In the Lizard, Phrynocephalus Przewalskii (Agamidae)
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J Comp Physiol B (2005) DOI 10.1007/s00360-005-0054-7 http://www.paper.edu.cn ORIGINAL PAPER Chongbin Liu Æ Rende Li Æ Zhonghu Liu Æ Shuming Yin Ziren Wang The role of prostaglandins and the hypothalamus in thermoregulation in the lizard, Phrynocephalus przewalskii (Agamidae) Received: 16 August 2005 / Revised: 24 October 2005 / Accepted: 1 November 2005 Ó Springer-Verlag 2005 Abstract Typically, small lizards rely heavily on behav- Introduction ioral thermoregulation rather than physiological mech- anisms to control their rates of warming and cooling. Most reptiles are ectothermic vertebrates, which regulate We tested the hypothesis that prostaglandins participate body temperature (T ) by behavioral and physiological in mediating the cardiovascular response to heating and b means. The effectiveness of behavioral thermoregulation cooling and temperature regulating neurons in the depends on the hypothalamus as a thermostat and car- hypothalamus of the small lizard Phrynocephalus prze- diovascular responses during heating and cooling (Grigg walskii. In vivo and in vitro treatments, heart rates et al. 1979; Tosini et al. 2001; Franklin and Seebacher (HRs) were all found to be higher during heating than 2003; Seebacher and Franklin 2003). Numerous studies during cooling, hysteresis was distinct below 30 and on different groups of reptiles have shown different heart 26°C, respectively. In vivo, as administration of COX rates (HRs) in warming and cooling (‘hysteresis’ pat- inhibitor, there were no differences in HR between tern), which allow reptiles to control rates of transient heating and cooling at any body temperature and heat transfer and peripheral flow between the environ- administration of agonist prostaglandins only produced ment and their core (Bartholomew and Tucker 1963, a significant effect on HR below 25°C. Single-unit 1964; Morgareidge and White 1972; Smith 1976; Grigg activity was recorded extracellularly in vitro with mi- et al. 1979; Bartholomew 1982; Grigg and Seebacher croelectrodes, found the firing rate of the continuous 1999; O’Connor 1999; Seebacher and Franklin 2003). unit increased 23% when the temperature of the artificial However, the physiological mechanisms of ‘hysteresis’ cerebrospinal fluid dropped from 30–20°C. We conclude pattern are not well known. that prostaglandins appear to play only a limited role in Prostaglandins are local hormones having diverse modulating heart activity in Phrynocephalus przewalskii and potent biological activities. Extensive investigation and suggest that cold-sensitive neurons in the preoptic concerning the physiological role of prostaglandins in and anterior hypothalamus (PO/AH) are involved in cardiovascular function have been carried out in mam- thermoregulatory control during heating or cooling. mals, yet only a few attempts have been made to eluci- date the role of these molecules in non-mammalian Keywords Body temperature Æ Hysteresis Æ vertebrates. Prostaglandin E (PGE ) is known to par- Hypothalamus Æ Lizard Æ Prostaglandins Æ 2 2 ticipate to the maintenance of organ blood flow and Thermoregulation regulation of blood pressure. Injection of PGE2 causes bradycardia in the eel Anguilla anguilla (Janvier 1997), but increases HRs in rainbow trout (Brown and Communicated by I.D. Hume Bucknall 1985). Prostaglandin F2a (PGF2a) is regarded as a vosoactive substance in amphibians and reptiles C. Liu Æ R. Li Æ Z. Liu Æ S. Yin Æ Z. Wang (&) (Robleto and Herman 1988; Altimiras et al. 1998). School of Life Sciences, Lanzhou University, Hysteresis in reptiles is triggered by prostaglandins, and Lanzhou, 730000 Gansu, People’s Republic of China E-mail: [email protected] to a lesser extent by the autonomic nervous system, but Tel.: +86-931-3658646 there exists considerable variation in the control mech- Fax: +86-931-8913562 anisms among species (Morgareidge and White 1972; Altimiras et al. 1998; Seebacher and Franklin 2003, C. Liu 2004a, b). The importance of these differences in regu- Department of Life Science, Jinggangshan College, Ji‘an, 343009 Jiangxi, People’s Republic of China lating HRs of warming and cooling in reptiles have yet 转载 中国科技论文在线 http://www.paper.edu.cn to be thoroughly examined and we test for this in a small movements. We used a 100 W incandescent light bulb, lizard, Phrynocephalus przewalskii, increasing the phy- suspended 40 cm above the plastic container, to heat the logenetic diversity of reptiles considered for this box to a maximum temperature of 38°C (Li and Liu tendency. Moreover, HR and blood circulation in ver- 1992; Belliure and Carrascal 2002). During cooling, the tebrates are controlled by complex hormonal and chamber was allowed to cool convectively in a temper- nervous mechanisms (Wilson 1983; Robleto and ature-controlled room set at 20°C. All monitoring Herman 1988; Altimiras et al. 1998) and stress under equipment was controlled from a separate room adjoin- artificial conditions may confound laboratory data. ing the experimental chamber. In in vitro experiments, Therefore, this study attempts to determine the HR we prepared an isolated perfused lizard heart-lung model effects of heating and cooling in vivo and in vitro, in (IPLHLM). Following sodium pentobartital injection, order to elucidate the mechanisms underlying these re- the heart was exposed and the posterior cardinal vein was sponses. cut at 2 mm in the front of the liver and a cannula of this After discovery of thermosensitive neurons in the vein was used by binding the proximal end with a thread. hypothalamus (Nakayama et al. 1961), some neurons of Next, the pericardium was opened and the anterior car- efferent side in the thermoregulatory network have been dinal veins, carotid and systemic arteries, and the trachea identified (Nagashima et al. 2000). Reptiles, as well as were severed, but the lung was left intact. The IPLHLM other ectothermic vertebrates, depend fundamentally on was isolated from the inner thoracic cavity and placed external heat sources and behavioral adjustments to upside down in a tub containing Ringer solution of the alter their body temperature. However, the fact that the following composition (mM): 80.0 NaCl, 2.5 KCl, 2.0 circadian rhythm of behavioral temperature selection of CaCl2, 1.0 MgCl2, 40.0 NaHCO3, and 10.0 glucose and Podarcis sicula (Lacertidae) is not definitely abolished equilibrated with 3% CO2 + 97% O2, pH was 7.75 after both parietalectomy and pinealectomy, suggests (Wasser et al. 1997). We set the preload (filling pressure) that the suprachiasmatic nuclei (SCN) or neighboring and afterload conditions by adjusting the height of the hypothalamic areas may also be involved in driving this perfusion reservoirs and the outflow catheter relative to rhythm (Innocenti et al. 1993). Interestingly, in Dipso- the heart (30±0.5 cm water pressure). The flow rate was saurus dorsalis, the daily bout of voluntary hypothermia directly dependent on water pressure. disappears after lesion to the periventricular preoptic area of the hypothalamus (Berk and Heath 1975). Fur- thermore, in ectothermic vertebrates, the temperature Drugs and treatments regulating neurons in the hypothalamus are still un- known. For this purpose, we record single-unit activity In in vivo experiments, animals used in the laboratory extracellulary in vitro with microelectrodes, using con- heating and cooling trials were starved for 24 h prior to stantly perfused preoptic and anterior hypothalamus experimentation. The non-steroidal anti-inflammatory (PO/AH) tissue slices with artificial cerebrospinal fluid drug Diclofenac (Sigma, 8 mg kgÀ1 body mass), an (aCSF) at different temperatures. inhibitor of COX activity (Rainsford 2001), was injected intraperitoneally 2.5 h before experimentation (Seebacher and Franklin 2003). Agonists prostaglandin À1 Materials and methods E2 (Sigma, 5 lgkg ) and prostaglandin F2a (Sigma, 10 lgkgÀ1) were administered intravenously at different Animals body temperatures and the total amount of fluid injected did not exceed 0.5–0.7% of total blood volume. PGE2 was Healthy adult lizards, Phrynocephalus przewalskii dissolved in 95% ethanol at 10 mg lÀ1 and further dilu- (n=48, mass=8.67±1.40 [SE] g, SVL=50.6±4.2 tions were made in physiological saline just prior to [SE] mm) were used after being captured in the desert administration (Janvier 1997). The concentration and and semi-desert area in Gansu province (37°60¢S, amount of ethanol injected was too diluted to have a 103°05¢E). They were housed in the laboratory (25–28°C) physiological effect (Crashaw et al. 1988). PGF2a was and acclimated for at least 2 weeks and given free dissolved directly in physiological saline before the access to feed and water. Artificial illumination was experiment (Stenslokken et al. 2002). For the controls provided on a 12L:12D cycle. Only male adult lizards (injected saline), the same quantity of saline was injected were considered for this study to avoid influencing the as drugs in experimental treatments. reproductive status of females at this time of the year. Heart rate and temperature measurements Experimental setup and surgery In in vivo experiments, HRs were taken from electro- In in vivo experiments were conducted in a temperature- cardiograms (ECG). The ECG was recorded from 5 mm controlled black plastic container with a custom-made long stainless steel electrodes (Medtronic, France) box measuring 15·5·10 cm3, which was large enough for placed subcutaneously in the limbs. The electrodes were the animals to fit comfortably but restricting sideways connected to a computerized data-acquisition