Thyrotropin-releasing hormone (TRH) has independent excitatory and modulatory actions on lamina IX neurons of lumbosacral spinal cord slices from adult rats.
Coronal and horizontal slices of the lumbar and sacral spinal cord, respectively, of ovariectomized adult rats, either treated with estrogen (OVX+E) or untreated (OVX), were used to test the neuronal actions of TRH and its metabolite, cyclo(His-Pro) (or cHP). Both coronal slices, which possess only short stumps of ventral roots (VRs), and horizontal slices, in which long sections of VRs were preserved, were used for extracellular recording of single motor and other types of neurons. Methodological comparisons between these two types of slices showed that the length of VRs preserved had no significant effect on the characteristics of motoneurons (MNs). In coronal slices, MNs in medial and lateral lamina IX (MNM and MNL, respectively) were identified by antidromic activation. Of these lumbar MNs, estrogen treatment lowered the antidromic activation threshold for MNM but not MNL. Because MNM innervate the back muscles crucial for the execution of the estrogen-dependent lordosis, the observed estrogen effect may contribute to the hormone's induction of the sexual behavior. The recorded MNs and other types of neurons were subjected to bath applications of TRH, cHP, and neurotransmitters. TRH was found to be capable of evoking an early, shorter-lasting neuronal excitation and/or a late, longer-lasting modulation of neuronal responses to transmitters. Each neuronal action could occur with or without the other, and the occurrence of the excitation did not affect the probability of whether a modulation would occur later. The modulatory, but not the excitatory, action appeared to be shared by cHP, because cHP could also modulate neuronal responses in similar, if not identical, ways as TRH did, but could neither stimulate neurons nor mimic TRH in desensitizing TRH-evoked excitation. The modulatory actions of the two peptides were not affected by estrogen. Although the excitatory action was desensitized by repeated TRH applications, the modulatory action did not appear to be attenuated but instead was often enhanced by repeated administrations of TRH and/or cHP. These results, together with the essentially identical findings from our previous study on hypothalamic neurons, indicate that the excitatory and the modulatory actions of TRH are independent of each other and, hence, are mediated by different subcellular mechanisms.
Kow
L M
LM
Rockefeller University, New York, NY 10021, USA.
Pfaff
D W
DW
D000818:Animals; D004594:Electrophysiology; D004958:Estradiol / Q000494:pharmacology; D005260:Female; D066298:In Vitro Techniques; D009046:Motor Neurons / Q000187:drug effects / Q000502:physiology; D010052:Ovariectomy; D010456:Peptides, Cyclic / Q000494:pharmacology; D010879:Piperazines / Q000494:pharmacology; D011187:Posture; D051381:Rats; D017207:Rats, Sprague-Dawley; D012726:Sexual Behavior, Animal / Q000187:drug effects; D013116:Spinal Cord / Q000187:drug effects* / Q000502:physiology; D013973:Thyrotropin-Releasing Hormone / Q000494:pharmacology* / Q000502:physiology
D010456:Peptides, Cyclic; D010879:Piperazines; D004958:Estradiol; D013973:Thyrotropin-Releasing Hormone; C015924:histidyl-proline diketopiperazine