Baroreceptor-sensitive neurons in the rat paratrigeminal nucleus

Evidence for glial reactivity using positron-emission tomography imaging of translocator Protein-18 kD [TSPO] in both sham and nerve-injured rats in a preclinical model of orofacial neuropathic pain

Chronic neuropathic pain is a debilitating condition that results from damage to the nervous system. Current treatments are largely ineffective, with limited understanding of the underlying mechanisms hindering development of effective treatments. Preclinical models of neuropathic pain have revealed that non-neural changes are important for the development of neuropathic pain, although these data are derived almost exclusively from post-mortem histological analyses. Although these static snapshots have provided valuable data, they cannot provide insights into non-neural cell changes that could be also assessed in human patients with chronic pain. In this study we used translocator protein 18 kDa (TSPO) PET imaging with [18F]PBR06 to visualise in-vivo, the activity of macrophages and microglia in a rodent preclinical model of trigeminal neuropathic pain. Using chronic constriction injury of the infraorbital nerve (ION-CCI) we compared temporal changes in TSPO binding in male rats, prior to, and up to 28 days after ION-CCI compared with both sham-injured and naïve counterparts. Unexpectedly, we found significant increases in TSPO signal in both ION-CCI and sham-injured rats within the trigeminal ganglion, spinal trigeminal nucleus and paratrigeminal nucleus during the initial phase following surgery and/or nerve injury. This increased TSPO binding returned to control levels by day 28. Qualitative histological appraisal of macrophage accumulation and glial reactivity in both ION-CCI and sham-injured rats indicated macrophage accumulation in the trigeminal ganglion and microglial reactivity in the brainstem trigeminal complex. These findings show, glial changes in the peripheral nerve and brain in both nerve-injured and sham-injured rats in a preclinical model of neuropathic pain which provides a platform for translation into human patients.

Cardiovascular autonomic effects of transcutaneous auricular nerve stimulation via the tragus in the rat involve spinal cervical sensory afferent pathways

BACKGROUND

Electrical stimulation on select areas of the external auricular dermatome influences the autonomic nervous system. It has been postulated that activation of the Auricular Branch of the Vagus Nerve (ABVN) mediates such autonomic changes. However, the underlying neural pathways mediating these effects are unknown and, further, our understanding of the anatomical distribution of the ABVN in the auricle has now been questioned.

OBJECTIVE

To investigate the effects of electrical stimulation of the tragus on autonomic outputs in the rat and probe the underlying neural pathways.

METHODS

Central neuronal projections from nerves innervating the external auricle were investigated by injections of the transganglionic tracer cholera toxin B chain (CTB) into the right tragus of Wistar rats. Physiological recordings of heart rate, perfusion pressure, respiratory rate and sympathetic nerve activity were made in an anaesthetic free Working Heart Brainstem Preparation (WHBP) of the rat and changes in response to electrical stimulation of the tragus analysed.

RESULTS

Neuronal tracing from the tragus revealed that the densest CTB labelling was within laminae III-IV of the dorsal horn of the upper cervical spinal cord, ipsilateral to the injection sites. In the medulla oblongata, CTB labelled afferents were observed in the paratrigeminal nucleus, spinal trigeminal tract and cuneate nucleus. Surprisingly, only sparse labelling was observed in the vagal afferent termination site, the nucleus tractus solitarius. Recordings made from rats at night time revealed more robust sympathetic activity in comparison to day time rats, thus subsequent experiments were conducted in rats at night time. Electrical stimulation was delivered across the tragus for 5 min. Direct recording from the sympathetic chain revealed a central sympathoinhibition by up to 36% following tragus stimulation. Sympathoinhibition remained following sectioning of the cervical vagus nerve ipsilateral to the stimulation site, but was attenuated by sectioning of the upper cervical afferent nerve roots.

CONCLUSIONS

Inhibition of the sympathetic nervous system activity upon electrical stimulation of the tragus in the rat is mediated at least in part through sensory afferent projections to the upper cervical spinal cord. This challenges the notion that tragal stimulation is mediated by the auricular branch of the vagus nerve and suggests that alternative mechanisms may be involved.

The greater occipital nerve and its spinal and brainstem afferent projections: A stereological and tract-tracing study in the rat

The neuromodulation of the greater occipital nerve (GON) has proved effective to treat chronic refractory neurovascular headaches, in particular migraine and cluster headache. Moreover, animal studies have shown convergence of cervical and trigeminal afferents on the same territories of the upper cervical and lower medullary dorsal horn (DH), the so-called trigeminocervical complex (TCC), and recent studies in rat models of migraine and craniofacial neuropathy have shown that GON block or stimulation alter nociceptive processing in TCC. The present study examines in detail the anatomy of GON and its central projections in the rat applying different tracers to the nerve and quantifying its ultrastructure, the ganglion neurons subserving GON, and their innervation territories in the spinal cord and brainstem. With considerable intersubject variability in size, GON contains on average 900 myelinated and 3,300 unmyelinated axons, more than 90% of which emerge from C₂ ganglion neurons. Unmyelinated afferents from GON innervates exclusively laminae I-II of the lateral DH, mostly extending along segments C_2-3 . Myelinated fibers distribute mainly in laminae I and III-V of the lateral DH between C₁ and C₆ and, with different terminal patterns, in medial parts of the DH at upper cervical segments, and ventrolateral rostral cuneate, paratrigeminal, and marginal part of the spinal caudal and interpolar nuclei. Sparse projections also appear in other locations nearby. These findings will help to better understand the bases of sensory convergence on spinomedullary systems, a critical pathophysiological factor for pain referral and spread in severe painful craniofacial disorders.

Vagal Afferent Innervation of the Airways in Health and Disease

Vagal sensory neurons constitute the major afferent supply to the airways and lungs. Subsets of afferents are defined by their embryological origin, molecular profile, neurochemistry, functionality, and anatomical organization, and collectively these nerves are essential for the regulation of respiratory physiology and pulmonary defense through local responses and centrally mediated neural pathways. Mechanical and chemical activation of airway afferents depends on a myriad of ionic and receptor-mediated signaling, much of which has yet to be fully explored. Alterations in the sensitivity and neurochemical phenotype of vagal afferent nerves and/or the neural pathways that they innervate occur in a wide variety of pulmonary diseases, and as such, understanding the mechanisms of vagal sensory function and dysfunction may reveal novel therapeutic targets. In this comprehensive review we discuss historical and state-of-the-art concepts in airway sensory neurobiology and explore mechanisms underlying how vagal sensory pathways become dysfunctional in pathological conditions.

Repetitive Diving in Trained Rats Still Increases Fos Production in Brainstem Neurons after Bilateral Sectioning of the Anterior Ethmoidal Nerve

This research was designed to investigate the role of the anterior ethmoidal nerve (AEN) during repetitive trained diving in rats, with specific attention to activation of afferent and efferent brainstem nuclei that are part of this reflexive response. The AEN innervates the nose and nasal passages and is thought to be an important component of the afferent limb of the diving response. Male Sprague-Dawley rats (N = 24) were trained to swim and dive through a 5 m underwater maze. Some rats (N = 12) had bilateral sectioning of the AEN, others a Sham surgery (N = 12). Twelve rats (6 AEN cut and 6 Sham) had 24 post-surgical dive trials over 2 h to activate brainstem neurons to produce Fos, a neuronal activation marker. Remaining rats were non-diving controls. Diving animals had significantly more Fos-positive neurons than non-diving animals in the caudal pressor area, ventral medullary dorsal horn, ventral paratrigeminal nucleus, nucleus tractus solitarius, rostral ventrolateral medulla, Raphe nuclei, A5, Locus Coeruleus, and Kölliker-Fuse area. There were no significant differences in brainstem Fos labeling in rats diving with and without intact AENs. Thus, the AENs are not required for initiation of the diving response. Other nerve(s) that innervate the nose and nasal passages, and/or suprabulbar activation of brainstem neurons, may be responsible for the pattern of neuronal activation observed during repetitive trained diving in rats. These results help define the central neuronal circuitry of the mammalian diving response.

Xenobiotic pulmonary exposure and systemic cardiovascular response via neurological links

The cardiovascular response to xenobiotic particle exposure has been increasingly studied over the last two decades, producing an extraordinary scope and depth of research findings. With the flourishing of nanotechnology, the term "xenobiotic particles" has expanded to encompass not only air pollution particulate matter (PM) but also anthropogenic particles, such as engineered nanomaterials (ENMs). Historically, the majority of research in these fields has focused on pulmonary exposure and the adverse physiological effects associated with a host inflammatory response or direct particle-tissue interactions. Because these hypotheses can neither account entirely for the deleterious cardiovascular effects of xenobiotic particle exposure nor their time course, the case for substantial neurological involvement is apparent. Indeed, considerable evidence suggests that not only is neural involvement a significant contributor but also a reality that needs to be investigated more thoroughly when assessing xenobiotic particle toxicities. Therefore, the scope of this review is several-fold. First, we provide a brief overview of the major anatomical components of the central and peripheral nervous systems, giving consideration to the potential biologic targets affected by inhaled particles. Second, the autonomic arcs and mechanisms that may be involved are reviewed. Third, the cardiovascular outcomes following neurological responses are discussed. Lastly, unique problems, future risks, and hurdles associated with xenobiotic particle exposure are discussed. A better understanding of these neural issues may facilitate research that in conjunction with existing research, will ultimately prevent the untoward cardiovascular outcomes associated with PM exposures and/or identify safe ENMs for the advancement of human health.

Distinct brainstem and forebrain circuits receiving tracheal sensory neuron inputs revealed using a novel conditional anterograde transsynaptic viral tracing system

Sensory nerves innervating the mucosa of the airways monitor the local environment for the presence of irritant stimuli and, when activated, provide input to the nucleus of the solitary tract (Sol) and paratrigeminal nucleus (Pa5) in the medulla to drive a variety of protective behaviors. Accompanying these behaviors are perceivable sensations that, particularly for stimuli in the proximal end of the airways, can be discrete and localizable. Airway sensations likely reflect the ascending airway sensory circuitry relayed via the Sol and Pa5, which terminates broadly throughout the CNS. However, the relative contribution of the Sol and Pa5 to these ascending pathways is not known. In the present study, we developed and characterized a novel conditional anterograde transneuronal viral tracing system based on the H129 strain of herpes simplex virus 1 and used this system in rats along with conventional neuroanatomical tracing with cholera toxin B to identify subcircuits in the brainstem and forebrain that are in receipt of relayed airway sensory inputs via the Sol and Pa5. We show that both the Pa5 and proximal airways disproportionately receive afferent terminals arising from the jugular (rather than nodose) vagal ganglia and the output of the Pa5 is predominately directed toward the ventrobasal thalamus. We propose the existence of a somatosensory-like pathway from the proximal airways involving jugular ganglia afferents, the Pa5, and the somatosensory thalamus and suggest that this pathway forms the anatomical framework for sensations arising from the proximal airway mucosa.

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.

Modulation of paratrigeminal nociceptive neurons following temporomandibular joint inflammation in rats

To evaluate the involvement of paratrigeminal nucleus (Pa5) nociceptive neurons in temporomandibular joint (TMJ) inflammation-induced pain and its autonomic correlates, we conducted behavioral, single unit recording and Fos immunohistochemical studies in anesthetized rats. Nocifensive behaviors to mechanical, heat or cold stimulation of the lateral face over the TMJ region were significantly enhanced in the TMJ-inflamed rats for 10-14 days after injection of complete Freund's adjuvant (CFA) into the TMJ and gradually decreased at the end of the 14-day observation period. Lowering of the nocifensive threshold in TMJ-inflamed rats lasted longer in vagus nerve-transected rats than vagus nerve-intact rats. A large number of Fos-like immunoreactive (LI) cells were observed in the Pa5, and half of them were retrogradely labeled with Fluorogold (FG) injected into the parabrachial nucleus. Background activity of Pa5 wide dynamic range and nociceptive specific neurons was significantly higher in the TMJ-inflamed rats when compared with controls. Responses to mechanical stimuli were significantly higher in NS neurons in the TMJ-inflamed rats. All thermal responsive Pa5 neurons were exclusively sensitive to cold and the response to cold was significantly higher in the TMJ-inflamed rats compared with control rats. Vagus nerve stimulation significantly decreased responses to mechanical and cold stimuli as well as the background activity in TMJ-treated rats but not in TMJ-untreated rats. The present findings suggest that populations of Pa5 neurons are nociceptive and involved in TMJ inflammation-induced pain as well as in autonomic processes related to TMJ pain.

Visceral and orofacial somatic afferent fiber terminals converge onto the same neuron in paratrigeminal nucleus: An electron microscopic study in rats

The paratrigeminal nucleus (Pa5) receives visceral sensory inputs through the vagus (X) and glossopharyngeal (IX) nerves and somatic sensory inputs through the trigeminal (V) nerve. In the present study, transganglionic transport of the WGA-HRP and Wallerian degeneration was used to identify whether two kinds of primary afferent fiber terminals converge onto a single neuron in the Pa5 at the utrastructural level. It was found that HRP-labeled and degenerated terminals originating from the IX and/or X nerves and infraorbital nerve formed asymmetrical synapses with unlabeled dendrites in the Pa5. Furthermore, approximately 7% (43/630) HRP-labeled and 31% (43/137) degenerated terminals formed synaptic connections with the same dendritic profiles simultaneously in the dorsal division of the Pa5. These results may provide a neuroanatomical substrate for integration of viscerosomatic sensory inputs associated with visceral and cardiovascular reflexes in the Pa5.

Rostral parvicellular reticular formation neurons projecting to rostral ventrolateral medulla receive cardiac inputs in anesthetized rats

The rostral parvicellular reticular formation (rRFp) was explored electrophysiologically in urethane-chloralose anesthetized rats. Spontaneously-active neurons that exhibited a pulse-related activity were recorded and tested for their projections to the rostral ventrolateral medulla (RVLM). About one-third (10/29) of the rRFp neurons that exhibited a pulse-related activity were antidromically activated by RVLM stimulation with conduction velocities between 0.2-4.4m/s and fell within the B and C fibre range. A majority (8/10) of these neurons had a low (<10spikes/s) mean firing rate, whereas a small proportion (2/10) had a high (>15spikes/s) mean firing rate. These findings suggest a direct pathway from the rRFp to the RVLM and suggest that neurons projecting to the RVLM receive cardiac inputs and can modulate RVLM neuronal activity.

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.

The anterior ethmoidal nerve is necessary for the initiation of the nasopharyngeal response in the rat

Stimulation of the nasal passages with ammonia vapors can initiate a nasopharyngeal response that resembles the diving response. This response consists of a sympathetically mediated increase in peripheral vascular resistance, parasympathetically mediated bradycardia and an apnea. The current study investigated the role of the anterior ethmoidal nerve (AEN) in the nasopharyngeal response in the rat, as it is thought that the AEN provides the main sensory innervation of the nasal passages. When both AENs were intact, nasal stimulation caused significant bradycardia, hypertension, and apnea and produced Fos label ventrally within the ipsilateral medullary dorsal horn (MDH) and paratrigeminal nucleus just caudal to the obex. This labeling presumably represents activation of second-order trigeminal neurons. When only one AEN was intact, the nasopharyngeal response was slightly attenuated, and a similar pattern of Fos labeling was only seen in the trigeminal nucleus ipsilateral to the intact AEN. The trigeminal labeling contralateral to the intact AEN was significantly reduced. When both AENs were cut, the nasopharyngeal response to nasal stimulation consisted of only a slight apnea and an increase in arterial pressure; the resultant Fos labeling within the trigeminal nucleus was significantly reduced. Cutting both AENs but not stimulating the nasal passages also produced some Fos labeling within the trigeminal nucleus. These findings suggest that a single AEN can provide sufficient afferent input to initiate the cardiorespiratory changes consistent with the nasopharyngeal response. We conclude that the AEN provides a unique afferent contribution that is capable of producing the diving response.

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.

Autoradiographic distribution and alterations of kinin B2 receptors in the brain and spinal cord of streptozotocin-diabetic rats

This study investigates whether bradykinin (BK) B(2) receptor binding sites are increased in the brain and thoracic spinal cord of streptozotocin (STZ)-diabetic rats at 2, 7, and 21 days posttreatment by in vitro autoradiography with the radioligand [(125)I]HPP-Hoe 140. In control and diabetic rats, specific binding sites for B(2) receptors were detected in the brain and in various laminae of the spinal cord, predominantly in superficial laminae (K(d)=34 pM). In diabetic rats, B(2) receptor densities were significantly increased in lamina l of the dorsal horn (+35% at 7 and 21 days), spinal trigeminal nucleus (+70% at 7 and 21 days) and nucleus tractus solitarius (+100% at 2 and 7 days). B(2) receptor analogues D-Arg[Hyp(3),Thi(5),D-Tic(7),Oic(8)]-BK (Hoe 140), 3-(4 hydroxyphenyl)propionyl-Hoe 140 (HPP-Hoe 140), LF16-0687 mesylate ((2-Pyrrolidinecarboxamide, N-[3-[[4-aminoiminomethyl)benzoyl]amino]propyl]-1-[[2,4-dichoro-3-[[(2,4-dimethyl-8-quinolinyl)oxy]methyl]phenyl]sulfonyl]-(2S)-(9Cl)), and BK decreased binding of [(125)I]-HPP-Hoe 140 in the spinal dorsal horn, with K(i) values of 0.5, 1.5, 3.2, and 3.7 nM, respectively. These values were not significantly different in diabetic rats at 7 days (0.5 (Hoe 140), 0.7 (HPP-Hoe 140), 1.2 (BK), and 1.7 (LF16-0687) nM). While des-Arg(10)-Hoe 140 was three orders of magnitude less potent than Hoe 140, B(1) receptor agonist (des-Arg(9)-BK) and antagonist (AcLys[D-betaNal(7),Ile(8)]des-Arg(9)-BK, R-715) did not affect [(125)I]-HPP-Hoe 140 binding at 1 microM concentration. Data suggest a very discrete and temporal increase of B(2) receptor density (without affinity changes) in the spinal cord and hindbrain of STZ-diabetic rats. This contrasts with the early induction and over-expression of B(1) receptors reported in the brain and spinal cord of STZ-diabetic rats.

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 ambiguus 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.Key words: cardiovascular reflexes, baroreflex response, arterial blood pressure, multiple single unit recording.

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.Key words: bradykinin, arterial blood pressure, ensemble neuron recording, RVL, baroreflex.