Properties of presympathetic neurones in the rostral ventrolateral medulla in the rat: an intracellular study "in vivo'.

Abstract

1. Intracellular recordings were made in pentobarbitone-anaesthetized rats from sixty-eight neurones located in the rostral ventrolateral medulla (RVLM), which responded with inhibition (latency, 33-6 + 9-3 ms) after stimulation of the aortic depressor nerve with short bursts of pulses. This inhibition was due to chloride-and voltage-dependent IPSPs. 2. Seventeen neurones could be excited antidromically after stimulation in the T2 spinal segment (conduction velocity, 1 9-8-5 m s-1) and were classified as RVLM presympathetic vasomotor neurones.3. 'Spontaneously' active neurones (n = 29) displayed a largely irregular pattern of firing, with no clear relationship between the level of the membrane potential and cycles of phrenic nerve activity at end-tidal CO2 < 5 0%. Cardiac cycle-related shifts of the membrane potential were not considered indicative of baroreceptor input as they could be due to movement artifacts. 4. All neurones displayed large synaptic activity (EPSPs and IPSPs, peak-to-peak amplitude > 5 0 mV). The depolarizing IPSPs observed during injection of chloride and/or negative current consisted of a phasic and a tonic component. 5. The on-going activity of these neurones resulted from synaptic inputs, with individual action potentials usually preceded by identifiable fast EPSPs. 6. No evidence was found for the presence of gradual depolarizations (autodepolarizations) between individual action potentials, and therefore under these experimental conditions the activity of RVLM presympathetic neurones did not depend on intrinsic pacemaker properties. 7. These results are consistent with the 'network' hypothesis for the generation of sympathetic vasomotor tone.

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“…The ongoing activity of these neurons resulted of synaptic inputs, with individual action potentials usually preceded by identifiable fast EPSPs (Lipski et al 1996). These findings are in agreement with the network hypothesis of the generation of sympathetic vasomotor tone proposed by Barman and Gebber (1987).…”

“…As proposed by Lipski (Lipski et al 1996), it is possible that, under normal conditions, RVLM premotor neuron activity is determined by the balance between excitatory and inhibitory synaptic inputs, including amino acid excita-tory mechanisms. However, under special conditions, such as hypertension, an imbalance between these inputs, resulting in an increased excitatory activity within the RVLM, may enhance the tonic activity of RVLM neurons.…”

“…The RVLM neurons have been studied using extracellular recording (Brown and Guyenet 1984, 1985, Morrison et al 1988, Campos and McAllen 1999 and intracellular recordings (Lipski et al 1996). However, despite the large number of studies on the RVLM and its recognized importance on the sympathetic vasomotor tone generation, the basis for the tonic ongoing activity of these neurons is not yet fully understood.…”

Rhythmic Properties of Neurons in the Rostral Ventrolateral Medulla of the Rat In Vitro: Effects of Clonidine

“…This negative result could have been due to technical deficiencies, since R-type neurons tend to be smaller than I-type neurons, as reported by Oshima et al (2000), for the brain stem/spinal cord preparation. The small-sized neurons in the study of Lipski et al (1996) could have been preferentially damaged, because even some of the larger (I-type) neurons in that study showed signs of damage (Fig. 8C in Lipski et al 1996), thus indicating that the technique used was not suitable for penetration of the smaller R-type neurons in the RVLM.…”

“…Another group of investigators (Lipski et al 1996), using intracellular recordings in the in vivo rat preparation, has reported conflicting results: they could not find regularly firing bulbospinal barosensitive neurons. This negative result could have been due to technical deficiencies, since R-type neurons tend to be smaller than I-type neurons, as reported by Oshima et al (2000), for the brain stem/spinal cord preparation.…”