The Journal of Neurophysiology Vol. 80 No. 3 September 1998, pp. 1033-1041
Copyright ©1998 The American Physiological Society

5-Hydroxytryptamine Responses in Immature Rat Rostral Ventrolateral Medulla Neurons In Vitro

L. L. Hwang and N. J. Dun

Department of Anatomy and Neurobiology, Medical College of Ohio, Toledo, Ohio 43614

Hwang, L. L. and N. J. Dun. 5-Hydroxytryptamine responses in immature rat rostral ventrolateral medulla neurons in vitro. J. Neurophysiol. 80: 1033-1041, 1998. Whole cell patch recordings were made from rostral ventrolateral medulla (RVLM) neurons of brainstem slices from 8- to 12-day-old rats. By superfusion or pressure ejection to RVLM neurons, 5-hydroxytryptamine (5-HT) elicited three types of membrane potential changes: a slow hyperpolarization (5-HT_H), a slow depolarization (5-HT_D) and a biphasic response, which persisted in a tetrodotoxin (TTX, 0.3 µM)-containing solution. 5-HT_H were accompanied by a decrease of input resistance in the majority of responsive neurons. Hyperpolarization reduced and depolarization increased the 5-HT_H; the mean reversal potential was 92.3 mV in 3.1 mM and shifted to 69.3 mV in 7 mM [K⁺]_o. Barium (Ba²⁺, 0.1 mM) but not tetraethylammonium (TEA, 10 mM) suppressed 5-HT_H. The 5-HT_1A receptor agonist (±)-8-hydroxy-dipropylamino-tetralin (8-OH-DPAT; 5-50 µM) hyperpolarized RVLM neurons. The 5-HT_1A antagonist pindobind-5-HT_1A (PBD; 1-3 µM) and the 5-HT₂/5-HT₁ receptor antagonist spiperone (1-10 µM) suppressed 5-HT_H and the hyperpolarizing phase of biphasic responses; the 5-HT₂ receptor antagonist ketanserin (3 µM) was without significant effect. 5-HT_D were associated with an increase or no apparent change of input resistance in RVLM neurons. Hyperpolarization of the membrane decreased or caused no apparent change in 5-HT_D. 5-HT_D were reduced in an elevated [K⁺]_o (7.0 mM) solution and >60% in a low Na⁺ (26 mM) solution and were not significantly changed in a low Cl (6.7 mM) or Ca²⁺-free/high Mg²⁺ (10.9 mM) solution. The 5-HT₂ receptor agonist -methyl-5-HT (50 µM) depolarized RVLM neurons, and the 5-HT₂ antagonist ketanserin (1-10 µM) attenuated the 5-HT_D and the depolarizing phase of biphasic responses, whereas the 5-HT_1A receptor antagonist PBD (2 µM) was without effect. Inclusion of the hydrolysis resistant guanine nucleotide GDP--S in patch solution significantly reduced the 5-HT_H as well as the 5-HT_D. The present study shows that, in the immature rat RVLM neurons, 5-HT causes a slow hyperpolarization and depolarization probably by interacting with 5-HT_1A and 5-HT₂ receptors, which are G-proteins coupled. 5-HT_H may involve an increase of an inwardly rectifying K⁺ conductance, and 5-HT_D appear to be caused by a decrease of K⁺ conductance and/or increase of nonselective cation conductance.

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