Covariant maturation of nocifensive oral behaviour and c-fos expression in rat superior colliculus

Lamina I NK1 expressing projection neurones are functional in early postnatal rats and contribute to the setting up of adult mechanical sensory thresholds

BACKGROUND

A small proportion of lamina I neurons of the spinal cord project upon the hindbrain and are thought to engage descending pathways that modulate the behavioural response to peripheral injury. Early postnatal development of nociception in rats is associated with exaggerated and diffuse cutaneous reflexes with a gradual refinement of responses over the first postnatal weeks related to increased participation of inhibitory networks. This study examined the postnatal development of lamina I projection neurons from postnatal day 3 (P3) until P48.

RESULTS

At P3, a subset of lamina I neurons were found to express the neurokinin 1 (NK1) receptor. Using fluorogold retrograde tracing, we found that the NK1 positive neurons projected upon the parabrachial nucleus (PB) within the hindbrain. Using c-fos immunohistochemistry, we showed that lamina I and PB neurons in P3 rats responded to noxious stimulation of the periphery. Finally, ablation of lamina I neurons with substance-P saporin conjugates at P3 resulted in increased mechanical sensitivity from P45 onwards compared to control animals of the same age.

CONCLUSIONS

These results suggest that the lamina I pathway is present and functional at least from P3 and required for establishing and fine-tuning mechanical sensitivity in adult rats.

Formalin-induced c-fos expression in the brain of infant rats

In the fetal, infant, and adult rat, injury induces a well-defined behavioral response and induces c-fos expression in the spinal cord dorsal horn. There is more limited information about the processing of noxious stimulation in the infant brain. We describe here the appearance of the Fos protein in the brain of fetal and infant rats following formalin-induced injury. Regions were chosen for analysis with a special focus on brain loci that express c-fos in the adult. No Fos positive cells were found in the brains of fetuses; newborns did not show increased Fos expression after formalin injection in any structure examined. At 3 and 14 days of age, there was a significant increase in Fos staining induced by formalin in the ventral lateral medulla. In contrast, paraventricular and medial dorsal nuclei of the thalamus, the paraventricular nucleus of the hypothalamus, and periaqueductal gray of the midbrain showed increased levels of Fos protein only at 14 days of age. We hypothesize that this developmental pattern is related not only to the maturation of pain perception but also to development of autonomic and defensive reactions to pain in the infant.

PERSPECTIVE

Because the infant processes pain differently than the adult, knowledge of those differences informs pediatric clinical practice. Using Fos expression as a marker of neural activity in the rat, we show that the pattern of brain activation is immature at birth but is in place by 14 days of age.

Vibrissa sensation in superior colliculus: wide-field sensitivity and state-dependent cortical feedback

Rodents use their vibrissae (whiskers) to sense and navigate the environment. A main target of this sensory information is the superior colliculus in the midbrain, which rats can use to detect meaningful whisker stimuli in behavioral contexts. Here, we used field potential, single-unit, and intracellular recordings to show that, although cells in the intermediate layers of the superior colliculus respond relatively effectively to single whiskers, the cells respond much more robustly to simultaneous, or nearly simultaneous, wide-field (multiwhisker) stimuli. The enhanced multiwhisker response is temporally stereotyped, consisting of two short latency peaks caused by convergent trigeminal synaptic inputs and cortical feedback, respectively. The cells are highly sensitive to the degree of temporal dispersion and contact order of multiwhisker stimuli, which makes them excellent detectors of initial multiwhisker contact. In addition, their output is most robust during quiescent states because of the dependence of cortical feedback on forebrain activation, and this may serve as an alerting signal to drive orienting responses.

Light-dependent retinal innervation of the rat superior colliculus

Mammalian retinal projections are divided into two anatomically and functionally distinct systems: the primary visual system, which mediates conscious visual processing, and the subcortical visual system, which mediates nonconscious responses to light. Light deprivation during a critical period in development alters the anatomy, physiology, and function of the primary visual system in many mammalian species. However, little is known about the influence of dark-rearing on the development of the subcortical visual system. To evaluate whether the early lighting environment alters the anatomy of the subcortical visual system, we examined the retinas and retinofugal projections of rats reared in a 12:12 light/dark cycle or in constant dark from birth to 4 months of age. We found that dark-rearing was associated with a reduction in the distribution of retinal fibers in the stratum opticum of the contralateral superior colliculus. In contrast to the plasticity of the retinocollicular projection, retinal input to sleep, circadian, and pupillary control centers in the hypothalamus, pretectum, and lateral geniculate complex was unaffected by dark-rearing. A decrease in retinal innervation of the stratum opticum and intermediate layers of the superior colliculus may account for some of the deficits in multisensory integration that have been observed in dark-reared animals of several species.

The orofacial formalin test

The subcutaneous injection of formalin into the rat upper lip generates behavioral responses that last several minutes. The time course of the response is similar to what is observed following formalin injection into the paw, i.e. biphasic, with an early and short-lasting first phase followed, after a quiescent period by a second, prolonged (tonic) phase. The applied chemical stimulus (formalin) can be qualified as noxious since it produces tissue injury, activates Adelta and C nociceptors as well as trigeminal and spinal nociceptive neurons and is felt as painful in man. In addition, increasing the concentration of formalin causes a parallel aggravation of histological signs of tissue inflammation and injury. The measured behavioral response (face rubbing) is a relevant end-point: prolonged face rubbing is evoked by formalin but not saline injection and a positive relationship between the amplitude of the response and the formalin concentration is observed, at least up to 2.5%. At higher formalin concentrations, the use of other or additional end-points should be considered. Finally, the behavioral response in the orofacial formalin test is sensitive to various opioid and non-opioid analgesics. The orofacial formalin test can then be considered as a reliable way of producing and quantifying nociception in the trigeminal region of the rat.

Immunocytochemical localization of extracellular signal-regulated kinases 1 and 2 phosphorylated neurons in the brainstem of rat following visceral noxious stimulation

The aim of the present study was to determine the activation of the extracellular signal-regulated kinases (ERKs) 1 and 2 in brainstem neurons following noxious visceral stimulation. Ether and urethane anaesthetized rats received an intraperitoneal injection of acetic acid (ENS, UNS) or were left untreated (ECT, UCT). Paraffin embedded brain sections immunoreacted with an antibody specific for phosphorylated ERKs. In noxious stimulated rats ERKs activated neuron profiles in the periaqueductal gray matter, parabrachial, dorsal raphe, solitary tract nucleus, area postrema and superior colliculus suggest that ERKs activation takes place mainly in brainstem nuclei in which nociception and visceral activities interact. The comparison between ENS and UNS rats shows that the long acting anaesthetic urethane attenuates the number of the ERKs activated neurons compared to the short acting ether.