Effects of electrical stimulation of vagal afferents on spinothalamic tract cells in the rat

The effects of electrical stimulation of cervical vagal afferents (VAS) on the background activity and on the responses of 25 spinothalamic tract (STT) neurons to noxious stimuli were studied in anesthetized rats. Background (spontaneous) activity of 9 (36%) STT neurons was inhibited by all intensities of VAS. 6 (24%) units were facilitated at lesser and inhibited at greater intensities of VAS, 5 (20%) units were only facilitated by all intensities of VAS, and 5 (20%) units were not affected by VAS. Responses of 8 (36%) STT neurons to noxious stimuli were only inhibited by VAS, 9 (41%) were facilitated at lesser and inhibited at greater intensities of VAS, and 5 units (23%) were only facilitated by VAS. There were no significant differences in VAS-produced modulatory effects between STT neurons and 16 unidentified lumbar spinal dorsal horn neurons studied under the same conditions. These results reveal that descending facilitatory and inhibitory pathways engaged by activation of vagal afferents modulate rostrally projecting nociceptive transmission neurons in the spinal cord, constituting an important regulatory network for nociception.

Electrical stimulation of cervical vagal afferents. I. Central relays for modulation of spinal nociceptive transmission

1. Supraspinal relays for vagal afferent modulation of responses of spinal dorsal horn neurons to 50 degrees C heating of the skin were examined by the use of nonselective, reversible local anesthesia or soma-selective, irreversible neurotoxic damage of neural tissue. Eighty-five neurons were isolated in the lumbar spinal dorsal horn of 80 pentobarbital-anesthetized, paralyzed rats. All neurons studied had receptive fields on the glabrous skin of the plantar surface of the ipsilateral hind paw and responded to mechanical stimuli of both low and high intensity as well as noxious thermal stimulation. 2. Intensity-dependent modulation by vagal afferent stimulation (VAS) of neuronal responses to heating of the skin was established. Responses of 40 units were facilitated by low and inhibited by greater intensities of VAS. Another 36 units were only inhibited by VAS, and four were only facilitated. 3. Local anesthesia of the dorsolateral pons by bilateral microinjections of lidocaine (4%, 0.5 microliter) were made to examine the contribution of this area to VAS-produced spinal modulation. The microinjection of lidocaine bilaterally into the ventral locus coeruleus/subcoeruleus (LC/SC) reversibly and significantly attenuated VAS-produced inhibition of unit responses to heat from 63 to 89% of control and abolished VAS-produced facilitation. The microinjection of lidocaine bilaterally into the dorsal LC had no significant effect on VAS-produced modulation of spinal dorsal horn neurons. 4. Ibotenic acid (10 micrograms, 0.5 microliter) was microinjected into the dorsolateral pons to determine the relative contributions of cell bodies in this area to VAS-produced spinal modulation. Unilateral microinjection of ibotenic acid into the LC/SC ipsilateral to the vagus nerve stimulated had no significant effect on VAS-produced inhibition but significantly attenuated VAS-produced facilitation of unit responses to heat. Bilateral microinjections of ibotenic acid significantly attenuated VAS-produced inhibition of unit responses to heat from 48 to 94% of control. 5. Local anesthesia of the medial rostroventral medulla (RVM), primarily the nucleus raphe magnus (NRM), significantly attenuated VAS-produced inhibition of unit responses to heat from 55 to 87% of control but had no significant effect on VAS-produced facilitation. Microinjection of ibotenic acid into the RVM also significantly reduced VAS-produced inhibition of unit responses to heat. No significant change in VAS-produced spinal modulation was found after lidocaine microinjection into areas dorsal to the NRM, the nucleus raphe pallidus, or the olivary nucleus.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrical stimulation of cervical vagal afferents. II. Central relays for behavioral antinociception and arterial blood pressure decreases

1. Supraspinal substrates mediating vagal afferent stimulation (VAS)-induced inhibition of the nociceptive tail-flick reflex were examined by the use of the soma-selective neurotoxin ibotenic acid and the nonselective local anesthetic lidocaine. Fifty rats were studied in the lightly anesthetized state maintained with pentobarbital sodium. 2. The threshold intensity of VAS required to inhibit the tail-flick reflex to a cut-off latency of 10 s was established in all rats. Ibotenic acid (5 or 10 micrograms, 0.5 microliter) or lidocaine (4%, 0.5 microliter) was then microinjected into various regions of the brain stem followed by reestablishment of the intensity of VAS required to produce inhibition of the tail-flick reflex. 3. Microinjections of ibotenic acid into the ipsilateral nucleus tractus solitarius (NTS), medial rostroventral medulla (principally the nucleus raphe magnus; NRM), or bilaterally into the dorsolateral pons (principally the locus coeruleus/subcoeruleus; LC/SC), significantly increased the threshold intensity of VAS required to inhibit the tail-flick reflex. Microinjections of ibotenic acid into either the rostral or caudal ventrolateral medulla (RVLM or CVLM, respectively) ipsilateral to the vagus nerve stimulated or ipsilateral LC/SC did not significantly affect the inhibition produced by VAS. Arterial blood pressure decreases produced by VAS were significantly attenuated or eliminated after microinjections of ibotenic acid into the NTS, RVLM, CVLM, or NRM. Lidocaine microinjected into the ipsilateral CVLM also significantly increased the intensity of VAS required to inhibit the tail-flick reflex. 4. These outcomes obtained with behavioral measures are consistent with the outcomes of the preceding study using electrophysiological measures in establishing that cells in the NTS, LC/SC, and NRM regions and fibers of passage in the CVLM are important in mediating the inhibitory effects of VAS. The present studies confirm previous reports of the importance of the RVLM and CVLM in VAS-produced depressor responses but also demonstrate that the NRM is critical for this cardiovascular response.

Plasticity of some spinal dorsal horn neurons as revealed by pentobarbital-induced disinhibition

Extracellular activity was recorded from single spinal dorsal horn neurons in physiologically intact, awake, drug-free cats before and after the intravenous administration of 20 mg/kg pentobarbital (Pb). Pb produced a series of changes in response properties that reflect a significant moment-to-moment plasticity of some spinal dorsal horn neurons. Pb administration unmasked the ability of some low-threshold (LT) neurons to respond to noxious mechanical or thermal stimuli resulting in their being reclassified as wide dynamic range (WDR) neurons. Pb also appeared to unmask an afterdischarge in some neurons following noxious mechanical stimulation. In addition, some neurons appeared to be better able to signal changes in the intensity of mechanical stimulation after Pb. Neuronal receptive fields for low threshold stimulation were reduced in many instances but enlargement was also observed. The responses of some neurons to peripheral stimulation were unchanged by Pb. We hypothesize that the relatively low doses of Pb used in the study reduced tonic inhibition of some spinal dorsal neurons although the observed effects could have been produced by excitation.

Mononuclear cells in glomeruli and cytokines in urine reflect the severity of experimental proliferative immune complex glomerulonephritis

Immunohistochemical methods were used to investigate the role of macrophages in the progression of proliferative immune complex glomerulonephritis. The mononuclear cell component of glomerular inflammation was analysed in three different stages of chronic serum sickness, each of which was clearly distinguished by criteria of kidney function. Urinary excretion of the macrophage secretory products interleukin-1 and tumour necrosis factor was also evaluated in relation to the functional severity of kidney disease. T lymphocytes and macrophages began to accumulate in glomeruli at the onset of proteinuria, but not before. Urinary excretion of interleukin-1 also began with proteinuria. Proteinuria increased in direct correlation with increases in the number of glomerular macrophages. Development of the most severe stage of glomerulonephritis, characterized by cachexia, declining kidney function, and necrotizing glomerular pathology, was accompanied by the disappearance of T cells from glomeruli and the expression of highly abnormal phenotypes by most macrophages. In addition, there was a switch from urinary excretion of interleukin-1 to excretion of tumour necrosis factor. The progression of proliferative immune complex glomerulonephritis was associated with qualitative as well as quantitative changes in glomerular macrophage populations. Differentiation and/or activation of those glomerular macrophages may have resulted from local T cell-mediated immunoregulation. Measurements of urinary cytokine excretion provided a reliable means of monitoring disease progression. The local action of tumour necrosis factor probably contributed to declining kidney function in the most severe stage of disease.

Modulation of spinal nociceptive transmission from nuclei tractus solitarii: a relay for effects of vagal afferent stimulation

1. The effects of electrical and chemical stimulation in the nucleus tractus solitarii (NTS) on spinal nociceptive transmission were examined in pentobarbital sodium-anesthetized, paralyzed rats. These studies also examined the role of the NTS as a relay for the effects of vagal afferent stimulation (VAS) on spinal nociceptive transmission. All 75 neurons studied were located in laminae I-VI in the L3-L5 spinal segments, with receptive fields on the glabrous skin of the plantar surface of the ipsilateral hindpaw. The units responded to mechanical (low and/or high intensities) and thermal stimuli (42-52 degrees C). 2. Electrical stimulation in the NTS either ipsilateral or contralateral to the spinal unit inhibited neuronal responses to noxious thermal stimuli. The magnitude of inhibition did not significantly differ as a function of either ipsilateral (15 units) or contralateral (12 units) NTS stimulation (NTSS) as indicated by extrapolated thresholds for inhibition of responses to heat, intensities to produce 50% inhibition of responses to heat, and the slope of recruitment lines for inhibition. Tracking experiments also revealed that stimulation in the area ventral to the NTS produced a greater magnitude of inhibition of these units than did NTSS. 3. NTSS significantly decreased the slope of the stimulus-response functions (SRFs) of dorsal horn units to graded thermal stimuli (42-52 degrees C), whereas response threshold was unaffected by NTSS. The apparent latency of NTSS to produce inhibition of unit responses to heating of the hindpaw was determined to be 50 +/- 10 ms (mean +/- SE). 4. Microinjection of 50 nmol of glutamate into the NTS ipsilateral to the spinal unit also inhibited neuronal responses to thermal stimuli in 17/21 units; responses of 2/21 units were facilitated. Inhibition typically lasted 4-7 min and was shown to be dose-volume dependent. 5. The effects of VAS and NTSS on spinal nociceptive transmission were directly compared. The responses of 17 units to 50 degrees C heating of the hindpaw were facilitated by low and inhibited by greater intensities of VAS (Biphasic units); the responses of 12 units were only inhibited by VAS (Inhib units); three were only facilitated (Facil units), and 2 were unaffected by VAS. In contrast, NTSS generally inhibited the same spinal units, although modest facilitation was produced by NTSS contralateral to the recording site. NTSS produced greater inhibition of the Biphasic units than did VAS, shown by a leftward shift of the recruitment line of inhibition and greater inhibition at the same intensity of electrical stimulation.(ABSTRACT TRUNCATED AT 400 WORDS)

Vagal afferent modulation of spinal nociceptive transmission in the rat

1. The effects of vagal afferent stimulation (VAS) on spinal nociceptive transmission and the spinal pathway(s) mediating VAS-produced effects were examined in pentobarbital sodium- and urethane-anesthetized, paralyzed rats. The 60 units studied responded to mechanical stimuli and noxious heating (50 degrees C) of cutaneous receptive fields confined to the glabrous skin of the toes and footpads. Recording sites were located in laminae I-VI of the L3-L5 spinal segments. 2. VAS facilitated and inhibited neuronal responses to heat. In pentobarbital-anesthetized rats, responses of most (24/44) units were facilitated by low and inhibited by higher intensities of VAS. Responses of some units (15/44) were only inhibited and others (4/44) only facilitated by VAS. Inhibition produced by VAS was intensity-, pulse width-, frequency-, and stimulation duration-dependent. In urethane-anesthetized rats, responses of 6/16 units were initially facilitated, then inhibited as the intensity of VAS was increased; responses of nine units were inhibited by VAS. Quantitative comparisons of recruitment indices, mean thresholds for inhibition and mean intensities to inhibit unit responses to heat to 50% of control revealed no significant differences between the two anesthetic conditions. 3. The effects of VAS on neuronal responses to heat were dissociable from its effect on blood pressure. Regardless of the effect of VAS on unit responses to noxious heat, VAS consistently produced intensity-dependent depressor responses. The latencies to onset of inhibition and facilitation by VAS were determined by a cumulative sum technique and bin-by-bin analysis of peristimulus time histograms. The apparent latencies were 91 +/- 11 (SE) ms for inhibition and 278 +/- 59 ms for facilitation, both of which occurred before changes in blood pressure. Finally, microinjections of lidocaine into the ventrolateral funiculus (VLF) or transections of the dorsolateral funiculus (DLF) of the thoracic spinal cord attenuated VAS-produced effects on neuronal responses, but did not affect VAS-induced depressor responses. 4. The responses of 11 dorsal horn units to graded noxious heating of the skin were studied; the stimulus-response functions (SRF) were linear and monotonic throughout the temperature range examined (42-52 degrees C). VAS at intensities which inhibited unit responses to heat significantly decreased the slope of the SRF. VAS at intensities which facilitated unit responses to heat produced a leftward, parallel shift of the SRF.(ABSTRACT TRUNCATED AT 400 WORDS)

Vagal afferent modulation of a nociceptive reflex in rats: involvement of spinal opioid and monoamine receptors

Modulation of the spinal nociceptive tail flick (TF) reflex by electrical stimulation of subdiaphragmatic or cervical vagal afferent fibers was characterized in rats lightly anesthetized with pentobarbital. Cervical vagal afferent stimulation (VAS) inhibited the TF reflex in a pulse width-, frequency-, and intensity-dependent fashion. The optimum parameters for inhibition of the TF reflex were determined to be 2.0 ms pulse width, 20 Hz frequency with a threshold (T) current of 60 microA. Cervical VAS at 0.2-0.6 T facilitated the TF reflex. Cervical VAS at T typically produced a depressor arterial blood pressure response, but inhibition of the TF reflex by VAS was not due to changes in blood pressure. Subdiaphragmatic VAS also inhibited the TF reflex and generally produced a pressor effect, but did not facilitate the TF reflex at intensities of stimulation less than T as did cervical VAS. The parameters of cervical VAS required for inhibition of TF reflex suggest that excitation of high-threshold, unmyelinated fibers are important in VAS-induced descending inhibition. The intrathecal administration of pharmacologic receptor antagonists into the subarachnoid space of the lumbar enlargement indicated that the opioid receptor antagonist naloxone produced a dose-dependent antagonism of cervical VAS-produced inhibition of TF reflex, but single doses of either phentolamine or methysergide (30 micrograms each) failed to affect the inhibition by VAS. Combined intrathecal injection of both phentolamine and methysergide (30 micrograms each), however, significantly attenuated inhibition of the TF reflex by cervical VAS. These results suggest that cervical VAS engages a spinal opioid system and co-activates descending serotonergic and noradrenergic systems to modulate spinal nociceptive processing.

Lack of spontaneous activity of cutaneous spinal dorsal horn neurons in awake, drug-free, spinally transected cats

Extracellular single-unit activity was recorded from neurons with cutaneous input in the dorsal horn of the spinal cord (L4-L6) of awake, drug-free cats before (for several weeks) and after (day 1 through day 7) cord transection (T12). The spontaneous activity of the neurons was minimal or nonexistent in both recording conditions. The lack of spontaneous activity following spinal cord transection contrasts sharply with activity recorded in acute spinal-cord transected preparations in which high rates of spontaneous activity have been reported. This discrepancy may reflect an important difference between the chronic, awake, drug-free and acute preparations.