Developmental regulation of metabotropic glutamate receptor 1 splice variants in olfactory bulb mitral cells

Alternative splicing of the metabotropic glutamate receptor 1 (mGluR1) receptor gene generates two major receptor isoforms, mGluR1a and mGluR1b, differing in intracellular function and distribution. However, little is known on the expression profiles of these variants during development. We examined the mRNA expression profile of mGluR1a/b in microdissected layers and acutely isolated mitral cells in the developing mouse olfactory bulb. This analysis showed that the two mGluR1 variants are differentially regulated within each bulb layer. During the first postnatal week, the mGluR1a isoform replaces GluR1b in the microdissected mitral cell layer (MCL) and in isolated identified mitral cells, coinciding with a developmental epoch of mitral cell dendritic reorganization. Although mGluR1a mRNA is expressed at high levels in both the adult external plexiform layer (EPL) and MCL, Western blotting analysis reveals a marked reduction of the mGluR1a protein in the MCL, where mitral cell bodies are located, and strong labeling in the EPL, which contains mitral cell dendrites. This suggests that there is increased dendritic trafficking efficiency of the receptor in adult. The temporal and spatial shift in mGluR1b/a expression suggests distinct roles of the mGluR1 isoforms, with mGluR1b potentially involved in the early mitral cell maturation and mGluR1a in dendritic and synapse function.

Two GABAergic intraglomerular circuits differentially regulate tonic and phasic presynaptic inhibition of olfactory nerve terminals

Olfactory nerve axons terminate in olfactory bulb glomeruli forming excitatory synapses onto the dendrites of mitral/tufted (M/T) and juxtaglomerular cells, including external tufted (ET) and periglomerular (PG) cells. PG cells are heterogeneous in neurochemical expression and synaptic organization. We used a line of mice expressing green fluorescent protein under the control of the glutamic acid decarboxylase 65-kDa gene (GAD65+) promoter to characterize a neurochemically identified subpopulation of PG cells by whole cell recording and subsequent morphological reconstruction. GAD65+ GABAergic PG cells form two functionally distinct populations: 33% are driven by monosynaptic olfactory nerve (ON) input (ON-driven PG cells), the remaining 67% receive their strongest drive from an ON-->ET-->PG circuit with no or weak monosynaptic ON input (ET-driven PG cells). In response to ON stimulation, ON-driven PG cells exhibit paired-pulse depression (PPD), which is partially reversed by GABA(B) receptor antagonists. The ON-->ET-->PG circuit exhibits phasic GABA(B)-R-independent PPD. ON input to both circuits is under tonic GABA(B)-R-dependent inhibition. We hypothesize that this tonic GABA(B)R-dependent presynaptic inhibition of olfactory nerve terminals is due to autonomous bursting of ET cells in the ON-->ET-->PG circuit, which drives tonic spontaneous GABA release from ET-driven PG cells. Both circuits likely produce tonic and phasic postsynaptic inhibition of other intraglomerular targets. Thus olfactory bulb glomeruli contain at least two functionally distinct GABAergic circuits that may play different roles in olfactory coding.

Quantitative analysis of neuronal diversity in the mouse olfactory bulb

Olfactory sensory information is processed and integrated by circuits within the olfactory bulb. Golgi morphology suggests the olfactory bulb contains several major neuronal classes. However, an increasingly diverse collection of neurochemical markers have been localized in subpopulations of olfactory bulb neurons. While the mouse is becoming the animal model of choice for olfactory research, little is known about the proportions of neurons expressing and coexpressing different neurochemical markers in this species. Here we characterize neuronal populations in the mouse main olfactory bulb, focusing on glomerular populations. Immunofluorescent labeling for: 1) calretinin, 2) calbindin D-28K (CB), 3) parvalbumin, 4) neurocalcin, 5) tyrosine hydroxylase (TH), 6) the 67-kDa isoform of GAD (GAD67), and 7) the neuronal marker NeuN was performed in mice expressing green fluorescent protein under the control of the glutamic acid decarboxylase 65kDa (GAD65) promoter. Using unbiased stereological cell counts we estimated the total numbers of cells and neurons in the bulb and the number and percentage of neurons expressing and coexpressing different neurochemical populations in each layer of the olfactory bulb. Use of a genetic label for GAD65 and immunohistochemistry for GAD67 identified a much larger percentage of GABAergic neurons in the glomerular layer (55% of all neurons) than previously recognized. Additionally, while many glomerular neurons expressing TH or CB coexpress GAD, the majority of these neurons preferentially express the GAD67 isoform. These data suggest that the chemospecific populations of neurons in glomeruli form distinct subpopulations and that GAD isoforms are preferentially regulated in different neurochemical cell types.

Spontaneous field potentials in the glomeruli of the olfactory bulb: the leading role of juxtaglomerular cells

Field potentials recorded in the olfactory bulb glomerular layer (GL) are thought to result mainly from activation of mitral and tufted cells. The contribution of juxtaglomerular cells (JG) is unknown. We tested the hypothesis that JG are the main driving force to novel spontaneous glomerular layer field potentials (sGLFPs), which were recorded in rat olfactory bulb slices maintained in an interface chamber. We found that sGLFPs have comparable magnitudes, durations and frequencies both in standard horizontal slices, where all layers with all cell types were present, and in isolated GL slices, where only JG cells were preserved. Hence, the impact of mitral and deep/medium tufted cells to sGLFPs turned out to be minor. Therefore, we propose that the main generators of sGLFPs are JG neurons. We further explored the mechanism of generation of sGLFPs using a neuronal ensemble model comprising all types of cells associated with a single glomerulus. Random orientation and homogenous distribution of dendrites in the glomerular neuropil along with surrounding shell of cell bodies of JG neurons resulted in substantial spatial restriction of the generated field potential. The model predicts that less than 20% of sGLFP can spread from one glomerulus to an adjacent one. The contribution of JG cells to the total field in the center of the glomerulus is estimated as approximately 50% ( approximately 34% periglomerular and approximately 16% external tufted cells), whereas deep/medium tufted cells provide approximately 39% and mitral cells only approximately 10%. Occasionally, some sGLFPs recorded in adjacent or remote glomeruli were cross-correlated, suggesting involvement of interglomerular communication in information coding. These results demonstrate a leading role of JG cells in activation of the main olfactory bulb (MOB) functional modules. Finally, we hypothesize that the GL is not a set of independent modules, but it represents a subsystem in the MOB network, which can perform initial processing of odors.

Centre-surround inhibition among olfactory bulb glomeruli

Centre-surround inhibition--the suppression of activity of neighbouring cells by a central group of neurons--is a fundamental mechanism that increases contrast in patterned sensory processing. The initial stage of neural processing in olfaction occurs in olfactory bulb glomeruli, but evidence for functional interactions between glomeruli is fragmentary. Here we show that the so-called 'short axon' cells, contrary to their name, send interglomerular axons over long distances to form excitatory synapses with inhibitory periglomerular neurons up to 20-30 glomeruli away. Interglomerular excitation of these periglomerular cells potently inhibits mitral cells and forms an on-centre, off-surround circuit. This interglomerular centre-surround inhibitory network, along with the well-established mitral-granule-mitral inhibitory circuit, forms a serial, two-stage inhibitory circuit that could enhance spatiotemporal responses to odours.

Dopamine D2 receptor-mediated presynaptic inhibition of olfactory nerve terminals

Olfactory receptor neurons of the nasal epithelium project via the olfactory nerve (ON) to the glomeruli of the main olfactory bulb, where they form glutamatergic synapses with the apical dendrites of mitral and tufted cells, the output cells of the olfactory bulb, and with juxtaglomerular interneurons. The glomerular layer contains one of the largest population of dopamine (DA) neurons in the brain, and DA in the olfactory bulb is found exclusively in juxtaglomerular neurons. D2 receptors, the predominant DA receptor subtype in the olfactory bulb, are found in the ON and glomerular layers, and are present on ON terminals. In the present study, field potential and single-unit recordings, as well as whole cell patch-clamp techniques, were used to investigate the role of DA and D2 receptors in glomerular synaptic processing in rat and mouse olfactory bulb slices. DA and D2 receptor agonists reduced ON-evoked synaptic responses in mitral/tufted and juxtaglomerular cells. Spontaneous and ON-evoked spiking of mitral cells was also reduced by DA and D2 agonists, and enhanced by D2 antagonists. DA did not produce measurable postsynaptic changes in juxtaglomerular cells, nor did it alter their responses to mitral/tufted cell inputs. DA also reduced 1) paired-pulse depression of ON-evoked synaptic responses in mitral/tufted and juxtaglomerular cells and 2) the amplitude and frequency of spontaneous, but not miniature, excitatory postsynaptic currents in juxtaglomerular cells. Taken together, these findings are consistent with the hypothesis that activation of D2 receptors presynaptically inhibits ON terminals. DA and D2 agonists had no effect in D2 receptor knockout mice, suggesting that D2 receptors are the only type of DA receptors that affect signal transmission from the ON to the rodent olfactory bulb.

Direct excitation of mitral cells via activation of alpha1-noradrenergic receptors in rat olfactory bulb slices

The main olfactory bulb receives a significant modulatory noradrenergic input from the locus coeruleus. Previous in vivo and in vitro studies showed that norepinephrine (NE) inputs increase the sensitivity of mitral cells to weak olfactory inputs. The cellular basis for this action of NE is not understood. The goal of this study was to investigate the effect of NE and noradrenergic agonists on the excitability of mitral cells, the main output cells of the olfactory bulb, using whole cell patch-clamp recording in vitro. The noradrenergic agonists, phenylephrine (PE, 10 microM), isoproterenol (Isop, 10 microM), and clonidine (3 microM), were used to test for the functional presence of alpha1-, beta-, and alpha2-receptors, respectively, on mitral cells. None of these agonists affected olfactory nerve (ON)-evoked field potentials recorded in the glomerular layer, or ON-evoked postsynaptic currents recorded in mitral cells. In whole cell voltage-clamp recordings, NE (30 microM) induced an inward current (54 +/- 7 pA, n = 16) with an EC(50) of 4.7 microM. Both PE and Isop also produced inward currents (22 +/- 4 pA, n = 19, and 29 +/- 9 pA, n = 8, respectively), while clonidine produced no effect (n = 6). In the presence of TTX (1 microM), and blockers of excitatory and inhibitory fast synaptic transmission [gabazine 5 microM, 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) 10 microM, and (+/-)-2-amino-5-phosphonopentanoic acid (APV) 50 microM], the inward current induced by PE persisted (EC(50) = 9 microM), whereas that of Isop was absent. The effect of PE was also observed in the presence of the Ca(2+) channel blockers, cadmium (100 microM) and nickel (100 microM). The inward current caused by PE was blocked when the interior of the cell was perfused with the nonhydrolyzable GDP analogue, GDPbetaS, indicating that the alpha1 effect is mediated by G-protein coupling. The current-voltage relationship in the absence and presence of PE indicated that the current induced by PE decreased near the equilibrium potential for potassium ions. In current-clamp recordings from bistable mitral cells, PE shifted the membrane potential from the downstate (-52 mV) toward the upstate (-40 mV), and significantly increased spike generation in response to perithreshold ON input. These findings indicate that NE excites mitral cells directly via alpha1 receptors, an effect that may underlie, at least in part, increased mitral cell responses to weak ON input during locus coeruleus activation in vivo.

Membrane bistability in olfactory bulb mitral cells

Whole-cell patch-clamp recordings were used to investigate the electrophysiological properties of mitral cells in rat main olfactory bulb brain slice preparations. The majority of mitral cells are bistable. These cells spontaneously alternate between two membrane potentials, separated by approximately 10 mV: a relatively depolarized potential (upstate), which is perithreshold for spike generation, and a relatively hyperpolarized potential (downstate), in which spikes do not occur. Bistability occurs spontaneously in the absence of ionotropic excitatory or inhibitory synaptic inputs. Bistability is voltage dependent; transition from the downstate to the upstate is a regenerative event activated by brief depolarization. A brief hyperpolarization can switch the membrane potential from the upstate to the downstate. In response to olfactory nerve (ON) stimulation, mitral cells in the upstate are more likely to fire an action potential than are those in the downstate. ON stimulation can switch the membrane potential from the downstate to the upstate, producing a prolonged and amplified depolarization in response to a brief synaptic input. We conclude that bistability is an intrinsic property of mitral cells that is a major determinant of their responses to ON input.

Radial glia development in the mouse olfactory bulb

Radial glia are critical for cell migration and lamination of the cortex. In most developing cortical structures, radial glia, as their name suggests, extend processes from the ventricle to the pia in regular parallel arrangements. However, immunohistochemical labeling from several laboratories suggests that radial glia have a more branched morphology in the olfactory bulb. To investigate the morphology of radial glia in the mouse olfactory bulb we (1) labeled radial glia and olfactory receptor neuron axons at 24-hour intervals by immunohistochemistry; and (2) developed a novel method of generating and applying "nanocrystals" of 1,1'-dioctadecyl-3,3,3',3'- tetramethylindocarbocyanine perchlorate (DiI) to the ventricle surface such that the processes of single olfactory bulb radial glia are labeled in the embryonic olfactory bulb. We examined the structure and interactions of radial glia with ingrowing olfactory receptor neuron (ORN) axons in late embryonic olfactory bulb development. These results showed that olfactory bulb radial glia do not form straight parallel structures as do radial glia in the neocortex but rather have a convoluted trajectory from the ventricle to the bulb surface. Moreover, olfactory bulb radial glia consistently extend tangential branches at the level of the internal plexiform layer. Beginning at embryonic day 17.5, two types of radial glia can be distinguished: type I radial glia have a process that extends from the ventricle into the glomerular layer. These apical processes form highly restricted tufts, or "glial glomeruli" at the same time that ORN axons are forming "axonal glomeruli." In type II radial glia the apical process does not enter the glomerular layer but instead ramifies within the external plexiform layer. The tight spatiotemporal relationship between the glomerulization of radial glia processes and ORN axons during development suggest that radial glia processes could play a role in the formation and/or stabilization of mammalian glomeruli.

Tonic and synaptically evoked presynaptic inhibition of sensory input to the rat olfactory bulb via GABA(B) heteroreceptors

Olfactory receptor neurons of the nasal epithelium send their axons, via the olfactory nerve (ON), to the glomeruli of the olfactory bulb (OB), where the axon terminals form glutamatergic synapses with the apical dendrites of mitral and tufted cells, the output cells of the OB, and with juxtaglomerular (JG) interneurons. Many JG cells are GABAergic. Here we show that, despite the absence of conventional synapses, GABA released from JG cells activates GABA(B) receptors on ON terminals and inhibits glutamate release both tonically and in response to ON stimulation. Field potential recordings and current-source density analysis, as well as intracellular and whole cell recording techniques were used in rat OB slices. Baclofen (2-5 microM), a GABA(B) agonist, completely suppressed ON-evoked synaptic responses of both mitral/tufted cells and JG cells, with no evidence for postsynaptic effects. Baclofen (0.5-1 microM) also reversed paired-pulse depression (PPD) of mitral/tufted cell responses to paired-pulse facilitation (PPF), and reduced depression of JG cell excitatory postsynaptic currents (EPSCs) during repetitive ON stimulation. These results suggest that baclofen reduced the probability of glutamate release from ON terminals. The GABA(B) antagonists CGP35348 or CGP55845A increased mitral/tufted cell responses evoked by single-pulse ON stimulation, suggesting that glutamate release from ON terminals is tonically suppressed via GABA(B) receptors. The same antagonists reduced PPD of ON-evoked mitral/tufted cell responses at interstimulus intervals 50-400 ms. This finding suggests that a single ON impulse evokes sufficient GABA release, presumably from JG cells, to activate GABA(B) receptors on ON terminals. Thus GABA(B) heteroreceptors on ON terminals are activated by ambient levels of extrasynaptic GABA, and by ON input to the OB. The time course of ON-evoked, GABA(B) presynaptic inhibition suggests that neurotransmission to M/T cells and JG cells will be significantly suppressed when ON impulses arrive in glomeruli at 2.5-20 Hz. GABA(B) receptor-mediated presynaptic inhibition of sensory input to the OB may play an important role in shaping the activation pattern of the OB glomeruli during olfactory coding.

Expression of semaphorins in developing and regenerating olfactory epithelium

Semaphorins provide signals that guide growing axons to their appropriate destinations. The secreted semaphorin, Sema3A, mediates repulsive effects on axons from various neuronal populations in embryonic rats. The authors localized Sema3A mRNA expression in the primary olfactory pathway during development, in adult rats, and in adult rats that were subjected to a unilateral olfactory bulbectomy. Developing rats at ages from embryonic day 14 (E14) to E19 expressed Sema3A in the olfactory receptor neurons (ORNs) of the olfactory epithelium and in chondrogenic structures surrounding the nasal cavity. In vitro, ORN axons at E14 avoided substrate-bound Sema3A. Low levels of Sema3A expression persisted in the normal adult epithelium both in ORNs scattered throughout the epithelium and in small clusters. Three days after a unilateral olfactory bulbectomy, Sema3A transcript levels increased in regenerating neurons. High levels of Sema3A transcript were found at 1 week postbulbectomy, persisted for 2 weeks, and diminished by 3 weeks. Several other murine semaphorins (Sema4A, Sema4B, and Sema4C) were expressed differentially in the primary olfactory pathway both during development and regeneration. These findings suggest that Sema3A and perhaps other semaphorins play a role in directing ORNs out of the epithelium and to the olfactory bulb, their target structure, during both development and regeneration.

Ultrasensitive pheromone detection by mammalian vomeronasal neurons

The vomeronasal organ (VNO) is a chemoreceptive organ that is thought to transduce pheromones into electrical responses that regulate sexual, hormonal and reproductive function in mammals. The characteristics of pheromone signal detection by vomeronasal neurons remain unclear. Here we use a mouse VNO slice preparation to show that six putative pheromones evoke excitatory responses in single vomeronasal neurons, leading to action potential generation and elevated calcium entry. The detection threshold for some of these chemicals is remarkably low, near 10(-11) M, placing these neurons among the most sensitive chemodetectors in mammals. Using confocal calcium imaging, we map the epithelial representation of the pheromones to show that each of the ligands activates a unique, nonoverlapping subset of vomeronasal neurons located in apical zones of the epithelium. These neurons show highly selective tuning properties and their tuning curves do not broaden with increasing concentrations of ligand, unlike those of receptor neurons in the main olfactory epithelium. These findings provide a basis for understanding chemical signals that regulate mammalian communication and sexual behaviour.

Ultrastructural characteristics and conduction velocity of olfactory receptor neuron axons in the olfactory marker protein-null mouse

Olfactory receptor neuron (ORN) axon diameters and the conduction velocity of the compound action potential along ORN axons were studied in olfactory marker protein (OMP)-null mice and genotypically matched controls. The compound action potential was distinguished from postsynaptic field potentials by its shorter latency, its persistence following application of cobalt or kynurenic acid that blocked postsynaptic responses, and its ability to follow paired-pulse stimulation at 300 Hz. Blockade of the postsynaptic field responses by kynurenic acid indicates that in the mouse, as in the rat, glutamate is the olfactory nerve transmitter. The mean conduction velocity of ORNs in wild-type control mice was 0. 47+/-0.19 (S.E.M.) m/s (n=5), similar to the conduction velocity reported for other mammals. The mean diameter of ORN axons in control mice was 0.202+/-0.005 and 0.261+/-0.006 microm in the OMP-null mice. This increase in fiber diameter in the OMP-nulls predicts an increase in impulse conduction velocity. However, the mean conduction velocity of OMP-null mice, 0.38+/-0.03 m/s (n=6), was not significantly different from control (P>0.1). The conduction velocity predicted by the increase in fiber diameter in OMP-null mice was within the 95% confidence interval of the measured value. Thus, OMP-null ORNs are normal with respect to the conduction velocity of their axons. The number of axodendritic synapses in the glomeruli of OMP-null mice is higher than in congenic wild-type mice.

Expression of NR1, NR2A-D, and NR3 subunits of the NMDA receptor in the cerebral cortex and olfactory bulb of adult rat

Quantitative reverse transcriptase - polymerase chain reaction was used to analyze the relative expressions of NR1, NR2A, NR2B, NR2C, NR2D, and NR3 subunits of the NMDA receptor in the piriform, entorhinal, visual, and motor cortices as well as in the olfactory bulb of adult rat. The analysis detected clear differences in the relative proportions of the NMDA receptor subunits between the five forebrain regions examined. These differences were particularly striking when the piriform and motor cortices were compared. In the piriform cortex, NR1 was the predominant transcript. The expression of NR2A was only slightly higher than half of that of NR1. NR2B was expressed even at lower levels ( approximately 30% of NR1). NR2C and NR3 were expressed at levels which were approximately 15% of those of NR1. NR2D had the lowest levels of expression ( approximately 3% of NR1). In contrast, NR2B was the predominant transcript in the motor cortical region, where it was expressed at the levels close to 135% of those of NR1 message. NR2A had the levels of expression of approximately 50% of those of NR1. The NR2C expression was close to 25% that of NR1, and the NR2D and NR3 transcripts were totally absent from this cortical area. These findings suggest a significant regional variability of the NMDA receptors in the adult rat forebrain.

Long-lasting depolarizations in mitral cells of the rat olfactory bulb

We investigated the mechanisms of long-lasting depolarizing potentials (LLDs) generated in mitral cells with whole-cell patch recordings in the rat olfactory bulb slice. LLDs occur spontaneously and are evoked by either orthodromic stimulation of the olfactory nerve or antidromic stimulation of mitral and tufted (M/T) cells. LLDs are followed by a long refractory period, limiting LLD generation to approximately 1 Hz. LLD production does not appear to involve either intrinsic voltage-activated or metabotropic mechanisms. The initiation of LLDs requires activation of non-NMDA but not NMDA receptors. Dual recordings from the apical dendrites and somata of mitral cells show that LLDs are generated in the distal portion of the apical dendrite, most likely in the glomerulus. The rising phase of LLDs shows characteristics of polyneuronal input, including a high variability and sensitivity to charge screening. Paired recordings from adjacent mitral cells suggest that LLDs occur synchronously only in cells whose apical dendrites ramify in the same glomerulus. These findings suggest that LLDs involve recurrent, intraglomerular dendrodendritic interactions among M/T cells.

Development of the olfactory bulb: evidence for glia-neuron interactions in glomerular formation

Olfactory bulb (OB) glomeruli have long been considered functional units in the processing of odor information. Recently, it has been shown that axons from olfactory receptor neurons (ORNs) expressing the same odorant receptor gene converge onto two or a few topographically fixed glomeruli in the OB. The interactions between ORN axons, mitral/tufted cell dendrites, juxtaglomerular (JG) cells, and glial cells during the development of glomeruli is of great importance in light of this receptor gene glomerular topography in the primary olfactory projection. To explore the development of mammalian olfactory glomeruli, we investigated the relationships among radial glia (RG), astrocytes, ORNs, JG cells, mitral/tufted cell dendrites, and olfactory Schwann cells throughout embryonic and early postnatal development. Our results indicate that glomeruli are formed through an invariant sequence of cellular events: (1) pioneering ORN axons contact the rostral telencephalon at approximately E11-14, which coincides with the onset of morphologic changes in telencephalic RG; (2) at E15-16, RG branch and begin to form two plexuses, one located in the subventricular layer and the other superficial to the presumptive mitral cell layer; (3) at E17-18, ORN axons accumulate in a dense band superficial to the outer radial glia plexus; (4) at E19-20, processes from RG and astrocytes begin to ramify to form glial tufts, or glial glomeruli. Coincident with the formation of these glial glomeruli, ORN axons intermingle with the glial processes and form proto-glomeruli; (5) at E21 to P0, JG cells begin to migrate into position surrounding glomeruli, (6) and at P4, the apical tuft of mitral cells becomes restricted to a single glomerulus. Interestingly, glomerular development also occurs in a distinct rostral to caudal gradient. That is, glomeruli in the rostral OB develop earlier than those in the caudal OB, but the sequence of cellular events at any point in the bulb is invariant. These results demonstrate that glomeruli are formed in a specific spatiotemporal sequence beginning with ORN axon-glia contacts, then JG cell arrival, and finally mitral cell apical dendrite restriction.

Olfactory receptor neurons express D2 dopamine receptors

The role of the dopamine (DA) in the olfactory bulb (OB) was explored by determining which of the potential target cells express dopamine receptors (DARs). Previously, it was reported that D2-like DAR (D2, D3, and D4 subtypes) radioligand binding is restricted to the outer layers of the OB. The neuronal elements present only in these layers are the axons of the olfactory receptor neurons (ORNs) and the juxtaglomerular (JG) neurons of the glomerular layer. Based on this pattern of D2-like ligand binding, it was suggested that D2-like receptors might be located presynaptically on ORN terminals. The present study was undertaken to investigate this hypothesis. In the outer bulb layers of rats in which the ORNs were destroyed by nasal lavage with ZnSO(4), D2-like radioligand binding was reduced severely. The receptor subtype D2 mRNA, but not D3 mRNA, was detected in adult rat olfactory epithelial tissue. By using in situ hybridization, this D2 mRNA was located preferentially in epithelial layers that contain ORN perikarya. D2 mRNA was eliminated after bulbectomy, a manipulation known to cause retrograde degeneration of the mature ORNs. Taken together, the surgical manipulations indicate that mature ORNs express D2 DARs and are consistent with the hypothesis that functional receptors are translocated to their axons and terminals in the bulb. This suggests that dopamine released from JG interneurons could be capable of presynaptically influencing neurotransmission from the olfactory nerve terminals to OB target cells through the D2 receptor.

Evidence for cholinergic regulation of basal norepinephrine release in the rat olfactory bulb

The effects of locally infused cholinergic agonists on extracellular levels of norepinephrine in the olfactory bulb of anesthetized rats were determined using in vivo microdialysis coupled with high-performance liquid chromatography and electrochemical detection. Using chronically implanted microdialysis probes, the basal norepinephrine level in the olfactory bulb was 0.55 pg/10 microl dialysate. Local infusion of K+ (30 mM) or the norepinephrine re-uptake inhibitor desipramine (1 microM) through the dialysis probe significantly increased basal norepinephrine levels. Focal activation of noradrenergic locus coeruleus neurons, the sole source of norepinephrine innervation of the olfactory bulb, increased norepinephrine levels by 247% of control. Local infusion of the acetylcholinesterase inhibitor soman (0.4 mM) into the olfactory bulb increased basal norepinephrine levels by 134% of control, suggesting that endogenously released acetylcholine modulates norepinephrine release. Intrabulbar infusion of acetylcholine (40 mM) or nicotine (40 mM) increased norepinephrine levels (317% and 178% of control, respectively), while infusion of the muscarinic receptor agonist pilocarpine (40 mM) reduced norepinephrine levels (54% of control). These results demonstrate that basal norepinephrine release in the olfactory bulb is potently modulated by stimulation of local cholinergic receptors. Nicotinic receptors stimulate, and muscarinic receptors inhibit, norepinephrine release from locus coeruleus terminals.

Diagonal band stimulation increases piriform cortex neuronal excitability in vivo

The effects of diagonal band (NDB) stimulation on the spontaneous discharge of pyramidal cells and evoked field potentials (FPs) in piriform cortex (PC) were investigated in vivo. NDB stimulation increased the spontaneous firing rate of PC cells, and increased the disynaptic excitatory (B1) and decreased the disynaptic inhibitory (P2) FP components following lateral olfactory tract (LOT) stimulation. NDB stimulation decreased the P2 component following activation of association fibers in caudal PC. NDB stimulation reduced the paired-pulse inhibition of the P2 component following LOT and caudal PC shocks. The effects of NDB stimulation were reversed by scopolamine, suggesting the involvement of muscarinic receptors. These results suggest that activation of cholinergic inputs to PC increases the excitability of pyramidal cells, probably by a disinhibitory mechanism.

The organization of preoptic-medullary circuits in the male rat: evidence for interconnectivity of neural structures involved in reproductive behavior, antinociception and cardiovascular regulation

The present studies used anatomical tract-tracing techniques to delineate the organization of pathways linking the medial preoptic area and the ventral medulla, two key regions involved in neuroendocrine, autonomic and sensory regulation. Wheatgerm agglutinin-horseradish peroxidase injections into the ventromedial medulla retrogradely labeled a large number of neurons in the medial preoptic area, including both the median and medial preoptic nuclei. The termination pattern of preoptic projections to the medulla was mapped using the anterograde tracers Phaseolus vulgaris leucoagglutinin and biotinylated dextran amine. Tracer injections into the preoptic area produced a dense plexus of labeled fibers and terminals in the ventromedial and ventrolateral pons and medulla. Within the caudal pons/rostral medulla, medial preoptic projections terminated heavily in the nucleus raphe magnus; strong anterograde labeling was also present in the pontine reticular field. At mid-medullary levels, labeled fibers focally targeted the nucleus paragigantocellularis, in addition to the heavy fiber labeling present in the midline raphe nuclei. By contrast, very little labeling was observed in the caudal third of the medulla. Experiments were also conducted to map the distribution of ventral pontine and medullary neurons that project to the medial preoptic area. Wheatgerm agglutinin-horseradish peroxidase injections in the preoptic area retrogradely labeled a significant population of neurons in the ventromedial and ventrolateral medulla. Ascending projections from the medulla to the preoptic area were organized along rostral-caudal, medial-lateral gradients. In the caudal pons/rostral medulla, retrogradely labeled cells were aggregated along the midline raphe nuclei; no retrograde labeling was present laterally at this level. By contrast, in the caudal half of the medulla, cells retrogradely labeled from the medial preoptic area were concentrated as a discrete zone dorsal to the lateral reticular nucleus; labeled cells were not present in the ventromedial medulla at this level. The present findings suggest that the medial preoptic area and ventral midline raphe nuclei share reciprocal connections that are organized in a highly symmetrical fashion. By contrast, preoptic-lateral medullary pathways are not reciprocal. These preoptic-brainstem circuits may participate in antinociceptive, autonomic and reproductive behaviors.

Dendrodendritic recurrent excitation in mitral cells of the rat olfactory bulb

Most neuronal interactions within the olfactory bulb network are mediated by dendrodendritic synapses. Dendritic transmitter release potentially could affect the parent dendrite as well as local neuronal elements that have receptors for the released transmitter. Here we report that under conditions that facilitate N-methyl-D-aspartate (NMDA) receptor activity (reduced GABAA inhibition and extracellular Mg2+), a single action potential evoked by brief intracellular current pulses in mitral cells is followed by a prolonged depolarization, which is blocked by an NMDA receptor antagonist. This depolarization also is evoked by a presumed calcium spike in the presence of tetrodotoxin. A similar NMDA-receptor-dependent prolonged depolarization is elicited by stimulation of the lateral olfactory tract at current intensities subthreshold for antidromic activation of the recorded neuron. These observations suggest that glutamate released from the dendrites of mitral cells excites the same and neighboring mitral cell dendrites. Further evidence suggests that both the apical and lateral dendrites of mitral cells participate in this recurrent excitation. These dendrodendritic interactions may play a role in the prolonged, NMDA-receptor-dependent depolarization of mitral/tufted cells evoked by olfactory nerve stimulation.

Phenylmethylsulfonyl fluoride inhibitory effects on acetylcholinesterase of brain and muscle

Differential inhibition of brain versus peripheral acetylcholinesterase (AChE) by phenylmethylsulfonyl fluoride (PMSF) suggested that PMSF might preferentially inhibit different AChE molecular forms. AChE inhibition was examined after systemic and in vitro PMSF treatment. Systemic administration resulted in no overt behavioral changes but produced a 71% reduction in brain AChE; hemidiaphragm, extensor digitorum longus and soleus muscles showed 65, 50 and 41% reductions. Muscle asymmetric AChE was reduced to the greatest extent (50-80%). The tetrameric form was inhibited in brain and hemidiaphragm (60-76%) but spared in other muscles (18-22%). Monomeric AChE was spared in all tissues. When PMSF was added to a muscle homogenate all forms were inhibited equally. Purified monomer and tetramer forms were inhibited equally in vitro. These results suggest that PMSF inhibition of AChE is a consequence of a selective inhibition of membrane-associated forms and that the apparent brain selectivity is related to the greater fraction of membrane-associated AChE in brain.

Norepinephrine increases rat mitral cell excitatory responses to weak olfactory nerve input via alpha-1 receptors in vitro

A rat olfactory bulb in vitro slice preparation was used to investigate the actions of norepinephrine on spontaneous and afferent (olfactory nerve) evoked activity of mitral cells. Single olfactory nerve shocks elicited a characteristic mitral cell response consisting of distinct, early and late spiking components separated by a brief inhibitory epoch. Bath-applied norepinephrine (1 microM) increased the early spiking component elicited by perithreshold (79% increase, P<0.02), but not by suprathreshold (3% decrease, P>0.05), intensity olfactory nerve shocks. The facilitatory effect of norepinephrine was due to a reduction in the incidence of response failures to perithreshold intensity shocks. Norepinephrine also decreased the inhibitory epoch separating the early and late spiking components by 44% (P<0.05). By contrast, norepinephrine had no consistent effect on the spontaneous discharge rate of the mitral cells. The effects of norepinephrine were mimicked by the al receptor agonist phenylephrine (1 microM, P<0.001). Both norepinephrine and phenylephrine modulation of mitral cell responses were blocked by the al adrenergic antagonist WB-4101 (1 microM). These findings are consistent with observations that the main olfactory bulb exhibits the highest density of alpha1 receptors in the brain. The alpha2 receptor agonist clonidine (100 nM) and the beta receptor agonist isoproterenol (1 microM) had inconsistent effects on mitral cell spontaneous and olfactory nerve-evoked activity. These results indicate that norepinephrine increases mitral cell excitatory responses to weak but not strong olfactory nerve inputs in vitro via activation of al receptors. This is consistent with recent findings in vivo that synaptically released norepinephrine preferentially increases mitral cell excitatory responses to weak olfactory nerve inputs. Taken together, these results suggest that the release of norepinephrine in the olfactory bulb may increase the sensitivity of mitral cells to weak odors. Olfactory cues evoke norepinephrine release in the main olfactory bulb, and norepinephrine plays important roles in early olfactory learning and reproductive/maternal behaviors. By increasing mitral cell responses to olfactory nerve input, norepinephrine may play a critical role in modulating olfactory function, including formation and/or recall of specific olfactory memories.

Odor-induced, activity-dependent transneuronal gene induction in vitro: mediation by NMDA receptors

Expression of tyrosine hydroxylase (TH) by juxtaglomerular (JG) neurons of the olfactory bulb (OB) requires innervation of the bulb by olfactory receptor neurons (ORNs). ORN lesion selectively downregulates TH in JG neurons. In reversible odor deprivation, TH expression is downregulated as the naris is closed and then upregulated upon naris reopening. The mechanism or mechanisms regulating this dependence are unknown. TH expression could be regulated by trophic factor release and/or synaptic activity from ORN terminals. We investigated TH expression in cocultures of dissociated postnatal rat OB cells and embryonic olfactory neuroepithelium (OE) slice explants. TH-positive neurons in control dissociated OB cell cultures alone comprise only a small fraction of the total population of cells present in the culture. However, when OE slice explants are cocultured with dispersed OB cells, there is a mean 2.4-fold increase in the number of TH-positive neurons. ORNs in vivo use glutamate as a neurotransmitter. Broad spectrum excitatory amino acid antagonists (kyurenic acid) or selective antagonists of the NMDA receptor (APV) both prevent induction of TH expression in OE-OB cocultures. Furthermore, pulse application of NMDA stimulates TH expression in OB neurons in the absence of OE. In vitro, OB TH neurons express NMDA receptors, suggesting that NMDA stimulation is acting directly on TH neurons. Exposure of OE explants to natural odorants results in upregulation of TH, presumably through increased ORN activity, which could be blocked by APV. These findings indicate that odorant-stimulated glutamate release by ORN terminals regulates TH expression via NMDA receptors on JG dopaminergic neurons.

Current-source density analysis in the rat olfactory bulb: laminar distribution of kainate/AMPA- and NMDA-receptor-mediated currents

The one-dimensional current-source density method was used to analyze laminar field potential profiles evoked in rat olfactory bulb slices by stimulation in the olfactory nerve (ON) layer or mitral cell layer (MCL) and to identify the field potential generators and the characteristics of synaptic activity in this network. Single pulses to the ON evoked a prolonged (>/=400 ms) sink (S1ON) in the glomerular layer (GL) with corresponding sources in the external plexiform layer (EPL) and MCL and a relatively brief sink (S2ON) in the EPL, reversing in the internal plexiform and granule cell layers. These sink/source distributions suggested that S1ON and S2ON were generated in the apical dendrites of mitral/tufted cells and granule cells, respectively. The kainate/AMPA-receptor antagonist CNQX (10 microM) reduced the early phase of S1ON, blocked S2ON, and revealed a low amplitude, prolonged sink at the location of S2ON in the EPL. Reduction of Mg2+, in CNQX, enhanced both the CNQX-resistant component of S1ON and the EPL sink. This EPL sink reversed below the MCL, suggesting it was produced in granule cells. The NMDA-receptor antagonist APV (50 microM) reversibly blocked the CNQX-resistant field potentials in all layers. Single pulses were applied to the MCL to antidromically depolarize the dendrites of mitral/tufted cells. In addition to synaptic currents of granule cells, a low-amplitude, prolonged sink (S1mcl) was evoked in the GL. Corresponding sources were in the EPL, suggesting that S1mcl was generated in the glomerular dendritic tufts of mitral/tufted cells. Both S1mcl and the granule cell currents were nearly blocked by CNQX (10 microM) but enhanced by subsequent reduction of Mg2+; these currents were blocked by APV. S1mcl also was enhanced by gamma-aminobutyric acid-A-receptor antagonists applied to standard medium; this enhancement was reduced by APV. ON activation produces prolonged excitation in the apical dendrites of mitral/tufted cells, via kainate/AMPA and NMDA receptors, providing the opportunity for modulation and integration of sensory information at the first level of synaptic processing in the olfactory system. Granule cells respond to input from the lateral dendrites of mitral/tufted cells via both kainate/AMPA and NMDA receptors; however, in physiological concentrations of extracellular Mg2+, NMDA-receptor activation does not contribute significantly to the granule cell responses. The glomerular sink evoked by antidromic depolarization of mitral/tufted cell dendrites suggests that glutamate released from the apical dendrites of mitral/tufted cells may excite the same or neighboring mitral/tufted cell dendrites.

Identification of a long variant of mRNA encoding the NR3 subunit of the NMDA receptor: its regional distribution and developmental expression in the rat brain

A longer variant of rat mRNA encoding the NR3 subunit of the NMDA receptor has been identified. It contains a 60-bp insertion at the nucleotide position 3007 in the intracellular domain of the C-terminal of the previously cloned variant. Therefore, the NR3 mRNA exists in at least two variants--with the insert (NR3-long; NR3-l) and without the insert (NR3-short; NR3-s). The NR3-l variant is expressed throughout the adult rat brain. Moreover, this variant predominates in the occipital and entorhinal cortices, thalamus and cerebellum. Analysis of NR3-l development indicates that it is regulated in a region-specific manner.

Glutamate and synaptic plasticity at mammalian primary olfactory synapses

Glutamate is the transmitter at synapses from the olfactory nerve (ON) to mitral (Mi)/tufted cells, but very little is known about the functional properties of this synapse. This report summarizes in vitro physiological and computational modeling studies investigating glutamatergic neurotransmission at ON-->Mi cell synapses. Single ON shocks in rat main olfactory bulb (MOB) slices elicit distinct early and late spiking components triggered, respectively, by (RS)-alpha-amino-3-hydroxy-5-methyl-4-isoxazole-propionic acid (AMPA)/kainic acid (KA) and N-methyl-D-aspartate (NMDA) receptor activation. Modeling simulations showed that the placement of both AMPA/KA and NMDA receptors on Mi apical dendrites replicates the experimentally observed early and late Mi spiking responses to ON shocks. Brief, tetanic ON stimulation in vitro induced robust, selective long-term potentiation (LTP) of NMDA receptor-dependent spiking. Modeling experiments disclosed several potential mechanisms underlying the selective LTP of NMDA receptor-dependent spiking. These findings demonstrate that ON-->Mi cell transmission exhibits a novel form of plasticity whereby high frequency synaptic activity induces selective LTP of NMDA receptor-dependent spiking.

Nerve gas-induced seizures: role of acetylcholine in the rapid induction of Fos and glial fibrillary acidic protein in piriform cortex

Soman (pinacolymethylphosphonofluoridate), a highly potent irreversible inhibitor of acetylcholinesterase (AChE), causes seizures and rapidly increases Fos and glial fibrillary acidic protein (GFAP) staining in piriform cortex (PC). This suggests that the inhibition of AChE by soman leads to increased acetylcholine (ACh) and neuronal excitability in PC. The sole source of cholinergic input to PC is from the nucleus of the diagonal band (NDB). To investigate the role of ACh in soman-induced seizures, we lesioned cholinergic neurons in NDB unilaterally with 192-IgG-saporin. By 10 d, saporin eliminated staining for choline acetyltransferase (ChAT), the synthetic enzyme for ACh, in NDB ipsilateral to the lesion. Staining for AChE, the degradative enzyme for ACh, was eliminated in PC ipsilateral to the lesioned NDB. By 45-60 min after soman, increased Fos and GFAP staining in PC was evident only ipsilateral to the unlesioned NDB. By 90-120 min after soman, Fos and GFAP staining increased bilaterally in PC. In a second experiment, electrical stimulation electrodes were implanted unilaterally in the NDB to activate focally the projections to PC in unanesthetized rats. Within 5 min of NDB stimulation, there were clear behavioral and EEG signs of convulsions. After 45-60 min of NDB stimulation, there was increased Fos and GFAP staining in layer II of PC ipsilateral to the stimulation site. Pretreatment with the selective muscarinic receptor antagonist scopolamine blocked the convulsions and prevented increased Fos and GFAP staining in PC. These results suggest that ACh release in PC triggers the initiation of seizures and gliosis after soman administration, predominantly by the activation of muscarinic receptors.

Fos expression in rat pontine tegmental neurons following activation of the medial preoptic area

Fos immunohistochemistry was used to map the distribution of pontine neurons excited by activation of the medial preoptic area (MPO). Although we have previously shown that Barrington's nucleus receives a very dense focal input from the MPO, electrical stimulation of the preoptic area unexpectedly induced very little Fos expression in Barrington's neurons. These results suggest that the MPO-->Barrington's projection utilizes a transmitter(s) that does not involve transduction of the Fos protein; alternatively, MPO afferents to Barrington's nucleus may be inhibitory in nature. As Barrington's nucleus plays a critical role in micturition, MPO projections to Barrington's nucleus may regulate voiding reflexes during sexual behavior. Interestingly, while the locus coeruleus (LC) proper receives only a sparse projection from the MPO, extensive Fos expression was present in LC. The finding of Fos immunoreactive LC neurons suggests that the excitatory influence of MPO may regulate LC neuronal activity and NE release during reproductive behaviors.

Functional organization of rat olfactory bulb glomeruli revealed by optical imaging

The functional organization and synaptic physiology of olfactory bulb glomeruli were studied in rat in vitro slice preparations stained with the voltage-sensitive dye RH-155. Optical signals were recorded with a 100-element photodiode array at high temporal resolution. Pharmacological and ionic manipulations were used to investigate synaptic responses to stimulation of the olfactory nerve layer (ONL). ONL stimulation evoked a sodium-mediated compound action potential that propagated across the ONL and invaded individual glomeruli. This presynaptic volley evoked calcium-dependent synaptic responses the amplitudes of which were largest within the glomerular layer (GL); smaller amplitude responses were recorded in deeper layers of the olfactory bulb. Synaptic responses in the GL were attenuated by the non-NMDA ionotropic glutamate receptor antagonist CNQX; the residual component was suppressed by the NMDA glutamate receptor antagonist AP-5. The GABAA receptor antagonist bicuculline methiodide had little effect, whereas the GABAB receptor agonist baclofen dramatically attenuated ONL-evoked synaptic responses. The effects of baclofen were reversed by the GABAB receptor antagonist CGP35348. Paired-pulse depression of ONL-evoked synaptic responses in the GL was partially reversed by CGP35348. These findings suggest that olfactory nerve axons release glutamate to activate both NMDA and non-NMDA receptors on GL neurons, that GABAA receptor-mediated inhibition has little effect on these responses, and that GABAB receptor-mediated inhibition may act presynaptically on olfactory nerve axons to modulate their inputs to olfactory bulb neurons.

Glomerular synaptic responses to olfactory nerve input in rat olfactory bulb slices

In olfactory bulb slices from young rats, the field potential evoked in the glomerular layer by stimulation in the olfactory nerve layer consisted of two negative components: an early component (N1) which was blocked by bath application of the kainate/amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX, 10 microM), and a late, prolonged component (N2; duration > or = 350 msec) which was unaffected by CNQX, was enhanced by reduction of Mg2+ in the medium, and was blocked by the N-methyl-D-aspartate receptor antagonist DL-2-amino-5-phosphonovalerate (50 microM). A comparison of the glomerular field potentials before and after knife cuts that isolated the glomerular layer from the deeper layers of the olfactory bulb indicated that both N1 and N2 were produced by currents generated, for the most part, within the glomeruli. A laminar analysis of the field potential profiles evoked by olfactory nerve stimulation in standard medium, or in the presence of CNQX, showed that N1 and N2 reversed polarity in the external plexiform and mitral cell layers, suggesting that both components reflected synaptic responses in the distal, apical dendrites of mitral/tufted cells. Simultaneous field potential recordings in the glomerular layer and intracellular recordings in the mitral cell layer showed that: (i) N1 is associated with a brief, short-latency spiking activity of mitral cells, and (ii) N2 is associated with prolonged mitral cell spiking, since N2 and the late cell firing had similar time-courses, and both were blocked by bath applied DL-2-amino-5-phosphonovalerate. Application of the GABA(A) receptor antagonist bicuculline methiodide (10 microM) to standard medium selectively enhanced N2. The enhanced N2 was significantly reduced by DL-2-amino-5-phosphonovalerate. Strychnine, an antagonist of glycine receptors, had similar effects to those of bicuculline, but only at high concentrations that have been previously shown to block GABA(A) receptors; at low concentrations strychnine had no effect. The effects of all drugs tested were reversible. In the rat olfactory bulb, activation of the olfactory nerve evokes a kainate/AMPA receptor-mediated response in the distal, apical dendrites of mitral/tufted cells, followed by a slow N-methyl-D-aspartate receptor-mediated response which triggers prolonged discharge of mitral cells. GABA(A) receptor-mediated inhibition appears to suppress, preferentially, this N-methyl-D-aspartate receptor-mediated component. The presence of prolonged N-methyl-D-aspartate receptor-mediated postsynaptic activity at the primary synapses of the olfactory system may play a key role in olfactory processing by facilitating synaptic integration and plasticity.

Soman-induced seizures rapidly activate astrocytes and microglia in discrete brain regions

Neurons in the piriform cortex and the pontine nucleus locus coeruleus express elevated levels of the immediate early gene protein product, Fos, within 30-45 minutes of a seizurogenic dose of the anticholinesterase, soman (Zimmer et al., [1997] J. Comp. Neurol. 378:468-481). By 24 hours following soman injection, there is marked neuropathology in the piriform cortex. These findings suggest selective, regional vulnerability in response to the seizurogenic actions of soman. In the present study, we determined that soman-induced seizures also cause selective, rapid activation of astrocytes and microglia in the piriform cortex and other brain regions. Animals were killed at different intervals between 1 hour and 24 hours after a convulsive dose of soman. Brain sections were processed for immunocytochemical detection of astrocytes with antibodies against glial fibrillary acidic protein, and microglia and macrophages with antibodies against the complement receptor 3 protein, OX-42. The results demonstrate that following soman administration: (1) there is a rapid increase in glial fibrillary acidic protein staining in astrocytes of the piriform cortex (1 hour); (ii) reactive astrocytes are specifically restricted to layer II and the superficial boundaries of layer III of the piriform cortex. These are the same layers in which neurons express Fos within 30-45 minutes following soman administration; (3) between 1 and 4 hours, resting (ramified) microglia in the piriform cortex and the hippocampus alter their morphology to resemble active microglia. From 4-8 hours, active microglia undergo morphological changes characteristic of reactive microglia that resemble macrophages. Taken together, these observations indicate that astrocytes and microglia in brain regions susceptible to soman become rapidly "reactive" in response to seizures. The highly specific anatomical codistribution of reactive glia and Fos-expressing neurons suggests that intensely active neurons provide local signals that trigger reactive changes in neighboring glia.

Anatomical localization and time course of Fos expression following soman-induced seizures

Soman (pinacolymethylphosphonofluoridate), a highly potent, irreversible inhibitor of cholinesterase, causes intense convulsions, neuropathology and, ultimately, death. There is evidence that certain brain structures are selectively vulnerable to the pathological consequences of soman-induced seizures. A working hypothesis is that central nervous system (CNS) structures with the earliest and most severe signs of neuropathology may be key sites for the initiation of the seizures. Fos, the immediate-early gene product, increases rapidly in several animal seizure models. Thus, we reasoned that the earliest brain regions to express Fos might be involved in the initiation and maintenance of soman-induced convulsions. To assess this, rats were injected with a single, convulsive dose of soman (77.7 micrograms/kg, i.m.). The animals were euthanized and processed for immunocytochemical analysis at several time points. Robust Fos expression was seen in layer II of the piriform cortex and the noradrenergic nucleus locus coeruleus within 30-45 minutes. One hour following soman injection, staining was more intense in the piriform cortex layer II and in the locus coeruleus. In addition, Fos was evident in the piriform cortex layer III, the entorhinal cortex, the endopiriform nucleus, the olfactory tubercle, the anterior olfactory nucleus and the main olfactory bulb. By 2 hours, Fos staining was present throughout the cerebral cortex, thalamus, caudate-putamen and the hippocampus. At 8 hours and beyond, Fos expression returned to control levels throughout the CNS except for the piriform cortex and the locus coeruleus which still had robust labeling. By 24 hours, neuropathology was evident throughout the rostral-caudal extent of layer II of the piriform cortex. The rapid induction of Fos in the piriform cortex and the locus coeruleus, taken together with previous anatomical, eletrophysiological and neurochemical studies, suggests that prolonged, excessive exposure to synaptically released acetylcholine and norepinephrine triggers the production of soman-induced seizures initially in the piriform cortex and subsequently in other cortical and subcortical structures.

Olfactory epithelial organotypic slice cultures: a useful tool for investigating olfactory neural development

An in vitro slice culture was established for investigating olfactory neural development. The olfactory epithelium was dissected from embryonic day 13 rats; 400 microns slices were cultured for 5 days in serum-free medium on Millicell-CM membranes coated with different substrates. The slices were grown in the absence of their appropriate target, the olfactory bulb, or CNS derived glia. The cultures mimic many features of in vivo development. Cells in the olfactory epithelium slices differentiate into neurons that express olfactory marker protein (OMP). OMP-positive cells have the characteristic morphology of olfactory receptor neurons: a short dendrite and a single thin axon. The slices support robust axon outgrowth. In single-label experiments, many axons expressed neural specific tubulin, growth-associated protein 43 and OMP. Axons appeared to grow equally well on membranes coated with type I rat tail collagen, laminin or fibronectin. The cultures exhibit organotypic polarity with an apical side rich in olfactory neurons and a basal side supporting axon outgrowth. Numerous cells migrate out of the slices, of which a small minority was identified as neurons based on the expression of neural specific tubulin and HuD, a nuclear antigen, expressed exclusively in differentiated neurons. Most of the migrating cells, however, were positive for glial fibrillary acidic protein and S-100, indicating that they are differentiated glia. A subpopulation of these glial cells also expressed low-affinity nerve growth factor receptors, indicating that they are olfactory Schwann cells. Both migrating neurons and glia were frequently associated with axons growing out of the slice. In some cases, axons extended in advance of migrating cells. This suggests that olfactory receptor neurons in organotypic cultures require neither a pre-established glial/neuronal cellular terrain nor any target tissue for successful axon outgrowth. Organotypic olfactory epithelial slice cultures may be useful for investigating cellular and molecular mechanisms that regulate early olfactory development and function.

Activation of locus coeruleus enhances the responses of olfactory bulb mitral cells to weak olfactory nerve input

The main olfactory bulb (MOB) receives a dense projection from the pontine nucleus locus coeruleus (LC), the largest collection of norepinephrine (NE)-containing cells in the brain. LC is the sole source of NE innervation of MOB. Previous studies of the actions of exogenously applied NE on mitral cells, the principal output neurons of MOB, are contradictory. The effect of synaptically released NE on mitral cell activity is not known, nor is the influence of NE on responses of mitral cells to olfactory nerve inputs. The goal of the present study was to assess the influence of LC activation on spontaneous and olfactory nerve-evoked activity of mitral cells. In methoxyflurane-anesthetized rats, intracoerulear microinfusions of acetyicholine (ACh) (200 mM; 90-120 nl) evoked a four- to fivefold increase in LC neuronal discharge, and a transient EEG desynchronization and decrease in mitral cell discharge. LC activation increased excitatory responses of mitral cells evoked by weak (i.e., perithreshold) nasal epithelium shocks (1.0 Hz) in 17/18 cells (mean Increase = 67%). The discharge rate of mitral cells at the time that epithelium-evoked responses were increased did not differ significantly from pre-LC activation baseline values. Thus, changes in mitral baseline activity do not account for the increased response to epithelium stimulation. These findings suggest that increased activity in LC-NE projections to MOB may enhance detection of relatively weak odors.

Olfactory nerve stimulation activates rat mitral cells via NMDA and non-NMDA receptors in vitro

The neurotransmitter(s) and receptors mediating excitatory transmission at the mammalian olfactory nerve-mitral cell synapse were investigated using extracellular recordings in rat olfactory bulb slices. Single shocks applied to the olfactory nerve elicited both a short latency and a delayed excitatory response in mitral cells. Both responses were blocked after bath application of kynurenic acid, a broad-spectrum glutamate receptor antagonist, or DNQX, a preferential non-NMDA receptor antagonist. The specific NMDA receptor antagonist AP5 selectively attenuated the delayed, but not the initial excitation. These results suggest that glutamate is the major excitatory transmitter in the mammalian olfactory nerve, and excites mitral cells via NMDA and non-NMDA receptors.

Expression of extracellular matrix molecules and cell surface molecules in the olfactory nerve pathway during early development

The spatial and temporal expression patterns of several extracellular matrix molecules--laminin and fibronectin and cell surface molecules, neural cell adhesion molecule (NCAM), L1, tenascin, chondroitin sulfate proteoglycan, and peanut agglutinin (PNA) binding sites--were investigated during early olfactory nerve development. NCAM and L1 have similar patterns: They are expressed in the olfactory nerve and on the olfactory receptor neurons (ORNs) commencing with the earliest olfactory axon outgrowth (E12-E15). Their expression patterns suggest that both NCAM and L1 are associated with extension and fasciculation of olfactory axons. A comparison of L1 and olfactory marker protein suggests that L1 is expressed predominantly on immature ORNs. Laminin has an unique punctate staining pattern in the developing olfactory pathway as early as E12. These laminin puncta might play a role in olfactory neurite outgrowth and guidance. At E14, when pioneer olfactory axons enter the brain, the laminin-positive meninges on the surface of the olfactory bulb primordium break down but remain intact in the rest of the telencephalon. This suggests a functional interaction between the olfactory axons and the glial-pial barrier. Fibronectin staining is diffuse throughout the cranial mesenchyme but is absent from the olfactory nerve pathway. No specific patterns of tenascin or chondroitin sulfate, were observed during early olfactory development. PNA binding sites were associated with olfactory axon fasciculation. The expression of several extracellular matrix molecules and cell surface molecules is spatially and temporally regulated in the developing olfactory system. These molecules, thus, may play functional roles in olfactory axon outgrowth, fasciculation, and/or guidance.

Dendrites of locus coeruleus neurons extend preferentially into two pericoerulear zones

The intrinsic cytoarchitecture and neurochemical organization of the nucleus locus coeruleus have been characterized extensively, but there is little information about the organization of locus coeruleus neuronal processes extending outside of the nucleus proper. Light and electron microscopic immunocytochemical techniques were used to investigate the distribution of dopamine-beta-hydroxylase- or tyrosine-hydroxylase-labeled extranuclear processes in the rat pericoerulear region. The vast majority of these processes extended preferentially into two zones: (1) the pontine tegmentum medial and rostral to locus coeruleus, here termed the rostromedial pericoerulear region; and (2) a narrow region adjacent to the IVth ventricle caudomedial to locus coeruleus, designated here as the caudal juxtaependymal pericoerulear region. Far fewer labeled processes extended into the lateral and ventral pericoerulear regions. Seventy-seven percent of the labeled profiles in the pericoerulear region were dendrites. All labeled profiles in the rostromedial pericoerulear region and 94% of the labeled profiles in the caudal juxtaependymal zone were dendrites. By contrast, in the rostroventral pericoerulear region, 25% of the labeled profiles were axons. Locus coeruleus extranuclear dendrites were never presynaptic to other structures but were often contacted by several unlabeled presynaptic terminals. These results indicate that the dendrites of locus coeruleus neurons extend preferentially into two pericoerulear zones. Extranuclear dendrites in all pericoerulear regions receive extensive, nonnoradrenergic synaptic contacts. Thus, pericoerulear dendrites, particularly in the rostromedial and caudal juxtaependymal zones, are important sites for the integration of inputs to locus coeruleus neurons.

Medial preoptic area afferents to periaqueductal gray medullo-output neurons: a combined Fos and tract tracing study

We have shown recently that the medial preoptic area (MPO) robustly innervates discrete columns along the rostrocaudal axis of the midbrain periaqueductal gray (PAG). However, the location of PAG neurons responsive to MPO activation is not known. Anterograde tract tracing was used in combination with Fos immunohistochemistry to characterize the MPO --> PAG pathway anatomically and functionally within the same animal. Focal electrical or chemical stimulation of MPO in anesthetized rats induced extensive Fos expression within the PAG compared with sham controls. Fos-positive neurons were organized as 2-3 longitudinal columns. The organization and location of these columns overlapped remarkably well with the distribution of fibers and terminals in PAG labeled by Phaseolus vulgaris leucoagglutinin (PHA-L) injected into the same MPO stimulation site. This indicates that MPO inputs may terminate on the soma or proximal dendrites of neurons exhibiting elevated Fos. A second series of experiments investigated whether MPO stimulation excited PAG neurons with descending projections to the medulla. Retrograde labelling of PAG neurons projecting to the medial and lateral regions of the rostroventral medulla (RVM) was combined with MPO-induced Fos expression. The results showed that a substantial population (37-53%) of Fos-positive PAG neurons projected to the ventral medulla. This indicates that MPO stimulation engages PAG-medullary output neurons. Taken together, these results suggest that the MPO --> Pag --> RVM projection constitutes a functional pathway. This circuit may coordinately regulate neuroendocrine, motor, and autonomic adjustments necessary for the elaboration of sexual behaviors.

Orthodromic synaptic activation of rat olfactory bulb mitral cells in isolated slices

Axons of olfactory receptor neurons terminate in the glomerular layer of the olfactory bulb, where they synapse with the apical dendrites of mitral cells. Although the mitral cell and its excitation by the olfactory nerve have been the subject of numerous experimental investigations, in vitro studies of these neurons have primarily used nonmammalian preparations. We have recorded the responses of rat olfactory bulb mitral cells to stimulation of the olfactory nerve layer in vitro using extracellular and whole cell patch techniques. Olfactory bulbs were cut into 400-microns thick slices in approximately horizontal section and submerged in a recording chamber. Patch clamp electrodes were guided into the mitral cell layer, which was visible under a dissecting microscope. A stimulating electrode was placed onto the olfactory nerve layer (ONL) rostral to the recording electrode. In extracellular recordings, mitral cells typically responded to ONL stimulation with a prolonged excitation lasting 1 s or longer. With whole cell patch recordings, membrane resistances (mean 272 M omega) were substantially higher than those reported in previous intracellular studies that used sharp electrodes. Small spontaneous excitatory potentials were present in some mitral cells. ONL stimulation caused a prolonged depolarization comparable to the duration of the period of excitation observed in extracellular recordings. At membrane potentials near -55 mV, ONL stimulation evoked a train of spikes. All but the first of these spikes were blocked by hyperpolarization of the membrane to -65 mV.

Fos expression induced by changes in arterial pressure is localized in distinct, longitudinally organized columns of neurons in the rat midbrain periaqueductal gray

The distribution of neurons expressing Fos within the periaqueductal gray (PAG) following pharmacologically induced high or low blood pressure was examined to determine (1) if PAG neurons are responsive to changes in arterial pressure (AP) and (2) the relationship of these cells to the functionally defined hypertensive and hypotensive columns in PAG. Changes in AP differentially induced robust Fos expression in neurons confined to discrete, longitudinally organized columns within PAG. Increased AP produced extensive Fos-like immunoreactivity within the lateral PAG, beginning at the level of the oculomotor nucleus. At the level of the dorsal raphe, Fos expression induced by increased AP shifted dorsally, into the dorsolateral division of PAG; this pattern of Fos labeling was maintained throughout the caudal one-third of PAG. Double-labeling for Fos and nicotinamide adenine dinucleotide phosphate diaphorase confirmed that Fos-positive cells induced by increased AP were located in the dorsolateral division of PAG at these caudal levels. Fos positive cells were codistributed, but not colocalized, with nicotinamide adenine dinucleotide phosphate diaphorase-positive cells. Decreased AP evoked a completely different pattern of Fos expression. Fos-positive cells were predominantly located within the ventrolateral PAG region, extending from the level of the trochlear nucleus through the level of the caudal dorsal raphe. Double-labeling studies for Fos and serotonin indicated that only 1-2 double-labeled cells per section were present. Saline infusion resulted in very few Fos-like immunoreactive cells, indicating that volume receptor activation does not account for Fos expression in PAG evoked by changes in AP. These results indicate that (1) substantial numbers of PAG neurons are excited by pharmacologically induced changes in AP and (2) excitatory barosensitive PAG neurons are anatomically segregated based on their responsiveness to a specific directional change in AP.

Acidic pH rapidly increases immunoreactivity of glial fibrillary acidic protein in cultured astrocytes

Neuroepithelial progenitor cells from forebrains of newborn rat pups develop into "mature" astrocytes in an epidermal growth factor-containing medium free of serum (Von Visger et al: Exp Neurol 128:34, 1994). Eight-week-old "mature" astrocyte cultures on poly-L-lysine-coated dishes were exposed to an acidic medium (pH 5.8-6.0) for 2-6 h. Immunoreactivity for glial fibrillary acidic protein (GFAP) dramatically and rapidly increased; this immediate increase was not affected by pretreatment with cycloheximide. In further experiments we found that the increase in GFAP was undiminished for 24-48 h after the acid-treated astrocytes were returned to normal growth medium. The Ca2+ channel antagonists nifedipine and diltiazem attenuated the increase in GFAP immunoreactivity. These results suggest that extracellular acidosis may produce a rapid increase in GFAP immunoreactivity in astrocytes independent of de novo protein synthesis, possibly by increasing intracellular levels of free Ca2+ ions.

Evidence that pioneer olfactory axons regulate telencephalon cell cycle kinetics to induce the formation of the olfactory bulb

Early olfactory axons follow a specific pathway to reach the developing telencephalon. We observed that a subpopulation of these axons, the pioneer olfactory axons, penetrate into the ventricular zone of a highly restricted region of the telencephalon at E13 and E14. At E15, this same telencephalic region evaginates to form the olfactory bulb. To investigate the possibility that the pioneer olfactory axons induce the olfactory bulb by influencing precursor cell populations, we compared cell cycle kinetics and differentiation in the olfactory bulb primordium and the adjacent neocortex using cumulative bromdeoxyuridine labeling. The results showed that, 24 hr after the arrival of the first pioneer axons, the duration of the cell cycle is prolonged significantly in the olfactory bulb primordium. In addition, twice as many cells have exited the mitotic cycle in the olfactory bulb primordium versus the adjacent cortex. These findings suggest that pioneer olfactory axons play a role in the induction of the olfactory bulb by selectively modulating cell cycle kinetics in the olfactory bulb primordium. Afferent axons may influence target morphogenesis by modulating target precursor cell proliferation in other developing neural structures.

Directionally specific changes in arterial pressure induce differential patterns of fos expression in discrete areas of the rat brainstem: a double-labeling study for Fos and catecholamines

Although the nucleus tractus solitarii (NTS) has been established as the primary site of synaptic integration for the baroreceptor reflex, the higher-order pathways responsive to, and mediating, changes in vasomotor tone are not well characterized. We used immunohistochemistry to determine the distribution of cells expressing the Fos protein following pharmacologically induced, directionally specific changes in arterial pressure. The goal of this investigation was to determine if this immediate early gene product is differentially expressed in neurons of the rat brainstem following increased (pressor) versus decreased (depressor) arterial blood pressure (AP). Because brainstem catecholaminergic (CA) cell groups have been implicated in cardiovascular regulation, a double-labeling immunohistochemical procedure was used to examine the distribution of Fos in CA cells. Animals received continuous intravenous infusion of either a vasoconstrictor (l-phenylephrine hydrochloride), a vasodilator (sodium nitroprusside), or physiological saline. Extensive Fos-like immunoreactivity (FLI) was induced in both the pressor and depressor conditions in the NTS, caudal ventrolateral medulla (CVLM), rostral ventrolateral medulla (RVLM), A5, locus coeruleus (LC), Kolliker-Fuse, and parabrachial nucleus (PBN). These regions have all been implicated in central cardiovascular regulation. There were differences in the anatomical distribution of Fos-positive cells along the rostrocaudal axis of CVLM in the pressor and depressor conditions. Specifically, increased AP induced significantly more FLI cells within the rostral aspects of CVLM, whereas decreased AP resulted in a significantly greater number of FLI cells within the caudal CVLM. This result suggests that selective vasomotor responses differentially engaged discrete subsets of neurons within this brainstem region. Overall, approximately 50% of CA-immunoreactive cells were also FLI (CA-FLI) in the A1, A5, and A7 regions. Interestingly, increased AP produced significantly more CA-FLI double-labeled cells within the caudal than rostral A1 compared with depressor and control groups. Additionally, increased AP yielded significantly less CA-FLI double-labeled cells within the caudal A2 region. This suggests that CA barosensitive neurons in the CVLM/A1 and NTS/A2 regions are functionally segregated along the rostrocaudal axis of these structures. While twice as many PNMT-FLI double-labeled neurons were found in the C1-C3 regions following vasomotor changes versus saline control, there were no differences in the numbers or anatomical locations of labeled cells between pressor versus depressor groups. The results of this study indicate that (1) tonic changes in AP induce robust Fos expression in brainstem cardiovascular areas and (2) neurons responsive to specific directional changes in arterial pressure are segregated in some brainstem regions.

Expression of the neural cell adhesion molecule (NCAM) and polysialic acid during taste bud degeneration and regeneration

Taste receptor cells are replaced throughout life, accompanied by continuing synaptogenesis between newly formed taste cells and first-order gustatory fibers. The neural cell adhesion molecule (NCAM) is expressed by a subset of taste cells in adult rodents and appears on gustatory nerve fibers during development prior to differentiation of the taste buds. We employed antibodies against the extracellular domain of the NCAM polypeptide (mAb 3F4) and against polysialic acid (PSA) residues found on embryonic forms of NCAM (mAb 5A5) to investigate the relationship between the expression of these molecules and the innervation of taste buds in adult rats. In unoperated rats, anti-NCAM recognized a subset of cells within the vallate taste buds and also the fibers of the glossopharyngeal (IXth) nerve, including those innervating the gustatory epithelium. Taste bud cells did not express PSA but mAb 5A5 immunoreactivity was observed on some fibers of the IXth nerve, including a few that entered the taste buds. Bilateral crush of the IXth nerve resulted in the loss of NCAM expression from the gustatory epithelium within 8 days. As IXth nerve fibers reinnervated the epithelium, NCAM expression was seen first in the nerve, followed by increased expression in the epithelium as the taste cells differentiated from their precursors. PSA expression by fibers of the IXth nerve did not return to normal until well after the regeneration of the vallate taste buds. The present results demonstrate that taste cell expression of NCAM is dependent upon innervation by the IXth nerve and that NCAM expression appears in the nerve prior to its expression in the differentiating epithelium during regeneration.(ABSTRACT TRUNCATED AT 250 WORDS)