Rats make abundant 50kHz ultrasonic vocalizations (USVs) when they play and exhibit other positive social interactions. This response can be dramatically increased by tickling animals, especially when directed toward bodily areas toward which animals direct their own play solicitations (e.g., nape of the neck). The analysis of this system indicates that the response largely occurs in positive, playful social situations, and may index willingness for social engagement, similar to human infantile laughter, which may mature into productive adult socio-sexual behaviors. There are now enough formal similarities between rat 50kHz USVs and human laughter, to realistically hypothesize that they are neurally and functionally homologous at the subcortical level of brain organization. [Panksepp J. Neuroevolutionary sources of laughter and social joy: Modeling primal human laughter in laboratory rats.You can watch a short video of Panksepp tickling a rat here:
Behav Brain Res. 2007 Feb 20.]
There are ethical implications regarding the use of rats in science and our dealings with them generally that stem from Panksepp's work. Panksepp's own decision to continue using them in ways that harm them could be an example of focal bilateral damage to his ventromedial prefrontal cortex. He began writing about the fact that rats makes these 50kHz ultrasonic vocalizations in about 1998. He began speculating that this is laughter in 2000. (Panksepp J, Burgdorf J. 50-kHz chirping (laughter?) in response to conditioned and unconditioned tickle-induced reward in rats: effects of social housing and genetic variables. Behav Brain Res. 2000 Oct;115(1):25-38.)
Since then, he has concurrently argued that rats laugh and has been torturing rats (here, here, and here. He argues that "the bedrock of emotional feelings is contained within the evolved emotional action apparatus of mammalian brains."
There is something twisted about vivisectors. They argued for decades that animals could be treated anyway whatsoever because they didn't have emotions. Now that they've been admitting (and claiming that it's they who made these "discoveries") that animals have emotional feelings, they claim that this is justification for experimenting on them.