DENDRITIC ORIGIN OF LATE EVENTS IN OPTICAL RECORDINGS FROM SALAMANDER OLFACTORY-BULB

1. Optical recordings of membrane-potential changes were used to characterize the origin and proterties of the electrical signals from the dendritic level in slices of the salamander olfactory bulb. 2. The optical events were correlated with field-potential waves recorded simultaneously. Both responses exhibited patterns similar to those found in other species. 3. Orthodromic stimulation evoked a compound action potential in the olfactory nerve fibers, followed by two additional principal waves (N1 and N2). These field-potential waves reflected excitatory postsynaptic potentials at the primary mitral/tufted and granule cell dendrites, respectively. 4. Extrinsic optical signals from horizontal slices stained with the pyrazo-ozonal dye RH-155 showed a characteristic sequence of depolarizing and hyperpolarizing events. All of the signals exhibited a wavelength dependence expected for this dye and were abolished in the presence of high K+ in the bath. 5. According to their time courses, depolarizing responses under normal recording conditions were divided into two components, fast and slow. Orthodormic stimuli evoked a fast presynaptic response that represents synchronous compounds action protentials from olfactory nerve fibers. At subglomerular levels, additional fast responses could often be recorded at the peri/subglomerular level and in the mitral/tufted somata region. These postsynaptic responses partially conincided with the rising phase of a different depolarizing signal, a slow component charcaterized by its prolonged time course. 6. With orthodromic stimulation, this slow signal attained its largest amplitude in the zone between the glomeruli and the superficial part of the external plexiform layer (EPL). Antidromic stimuli evoked a signal with some similarities to the one evoked orthodromically, but originating in deeper EPL regions. 7. Slow components were characterized by their Ca dependence. Low Ca2+ medium, or calcium channel blockers, suppressed this optical component, whether evoked orthodromically, antidromically, or by direct stimulation. In addition, Ba2+ (2.5-3.6 mM) in the bath did not abolish these responses, suggesting that they do not reflect a glial depolarization in response to elevated extracellular K+ concentration )[K+]o). 8. Locally applied stimuli next to the glomerular layer elicited these signals in 5-10 muM tetrodotoxin (TTX) or in low extracellular Na+ concentration ([Na+]o) medium, but antidromic or orthodromic stimuli failed to evoke the responses under these conditions. The sizes of the responses to local ttimuli reamained constant, but an increase in their duration was observed in either TTX or low [Na+]o. 9. Gamma-Aminobutyric acid (GABA) and baclofen reduced the size of the slow components in a dose-dependent manner. Muscimol (GABA(a) agonist) did not reproduce this effect but did alter the genreal pattern of th responses. A suppressive GABA effect was also observed under local stimulation, indicating that the responsiveness of mitral/tufted cells is essential for the genreation of these optical signals. 10. Removal of Mg2+ from the bath depressed the slow component, and the opposite effect was observed in the presence of an N-methyl-D-aspartate (NMDA) receptor blocker [2-amino-4-phosphonovaleric acid (APV)]. This suggests that not only is the NMDA receptor involved in depressing the elements that generate the slow componenets of the optical signals, but also that these optical responses most likely arise from mitral/tufted dentrites. 11. During orthodromic paired stimulation, moderate conditions responses usually gave rise to a period of partial or complete suppression of the subsequent test slow responses. On the other hand, just threshold stimuli evoked, instead, a period of facilitation. The magnitude and the duration of htis period were related to the intensity of the stimulation and also to the condition/test interval. 12. Moderate stimuli could generate an additional late optical signal at low bath temperature (11-degrees-C). The latency and the size of these late depolarizations were inversely related to the intensity of the stimulus. During pair pulse stimulation, test responses were suppressed, but the second stimulus shortened the latency of the late response. 13. The properties of the slow components of the optical signals indicate that theyare predominantly generated on mitral/tufted processes by long-lasting regenerative Ca2+ currents. These rvents can be associated not only with the presence of a facilitatory response and the period of late reexcitation, but also with the period of suppression that follows andy orthodromic activity. Probably, inhibitory actions also contribute to the regulation to these depolarizations.