Early-life environmental manipulations are thought to influence brain maturation and development throughout the rest of life. Childhood exposure to adversity has been shown to increase vulnerability to a variety of psychophysiological disorders in adulthood, including depression, anxiety, social impairment, and cognitive deficits(Baker et al., 2013; Saleh et al., 2017). In particular, there is a significant association between early life stress and alterations in the transmission and processing of nociceptive signals in later life (Baker et al., 2013, Caes and Roche, 2020). Early postnatal experiences resulted in significant anatomical and functional changes in brain networks involved in nociceptive modulation (Baker et al., 2013, Fan et al., 2014, VanTieghem and Tottenham, 2017).

MD is a systematic term that describes the effects of the loss of a mother on newborns and young children developing in later life. Clinical and preclinical studies have shown long-lasting negative effects of maternal deprivation on neuronal competence in later life (Fitzgerald et al., 2020, Mackes et al., 2020, Medeiros et al., 2022). MD with district protocols has been used frequently to evaluate the consequences of early life stresses on adult performances. On the other hand, in rodents, nonstressful, gentle and positive handling of pups in the late prenatal and early postnatal periods increased the behavioral performance of offspring in later life (Olsavsky et al., 2013, Raineki et al., 2014, Río-Ȧlamos et al., 2015). Neonatal handled rats indicated low anxiety-like behavior in open field as well as the elevated plus-maze tests (Kiosterakis et al., 2009, Madruga et al., 2006). In addition, neonatal handling has been associated with an elevation in the spatial learning and memory performance of adult rats (Fenoglio et al., 2005, Lehmann et al., 2002, Stamatakis et al., 2008). Nociceptive thresholds and morphine analgesia have been increased in postnatal handled mice (Sternberg & Ridgway, 2003). Nonetheless, some studies indicate that neonatal handling has a detrimental effect on neurobehavioral performance in rodents or has no effect at all (Karkow and Lucion, 2013, Madruga et al., 2006, Silveira et al., 2005, Siviy and Harrison, 2008).

Orofacial pain is a common and distressing type of pain that affects the quality of life for millions of people worldwide (Jin et al., 2016, Okeson, 2008). Orofacial pain occurs as a result of abnormal trigeminal nerve activation, which transmits nociceptive signals to second and third order neurons in the spinal trigeminal nucleus and thalamic nuclei, respectively (Olszewski, 1950, Sessle, 1999). Activation of trigeminal neurons increases the synthesis and release of pro-nociceptive and vasoactive peptides, such as substance P and calcitonin gene-related peptide (CGRP), in the brainstem nuclei and primary sensory nerves of the trigeminal ganglion (Benemei et al., 2009, Goadsby et al., 2009, Hökfelt et al., 2007).

For the study of various features of trigeminal nociception and pain-related neurobehavioral dysfunctions, experimental models of orofacial pain have been developed (Clavelou et al., 1989, Erfanparast et al., 2017, Polson and Fuji, 2012). In particular, intra-dental administration of chemical compounds like capsaicin or formalin has been shown to induce inflammatory pulpal pain in rodents. The agents stimulate primary polymodal C and A-delta nociceptive fibers that are associated with specific nociceptive criteria, including exacerbated facial rubbing, grooming, and head shaking behaviors (Chidiac et al., 2002, Rahbar et al., 2019). In addition, NTG administration evokes migraine-like profiles characterized by thermal hyperalgesia, mechanical allodynia, and c-fos over-expression in the brainstem nociceptive neurons (Farkas et al., 2016, Pradhan et al., 2014). NTG has been shown to increase cerebral meningeal blood flow and to produce photophobia (Farajdokht et al., 2016, Greco et al., 2011).

Previous studies support an association between neonatal handling and subsequent alterations in the development of nociceptive circuitries in rats’ brains. However, the effect of early life handling experiences on trigeminal pain processing has not yet been evaluated. Here, three different rat models of orofacial pain (capsaicin-induced dental pulp pain, formalin-induced trigeminal pain, and NTG-induced migraine-like headaches) were used to assess whether orofacial spontaneous pain behaviors is altered in adult rats exposed to MD stress or neonatal handling.