Steady-state centrifugal input via the lateral olfactory tract modulates spontaneous activity in the rat main olfactory bulb.

Mitral and tufted cells in the main olfactory bulb (MOB) of anesthetized rats exhibit vigorous spontaneous activity, action potentials produced in the absence of odor stimuli. The central hypothesis of this paper is that tonic activity of centrifugal input to the MOB modulates the spontaneous activity of MOB neurons. The spontaneous activity of centrifugal fibers causes a baseline of steady-state neurotransmitter release, and odor stimulation produces transient changes in the resulting spontaneous activity. This study evaluated the effect of blocking centrifugal axon conduction in the lateral olfactory tract (LOT) by topically applying 2% lidocaine. Mean spontaneous activity of single bulbar neurons was recorded in each MOB layer before and after lidocaine application. While the spontaneous activity of most MOB neurons reversibly decreased after blockade of the LOT, the spontaneous activity of some neurons in the mitral, tufted and granule cell layers increased. The possible mechanisms producing such changes in spontaneous activity are discussed in terms of the tonic, steady-state release of excitatory and/or inhibitory signals from centrifugal inputs to the MOB. The data show for the first time that tonic centrifugal input to the MOB modulates the spontaneous activity of MOB interneurons and projection neurons. The present study is one of the few that focuses on steady-state spontaneous activity. The modulation of spontaneous activity demonstrated in this study implies a behaviorally relevant, state-dependent regulation of the MOB by the CNS.

Changing undergraduate human anatomy and physiology laboratories: perspectives from a large-enrollment course.

In the present article, a veteran lecturer of human anatomy and physiology taught several sections of the laboratory component for the first time and shares his observations and analysis from this unique perspective. The article discusses a large-enrollment, content-heavy anatomy and physiology course in relationship to published studies on learning and student self-efficacy. Changes in the laboratory component that could increase student learning are proposed. The author also points out the need for research to assess whether selective curricular changes could increase the depth of understanding and retention of learned material.

Fractal analysis reveals subclasses of neurons and suggests an explanation of their spontaneous activity.

The present work used fractal time series analysis (detrended fluctuation analysis; DFA) to examine the spontaneous activity of single neurons in an anesthetized animal model, specifically, the mitral cells in the rat main olfactory bulb. DFA bolstered previous research in suggesting two subclasses of mitral cells. Although there was no difference in the fractal scaling of the interspike interval series at the shorter timescales, there was a significant difference at longer timescales. Neurons in Group B exhibited fractal, power-law scaled interspike intervals, whereas neurons in Group A exhibited random variation. These results raise questions about the role of these different cells within the olfactory bulb and potential explanations of their dynamics. Specifically, self-organized criticality has been proposed as an explanation of fractal scaling in many natural systems, including neural systems. However, this theory is based on certain assumptions that do not clearly hold in the case of spontaneous neural activity, which likely reflects intrinsic cell dynamics rather than activity driven by external stimulation. Moreover, it is unclear how self-organized criticality might account for the random dynamics observed in Group A, and how these random dynamics might serve some functional role when embedded in the typical activity of the olfactory bulb. These theoretical considerations provide direction for additional experimental work.

A selective PMCA inhibitor does not prolong the electroolfactogram in mouse.

BACKGROUND: Within the cilia of vertebrate olfactory receptor neurons, Ca(2+) accumulates during odor transduction. Termination of the odor response requires removal of this Ca(2+), and prior evidence suggests that both Na(+)/Ca(2+) exchange and plasma membrane Ca(2+)-ATPase (PMCA) contribute to this removal. PRINCIPAL FINDINGS: In intact mouse olfactory epithelium, we measured the time course of termination of the odor-induced field potential. Replacement of mucosal Na(+) with Li(+), which reduces the ability of Na(+)/Ca(2+) exchange to expel Ca(2+), prolonged the termination as expected. However, treating the epithelium with the specific PMCA inhibitor caloxin 1b1 caused no significant increase in the time course of response termination. CONCLUSIONS: Under these experimental conditions, PMCA does not contribute detectably to the termination of the odor response.

The source of spontaneous activity in the main olfactory bulb of the rat.

INTRODUCTION: In vivo, most neurons in the main olfactory bulb exhibit robust spontaneous activity. This paper tests the hypothesis that spontaneous activity in olfactory receptor neurons drives much of the spontaneous activity in mitral and tufted cells via excitatory synapses. METHODS: Single units were recorded in vivo from the main olfactory bulb of a rat before, during, and after application of lidocaine to the olfactory nerve. The effect of lidocaine on the conduction of action potentials from the olfactory epithelium to the olfactory bulb was assessed by electrically stimulating the olfactory nerve rostral to the application site and monitoring the field potential evoked in the bulb. RESULTS: Lidocaine caused a significant decrease in the amplitude of the olfactory nerve evoked field potential that was recorded in the olfactory bulb. By contrast, the lidocaine block did not significantly alter the spontaneous activity of single units in the bulb, nor did it alter the field potential evoked by electrical stimulation of the lateral olfactory tract. Lidocaine block also did not change the temporal patters of action potential or their synchronization with respiration. CONCLUSIONS: Spontaneous activity in neurons of the main olfactory bulb is not driven mainly by activity in olfactory receptor neurons despite the extensive convergence onto mitral and tufted cells. These results suggest that spontaneous activity of mitral and tufted is either an inherent property of these cells or is driven by centrifugal inputs to the bulb.

Physiological evidence for two classes of mitral cells in the rat olfactory bulb.

The spontaneous activity of mitral cells was recorded in vivo from the main olfactory bulb of freely breathing anesthetized rats. Single units recorded extracellularly from the mitral cell body layer were further identified as mitral cells by antidromic activation of the lateral olfactory tract and the posterior piriform cortex. Hierarchical cluster analysis of their spontaneous activity showed that at least two classes of mitral cells could be distinguished. A post-hoc multivariate analysis of variance indicated significant differences between the two groups based on mean rate, latency, and the coefficient of variation in interspike interval. Univariate tests showed that the groups differed in mean rate, but not in latency, or in the coefficient of variation in interspike interval. Autocorrelation analysis showed that the high frequency group tended to fire in bursts. Functional implications of these putative subclasses of mitral cells are discussed.

A housefly sensory-motor integration laboratory.

Insects have many interesting behaviors that can be observed in an introductory biology laboratory setting. In the present article, we describe several reflexes using the housefly Musca domestica that can be used to introduce students to sensory and motor responses and encourage them to think about the underlying neural circuits and integration of sensory information that mediate the behaviors.

Comparison of identified mitral and tufted cells in freely breathing rats: II. Odor-evoked responses.

Mitral and tufted cells are the 2 types of output neurons of the main olfactory bulb. They are located in distinct layers, have distinct projection patterns of their dendrites and axons, and likely have distinct relationships with the intrabulbar inhibitory circuits. They could thus be functionally distinct and process different aspects of olfactory information. To examine this possibility, we compared the odor-evoked responses of identified single units recorded in the mitral cell layer (MCL units), in the core of the external plexiform layer (not at the glomerular border tufted cells), or at the glomerular border of this layer (GB tufted cells) of the entire olfactory bulb. Differences between mitral and tufted cells were observed only when subtle aspects of the responses were explored, such as the firing rate per respiratory cycle or the distribution of firing activity along the respiratory cycle. By contrast, more clear differences were found when the 2 subtypes of tufted cells were examined separately. GB units were significantly more responsive, had significantly higher firing activity, and showed greater activity at the transition between inspiration and expiration. The projection-type tufted cells situated closer to the entrance of the olfactory bulb may thus form a distinct physiological class of output neurons and differ from mitral cells and other tufted cells in the manner of processing olfactory information.

Comparison of identified mitral and tufted cells in freely breathing rats: I. Conduction velocity and spontaneous activity.

The spontaneous activity and impulse conduction velocities of mitral and tufted cells were compared in the entire main olfactory bulb of freely breathing, anesthetized rats. Single units in the mitral cell body layer (MCL) and external plexiform layer (EPL) were identified by antidromic activation from the lateral olfactory tract (LOT), electrode track reconstructions based on dye marking, and the waveform of LOT-evoked field potentials. Using the track reconstructions, EPL units were further subdivided into glomerular border (GB) and not at the glomerular border (notGB) cells. For conduction velocity, significant differences were only found between MCL and GB units and not between MCL and all EPL units or between MCL and notGB units. For spontaneous activity, no significant differences were found between the different unit groups regarding the mean, maximum, or relative maximum rate per 100-ms bin. By contrast, they showed a differential modulation of their firing activity by respiration. GB but not notGB units had a significantly higher mean rate during the respiratory cycle than MCL units with significantly more activity during inspiration. Thus, mitral and tufted cells are similar in their impulse conduction velocity and spontaneous activity, though the more superficially placed GB cells exhibit differences. A comparison of odor responses in these cell types in the companion paper also points to differences between mitral and superficial projection tufted cells.

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.

In vivo preparation and identification of mitral cells in the main olfactory bulb of the mouse.

The mouse main olfactory bulb (MOB) is commonly used as a mammalian model to study olfactory processing. The genetic techniques available with the mouse make its MOB a powerful model for analysis of neuronal circuitry. The mouse has been used as a mammalian model for all types of MOB neurons, but especially to study the activity of mitral cells. However, mouse mitral cell activity is most commonly studied in vitro. Therefore, we aimed to develop a protocol to record the activity of antidromically identified mitral cells in mouse in vivo. Currently, such a protocol does not exist. Using extracellular techniques, we report a protocol that is able to record neurons from all mouse MOB layers. Specifically, mitral cell single-units were identified by antidromic activation from the posterior piriform cortex, and their spontaneous activity was recorded for more than 30 min. This protocol is stable enough to record from single-units while buprenorphine was applied both topically to the surface of the MOB and injected systemically.

Impulse conduction of olfactory receptor neuron axons.

Compound action potentials were recorded from rat olfactory receptor neuron axons at measured distances from the stimulation electrode along the lateral surface of the main olfactory bulb. Distances were plotted as a function of the latencies measured from stimulus onset to the prominent negative trough of the triphasic compound action potential. A straight line was fitted to these data to calculate impulse conduction velocity, 0.42 +/- 0.01 m/s (n = 25). Two procedures were used to investigate whether those axons that project to caudal regions of the bulb had faster conduction velocities than axons projecting to rostral bulb. First, the stimulating electrode was moved to mid-bulb and the recording electrode was placed on the caudal bulb. Alternatively, axons were stimulated antidromically at the caudal bulb. These two procedures stimulate those axons projecting to caudal bulb and bypass olfactory receptor neuron axons that synapse in the rostral bulb. The mean impulse conduction velocities from these caudal and antidromic recordings were 0.58 +/- 0.19 m/s (n = 8) and 0.57 +/- 0.19 m/s (n = 9), respectively. Though both of these means are higher than the impulse conduction velocity calculated for stimulation at the rostral bulb, the differences were not statistically significant.