W .-H. Cao, S.F. Morrison Brain Research 887 2000 46 –52
47
SPNs have been divided into two physiologically distinct effect. The position of the atrial catheter was verified after
populations: EPI ADR SPNs, which putatively regulate the each experiment.
activity of adrenal epinephrine-secreting chromaffin cells, and NE ADR SPNs that control adrenal norepinephrine-
2.3. Recording and identification of adrenal sympathetic secreting chromaffin cells [17]. In the present study, we
preganglionic neurons distinguished between these two groups of adrenal SPNs to
ascertain if PBG might evoke differential responses in The extracellular action potentials of adrenal SPNs were
those regulating adrenal epinephrine secretion and those recorded with glass pipettes containing an electropho-
controlling norepinephrine release. The results suggest that retically sharpened 7-mm-diameter carbon filament. The
the cardiopulmonary receptors activated by intra-atrial reference electrode was inserted into muscle lateral to the
PBG evoke a uniform inhibition of adrenal SPNs. A vertebral column or lateral to the occipital bone. Spinal
preliminary report of these results has been published [4]. penetrations were made along the dorsal root entry zone,
and adrenal SPNs were located 0.7–1.1 mm below the dorsal surface. Neuronal signals were amplified, filtered
2. Materials and methods
bandpass frequencies 300–3000 Hz, monitored on an oscilloscope and recorded on VCR tape. As previously
2.1. General procedures described [17], adrenal SPNs were antidromically iden-
tified with stimuli applied to the left adrenal nerve. Three Experiments were performed on 19 male Sprague–Daw-
criteria were used to establish the antidromic nature of the ley rats 300–500 g anesthetized with urethane 800
response of spinal neurons to adrenal nerve stimulation: mg kg, i.p. and chloralose 70 mg kg, i.v.. The trachea
1 constant onset latency, 2 high following frequency, and femoral vein were cannulated for artificial ventilation
and 3 collision with spontaneous action potentials or and drug administration, respectively. Arterial pressure
those evoked by medullary stimuli. To perform a time- AP was measured from the abdominal aorta by a
controlled collision test, pulses coincident with neuronal polyethylene catheter inserted through the femoral artery,
action potentials were obtained from a window dis- and heart rate HR was determined from the AP signal
criminator and used to trigger a stimulator at a specified with a biotach. The animals were paralyzed with d-
delay. tubocurarine chloride initially 0.6 mg rat, i.v., thereafter,
To distinguish between the two physiologically distinct 0.2 mg h, i.v. and then artificially ventilated with 100
groups of rat adrenal SPNs [17], the following were O at a minute volume of 140–180 ml. Respiratory rate
2
determined for antidromically activated adrenal SPNs: 1 was adjusted to maintain end-tidal CO between 3.5 and
2
their response to RVLM stimulation, 2 the relationship of 4.5. Body temperature was maintained at 378C with a
their spontaneous activity to the cardiac cycle, 3 their thermostatically controlled heating lamp. Rats were placed
different sensitivities to baroreflex, and 4 their responses prone in a stereotaxic apparatus and spinal investigation
to the glucopenia induced with intravenous 2-deoxy-glu- unit with the bite bar 11 mm below the interaural line and
cose. a spinal clamp on the T
and T vertebral processes. An
10 11
occipital craniotomy and T –T laminectomy were per-
7 9
formed. Bilateral pneumothoraces reduced respiratory 2.4. Neural stimulation
pump-related movements of tissue near the recording and stimulating electrodes.
To orthodromically activate adrenal SPNs by stimulation of their premotor input, paired-pulse 6 ms interpulse
2.2. Bezold-Jarisch reflex activation interval, 50–300 mA, 1 ms duration, 0.25 Hz stimuli were
delivered to the RVLM coordinates: 2.6 mm rostral, 1.9 PBG was selected because it directly activates chemical-
mm lateral and 2.3 mm ventral to the calamus scriptorius ly sensitive cardiopulmonary vagal afferents through
through a monopolar tungsten electrode 50 mm exposed serotonin 5-HT membrane receptors. PBG is more
tip. The RVLM contains spinally projecting, sympathetic
3
specific and has fewer systemic effects than other physio- premotor neurons that regulate adrenal medullary function
logical activators of cardiopulmonary receptors [2]. A [22,29]. Histological examination of the locations of
polyethylene catheter PE 50 was inserted into the right marker lesions made through the stimulating electrode
external jugular vein and advanced until its tip reached the indicated that they were consistently within 400 mm of the
entrance of the right atrium. This right atrial catheter was caudal pole of the facial nerve nucleus.
pre-filled with PBG dissolved in 0.9 saline. Bolus To antidromically identify adrenal SPNs with electrical
injections of PBG 100 mg kg, 50 ml, .10 min intervals stimulation of the adrenal nerve 1 ms duration, 50–500
were administered into the right atrium. Equal volumes of mA, the left adrenal nerve was approached retroperitoneal-
saline administered into the right atrium did not cause ly, dissected distal to the suprarenal ganglion and placed
observable responses in AP or HR, indicating no volume on a monopolar hook electrode at its entry to the adrenal
48 W
gland. The anode was clipped to the muscle near the to RVLM stimulation and their differential sensitivity to
adrenal nerve. the baroreceptor reflex. In the upper trace of Fig. 1A, the
To activate the baroreceptor reflex, the left aortic peristimulus time interval histogram for an EPI ADR SPN
depressor nerve ADN was identified in the neck and the responding to paired-pulse stimulation of the RVLM shows
central cut end was placed on a pair of platinum hook the characteristic early inhibition of spontaneous discharge
electrodes spaced 2 mm apart. Electrode stimuli were triad during the first 50–80 ms following RVLM stimulation,
square-wave pulses 7 ms interpulse interval, 10 mA, 1 ms, followed by a prominent excitation at a longer latency
0.25 Hz. Natural stimulation of arterial baroreceptors mean response latency, 150 ms. In contrast, the RVLM
consisted of the rise in arterial pressure following an stimulus-evoked responses of NE ADR SPNs Fig. 1B,
intravenous bolus of phenylephrine 50–100 mg ml in a upper trace consisted of an early, marked increase in
volume of 0.1 ml of saline over 1–2 s. discharge probability mean response latency, 25 ms,
which was often followed by a period of reduced firing lasting 100–200 ms. The average mean response latency
2.5. Data analysis for the 23 EPI ADR SPNs in this study was 14164.3 ms
range, 87 to 172 ms, which was significantly P,0.001 The action potentials of adrenal SPNs and the arterial
longer than that of the 25 NE ADR SPNs average, 3061.9 blood pressure were digitized at 22 kHz and recorded on
ms; range, 17–53 ms. VCR tape, along with trigger pulses coincident with
As illustrated in the lower traces of Fig. 1, the sponta- stimulus delivery. Computer-aided data analysis consisted
neous discharge probability of EPI ADR SPNs usually of peristimulus and perisystolic time interval histograms of
exhibited no modulation over the time course of the the discharges of adrenal SPNs. Results were expressed as
