Neurophysiology of magnocellular forebrain inputs to the olfactory bulb in the rat: frequency potentiation of field potentials and inhibition of output neurons.

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.