Odor discrimination of "IP 3-" and “cAMP-increasing” odorants in the turtle olfactory bulb

The ability of the turtle olfactory system to discriminate between various odorants that increase levels of adenosine 3′,5′-cyclic monophosphate (cAMP) and inositol trisphosphate (IP ₃) in the olfactory bulb was examined by the cross-adaptation technique and analyzed by multidimensional scaling. The mean values of the degree of discrimination among the IP ₃-increasing odorants were higher than those among the cAMP-increasing odorants, and were similar to those between cAMP- and IP ₃-increasing odorants, suggesting that the features of the receptors of cAMP-increasing odorants are different from those which respond to IP ₃-increasing odorants. Analysis by multidimensional scaling suggested that differences in second messenger pathways are not related to detecting odor quality in the turtle olfactory system. Received: 21 July 1995/Received after revision: 20 October 1995/Accepted: 6 November 1995     

Odor response properties of neighboring mitral/tufted cells in the rat olfactory bulb

Olfactory perception initiates in the nasal epithelium wherefrom olfactory receptor neurons--expressing the same receptor protein--project and converge in two different glomeruli within each olfactory bulb. Recent evidence suggests that glomeruli are isolated functional units, arranged in a chemotopic manner in the olfactory bulb. Exposure to odorants leads to the activation of specific populations of glomeruli. In rodents, about 25-50 mitral/tufted cells project their primary dendrites to a single glomerulus receiving similar sensory input. Yet, little is known about the properties of neighboring mitral/tufted cells connected to one or a few neighboring glomeruli. We used tetrodes to simultaneously record multiple single-unit activity in the mitral cell layer of anesthetized, freely breathing rats while exposed to mixtures of chemically related compounds. First, we characterized the odorant-induced modifications in firing rate of neighboring mitral/tufted cells and found that they do not share odorant response profiles. Individual units showed a long silent (11.01 ms) period with no oscillatory activity. Cross-correlation analysis between neighboring mitral/tufted cells revealed negligible synchronous activity among them. Finally, we show that respiratory-related temporal patterns are dissimilar among neighboring mitral/tufted cells and also that odorant stimulation results in an individual modification that is not necessarily shared by neighboring mitral/tufted cells. These results show that neighboring mitral/tufted cells frequently exhibit dissimilar response properties, which are not consistent with a precise chemotopic map at the mitral/tufted cell layer in the olfactory bulb.     

Calcium-binding protein parvalbumin-immunoreactive neurons in the rat olfactory bulb

The laminar distribution and morphological features of parvalbumin-immunoreactive [PV(+l)] neurons, one of the subpopulations of GABAergic neurons, were studied in the rat olfactory bulb at a light microscopic level. In the main olfactory bulb of adult rats, PV(+) neurons were mainly located in the external plexiform layer (EPL), and a few were scattered in the glomerular layer (GL), mitral cell layer (ML), and granule cell layer (GRL); whereas PV(+) neurons were rarely seen in the accessory olfactory bulb. The inner and outer sublayers of the EPL (ISL and OSL) appeared to be somewhat different in the distribution of PV(+) somata and features of PV(+) processes. PV(+) somata were located throughout the OSL, and PV(+) processes intermingled with one another, making a dense meshwork in the OSL; whereas, in the ISL, PV(+) somata were mainly located near the inner border of the EPL, and PV(+) processes made a sparser meshwork than that in the OSL. PV(+) neurons in the EPL were apparently heterogeneous in their structural features and appeared to be classifiable into several groups. Among them there appeared five distinctive types of PV(+) neurons. The most prominent group of PV(+) neurons in the OSL were superficial short-axon cells, located in the superficial portion of this sublayer and giving rise to relatively thick processes, in horizontal or oblique directions, which usually bore spines and varicosities. Another prominent group of PV(+) neurons extended several short, branched dendrites with spines and varicosities, which appeared to intermingle with one another, making a relatively small, spherical or ovoid dendritic field around the cell bodies; most of them resembled Van Gehuchten cells reported in previous Golgi studies. A third distinctive and most numerous group of PV(+) neurons were of the multipolar type; their somata and processes were located throughout the EPL. Their relatively smooth processes with frequent varicosities and a few spines were extended horizontally or diagonally throughout the EPL. A fourth group, which could be a subtype of the multipolar type, were located in or just above th ML and extended several thin, smooth dendrites in the EPL, some of which appeared to reach the border between the GL and EPL. Occasionally, axonlike processes arose from their cell bodies and extended into the ML. This fourth type of PV(+) neuron was named inner short-axon cells. A fifth group of neuron was located in the ML; processes of these neurons were extended horizontally, so they were named inner horizontal cells. PV(+) processes from the fourth and the fifth group of cells appeared to make contacts on mitral cell somata. In the GL some presumably periglomerular cells were also PV(+). In the GRL, PV(+) neurons were small in number, but they were also heterogeneous in their structural features; Some were identified as Golgi cells. This study shows a tremendous heterogeneity in morphological features of a chemically defined subpopulation of GABAergic interneurons in the olfactory bulb.     

The projection from the olfactory epithelium to the olfactory bulb in the salamander, Ambystoma tigrinum

Summary Odor quality may be represented as a topographic code of responses of receptor cells throughout the olfactory epithelium, with this code conveyed to the central nervous system by a topographic projection from the olfactory epithelium to the olfactory bulb. There is good evidence for topographic differences in odor-induced receptor cell activity in the tiger salamander but there is no evidence for a topographic epithelium-to-bulb projection in this species. In the present study ³H-leucine autoradiography was used to trace the projections of olfactory receptor neurons in the tiger salamander. Thirteen animals received small injections of tritiated leucine into different regions of the dorsal or the ventral olfactory epithelium, or into the ventrolateral, vomeronasal organ. The results show that the anterior-to-posterior axes in the dorsal and ventral epithelia are represented along the ventral-to-dorsal axis in the rostral end of the olfactory bulb. The vomeronasal organ projects to the caudal end of the bulb. We conclude that the central projection of the olfactory epithelium in the tiger salamander is topographically organised only along the antero-posterior axis and not the medio-lateral axis. Thus epithelial receptor cell activity along the antero-posterior axis would be represented in the glomerular layer of the bulb by activity along its ventro-dorsal axis.     

Reactions of olfactory bulb neurons to different stimulus intensities in laboratory mice

Summary The activity of 219 cells in the olfactory bulb of waking, freely breathing mice was analysed, and it was found that their spontaneous discharge rate varied between 0.3 and 33 AP/s. Both butyric acid and geraniol elicited responses of four types: 38% of the 98 responses were of type 1 (excitation), 43% of type 2 (inhibition), 10% of type 3 (complex structure), and 8% were characterized by a change in the temporal pattern of the activity. Response duration varied from less than 500 ms to more than 1 min. 52 secondary neurons were stimulated with four different concentrations of the odor substances. All of the responsive cells showed a clear ability to discriminate concentration. That is, response magnitude varied with intensity, producing non-monotonie curves. Most of the neurons responded only in a region of a more or less limited concentration, and in no case was a saturation curve observed. Approx. 40% of the neurons responded to the lowest concentration tested (10^–8 vol. % of butyric acid or geraniol). The strongest stimuli, 10^–2 vol.%, were relatively ineffective.The work was supported by the Deutsche Forschungsgemeinschaft (Schm 322/7).     

Ketamine and MK801 attenuate paired pulse inhibition in the olfactory bulb of the rat

Summary We have investigated the effects of the phencyclidine like-compounds ketamine and MK801 on the evoked field potentials of rat olfactory bulb. Low doses of ketamine (3–6 mg/kg) blocked the inhibition of mitral cells by granule cells evoked by stimulation of lateral olfactory tract fibres or by stimulation of olfactory nerve. This blockade was not accompanied by a decrease in granule cell excitation as revealed by field potential recording. MK801 had a similar effect on the inhibition of mitral cells evoked by stimulation of the lateral olfactory tract. As ketamine does not influence the inhibitory action of GABA (Anis et al. 1983) these results suggest that both ketamine and MK801 block inhibition by an action on intrinsic excitatory feed-back circuits in the olfactory bulb.     

A centrifugal respiratory modulation of olfactory bulb unit activity: a study on acute rat preparation

Summary In behaving rats, unit activity in the mitral and granule cell layers of the olfactory bulb (OB) can be modulated by respiration. In order to determine whether central influences could take part in this phenomenon, respiratory rhythm and the activity of OB units were recorded in the present experiment and analyzed temporally in 18 anaesthetized tracheotomized rats. In spite of the interrupted nasal airflow, 30 of the 80 cells recorded in the mitral and granule cell layers, still displayed a significant respiratory patterning of their activity. Maximal neuronal discharges were time-locked with different phases of the respiratory cycle, most often synchronized with the end of expiration. This is in contrast with previous observations in intact animals. Possible underlying mechanisms are discussed.     

Inputs from the olfactory bulb and olfactory cortex to the entorhinal cortex in the cat

Summary Field potentials and unit activity elicited by electrical stimulation of the olfactory bulb (OB) and anterior and posterior prepiriform cortex (PPCa and PPCp) were measured extracellularly in the entorhinal cortex (EC) of the cat. Different topographic distributions of the amplitude and peak latency of average evoked potentials (AEPs) were obtained depending on the stimulated area. The maximal evoked activity in the EC showed a gradient in a latero-medial direction with the extremes corresponding to the stimulation of OB and PPCp respectively. Analysis of firing patterns of units in the EC in response to stimulation of the OB, PPCa and PPCp showed that an appreciable number of units responded to stimulation of different areas, mainly PPC- a and PPCp. It was found that the pathways being stimulated differed in conduction velocities with the PPCp-EC being the slowest. Most responding units lay in layer I and II of the EC. The AEPs to PPCaand PPCp-stimulation presented different types of depth profiles. Stimulation of the PPCa evoked an initial surface-negative depth-positive potential whereas the PPCp evoked a different type of AEP with an initial positive component at the surface and negative in depth. It is assumed that the stimulated fibres have their active synapses at different levels within the superficial layers of the EC. The possibility of direct influence of olfactory inputs on the hippocampus mediated by one synapse in the EC is discussed.     

Capillaries in aging rat olfactory bulb: A quantitative light and electron microscopic analysis

Olfactory bulbs from Charles River (Crl) rats from 3 to 36 months have been examined with light and electron microscopy. Total capillary length, surface, and volume, as well as number of endothelial cells, increases during the twofold increase in olfactory bulb volume from 3 to 18 months, but the relative density of these parameters shows no change during this time; from 18 to 36 months when neuronal cell body and dendrites are decreasing markedly in size, the relative density of capillaries shows only a modest decrease. Capillary lumen size and capillary wall thickness remain the same throughout life, but basal lamina thickness doubles from 3 to 24 months and then remains constant from 24 to 36 months. The incidence of several unusual ultrastructural features of the outer capillary basal lamina has been shown to increase with age.     

Effects of olfactory bulb removal on fear responses and passive avoidance in the rat

Following complete bulbectomies, male hooded rats showed an increase in irritability and difficulty of handling, but a decrease in timidity or fear responses. After rats had learned to drink in an open field, a cat was confined in the center, and fear was defined by the behavior of controls, viz., almost total suppression of drinking and long periods of freezing, broken by brief bursts of high-speed activity. Bulb animals could not have differed more radically. Bulbs showed neither freezing nor suppression of drinking. The present results could not be attributed to differences in shock reactivity; nor could they be attributed to differences in learning or retention. Bulb animals also showed an impairment on the acquisition of step-down passive avoidance but no difference in step-downs after acquisition. These results argue against an impairment of response inhibition. The present study suggests that bulbectomy increases irritability and decreases timidity.     

传入神经阻滞对成年大鼠嗅球calbindin表达的影响

目的：研究传入神经阻滞对成年SD大鼠嗅球中calbindin（CB）表达的影响。 方法： 成年SD大鼠，ZnSO4 灌流单边鼻孔，10、20、30和60 d后用免疫组化方法检测嗅球的CB表达。 结果： 损伤后10、20、30和60 d，与对照组相比，损伤组嗅球的CB阳性细胞密度和染色强度显著下降，并且从10、20到30 d逐渐下降，损伤后60 d的结果与30 d接近。 结论： 大鼠嗅球中CB的表达受传入神经阻滞的影响。     

Postnatal X-ray irradiation effects on glomerular layer of rat olfactory bulb: quantitative and immunocytochemical analysis

Summary In the rat olfactory bulb, the majority of interneurons in the glomerular layer (GL) are supposed to be generated during first postnatal week. Low and repeated doses of X-rays (200 rad x 4 and 200 rad x 6) were used during this period to impair the development of interneurons. The resulting effects on olfactory bulb neurons were examined stereologically and immunocytochemically in animals of 4 and 12 weeks of age. Quantitative analysis showed that, 1) the volume of the GL decreased to 55% (1200 rad) – 70% (800 rad) of control, 2) numerical cell densities in GL decreased to 40% (1200 rad) – 60% (800 rad) of control, thus resulting in 3) a decrease of the total cell number in GL to 20% (1200 rad) – 40% (800 rad) of control in irradiated olfactory bulbs of animals 4 weeks old. In comparison, mitral cells, which are generated prenatally, were much less affected (total cell number: 70–80% of control), indicating a selective loss of cells generated during the first postnatal week in GL. Effects on somata and processes immunoreactive for GABA, tyrosine hydroxylase (TH), calbindin D-28K and parvalbumin (PV) were examined in irradiated bulbs of both 4 and 12 week-old rats. All of these immunoreactive elements showed a drastic decrease in all layers. Semiquantitative analysis showed that in the GL, calbindin D-28K immunoreactive (calbindin D-28K(+)) neurons decreased more extensively than TH immunoreactive (TH(+)) and GABA-like immunoreactive (GABA(+)) neurons; that is, TH(+) and GABA(+) neurons decreased to 20% (1200 rad) – 40% (800 rad) of control, whereas calbindin D-28K(+) neurons decreased to 10% (1200 rad) – 30% (800 rad) of control in the GL of irradiated bulbs. These findings indicated that larger proportions of calbindin D-28K(+) neurons might be generated during the first postnatal week than those of GABA(+) and TH(+) neurons. Furthermore, in irradiated bulbs the proportion of GABA(-)TH(+) cells in TH(+) cells increased to about twice of control, and the estimated total numbers of GABA(-)TH(+) cells in irradiated rats were 95% (800 rad) and 40% (1200 rad) of control. These observations suggest that the majority of GABA(-)TH(+) neurons were less affected by X-ray irradiation during the first postnatal week and thus that they might be generated in the prenatal period. Since during the first 2 postnatal weeks, neurons showing GABA(-)TH(+) were not seen in GL (Kosaka et al. 1987a), the majority of GABA(-)TH(+) neurons in adult olfactory bulb were assumed to change their phenotype at some postnatal developmental period.     

Effect of bilateral olfactory bulbectomy on discrimination avoidance conditioning in rats

Studies were made with the two-way shuttle box method on the acquisition of discrimination avoidance learning by olfactory bulbectomized rats in relation to changes in emotional behavior. Bulbectomized rats showed a marked increase in locomotor activity, with accompanying augmentation of the reactivity and the appearance of muricidal behavior. Initially, the bulbectomized rats showed elevated conditioned avoidance responses to both the CS+ and the CS-. In later stages, there was a continued slow increase in responses to the CS+ accompanied by a decrease in responses to the CS-, until responses to both stimuli were only slightly elevated above the levels shown by control rats. This result suggests that olfactory bulbectomy does not affect discrimination ability itself, but the impairment of discrimination during the initial stages is resulted from hyperemotionality induced by olfactory bulbectomy.     

The effects of cryogenic blockade of the centrifugal,bulbopetal pathways on the dynamic and static response characteristics of goldfish olfactory bulb mitral cells

Summary The responses of single goldfish olfactory bulb mitral cells were studied by extracellular recordings before and during cryogenic blockade of the efferent, centrifugal pathways in the ipsilateral olfactory tract. In each experiment the same odour was presented 40 times before and then 40 times during cooling. Each stimulus period (at least 30 s) was preceded by a stimulus-free interval (at least 30 s), during which a steady stream of tap water was applied. These procedures allow the investigation of activity changes of single neurons and of cell ensembles using statistical methods. i) In comparison with the pre-cooling activity, cooling of the efferent pathways did not cause a generalized disinhibition in mitral cell responses. Significant disinhibitory, significant inhibitory and indifferent effects occurred in about the same proportion during repetitive water and odour applications. ii) Abrupt or slow changes of single mitral cell discharge patterns during the 40 water and odour applications were observed before and during blocking of the efferent fibre systems: These pattern changes are therefore not necessarily a consequence of the efferent signals, and may thus have been a result of intrabulbar plasticity. iii) The most notable effect of efferent fibre blockade across all experiments was a significant (Wilcoxon-ranktest, P = 0.01) decrease of the signal to noise ratio i.e., the ratio between the activity during the spontaneous (water) and the stimulus (odour) phase, which could be demonstrated for both the phasic (immediately after stimulus onset) and tonic (during long term stimulation) components of the mitral cell responses.     

多巴胺受体在大鼠嗅球的表达及其在帕金森病大鼠嗅觉障碍中的作用

目的 观察多巴胺受体在大鼠嗅球（OB）的表达与分布,探讨左旋多巴（L-DOPA）治疗对帕金森病（PD）大鼠嗅觉的影响。 方法 采用免疫印迹、免疫荧光等方法观察多巴胺受体在大鼠OB中的表达;6-羟多巴胺（6-OHDA）双侧注射建立PD大鼠模型,检测L-DOPA治疗对PD大鼠嗅觉功能及谷氨酸脱羧酶（GAD）和脑源性神经营养因子（BNDF）表达的影响。 结果 嗅球内D1和D2两种多巴胺受体亚型表达含量高。D1和D2在颗粒细胞层（GCL）内GAD阳性的γ-氨基丁酸（GABA）能神经元上大量表达,被酪氨酸羟化酶（TH）阳性神经纤维终末包绕。PD大鼠OB内GCL层TH蛋白表达明显下降（0.05±0.01 vs 0.01±0.00,P<0.001）。L-DOPA治疗后,PD大鼠找寻食物小球时间显著降低［（624.4±113.4）s vs(312.4±79.35)s,P<0.05］,OB内BDNF表达显著升高（0.02±0.01 vs 0.07±0.01,P<0.01）。 结论 D1和D2在GCL层GABA能神经元大量表达。L-DOPA治疗可缓解PD大鼠嗅觉障碍,可能与激活OB内GABA能神经元上的D1和D2复合体,进而改善BDNF表达有关。     

Induction and maintenance of epileptiform activity in the rabbit olfactory bulb depends on centrifugal input

Summary A technique of cryogenic blockade was used in waking rabbits to produce complete and reversible isolation of the olfactory bulb from the rest of the brain. During cooling of the olfactory peduncle epileptiform activity occurred spontaneously in the pyriform cortex in 3 out of 20 sessions, but never in the bulb. Following removal of the cryoblockade, during the seizure state, epileptiform discharges appeared simultaneously in the bulb and pyriform cortex. In the control state, without cooling of the peduncle, epileptiform activity could be evoked in the bulb and cortex by intense electrical stimulation of either the bulb or the lateral olfactory tract. During the cryoblockade, however, intense stimulation of the bulb failed to evoke seizure-like discharges. The results demonstrate a dependency on more central olfactory structures for the induction and maintenance of epileptiform activity in the olfactory bulb.This project was supported by a grant no. HL31164 from NIH     

Impact of apoE deficiency during synaptic remodeling in the mouse olfactory bulb

In this study we examined the role of apoE on the rate of synaptic recovery in the olfactory bulb (OB) following olfactory epithelium (OE) lesioning in mice. We used both immunoblotting and immunohistochemical techniques to compare the density of OB synaptophysin (Syn, a synaptic marker) in apoE-gene deficient/knockout (KO) mice and wild-type (WT) mice following OE lesion. We found that the whole bulb concentrations of Syn, measured by immunoblotting, declined sharply following injury in both WT and KO mice during the degenerative phase (3-7 days). After this initial decline, the Syn concentration gradually increased to normal levels by 56 days in WT mice. In contrast, Syn concentration in KO mice did not recover by day 56 when Syn density in WT was essentially normal. Glomerular Syn density, measured by immunohistochemistry, found a lower density in KO mice at all time points post-lesion. This lower concentration of whole bulb Syn parallels the slower recovery of glomerular area in KO mice. The data indicate that apoE deficiency in KO mice is associated with a delayed recovery of the glomerular area and a slower recovery in Syn concentration in the OB.     

Effects of dopamine and fluphenazine on field potential amplitude in the salamander olfactory bulb

The effects of dopamine (DA) and fluphenazine (FLU), a phenothiazine DA receptor antagonist, were examined in the salamander olfactory bulb. Field potentials were recorded in the granule cell layer of superfused hemibrain preparations, and the amplitude of the large positive peak was compared following stimulation of the olfactory nerve (ON) and lateral olfactory tract (OT). In preparations superfused with normal amphibian Ringer's solution, the large peak occurred 14–21 ms after either ON or OT stimulation. It therefore appeared to reflect the activation of granule cell synapses with mitral cells, as in olfactory bulbs of other species. In three groups of preparations that were superfused with single concentrations of DA, significant decreases were observed in the amplitude of the large peak of ON- and OT-evoked potentials with increases in concentration from 5–200 M. Moreover, with 5 M DA and 50 M DA, significant decreases were observed in the amplitude of the large peak of ON-evoked potentials with increases in superfusion time from 1–15 min. With each DA concentration tested, the mean percentage decrease in the ON-evoked potentials was significantly larger than the mean percentage decrease in the OT-evoked potentials. In five groups of preparations that were superfused with single concentrations of FLU, significant decreases were also observed in the amplitude of the large peak of ON- and OT-evoked potentials with increases in concentration from 0.5–150 M. With 100 M FLU and 150 M FLU, significant decreases were observed in the amplitude of the large peak of both ON- and OT-evoked potentials with increases in superfusion time from 5–10 min. With each FLU concentration tested, the mean percentage change in the ON-evoked potentials was significantly larger than the mean percentage change in the OT-evoked potentials. The stronger effects of DA and FLU on the ON-evoked than OT-evoked potentials suggested that both drugs target receptors in the rostral (superficial) layers of the salamander olfactory bulb which have a higher density or affinity for DA and FLU than receptors in the more caudal (deep) layers of the bulb. When preparations were superfused with an equimolar mixture of DA and FLU at the ED₅₀ concentrations (50 M for both), FLU blocked approximately 50% of the decrease in the amplitude of the ON-evoked potentials relative to the decrease measured in preparations superfused with DA alone. Since FLU depressed the amplitude of ON-evoked potentials when it was tested alone, however, the rostral DA receptors could occur on both the olfactory receptor cell axons and their postsynaptic targets, or FLU could limit mitral/tufted cell excitation by affecting other types of receptors or voltage-sensitive Ca²⁺ channels. Results of this study which show that DA and FLU reduce the amplitude of evoked potentials in the salamander olfactory bulb provide evidence for the occurrence of DA receptors in the amphibian brain. More importantly, the stronger effects of DA and FLU on the ON-evoked than OT-evoked potentials suggest that the DA receptors could function to limit the excitation of cells at an early synaptic level in the salamander bulb. By modulating spatiotemporal patterns of synaptic activity in the glomerular layer, the receptors could profoundly influence the initial encoding of information about odors.