Damped oscillatory activity in the guinea pig accessory olfactory bulb slice

Effects of N-methyl-D-aspartate glutamate receptor antagonists on oscillatory signal propagation in the guinea-pig accessory olfactory bulb slice: characterization by optical, field potential and patch clamp recordings

To characterize the role of N-methyl-d-aspartate glutamate receptors in oscillations induced by a single electrical stimulation of the vomeronasal nerve layer, optical, field potential and patch clamp recordings were carried out in guinea-pig accessory olfactory bulb slice preparations. Bath application of the N-methyl-D-aspartate receptor antagonists, 2-amino-5-phosphonovaleric acid or MK-801, produced an increase in frequency of oscillating waves (oscillation) in external plexiform and mitral cell layers. The removal of Mg2+ from perfusate abolished oscillations, while subsequent application of 2-amino-5-phosphonovaleric acid or MK-801 restored oscillations. Vomeronasal nerve layer-evoked postsynaptic currents were analyzed by whole-cell clamp recordings from mitral and granule cells. A long-lasting excitatory postsynaptic current and periodic inhibitory postsynaptic currents, which were superimposed on the long excitatory postsynaptic current, were observed in mitral cells. The frequency of the periodic inhibitory postsynaptic currents correlated with the frequency of oscillations observed in the optical and field potential recordings. Furthermore, periodic inhibitory postsynaptic currents were blocked by puff application of bicuculline to the external plexiform layer/mitral cell layer, where mitral cells make dendrodendritic synapses with granule cells. In addition, puff application of the non-N-methyl-D-aspartate antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, to the external plexiform layer/mitral cell layer suppressed an early phase of periodic inhibitory postsynaptic currents (membrane oscillation), whereas 2-amino-5-phosphonovaleric acid suppressed the late phase of periodic inhibitory postsynaptic currents. These data indicate that periodic excitatory postsynaptic currents of granule cells induce relevantly periodic inhibitory postsynaptic currents in mitral cells via dendrodendritic synapses and suggest that feedback inhibition regulates generation of oscillation via activation of non-N-methyl-d-aspartate glutamate receptors and gradual attenuation of oscillation via activation of N-methyl-D-aspartate receptors on granule cells.

Developmental changes in oscillatory and slow responses of the rat accessory olfactory bulb

Field potential, patch-clamp and optical recordings were performed in accessory olfactory bulb slices of postnatal rats following single electrical stimulation of the vomeronasal nerve layer. On the basis of differences in the components of the field potential, postnatal days were divided into three periods: immature (until postnatal day 11), transitional (postnatal days P12-17) and mature periods (after postnatal day 18). During the immature period, vomeronasal nerve layer stimulation provoked a characteristic damped oscillatory field potential, and the field potential recorded in the glomerular layer consisted of a compound action potential followed by several periodic negative peaks superimposed on slow components. Reduction in [Mg2+] enhanced slow components but did not affect oscillation, whereas an NMDA receptor antagonist, D-2-amino-5-phosphonovalerate, depressed slow components but did not affect the oscillation. During the mature period, slow components and the periodic waves (oscillation) disappeared. The time course of the field potential was similar to that in adults, suggesting that the accessory olfactory bulb reached electrophysiologically maturity at postnatal day 18. A non-NMDA receptor antagonist, 6-cyano-7-nitroquinoxaline-2,3-dione, inhibited vomeronasal nerve layer-induced responses, while D-2-amino-5-phosphonovalerate had no effect, suggesting that NMDA and non-NMDA receptors are active in immature tissues, whereas non-NMDA receptors predominated in mature tissue. Results from whole-cell patch recordings in mitral and granule cells yielded results consistent with those from field potential and optical recordings. Further, a gradual decrease in number and frequency of oscillating waves was observed until postnatal day 17. Analyses of the depth profile of field potentials and current source density in immature tissue suggested that the oscillation and slow components originated in the glomerular layer but not in the external plexiform/mitral cell layer. Further, a new type of oscillation, which was independent of the reciprocal dendrodendritic synapses between mitral and granule cells, was detected. These data indicate that the lack of oscillatory suppression by immature NMDA receptors may play a critical role in the dynamic alteration of bulbar conditions.

Novel subdivisions of the rat accessory olfactory bulb revealed by the combined method with lectin histochemistry, electrophysiological and optical recordings

Wistaria floribunda agglutinin and peanut agglutinin were found to bind histochemically to the anterior and posterior regions, respectively, of the vomeronasal nerve and glomerular layers in the rat accessory olfactory bulb. Furthermore, Ricinus communis agglutinin showed strong binding to the anterior region of the vomeronasal nerve and glomerular layers, whereas it bound weakly and/or moderately to the rostral two-thirds of the posterior glomerular layer but not at all to the caudal one-third. This suggests that the posterior region is further divided into two subregions. An electrophysiological mapping study in sagittal slice preparations demonstrated that stimulation given within the anterior vomeronasal nerve layer elicited field potentials within the anterior region of the external plexiform layer, whereas shocks to the rostral two-thirds and the caudal one-third of the posterior vomeronasal nerve layer provoked field responses within the rostral two-thirds and within the caudal one-third of the posterior external plexiform layer, respectively, indicating that the posterior external plexiform layer is also divided into two subregions. Real-time optical imaging showed similar results as above, except that neural activity also spread into mitral cell layers. Furthermore, the most anterior and posterior ends of the neural activity evoked in the rostral two-thirds of the posterior region immediately adjoined the posterior border of that evoked in the anterior region and the anterior border of that evoked in the caudal one-third of the posterior region, respectively. Moreover, the granule cell layer was also found to have similar boundaries. Thus, optical imaging studies demonstrated individual precise boundaries of these subdivisions, which were positioned right beneath those defined by Ricinus communis agglutinin histochemistry. The presence of functional segregation in each layer leads us to conclude that there are at least three different input-output pathways in the rat vomeronasal system.

Effects of GABAergic agonists and antagonists on oscillatory signal propagation in the guinea-pig accessory olfactory bulb slice revealed by optical recording

To investigate the action of GABAergic agents on oscillatory signal propagation induced by electrical stimulation of the vomeronasal nerve layer, optical and electrophysiological recordings were carried out in slice preparations of the guinea-pig accessory olfactory bulb. In response to electrical stimuli, characteristic optical signals appeared in each layer: in the vomeronasal nerve layer, a transient presynaptic response; in the glomerular layer, pre- and postsynaptic responses; in the external plexiform, mitral cell and granule cell layers, a damped oscillatory response. Application of the GABAergic agonists, that is, GABA, muscimol (a GABAA receptor agonist) and baclofen (a GABAB receptor agonist), suggested that the GABAB action existed mainly in the glomeruli, whereas the GABAA action was present in both the glomeruli and the external plexiform layer. Bicuculline (a GABAA receptor antagonist) produced long-lasting but nonoscillating excitation in the external plexiform and mitral cell layers, indicating that the GABAA action contributes to the formation of oscillatory responses. When double-pulse stimulation was applied to the vomeronasal nerve layer, the test responses in the glomerular layer and external plexiform and mitral cell layers were depressed, but those in the vomeronasal nerve layer were not. Application of 2-hydroxysaclofen (a GABAB receptor antagonist) mostly blocked paired-pulse depression occurring in the glomerular layer and restored the reduced transmission to mitral cells, but had only a small effect on the depressed oscillatory response in the external plexiform and mitral cell layers. These observations suggest that GABAB action in the glomerular layer might, at least, regulate information flow from vomeronasal afferents to apical dendrites of mitral cells, like a gate inhibition. However, actions other than GABAB could also be involved in the depression of the oscillation in the external plexiform and mitral cell layers.

Subdivisions of the guinea-pig accessory olfactory bulb revealed by the combined method with immunohistochemistry, electrophysiological, and optical recordings

The presence of subgroups in vomeronasal sensory neurons has been known in various animals. To elucidate possible functional subdivisions in the guinea-pig accessory olfactory bulb, the combined studies with GTP-binding protein immunohistochemistry, electrophysiological and optical recordings were carried out. Gi2 alpha and Go alpha proteins were immunohistochemically localized, respectively, in the anterior and posterior regions of the vomeronasal nerve and glomerular layers, indicating that the guinea-pig accessory olfactory bulb receives at least two different inputs. This suggests that an anatomical boundary exists in these two layers. A mapping study of field potentials in sagittal slice preparations demonstrated that stimulation of the anterior vomeronasal nerve layer elicited field potentials with weak oscillatory responses exclusively in the anterior region of the external plexiform layer, whereas shocks to the posterior vomeronasal nerve layer provoked distinct oscillatory responses within the posterior one. The damping factors of oscillations in the anterior and posterior regions were 0.064+/-0.028 and 0.025+/-0.014, respectively. These electrophysiological results suggest that the accessory olfactory bulb consists of two functionally different subdivisions. Real-time optical imaging showed that anterior vomeronasal nerve layer shocks produced neural activity which spread horizontally from anterior to posterior only within the anterior region of the external plexiform and mitral cell layers, whereas shocks to the posterior vomeronasal nerve layer evoked periodic neural activity which spread horizontally from posterior to anterior only within the posterior region. Furthermore, the most posterior extent of the optical response evoked in the anterior region immediately adjoined the most anterior extent of that evoked in the posterior region. The maximal distance of signal propagation in the granule cell layer corresponded to that in the overlying external plexiform and mitral cell layers, indicating that the granule cell layer also has a similar boundary. Thus, these optical imaging studies not only demonstrated a precise boundary in each layer of the accessory olfactory bulb, which was positioned right beneath the boundary defined by GTP-binding protein immunohistochemistry, but also confirmed the observations from electrophysiological mapping that evoked field potentials are independently distributed in each of two subdivisions. The presence of the functional subdivision in each layer leads us to conclude that the accessory olfactory bulb in the guinea-pig is distinctly segregated into the anterior and posterior subdivisions, and to suggest that there are at least two different input output pathways in the vomeronasal system.