Chronic dihydroergotoxine administration sets on receptors for enkephalin and thyrotropin releasing hormone in the aged-rat brain

Effects of dihydroergotoxine on central cholinergic neuronal systems and discrimination learning test in aged rats

We evaluated changes in the cholinergic neuronal system and learning ability with aging. Choline acetyltransferase (ChAT) activity, a presynaptic index of the cholinergic system, was decreased in the cerebral cortex, hippocampus, striatum, and hypothalamus in the brain of aged rats compared with young adults. Muscarinic cholinergic binding sites (receptors, MCR), a postsynaptic index of the cholinergic system, were markedly decreased in all areas of the brain. However, intraperitoneal injection of 1 mg/kg of dihydroergotoxine (DHET) for 14 days normalized both ChAT and MCR in the cerebral cortex and hippocampus. In the striatum, ChAT was normalized, but MCR did not recover. Aged rats showed marked learning impairment in a 30-day operant type brightness discrimination learning test. Daily DHET administration restored the discrimination ability in the aged rats to nearly the young adult level. DHET had no effects on central cholinergic indices or learning test results in young adult rats. These findings suggest that learning is impaired in aged rats due to impairment in the central cholinergic neuronal system, and that DHET normalizes the decreased function in this system, restoring the learning ability.

Changes of neuropeptides and their receptors in experimental stroke gerbil brains

Eight kinds of neuropeptides and four kinds of neuropeptide receptors were examined in the right and left hemispheres of mongolian gerbils after unilateral carotid ligation-induced stroke and in normal controls. Five hours after ligation of the right common carotid artery, beta-endorphin concentration in the right hemisphere (ischemic side) of the stroke group was significantly increased compared with that in the contralateral hemisphere (non-ischemic side), but there were no differences between sides in other neuropeptides either with or without stroke. Furthermore, although there were no differences in [3H]naloxone binding, [3H]thyrotropin-releasing hormone binding or 125I-vasoactive intestinal polypeptide binding in the brain in this model of stroke, [3H]enkephalin binding was significantly lower on the ischemic side than on the non-ischemic side in the stroke group. These results suggest that increased activity in the beta-endorphinergic system in the brain might be partly caused by ischemic brain failure.

Changes in brain thyrotropin-releasing hormone (TRH) of seizure-prone El mice

We examined the anticonvulsant effects of DN-1417 an analog of the thyrotropin-releasing hormone (TRH) in seizure-prone El mice. Changes in both immunoreactive TRH (IR-TRH) and TRH receptor binding activity in discrete brain regions of El mice were also measured before and after sensitization and during the postictal period, and they were compared with those in the ddY mice. Intraperitoneal injection of DN-1417 with 150 and 450 mg/kg significantly increased the El mouse seizure threshold in a dose-dependent manner. IR-TRH in the hippocampus of El mice, which was significantly lower than in ddY mice, significantly increased after sensitization. During the postictal period, however, it slowly decreased again and then gradually recovered to the preconvulsive level without any change in TRH receptor binding. In the striatum of El mice, although TRH receptor binding was significantly higher than in ddY mice, it was not affected by sensitization. These findings indicate that the hippocampal TRH system may play an inhibitory role in El mouse seizures whereas the striatal TRH system may be important for its seizure susceptibility.

The effects of hemorrhagic shock on thyrotropin-releasing hormone and its receptors in discrete regions of rat brain

The effects of hemorrhagic shock on thyrotropin-releasing hormone (TRH) levels and its receptors were studied in different regions of the rat brain. Rats were bled for 30 min from the left femoral artery, and their mean arterial pressure was kept at 40 mmHg for the following hour. The rats were killed by decapitation. Rat brains were immediately removed and dissected into 7 regions. Hemorrhagic shock decreased TRH significantly in the frontal cortex, septum, hippocampus, and hindbrain but TRH was not changed in the striatum, hypothalamus, and midbrain. Hemorrhagic shock significantly decreased TRH receptor binding in the septum and hindbrain. Scatchard analysis of saturation isotherms of specific TRH binding showed that the decreased specific TRH binding in the hindbrain resulted not from an increase of the dissociation constant (Kd), but from a decrease in the maximum number of binding sites (Bmax). In the septum, the decrease in specific binding was due both to a decrease in Bmax and an increase in Kd. The findings indicate that TRH plays a role in the physiological response to hemorrhagic shock.

Reduction of rat striatal thyrotropin-releasing hormone receptors produced by repeated methamphetamine administration

It has been reported previously that repeated, but not continuous, administration of methamphetamine (MAP) to animals produces progressive and sustained enhancement of MAP-induced behavior (behavioral sensitization), which may be related to functional changes in central dopamine (DA) systems. To investigate the possible involvement of thyrotropin-releasing hormone (TRH), a neuromodulator of DA, both immunoreactive TRH (IR-TRH) levels and specific TRH binding were examined in rat brain regions after MAP administration either repeatedly (4 mg/kg intraperitoneally once a day for 14 consecutive days) or continuously (about 4 mg/kg/day for 13 consecutive days). Although no significant changes were observed in IR-TRH levels in any regions of the brain following repeated MAP injections, specific TRH binding in the striatum significantly decreased. Scatchard analysis revealed that the decrease was due to a reduction in the maximum number of binding sites (Bmax). Pretreatment with haloperidol prior to each MAP injection prevented this decrease. Continuous MAP administration had no effect on regional specific TRH binding. These results suggest that repeated MAP administration caused lasting dysfunction in the brain TRH system, which may be implicated in the behavioral sensitization.

Long-term increase in striatal thyrotropin-releasing hormone receptor binding caused by amygdaloid kindling

Our previous finding that intracerebroventricular (i.c.v.) administration of both thyrotropin-releasing hormone (TRH) and its analogue, gamma-butyrolactone-gamma-carbonyl-L-histidyl-L-prolinamide citrate (DN-1417), suppressed seizure development of amygdaloid (AM) kindling and kindled AM seizures leads to a new hypothesis that endogenous TRH may be an antiepileptic substance in the brain. In this study, we examined postictal chronological changes in both immunoreactive TRH (IR-TRH) and TRH receptor binding activity in discrete brain regions of AM-kindled rats to study the relationship of the brain TRH system to kindling-induced seizure susceptibility. AM-kindled rats were decapitated 30 min, 24 h, 48 h, 7 days, and 21 days after the last kindled convulsion. IR-TRH increased markedly in the AM/pyriform cortex and hippocampus 24 and 48 h after the last convulsion, and returned to the control (unstimulated, sham-operated) value within 3 weeks after the convulsions ended. In contrast, a significant increase in the striatal TRH binding sites was evident 24 h after the cessation of convulsions which lasted 21 days. A lasting change in the striatal TRH neural system may be related to kindling-induced seizure susceptibility.

Effect of immobilization stress on neuropeptides and their receptors in rat central nervous system

The effect of immobilization stress (IM-stress) on the concentration and the receptor binding of substance P (SP), methionine-enkephalin (ME) and thyrotropin-releasing hormone (TRH) was determined in eight brain regions and the spinal cord. The concentration of SP was decreased in the septum, striatum and hippocampus, and SP receptor binding was decreased in the septum, amygdala + pyriform cortex and hypothalamus. Scatchard analysis indicated that the decrease in the SP binding is mainly due to the decrease in the number of receptors. The concentration of ME was not changed, but ME receptor binding was decreased in the septum. The concentration of TRH was decreased in the frontal cortex, septum, amygdala + pyriform cortex and pons + medulla oblongata, but increased in the spinal cord. TRH receptor binding was decreased in the septum, amygdala + pyriform cortex and hypothalamus. Scatchard analysis indicated that the decrease in TRH binding is due to the decrease in the number of receptors. These results show that IM-stress affects the neuropeptide receptor as well as neuropeptide concentration, and that the septum is a very important region under IM-stress.

Up-regulation of thyrotropin-releasing hormone (TRH) receptors in rat spinal cord after codepletion of serotonin and TRH

Immunohistochemical evidence indicates the coexistence of serotonin and TRH in many raphe neurons. We examined the biochemical changes in TRH receptors after destruction of the serotonergic pathways by 5,7-dihydroxytryptamine (5,7-DHT). 2 weeks after an intracerebroventricular injection of 5,7-DHT, rats were killed, and specific brain regions were dissected on ice. Serotonin levels in the CNS of lesioned rats was reduced by 50-85% in all regions, with the highest reduction in the spinal cord and hippocampus. Immunoreactive TRH was reduced in the spinal cord by 70%, but other brain regions contained normal levels of TRH. TRH receptor binding was increased by 40% in the spinal cord of lesioned rats, but appeared unchanged in rostral brain regions in which no decrease in TRH content was detected. Scatchard plots of TRH receptor binding in the spinal cord indicated that the increased binding after 5,7-DHT administration reflected an increased receptor number. These findings suggest that biochemical up-regulation of TRH receptors occurs in the spinal cord following depletion of TRH.

Effects of guanine nucleotides, transition metals and temperature on enkephalin receptors of rat brain membranes

The interactions among guanine nucleotides, transition metals and enkephalin (ENK) receptors were investigated by assaying the [3H]ENK binding to brain synaptic membranes. In the presence of 100 mM NaCl, GTP and Gpp(NH)p reduced the binding of [3H]ENK at 0 degree C and 25 degrees C, reflecting a decrease mainly in the affinity of the binding sites. In the absence of sodium, however, guanine nucleotides only slightly lowered the ENK binding at 25 degrees C, and at 0 degree C they strongly increased the ENK binding dose-dependently. This nucleotide-induced increase of specific ENK binding reflects an increase in the affinity and the number of the binding sites. In micromolar concentrations, zinc and cupric ions inhibited the ENK binding to opioid receptors by respectively reducing the affinity and the number of binding sites. In the absence of sodium at 0 degree C, cupric ions completely antagonized the nucleotide-induced increase in the ENK binding, although zinc ions did not affect it. The results suggest complex interactions of transition metals and guanine nucleotides with ENK binding sites.