| The amygdala is a telencephalic group of nuclei in the mammalian brain, which consists of several anatomically and functionally distinct nuclei, including the lateral(LA)and basal(BA) amygdala and the central amygdala(Ce A). The central amygdala contains glucose-sensing neurons, which is therefore essentially involved in the regulation of feeding and body weight. Secretin is a 27-amino acid peptide which belongs to the secretin/glucagon superfamily. Secretin and secretin receptors are widely expressed in the central amygdala. Recent studies have revealed a suppression of food intake in fasted mice by either intraperitoneal(IP) or intracerebroventricular(ICV) secretin administration.Object: To evaluate the electrophysiological effects and feeding behavior of secretin in central amygdala of rats. Methods: In vivo extracellular recordings and behavioral test were used in the present study. Results: 1. In 13 out of the 27 central amygdala neurons recorded, micro-pressure ejection of secretin(0.1μM) increased the spontaneous firing rate from 2.30±0.91 Hz to 3.20±1.07 Hz significantly(P<0.001). The average increase was104.22±26.18%, which was significantly different from that of normal saline control group(P<0.01). However, in 6 out of the 27 neurons, secretin decreased the spontaneous firing rate from 6.03±2.13 Hz to 2.20±1.39Hz(P<0.05). The average decrease was68.80±12.10%, which was significantly different from that of normal saline control group(P<0.001). Further studies were performed to observe the effects of a higher concentration of secretin on central amygdala neurons. Secretin at 0.1m M caused a228.42±48.40% increase in the firing rate of 15 neurons, which was significantly stronger than that of 0.1μM secretin(P<0.05). 2. Two major firing patterns of central amygdala neurons(i.e. bursting firing with moderate and variable firing rate, and irregular firing with low and variable firing rate) were classified in the present recordings. To observe the effects of secretin on the firing patterns, we further analyzed the coefficient of variation(CV) and fano factor(FF) of inter-spike interval(ISI) before and after secretin application.In the neurons with secretin-induced increase in firing rate, micro-pressure ejection of secretin did not change the CV(basal: 1.19±0.16; secretin: 1.17±0.16; P>0.05) and the FF(basal: 2.74±0.80; secretin: 1.79±0.52; P>0.05) significantly. In the neurons with secretin-induced decrease in firing rate, secretin did not change the CV(basal: 1.03±0.08;secretin: 1.29±0.12; P>0.05), as well as the FF(basal: 0.64±0.43; secretin: 2.60±1.53;P>0.05) significantly. 3. We next observed the effects of secretin on glucose-sensitiveneurons in the central amygdala. In total 117 central amygdala neurons, micro-pressure application of 0.5M glucose significantly decreased the basal firing rate in 16 neurons.Therefore, in the present study, 13.68% of central amygdala neurons belong to glucose-sensitive neurons. In 7 out of the 16 glucose-sensitive neurons(basal:4.15±1.28 Hz, glucose: 2.15±0.79 Hz, P<0.05, n=7) with long time stable recording,further application of secretin(0.1μM) decreased the firing rate from 5.31±1.48 Hz to1.74±1.01Hz(P<0.01). The average decrease was 75.32±10.47%(P<0.001 compared with normal saline). 4. Feeding behavioral tests were performed to study the possible involvement of secretin in the central amygdala. Bilateral microinjection of 0.1μM secretin into the central amygdala significantly decreased cumulative food intake. Food intake was significantly lower in rats treated with secretin as compared to vehicle-treated rats. The secretin-induced decrease in food intake at the time points of 1h, 2h, 4h and 12 h was 46.08±8.54%(P<0.001), 44.41±8.87%(P<0.01), 54.00±6.76%(P<0.001) and35.66±9.49%(P<0.01), respectively. In another group of experiment, we observed the effects of H89(an inhibitor of c AMP-activated protein kinase) on secretin-induced change of cumulative food intake. Compared with H89(10μM) alone, co-injection of H89 and secretin(0.1μM) did not change the cumulative food intake significantly(P>0.05 at the time points of 1h, 2h, 4h and 12 h, n=11), suggesting the possible involvement of c AMP-PKA pathway in secretin-induced decrease of food intake. Conclusion: The present studies indicated that secretin increased the neuronal activity in most central amygdala neurons. However, in glucose-sensitive neurons, secretin decreased the firing rate. Bilateral microinjection of secretin into the central amygdala significantly reduced cumulative food intake through c AMP-activated protein kinase activation. The present electrophysiological and behavioral findings may provide a rationale for further investigations into the biological effects of secretin in central nervous system. |
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