Group I metabotropic glutamate receptor activation produces a direct excitation of identified septohippocampal cholinergic neurons.
Septohippocampal cholinergic neurons innervate the hippocampus and provide it with almost its entire acetylcholine. Axon collaterals of these neurons also release acetylcholine within the septum and thereby maintain the firing activity of septohippocampal GABAergic neurons. A loss of septohippocampal cholinergic neurons occurs in various neurodegenerative disorders associated with cognitive dysfunctions. group I metabotropic glutamate receptors have been implicated in septohippocampal-dependent learning and memory tasks. In the present study, we examined the physiological and pharmacological effects of a potent and selective group I metabotropic glutamate receptor (mGluR) agonist S-3,5-dihydroxyphenylglycine (DHPG) on rat septohippocampal cholinergic neurons that were identified in brain slices using a selective fluorescent marker. In whole cell recordings, DHPG produced a reversible, reproducible and a direct postsynaptic and concentration-dependent excitation in 100% of septohippocampal cholinergic neurons tested with an EC(50) of 2.1 microM. Pharmacologically, the effects of DHPG were partially/completely reduced by the mGluR1 antagonists, 7-hydrox-iminocyclopropan[b]chromen-1a-carboxylic acid ethyl ester and (+)-2-methyl-4-carboxyphenylglycine. Addition of the mGluR5 antagonist, 2-methyl-6-(phenylethnyl)pyridine hydrochloride, reduced the remaining response to DHPG, suggesting involvement of both receptor subtypes in a subpopulation of septohippocampal cholinergic neurons. In double-immunolabeling studies, 74% of septohippocampal cholinergic neurons co-localized mGluR1alpha-immunoreactivity and 35% co-localized mGluR5-immunoreactivity. Double-immunolabeling studies at the light and electron-microscopic levels showed that vesicular glutamate transporter 2 terminals make asymmetric synaptic contacts with septohippocampal cholinergic neurons. These findings may be of significance in treatment of cognitive deficits associated with neurodegenerative disorders as a group I mGluR-mediated activation of septohippocampal cholinergic neurons would enhance the release of acetylcholine both in the hippocampus and in the septum.
Wu
Min
M
Dept. of Psychiatry, CMHC 335A, Yale University School of Medicine, 34 Park Street, New Haven, CT 06508.
Hajszan
Tibor
T
Xu
Changqing
C
Leranth
Csaba
C
Alreja
Meenakshi
M
D000818:Animals; D002352:Carrier Proteins / Q000378:metabolism; D002799:Cholinergic Fibers; D004305:Dose-Response Relationship, Drug; D004594:Electrophysiology; D018690:Excitatory Amino Acid Agonists / Q000008:administration & dosage / Q000494:pharmacology; D005625:Frontal Lobe / Q000378:metabolism; D005998:Glycine / Q000008:administration & dosage / Q000031:analogs & derivatives* / Q000494:pharmacology; D006624:Hippocampus / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D007158:Immunologic Techniques; D066298:In Vitro Techniques; D008297:Male; D026901:Membrane Transport Proteins; D009474:Neurons / Q000187:drug effects; D009994:Osmolar Concentration; D017729:Presynaptic Terminals / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D064529:Receptor, Metabotropic Glutamate 5; D018094:Receptors, Metabotropic Glutamate / Q000819:agonists / Q000378:metabolism*; D012118:Resorcinols / Q000008:administration & dosage / Q000494:pharmacology; D020665:Septum of Brain / Q000166:cytology / Q000187:drug effects / Q000502:physiology*; D013569:Synapses / Q000502:physiology; D014018:Tissue Distribution; D050599:Vesicular Glutamate Transport Protein 2; D033921:Vesicular Transport Proteins
D002352:Carrier Proteins; D018690:Excitatory Amino Acid Agonists; C578937:Grm5 protein, rat; D026901:Membrane Transport Proteins; D064529:Receptor, Metabotropic Glutamate 5; D018094:Receptors, Metabotropic Glutamate; D012118:Resorcinols; C493475:Slc17a6 protein, rat; D050599:Vesicular Glutamate Transport Protein 2; D033921:Vesicular Transport Proteins; C104077:metabotropic glutamate receptor type 1; C079215:3,5-dihydroxyphenylglycine; D005998:Glycine