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Investigation of muscarinic and metabotropic (glutamate) response properties of adult and immature olfactory cortical brain slices in vitro.

Postlethwaite, Michael; (1999) Investigation of muscarinic and metabotropic (glutamate) response properties of adult and immature olfactory cortical brain slices in vitro. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

Muscarinic cholinergic- and metabotropic glutamate receptor-evoked responses of adult and immature olfactory cortical neurones were studied in brain slices using intracellular electrophysiological recordings. An attempt was made to study the ontogeny of the cortical slow post-stimulus afterdepolarization (sADP) induced by muscarinic cholinergic or metabotropic glutamate agonists in immature (14-22 days) and adult (greater than 40 day old) rats. In adult neurones, 10 μM oxotremorine-M (OXO-M, a muscarinic agonist) or 1S,3R- ACPD (10-50 μM, a metabotropic glutamate receptor agonist) displayed postsynaptic excitatory and presynaptic inhibitory effects. In contrast, immature rat olfactory neurones responded to 10 μM OXO-M with spontaneous epileptiform activity which was blocked by pharmacological agents such as atropine (1 μM), pirenzepine (a muscarinic M1 receptor antagonist; 300 nM), tetrodotoxin (1 μM) or the glutamate receptor antagonists DL-APV (100 μM) or CNQX (20 μM). Inhibition of either GABAA or GABAB receptor activity with bicuculline (10 μM) or CGP52432 (1 μM) respectively, actively promoted bursting behaviour, working synergistically with lower doses of OXO-M. Conversely, augmenting GABAA or GABAB receptor function with pentobarbitone (100 μM) or baclofen (10 μM), respectively, blocked the bursting. The induced bursting was accompanied by a dramatic prolongation of evoked postsynaptic potentials (PSPs), exhibiting recurrent superimposed spike discharges, not seen in control. 1S,3R-ACPD (10-50 μM) induced typical adult-type responses in immature cells, i.e. presynaptic inhibition and postsynaptic excitation, with no evidence of epileptiform behaviour. During experiments designed to investigate the possible involvement of release of calcium from intracellular stores in the generation of the cholinergic/metabotropic glutamate agonist-induced sADP and its underlying tail current (IADP) adult guinea- pig olfactory neurones, it was found that the sADP induced by OXO-M (10 μM) or 1S,3R ACPD (10-50 μM), could be reversibly blocked by 0.5-3 mM caffeine, a compound known to promote release of calcium from intracellular stores. However, when this inhibitory action was investigated further using compounds with more specific effects on intracellular calcium release, i.e. ryanodine (10 μM), thapsigargin (3 μM) or dantrolene (10 μM), no consistent inhibition of IADP was observed. In addition, other possible effects of caffeine were discounted as being important by use of more specific pharmacological tools, i.e. IBMX (100 μM; an inhibitor of phosphodiesterase activity) or adenosine (100 μM; to reverse any inhibition of adenosine receptors imposed by caffeine). In caesium-loaded cells, a direct block of voltage-sensitive calcium entry by 3 mM caffeine was found, but this could not account for the full inhibition of IADP in this system; a direct blockade of the proposed IADP K+ channels by caffeine was therefore proposed. In conclusion, the muscarinic response profile of immature rat olfactory neurones was found to differ from adult neurones, since they displayed bursting behaviour in response to muscarinic (but not metabotropic) activation, thought to be generated through a local network mechanism. Furthermore, release of calcium from intracellular stores was unlikely to play an important role in sADP or IADP generation. Caffeine, a known modulator of intracellular calcium release, however, blocked IADP through a mechanism which did not involve intracellular calcium stores, most likely having a direct action on calcium entry and/or the proposed IADP K+ conductance.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Investigation of muscarinic and metabotropic (glutamate) response properties of adult and immature olfactory cortical brain slices in vitro.
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Biological sciences; Olfactory neurons
URI: https://discovery.ucl.ac.uk/id/eprint/10103352
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