear population-specific actions of AM on separate identified groups of PVN neurons. Such effects on magnocellular neurons likely contribute to the hypotensive actions of this peptide in PVN. Although the effects on parvocellular neurons may also contribute to such cardiovascular effects of AM, it is more likely that actions on this population of PVN neurons underlie the previously demonstrated activational effects of AM on the hypothalamic-pituitary-adrenal axis.

Maternity leads to morphological synaptic plasticity in the oxytocin system

The oxytocinergic system, which plays a major role in the control of different aspects of maternity, undergoes extensive synaptic and neuronal-glial remodelling during parturition and lactation and has thus become a remarkable example of activity-dependent morphological synaptic plasticity in the adult mammalian brain. The use of different comparative ultrastructural analyses on the rat supraoptic and paraventricular nuclei, together with identification of pre- and post-synaptic elements, has allowed us to show that there is a significant increase in the number of GABAergic, glutamatergic and noradrenergic synapses impinging on oxytocin neurons, concomitant with a reduction of glial coverage of the neurons. This synaptic plasticity involves axo-dendritic and axo-somatic contacts originating from terminals making one or several synaptic contacts in one plane of section. While noradrenergic afferents arise from medullary catecholaminergic neurons, our recent in vitro observations indicate that GABAergic and glutamatergic afferents derive, at least partly, from local intrahypothalamic neurons, in close proximity to oxytocin neurons. The cellular mechanisms underlying this morphological synaptic plasticity remain to be determined but it is highly likely that they depend on increased activity in both pre- and post-synaptic elements. Moreover, the oxytocin system continues to express 'embryonic' molecular features that may allow the morphological plasticity to occur. In particular, it expresses high levels of cell surface adhesion molecules currently thought to intervene in synaptic remodelling in the developing and lesioned central nervous system, including the weakly adhesive polysialylated isoform of the Neural Cell Adhesion Molecule, the axonal glycoprotein F3 and its ligand, the extracellular matrix glycoprotein, tenascin-C.

Synaptic contacts between serotonergic and cholinergic neurons in the rat dorsal raphe nucleus and laterodorsal tegmental nucleus

We examined synaptic connectivity between cholinergic and serotonergic neurons in the dorsal raphe nucleus and the laterodorsal tegmental nucleus of the rat. To this purpose we employed two variations (the combination of pre-embedding immunogold-silver intensification with avidin-biotin-peroxidase complex technique and the combination of avidin-biotin-peroxidase/3, 3'-diaminobenzidine/silver-gold intensification with avidin-biotin-peroxidase/3,3'-diaminobenzidine reaction) of a double pre-embedding immunoelectron procedure, using primary antibodies against vesicular acetylcholine transporter and serotonin. At the light-microscopic level, serotonin-like immunoreactive neurons in the dorsal raphe nucleus appeared as reddish black and vesicular acetylcholine transporter-like immunoreactive axon terminals were brown colored using a combination of pre-embedding immunogold-silver technique and avidin-biotin-peroxidase complex technique. Serotonin-like immunoreactive fibers projected to the laterodorsal tegmental nucleus. At the electron microscopy level, with both methods we observed in the dorsal raphe nucleus vesicular acetylcholine transporter-immunopositive axon terminals in synaptic contact with serotonin-like immunoreactive dendrites and, to a lesser degree, with serotonin-like immunoreactive cell bodies. These synapses usually were of the symmetrical type. Occasionally we noted, next to vesicular acetylcholine transporter-immunopositive axon terminals, also immunonegative terminals synapsing with the serotonin-like immunoreactive dendrites. In the laterodorsal tegmental nucleus we found serotonin-like immunoreactive axon terminals and immunonegative terminals forming synapses with vesicular acetylcholine transporter-immunoreactive dendrites. Most synapses formed by the serotonin-like immunopositive terminals were of the asymmetrical type. Our results suggest that serotonergic neurons in the dorsal raphe nucleus and cholinergic neurons in the laterodorsal tegmental nucleus may reciprocally influence each other by means of synaptic connectivity. Such connectivity may serve to regulate pain sensation, or be involved in the regulation of the sleeping-waking cycle.

Local circuitry regulates the excitability of rat neurohypophysial neurones

The importance of angiotensin II (AII) and glutamate has long since been recognized in neuroendocrine regulation. However, the mechanisms by which AII and glutamate modulate the excitability of the paraventricular nucleus (PVN) have largely remained a mystery until recently. It is now apparent that AII and glutamate are potent stimulators of both magnocellular and parvocellular neurones in the rat PVN. While glutamate, the predominant excitatory neurotransmitter in the CNS, ubiquitously excites PVN neurones, AII appears to mediate excitability of the PVN by both direct and indirect mechanisms. Interestingly, both of these neurotransmitters, upon exciting the PVN, activate an inhibitory feedback system, which is capable of diminishing the initial stimulus. Physiologically, this moderates the output signals from the PVN, and probably also regulates neuropeptide release from the neurohypophysis. The importance of this negative-feedback loop is evident in the pathophysiological implications of a disruption in the system. Evidence suggests that a breakdown in this system may be responsible in part for the onset and maintenance of both congestive heart failure and hypertension. Future studies will continue to characterize both the actions of glutamate and AII in the PVN, and to further elucidate the mechanisms which control the excitability of the PVN.

The neurotensinergic synaptic innervation of vasopressin containing neurons in the rat hypothalamic paraventricular nucleus

A recent physiological report suggested that neurotensin could inhibit the vasopressin releasing from vasopressin-producing neurons in the hypothalamic paraventricular nucleus but not in the supraoptic nucleus. In the present study, the synaptic relationship between the neurotensin-like immunoreactive and vasopressin-like immunoreactive neurons has been examined using a pre-embedding double immunostaining technique in the rat hypothalamic paraventricular nucleus. At the light microscopic level, many neurotensin-like immunoreactive fibers were found near the vasopressin-like immunoreactive neurons. At the electron microscopic level, the neurotensin-like immunoreactive fibers were identified as axon terminals that made many synapses on the vasopressin-like immunoreactive perikarya and dendrites. The synapses were both asymmetrical and symmetrical. These findings of the present study suggest that the inhibitory effect of neurotensin on the vasopressin neurons in the hypothalamic paraventricular nucleus may be due to the direct synapses made by neurotensin-like immunoreactive axon terminals on the vasopressin-like immunoreactive neurons.

Nitric oxide regulates NMDA-driven GABAergic inputs to type I neurones of the rat paraventricular nucleus

1. Whole-cell recordings were obtained from type I paraventricular nucleus (PVN) neurones in coronal slices of rat hypothalamus to study the involvement of nitric oxide (NO) in the modulation of inhibitory transmission resulting from the activation of N-methyl-D-aspartate (NMDA) receptors by the high affinity receptor agonist D,L-tetrazol-5-ylglycine. 2. A brief pulse of NMDA agonist (0.1-10 microM) faithfully elicited increases in action potential firing frequency in all type I cells tested (n = 55). In cells with membrane potentials positive to -75 mV, this excitation was accompanied by an underlying depolarization (> 2 mV) in the majority of cases (n = 45). At membrane potentials negative to -75 mV, NMDA agonist application elicited an initial monotonie depolarization, which was auxiliary to profound, rhythmic oscillations of the membrane potential, resulting in the emergence of burst-like activity in these cells (n = 8). 3. In addition to depolarizing the neurones, the NMDA agonist also elicited inhibitory postsynaptic potentials (IPSPs) in 40% (n = 22) of the cells tested. The IPSPs were inhibited by the GABAA receptor antagonist bicuculline methiodide (BMI). 4. Microdialysis of NO into the PVN has been shown to increase local levels of inhibitory neurotransmitters, including GABA. The possibility that NO-induced increases in GABA lead to an increase in inhibitory synaptic activity in PVN was investigated by administering NO by three different methods. Bath application of the donor compound, S-nitroso-N-acetyl-penicillamine (SNAP; n = 7), bubbled NO solution (n = 5), or the NO precursor L-arginine (n = 6) all elicited increases in IPSP frequency. 5. Production of NO in other brain centres has been linked to the activation of the NMDA receptor. In order to determine whether the increase in IPSPs following NMDA was the result of activation of NO, the production of NO was blocked with the NO synthase inhibitor N omega-nitro-L-arginine methylester (L-NAME). Subsequent NMDA receptor activation elicited more pronounced depolarizations, but there was no accompanying increase in IPSP frequency (n = 5). 6. This study demonstrates that GABAergic inhibition resulting from NMDA receptor activation can be regulated profoundly by NO. By increasing inhibitory transmission within a nucleus, NO may serve as an important intermediary in the regulation of neuronal excitability in the central nervous system.

A new fixation procedure for study of the histaminergic neurons by immunoelectron microscopy using the direct antiserum against histamine

A new perfusion protocol was developed to detect histamine-like immunoreactive neurons at the electron microscopic level. By stepwise perfusion of 1-ethyl-3(3-diamethylaminopropyl)-carbodiimide and paraformaldehyde solutions, the brain block could be cut with a vibratome and the immunoreactivity could be detected using the avidin-biotin-peroxidase-complex method. We used this method to study the ultrastructure and synaptic relations of the histaminergic neurons in the postmammillary caudal magnocellular nucleus of the rat hypothalamus. This method should also be useful for examination of histaminergic neurons in other tissues and the synaptic relations of histaminergic neurons with other neurotransmitter-containing neurons by double immunostaining.

Taurine immunoreactivity in the rat supraoptic nucleus: prominent localization in glial cells

Taurine is an inhibitory amino acid that hyperpolarizes magnocellular neurosecretory neurons. To determine which cell types in the rat supraoptic nucleus contain taurine, we used a monoclonal antibody raised against a taurine conjugate. Preembedding immunocytochemistry was carried out at the light and electron microscopic levels using diaminobenzidine and gold-substituted silver-intensified peroxidase as markers. We report the presence of taurine in all cellular compartments of the supraoptic nucleus, except axons, with variable labeling intensities among the different compartments. Few cell bodies of magnocellular neurons were immunoreactive, but many distal dendrites and some proximal ones showed weak-to-moderate levels of immunoreactivity. Strong immunoreactivity was found over glial cell bodies and their processes, in particular in the ventral glial lamina of the supraoptic nucleus. Large astrocytic processes labeled with the taurine antibody included the endfeet participating in the glial limitans around capillaries and at the ventral surface of the hypothalamus. Other types of immunoreactive astrocytic profiles were found scattered within the neuropil where these processes participated in different interactions with the neuronal elements of the supraoptic nucleus. Immunoreactive glial expansions, sometimes even the main process of the glial cell, engulfed axonal boutons. Other labeled glial processes were found between two magnocellular perikarya or closely apposed to the membrane of axonal boutons contacting the neuronal cell bodies. The frequent finding of closely apposed glial and dendritic elements bearing different levels of taurine-like immunoreactivity suggests that exchange of taurine between those two compartments may occur. We propose that taurine could be released from supraoptic glia by a small decrease in osmolarity or by receptor-mediated mechanisms during conditions of low hormonal (vasopressin and/or oxytocin) needs. Such released taurine could then act on presynaptic or postsynaptic sites, or both, to exert its neuromodulatory actions.

Synaptic relations of neurotensinergic neurons in the dorsal raphe nucleus

The ultrastructure and synaptic relations of neurotensinergic neurons in the rat dorsal raphe nucleus (DRN) were examined. The neurotensin-like immunoreactive (NT-L1) neurons in the DRN were fusiform or spherical. The NT-LI perikarya could only be detected in colchicine-treated animals whereas the immunoreactive axon terminals could only be found in the animals not treated with colchicine. Although many NT-LI dendrites received synapses from nonimmunoreactive axon terminals, the NT-LI perikarya received few synapses. NT-LI axon terminals also made synapses on nonimmunoreactive dendrites. Occasionally, synapses were found between the NT-LI axon terminals and NT-LI dendrites in the cases in which the animals were not treated with colchicine.

Noradrenergic innervation of vasopressin- and oxytocin-containing neurons in the hypothalamic paraventricular nucleus of the macaque monkey: quantitative analysis using double-label immunohistochemistry and confocal laser microscopy

Previous reports on the rat and monkey hypothalamus have revealed a dense noradrenergic innervation within the hypothalamic paraventricular nucleus as assessed by dopamine-beta-hydroxylase immunohistochemistry. These single-label analyses were unable to delineate the cellular structures which receive this catecholaminergic innervation. Double-label preparations in the rat hypothalamic paraventricular nucleus have demonstrated synaptic interactions between noradrenergic varicosities and magnocellular neurons. However, the density and distribution of varicosities contacting chemically identified magnocellular neurons have not been assessed at the light or electron microscopic level. In this report, single-label immunohistochemistry was used to assess the morphology and distribution of vasopressin- and oxytocin-immunoreactive neurons within the macaque hypothalamic paraventricular nucleus. In addition, double-label immunohistochemistry was combined with confocal laser scanning microscopy to quantify the number of dopamine-beta-hydroxylase-immunoreactive varicosities in apposition to magnocellular neurons expressing vasopressin or oxytocin immunoreactivity. The morphology of chemically identified neurons was also compared to magnocellular neurons in the monkey hypothalamic paraventricular nucleus which were filled with Lucifer Yellow in order to assess the somatodendritic labeling of the immunohistochemical preparation. Qualitative assessment of immunohistochemically identified magnocellular cells indicated that vasopressin- and oxytocin-containing neurons are observed throughout the rostrocaudal extent of the monkey hypothalamic paraventricular nucleus, demarcating this structure from the surrounding anterior hypothalamus. The distribution of the two nonapeptides is complementary, with vasopressin-immunoreactive neurons having a greater somal volume and located in a more medial aspect of the mid and caudal hypothalamic paraventricular nucleus relative to oxytocin-immunoreactive perikarya. For the double-label preparations, a series of confocal optical sections was assessed through the total somal volume of vasopressin- and oxytocin-immunoreactive neurons along with the corresponding dopamine-beta-hydroxylase-immunoreactive varicosities in the same volume of tissue, generating a varicosity-to-neuron ratio which was further characterized morphologically to assess afferent input to the soma and proximal dendrites. Quantitative analysis revealed that vasopressin-immunoreactive neurons received approximately two thirds of their dopamine-beta-hydroxylase-immunoreactive varicosities in apposition to the proximal dendrites and one third in apposition to the somata. Furthermore, vasopressin-immunoreactive neurons received a greater innervation density than oxytocin-immunoreactive neurons, which did not have a differential distribution of varicosities on the proximal dendrites and somata. The distribution of dopamine-beta-hydroxylase-immunoreactive afferents on magnocellular neurons in the hypothalamic paraventricular nucleus may reflect a physiological role of this circuit in terms of preferential release of vasopressin from magnocellular neurons upon noradrenergic stimulation.

Local inhibitory synaptic inputs to neurones of the paraventricular nucleus in slices of rat hypothalamus

1. Intracellular recordings were obtained from neurones in the region of the paraventricular nucleus in slices of rat hypothalamus. Glutamate microdrops were applied to the surface of the slices at sites dorsal, lateral and ventral to the paraventricular nucleus to selectively activate local presynaptic neurones. The gamma-aminobutyric acidA (GABAA)-receptor antagonists picrotoxin or bicuculline were bath-applied to block synaptic inhibition. 2. Glutamate microapplication caused a tonic depolarization and often repetitive action potentials in twenty of forty-seven recorded cells. This was probably caused by the direct exposure of the dendrites of the recorded cells to the glutamate microdrops. 3. Glutamate microstimulation elicited inhibitory synaptic responses in nine of forty-seven neurones tested. Glutamate microdrops caused discrete, hyperpolarizing postsynaptic potentials (PSPs) in four cells recorded with microelectrodes containing potassium acetate and evoked depolarizing PSPs in four cells recorded with KCl-filled microelectrodes. Glutamate microapplication inhibited spontaneous spike firing in another cell recorded with a potassium acetate microelectrode. 4. Bath application of GABAA-receptor antagonists completely blocked the hyperpolarizing PSPs elicited by glutamate microstimulation in three of three cells recorded with potassium acetate electrodes and the depolarizing PSPs in two of two cells recorded with KCl electrodes, indicating they were inhibitory PSPs caused by the release of GABA. Suppression of GABAA-mediated synaptic inhibition did not reveal any glutamate-evoked excitatory PSPs. 5. Recorded cells were identified as magnocellular, parvocellular or non-paraventricular bursting neurones on the basis of their electrophysiological properties. Direct depolarization and local inhibitory synaptic responses were observed in all three cell types. 6. Several conclusions can be drawn from these data: (1) functional glutamate receptors are distributed throughout neuronal populations in the paraventricular region of the hypothalamus, confirming and extending previous observations; (2) local synaptic inputs to neurones in the paraventricular nucleus are primarily inhibitory, supplied by perinuclear GABAergic neurones; (3) both magnocellular and parvocellular subpopulations receive local inhibitory synaptic inputs. The possibility that these local GABAergic circuits mediate inhibitory inputs to paraventricular neurones from limbic structures is discussed.

Local origins of some GABAergic projections to the paraventricular and supraoptic nuclei of the hypothalamus in the rat

Axonal transport and immunohistochemical methods were used to characterize the organization of glutamic acid decarboxylase-immunoreactive (GAD-ir) projections to the paraventricular (PVH) and supraoptic (SO) nuclei of the hypothalamus in the rat. In line with prior reports, GAD-ir varicosities were found to be densely and quite uniformly distributed throughout the hypothalamus, including the PVH and the SO. Nonetheless, the periventricular part of the PVH was consistently found to contain a disproportionately high density of GAD-ir elements. Small crystalline implants of the retrograde tracer, true blue, into the PVH labeled GAD-ir cells in the anterior perifornical region, portions of the anterior hypothalamic area immediately ventral to the PVH, a region just dorsal to the rostral SO and extending caudomedially over the optic chiasm and tract, and within the anterior one-third of the PVH itself. Because possible uptake of retrograde tracer by local dendritic processes might have yielded false positive filling of nearby GAD-ir cells, anterograde transport, Phaseolus vulgaris-leucoagglutinin, and combined anterograde transport-immunohistochemical methods were used to attempt to confirm these four putative local sources of GAD-ir inputs. Tracer injections in each of the above mentioned regions labeled sparse to moderate axonal projections to the PVH, which ramified preferentially in the parvicellular division of the nucleus. Projections to the magnocellular division of the PVH and the SO were generally sparse and inconsistently observed in this material. A variable, and generally small, proportion of anterogradely labeled axons and terminals in the PVH also displayed GAD-ir. These results suggest that GABAergic projections to visceromotor cell types in the PVH and SO arise, at least in part, from several diffusely distributed local sources. The fact that these afferents were found to terminate preferentially in the parvicellular division of the PVH makes it likely that additional sources of GABAergic projections to the magnocellular neurosecretory system remain to be identified. Peri- and intranuclear GABAergic neurons could provide an intermediary by which documented (and generally inhibitory) limbic system influences on neuroendocrine function are exerted.

Converging GABA- and glutamate-immunoreactive axons make synaptic contact with identified hypothalamic neurosecretory neurons

To study the neurochemical identity of axons in synaptic contact with identified hypothalamic neurosecretory neurons in rats, we combined retrograde axonal transport of a marker molecule with postembedding immunogold staining for amino acid neurotransmitters. After intravenous injections of horseradish peroxidase, neurosecretory neurons with axons in the median eminence or neurohypophysis transported the peroxidase retrogradely back to the cell body of origin. Serial ultrathin sections from the paraventricular and arcuate nuclei were immunostained with glutamate or GABA antisera. Peroxidase-labeled neurons and their dendrites received synaptic contact from colloidal gold-labeled axons immunoreactive for GABA or for glutamate. Axons which were highly immunoreactive for GABA and other axons immunoreactive for glutamate but not for GABA consistently made converging synaptic contact with the same peroxidase-labeled cell. Some of the peroxidase-labeled neurons from the arcuate nucleus which were postsynaptic to both GABA and glutamate axons were themselves identified as being GABA immunoreactive. Serial ultrathin sections revealed that multiple presynaptic axons immunoreactive for glutamate or GABA made repeated contacts with single neurons. These results suggest a widespread convergence of the major inhibitory and excitatory amino acid transmitter on the neurons which control both the anterior and posterior pituitary hormones.

Evidence of GABA-immunopositive neurons in the dorsal part of the lateral geniculate nucleus of reptiles: morphological correlates with interneurons

The distribution and staining pattern of gamma-aminobutyric acid immunoreactivity have been examined by both light and electron microscopy in the dorsal part of the lateral geniculate nucleus of three reptilian species: the turtle Chinemys reevesi, the lizard Ophisaurus apodus and the snake Vipera aspis. After perfusion of the animals with 1% paraformaldehyde and 1% glutaraldehyde and polyethyleneglycol embedding of the brains, the analysis of sections processed immunocytochemically with an anti-GABA antiserum has revealed a moderate-to-dense labeling of the neurons of the dorsal part of the lateral geniculate complex in these species. Labeled cell bodies are small-sized, either rounded or fusiform and the GABA-positive dendrites emerging from them are not preferentially oriented in any particular direction. Quantitative studies in Vipera indicate that GABA-positive neurons make up about 14% of the population of neurons of the dorsal part of the lateral geniculate nucleus. Electron microscopy of specimens treated by either pre- or post-embedding techniques has confirmed that these cells corresponded to neurons. No glial cells were ever observed to be immunopositive. These GABA-positive neurons, characterized by the presence of pleiomorphic synaptic vesicles localized either in their perikaryon or more often in presynaptic dendrites, established symmetrical synaptic contacts. In this case, the latter were involved both pre- and postsynaptically in serial and, more rarely, in triadic arrangements, a synaptic organization specific to interneurons. The involvement of such GABA-positive neurons in local circuits is discussed.

Electron microscopic study of GABA-immunoreactive neuronal processes in the superficial gray layer of the rat superior colliculus: their relationships with degenerating retinal nerve endings

GABA-immunoreactive neuronal elements were detected in the stratum griseum superficiale or superficial gray layer of the rat superior colliculus in an electron microscopic study, using postembedding immunocytochemistry with protein A-gold as a marker. In addition to neuronal somata, two types of GABA-immunoreactive neuronal processes were observed. Numerous profiles of axon terminals (1 microns in diameter) with clear round or pleomorphic synaptic vesicles and mitochondria were found to establish mostly symmetrical synaptic contacts with GABA-immunonegative dendrites of various diameters. Some axosomatic synapses could also be observed. The gold particle density in this axon terminal compartment was between seven and 13 times the background level. The stratum griseum superficiale also included GABA-immunoreactive dendrites, some of which contained clear synaptic vesicles. These dendritic profiles always formed the presynaptic component of dendrodendritic synaptic contacts. The density of the gold particles in the dendritic compartment, taken as a whole, was between three and 13 times the background level. Furthermore, the relationship between the GABA-immunoreactive neuronal elements and degenerating retinal nerve endings identified in the left stratum griseum superficiale following enucleation of the right eye was investigated after a 7-day survival period. The profiles of degenerating retinal nerve endings (0.7 microns in diameter) were found to be devoid of any specific labelling. Most of the retinal boutons established axodendritic synapses of the asymmetrical type with an immunonegative dendrite, which was also contacted in some cases by a GABA-immunopositive axon terminal. Other retinal endings were presynaptic to GABA-immunopositive dendritic profiles with synaptic vesicles, some of which were found to contact in turn an unlabelled dendrite, thereby completing serial synaptic relationships. More rarely, retinal endings formed the presynaptic component of possible axoaxonic synapses with GABA-positive terminals presumed to be axonic in nature. It can be concluded that the retinal input to the superficial gray layer often converges with a GABAergic axonal input on a dendritic target, the neurotransmitter specificity of which is unknown. In other cases, retinal terminals synaptically contact GABA-immunolabelled conventional and presynaptic dendrites and probably also some axon terminals; this might provide an anatomical substrate for the control of GABA release from these GABAergic processes. These results indicate that transmitter GABA plays an important role in retinocollicular transmission.

GABA: a dominant neurotransmitter in the hypothalamus

To study the organization and distribution of the inhibitory amino acid neurotransmitter GABA in the medial hypothalamus, we used a postembedding immunocytochemical approach with colloidal gold. Quantitative analysis showed that half (49%) of all synapsing boutons studied were immunoreactive for GABA, based on immunogold staining of the suprachiasmatic, arcuate, supraoptic, and paraventricular nuclei. This was corroborated with pre-embedding peroxidase immunostaining with antisera against glutamate decarboxylase, the GABA synthetic enzyme. These data suggest that GABA is the numerically dominant neurotransmitter in the hypothalamus, and emphasize the importance of inhibitory circuits in the hypothalamus. Serial ultrathin sections were used to reconstruct GABA immunoreactive boutons and axons in three dimensions. With this type of analysis we found less morphological heterogeneity between GABA immunoreactive boutons than with single ultrathin sections. Single sections sometimes showed boutons containing only small clear vesicles, and other with both clear vesicles and small dense core vesicles. However, with serial sections through individual boutons, dense core vesicles were consistently found at the periphery of the pre-synaptic GABA immunoreactive boutons, suggesting probable co-localization of GABA with unidentified peptides in most if not all boutons throughout the hypothalamus. A positive correlation was found between the density of small clear vesicles and the intensity of immunostaining with colloidal gold particles. GABA immunoreactive axons generally made symmetrical type synaptic specializations, although a small percentage made strongly asymmetrical synaptic specializations. Vesicles in GABA immunoreactive boutons were slightly smaller than those in non-reactive boutons. Synaptic efficacy is related to the position of the synapse on the post-synaptic neuron. While the majority of GABA immunoreactive axons made synaptic contact with dendrites, the distribution of GABA immunoreactive synapses on somata and dendrites was the same as would be expected from a random distribution of all boutons. No preferential innervation of cell bodies by GABA immunoreactive terminals was found. Serial ultrathin sections showed that a GABA immunoreactive axon would sometimes make repeated synaptic contacts with a single postsynaptic neuron, indicating a high degree of direct control by the presynaptic GABAergic cell. Other immunoreactive axons made synaptic contact with a number of adjacent dendrites and cells, suggesting a role for GABA in synchronizing the activity of hypothalamic neurons. Based on the density of immunogold particles per unit area, varying concentrations of immunoreactive GABA were found in different presynaptic boutons in the hypothalamus.

Topographical relationships between catecholamine- and neuropeptide-containing fibers in the median eminence of the newt, Triturus alpestris. An ultrastructural immunocytochemical study

Dopaminergic and peptidergic nerve fibers were simultaneously demonstrated with a double-labeling technique at the ultrastructural level. The first antibody, raised against tyrosine hydroxylase, was applied during the preembedding phase and visualized with the peroxidase method. The second antibody, raised against one of the peptides met-enkephalin, somatostatin or gonadotropin-releasing hormone (GnRH), was applied to the ultrathin sections and visualized with gold-labeled goat anti-rabbit IgG. The fibers of both categories were present in the zona externa of the median eminence, frequently contacting the basal lamina of the portal vessels. In addition, topographical relationships between different types of nerve fibers were observed in the perivascular areas, although there were no morphological signs of synaptic specializations. Using serial sections, it could be established that one GnRH-fiber contacted both a dopaminergic fiber and a fiber immunoreactive for met-enkephalin. The observations support earlier physiological data concerning the regulation of the hypothalamo-hypophyseal axis, with special emphasis on the release of neurohormones in the median eminence of the newt.

Emerging concepts of structure-function dynamics in adult brain: the hypothalamo-neurohypophysial system

As the first known of the mammalian brain's neuropeptide systems, the magnocellular hypothalamo-neurohypophysial system has become a model. A great deal is known about the stimulus conditions that activate or inactivate the elements of this system, as well as about many of the actions of its peptidergic outputs upon peripheral tissues. The well-characterized actions of two of its products, oxytocin and vasopressin, on mammary, uterine, kidney and vascular tissues have facilitated the integration of newly discovered, often initially puzzling, information into the existing body of knowledge of this important regulatory system. At the same time, new conceptions of the ways in which neuropeptidergic neurons, or groups of neurons, participate in information flow have emerged from studies of the hypothalamo-neurohypophysial system. Early views of the SON and PVN nuclei, the neurons of which make up approximately one-half of this system, did not even associate these interesting, darkly staining anterior hypothalamic cells with hormone secretion from the posterior pituitary. Secretion from this part of the pituitary, it was thought, was neurally evoked from the pituicytes that made the oxytocic and antidiuretic "principles" and then released them upon command. When these views were dispelled by the demonstration that the hormones released from the posterior pituitary were synthesized in the interesting cells of the hypothalamus, the era of mammalian central neural peptidergic systems was born. Progress in developing an ever more complete structural and functional picture of this system has been closely tied to advancements in technology, specifically in the areas of radioimmunoassay, immunocytochemistry, anatomical tracing methods at the light and electron microscopic levels, and sophisticated preparations for electrophysiological investigation. Through the judicious use of these techniques, much has been learned that has led to revision of the earlier held views of this system. In a larger context, much has been learned that is likely to be of general application in understanding the fundamental processes and principles by which the mammalian nervous system works.(ABSTRACT TRUNCATED AT 400 WORDS)