Role of the Basal Forebrain Neurons in Cortical Activation and Behavioural State Regulation

Re: BF projections to the thalamus

B.E. Jones

On Mon Dec 7, kazue semba wrote
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>To Dr. Jones (#0891): Thank you for a stimulating and scholarly poster. The new data are very exciting, and I appreciate a historical perspective as well. My question is about the BF projections to the thalamus. Could you comment on how they fit into your theory? My understanding is that the BF projects to the reticular (in part, cholinergic and GABAerigc) and mediodorsal (in part, GABAergic) thalamic nuclei in rat, and in cat, to the anterior and ventromedial nuclei as well (in part, cholinergic). BF neurons could therefore influence cortical neurons not only through their direct projections to the cortex, but also through the thalamus. Some cholinergic BF neurons actually project to both the thalamus and cortex (Jourdain et al., 1989). Do the BF projections to the thalamus represent one mechanism for interaction between thalamocortical and basalo-cortical systems?

Thank you, Dr. Semba for your highly pertinent comments and questions concerning the potential interaction between the thalamocortical and basalocortical systems. At this time, I must say that we will only be able to examine and understand the interaction of these systems after we have learned more about the modulation of the cortex by the basal forebrain cell populations. This being said, I would posit that the basalo-thalamic modulation most likely functions in a parallel fashion to the basalo-cortical modulation. 1) Accordingly, I would speculate that the input to the dorsomedial (and anterior) nuclei would permit a parallel facilitatory input to the thalamic projection nuclei that project to the orbitofrontal (and cingulate) cortices, which also represent major target areas of the basalo-cortical projection. This influence could allow parallel synchronized pacing (in a theta range) in the cortex and in its specific thalamo-cortical input for coherent processing. 2) The input to the ventromedial nucleus is also interesting since, like the basalo-cortical projection, the cortical projection from this thalamic nucleus is very widespread. The input from the basal forebrain would allow coordination of parallel widespread influences from basal forebrain and thalamus. 3) Finally, the role of basalis input to the reticularis nucleus of the thalamus is most difficult to interpret in a simple manner. Acetylcholine hyperpolarizes the GABAergic reticularis neurons. The basalis influence could thus be viewed most simply as attenuating inhibitory input from the reticularis neurons upon thalamo-cortical projection neurons. Phasic influences however could also be associated with rebound low threshold calcium spike bursts in the reticularis neurons, which up to now are thought to occur only in sleep. Understanding of these inputs will thus only be possible with recording of identified cholinergic and non-cholinergic basal forebrain neurons in association with recording of thalamic neurons.

Wed Dec 9