Effects of olfactory peduncle sectioning on the single unit responses of olfactory bulb neurons to odor presentation in awake rabbits

The influences of centrifugal inputs to the olfactory bulb werestudied by recording singlecell responses evoked by olfactorystimuli in intact and peduncle-sectioned bulbs of awake freebreathingrabbits. Responses of intact animals were mainly characterizedby a temporal reorganization of the single unit discharge -responsive second order neurons increased their firing activityduring inspirations and were silent during expirations. Thissynchronization of firing discharge with respiration occurredin the absence of any significant change in the overall firingactivity measured over intervals which included both the inspiratoryand expiratory phases of the respiratory cycle. By contrast,neurons recorded in isolated olfactory bulbs exhibited eithera significant increase or a decrease in firing activity duringodor presentation, and, furthermore, the synchronization ofthese units to the respiratory cycle was markedly reduced comparedwith that in intact animals. Comparison of cell responsivenessbetween intact and isolated olfactory bulbs indicated that thelesion increased the number of odors which induced a response,but did not change the percentage of cells which failed to respondto any of the 5 odorants used in this study. The cell responsivenessincreased for camphor and isoamyl acetate, and to a lesser extentfor food odor. The results indicate that high order nervousstructures exert a powerful inhibitory influence on the responsesof olfactory bulb second-order neurons to odor stimuli. Theyalso suggest that, in intact rabbits, centrifugal inputs playa role in the odor-induced synchronization of the single unitactivity with respiration.     

Spontaneous calcium transients in the immature adult-born neurons of the olfactory bulb

Spontaneous neuronal activity and concomitant intracellular Ca²⁺ signaling are abundant during early perinatal development and are well known for their key role in neuronal proliferation, migration, differentiation and wiring. However, much less is known about the in vivo patterns of spontaneous Ca²⁺ signaling in immature adult-born cells. Here, by using two-photon Ca²⁺ imaging, we analyzed spontaneous in vivo Ca²⁺ signaling in adult-born juxtaglomerular cells of the mouse olfactory bulb over the time period of 5 weeks, from the day of their arrival in the glomerular layer till their stable integration into the preexisting neural network. We show that spontaneous Ca²⁺ transients are ubiquitously present in adult-born cells right after their arrival, require activation of voltage-gated Na⁺ channels and are little sensitive to isoflurane anesthesia. Interestingly, several parameters of this spontaneous activity, such as the area under the curve, the time spent in the active state as well as the fraction of continuously active cells show a bell-shaped dependence on cell’s age, all peaking in 3–4 weeks old cells. This data firmly document the in vivo presence of spontaneous Ca²⁺ signaling during the layer-specific maturation of adult-born neurons in the olfactory bulb and motivate further analyses of the functional role(s) of this activity.     

Characteristics of electrical activity of olfactory bulb neurons of fish

It was shown by intracellular recording of the activity of olfactory bulb neurons of the carp that their dendrites are excited both by synaptic activation and by direct stimulation with an electric current. The dendrites generate an action potential and probably conduct it for some distance toward the soma. The neurons can be divided into two groups: one responds to ortho- and antidromic stimuli with one, rarely with two peaks, the other responds with a rhythmical discharge. The presence of early and late IPSP is characteristic of neurons of both groups. Rhythmical variations in potential with a frequency of 26–33/sec, so-called oscillations, are recorded; they may be excitatory (in secondary neurons they correspond to EPSP) or inhibitory (they correspond to IPSP). Possible mechanisms of the excitatory oscillations and the rhythmical discharge in olfactory bulb neurons of the carp are discussed.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol.3, No.5, pp. 505–511, September–October, 1971.     

The age of olfactory bulb neurons in humans

Continuous turnover of neurons in the olfactory bulb is implicated in several key aspects of olfaction. There is a dramatic decline postnatally in the number of migratory neuroblasts en route to the olfactory bulb in humans, and it has been unclear to what extent the small number of neuroblasts at later stages contributes new neurons to the olfactory bulb. We have assessed the age of olfactory bulb neurons in humans by measuring the levels of nuclear bomb test-derived (14)C in genomic DNA. We report that (14)C concentrations correspond to the atmospheric levels at the time of birth of the individuals, establishing that there is very limited, if any, postnatal neurogenesis in the human olfactory bulb. This identifies a fundamental difference in the plasticity of the human brain compared to other mammals.     

Response patterns of single neurons in the tortoise olfactory epithelium and olfactory bulb

The responses to odor stimulation of 40 single units in the olfactory mucosa and of 18 units in the olfactory bulb of the tortoise (Gopherus polyphemus) were recorded with indium-filled, Pt-black-tipped microelectrodes. The test battery consisted of 27 odorants which were proved effective by recording from small bundles of olfactory nerve. Two concentrations of each odorant were employed. These values were adjusted for response magnitudes equal to those for amyl acetate at –2.5 and –3.5 log concentration in olfactory twig recording. Varying concentrations were generated by an injection-type olfactometer. The mucosal responses were exclusively facilitory with a peak frequency of 16 impulses/sec. 19 mucosal units responded to at least one odorant and each unit was sensitive to a limited number of odorants (1–15). The sensitivity pattern of each unit was highly individual, with no clear-cut types, either chemical or qualitative, emerging. Of the 18 olfactory bulb units sampled, all responded to at least one odorant. The maximum frequency observed during a response was 39 impulses/sec. The bulbar neurons can be classified into two types. There are neurons that respond exclusively with facilitation and others that respond with facilitation to some odorants and with inhibition to others. Qualitatively or chemically similar odorants did not generate similar patterns across bulbar units.     

Generation of spike activity by dendrites of secondary neurons of the rat olfactory bulb

Experiments on secondary neurons of the rat olfactory bulb showed the existence of a third region of action potential generation. It evidently consists of dendrites. This is shown by the distance from the soma of the point where action potentials arise initially and by the recording of spontaneous action potentials of comparatively low amplitude, not spreading into the axon. Action potentials are generated by apical dendrites and also, perhaps, by basal dendrites. Besides partial action potentials with stable amplitude, partial action potentials with, for practical purposes, a stepwise changing amplitude also were recorded. It is suggested that the amplitude of the partial action potentials is modified by IPSPs in the spike-generating zones.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 8, No. 3, pp. 282–290, May–June, 1976.     

Cholinergic modulation of dopaminergic neurons in the mouse olfactory bulb

Considerable evidence exists for an extrinsic cholinergic influence in the maturation and function of the main olfactory bulb. In this study, we addressed the muscarinic modulation of dopaminergic neurons in this structure. We used different patch-clamp techniques to characterize the diverse roles of muscarinic agonists on identified dopaminergic neurons in a transgenic animal model expressing a reporter protein (green fluorescent protein) under the tyrosine hydroxylase promoter. Bath application of acetylcholine (1 mM) in slices and in enzymatically dissociated cells reduced the spontaneous firing of dopaminergic neurons recorded in cell-attached mode. In whole-cell configuration no effect of the agonist was observed, unless using the perforated patch technique, thus suggesting the involvement of a diffusible second messenger. The effect was mediated by metabotropic receptors as it was blocked by atropine and mimicked by the m2 agonist oxotremorine (10 muM). The reduction of periglomerular cell firing by muscarinic activation results from a membrane-potential hyperpolarization caused by activation of a potassium conductance. This modulation of dopaminergic interneurons may be important in the processing of sensory information and may be relevant to understand the mechanisms underlying the olfactory dysfunctions occurring in neurodegenerative diseases affecting the dopaminergic and/or cholinergic systems.     

Maturation of vomeronasal receptor neurons in vitro by coculture with accessory olfactory bulb neurons

To analyze the mechanisms of perception and processing of pheromonal signals in vitro, we previously developed a new culture system for vomeronasal receptor neurons (VRNs), referred to as the vomeronasal pocket (VN pocket). However, very few VRNs were found to express the olfactory marker protein (OMP) and to have protruding microvilli in VN pockets, indicating that these VRNs are immature and that VN pockets are not appropriate for pheromonal recognition. To induce VRN maturation in VN pockets, we here attempted to coculture VN pockets with a VRN target-accessory olfactory bulb (AOB) neurons. At 3 weeks of coculture with AOB neurons, the number of OMP-immunopositive VRNs increased. By electron microscopy, the development of microvilli in VRNs was found to occur coincidentally with OMP expression in vitro. These results indicate that VRN maturation is induced by coculture with AOB neurons. The OMP expression of VRNs was induced not only by AOB neurons but also by neurons of other parts of the central nervous system (CNS). Thus, VRN maturation requires only CNS neurons. Since the maturation of VRNs was not induced in one-well separate cultures, the nonspecific induction of OMP expression by CNS neurons suggests the involvement of a direct contact effect with CNS in VRN maturation.     

Local neurons play key roles in the mammalian olfactory bulb

Over the past few decades, research exploring how the brain perceives, discriminates, and recognizes odorant molecules has received a growing interest. Today, olfaction is no longer considered a matter of poetry. Chemical senses entered the biological era when an increasing number of scientists started to elucidate the early stages of the olfactory pathway. A combination of genetic, biochemical, cellular, electrophysiological and behavioral methods has provided a picture of how odor information is processed in the olfactory system as it moves from the periphery to higher areas of the brain. Our group is exploring the physiology of the main olfactory bulb, the first processing relay in the mammalian brain. From different electrophysiological approaches, we are attempting to understand the cellular rules that contribute to the synaptic transmission and plasticity at this central relay. How olfactory sensory inputs, originating from the olfactory epithelium located in the nasal cavity, are encoded in the main olfactory bulb remains a crucial question for understanding odor processing. More importantly, the persistence of a high level of neurogenesis continuously supplying the adult olfactory bulb with newborn local neurons provides an attractive model to investigate how basic olfactory functions are maintained when a large proportion of local neurons are continuously renewed. For this purpose, we summarize the current ideas concerning the molecular mechanisms and organizational strategies used by the olfactory system to encode and process information in the main olfactory bulb. We discuss the degree of sensitivity of the bulbar neuronal network activity to the persistence of this high level of neurogenesis that is modulated by sensory experience. Finally, it is worth mentioning that analyzing the molecular mechanisms and organizational strategies used by the olfactory system to transduce, encode, and process odorant information in the olfactory bulb should aid in understanding the general neural mechanisms involved in both sensory perception and memory. Due to space constraints, this review focuses exclusively on the olfactory systems of vertebrates and primarily those of mammals.     

Calbindin D-28k-positive neurons in the rat olfactory bulb

Summary We have studied the distribution of calbindin D-28k immunoreactivity in the rat olfactory bulb using specific monoclonal antibodies and the avidin-biotin-immunoperoxidase method. The largest number of positive neurons was located in the periglomerular layer. These neurons were identified as periglomerular cells; they have been described also by other authors as calbindin-positive elements. Close to these neurons, a second population of nerve cells was identified as superficial shortaxon neurons. The remaining layers showed a smaller number of stained elements. Other labeled neurons were located along the external border of the external plexiform layer; the scarce neurons marking its internal border were identified as van Gehuchten cells. No immunoreactive structures were found in the mitral cell layer, although we observed another population of immunostained short-axon cells at its internal border. Some reactive structures, identified by us as horizontal and vertical cells of Cajal, were located in the boundary zone between the internal plexiform layer and the granule layer. In the white matter, we found a neuronal type characterized by its large size and oriented arborization of varicose dendrites.     

The effects of analgesic supplements on neural activity in the main olfactory bulb of the mouse

We evaluated ketoprofen, a nonsteroidal anti-inflammatory drug (NSAID), as an antinociceptive supplement to chloral hydrate anesthesia in mouse. Effects of ketoprofen on main olfactory bulb (MOB) neuronal spontaneous activity were investigated using extracellular recordings in mouse in vivo. These effects were compared with those of another nociceptive supplement, the mu-opioid agonist buprenorphine. Ketoprofen (100 or 200 mg/kg) did not significantly alter MOB single-unit spontaneous rates in either ICR or C57BL/6J mice. In contrast, buprenorphine, at doses of 0.02, 0.05, and 0.20 mg/kg, inhibited MOB neuronal spontaneous rates by 19%, 49%, and 57%, respectively. Neither drug altered the temporal patterning of single-unit spike trains, as measured by the interspike interval (ISI) coefficient of variation (CV). We also investigated the ability of ketoprofen and buprenorphine to induce antinociception in the anesthetized mouse. The electroencephalogram (EEG) was used to measure the anesthetic plane. Both ketoprofen and buprenorphine altered the EEG trace and ketoprofen altered the power spectrum in a manner consistent with deepening anesthesia. Lastly, when applied at the time of anesthesia induction, ketoprofen decreased the amount of chloral hydrate necessary to maintain a defined anesthetic plane during the rest of the experiment. These results suggest that ketoprofen induces antinociception under chloral hydrate anesthesia without significantly inhibiting spontaneous activity of MOB neurons. Ketoprofen is therefore suitable as an antinociceptive supplement to chloral hydrate anesthesia during in vivo electrophysiologic recordings of the mouse MOB.     

Modification of synapse formation of accessory olfactory bulb neurons by coculture with vomeronasal neurons

Previously, a coculture system of accessory olfactory bulb (AOB) neurons and vomeronasal (VN) neurons was established for studying the functional roles of AOB neurons in pheromonal signal processing. In this study, the effect of VN neurons on the development of AOB neurons was examined in a coculture system. Spine density was quantitatively measured for various culture periods of 21, 28, 36, and 42 days in vitro. The densities of dendritic spines were lower in the coculture than in single culture for all periods in vitro. Synapse formation on spines was analyzed immunocytochemically using an anti-synaptophysin antibody. The ratio of the density of synaptophysin-immunopositive spine/total spine density was larger in the coculture than in the single culture. The volume of spine head was larger in the coculture than in single culture. These changes were not observed in the coculture in which there was no physical contact between AOB neurons and VN neurons. These observations suggest that synapse formation on the spines of AOB neurons is modified by physical contact with VN neurons.     

Response correlation maps of neurons in the mammalian olfactory bulb.

To define the relationship between glomerular activation patterns and neuronal olfactory responses in the main olfactory bulb, intracellular recordings were combined with optical imaging of intrinsic signals. Response correlation maps (RCMs) were constructed by correlating the fluctuations in membrane potential and firing rate during odorant presentations with patterns of glomerular activation. The RCMs indicated that mitral/tufted cells were excited by activation of a focal region surrounding their principal glomerulus and generally inhibited by activation of more distant regions. However, the structure of the RCMs and the relative contribution of excitatory and inhibitory glomerular input evolved and even changed sign during and after odorant application. These data suggest a dynamic center-surround organization of mitral/tufted cell receptive fields.     

Electrotonic interaction between secondary neurons of the carp olfactory bulb

Responses of secondary neurons of the carp olfactory bulb evoked by electrical stimulation of the olfactory tract were investigated by intracellular recording. In most neurons spike responses were identified as antidromic. Their latent periods varied from 2.5 to 55 msec. Two other types of responses of secondary neurons had constant latent periods: the pseudo-antidromic spike and a fast low-amplitude depolarization potential. It is concluded that these responses are generated by the antidromic spike of a neighboring neuron, connected electrotonically with the recorded neuron.M. V. Lomonosov Moscow State University. Translated from Neirofiziologiya, Vol. 8, No. 5, pp. 490–496, September–October, 1976.     

Receptive fields of second-order neurons in the olfactory bulb of the hamster

Electrical stimulation of nerve fibers emerging from different positions of the olfactory epithelium was used to determine the receptive fields for 52 olfactory bulb neurons in the hamster. The responses of olfactory bulb neurons were recorded extracellularly with metal-filled micropipettes. Suprathreshold stimuli (500 microA) were applied to each of eight standard epithelial positions spaced approximately 250 microns apart, and the position, or positions, which, when stimulated, produced a response in the bulb were taken as an index of the neuron's receptive field. The results indicate that most bulb neurons have very localized receptive fields limited to only one or two stimulating positions. Furthermore, there was a statistically significant correlation between the location of a neuron's receptive field in the olfactory epithelium and the recording depth of the neuron in the olfactory bulb (Spearman rank correlation coefficient, rs, 0.67, P < 0.001). These findings demonstrate that in the mammalian olfactory system there exists a topographical projection of input from localized regions in the epithelium onto the second-order neurons in the olfactory bulb.     

Neocortical and olfactory bulb activity, in armadillos submitted to covering with soil

Armadillos were submitted to an experimental condition in which their head and thorax were covered by a layer of soil 30 cm high. In this condition the electrical activity of the neocortex and of the olfactory bulbs was studied. Neocortical activity did not show signs of damage remaining without gross changes before, during and after the covering with soil. The olfactory bulbs showed a great decrease of bursting sinusoidal activity which almost instantaneously reversed when the soil was removed. The mechanisms of these changes are discussed.