Extracellular single-unit recordings of piriform cortex neurons in rats: Influence of different types of anesthesia and characterization of neurons by pharmacological manipulation of serotonin receptors

In epilepsy research, there is a growing interest in the role of the pirifo rm cortex (PC) in the development and maintenance of limbic kindling and ot her types of limbic epileptogenesis leading to complex partial seizures. Ne urophysiological studies on PC or amygdala-PC slice preparations from kindl ed rats showed that kindling of the amygdala induces long-lasting changes i n synaptic efficacy in the ipsilateral PC, including spontaneous discharges and enhanced susceptibility of PC neurons to evoked burst responses. These long-lasting electrophysiological changes in the PC during kindling appear to be due, at least in part, to impaired function of gamma-aminobutyric ac id (GABA)ergic interneurons. The aim of the present study was to develop an anesthetic protocol allowing electrophysiological single-unit recordings f rom inhibitory, presumably GABAergic PC interneurons in vivo. In addition t o recording of spontaneously active PC neurons, microiontophoretic applicat ion of glutamate was used to activate silent neurons. Anesthesia of rats wi th ketamine/xylazine was not suited for single-unit recordings in the PC be cause of marked cardiovascular depression. Anesthesia with chloral hydrate allowed recording of spontaneous or glutamate-driven single-unit activity i n similar to 40% of all animals. A similar percentage was obtained when rec ordings were done with the narcotic opioid fentanyl (plus gallamine), after all surgical preparations were performed under anesthesia with repeated ad ministration of the barbiturate methohexital. To avoid brain accumulation o f methohexital by repeated applications, we modified the anesthetic protoco l in that methohexital was only injected once for initiation of surgical an esthesia, followed by the short-acting anesthetic propofol which does not a ccumulate upon repeated application. Again, after surgical preparation, ele ctrophysiological recordings were done under fentanyl (plus gallamine). By this procedure, spontaneous or glutamate-driven single-unit activity could be measured in all rats in either layer II or III of the PC. Based on shape and frequency of action potentials, two types of neurons mere recorded. Th e predominant type was similar in its firing characteristics to GABAergic n eurons in other brain regions, was mainly located in layer III, and could b e suppressed by the serotonin(2A) receptor antagonist MDL 100,907, suggesti ng that this type of PC neuron represents inhibitory, putative GABAergic in terneurons. This new in vivo preparation may be useful for evaluation of PC neurons in kindled rats. (C) 1999 Wiley-Liss, Inc.