Normal circadian rhythms of behavior are disrupted in disorders involving the dopamine (DA) system, such as Parkinson's disease. We have reported previously using unilateral injections of the catecholamine toxin, 6-hydroxydopamine (6-OHDA), into the medial forebrain bundle that DA signaling regulates daily expression of the clock protein, PERIOD2 (PER2), in the dorsal striatum of the rat. In the present study, we made widespread lesions of DA fibers using large injections of 6-OHDA into the third ventricle to determine the involvement of DA in normal daily rhythms of wheel-running activity and PER2 patterns in the suprachiasmatic nucleus (SCN) and several regions of the limbic forebrain. Rats injected with 6-OHDA and housed in constant darkness were less active in the wheel and showed a disorganized pattern of activity in which wheel running was not confined to a specific phase over 24 h. The 6-OHDA injection had no effect on the daily PER2 pattern in the SCN, but blunted the normal rise in PER2 in the dorsal striatum. 6-OHDA also blunted PER2 expression in the periventricular nucleus of the hypothalamus, a region in which a daily PER2 pattern has not been previously reported in male rats, and in the oval nucleus of the bed nucleus of the stria terminalis, but not in the central nucleus of the amygdala. These results indicate that DA plays a prominent role in regulating circadian activity at both behavioral and molecular levels.

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Intraperitoneal injection is a common route for parenteral administration of drugs in rodents. A serious consequence associated with this technique, however, is the puncture of vital organs such as the cecum, which causes pain and occasionally peritonitis. Reports have described the cecum as located on either side of the lower abdominal cavity, contributing to the idea that intraperitoneal injections can be performed in either side. The authors investigated the location of the cecum in adult male and female albino and pigmented rat strains, and evaluated the consequences of intraperitoneal injections in the right and left portion of the lower abdomen. Of the rats they investigated, 71.8% had ceca on the left side of the abdomen. The authors also found that injections on the left side were more likely to result in punctured ceca.Intraperitoneal (i.p.) injection is a common route for parenteral administration of drugs in laboratory rodents. The i.p. technique allows for the deposition of substances into the peritoneal cavity, which is formed by the peritoneum, a serous membrane that covers the internal walls of the abdominal cavity (parietal peritoneum) and most of the abdominal organs (visceral peritoneum). Under normal circumstances the peritoneum is transparent and smooth, functioning to prevent friction between the moving viscera, such as the intestines. The peritoneum also maintains the abdominal organs in place and acts as a medium for their blood and lymphatic vessels.In rats, substances deposited into the peritoneal cavity are absorbed by vessels such as the colic, the intestinal, and the mesenteric veins, which converge into the anterior mesenteric vein that carries blood via the portal system into the liver 1 . After being metabolized in the liver, i.p.-injected substances are carried with deoxygenated blood by the inferior cava vein into the right atrium of the heart. The pulmonary artery distributes this blood to the lungs to be oxygenated, returning via the pulmonary vein into the left atrium and then to the left ventricle and finally into the circulatory system at large. Despite this seemingly lengthy process, the speed of absorption following i.p. injections is between 25% and 50% the speed of intravenous (i.v.) injection 2 . For this reason, i.p. injections are preferred over i.v. injections in rats when the breakdown of drugs in the liver is not an issue for the research.The i.p. technique is simple and does not require much training. Rats can be injected daily over the course of several days (for 3-4 weeks, for example), and with the proper precautions this daily procedure does not cause any serious complications. However, even a single poorly administered i.p. injection can have adverse consequences, including lesions of the internal organs and peritonitis (inflammation of the peritoneum). This is not only painful for the rats, but can lead to death if bacteria from the intestines enter the circulatory system, causing bacteremia and septic shock.Given the potential for complications from i.p. in...

The effects of the opioid antagonist naloxone were examined on the development of conditioned partner preference induced by paced copulation in female rats. In Experiment 1, ovariectomized, hormone-primed rats were conditioned to associate scented and unscented male rats with paced and nonpaced copulation, respectively. Female rats in Experiment 2 associated albino or pigmented male rats with paced or nonpaced copulation. Naloxone or saline was administered before each conditioning trial. During a final drug-free preference test, female rats could choose to copulate with either a pacing related or unrelated male. Saline-trained female rats in the paired group copulated preferentially with the pacing-related male rat, whereas naloxone-trained female rats did not show a preference. The authors concluded that opioids mediated the conditioned partner preference induced by paced copulation.

The effects of the dopamine receptor antagonist flupenthixol were examined on the development of conditioned partner preference induced by paced copulation in female rats. In Experiment 1, ovariectomized, hormone-primed rats were conditioned to associate scented and unscented males with paced and nonpaced copulation, respectively. Females in Experiment 2 associated albino or pigmented males with paced or nonpaced copulation. Flupenthixol or saline was administered before each conditioning trial. During a final drug-free preference test, females could choose to copulate with either a pacing-related or nonpacing-related male. Saline-trained females copulated preferentially with the pacing-related male, whereas flupenthixol disrupted odor but not strain conditioning. The role of dopamine in conditioned partner preference depends on the type of stimuli to be learned.