We observed and analyzed the success rate of two kinds of jugular vein catheterizations in a drug self-administration (SA) rat model. The success rate of the jugular vein catheterization in the Optimized Operation group (OO group) was significantly higher than that in the Traditional Operation group (TO group) (93.33% vs 46.67%, χ2 = 31.11, P < 0.001). Thus, the optimized jugular vein catheterization could make the construction of a drug self-administration model in rats more stable and reliable and was more efficient in successfully creating SA models than the traditional operation.
A good animal model serves as the basis for the study of specific mechanisms for the occurrence and development of related diseases, and it is also an important tool for scientific research, especially basic research. In animal experiments on drug addiction, a large number of studies have shown that compared with the CPP model, the SA model is more stable and more reliable, results in fewer individual differences and is considered to be one of the most effective models for studying animal drug-craving behavior . Animals used to create SA models for drug addiction mainly include rats, mice and nonhuman primates.
In the middle of the 20th century, many researchers began to perform drug-related behavioral studies with animals to better understand the human addiction process. Professor James R. Weeks used automatic intravenous injection to simulate morphine addiction in non-free-living rats in 1962 . This signified the start of studies using SA models with jugular vein catheterization. The industry has adopted its own drug delivery model as a behavioral method for studying drug addiction . In 1969, Professors Deneau, Yanagita, and Seevers conducted a study on drug abuse for free-living cynomolgus monkeys to explore whether they would perform voluntary and active self-administration, which was much closer to the state observed in human addiction .
Under the SA training procedure, when the experimental animal performs a response, such as performing a valid nose-poke or pressing a valid lever, a certain dose of drug is delivered via an intravenous catheter. However, the typical operation for creating the SA model is relatively complicated, and its technical requirements are steep. In a SA model in free-moving rats, it is most important to ensure that the jugular vein is unobstructed, not leaking, and is free of infection and other problems during the whole training process. The most challenging and critical technique is the jugular vein catheterization .
The traditional tube operation primarily includes a PE tube (the extravascular part), a silicone tube placed into the blood vessel, and a fixing point created by creating a knot at the connection between the two sections. However, due to the instability of the structure itself, the possibility of leaking at the joint is high [13, 14]. This method has many drawbacks, and it is prone to problems such as blockage, leakage, bleeding, infection, significant weight loss, and even death. If any of the above problems occurs, the jugular vein catheterization would fail, directly and negatively impacting the construction of additional animal models .
There are two main reasons for the low success rate of the traditional jugular vein catheterization surgery: 1) The components of the implanted catheter are complex in structure, the flexibility and strength are not suitable for long-term implantation, and the two components are not perfectly matched, easily resulting in blockage or leakage; 2) the surgical operation is not standard or rigorous, resulting in an unsuccessful implantation.
In response to the above problems, our experiment made several following improvements in the materials and techniques used for performing jugular vein catheterization to explore the feasibility and effectiveness of a more optimized scheme:
1) Uniformity: The entire tube is composed of soft silicone, which eliminates the need for a secondary joint. The possibility of leakage is also decreased, and the soft silicone tube has little effect on the neck flexibility of the rats. The traditional catheter, however, uses a PE tube (the extravascular part) and a silicone tube (placed in the blood vessel), which may increase the possibility of leakage at the joint.
2) Individualized variability of the S-type positioning nail: the S-type positioning nail is made from a common stapler nail. In our opinion, this is the most important and satisfying improvement, because we could adjust the position of the fixed site according to our actual needs after adding the small S-type positioning nail. The traditional method uses one knot directly at the connection between the two parts. The stability of this knot is not as good as a solid knot in a rope. If the knot is too tight, the catheter could be easily blocked. If it is too loose, the tube could easily be disengaged or affect the flexibility of the neck. The most serious consequence would be dragging of the silicone tube placed in the vein. In addition, the proposed operation might be relatively easy because of the use of the S-type positioning nail, which could improve the surgical efficiency.
3) The length matching of the implanted catheter: it seems impossible to prepare precise implanted catheters in advance due to individual differences in the experimental rats. In our optimized materials, because of the S-type positioning nail, both the length of the catheter and the position of the fixed site could be adjusted according to our actual needs. However, the traditional materials cannot be freely adjusted due to the existence of a uniform fixed position, so it is impossible to match the implantation perfectly for each rat.
4) The implanted blood vessel portion of the catheter: The length of the implanted blood vessel portion of the optimized catheter is approximately 4.5 cm. At this length, the end of the catheter just reaches the right atrium, which can reduce direct damage to blood vessels. To some extent, the length of the implanted portion also reduces the risk of blockage and leakage. However, in the traditional procedure, the implanted blood vessel portion is generally 1.2 cm in length, increasing the chance of damaging blood vessels and leading to thrombi and blockage.
5) The shape of the end of the implanted catheter: In our procedure, the end is cut into the shape of a fish mouth shape so that the implanted catheter cannot easily adhere to the vessel wall and damage it. However, in the traditional procedure, the end of the implanted catheter has a single inclined surface (similar to a syringe needle), making it easy to damage the blood vessels and ultimately resulting in leakage.
6) The connection between the jugular vein catheter and the drug self-administration device: in our procedure, an M4 threaded base with an internal bore is used to connect the jugular vein catheter to the drug self-administration device. The M4 threaded base is small, the base is completely embedded in the back of the experimental rat, and the upper thread is only partially exposed. The connection between the upper thread portion and the drug self-administration device is stable and cannot easily fall off, and it does not affect the flexibility of the rats. In other studies, experimental rats needed a small back clip to connect the jugular vein catheter to the drug self-administration device. However, free-moving rats sometimes easily shake off the clip because it makes them uncomfortable. Furthermore, in the traditional operation, it is easier to drag the jugular vein catheter, which eventually leads to failure of the model .
7) Low cost of materials: the optimized jugular vein catheters are mainly composed of several simple and inexpensive components. It should be emphasized that simplicity, low cost and high efficiency have been effectively unified due to the application of the S-type positioning nail.
The success of the jugular vein catheterization is the most important factor for the SA model. Our results showed that the optimized jugular vein catheterization for rats was significantly better than the traditional procedure. The success rate of jugular vein catheterization was significantly improved after optimization.
In summary, the construction of scientific and efficient animal models is the basis for basic scientific research. The SA model represents one of the oldest methods for studying drug addiction, but it will continue to provide very valuable contributions to research on drug addiction well in the future. Jugular vein catheterization is one of the most important and critical techniques in this animal model. In the present study, we provided numerous details, including materials, approaches, updated techniques and protocols, for the development of a more optimal SA model. Our experiment explored an optimized scheme for animal jugular vein catheterization for the creation of a drug self-administration model, which could provide a more stable and efficient tool for basic research on drug addiction.