Putative transmitters in denervated olfactory complex

Dopaminergic modulation of synaptic transmission and neuronal activity patterns in the zebrafish homolog of olfactory cortex

Dopamine (DA) is an important modulator of synaptic transmission and plasticity that is causally involved in fundamental brain functions and dysfunctions. We examined the dopaminergic modulation of synaptic transmission and sensory responses in telencephalic area Dp of zebrafish, the homolog of olfactory cortex. By combining anatomical tracing and immunohistochemistry, we detected no DA neurons in Dp itself but long-range dopaminergic input from multiple other brain areas. Whole-cell recordings revealed no obvious effects of DA on membrane potential or input resistance in the majority of Dp neurons. Electrical stimulation of the olfactory tracts produced a complex sequence of synaptic currents in Dp neurons. DA selectively decreased inhibitory currents with little or no effect on excitatory components. Multiphoton calcium imaging showed that population responses of Dp neurons to olfactory tract stimulation or odor application were enhanced by DA, consistent with its effect on inhibitory synaptic transmission. These effects of DA were blocked by an antagonist of D2-like receptors. DA therefore disinhibits and reorganizes sensory responses in Dp. This modulation may affect sensory perception and could be involved in the experience-dependent modification of odor representations.

Differential involvement of hippocampal vasoactive intestinal peptide in nociception of rats with a model of depression

The effects of VIP microinjected unilaterally (left or right) into the hippocampal CA1 area at a dose of 10 and 100 ng or bilaterally (10 ng), on nociception of male Wistar rats with a model of depression (bilateral olfactory bulbectomy-OBX) were studied. Nociception was examined applying mechanical pressure on the left hind paw of the rat (analgesy-meter test). It was found that in OBX rats the pain threshold is increased. VIP showed differential effects depending on the side and dose of administration. The pain threshold after left-side microinjections of VIP into the hippocampal CA1 area of OBX rats was significantly higher than that after injections into right-side. There are no significant differences between right-side VIP-treated and OBX rats. Bilateral microinjections of VIP also exerted antinociceptive effect. These findings suggest that the hippocampal lateralized antinociceptive effect of VIP in OBX rats depends on the hemisphere of injection and suggest that VIP-ergic neurons in the hippocampal CA1 area may play differential role in nociception of rats with a model of depression.

Localization of putative transmitters in the hippocampal formation: with a note on the connections to septum and hypothalamus

Biochemical assays on microdissected samples, denervation studies, subcellular fractionation, and light and electron microscopic autoradiography of high affinity uptake have been performed to study the cellular localization of transmitter candidates in the rat hippocampal formation. High affinity uptake of glutamate and aspartate is localized in the terminals of several excitatory systems, such as the entorhino-dentate fibres (perforant path), mossy fibres (from granular cells) and pyramidal cell axons. Thus, in stratum radiatum and oriens of CA1, 85% of glutamate and asparate uptake and 40% of glutamate and aspartate content are lost after lesions of ipsilateral plus commissural fibres from CA3/CA4. Hippocampal efferents also take up aspartate and glutamate, since these activities are heavily reduced in the lateral septum and mamillary bodies after transection of fimbria and the dorsal fornix. The synthesis (by glutamic acid decarboxylase), content and high affinity uptake of gamma-aminobutyrate (GABA) are not reduced after lesions of these or other projection fibre systems. A localization in intrinsic neurons is confirmed by a selective loss of glutamic acid decarboxylase after local injections of kainic acid. Peak concentrations of the enzyme occur near the pyramidal and granular cell bodies, corresponding to the site of the inhibitory basket cell terminals, and in the outer parts of the molecular layers. Some 85% of glutamic acid decarboxylase is situated in 'nerve ending particles'. Acetylcholine synthesis (by choline acetyltransferase) disappears after lesions of septo-hippocampal fibres. Since 80% of the hippocampal choline acetyltransferase is in 'nerve ending particles', the characteristic topographical distribution of this enzyme should reflect the distribution of cholinergic septo-hippocampal afferents. Serotonin, noradrenaline, dopamine and histamine are located/synthesized in afferent fibre systems. Some monoamine-containing afferents to the hippocampal formation pass via the septal area, others via the amygdala. The hippocampal formation also contains nerve elements reacting with antibodies against neuroactive peptides, such as enkephalin, substance P, somatostatin and gastrin/cholecystokinin.

The olfactory bulbectomised rat as a model of depression

Bilateral olfactory bulbectomy results in changes in behavior, and in the endocrine, immune and neurotransmitter systems, that simulates many of those seen in patients with major depression. The olfactory system in the rat forms a part of the limbic region in which the amygdala and hippocampus contribute to the emotional and memory components of behavior. However, the loss of olfaction alone, which results from bulbectomy, is not the major factor that contributes to the behavioral abnormalities as peripherally induced anosmia does not cause the same behavioral changes. Thus it would appear that bulbectomy causes a major dysfunction of the cortical-hippocampal-amygdala circuit that underlies the behavioral and other changes. These neuroanatomical areas also seem to be dysfunctional in the patient with major depression. Chronic, but not acute, administration of antidepressants largely corrects most the behavioral, endocrine, immune and neurotransmitter changes that occur following bulbectomy. Thus the olfactory bulbectomized rat is not only a model for detecting antidepressant activity but also one for exploring the inter-relationships between these systems that are also dysfunctional in patients with major depression.

Calcium-binding proteins: differential expression in the rat olfactory cortex after neonatal olfactory bulbectomy

Calbindin, parvalbumin, and calretinin, members of EF-hand calcium-binding proteins, play important roles in buffering intracellular calcium ions. These proteins are localized in distinct populations of cells in the olfactory bulb (the primary sensory relay in the olfactory system) and its major synaptic target, the primary olfactory cortex (POC). In the present study, the postnatal expression of these calcium-binding proteins in layer III of POC was quantitatively examined 30 days after neonatal bulbectomy, a manipulation known to cause cell death and neurotransmitter changes. The numbers of both calbindin and parvalbumin-immunoreactive profiles showed significant increases (68% and 163%, respectively), while calretinin-immunoreactive profiles exhibited a 46% reduction. The data demonstrate that the expression of these calcium-binding proteins is regulated in part by the afferent input from the olfactory bulb. Furthermore, the resultant increase in calbindin and parvalbumin expression may provide neuroprotective support necessitated by possible alterations in intracellular calcium ions and other neurochemical factors that accompany neonatal bulb removal.

The olfactory bulbectomized rat as a model of depression: an update

The olfactory bulbectomized (OB) rat has been proposed as an animal model of depression. The following behavioural changes have been observed following bilateral olfactory bulbectomy: hyperactivity in an enclosed arena, such as the open-field; enhanced nocturnal hyperactivity in a 24-hr home cage activity monitor; deficits in memory, as shown by passive avoidance behaviour and in the Morris maze and the 8-arm radial maze; increased open arm entries in the elevated plus-maze; and changes in food motivated and conditioned taste aversion behaviour. Alterations in the noradrenergic, serotonergic, cholinergic, gamma-aminobutyric acid (GABA)ergic and glutamatergic neurotransmitter systems are also associated with olfactory bulbectomy. The variety of immune changes following olfactory bulbectomy includes reduced neutrophil phagocytosis, lymphocyte mitogenesis, lymphocyte number and negative acute phase proteins, increased leucocyte adhesiveness/aggregation, monocyte phagocytosis, neutrophil number and positive acute phase proteins. An enhanced nocturnal secretion of corticosterone is observed in OB rats, which is normally suppressed by dexamethasone. The most commonly employed behavioural indicator of antidepressant activity is attenuation of the OB-related hyperactivity in the open-field. However, many of the other behavioural, neurotransmitter and immune changes have been shown to be attenuated by chronic (but not acute) antidepressant treatment. Tricyclic antidepressants (amitriptyline, desipramine), atypical agents (mianserin), selective serotonin reuptake inhibitors (paroxetine, sertraline, fluvoxamine), reversible inhibitors of monoamine oxidase A (moclobemide), as well as putative antidepressants such as 5-hydroxytryptamine1A agonists (zalospirone, ipsapirone), noncompetitive N-methyl-D-aspartate antagonists (MK-801) and triazolobenzodiazepines (alprazolam, adinazolam), have demonstrated antidepressant-like activity in this model. As many of the changes exhibited by the OB rat are qualitatively similar to those observed in depressed patients, it may be concluded that the OB rat is a model of depression and not just a means whereby putative antidepressants may be tested.

Intrabulbar associational system in the rat olfactory bulb comprises cholecystokinin-containing tufted cells that synapse onto the dendrites of GABAergic granule cells

The intrabulbar associational system (IAS) originates from tufted cells whose axons terminate in the internal plexiform layer (IPL) on the opposite side of the same olfactory bulb. The postsynaptic targets of the IAS are unknown. Subpopulations of tufted cells contain different neuropeptides and transmitters but it is not known if tufted cells forming the IAS are homogeneous with respect to neurotransmitters. Therefore, the goals of the present study were to identify the postsynaptic targets of the IAS and to determine the major transmitter in this intrabulbar circuit. Biocytin anterograde tracing revealed that the axons of superficially situated tufted cells coursed directly to the IPL where they turned abruptly to run ventrally and dorsally to terminate in the IPL on the opposite side of the olfactory bulb. WGAapoHRP-Au retrograde tracing combined with immunohistochemistry for CCK revealed that all tufted cells retrogradely labeled by WGAapoHRP-Au injection in the IPL were immunoreactive for CCK. Anterograde transport of biocytin combined with postembedding immunocytochemical gold-labeling for GABA demonstrated that labeled IAS axons terminate predominantly, if not exclusively, on GABAergic granule cell dendrites in the IPL. These results confirm that the IAS arises from tufted cells and is topographically organized. We further demonstrate that tufted cells forming the IAS use the neuropeptide CCK as a transmitter. In addition, we show that the postsynaptic targets of the CCKergic IAS are the dendrites of GABAergic granule cells coursing through the IPL toward the EPL. As CCK is generally an excitatory neuropeptide, we suggest that the IAS functions to excite topographically discrete populations of granule cells. This action may lead to inhibition of equally discrete populations of mitral/tufted cells. Thus, the IAS may be an intrabulbar inhibitory circuit that coordinates topographically organized neural networks in the olfactory bulb.

Release of gamma-[3H]aminobutyric acid from rat olfactory bulb and substantia nigra: differential modulation by glutamic acid

We have studied the glutamate modulation of gamma-[3H]aminobutyric acid ([3H]GABA) release from GABAergic dendrites of the external plexiform layer of the olfactory bulb and from GABAergic axons of the substantia nigra. In the olfactory bulb, [3H]GABA release was induced by high K+ and kainate, and not by aspartate and glutamate alone. However, when the tissue was conditioned by a previous K+ depolarization, glutamate and aspartate caused [3H]GABA release. The effect of glutamate was significantly enhanced when the GABA uptake mechanism was blocked by nipecotic acid. N-Methyl-D-aspartate and quisqualate did not cause [3H]GABA release under the same conditions. The acidic amino acid receptor antagonist 2-amino-4-phosphonobutyric acid and the N-methyl-D-aspartate receptor antagonist 2-amino-5-phosphonovaleric acid significantly inhibited the K+-glutamate- and the kainate-induced [3H]GABA release. Mg2+ (5 mM), which blocks the N-methyl-D-aspartate receptors, significantly inhibited the K+-glutamate-induced but not the kainic acid-induced [3H]GABA release. The K+-glutamate-stimulated release, but not the K+-stimulated [3H]GABA release, was strongly inhibited by Na+-free solutions or by 300 nM tetrodotoxin. Apparently the glutamate-induced release of [3H]GABA occurs through an interneuron because it is dependent on the presence of nerve conduction. In the substantia nigra no [3H]GABA release was elicited by any of the glutamate agonists tested. The present results clearly differentiate between the effects of glutamate on the release of [3H]GABA from the substantia nigra and from the olfactory bulb.(ABSTRACT TRUNCATED AT 250 WORDS)

Cysteine: depolarization-induced release from rat brain in vitro

Compounds released on depolarization in a Ca2+-dependent manner from rat brain slices were screened to identify candidates for neuroactive substances. Lyophilized superfusates were analyzed by reversed-phase HPLC after derivatization with 9-fluorenyl N-succinimidyl carbonate. One of the compounds that showed an increase of concentration in superfusates in the presence of iodoacetamide was identified as the cysteine (Cys) derivative, S-carboxamidomethylcysteine, by fast atom bombardment mass spectrometry and other methods. This stable Cys derivative originates from endogenous, extracellular Cys. The finding led to a method for quantification of Cys in superfusates by immediate cooling of the superfusates to 0 degrees C and reaction of Cys with N-ethylmaleimide. Depolarization-induced Ca2+-dependent release of Cys was most prominent in the neocortex, followed by the mesodiencephalon, striatum, and cerebellum. This suggests that Cys is released from a neuronal compartment and might be involved in neurotransmission.

Glutamate-like immunoreactivity revealed in rat olfactory bulb, hippocampus and cerebellum by monoclonal antibody and sensitive staining method

Although there is good evidence favoring L-glutamate as a major excitatory amino acid transmitter, relatively little is known about the distribution of nerve terminals using this substance. A method visualizing glutamate-like immunoreactivity at the light microscopic level by means of a monoclonal antibody, mAb 2D7, is described. --The antigen used for immunization was a glutaraldehyde-linked glutamate-BSA conjugate, and hybridomas were differentially screened by ELISA for production of antibodies recognizing glutamate- but not aspartate-BSA. The crossreactivity of 'anti-glutamate' mAb 2D7 as estimated in absorption tests was low even with conjugates closely related to glutamate-BSA.--Semithin sections from rapidly perfusion-fixed, plastic-embedded rat brain tissues were etched and stained by a combination of the peroxidase-antiperoxidase method and silver enhancement of the diaminobenzidine reaction product. Only this amongst several other immunohistochemical methods tried produced labeling patterns which showed terminal-like elements in brain regions such as olfactory bulb, hippocampus and cerebellum, and which were mostly consistent with already available information on systems using glutamate as neurotransmitter. Particularly striking was the staining of elements reminiscent of mossy fiber terminals in hippocampus and cerebellum as well as of cerebellar parallel fiber terminals.

Ultrastructure of synaptic remodeling in piriform cortex of adult rats after neonatal olfactory bulb removal: an immunocytochemical study

The purpose of this investigation was to study possible remodeling in synaptic structures of the piriform cortex (PC) of adult rats following neonatal deafferentation by removal of the olfactory bulb (OB) at birth. Emphasis was placed on possible qualitative changes in the ultrastructure and immunocytochemical localization of cholecystokinin (CCK, a possible excitatory neurotransmitter or modulator) and glutamic acid decarboxylase (GAD, precursor enzyme to the inhibitory transmitter GABA) in axons, terminals, and synaptic complexes. Light microscopic results in normal adult material show that GAD-positive terminals form a dense band subjacent to the lateral olfactory tract (LOT), become less dense in deeper Ib, and are rare in layer II. Following deafferentation, GAD-positive terminals appear denser and more homogeneously distributed throughout layer I and are also more prevalent in layer II. Ultrastructural results of normals and controls indicate GAD-positive terminals normally contain pleomorphic or flattened vesicles and form symmetric contacts onto dendritic shafts and branches throughout layer I. In deafferented layer I not only do there appear to be greater numbers of symmetric GAD-positive contacts, but in contrast to normals, asymmetric contacts mainly onto spines are now present. Light microscopic results from deafferented material also show an apparent proliferation with spread or sprouting of CCK-positive fibers or axonlike structures mainly into layer Ia, whereas these fibers are normally observed only in the LOT and are generally few in number. Also in normals the few CCK-positive terminals in the area subjacent to the LOT contain flattened or pleomorphic vesicles and form symmetric contacts. Deafferentation results in CCK-positive terminals throughout layer I with a greater frequency of synaptic contacts which now also include a few asymmetric contacts onto spines. The findings clearly show modifications in synaptic patterns of immunocytochemical-labeled terminals that might be compatible with the process of atypical reinnervation of deafferented postsynaptic sites and possible ingrowth of new axons.

Brain output dysregulation induced by olfactory bulbectomy: an approximation in the rat of major depressive disorder in humans?

Mounting evidence indicates that the emotional, cognitive, neurovegetative and behavioral symptoms of patients with major depressive disorder are due to abnormal neurochemical substrates in the brain. Although the specific neurochemical abnormalities responsible have not been identified, the presenting symptoms of major depression are consistent with a disruption of normal neural communications between the limbic system and hypothalamus. Following removal of the olfactory bulbs, rats display a syndrome of behavioral deficits that also reflect a disruption of the limbic-hypothalamic axis. Moreover, the bulbectomy induced deficits are selectively reduced by the chronic administration of the same drugs that alleviate the symptoms of depression when given chronically to the patients. In addition to this pharmacological similarity, there are also numerous behavioral parallels between bulbectomized rats and major depression patients. The bulbectomized rat provides a good model in which to study antidepressant drugs and also may provide neurochemical and neuroanatomical data that are relevant to understanding the biological substrates of emotion and the causes of depression in humans.

Selective immunocytochemical staining of mitral cells in rat olfactory bulb with affinity purified antibodies against N-acetyl-aspartyl-glutamate

Polyclonal antibodies were raised in rabbits against N-acetyl-aspartyl-glutamate (NAAG) coupled to bovine serum albumin (BSA) with carbodiimide and were purified by affinity chromatography sequentially over BSA-agarose and NAAG-agarose resins. Solid-phase RIA revealed a distinct pattern of specificity of the antibodies for N-acetylated acidic peptides, with highest signal obtained for NAAG, and essentially no immunoreactivity demonstrable for aspartate or glutamate. Coronal sections through carbodiimide-fixed rat olfactory bulb were incubated with the purified antiserum and antigen localization visualized by the avidin-biotin peroxidase techniques. Immunoreactivity was restricted to the mitral cells, the major excitatory projection neurons of the lateral olfactory tract, a putative glutamatergic pathway. Immunoreactivity was selectively blocked by preincubation of the antibody with 1 microgram/ml of NAAG-BSA. These results demonstrate a specific neuronal localization of NAAG-like immunoreactivity and support the candidacy of NAAG as a neurotransmitter of the lateral olfactory tract.

Olfactory bulbectomy protects against pilocarpine-induced motor limbic seizures in rats

Systemic injection of pilocarpine (380 mg/kg) results in the development of motor limbic seizures in rats. Olfactory bulbectomy performed 7 days prior to pilocarpine administration has a strong protective effect against the seizures. Neuropathological changes which normally occur in this seizure model do not develop in the brains of bulbectomized animals.

Binding characteristics of the selective alpha 2-adrenoceptor antagonist [3H]idazoxan to rat olfactory cortex membranes

[3H]Idazoxan binding to membranes prepared from rat olfactory cortex obeyed saturation kinetics and was to a single population of sites. Although the density of sites was dependent on the incubation medium, binding was of high affinity (KD approximately 5.5 nM) with a Hill coefficient close to unity. Competition studies with a range of adrenoceptor agonists and antagonists confirmed that [3H]idazoxan binding was to alpha 2-adrenoceptors. Neither chemical lesions with the neurotoxin kainic acid nor chronic unilateral bulbectomy significantly altered any of the [3H]idazoxan binding parameters. These findings suggest that alpha 2-adrenoceptors are not located on the lateral olfactory tract terminals or pyramidal cells of the olfactory cortex.

Norepinephrine and amino acids in prepyriform cortex of rats fed imbalanced amino acid diets

Monoamines and amino acids were measured in anterior prepyriform cortex (PPC) and anterior cingulate cortex (CC) of male Sprague-Dawley rats after they were offered basal, imbalanced (IMB) or corrected amino acid diets, limited in threonine (THR) or isoleucine (ILE). In the THR study, brains were taken after 2.5 hr of feeding, when intake of THR-IMB was just depressed. In the ILE study the brains were taken after 3.5 hr on ILE-IMB, a less severely imbalanced ration, before the onset of food intake depression. The PPC has been shown to be involved in the acute response of animals to imbalanced amino acid diets. In the PPC from the IMB diet groups, NE was reduced by 30%, but the other monoamines were unchanged. In CC, an area involved in the adaptive, but not the acute feeding response to imbalanced diets, the monoamines were unchanged in the IMB diet groups. In both studies, in both tissues, the limiting amino acids were decreased in the IMB groups, although the decrease of ILE in the CC failed to reach significance. The remaining indispensable amino acids, added to create the imbalance, were slightly reduced in the THR-IMB group, but not in the ILE-IMB group in both tissues. Thus, the amino acid patterns were altered in the PPC and CC, as they are in whole brains from animals fed imbalanced amino acid diets. These results also suggest that the concentration of NE in the PPC may be associated with the initial food intake response of animals to imbalanced amino acid diets.

N-Acetylaspartylglutamate: possible role as the neurotransmitter of the lateral olfactory tract

N-Acetylaspartylglutamate, an endogenous brain peptide that binds with high affinity to a subpopulation of glutamate-binding sites in rat brain, is excitatory on rat piriform cortex pyramidal cells studied in a perfused brain slice. Both the monosynaptic excitation of the pyramidal cells elicited by stimulation of the lateral olfactory tract and the response to N-acetylaspartylglutamate were blocked by DL-2-amino-4-phosphonobutyrate but not by other excitatory amino acid antagonists. Responses to glutamate and aspartate, previously considered to be candidates as the lateral olfactory tract transmitter, were unaffected by 2-amino-4-phosphonobutyrate. Three days after unilateral bulbectomy there was a significant decrease in concentrations of N-acetylaspartylglutamate as well as aspartate, N-acetylaspartate, and gamma-aminobutyrate in the pyriform cortex of the side from which the olfactory bulb had been removed. These results are consistent with the possibility that N-acetylaspartylglutamate is the endogenous transmitter of the lateral olfactory tract.

Effect of lesions of the olfactory bulb on the levels of amino acids and related enzymes in the olfactory cortex of the guinea pig

Olfactory bulb removal and consequential degeneration of the lateral olfactory tract led to a decrease in the levels of glutaminase and malate dehydrogenase in the ipsilateral olfactory cortex. These changes in enzyme activity may account for the well established decrease in the levels of aspartate and glutamate in the olfactory cortex following ipsilateral bulbectomy. The level of glutamine synthetase, a glial marker enzyme, was slightly increased while the activities of glutamate decarboxylase, glutamate dehydrogenase, and glutamate oxaloacetic transaminase were unchanged.

Aspartate aminotransferase activity in fiber tracts of the rat brain

Activity of aspartate aminotransferase, an enzyme which catalyzes the interconversion of the excitatory transmitter candidates, glutamate and aspartate, has been measured in fiber tracts of rat, with an emphasis on sensory and motor systems of the brain. Most tracts had significantly higher activities than the cholinergic facial nerve root, consistent with the possibility that a component of aspartate aminotransferase activity might serve as a marker for neurons using glutamate and/or aspartate as neurotransmitter. Highest activity was in the auditory nerve root. On the other hand, a close correlation was found between aspartate aminotransferase and malate dehydrogenase activities in the fiber tracts, raising the question whether aspartate aminotransferase activity may be more closely related to energy metabolism than to transmitter metabolism.

Amino acid transmitter candidates in various regions of the primary olfactory cortex following bulbectomy

The effect of bulbectomy on amino acid levels in 5 regions of the rat primary olfactory cortex has been monitored. Glutamate levels were significantly lowered in the lateral olfactory tract only. In contrast, aspartate levels were significantly reduced in all regions except the periamygdaloid cortex. The results suggest transmitter heterogeneity of the tract fibres and confirm a likely transmitter role for aspartate for some tract terminals.

Physiology and pharmacology of olfactory bulb neurons in dissociated cell culture

Cells from olfactory bulbs of embryonic rats were grown in dissociated cell culture for up to 5 weeks. Both neurons and non-neuronal cells grew in these cultures, with a variety of neuronal populations appearing. A population of 20-25% of the neurons were GABAergic by the criterion of [3H]GABA uptake. Electrophysiologic measurements were made of the baseline activity of the cultured neurons. Cells showed a mean resting potential of 60.1 +/- 1.2 mV and a mean input resistance of 87.6 +/- 9.5 M omega. All cells were sensitive to microperfusion of GABA with half-maximal effect occurring at about 20 microM. Glutamate was universally excitatory but with variations in degree. Carnosine (beta-Ala-L-His), tested over the concentration range of 10 nM to 100 microM, had no effect on input resistance, resting potential, action potential shape, on-going synaptic activity or the responsiveness to either GABA or glutamate. These results are further evidence against a role for carnosine as the excitatory transmitter of the primary olfactory afferents.

Effect of regional cortical ablations on high-affinity D-aspartate uptake in striatum, olfactory tubercle, and pyriform cortex of the rat

The ablation of medial frontal cortex, but not of other separate cortical regions, was accompanied by a significant decrease in high affinity uptake of D-Asp in neostriatum, olfactory tubercle and pyriform cortex. The results indicate that the medial frontal cortex may be an important region for glutamergic/aspartergic cell bodies.

Levels and synthesis of glutamate and aspartate in the olfactory cortex following bulbectomy

Guinea pigs were unilaterally bulbectomised and the contents of aspartate, glutamate and GABA measured in slices of olfactory cortex taken from the lesioned and intact hemispheres. Two days after the operation there was a fall in the aspartate and glutamate levels, which persisted for over 120 days, whereas gamma-aminobutyric acid (GABA) showed a transient fall followed by a small rise. The fall in glutamate and aspartate was much greater in small, thin slices containing a high density of nerve terminals. The synthesis of 13C aminoacids from [13C]glucose during electrical stimulation was greater in the slices taken from the normal side than in those from the operated side. The GABA synthesis, however, was four times greater on the lesioned side. This time-course for the fall in acidic amino acids correlates with the fall in electrical responses, and this lends weight to the idea that aspartate and/or glutamate mediate synaptic transmission in the area.

Autoradiographic analysis of [3H]dopamine and [3H]dopa uptake in the turtle olfactory bulb

Uptake and retention of exogenous tritiated dopamine and L-dopa was observed within turtle olfactory bulb slices. In the more superficial layers, periglomerular and superficial tufted cells, as well as their processes, and intraglomerular dendrites were recognized as labeled. Within the deeper part of the bulb, some labeled cells between the tanycytes, as well as nerve fibers and terminals within the granule cell layer, are reported. The results confirm the presence of specific intrinsic dopaminergic cells within the reptilian olfactory bulb.

Biochemical and electrophysiological demonstrations of the actions of beta-bungarotoxin on synapses in brain

Homogeneous beta-bungarotoxin interacts irreversibly with rat olfactory cortex and produced permanent inhibition of neurotransmission (half-time of blockade for 230 nM toxin in 25 min). Binding occurs in the absence of divalent cations, but the rate of synaptic blockade is increased by Ca2+, which activates the intrinsic phospholipase A2 activity of the toxin. Other observable actions of the toxin, seen with rat cerebrocortical synaptosomes, are an increase in the release of acetylcholine, glutamate and gamma-aminobutyrate and impairment of transmitter uptake, which are all insensitive to tetrodotoxin. Inactivation of the toxin's phospholipase activity by chemical modification with p-bromophenacyl bromide diminishes the observed concomitant efflux of the neurotransmitters and lactate dehydrogenase. Collectively, the results support the idea that the toxin binds specifically and irreversibly to component(s) on nerve terminals and this together with the resultant phospholipolysis leads eventually to synaptic blockade. Such a proposal would account for the unique toxicity of the protein relative to phospholipase A2 enzymes.

Carnosine release from olfactory bulb synaptosomes is calcium-dependent and depolarization-stimulated

The dipeptide carnosine (beta-alanyl-L-histidine) has been proposed as a neurotransmitter in the mammalian olfactory pathway. Therefore, the efflux of in vivo-synthesized [14C]carnosine from mouse olfactory bulb synaptosomes was investigated. Carnosine was found to be released from the olfactory bulb synaptosomes by two mechanisms. The first is a slow spontaneous process that is independent of depolarization. The rate of this release was doubled in the presence of 1 mM external carnosine. Release by the second mechanism was markedly stimulated in the presence of calcium by depolarization with either 60 mM K+ or 300 microM veratridine. Omission of calcium abolished the stimulatory effect of both of these agents. Further, blockage of the veratridine-induced depolarization by tetrodotoxin also inhibited carnosine release. These results are consistent with the hypothesis that carnosine acts as a neurotransmitter in the mouse olfactory pathway.

Electrophysiological analysis of the presynaptic action of beta-bungarotoxin in the central nervous system

In view of previously reported actions of beta-bungarotoxin (beta-BuTX) on central brain synaptosomes, the effects on this toxin on the electrical activity of two brain slice preparations have been examined in vitro. beta-BuTX initially suppresses the synaptic component of the field responses to electrical stimulation in olfactory cortex and hippocampal slices. Intracellular recordings demonstrate that this synaptic depression occurs without detectable reduction in the sensitivity of the postsynaptic neuron to putative neurotransmitters. Following longer exposure to the toxin, reduced neuronal excitability is observed both pre- and post-synaptically. Elimination of the phospholipase A2 activity of beta-BuTX, by chemical modification or removing the Ca2+ necessary for enzymic activity, greatly reduces but does not totally eradicate the toxin's ability to block neurotransmission in the olfactory cortex. In the absence of enzymic activity beta-BuTX has no obvious effect on axonal conduction. Pure phospholipases A2, such as that from Naja melanoleuca mimic the transmission-blocking action of beta-BuTX, but with lower potency and without the effects on fibre excitability. Collectively, these results are taken as evidence that beta-BuTX initially suppresses transmitter release, a notion supported by the observed loss of spontaneous synaptic activity in hippocampal cells. Prolonged exposure to the toxin induces apparently less specific effects on neuronal excitability which are dependent on phospholipase A2 activity and are discussed with reference to the selective action of beta-BuTX on hippocampal fibre systems which possess release sites.

Aspartate and not glutamate is the likely transmitter of the rat lateral olfactory tract fibres

The protoveratrine A-evoked release of endogenous amino acid neurotransmitter candidates from olfactory cortex slices taken from control and bulbectomized rats has been monitored. Bulbectomy is accompanied by a statistically significant and specific attenuation of drug-evoked aspartate release suggesting that the excitatory transmitter of the lateral olfactory tract fibres is aspartate rather than glutamate.

Lateral olfactory tract transmitter: glutamate, aspartate, or neither?

Aspartate and glutamate are the principal candidates for the excitatory neurotransmitter released by the lateral olfactory tract (LOT) in prepyriform cortex of the rat. Identity of action of the natural transmitter with exogenous glutamate and/or aspartate, however, has not yet been demonstrated. We show that bath-applied 2-amino-4-phosphonobutyric acid, a presumed specific glutamate antagonist, blocks LOT-stimulated prepyriform field potentials and single unit activity but not the single unit response to ionophoretically applied glutamate or aspartate in rat olfactory cortex slices. These results suggest that neither aspartate nor glutamate is the LOT transmitter. Responses to ionophoretically applied N-methyl-DL-aspartate, kainic acid, and DL-homocysteate were clearly decreased by 2-amino-4-phosphonobutyric acid. This suggests that these agents, usually presumed to be aspartate or glutamate agonists, act at different receptors than aspartate and glutamate.

Patterns of endogenous amino acid release from slices of rat and guinea-pig olfactory cortex

A study has been made of the effects of depolarizing stimuli on the release of endogenous amino acid neurotransmitter candidates (aspartate, glutamate, GABA and taurine) from in vitro preparations of rat and guinea pig olfactory cortex. Exposure of small cubes of olfactory cortex tissue from either species to potassium chloride (50 mM) was accompanied by a calcium-dependent release of aspartate, glutamate and GABA. A similar release pattern was evoked by protoveratrine A (100 muM) although the release was largely calcium-independent. Neither agent led to increased release of taurine. Electrical stimulation of the excitatory input (lateral olfactory tract) of freshly prepared, synaptically intact olfactory cortex slices of both species induced significant release of aspartate and GABA from the uncut pial surface and of aspartate, GABA and glutamate from the cut surface. Evoked taurine release occurred from both surfaces of rat olfactory cortex slices but no release was detected from guinea pig olfactory cortex slices. These patterns of release were unaffected by changes in stimulus frequency and were mimicked by protoveratrine A (100 muM) applied to one or other surface. Preincubation of slices from rats for 2 led to loss of tissue amino acids and to changes in their release patterns; the presence of glutamine (5 mM) during preincubation prevented the loss of amino acids but did not alter their pattern of release. Because of the close similarities between both the electrophysiological properties and the patterns of amino acid release it is concluded that there is probably an identity of amino acid neurotransmitters (aspartate, glutamate and GABA) in rat and guinea pig olfactory cortex. The role of taurine in the rat olfactory cortex is unknown but would seem unlikely to be that of a neurotransmitter. The results are discussed: (i) in terms of the cellular origins of the released amino acids; and (ii) wit respect to apparent experimental discrepancies which have appeared in the literature.

Influence of unilateral olfactory bulbectomy on opiate and other binding sites in the contralateral bulb

The neurochemical consequences of unilateral olfactory bulbectomy (UBX) in mice were determined in the remaining olfactory bulb at various times after surgery. The most significant finding was a progressive decline in opiate ligand (dihydromorphine) and naloxone) binding that appeared within 11 days after surgery and persisted throughout the study. Statistically significant declines in spiroperidol (-67%), clonidine (-48%) and muscimol (-16%) binding were also observed 90 days after surgery. At 180 days postsurgery we observed a 20% increase in diazepam binding. No effect of UBX on dihydroalprenolol, quinucludinylbenzilate or serotonin ligand binding was observed. Bulbectomy resulted in a moderate decrease (-28%) in DOPA decarboxylase activity 14 days after surgery, which returned to normal by 30 days. Glutamic acid decarboxylase activity decreased by 37% 7 days after UBX, returned to normal by 14 days after surgery and then increased by 25% 90 days after UBX. Unilateral bulbectomy had no effect on cholineacetyltransferase activity in the remaining bulb. Thus, following a unilateral procedure, one bulb cannot necessarily serve as a valid control for the other. Possible explanations for the neurochemical changes observed are discussed.

Opiate and muscarinic ligand binding in five limbic areas after bilateral olfactory bulbectomy

Bilateral olfactory bulbectomy (BBX) in mice leads to a variety of neutrochemical changes in 5 limbic areas associated with the bulbs. Within one week after BBX, opiate ligand binding declined by 73% in the amygdala, rose by 82% in the hypothalamus and then returned to sham levels by 4 weeks in both areas. Opiate binding also declined by 47% in the olfactory tubercle and to essentially zero in the piriform cortex and the olfactory peduncle after 16 weeks. Muscarinic cholinergic binding declined in the amygdala and hypothalamus at 16 weeks after BBX, but reductions in muscarinic binding were observed in the piriform cortex and the olfactory peduncle by two weeks postsurgery. Muscarinic binding in the olfactory tubercle was unaffected by BBX, as was binding of beta-adrenergic and benzodiazepine ligands in the limbic areas. Binding of [3H]spiroperidol rose 61% in the olfactory tubercle two weeks after surgery and then declined to normal levels. Choline acetyltransferase activity rose by 64% within one week after BBX in the piriform cortex and remained elevated throughout the study. Activity of this enzyme also rose in the olfactory peduncle and the olfactory tubercle after surgery. BBX had only moderate effects on glutamic acid decarboxylase in the limbic areas, and enzyme activity increased 25% in the olfactory tubercle and the piriform cortex 4 weeks after BBX. BBX also resulted in a moderate decrease (22%) in DOPA decarboxylase activity in the olfactory tubercle two weeks after BBX. The implications of these neurochemical changes are discussed in terms of what is known about bulb-limbic system connections.

Time-course of declining electrical activity in guinea-pig olfactory cortex after olfactory bulb removal

An electrophysiological study has been made of the degeneration of the afferent axons of the lateral olfactory tract (L.O.T.) which give rise to excitatory synapses throughout the olfactory cortex. These fibres were severed by removing one or both olfactory bulbs. Evoked potentials were recorded from slices of olfactory cortex in vitro prepared at various times after bulbectomy. Up to 1.6 days later, all potentials were similar to those of the unoperated side. Between 1.6 and 1.8 days there was a complete loss of evoked potential from both L.O.T. axons and postsynaptic neurones, which occurred synchronously at all points along a 5-6 mm length of the tract.

Localization of acetylcholinesterase in the main and accessory olfactory bulbs of the mouse by light and electron microscopic histochemistry

Experiments were conducted to examine by light and electron microscopy the localization of acetylcholinesterase (AChE) in the main (MOB) and accessory (AOB) olfactory bulbs of the normal mouse. Evidence from the literature for cholinergic innervation of the mammalian olfactory bulb was then assessed in light of possible correlation between reported sites of termination of centrifugal fibers to the olfactory bulb and the localization of AChE. AChE-positive nerve fibers were concentrated in the periglomerular region and internal plexiform layer of the MOB. Stained fibers were also present in the granule cell, mitral cell, and external plexiform layers as well as within glomeruli. A few neurons in all layers of the MOB contained AChE reaction product. Unlike the MOB, AChE-positive fibers were not present in the glomerular layer of the AOB. AChE-positive fibers were concentrated in the inner plexiform layer, whereas fewer stained fibers were observed in the external plexiform and mitral cell layer and granule cell layer. Lightly stained neurons were found in the deeper portions of the external plexiform and mitral cell layer and granule cell layer. Ultrastructurally, AChE reaction product in the MOB and AOB was predominantly associated with small unmyelinated axons. Reaction product was also observed adjacent to axon terminals and dendrites. Occasionally within the MOB, AChE activity was found within periglomerular, tufted, short-axon, mitral, and granule cells. In the AOB, however, intracellular AChE activity was observed within some mitral/tufted cells and only a few granule cells. In conclusion, the AChE reaction product was mainly associated with axons in regions of the MOB where centrifugal fibers have been reported. Accessory olfactory bulb AChE localization was different from that of the MOB, suggesting a different pattern of cholinergic input to the AOB. The small amounts and sites of intraneuronal AChE reaction product in cells of the olfactory bulb indicate cholinoceptive rather than cholinergic function.

Release of endogenous amino acid neurotransmitter candidates from rat olfactory cortex slices: possible regulatory mechanisms and the effects of pentobarbitone

A study has been made of the regulation of evoked release of the amino acid neurotransmitter candidates (aspartate, GABA and taurine) from rat olfactory cortex slices. The effects of pentobarbitone (10-1000 microM) on release have also been assessed. Release of aspartate, the presumed excitatory transmitter of some of the lateral olfactory tract fibres, is reduced by muscimol (10 microM) and this effect is antagonized by picrotoxin (15 microM): it is concluded that presynaptic GABA receptors may modulate aspartate release. Low concentrations of pentobarbitone also reduce aspartate release, but this effect is picrotoxin-insensitive. Release of GABA, the presumed transmitter of inhibitory interneurones, is reduced by muscimol (10 microM) and this effect is antagonized by picrotoxin (15 microM): it is suggested that GABA release may be regulated by presynaptic autoreceptors. Pentobarbitone significantly increases release of GABA when slices are synaptically activated although the mechanism of this effect is unclear. Release of taurine, not hitherto considered a neurotransmitter in this brain area, is depressed by muscimol (10 microM) and pentobarbitone and increased by picrotoxin (15 microM). Results are discussed in terms of (i) mechanisms of regulation of amino acid release in the olfactory cortex, (ii) effects of pentobarbitone on release and (iii) the compatibility of the present results with previously published electrophysiological studies.

Neurotoxicity of kainic acid: evidence against an interaction with excitatory glutamate receptors in rat olfactory bulbs

The hypothetical interaction of kainic acid (KA) at excitatory glutamate (GLU) receptors was tested by injecting this neurotoxin into the olfactory bulb (OB), since the putatively GLUergic output neurons of the OB are among the few neurons in the CNS that are not excited by GLU. However, the mitral (and tufted) cells are found to be highly and preferentially destroyed by KA, whereas the GLUreceptive interneurons of the OB appear to be relatively resistant to the neurotoxin. These findings suggest that the toxic effect of KA on OB neurons may be mediated by mechanisms other than by action on GLUreceptors.