David M. Finch - US grants

The objective of this research is to establish in detail the physiological properties of inputs from the basal forebrain to the limbic system in adult rats. Three specific pathways will be studies: (1) projections from the nucleus of the diagonal band of Broca to the posterior cingulate cortex, (2) projections from the diagonal band to the entorhinal cortex, and (3) projections from the ventral pallidum/nucleus basalis to the basolateral nucleus of the amgydala. The pathways will be studied by the neurophysiological techniques of in vivo intracellular recording and labeling. The basal forebrain will be activated by electrical stimulation, and neuronal responses will be recorded intracellularly in the target areas. Target neurons in the cingulate and entorhinal cortices and the amygdala will be morphologically identified by labeling them with intracellular injections of horseradish peroxidase (HRP). The basal forebrain modulation of synaptic transmission in thalamo-cingulate projections will be studied. A final goal is to inject basal forebrain neurons intracellularly with HRP in order to characterize in detail their pattern of axonal projections and coilateralization. The studies will establish the in vivo physiological and morphological relationship between the extensive projection system of the basal forebrain, and three important components of the limbic system, the cingulate cortex (one of the important cortical targets of the hippocampal formation), the entorhinal cortex (a principal source of input to the dentate gyrus and hippocampus), and the amygdala. These intracellular studies will thus fill significant gaps in our knowledge about the synaptic organization and physiology of these systems.

The broad, long-term objectives of this project are to provide physiological and pharmacological analyses of the synaptic organization of efferent, afferent, and intrinsic connections of the rat subicular complex and entorhinal cortex. First, physiological studies of efferent projections from these structures will be continued. Work will concentrate on projections from the subicular complex and entorhinal cortex to the prefrontal cortex. In order to achieve this objective, the subicular complex and entorhinal cortex will be stimulated electrically, and synaptic potentials in response to this stimulation will be recorded intracellularly in the prefrontal cortex. Target neurons in the prefrontal cortex will be injected intracellularly with horseradish peroxidase (HRP) in order to establish their dendritic and axonal morphology. Second, a pharmacological analysis of projections from the subicular complex to the entorhinal cortex will be conducted, using multibarrel iontophoretic techniques. The pharmacological studies will be designed to test the hypothesis that the excitatory responses in entorhinal principal and inhibitory cells are mediated by glutamatergic transmission; and that inhibitory responses are mediated by GABAergic transmission, secondary to glutamatergic excitation of local inhibitory neurons (feedforward inhibition). Specific antagonists of GABA A and GABA B receptors will be used in order to determine the receptor subtype(s) responsible for the inhibitory responses. Third, a pharmacological analysis of the projections from the subicular complex and entorhinal cortex to the amygdala and to the prefrontal cortex will be conducted. It is hypothesized that glutamatergic and GABAergic receptors mediate feedforward inhibition within these targets, and effort will be concentrated on establishing the roles of receptor subtypes. Fourth, the physiological action of projections to the subicular complex and entorhinal cortex from the prefrontal cortex will be determined. In particular, it is proposed 1) to establish whether inhibitory responses to prefrontal stimulation are mediated by inhibitory neurons located within the subicular complex and entorhinal cortex and 2) to establish the dendritic morphology and pattern of axonal projections of responsive target neurons by injecting them intracellularly with HRP. The proposed intracellular and pharmacological studies attack problems that can best be studied at the systems level in the intact, in vivo preparation. The subicular complex and entorhinal cortex participate in medically-refractory temporal lobe seizures, and appear to be early targets of cell death in Alzheimer's disease. Accordingly, a better understanding of these neuronal systems can provide insight into the pathophysiology associated with these diseases.

DESCRIPTION (Adapted from applicant's abstract): The longer-term objective of this project is to study the synaptic interaction of two major components of the forebrain: the limbic cortex and the striatum. The specific aims are to establish in detail the physiological and pharmacological properties of projections from limbic cortex to the striatum at a cellular level. Research will focus on one particular pathway -- projections from the entorhinal cortex to the NAc. Research will be performed in the in vivo rat preparation using intracellular recording, intracellular labeling of single cells with horseradish peroxidase (HRP), and multibarrel iontophoresis. The combined application of these advanced procedures will be used to establish details about neuronal interactions. Hence, the present studies are designed to obtain physiological, morphological, and neuropharmacological data regarding the synaptic relationship of these areas, including information about their circuit properties and transmitter substances. The NAc is an important component of the ventral striatum. The entorhinal cortex is a key structure of the hippocampal formation, and much hippocampal input and output is funnelled through it. Direct projections from the entorhinal cortex to the NAc have been demonstrated by modern neuroanatomical tracing techniques. However, despite this close, one-way neuroanatomical relationship, the applicant claims that no publishable cellular physiological or pharmacological data have been forthcoming in this research area to date. The present project plans to focus on the following questions concerning: (1) the synaptic organization of projections from the entorhinal cortex to physiologically and morphologically defined NAc neurons; (2) which glutamatergic receptor subtypes mediate excitatory responses of NAc neurons to synaptic inputs from the entorhinal cortex; (3) which gamma aminobutyric acid (GABA)ergic receptor subtypes mediate the secondary inhibitor of striatal neurons to synaptic inputs from the entorhinal cortex; and (4) how excitatory inputs to the NAc from distinct regions of limbic cortex interact at the level of single NAc neurons. The NAc has been implicated in drug abuse and mental illness, including mania and schizophrenia. There is also some suggestive evidence that the entorhinal cortex may be involved in schizophrenia by virtue of histopathological abnormalities observed in the brains of schizophrenic patients. Accordingly, the broader significance of the current project may be its possible relevance to understanding interactions between the limbic cortex and striatum in the production of psychotic symptoms and drug treatment side-effects.