Cortical input to the basal forebrain
Section snippets
Animals and surgical procedures
Twelve male Sprague–Dawley rats (Zivic–Miller Laboratories, Inc., Zelienople, PA, U.S.A.; 275±25 g) were used in this study. All animals received Purina Mills Lab Chow No. 5001 and tap water ad libitum. The Rutgers University Research Animal Facility where the animals were housed, is maintained in accordance to NIH guidelines and is USDA registered and AAALAC accredited. Prior to surgery, the rats were anesthetized with sodium pentobarbital (50 mg/kg body weight). A 2.5% PHA-L (Vector Labs,
Nomenclature
For the localization of PHA-L injection sites in Fig. 1 and Fig. 5 cortical divisions were labelled according to the atlas of Swanson.[82]The prefrontal cortex (PFC) is generally defined as that part of the frontal cortex that has reciprocal connection with the mediodorsal thalamic nucleus35, 50and receives dense dopaminergic input from the ventral tegmental area.[20]The parcellation of the PFC used in this paper is largely adapted from Krettek and Price.[50]Accordingly, the PFC can be
Discussion
The present study provides evidence that i) among the corticofugal projections to the basal forebrain examined, only prefrontal, piriform and insular axons terminate in extended basal forebrain areas that are associated with cholinergic projection neurons; ii) PHA-L-labelled prefrontal boutons were found to be GABA-negative, and were confirmed to contain glutamate immunoreactivity; iii) the majority of prefrontal axons form asymmetric synapses with small dendritic shafts and spines that are
Conclusion
A comparison of the present results with data regarding the cortical targets of cholinergic neurons56, 71, 73, 92suggests that cholinergic neurons in specific forebrain subterritories may ultimately be interconnected with cortical regions sending projections to the same basal forebrain regions. For example, inputs from the piriform cortex were limited mainly to the ventral MCP–HDB regions where cholinergic projection neurons to the same cortical area have been described. Similarly, afferents
Acknowledgements
Dr Lennart Heimer (University of Virginia, NIH grant 17743) is gratefully acknowledged for allowing us to use the electron microscope facilities. Support was provided for this project by USPHS grant NS 23945 (L.Z.). The antibody against parvalbumin was a generous gift from Dr K. Baimbridge (University of British Columbia).
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