Some things are better left unexplored. What's next? Are scientists going to put other human parts into animals to try to make them more like people? If God wanted animals to be people, he would have simply made them as such. I wonder how many millions of dollars "scientists" spent on unethical experiments when they could have been trying to save lives. Millions of people still die from cancer ever day, but hey, Joe's brains can make a rat think better. Awesome research, if it even occurred.
This is the old, Everyone was so focused on if we could, that no one stopped to think about if we should. This kind of science is going to spiral out of control. Humans are the superior race, and as we try to combine humanity with other creatures and attempt to bring them closer, then we are denying our gift and wasting it on beasts. Soon enough(and already) people are going to want to give non-humans personhood.
True hybrids may be rare, but lab-made chimeras, in which cells from two strains or species coexist in a single organism, are utterly ubiquitous. To engineer the genome of a mouse—the starting point for many biomedical experiments—scientists must generate a chimeric blend of different breeds. The word hybrid, in the scientific sense, describes an organism that carries DNA from different species in each of its cells. Human-animal combos of this kind have indeed been created by several different labs—though only at the very early stages of development, and never in the hopes of making full-grown, crossbred monsters.
Human cells have been used to test things from the effect cancer has to testing new medicines. A couple human brain cells can't really rewire the way a mouse's brain works and make it "more conscious" than it is already. At best, they may improve memory and cognitive ability in the host mouse. At worst, nothing changes.
If the neither the human, nor the mouse were harmed by the experiment it's facially not morally objectionable. What if this leads to a breakthrough in treating Alzheimer's disease or dimentia? Scientific ethics are complicated, especially since some of medicine's greatest discoveries may not have been found during ethical procedures or experiments. Henrietta Lacks is a prime example of this, as using someone's cells to make a profit without their permission or without compensating them seems to happen often.
...If you replace the brain cells in an animal brain with human ones at some point you essentially have a human being in an animal's body, since the significance of a human being and a human being's rights come from our brains.
It's hard to know where that line would be crossed, just that it hasn't been crossed yet. If it's just a few cells and not replacing the entire brain then it may improve the animal's ability to learn but the animal is still not quite at the cognitive level to say that it should be afforded the same rights as a human.
If you do replace the entire or almost entire brain with human cells resulting in an animal whose intelligence is the same as a human then it wouldn't necessarily be unethical any more than artificial insemination is unethical. But the test subject would have to be afforded human rights. If it's as intelligent as a child then we would have to avoid causing any significant pain or suffering and give it freedom comparable to that you would give a child. If it's as intelligent as an adult then it should get the right to sign contracts and to vote. In some ways we'd ethically be able to do more with such a specimen if it was as intelligent as an adult since there are things it would be able to give informed consent to in exchange for money that couldn't be done ethically if the animal were as intelligent as a child.
Science has not crossed the ethical line yet, because these scientific research projects are aimed at improving the health and quality of life for humans. Science will not likely cross any ethical lines as long as scientists seek to use their discoveries for good purposes. We should not worry so much about scientific research.