Can baby rats really feel pain?
Rats are born in what is considered a 'altricial' state. This means that they are born mostly naked, have closed eyes and ears, and require weeks of care before being fully developed. During the neonatal state (the first 28 days of a rodent's life), rapid development occurs. By ten days, the fur has started to come in. At twelve to fourteen days hearing begins (Geal-Dor et al, 1993), Between fourteen to sixteen days, the eyes open.
With this in mind, the altricial state of rodents means that, developmentally, their nervous systems are further behind (for example, humans are born being able to hear and see, whereas this does not happen until nearly two weeks post birth in rats). The nervous system is responsible for taking sensory input and converting it to the correct response - this includes vision, hearing, and touch. Sensory neurones are responsible for transmitting all sensory input from the outside world to correctly produce a response in the animal (i.e. such as dilation of pupils at light, or the turning of ears to sound).
During development, neuronal stem cells (the precursors to neurons) undergo a process known as differentiation. This process essentially "determines" what job the neuron is destined to do (i.e., a sensory or motor neuron). It has been shown that the differentiation of neuronal stem cells to neurons in rats does not occur until two to three weeks post birth (Zelena & Soukup, 1977 and Milburn, 1973). With this in mind, the underdeveloped neuronal stem cells do not have a distinct purpose, and cannot differentiate between sensory information. For instance, touch versus pressure cannot be distinguished as the signals to transmit from the neuronal stem cells to the brain do not develop until the neurons are formed.
With this in mind, until around the age of 10 to 14 days, a neonatal rat cannot percieve pain. It is physically incapable of determining the pain due to the lack of neurones present.
Due to the fact that neurogenesis is still active in the first two weeks of postnatal life, rat pups have the potential to sustain injury and developmental issues if proper care is not taken as they can hurt themselves without realizing (Zeiss, 2021). Figure I depicts an interesting timescale to put this into perspective a tad.
So, a TLDR: rats are born with nerves technically, but they are underdeveloped.
*Please note: The Zelena and Soukup paper is a bit graphic in detail for experiments. please read at your discretion*
Sources
Comparative Milestones in Rodent and Human Postnatal Central Nervous System Development - Caroline J. Zeiss, 2021 (DOI: 10.1177/01926233211046933)
The Early Development of Muscle Spindles in the Rat - Alice Milburn, 1973 (DOI: 10.1242/jcs.12.1.175)
The development of Golgi tendon organs - Zelena and Soukup, 1977 (DOI: 10.1007/BF01261504)
Changing numbers of neuronal and non-neuronal cells underlie postnatal brain growth in the rat - Banderia, Lent, and Herculano-Houzel, 2009 (DOI: 10.1073/pnas.0804650106)
Development of hearing in neonatal rats: Air and bone conducted ABR thresholds - Geal-Dor, Freeman, Li, and Sohmer, 1993 (DOI: 10.1016/0378-5955(93)90113-f)
The early development of muscle spindles in the rat - Alice Milburn, 1973 (DOI: 10.1242/jcs.12.1.175)