Elementos de la secuencia didáctica

cervical vagotomy in the anaesthetized rat

The response of an NTS neurone to activation of cardiopulmonary afferents by right atrial administration of phenylbiguanide (PBG; left) and the response of the same neurone to right atrial administration of PBG following bilateral cervical vagotomy (right). Top panels - from the top, traces show blood pressure (BP; mmHg), a continuous rate histogram of neuronal activity (spikes bin'^) and the raw recording of neuronal activity (|iV). The administration of PBG is highlighted by a solid horizontal line ( — ; PBG - 1 2 pg k g '\ 20 pi). There is a gap in recording signified by the symbol \ and of the duration stated. Bottom panels - vagal stimulation was carried out at the identified points (•) and neuronal identity was confirmed between PBG injections by the similar shape of the evoked spike.

2 0 0 i^iV

PBG

PBG

3.3 Results - anaesthetized cat

Baseline values for systemic variables were measured and averaged for all 7 anaesthetized cats. They were (mean ± S.E.): - mean arterial pressure (MAP) 101 ± 5 mmHg; heart rate 160 ± 23 bpm; tracheal pressure - inflation 5.4 ± 2.5 mmHg, deflation 2.1 ± 0.7 mmHg; blood pH 7.32 ± 0.05; blood PO2 141 ± 2 7 mmHg; blood PCO2 38 ± 10 mmHg.

3.3.1 Cardiac vagal preganglionic neurones

Compound ‘piggyback’ glass microelectrodes were used to record a total of 11 cardiac vagal preganglionic neurones (CVPNs). These neurones had axons located in the cardiac branches of the vagus, confirmed by antidromic activation (figure 3.10 A), with conduction velocities in the B-fibre range (5.9-18.0 m s '\ mean: 11.4 ± 0.9 m s'^). The recording sites of 6 of these neurones were marked by ionophoretic Pontamine Sky Blue ejection (figure 3.11), a further 2 neurones were recorded in close proximity to one of these recording sites. In addition, based on the depth of recording and position of entry of the electrode through the dorsal surface of the brainstem, the positions of the remaining CVPNs were also judged as being close to the marked sites. Thus, the location of at least the majority of neurones recorded was found to be within or

ventrolateral to the nucleus ambiguus (figure 3.11).

Correlation of CVPN activitv

10 of the 11 CVPNs recorded had little or no ongoing activity, the remaining neurone had an ongoing activity of 6.3 spikes s '\ Therefore, in order to

examine correlation patterns of neuronal activity the excitatory amino acid DL- homocysteic acid (DLH; 10-120 nA) was ionophoretically applied to 7 of the 11 neurones.

All 11 CVPNs showed a strong correlation to ECG, with activity at its highest during the peak of the blood pressure wave (figure 3.10 B). Furthermore, all neurones had activity correlated to central respiratory drive, with the highest

period of activity in the post-inspiration and stage 2 expiration phases (figure 3.10 B). This correlation was maintained even in periods of high excitability evoked by ionophoretic DLH administration (60-120 nA; figure 3.12 C).

However, artificial removal of central respiratory drive by hyperventilation (data not shown) or low current (10 nA) pulmonary vagal branch stimulation (figure 3.13 B), which also considerably increased ongoing activity, did remove the respiratory-related modulation of these neurones. Finally, the ongoing activity of only 3 of these 11 neurones was correlated to tracheal pressure, at its highest during the period of lung deflation, the remaining 8 neurones did not display any tracheal pressure related rhythm (figure 3.10 B).

Ionophoretic application of DLH at high currents (30-160 nA) also produced a significant fall in both blood pressure and heart rate (MAP - 106 ± 4 to 90 ± 3 mmHg, P<0.05; heart rate - 156 ± 6 to 139 ±6 bpm, P<0.01 ; n=6) at 6 of 8 recording sites. In 3 cases CVPN activity was also recorded and increased inversely with the associated decreases in heart rate and blood pressure (figure 3.12).

3.3.2 Cardiopulmonary reflex (CVPN response)

Right atrial injections of phenylbiguanide (PBG; 14-32 pig k g '\ 100-200 pil) excited 9 of 11 CVPNs tested (figure 3.13 A). Increases occurred in both the number of spikes in each respiratory-related burst of activity (11 ± 4 to 27 ± 6; P<0.01 ), as well as in the duration of each burst (1.9 ± 0.5 to 3.6 ± 0.8 ms; P<0.01). In these 9 neurones right atrial PBG also caused a significant

bradycardia of 69 ± 6 bpm (159 ± 7 to 90 ± 6 bpm, P<0.01), a significant fall in blood pressure of 22 ± 1 mmHg (MAP, 96 ± 5 to 74 ± 4 mmHg, P<0.01), and a reduction in phrenic nerve activity (figure 3.13 A).

Furthermore, there was a significant (P<0.01) difference between the onset latency of the CVPN response (1.8-4.5 s; 3.4±0.3 s) and that of the bradycardic response (2.5-4.5 s; mean 3.7 ± 0.3 s) to right atrial PBG, the excitatory

Further analysis of this neuronal response revealed that in 8 of the 9 neurones the 1 St second of the evoked burst of activity (10 ± 1), which occurred within a 5 second window post-PBG injection, was significantly bigger (P<0.01) than the mean of the 1st second of the previous four control bursts (4 ± 1 ; figure 3.13 A). This excitation is therefore likely to be mediated by activation of C-fibres in the pulmonary circulation as it occurs inside the 5 second window i.e. inside the pulmonary circulation time. Additionally, although the 1st second of the response of the remaining neurone was unaffected, the total intensity and duration of the first burst post-PBG injection was increased.

As mentioned previously low current (10 \aA) pulmonary vagal branch

stimulation increased the ongoing activity and removed the respiratory phase pattern of firing of a single CVPN, in addition to inhibiting phrenic nerve activity (figure 3.12 B). However, right atrial PBG injection still caused excitation in this neurone (figure 3 .1 2B). This was also found to be the case in a second CVPN, where respiratory modulation was removed by hyperventilation (data not

shown).