Distribution of thyrotropin-releasing hormone binding sites: autoradiographic study in infant and adult human hippocampal formation

Thyrotropin-releasing hormone in the treatment of intractable epilepsy

Intractable seizures remain a significant therapeutic challenge despite current advances in the treatment of epilepsy. Thyrotropin-releasing hormone, the first neuroendocrine releasing factor to be isolated and fully characterized, was also the first releasing factor investigated as a possible neurotransmitter/neuromodulator outside the hypothalamus. Basic and clinical research has revealed a distinct neuroanatomic distribution and a neurochemical role for thyrotropin-releasing hormone in seizure modulation. Thyrotropin-releasing hormone and selected analogs were reported to have antiepileptic effects in several animal seizure paradigms, including kindling and electroconvulsive shock. Clinically, thyrotropin-releasing hormone treatment has been reported to be efficacious in such intractable epilepsies as infantile spasms, Lennox-Gastaut syndrome, myoclonic seizures, and other generalized and refractory partial seizures. Herein, we review evidence that suggests that thyrotropin-releasing hormone and selected thyrotropin-releasing hormone analogs may represent a new class of novel antiepileptic drugs, namely, antiepileptic neuropeptides and provide insights into potential new treatments for the intractable epilepsies.

Prolonged seizure suppression by a single implantable polymeric-TRH microdisk preparation

Thyrotropin-releasing hormone (TRH; Protirelin) is an endogenous neuropeptide known to have anticonvulsant effects in several seizure models and in intractable epileptic patients. Like most neuropeptides, its duration of action may be limited by a lack of sustained site-specific bioavailability. To attempt to provide long-term delivery, we attached TRH to a biodegradable polyanhydride copolymer as a sustained-release carrier. Utilizing the rat kindling model of temporal lobe epilepsy, a single TRH microdisk implanted stereotaxically into the seizure focus (amygdala) significantly suppressed kindling expression when assessed by the number of stimulations required to reach each behavioral stage and to become fully kindled (8.63 +/- 0.92 vs. 16.17 +/- 1.37; Mean +/- S.E.M.). Two indices of seizure severity, afterdischarge duration (Mean +/- S.E.M., sec.) (stimulated amygdala [87.40 +/- 5.47 vs. 51.80 +/- 15.65] and unstimulated amygdala [89.60 +/- 5.55 vs. 48.67 +/- 15.8] and clonus duration (71.2 +/- 5.94 vs. 29.40 +/- 8.87; Mean +/- S.E.M., sec.), were also significantly reduced by a single polymeric-TRH implant. Fifty days after initiation of the study a significant reduction in clonus duration (53.90 +/- 3.27 vs. 40.09 +/- 4.14) still remained in the TRH-implanted groups. This report is the first to provide evidence in support of in situ microdisk pharmacotherapy for potential neuropeptide delivery in intractable epilepsy and possibly other neurological disorders.

Effect of neuropeptides on cognitive function

Recent evidence indicates that, in addition to the involvement of cholinergic and other neurotransmitter systems, various neuropeptides that occur in cortical and subcortical brain regions have a role in cognitive behavior. This evidence results largely from behavioral studies in rodents and other animals, following peptide administration and only in a very few cases from similar studies in human subjects. Several neuropeptides studied appear to enhance or produce changes conducive to improvement in cognitive performance and these include vasopressin, corticotrophin-releasing hormone (CRH), somatostatin, substance P, neuropeptide Y, and thyrotrophin-releasing hormone (TRH), while one peptide, galanin, has been reported to inhibit cognitive processes. Of those neuropeptides that improve performance, only TRH has been shown recently to attenuate the memory impairment of human subjects and Alzheimer patients treated with an anticholinergic drug, and this review describes a series of complimentary studies in adult and aged rodents that contribute to our understanding of the possible mechanisms involved in the role of TRH in cognition.

Effects of the neuropeptide thyrotropin-releasing hormone on GABAergic synaptic transmission of CA1 neurons of the rat hippocampal slice during hypoxia

Because thyrotropin-releasing hormone (TRH) has been suggested to improve recovery of brain neurons from hypoxia, which strongly impairs GABAergic synaptic transmission, the present electrophysiological study used intracellular recording from CA1 neurons of the rat hippocampal slice to examine the cellular mechanisms underlying this phenomenon. Hypoxia induced by superfusion with a medium devoid of oxygen evoked typical membrane hyperpolarization, fall in input resistance, and strong depression of monosynaptic, GABAA receptor-mediated fast inhibitory postsynaptic potentials (IPSPs). The depression of fast IPSPs during hypoxia was found to be due to a combination of factors such as shift in the IPSP reversal potential and membrane hyperpolarization. GABAB receptor-mediated slow IPSPs were comparatively less sensitive to hypoxia. TRH (10 microM), applied 1 min prior to hypoxia, selectively accelerated recovery of membrane potential and delayed return of fast IPSPs to control amplitude without changing the mechanisms responsible for depression of GABAergic transmission. In conclusion, despite a slower recovery of IPSPs, TRH facilitated earlier return of neuronal excitability after the hypoxic period.

Radioimagers as an alternative to film autoradiography for in situ quantitative analysis of 125I-ligand receptor binding and pharmacological studies

Three radioimagers, the mu-imager, the beta-imager and the phosphorimager, were tested as alternatives to quantitative autoradiography on film, for receptor imaging and pharmacological in situ quantitative analysis. Two iodinated ligands 125I-interleukin-1 alpha and 125I-gonadotropin releasing hormone agonist were used for receptor characterization in mouse brain and pituitary sections. Due to the high number of the agonist receptors in rat pituitary gland, this tissue was used to compare measurements obtained from digital autoradiograms with classical gamma detector determination. This permits the evaluation of radioimager efficiency and absolute quantification. Radioimagers represent an improvement in terms of time of image acquisition. All the radioimagers are more sensitive than film for the detection of low levels of radioactivity. The spatial resolution provided by the mu-imager compares favourably with that obtained on film autoradiograms while digital autoradiograms from the phosphorimager and beta-imager did not show precise definition under our experimental conditions. Superimposition of histological structures from the stained sections with radiolabelled areas in the autoradiograms remains, at this time, the unique advantage of film. In conclusion, radioimagers represent an alternative to autoradiography on film or emulsion for in situ quantitative studies on tissue sections. They combine precise imaging for in situ binding studies with easy and direct access to counts in cpm. The improvement in radioimaging technology has, therefore, brought in situ analysis of iodinated ligand binding to the level of accuracy that is obtained with classical detectors of radioactivity.

Effects of thyrotropin-releasing hormone on GABAergic synaptic transmission of the rat hippocampus

The effect of thyrotropin-releasing hormone (TRH), a neuropeptide physiologically present in the mammalian hippocampus, on spontaneous, miniature and evoked GABAergic postsynaptic currents was investigated using whole-cell patch-clamp recording from pyramidal cells and interneurons of the rat hippocampal thin slice preparation. Bath application of 10 microM TRH induced an increase in the frequency of spontaneous postsynaptic currents from 1.07 +/- 0.68 to 3.16 +/- 0.73 Hz in pyramidal neurons and interneurons of the stratum lacunosum-moleculare (SL-M). In tetrodotoxin solution TRH did not change miniature postsynaptic currents. Application of TRH to minislices containing only the CA1 region still produced an increase in the spontaneous postsynaptic current frequency, indicative of an action by TRH upon a GABAergic circuitry. Paired recordings from one pyramidal cell and one stratum lacunosum moleculare interneuron displayed synchronous events whose frequency rose after TRH application, suggestive of a common, TRH-sensitive input. In a small subset of cells TRH induced the appearance of highly rhythmic large postsynaptic currents at a frequency of approximately 2 Hz, as confirmed by autocorrelation analysis. Postsynaptic currents electrically evoked by focal stimulation of stratum lacunosum-moleculare were depressed from 90 +/- 27 to 44 +/- 15 pA after application of TRH. This phenomenon was solely due to an increase in the number of synaptic failures. It is proposed that the effect of TRH on the GABAergic system was primarily exerted on a subset of interneurons controlling the activity of pyramidal cells as well as stratum lacunosum-moleuclare interneurons.

Cloning and functional characterisation of the human TRH receptor

Thyrotrophin-releasing hormone (TRH) is a hypothalamic tripeptide known to act via its receptor in the anterior pituitary gland to stimulate the release of thyrotrophin (TSH) from thyrotrophs and prolactin (PRL) from lactotrophs. It is also thought to act as a neurotransmitter/neuromodulator in the central and peripheral nervous systems. We have isolated a cDNA encoding the human pituitary TRH receptor (TRH-R) protein with a predicted amino acid sequence of 398 amino acids. Comparison of the amino acid sequences of the human TRH-R with the previously published rodent TRH-Rs showed that it is similar to both the rat and mouse TRH-Rs, except that each species has variant amino acids at the carboxy (COOH) terminus. The human TRH-R belongs to the family of seven transmembrane domain, G-protein-coupled receptors, and it is believed that the COOH terminal region of this family of receptors may play an important role in receptor downregulation/internalisation, and possibly G-protein coupling. COS-1 cells expressing the human TRH-R showed high affinity receptor binding. Stimulation of these cells with TRH produced a typical phosphoinositide response and mobilisation of intracellular calcium [Ca2+]i.