EFFECTS OF INTRASEPTAL CARBACHOL ON BURST PROPERTIES OF CA1 PYRAMIDAL NEURONS

1. -0.26mm +0.2mm +0.48mm +1mm +1.2mm Infusion Infusion Infusion 9 min 9 min 9 min 9 min 10 min 10 min 10 min 12 min 21 trials CA1 cc Amplitude of Third Spike Amplitude of First Spike Spike amplitude attenuation = BURSTINESS # SPIKES IN BURST Drug, p=0.062 Session, p=0.058 Proportion of spikes in burst # spikes in burst # SPIKES IN BURST INTERSPIKE INTERVAL Age, p=0.087 Drug, p=0.018 INTERSPIKE INTERVAL # SPIKES IN BURST INTERSPIKE INTERVAL SPIKE AMPLITUDE ATTENUATION Behav. State, p<0.001 Behav. State, p<0.001 Session by age, p=0.079 * * * * * * Drug, p=0.096 ISI (ms) # spikes in burst # spikes in burst ISI (ms) Amplitude attenuation CARBACHOL OLD CARBACHOL YOUNG * * YOUNG OLD Percentage increase in theta power SPIKE AMPLITUDE ATTENUATION BURSTINESS BURSTINESS SPIKE AMPLITUDE ATTENUATION Behav. State, p<0.001 Behav. State, p<0.001 Maze pre Maze post Maze pre Maze post Session, p=0.016 Young NS Old SubjectsSeven young (6-10 mo old) and eight old (22-24 mo) male Fischer 344 rats were used in this study. All animals were food deprived to approximately 90% of their ad libitum weights. Training ProcedureThe rats were given a 30 minute session each day to learn to alternate back and forth on a maze. After they reached criteria (80 alternations for at least 2 days of training), rats underwent surgery for microdrive implantation. SurgeryAnimals were anesthetized with ketamine-xylazine cocktail and implanted with a cannula aimed at the medial septum (0.5 mm anterior, 4.5 mm ventral form bregma), and a recording device to allow for extracellular single unit recordings aimed at the dorsal hippocampus (3.2 mm posterior, 2.2 mm lateral, 1 mm ventral from bregma). Each microdrive contained eight movable tetrodes. Re-training and recordings started one week after surgery. Proportion of spikes in burst Amplitude attenuation HOLDER Control Control Control Control Control Control Control Control Carbachol Carbachol Carbachol Carbachol Carbachol Carbachol Carbachol Carbachol Control Control Control Control Control Control Control Control Carbachol Carbachol Carbachol Carbachol Carbachol Carbachol Carbachol Carbachol YOUNG YOUNG YOUNG YOUNG YOUNG YOUNG YOUNG YOUNG OLD OLD OLD OLD OLD OLD OLD OLD Recording ProcedureAnimals wore a multi-channel headstage device that contained two arrays of infrared light emitting diodes. An overhead video tracking system recorded the rat's location and head direction. Analysis of the multi-single unit recordings from each probe was conducted off-line manually, using a spike parameter clustering method (McNaughton et al., 1989; Mizumori et al., 1989). The clustering was based on the relative amplitudes of the signals and the spike durations (see Wilson & McNaughton, 1993). During analysis a velocity filter was used to assure that the data was collected only if the animal moved faster than 2.4cm/sec(Tropp et al., 2004).Hippocampal EEG was also recorded. T-tests: *, p = 0.002 ISI (ms) Infusion ProcedureA 0.5 µl volume was infused at a rate of 0.125 µl/min. One minute was allowed for drug diffusion before the injector was removed. Total dose of carbachol infused was 0.125 µg. YOUNG OLD Amplitude attenuation Proportion of spikes in burst YOUNG OLD YOUNG OLD EFFECTS OF INTRASEPTAL CARBACHOL ON BURST PROPERTIES OF CA1 PYRAMIDAL NEURONS S. Sava1,2*,G.J. Peters1,3, E.J. Markus11Psych. Dept., Univ of Conn., Storrs, CT; 2McLean Hospital/Harvard Medical School, Belmont, MA; 3Psych. Dept., Univ of Delaware, Newark, DE. ABSTRACT HISTOLOGY EFFECT OF CARBACHOL ON BURST PROPERTIES EFFECT OF NOVELTY ON BURST PROPERTIES Single-unit recordings in the hippocampus show that, like cortical neurons, hippocampal pyramidal cells can fire single spikes or trains of high-frequency spikes, commonly called bursts (Rank, 1973). Hippocampal bursts are often assumed to have distinct physiological (Pike et al., 1999; Buzsáki et al., 2002) and/or computational functions (Lisman, 1997; Kepecs et al., 2002). The functional significance of bursts however is not clear. Tropp-Sneider and colleagues (2006) showed that the degree of CA1 bursting decreased when the animal ran on the maze compared to being at rest. These data may not indicate a functional change since bursting increases during sharp-wave oscillations (Buzsáki, 1989; Harris et al, 2001; Wong and Prince, 1978). The cholinergic agonist carbachol was infused into the medial septum, and hippocampal CA1 cells were recorded in freely moving rats in a familiar or novel environment. Septal activation disrupted the retrieval of a previously stored hippocampal place cell representation of a familiar environment regardless of age. When the environment was changed, medial septal activation disrupted the encoding process in young, but facilitated the encoding of the new information in aged rats. In contrast, burst properties of CA1 pyramidal cells were not affected by intraseptal carbachol. While burst firing was reduced when the animal was running on the maze, this was not related to the degree of environmental novelty or to septal activation. Supported by: UConn FRS445142; NIH #R29-A613941-01A1 to E.J.M. Carbachol increases hippocampal theta power during drinking After all single unit recordings were done, each rat received a series of 5 carbachol infusions. The hippocampal theta power was measured while the rat was drinking, before and after carbachol administration. INTRODUCTION Hippocampal pyramidal cells exhibit “place-specific” discharge in relation to the location of the animal (O'Keefe & Dostrovsky, 1971). CA1 pyramidal neurons can fire single spikes or a fast series of spikes commonly referred to as a burst. Typically bursts contain 2-6 spikes at short intervals, with a progressive attenuation in the amplitude of the spikes within the burst (Ranck, 1973; Quirk and Wilson, 1999; Quirk et al., 2001). Both the burst and attenuation phenomena have been postulated to play an important role in information processing (Lisman, 1997; Quirk et al., 2001). Previous studies showed that behavioral state can affect the bursting of CA1 neurons; specifically, CA1 neurons have a greater propensity to burst during awake immobility than during maze running (Harris al., 2001; Tropp Sneider et al., 2006). The present study examined differences in the bursting properties of CA1 neurons in awake-behaving young and old rats during distinct behavioral states (awake immobility and maze performance). We also investigated the effects of intraseptal carbachol and novelty on the burst properties of CA1 cells. EFFECT OF AGE AND BEHAVIORAL STATE . Are there differences in the burst properties of CA1 neurons as a function of behavioral state, age, or novelty? GENERAL METHODS T-test: *, p = 0.024 • Bursting of CA1 cells was not affected by the novel configuration. CONCLUSIONS CA1 neurons burst more during awake immobility than during maze running. Bursting of CA1 neurons does not seem to be affected by age, novelty, or intraseptal carbachol infusion. • Burst = spike train having an interspike interval less than 10 ms. • Within-cell examination of the effects of behavioral state, age and novelty: • Average number of spikes in burst • Burst spike interstimulus interval (ISI) • “Burstiness” (proportion of spikes in burst out of total number of spikes) • Attenuation (calculated only for bursts of 3 or more spikes) Summary of recorded cells by rat & condition REFERENCES T-tests: *, p < 0.006; #, p = 0.06 • On holder, CA1 neurons exhibited higher proportion of spikes in bursts; bursts had more spikes and shorter ISI. • There were no differences between burst properties of neurons recorded from young and old rats. Buzsáki G (1989). Two-stage model of memory trace formation: a role for "noisy" brain states. Neurosci. 31:551-570. Buzsáki G (2002). Theta oscillations in the hippocampus. Neuron 33:325-340. 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