Research reportEffect of fimbria-fornix lesion on 125I-angiotensin IV (Ang IV) binding in the guinea pig hippocampus
Introduction
The hexapeptide angiotensin IV (Ang IV), binds with high affinity to a specific binding site, termed the AT4 receptor [7], [31]. Central administration of Ang IV and other AT4 receptor ligands facilitates memory retention and retrieval in passive [4], [34] and active avoidance studies [4] and enhances performance in spatial memory paradigms [35]. In addition to their memory enhancing effects in normal animals, AT4 receptor ligands restore the deficit observed in various models of amnesia [25], [26], [35]. These observations suggest an important role for AT4 receptors in learning and memory processes.
In vitro autoradiographic studies with 125I-Ang IV and its analogues have localized AT4 receptors in the brain of various species including the guinea pig [19], macaque monkey [21] and human [5]. 125I-Ang IV binding is abundant in motor nuclei and in areas associated with cognitive and sensorimotor functions, such as the hippocampus, neocortex and cerebellum [19], [21]. Of particular interest is the occurrence of high density 125I-Ang IV binding in areas of basal forebrain cholinergic nuclei, including the medial septal complex and the basal nucleus of Meynert, and their terminal fields, such as the hippocampus and neocortex [19], [21]. In the hippocampus, 125I-Ang IV binding occurs in the granular layer of the dentate gyrus and the pyramidal cell layer of the CA1 to CA3 fields of Ammon’s horn, regions containing cholinergic terminal fields derived from basal forebrain neurons [19], [21], [34].
Pharmacological and anatomical evidence suggests that the AT4 receptor system might interact with the septo-hippocampal cholinergic pathway. Spatial learning impairments induced by the muscarinic antagonist, scopolamine, are attenuated by Ang IV analogues [25]. In addition, AT4 receptor ligands, Ang IV and LVV-hemorphin-7, potentiate potassium-evoked acetylcholine (ACh) release from hippocampal slices from rat [15] or guinea pig [16] brain.
Considering this evidence showing an interaction between AT4 receptors and the septo-hippocampal pathway, we hypothesized that AT4 receptors are produced in the cholinergic cell bodies in the medial septum and transported to a pre-synaptic site in the hippocampus to modulate release of ACh. To test this hypothesis, we used receptor autoradiography to examine the density of 125I-Ang IV binding sites in the guinea pig hippocampal formation following fimbria-fornix transection. The fimbria-fornix transection is a widely used model for removal of the septal cholinergic input to the hippocampus [1], [6], [12]. Staining for acetylcholinesterase (AChE), a well-established marker for cholinergic neurons and their terminal fields, was used to verify the success of the lesion.
Section snippets
Materials and methods
All experiments were performed in accordance with the Australian National Health and Medical Research Council ‘Code of Practice for the Care and Use of Animals for Scientific Purposes’ and were approved by the Animal Experimentation Ethics Committee of the Howard Florey Institute.
Results
As demonstrated previously [6], [11], a unilateral 7 mm knife-cut of the fimbria-fornix resulted in a reduction in cholinergic input to the hippocampal formation, as indicated by the loss of AChE staining in the ipsilateral hemisphere of the hippocampus, including the CA1, CA2 and CA3 fields of Ammon’s horn and the dentate gyrus (see Fig. 1, Fig. 2). A notable decrease in AChE staining was also observed in the dorsal aspect of the neocortex (see Fig. 1, Fig. 2). In the sham group, there was no
Effect of fimbria-fornix lesion on 125I-Ang IV binding in the guinea pig hippocampus
This study demonstrates that unilateral transection of the fimbria-fornix did not markedly affect 125I-Ang IV binding in the rostral hippocampus. The transection of the fimbria-fornix was accompanied by a significant loss of AChE staining in the ipsilateral hippocampus (and to some extent in the neocortex), indicating that the denervation of cholinergic input to the hippocampus was successful. Collectively, these results suggest that AT4 receptors in the hippocampal formation in the guinea pig
Conclusion
These results provide evidence that the AT4 receptor, which has been identified as IRAP, occur in a post-synaptic locus in the guinea pig hippocampus. Our autoradiographic data together with the IRAP in situ hybridization studies lead us to suggest that AT4 receptors are predominantly localised on the pyramidal cells of the Ammon’s horn and granule cells of the dentate gyrus. The results from this study should lead to the generation of novel hypotheses regarding the effect of AT4 receptor
Acknowledgements
This work was supported by the National Health and Medical Research Council block grant (No. 983 001).
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