Pre- and post-natal ontogeny of thyrotropin-releasing-hormone in the rat spinal cord: an immunocytochemical study

Synaptic control of motoneuronal excitability

Movement, the fundamental component of behavior and the principal extrinsic action of the brain, is produced when skeletal muscles contract and relax in response to patterns of action potentials generated by motoneurons. The processes that determine the firing behavior of motoneurons are therefore important in understanding the transformation of neural activity to motor behavior. Here, we review recent studies on the control of motoneuronal excitability, focusing on synaptic and cellular properties. We first present a background description of motoneurons: their development, anatomical organization, and membrane properties, both passive and active. We then describe the general anatomical organization of synaptic input to motoneurons, followed by a description of the major transmitter systems that affect motoneuronal excitability, including ligands, receptor distribution, pre- and postsynaptic actions, signal transduction, and functional role. Glutamate is the main excitatory, and GABA and glycine are the main inhibitory transmitters acting through ionotropic receptors. These amino acids signal the principal motor commands from peripheral, spinal, and supraspinal structures. Amines, such as serotonin and norepinephrine, and neuropeptides, as well as the glutamate and GABA acting at metabotropic receptors, modulate motoneuronal excitability through pre- and postsynaptic actions. Acting principally via second messenger systems, their actions converge on common effectors, e.g., leak K(+) current, cationic inward current, hyperpolarization-activated inward current, Ca(2+) channels, or presynaptic release processes. Together, these numerous inputs mediate and modify incoming motor commands, ultimately generating the coordinated firing patterns that underlie muscle contractions during motor behavior.

Two conductances mediate thyrotropin-releasing-hormone-induced depolarization of neonatal rat spinal preganglionic and lateral horn neurons

Thyrotropin-releasing hormone (TRH) has been recognized as a neuromodulator in several CNS regions, including the thoracolumbar spinal cord where an influence on cardiovascular autonomic function has been proposed. To identify the cellular mechanisms involved in the latter, whole cell patch-clamp recordings were obtained from 52 thoracolumbar lateral horn cells, including 17 sympathetic preganglionic neurons (SPNs), in spinal cord slices from neonatal rat (11-21 days). Under current clamp, bath applications of TRH (1-20 microM) induced a slowly rising and prolonged membrane depolarization in eight of nine cells tested. Under voltage clamp (holding potential -65 mV), 33 of 37 tested cells displayed a TRH-induced, tetrodotoxin-resistant inward current that was associated with either a reduction or an increase in membrane ion conductances. Current-voltage (I-V) relationships in 28 cells suggested two conductances. In 9 cells the current reversed at about -107 mV; in 10 cells the I-V lines remained parallel, whereas in 9 cells the current reversed at around -40 mV. In three of three cells, addition of 2 mM barium was associated with an inward current, and the TRH-induced inward current was also suppressed, suggesting the presence of a resting barium- and TRH-sensitive potassium conductance. A residual barium-insensitive conductance was seen to reverse near -40 mV. Intracellular dialysis with guanosine 5'-o-(3-thiotriphosphate) significantly enhanced the duration of the TRH effect, indicating that G protein activation participates in the TRH response. These observations not only reveal a direct, G-protein-mediated depolarizing action of TRH on neonatal rat SPNs and lateral horn cells but also imply that two separate conductances may be involved in the TRH responses in some neurons.

Effects of neonatal adrenalectomy on 5-HT and TRH-like immunoreactivity in the rat spinal cord

This study investigated the effects of neonatal unilateral adrenalectomy on the serotonin- (5-HT) and thyrotropin-releasing hormone-like immunoreactivities (TRH-LI) in the intermediolateral cell column (IML) of the rat spinal cord where the sympathetic preganglionic neurons innervating the adrenal medulla are located. The density of the innervation was measured by a computer-assisted image analysis. Two weeks after the lesion, only one rat (1/3) showed a 30% decrease of 5-HT-LI in the ipsilateral IML, although no modifications were observed for TRH-LI. At 1 month, we observed a variable decrease of 19% to 30% for either 5-HT- or TRH-LI in the ipsilateral IML. However, one animal (1/3) showed nonsignificant modifications. At 3 months, we showed a 17% mean loss of 5-HT-LI and TRH-LI in the ipsilateral IML. However, the decreases of 5-HT-LI and TRH-LI did not always appear similar. These results appear different from those obtained after neonatal removal of the superior cervical ganglion.