Journal of Neuroscience, Vol 8, 4492-4502, Copyright © 1988 by Society for Neuroscience
Neurophysiology of magnocellular forebrain inputs to the olfactory bulb in the rat: frequency potentiation of field potentials and inhibition of output neurons
WT Nickell and MT Shipley
Department of Anatomy and Cell Biology, University of Cincinnati College of Medicine, Ohio 45267.
Basal forebrain nuclei send projections, including cholinergic fibers, to
forebrain cortical targets. These systems have been associated with several
important functions, but their physiological actions are poorly understood.
We have studied the neurophysiological characteristics of one of these
systems, the projection from the nucleus of the horizontal limb of the
diagonal band (HDB) to the main olfactory bulb (MOB) in the rat. Single
shocks to HDB produce modest field potentials in MOB with no detectable
effect on the discharge characteristics of the principal output neurons of
the MOB, the mitral cells. By contrast, continuous stimulation at 10 Hz for
several seconds causes dramatic changes in the HDB field potential and
mitral cell firing. During this period of stimulation, there is an initial
facilitation of the field potential followed by a period of moderately
reduced response amplitude that lasts a few seconds. This brief period of
depression is succeeded by a sudden and marked potentiation of response
amplitude and duration. This potentiated response can be maintained
indefinitely by stimulation at lower frequencies than those required to
initiate the potentiation effect. Coincident with the onset of the
potentiated response, the spontaneous activity of the mitral cells is
completely inhibited. Both the potentiation and mitral cell inhibition can
be maintained indefinitely by continued stimulation at frequencies as low
as 6 Hz. These observations demonstrate that magnocellular basal forebrain
neurons exert powerful regulatory actions on specific neuronal populations
in cortical targets.