Ultrastructural transneuronal degeneration study of axonal elements within the paratrigeminal nucleus in sinoaortic deafferented rats.

OBJECTIVE: Morphological study that searched to authenticate the presence of sinoaortic baroreceptor inputs within the dorsolateral medullary nucleus under electron microscopy analysis. METHODS: After a 5-day survival period, 9 baroreceptor-denervated rats deeply anaesthetized with equithesin were transcardially perfused and their brains were histologically processed. RESULTS: The neuronal cytoarchitecture of the paratrigeminal nucleus comprehends afferent projections from other nuclei that have a distributive character regarding visceral and nociceptive functions in the cardiovascular reflex integration response. CONCLUSION: The medial portion of the nucleus receives afferent projections of the rostral ventrolateral medulla, as shown by retrograde neurotracing studies. The present results show that the medial extent of the paratrigeminal nucleus contains degenerated axoplasmic cellular components in sinoaortic deafferented rats. The number of degenerated axonal fibers was also larger in this area of the nucleus.

Ultrastructural transneuronal degeneration study of axonal elements within the paratrigeminal nucleus in sinoaortic deafferented rats / Estudo ultraestrutural de degeneração transneuronal dos elementos axonais do núcleo paratrigeminal de ratos com desnervação sino-aórtica

Objective: Morphological study that searched to authenticate the presence of sinoaortic baroreceptor inputs within the dorsolateral medullary nucleus under electron microscopy analysis. Methods: After a 5-day survival period, 9 baroreceptor-denervated rats deeply anaesthetized with equithesin were transcardially perfused and their brains were histologically processed. Results: The neuronal cytoarchitecture of the paratrigeminal nucleus comprehends afferent projections from other nuclei that have a distributive character regarding visceral and nociceptive functions in the cardiovascular reflex integration response. Conclusion: The medial portion of the nucleus receives afferent projections of the rostral ventrolateral medulla, as shown by retrograde neurotracing studies. The present results show that the medial extent of the paratrigeminal nucleus contains degenerated axoplasmic cellular components in sinoaortic deafferented rats. The number of degenerated axonal fibers was also larger in this area of the nucleus.

Objetivo: Estudo morfológico que buscou verificar, por meio de microscopia eletrônica, a presença de aferências de receptores sino-aórticos em núcleo localizado na região dorso-lateral bulbar. Métodos: Após 5 dias de sobrevida, 9 ratos com desnervação sinoaórtica anestesiados com equitesina foram submetidos à perfusão transcardíaca, e o encéfalo de cada um deles foi processado histologicamente. Resultados: A citoarquitetura neuronal do núcleo paratrigeminal compreende projeções aferentes de outros núcleos que apresentam uma característica distributiva em relação às funções viscerais e nociceptivas na integração do reflexo cardiovascular. Conclusão: A porção medial do núcleo recebe projeções aferentes da região rostro-ventrolateral do troncoencefálico, confirmadas por meio de estudos com rastreadores neuronais. Os resultados indicam que a região medial do núcleo paratrigeminal contém o maior número de fibras axonais degeneradas.

Discharge rate profiles of paratrigeminal nucleus neurons throughout a pressor event in non-anaesthetized rats.

Located in the lower brainstem, the paratrigeminal nucleus (Pa5) is related to cardiorespiratory autonomic reflex functions. To characterize the structures' role in blood pressure regulation and the cardiovascular reflex responses Pa5 unit activity was evaluated during a phenylephrine-produced pressor response in non-anaesthetized rats by means of simultaneous many-unit recording. Ninety five percent of the identified Pa5 responded to baroreceptor stimulation, 77% increasing and 23% decreasing firing rates. Cross-correlation analysis of neuron electrical behavior referenced to the heart beat event revealed that 65% of the featured cardiac cycle-locked rhythmic activity. The identification of neurons that change firing rates in response to increases of arterial pressure with cardiac cycle-locked rhythmic activity, further supports for a role for the nucleus in moment to moment control of blood pressure. The largest changes in firing rate occurred in the units with low resting firing rates in response to the ascending phase of the pressor event. Thus, the group displaying both cardiac cycle-locked and other rhythmic activities within the ranges of cardiac and respiratory rates or arterial pressure low frequencies, is probably the most influential regarding homoeostatic reflex responses. The findings advance the notion that the dynamic control of blood pressure involves lower brainstem integration of cardiac and respiratory reflexes.

Cardiovascular and baroreceptor functions of the paratrigeminal nucleus for pressor effects in non-anaesthetized rats.

Located in the lower brainstem, the paratrigeminal nucleus (Pa5) is related to cardiorespiratory autonomic reflex functions. To characterize the structures' role in blood pressure regulation and baroreflex response, both resting cardiovascular parameters and reflex responses were evaluated during phenylephrine-produced pressor responses in non-anaesthetized rats with or without bilateral chemical Pa5 ablation. The Pa5-ablated animals, in contrast to the Pa5-intact control animals, presented increased resting arterial pressure (115+/-4 vs. 100+/-3 mm Hg), decreased heart (293+/-10 vs. 315+/-7 bpm) and increase of the respiratory (104+/-3 vs. 94+/-5 rpm) rates, larger pressor responses and reduced baroreflex index (1.6+/-0.2 vs. 2.8+/-0.2, p<0.05). The cardiovascular changes, compatible to those produced by nucleus of the solitary tract (NTS) lesions in non-anaesthetized rats, indicate a reduction of both the sympathetic and cardiac components of the baroreflex response. Further analyses showed the Pa5 mediates reflex responses to smaller blood pressure increases, while the NTS would be predominantly active in surges over 40 mm Hg. Thus, the integrity of the Pa5 is important for resting blood pressure maintenance as for a full baroreceptor response.

Sensory sciatic nerve afferent inputs to the dorsal lateral medulla in the rat.

Investigations show the paratrigeminal nucleus (Pa5) as an input site for sensory information from the sciatic nerve field. Functional or physical disruption of the Pa5 alters behavioral and somatosensory responses to nociceptive hindpaw stimulation or sciatic nerve electrostimulation (SNS), both contralateral to the affected structure. The nucleus, an input site for cranial and spinal nerves, known for orofacial nociceptive sensory processing, has efferent connections to structures associated with nociception and cardiorespiratory functions. This study aimed at determining the afferent sciatic pathway to dorsal lateral medulla by means of a neuronal tract-tracer (biocytin) injected in the iliac segment of the sciatic nerve. Spinal cord samples revealed bilateral labeling in the gracile and pyramidal or cuneate tracts from survival day 2 (lumbar L1/L2) to day 8 (cervical C2/C3 segments) following biocytin application. From day 10 to day 20 medulla samples showed labeling of the contralateral Pa5 to the injection site. The ipsilateral paratrigeminal nucleus showed labeling on day 10 only. The lateral reticular nucleus (LRt) showed fluorescent labeled terminal fibers on day 12 and 14, after tracer injection to contralateral sciatic nerve. Neurotracer injection into the LRt of sciatic nerve-biocytin-treated rats produced retrograde labeled neurons soma in the Pa5 in the vicinity of biocytin labeled nerve terminals. Therefore, Pa5 may be considered one of the first sites in the brain for sensory/nociceptive inputs from the sciatic nerve. Also, the findings include Pa5 and LRt in the neural pathway of the somatosympathetic pressor response to SNS and nocifensive responses to hindpaw stimulation.

The role of the paratrigeminal nucleus in the pressor response to sciatic nerve stimulation in the rat.

The paratrigeminal nucleus (Pa5), an input site for spinal, trigeminal, vagus and glossopharyngeal afferents, is a recognized site for orofacial nociceptive sensory processing. It has efferent connections to brain structures associated with nociception and cardiorespiratory functions. This study aimed at determining the function of the Pa5 on the cardiovascular component of the somatosensory reflex (SSR) to sciatic nerve stimulation (SNS) in paralyzed and artificially-ventilated rats following Pa5 chemical lesions (ibotenic acid), synaptic transmission blockade (CoCl(2)), local anaesthetics (lidocaine) or desensitization of primary afferent fibers (capsaicin). The pressor response to sciatic nerve stimulation at 0.6 mA and 20 Hz (14+/-1 mm Hg) was strongly attenuated by contra- (-80%) or bilateral (-50%) paratrigeminal nucleus lesions. Ipsilateral Pa5 lesions only attenuated the response to 0.1 mA, 20 Hz SNS (-55%). Cobalt chloride or lidocaine injected in the contralateral paratrigeminal nucleus also attenuated the SSR. In capsaicin-treated animals, the pressor responses to 0.1 mA were abolished, whereas the responses to SNS at 0.6 mA were increased from 65 to 100% depending on the stimulus frequency. The paratrigeminal nucleus receives both, excitatory and inhibitory components; the later apparently involving capsaicin-sensitive fiber inputs mostly to the ipsilateral site whereas the capsaicin insensitive excitatory components that respond to high or low frequency stimulation, respectively, target the contralateral and ipsilateral sites. Thus, the paratrigeminal nucleus mediates excitatory and inhibitory components of the somatosensory reflex, representing a primary synapse site in the brain for nociceptive inputs from the sciatic innervation field.

Role of the paratrigeminal nucleus in nocifensive responses of rats to chemical, thermal and mechanical stimuli applied to the hind paw.

Anatomical and immunohystochemical data suggest the paratrigeminal nucleus (Pa5) may play a role in nociceptive processing. The current study examines the influence of unilateral Pa5 lesion on nocifensive responses of conscious rats to noxious thermal (Hargreaves test), mechanical (electronic von Frey and Randall-Selitto tests), and chemical (formalin 2.5%; 50 microl) stimuli applied to the hind paw. Lesion of the Pa5 induced by ibotenic acid did not affect the latency for radiant heat-induced withdrawal of either paw. In contrast, the mean mechanical threshold for withdrawal of the contralateral (but not ipsilateral) paw in Pa5-lesioned rats was reduced by approximately 45% and 20%, in electronic von Frey and Randall-Selitto tests, respectively, when compared to sham-operated animals. Conversely, animals with Pa5 lesions injected with formalin in the contralateral paw spent less time engaged in focused (licking, biting or scratching the injected paw) and total nocifensive behavior (i.e., focused nocifensive behavior plus protection of the injected paw during movements) in both the first and second phases of the test [ approximately 50% inhibition of each parameter during first phase (0-5 min) and at 20, 25, and 30 min of second phase, relative to the sham-operated group], but the number of paw-jerks was unaffected. Pa5 lesion also delayed the onset of second phase focused pain induced by formalin in the ipsilateral paw. The results suggest that the Pa5 integrates the supraspinal pain control system and plays a differential modulatory role in the central processing of mechanical and chemical nociceptive information.

Kinin and opioid receptors in the paratrigeminal nucleus modulate the somatosensory reflex to rat sciatic nerve stimulation.

The influence of kinin and opioid receptor blockade in the paratrigeminal nucleus (Pa5) on the somatosensory reflex (SSR) to sciatic nerve stimulation (SNS) was assessed in anaesthetized-paralyzed rats. SNS (square 1 ms pulses at 0.6 mA and 20 Hz for 10s) increased mean arterial pressure from 87+/-3 to 106+/-3 mmHg. Pressor responses to SNS were reduced 40-60% by HOE-140 and LF 16-0687 (B2 receptor antagonists; 20 and 100 pmol respectively), CTOP or nor-binaltorphimine (mu and kappa opioid receptor antagonists, respectively; 1 microg) but potentiated by naltrindole (delta opioid receptor antagonist) receptor antagonist microinjections into the contralateral (but not ipsilateral) Pa5. The SSR to sciatic nerve stimulation was not changed by B1 kinin receptor or NK1, NK2 and NK3 tachykinin receptor antagonists administered to the Pa5. Capsaicin pretreatment (40 mg/kg/day, 3 days) abolished the effects of the opioid receptor antagonists, but did not change the effect of kinin B2 receptor blockade on the SSR. Thus, the activity of B2 and opioid receptor-operated mechanisms in the Pa5 contribute to the SSR in the rat, suggesting a role for these endogenous peptides in the cardiovascular responses to SNS.

Barosensitive neurons in the rat tractus solitarius and paratrigeminal nucleus: a new model for medullary, cardiovascular reflex regulation.

The nucleus of the solitary tract (NTS), a termination site for primary afferent fibers from baroreceptors and other peripheral cardiovascular receptors, contains blood pressure-sensitive neurons, some of which have rhythmic activity locked to the cardiac cycle, making them key components of the central pathway for cardiovascular regulation. The paratrigeminal nucleus (Pa5), a small collection of medullary neurons in the dorsal lateral spinal trigeminal tract, like the NTS, receives primary somatosensory inputs of glossopharyngeal, vagal, and other nerves. Recent studies show that the Pa5 has efferent connections to the rostroventrolateral reticular nucleus (RVL), NTS, and ambiguous nucleus, suggesting that its structure may play a role in the baroreceptor reflex modulation. In the present study, simultaneous recording from multiple single neurons in freely behaving rats challenged with i.v. phenylephrine administration, showed that 83% of NTS units and 72% of Pa5 units were baroreceptor sensitive. Whereas most of the baroreceptor-sensitive NTS and Pa5 neurons (86 and 61%, respectively) increased firing rate during the ascending phase of the pressor response, about 16% of Pa5 and NTS baroreceptor-sensitive neurons had a decreased firing rate. On one hand, the decrease in firing rate occurred during the ascending phase of the pressor response, indicating sensitivity to rapid changes in arterial pressure. On the other hand, the increases in neuron activity in the Pa5 or NTS occurred during the entire pressor response to phenylephrine. Cross-correlational analysis showed that 71% of Pa5 and 93% of NTS baroreceptor-activated neurons possessed phasic discharge patterns locked to the cardiac cycle. These findings suggest that the Pa5, like the NTS, acts as a terminal for primary afferents in the medullary-baroreflex or cardiorespiratory-reflex pathways.

Bradykinin microinjection in the paratrigeminal nucleus triggers neuronal discharge in the rat rostroventrolateral reticular nucleus.

A small collection of neurons in the dorsal lateral medulla, the paratrigeminal nucleus (Pa5), projects directly to the rostroventrolateral reticular nucleus (RVL). Bradykinin (BK) microinjections in the Pa5 produce marked pressor responses. Also, the Pa5 is believed to be a component of the neuronal substrates of the somatosensory response and the baroreflex arc. Considering the developing interest in the functional physiology of the Pa5, the present study was designed to characterize RVL neuronal activity in response to BK microinjections in the Pa5 as well as to phenylephrine-induced blood pressure increases in freely behaving rats. Of the 46 discriminated RVL neurons, 82% responded with a 180% mean increase in firing rate after BK application to the paratrigeminal nucleus, before the onset of the blood pressure increase. Thirty (79%) of the RVL BK-excited neurons were baroreceptor-inhibited units that responded with a 30% decrease in firing rate in response to a phenylephrine-produced increase of blood pressure. Twenty-seven (71%) units of the latter population displayed cardiac-cycle-locked rhythmic activity. The findings demonstrate a BK-stimulated functional connection between the Pa5 and RVL that may represent the neural pathway in the BK-mediated pressor response. This pathway may be relevant to baroreflex mechanisms since it relates to cardiovascular pressure-sensitive neurons.

Baroreceptor-sensitive neurons in the rat paratrigeminal nucleus.

The paratrigeminal nucleus (Pa5) is a small collection of medullary neurons localized in the dorsal lateral spinal trigeminal tract. Electrophysiological and anatomical studies showed functional Pa5 efferent connections to the rostroventrolateral reticular nucleus (RVL) and the nucleus of the solitary tract (NTS), both well-studied components of the baroreflex arch. Similarly to the NTS, the main site for termination of cardiovascular peripheral afferents, the Pa5 receives primary sensory inputs of glossopharyngeal and vagus nerves, which suggests that the Pa5 may play a role in the baroreceptor reflex modulation. Simultaneous recording from multiple single neurons in 10 freely behaving rats showed that 37% of recorded Pa5 neurons altered firing rates (35% increased and 2% decreased) during the peak arterial blood pressure response to i.v. phenylephrine. Forty two percent of the 84 identified Pa5 baroreceptor-excited neurons showed high correlation to cardiac cycle denoting the synchronous phasicity to fast changes of blood pressure. Autocorrelation analysis revealed that 48 pressure-sensitive and 55 nonpressure-sensitive neurons have periodical activities which were not directly linked to cardiac cycle. We suggest that the Pa5, a yet unknown component of the baroreflex pathway, may relay baroreceptor information to the NTS and by passing other components of the baroreceptor reflex arch, directly to sympathetic premotor neurons in the RVL.