C. Y. Chai

Corticotrophin-releasing factor produces a long-lasting enhancement of synaptic efficacy in the hippocampus

We have previously demonstrated that intra‐hippocampal injection of corticotrophin‐releasing factor improved memory retention of an inhibitory avoidance learning in rats; while the electrophysiological effects corticotrophin‐releasing factor produces on hippocampal neurons are largely uncharacterized. In the present study, we found that corticotrophin‐releasing factor injected into the dentate gyrus of hippocampus produced a dose‐dependent and long‐lasting enhancement in synaptic efficacy of these neurons, as measured by an increase in the amplitude and slope of population excitatory postsynaptic potentials, as well as the amplitude of population spike. The onset of corticotrophin‐releasing factor‐induced potentiation was slow. It was observed approximately 40–60 min after corticotrophin‐releasing factor administration and lasted for more than 5 h. This effect of corticotrophin‐releasing factor was blocked by pretreatment with the cyclase‐adenosine‐3,5‐monophosphate (cAMP) inhibitor Rp‐adenosine‐3,5‐cyclic monophosphothiolate triethylamine (Rp‐cAMPS) and partially blocked by the N‐methyl‐D‐aspartate receptor antagonist MK‐801. Further, pretreatment with corticotrophin‐releasing factor receptor antagonist dose‐dependently diminished tetanization‐induced long‐term potentiation, and corticotrophin‐releasing factor and tetanic stimuli had an additive effect on hippocampal neuron excitation. Moreover, direct injection of corticotrophin‐releasing factor increased cAMP level in the dentate gyrus. These results together suggest that corticotrophin‐releasing factor‐induced potentiation simulates the late phase of tetanization‐induced long‐term potentiation and cAMP seems to be the messenger mediating this effect. Moreover, corticotrophin‐releasing factor‐induced potentiation and long‐term potentiation may share some similar mechanisms, and corticotrophin‐releasing factor is probably involved in the neural circuits underlying long‐term potentiation. Thus, corticotrophin‐releasing factor may play an important role in modulating synaptic plasticity in the hippocampus.

Stress selectively influences center region activity of mice in an open field

Effects of immobilization and footshock stress on locomotor activity in different areas of an open field were examined in mice. Center region activity, peripheral region activity and total activity were used as measurement indices. These results indicate that both immobilization and footshock stress significantly increased total activity across 24 min of behavioral testing. Further analyses revealed that the difference in total activity between the experimental and control groups were mainly attributal to an increase in center region activity. Both stress manipulations markedly augmented peripheral region activity for only the first 6 min. More important, when the proportion of center to peripheral activity was used as an index, both experimental groups manifested an inverted U shape relationship with the maximum effect occurring between 13-18 min of behavioral testing. Similarities of these activity measures in response to different stressors suggests that common neurochemical and/or neurohormonal mechanisms may mediate these behavioral changes.

Stress and corticotropin-releasing factor potentiate center region activity of mice in an open field

The effects of corticotropin-releasing factor (CRF) and previously published effect of stress on the locomotor activity of mice in different regions of an open field were compared. Intracerebroventricular (ICV) administration of 0.2 μg CRF, like stress, significantly increased center region activity; this effect was reversed by the benzodiazepine diazepam (DZP) at a dose of DZP having no significant effect alone. A dose of DZP that antagonized CRF-potentiated center region activity did not block amphetamine-stimulated center area activity. These results suggest that CRF may normally be responsible for many behavioral changes during conditions of stress.

The nNOS/cGMP signal transducing system is involved in the cardiovascular responses induced by activation of NMDA receptors in the rostral ventrolateral medulla of cats

Nitric oxide (NO) is synthesized from L-arginine by NO synthase (NOS). NO stimulates the soluble form of guanylyl cyclase (sGC) and induces accumulation of cyclic guanosine monophosphate (cGMP). The purpose of this study was to examine whether the cardiovascular responses induced by N-methyl-D-aspartate (NMDA) in the rostral ventrolateral medulla (RVLM) depend on the actions of NOS and sGC. In anesthetized cats, the extracellular NO level was measured by in vivo voltammetry using a nafion/porphyrine/o-phenylenediamine-coated carbon-fiber electrode. Microinjection of NMDA into the RVLM produced hypertension and bradycardia associated with NO formation. These NMDA-induced responses were attenuated by prior injections of 7-nitroindazole, a neuronal NO synthase (nNOS) inhibitor, and 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one, a sGC inhibitor. These findings suggest that NO is involved in the NMDA-induced cardiovascular responses in the RVLM.

Contribution between dorsal and ventrolateral regions of medulla oblongata in vasomotor function of cats

In cats anesthetized with alpha-chloralose and urethane, the medulla oblongata was explored with electrical and/or chemical stimulation for vasopressor actions. Chemical stimulation included semimicroinjection of sodium glutamate or microinjection of dl-homocysteic acid (DLH). The dorsal (DM), particularly its dorsomedial (DMM) portion, and the ventrolateral (VLM) regions were found to be most sensitive to both electrical and chemical stimulation. In between these two regions there was an area in the ventral portion of the gigantocellular and the parvocellular reticular nuclei that was sensitive to electrical and somewhat sensitive also to chemical stimulation. The responses to chemical stimulation indicate the existence of perikarya in these three areas for vasopressor actions. The most active response of VLM followed microinjection of DLH into the region of nu. paragigantocellularis lateralis (PGL) and subretrofacial nu. These responses were about 30% greater than those from DM where the most active area was found in the DMM region including the nu. and tractus solitarius. When an extensive area of VLM was lesioned bilaterally with kainic acid (KA, 1 micrograms/100 nl), the resting systemic arterial blood pressure (SAP) fell 42% and the pressor response on DM stimulation fell by more than 80%. In contrast, after similar KA bilateral destruction of DM the resting SAP fell 38% and the pressor responses from 67%. When only a limited area in the PGL/subretrofacial nu. of the rostral VLM (3-4 pressor sites, 1 mm apart) on one side was lesioned, the resting SAP remained unaltered while the pressor response on the ipsilateral DMM decreased 76%. On the other hand, when a similar smaller KA lesion (3-4 sites) was made in the DMM, although the resting SAP did not change, the pressor response on stimulation of the ipsilateral rostral VLM decreased 28%. Further lesioning of the contralateral DMM then decreased the rostral VLM pressor response 62% without much alternation in the resting SAP. Results of the present experiments are in accordance with our previous results that neuronal perikarya for vasopressor action exist not only in VLM but also in DM (18,34) with the function of VLM slightly predominant over DM. It seems highly possible that reciprocal innervation exists between DM and VLM, at least between DMM and the PGL/subretrofacial nu. region.

Sympathoadrenal excitation and inhibition by lower brainstem stimulation in cats

Effects of stimulation of brainstem sites on hemodynamics and plasma catecholamine levels were assessed in cats under chloralose-urethane anesthesia. Pressor areas of the dorsal medulla (DM) and ventrolateral medulla (VLM) and the depressor area of the paramedian reticular nucleus (PRN) were stimulated electrically using a monopolar electrode, or chemically using sodium glutamate microinjection. Plasma levels of norepinephrine (NE) and epinephrine (EPI) were measured in caval blood above the adrenal veins. Electrical stimulation of the DM and VLM produced increases in blood pressure and in plasma NE and EPI levels that were enhanced after acute vagotomies. The NE and EPI responses were attenuated after acute, bilateral adrenalectomies, confirming augmented adrenomedullary secretion, whereas the pressor responses were intact. Injection of sodium glutamate into the same pressor regions of the DM or VLM also produced pressor responses and elevated plasma catecholamine levels, indicating that the responses resulted from activation of neuronal perikarya. Stimulation of the PRN attenuated pressor and catecholamine responses during stimulation of the DM and VLM. The results indicate that pressor responses during stimulation of the DM and VLM are due at least partly to activation of perikarya in these regions, are associated with but not dependent on adrenomedullary activation, and are enhanced after vagotomy; and that neurons of the PRN exert inhibitory modulation of the pressor and adrenomedullary responses during stimulation of VLM and DM.

Reduction of common carotid resistance upon stimulation of an area dorsal to the facial nucleus of cats

In chloralose-urethane-anesthetized cats, electrical stimulation and glutamate injection on a small reticular area just dorsal to the facial nucleus (DFA) elicited an ipsilateral reduction in the common carotid resistance (CCR-reduction) with no or minimal change in other cardiovascular parameters. CCR-reduction was mediated via facial and glossopharyngeal nerves, involving partially muscarinic and partially non-muscarinic mechanisms.

A single minute lesion around the ventral respiratory group in medulla produces fatal apnea in cats

In 35 adult cats anesthetized with intraperitoneal chloralose and urethane, the ventrolateral medulla was explored by microinjection of kainic acid (KA, 24 mM, 200 nl) with metal electrode-tubing or glass micropipette to determine regions which elicit persistent apnea. Persistent apnea is defined as: (1) In spontaneously breathing cats, termination of respiration over 3 min with a decrease of the mean systemic arterial pressure (MSAP) to 25 mm Hg. (2) In animals under artificial ventilation and paralyzed by gallamine, cessation of bilateral phrenic nerve (PNA) activities over 25 min. The apnea producing area was located dorsal to the rostral pole of the lateral reticular nucleus, ventromedial to the ambiguous nucleus and immediately caudal to the retrofacial nucleus. Functionally, this region includes the rostral part of the ventral respiratory group (rVRG) encompassing the pre-BOtzinger area. We define this region as the VRG apnea producing area (VRG-Apa). Fatal apneusis was observed under following conditions: (1) Persistent apnea was produced after a single KA microinjection in one side of the VRG-Apa (5 animals). Microinjection of sodium glutamate (0.25 M, 70-200 nl) in the same area produced only brief apnea, while microinjection of kynurenic acid (0.1 M, 200 nl) showed little effect on the respiration but slightly increased the SAP. (2) Positioning an electrode nearby but not in the VRG-Apa with or without KA injection did not produce apnea. But when a second electrode insertion to the opposite VRG-Apa immediately produced persistent apnea even without KA injection (6 animals). (3) Midsagittal division of the medulla 0-5 mm rostral to the obex produced persistent silence of PNA on both sides in artificial ventilated animals (7 animals), while similar division 0-5 mm caudal to the obex (4 animals) produced a brief but reversible quiescence of PNA. In conclusion, findings of the present study support the existence of a restricted region of VRG-Apa. VRG-Apa on both sides are closely connected, and integrity of both VRG-Apa is essential for normal respiration.

Differential effects on sympathetic nerve activities elicited by activation of neurons in the pressor areas of dorsal and rostral ventrolateral medulla in cats.

Changes of the nerve activity of the sympathetic renal and vertebral nerves were elicited by microinjection of sodium glutamate (50 nmol/100 nl) into the pressor areas of the dorsal (DM) and rostral ventrolateral medulla (RVLM) in cats under urethane-chloralose anesthesia. Animals were bilaterally vagotomized, artificially ventilated, and paralyzed with gallamine triethiodide. The vertebral nerve activity always increased when pressor responses were induced by DM or RVLM stimulation. However, the effects of medullary stimulation on the renal nerve activity were variable. Three types of renal nerve responses concomitant with the pressor responses were observed in either baroreceptor-intact or baroreceptor-denervated cats. They were: (1) augmentation (type I); (2) attenuation (type II); and (3) insignificant change (type III). Type I responses were often elicited by RVLM stimulation whereas type II responses were often elicited by DM stimulation. Findings suggested that neurons integrating these sympathetic nerve activities were not equally distributed in the pressor areas of DM and RVLM. This result supports the notion that neurons located in different pressor areas of the brainstem exert differential effects over different sympathetic nerve activities.

Differential biochemical mechanisms mediate locomotor stimulation effects by caffeine and nicotine in rats.

Effects of caffeine and the interactive effects of caffeine and nicotine on locomotor activity in rats were examined in the present study. Other than confirming previous reports that both drugs enhanced locomotion, we have also found that their effects on activity were additive. Meanwhile, results of various biochemical measures have revealed that at the minimum effective doses of caffeine and nicotine which facilitated locomotor activity, only one biochemical system was preferentially influenced by either drug alone. The most significant findings were that caffeine stimulated the release of catecholamines and nicotine decreased the concentrations of tyrosine and tryptophan in brain. The combined effects of caffeine and nicotine on these brain amines were not different from those of each drug alone. Together with the report that caffeine and nicotine had differential actions on different activity measures, the present results support the hypothesis that caffeine and nicotine affect locomotor activity via different neurochemical mechanisms.