Convulsive seizures induced by N-methyl-D-aspartate microinjection into the mesencephalic reticular formation in rats

Effects of microinjections of a single 2 or 10 nmol dose of N-methyl-D-aspartate (NMDA) into the unilateral mesencephalic reticular formation (MRF) on behavior and electroencephalogram were examined in rats (n=18) during a 15 min period (Exp. 1), and subsequent effects of sound stimulation with key jingling applied at 15, 30, and 45 min after the injections were observed (Exp. 2). The microinjections of 2 nmol dose of NMDA (n=10) induced hyperactivity (9 of 10 rats) and running/circling (8 of 10 rats) in Exp. 1, and hyperactivity (3 of 10 rats) in Exp. 2. Moreover, the microinjections of 10 nmol dose of NMDA (n=8) induced not only hyperactivity (8 of 8 rats) and running/circling (7 of 8 rats) but also generalized tonic-clonic seizures (GTCS) (5 of 8 rats) in Exp. 1; these seizure patterns were also elicited by sound stimulation in Exp. 2. The seizure patterns were accompanied by electroencephalographic seizure discharges in the MRF and the motor cortex. In contrast, the control group rats (n=10) which received a single dose of saline microinjection into the unilateral MRF showed no behavioral or electroencephalographic changes in both Exp. 1 and 2. These findings suggest that the MRF has an important role in the development of GTCS, which follows hyperactivity and running/circling, and that potentiation of excitatory neurotransmission in the MRF participates in the development of audiogenic seizures as well as GTCS.     

Convulsive seizures induced by alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid microinjection into the mesencephalic reticular formation in rats

Effects of microinjections of a single 2 or 10 nmol dose of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) into the unilateral mesencephalic reticular formation (MRF) on behavior and on the electroencephalogram were examined in rats (n=30) over a 15-min period (Exp. 1); subsequent effects of sound stimulation with key jingling applied at 15, 30, and 45 min after the injection were observed (Exp. 2). The microinjections of a 2 nmol dose of AMPA (n=15) induced hyperactivity (15 of 15 rats) and running/circling (10 of 15 rats) in Exp. 1, and hyperactivity (5 of 15 rats) in Exp. 2. Moreover, the microinjections of a 10 nmol dose of AMPA (n=15) induced hyperactivity (15 of 15 rats), running/circling (13 of 15 rats), generalized tonic-clonic seizures (GTCS) (4 of 15 rats), and amygdala kindling-like seizures (AMKS) (8 of 15 rats) in Exp. 1; electroencephalographic seizure discharges were predominantly observed in the MRF during hyperactivity, running/circling and GTCS, while those predominantly observed in the amygdala were during AMKS. In Exp. 2, hyperactivity (15 of 15 rats), running/circling (14 of 15 rats) and GTCS (6 of 15 rats) were elicited by sound stimulation, although AMKS were not. The control group of rats (n=15) which received a single dose of saline microinjection into the unilateral MRF showed no behavioral or electroencephalographic changes in both Exp. 1 and 2. These findings suggest that potentiation of excitatory amino acid neurotransmission induced by AMPA injection into the MRF plays an important role not only in the development of hyperactivity, running/circling, GTCS and AMKS, but also in the development of audiogenic seizures.     

Convulsive seizures in rats induced by N-methyl-D-aspartate injection into the massa intermedia.

The behavioral and electroencephalographic effects of N-methyl-D-aspartate (NMDA, 25 nmol/1 microliter) injection into the massa intermedia (MI) was examined in rats. The injection caused violent running/jumping and shrill vocalization without evidence of EEG seizure in the hippocampus (HP) and amygdala (AM). Animals with the injection site located in the reuniens nucleus subsequently developed generalized tonic and then clonic seizure, leading to fatal status epilepticus in some animals. Intermittent or continuous EEG discharge in the limbic system was found during clonic seizures. These findings suggest that the NMDA receptor in the reuniens nucleus in the MI participates in the generation and expression of convulsive seizure in rats.     

N-methyl-D-aspartate unilaterally injected into the dorsal striatum of rats produces contralateral circling: antagonism by 2-amino-7-phosphonoheptanoic acid and cis-flupenthixol.

To evaluate the possible contribution of dorsal striatal glutamate receptors to motor behavior, circling responses were observed in rats following unilateral intrastriatal microinjections of the agonist, N-methyl-D-aspartate (NMDA) or the antagonist, 2-amino-7-phosphonoheptanoic acid (APH). The role of dopamine (DA) in NMDA-produced circling also was evaluated. In experiment 1, an NMDA dose of 5.0 micrograms (in 0.5 microliter), but not 0.5 or 0.05 microgram produced significant contraversive circling. In experiment 2, an APH dose of 10.0 micrograms but not 1.0 or 0.1 microgram produced significant ipsiversive circling. In experiment 3, microinjection of the ineffective 0.1 microgram dose of APH or a dose (20 micrograms) of the DA antagonist, cis-flupenthixol, that did not produce circling when administered alone, significantly reduced the circling response produced by the 5.0 micrograms dose of NMDA. As NMDA produced circling in the same direction as that seen following similar unilateral injections of locomotion-stimulating DA agonists, the present results suggest that glutamate, acting via NMDA receptors in the dorsal striatum, may exert an excitatory influence on motor systems. The observation that a DA receptor blocker antagonized the NMDA response further suggests that the observed motor excitatory effect of glutamate at NMDA receptors requires concurrent stimulation of DA receptors in the same region of the striatum.     

Characterization of audiogenic-like seizures in naive rats evoked by activation of AMPA and NMDA receptors in the inferior colliculus

The role of glutamate receptors in the inferior colliculus (IC) in audiogenic and audiogenic-like seizures was investigated in adult rats with transient neonatal hypothyroidism by 0.02% propylthiouracil (PTU) treatment through mother's milk (PTU rats) and in naive rats treated intracisternally with N-methyl-d-aspartate (NMDA), alpha-amino-3-hydroxy-5-methyl-4-isoxazole-proprionic acid (AMPA), or cyclothiazide, an inhibitor of rapid AMPA receptor desensitization. All rats showed audiogenic or audiogenic-like seizures characterized by running fit (RF) and generalized tonic-clonic seizures (GTCS). While systemically administered MK-801 inhibited GTCS, intracisternally administered NBQX inhibited RF and GTCS in both audiogenic and audiogenic-like seizures. Auditory stimulation shortened the latency to GTCS induced by AMPA, but not NMDA, at a subclinical dose and further elongated the shortened duration of RF, but not GTCS, induced by MK-801 pretreatment. Furthermore, Northern blot analysis was used to evaluate the expression of the immediate-early gene c-fos in the IC following induction of audiogenic or audiogenic-like seizures. The significant induction of c-fos mRNA by audiogenic seizures in PTU rats or by AMPA- or cyclothiazide-induced seizures in naive rats was prominent in the IC. MK-801 suppressed c-fos mRNA expression in the IC induced by audiogenic seizures in PTU rats or by AMPA-induced seizures in naive rats. NBQX suppressed the expression of c-fos mRNA in the IC induced by AMPA-induced seizures but did not suppress c-fos mRNA in PTU rats or rats with cyclothiazide-induced seizures. Auditory stimuli failed to affect c-fos mRNA induction by AMPA. The present study suggests that audiogenic-like seizures can be reproduced by glutamate receptor agonists in which AMPA receptors are primarily linked to the initiation of audiogenic seizures (RF) while NMDA receptors presumably located within the IC are involved in the propagation of GTCS in audiogenic seizures.     

Ventral pallidal microinjections of receptor-selective opioid agonists produce differential effects on circling and locomotor activity in rats

Locomotor activity was investigated following microinjections of receptor-selective opioid agonists into the ventral pallidum (VP) of rats. In Expt. 1, male Long-Evans rats were treated with unilateral microinjections of the μ agonist [d-Ala²-MePhe⁴, Gly-ol⁵]-enkephalin (DAGO), the σ agonist [d-Pen²,d-Pen⁵]-enkephalin (DPDPE) or the κ agonist U50,488H, and the rate and duration of circling behavior were measured. DAGO (0.01, 0.1, 1.0 nmol) procedure a dose-dependent increse in contralateral circling; pretreatment with 1.0 mg/kg naltrexone blocked the circling induced by the highest dose. The behavioral effect was largest when injections were targeted at the VP rather than structures dorsal to the VP. In contrast to DAGO, intrapallidal DPDPE (0.01, 0.1, 1.0, 10.0 nmol) produced a slight increase in contralateral circling only at the highest dose and U50, 488H (0.01, 0.1, 1.0, 10.0 nmol) produced no effect. In Expt. 2, the effects of bilateral injections of DAGO, DPDPE and U50,488H were tested in photocell activity ☐es. DAGO produced a dose-dependent increase in locomotor activity and this increase was decreased by 1.0 mg/kg naltrexone. A slight increase in activity was observed with the highest dose of DPDPE, and a slight decrease was observed with the highest dose of U50,488H. These findings confirm that opiate actions in the VP contribute to opiate-induced locomotion and suggest that μ and to some extent σ receptors are involved in this behavior.     

Effect of precollicular transection on audiogenic seizures in genetically epilepsy-prone rats

Previous studies have demonstrated that generalized tonic-clonic seizures (GTCS) consisting of running/bouncing clonic and tonic extension can still be elicited in rats after brain transections which separate forebrain from brain stem, showing that forebrain circuitry is not required for GTCS. Inasmuch as sound-induced generalized tonic-clonic seizures in rodents are characterized by running-bouncing clonic and tonic convulsions, we have hypothesized that these are brain stem seizures that can occur independently of the forebrain. To test this hypothesis, we examined the response of two strains of genetically epilepsy-prone rats (GEPR-3s and GEPR-9s) to seizure-evoking auditory stimuli 3 h after a precollicular transection or sham surgery performed under ether anesthesia. In addition, the effect of a precollicular transection on audiogenic seizures was evaluated in normal rats made susceptible to such seizures by infusing NMDA into the inferior colliculus. Following the transection 58% of GEPR-9s displayed a sound-induced tonic-clonic convulsion and the remaining 42% exhibited a sound-induced seizure when subjected to stimulation 5 min after a subconvulsant dose of pentylenetetrazol (PTZ). While sham surgery and the precollicular transection both reduced sound-induced seizure severity in GEPR-3s, the full seizure response could be elicited by sound stimulation following a subconvulsant dose of PTZ. Moreover, the audiogenic seizures in normal rats rendered susceptible by NMDA were unaltered by the precollicular transection. These findings show that the anatomical circuitry required for generalized tonic-clonic seizures evoked by sound stimulation in rodents resides within the brain stem.     

Differential effects of NMDA antagonists microinjections into the nucleus reticularis pontis caudalis on seizures induced by pentylenetetrazol in the rat

It has been shown that NMDA antagonists block the tonic but not the clonic component of seizures when they are injected in the oral region of the rat pontine reticular formation (PRF). The participation of the caudal PRF in the effects of NMDA antagonists upon the tonic and the clonic components of generalized seizures induced by pentylenetetrazol (PTZ) is unknown. The aim of the present study was to evaluate the effects of unilateral microinjections of competitive and non-competitive NMDA antagonists, 2-amino-7-phosphonoheptanoic acid (AP-7) and dizocilpine (MK-801), respectively, into the nucleus reticularis pontis caudalis of the rat PRF upon seizures induced by PTZ (70 mg/kg i.p.). MK-801 induced a dose-related decrease both in the incidence of generalized tonic-clonic seizures (GTCS) and in the presence of spikes in the EEG. MK-801 also increased GTCS latency. On the contrary, AP-7 did not have effects on GTCS. Interestingly, it induced ipsilateral circling behavior. These results suggest that in the caudal region of the rat PRF only non-competitive NMDA antagonists should block the generation of tonic and clonic components of generalized seizures.     

Reverse microdialysis of N-methyl- -aspartic acid into the lateral hypothalamus of rats: effects on feeding and other behaviors

The effects of reverse microdialysis of N-methyl- -aspartic acid (NMDA) into the lateral hypothalamus (LH) on feeding and other behaviors were examined. Consistent with similar studies utilizing central microinjections, NMDA reverse microdialysed into the rat LH rapidly elicited a strong concentration-dependent stimulation of feeding. The minimum perfusate concentration of NMDA needed to elicit feeding with reverse microdialysis was 660 μM, a concentration 1/50 of that needed with pressure injections. Further, eating responses could be consistently elicited in sequential tests separated by 2–4 h in the same subject, and the magnitude of the eating in the first and second tests was highly correlated (r=0.87). Behavioral analysis revealed that the main response to NMDA consisted of eating without the concomitant hyperactivity produced by central microinjections of this agonist. The other behaviors exhibited during NMDA administration were those that normally occur during spontaneous feeding in rats. Also, rats precisely compensated for the increased food intake elicited by NMDA by reducing spontaneous feeding during the subsequent nocturnal phase, so as to maintain normal daily intakes. In contrast, N-methyl- -aspartate (NMLA) reverse microdialysed in to the LH (660 μM) did not elicit feeding nor affect any other behaviors we examined. These data support a role for LH glutamate and NMDA receptors in the control of feeding.     

Focal microinjection of carbachol into the periaqueductal gray induces seizures in the forebrain of the rat

Previous studies have reported that the repetition of running-bouncing and tonic-clonic seizures mediated by brainstem structures eventually elicits seizure activity in the forebrain. The purpose of the present study was to determine if the periaqueductal gray (PAG) region is a component of the neural network through which brainstem seizures elicit forebrain seizures. Bilateral microinjection of 40 nmol carbachol into the PAG region of rats induced arrested, staring behavior accompanied by epileptiform electrocorticogram (ECoG) afterdischarge recorded from the parietal cortex. In two animals limbic seizure activity similar to kindled amygdala seizures was also induced. The carbachol effect was dose-related as the 40 nmol dose induced a significantly greater duration of ECoG afterdischarge than a 20 nmol dose. The carbachol effect was mediated by muscarinic receptors as bilateral 50 nmol atropine microinjection 1 min prior to 40 nmol carbachol microinjection inhibited all seizure activity. Immunohistochemical detection of the proto-oncogene c-fos was used to verify that seizure activity was induced in forebrain regions. Rats with seizures induced by PAG carbachol microinjections exhibited dense c-fos-like immunoreactivity in the dentate gyrus but not the CA(1) or CA(3) regions, amygdala, piriform cortex, perirhinal cortex or hypothalamus. In addition, PAG microinjection of 10 nmol N-methyl-D-aspartic acid (NMDA) induced wild-running convulsions while 400 pmol bicuculline induced clonic spasms, myoclonic activity or limbic seizures. These results indicate that stimulation of the PAG, a brainstem structure, is sufficient to induce forebrain seizures. Since the forebrain seizures were induced by a single carbachol administration, it is proposed that the PAG serves as a pathway for caudal-rostral seizure generalization.     

Fos expression in GABAergic cells and cells immunopositive for NMDA receptors in the inferior and superior colliculi following audiogenic seizures in rats

Given the evidence that the inferior colliculus (IC) and superior colliculus (SC) seem to play key roles in connecting auditory pathways and seizure output pathways in the neuronal network for audiogenic seizures (AS) in rats, we examined Fos activation in GABAergic cells and cells immunopositive for glutamate N-methyl-D-aspartate (NMDA) receptors in the IC and SC following AS using the double-labeling procedure. Generalized tonic-clonic seizures (GTCS), which developed as an advanced form of AS in some of the susceptible rats, induced an increase in Fos expression in three IC substructures-the dorsal cortex of IC (DCIC), central nucleus of IC (CIC), and external cortex of IC (ECIC)-and in one SC substructure, the deep gray layer of SC (DpG). Compared with the rats showing GTCS, rats exhibiting wild running (WR) without proceeding to GTCS showed a different pattern of AS-induced Fos expression. The DpG in the WR animals showed no significant increase in the levels of Fos-like immunoreactivity. The degrees of Fos activation that occurred in GABAergic cells and cells immunopositive for NMDA receptors were similar in the DCIC, CIC, ECIC, and DpG following AS. These results suggest that Fos activation in the DpG is involved in the development from WR to GTCS in AS-susceptible rats. They also provide some evidence that some GABAergic neurons in the IC and SC and glutamatergic afferents (via NMDA receptors) to these structures are activated by AS.     

Increased NMDA-induced excitability during ethanol withdrawal: a behavioural and histological study

Intrahippocampal injections ofN-methyl-d-aspartic acid (NMDA) leads to neurodegeneration in a dose-dependent manner. Chronic administration of ethanol to animals leads to CNS tolerance and dependence. Hyperexcitability following ethanol withdrawal is thought to be related to increased sensitivity of the NMDA receptors. The purpose of this study was to investigate this predisposition to hyperexcitability by intrahippocampal injection of low doses of NMDA. Using control and ethanol-withdrawn male Wistar rats, behavioural indices were determined immediately after injection and morphological damage was assessed after a period of recovery. There was significantly increased hyperactivity in the ethanol-treated rats immediately after injection. Morphological damage resulting from 5 nmol of NMDA was significantly greater in the CA3 region of the hippocampus in these animals. These data support the hypothesis that ethanol dependence and subsequent withdrawal is associated with increased sensitivity to NMDA which may underlie ethanol withdrawal-associated brain damage.     

Specific group II metabotropic glutamate receptor activation inhibits the development of kindled epilepsy in rats

The effects of intracerebral administration of the group II metabotropic glutamate receptor agonist, 2R,4R-APDC, were tested on both the development of amygdaloid kindling and on fully developed stage 5 amygdala kindled seizures. The development of amygdaloid kindling was significantly retarded in 2R,4R-APDC (10 nmol in 0.5 μl) treated animals compared to control animals over a period of 8 days. At a low dose, 2R,4R-APDC (0.1 nmol) caused a 42.5±26.6% increase of the generalised seizure threshold in fully kindled animals. As higher doses were administered, however, the changes in generalised seizure threshold were less marked, and even a small decrease in the threshold was seen (−19.6±5.36% at 10 nmol). The agonist 2R,4R-APDC inhibited depolarization-induced release of [³H]d-aspartate from cortical synaptosomes with an IC₅₀ value of 0.29 μM. This effect was maximal at 1 μM, and decreased with dose thereafter. These findings suggest that the selective activation of the group II metabotropic glutamate receptors by agonists such as 2R,4R-APDC may be of therapeutic potential in the treatment of seizure disorders.     

The gain of the baroreflex bradycardia is reduced by microinjection of NMDA receptor antagonists into the nucleus tractus solitarii of awake rats

The baroreflex activation with phenylephrine infusion produces a bradycardic response. In the present study, the role of NMDA receptors in the nucleus tractus solitarii (NTS) in the processing of the parasympathetic component of the baroreflex was evaluated using acid phosphonivaleric (AP-5), a selective NMDA receptor antagonist. Baroreflex activation was performed before and after bilateral microinjection of AP-5 into the intermediate commissural NTS (0.5 mm lateral to the midline). Microinjection of the vehicle (saline, 0.9%) or a dose of 2 nmol/50 nl of AP-5 into the NTS produced no effect on the gain of the baroreflex while a dose of 10 nmol/50 nl of AP-5 produced a significant reduction in the gain of the baroreflex 2 min after microinjection [-1.43+/-0.22 vs. -0. 43+/-0.03 bpm/mmHg, (n=6)], with a return to control levels 10 min after the microinjections. The dose of 10 nmol/50 nl was selective for NMDA receptors considering that the cardiovascular responses to microinjection of AMPA (0.05 pmol/50 nl), a non-NMDA receptor agonist, were not affected by this dose of AP-5 and the responses to microinjection of NMDA (2 nmol/50 nl) were blocked. The data show that the bradycardic response to baroreflex activation was blocked by AP-5 microinjected into the NTS, indicating that the neurotransmission of the parasympathetic component of the baroreflex is mediated by NMDA receptors in the NTS.     

Role of α1-adrenergic receptors in the intermediolateral column in mediating the pressor responses elicited by the stimulation of ventrolateral medullary pressor area

Microinjections of α1-adrenergic receptor agonists into the intermediolateral cell column of the spinal cord (IML) elicit sympathoexcitatory responses. This observation, together with the identification of projections of epinephrine-containing cells in the rostral ventrolateral medullary pressor area (VLPA) to the IML, has prompted speculation that epinephrine may mediate pressor responses to the stimulation of the VLPA. This hypothesis was tested in pentobarbital-anesthetized, artificially ventilated, male Wistar rats. A mesenteric arterial branch was cannulated for monitoring blood pressure. Pressor responses were elicited predominantly from T8–T10 by injections (1.7 nmol/20 nl) ofl-glutamate into the IML; maximum pressor responses(29.3 ± 4mmHg) were elicited from T9. Pressor responses were also elicited by injections of epinephrine into the IML at T9; maximum pressor effect(16.3 ± 1.2mmHg) was elicited by a dose of 0.05 pmol/20 nl. This effect of epinephrine at T9 was blocked by prior injections of prazosin (a selective α1-adrenergic receptor blocker; 0.125 pmol/20 nl) at the same site. Stimulation of the VLPA by unilateral microinjections of glutamate elicited pressor responses(56 ± 12mmHg). Bilateral injections of prazosin at T8–T10, in the dose ( (0.125 pmol) that blocked a maximally effective dose of epinephrine, did not block the pressor responses to subsequent injections of glutamate into the VLPA. On the other hand, bilateral microinjections of AP-7 (an NMDA receptor blocker; 1 nmol/20 nl), but not DNQX (10 pmol; a non-NMDA receptor blocker), into the IML at T8–T10 blocked the pressor effects of the subsequent injections of glutamate into the VLPA. At the dose used, AP-7 did not alter pressor responses to injections of kainic acid or AMPA into the IML at T9. These results suggest that under the experimental conditions in this study, the pressor responses following the stimulation of VLPA are not mediated by α1-adrenergic receptors in the IML. On the other hand, NMDA receptors in the IML do mediate these pressor responses.     

The effect of l-arginine and l-NAME on pentylenetetrazole induced seizures in ovariectomized rats,an in vivo study

The role of ovarian hormones and nitric oxide (NO) on seizure and their interaction have been widely investigated. The present study carried out to evaluate the effect of chronic administration of l-arginine (lA) and l-NAME (lN) on pentylenetetrazole (PTZ) induced epilepsy in ovariectomized (OVX) and naïve female rats.Fourty-eight female rats were randomly divided into six groups (n = 8) as follows: (1) sham, (2) ovarectomized (OVX), (3) sham-lA, (4) sham-lN, (5) OVX-lA, and (6) OVX-lN.The animals of sham-lA and OVX-lA received daily injection of 500 mg/kg l-arginine (i.p.) during 4 weeks. Sham-lN and OVX-lN were treated by 10 mg/kg l-NAME (i.p.) daily for 4 weeks. The animals of sham and OVX groups received 1 ml/kg saline (i.p.) instead of l-arginine and l-NAME. The latencies to minimal clonic seizures (MCS) and generalized tonic–clonic seizures (GTCS) after intraperitoneal injection of penetylenetetrazole (PTZ, 90 mg/kg) was recorded and compared between groups.A significant increase in the GTCS, but not MCS, latency was seen in OVX rats in comparison with sham-operated animals. Pretreatment of animals with l-NAME resulted in a significant increase in the GTCS and MCS latencies in sham group while no significant effects were seen in OVX rats. On the contrary, while pretreatment with l-arginine had no effects on MCS and GTCS latencies in sham group, a significant decrease in GTCS latency was observed in OVX rats.It is concluded that ovarian sex hormones affect seizure thresholds induced by PTZ and NO has a role on seizures susceptibility following PTZ administration. This NO effect might be differing in the presence or absence of ovarian hormones, but further investigations need to be done.     

Evidence for a functional relationship between noradrenaline and neurohypophyseal peptides in the brainstem of rats

The possibility of a functional relationship between noradrenaline and neurohypophyseal peptides in the control of cardiovascular function in the nucleus tractus solitarius of rats has been investigated. The hypotensive response to microinjections of noradrenaline (20 nmol) was abolished by simultaneous (but not prior) microinjections of [Arg8]vasopressin (AVP) and oxytocin at doses (0.9 pmol) which alone had no effects on cardiovascular parameters. AVP plus noradrenaline actually resulted in a transient pressor effect. Simultaneous administration of [deamino-D-Arg8]vasopressin, a selective agonist of AVP V2 receptors, did not modify the effect of noradrenaline, whereas the specific V1 antagonist D-(CH2)5-Tyr(Me)-AVP partially decreased its intensity. When subthreshold doses of both AVP (0.9 pmol) and noradrenaline (10 pmol) were administered simultaneously, a pressor response was observed. In vasopressin-deficient Brattleboro rats, microinjections of 0.9 pmol AVP had no effects, but a marked pressor response was observed after the administration of a higher dose (9 pmol). In parent strain Long-Evans rats, noradrenaline (20 nmol) also produced a hypotensive response, but in Brattleboro rats microinjections of this amine elicited a marked pressor effect. In these rats, simultaneous administration of a subthreshold dose of AVP (0.9 pmol) reversed this response in such a way that a fall in blood pressure, similar to that observed in Long-Evans rats after injection of noradrenaline alone, was registered. These results provide evidence for a functional interaction between noradrenaline and neurohypophyseal peptides in the control of cardiovascular function in the brainstem.     

Enhancement of NMDA-induced increases in levels of endogenous adenosine by adenosine deaminase and adenosine transport inhibition in rat striatum

Unilateral microinjection of N-methyl-d-aspartate (NMDA) into striatum of rats subsequently killed by high-energy focused microwave irradiation significantly increased in vivo levels of endogenous adenosine. At a dose of 25 nmol NMDA, levels of adenosine in injected striata were 263% of levels in uninjected contralateral striata. An inhibitor of adenosine deaminase (deoxycoformycin, DCF) in combination with an inhibitor of adenosine transport (dilazep, DLZP) at a dose that did not affect levels of endogenous adenosine, potentiated NMDA-induced increases in adenosine levels to 426% of contralateral striata. In the presence of DCF and DLZP, NMDA dose-dependently increased levels of adenosine (% of contralateral striatum) from 166% at 10 nmol to 622% at 100 nmol. NMDA-induced increases in levels of endogenous adenosine were completely blocked by prior administration of the NMDA receptor antagonist MK 801 (dizocilpine). Inhibitors of adenosine metabolism and transport may provide therapeutic benefit by potentiating excitatory amino acid-induced increases in levels of endogenous adenosine in vivo.     

Intraventricular infusion of N-methyl-d-aspartate

Summary The purpose of this study was to document the early cerebrovascular consequences of excessive N-methyl-d-aspartate (NMDA) receptor activation. Five microliters of NMDA (100 nmol/l) or vehicle was infused over a 15-min period into the lateral ventricle of adult rats. The protein tracer horseradisch peroxidase (HRP) was injected intravenously for blood-brain bartier (BBB) studies. The intraventricular infusion of vehicle (n=5) caused no alterations in arterial blood pressure or microvascular damage away from the intraventicular probe tract. In contrast, NMDA infusion (n=8) led to a gradual increase in arterial blood pressure (mean 36 mm Hg). Multifocal regions of HRP extravasation were observed bilaterally throughout the neuraxis following NMDA infusion. Sites of BBB disruption and hemorrhage included brain regions bordering ventricular spaces. In addition, isolated foci of protein extravasation were commonly detected in the cerebral cortex, thalamus, basal forebrain, septum and cerebellum. Pretreatment with the noncompetitive NMDA antagonist MK-801 (2 mg/kg) substantially reduced the BBB responses to NMDA. However, microvascular abnormalities were seen in NMDA-infused rats where blood pressure elevations were inhibited by blood removal. In addition to neurons, cerebral blood vessels are also acutely affected by NMDA receptor activation. Blockage of NMDA receptor channels following brain injury may potentially provide protection by attenuating BBB breakdown and subsequent brain edema.Supported by USPHS Grants NS-05820, NS-30291 and NS-27127 and by the American Heart Association Grant-in-Aid 90-1133, with funds contributed by the Flordia Affiliate.     

Involvement of NMDA receptors in the hypotensive response to the injection of L-glutamate into the lateral hypothalamus of unanesthetized rats

We report that microinjections of L-glutamate (L-glu) or N-methyl-D-aspartic acid (NMDA) in the lateral hypothalamus (LH) of unanesthetized rats caused a hypotensive response. Guide cannulas were stereotaxically placed in the LH 3 days before the experiments, under tribromoethanol anesthesia. One day before the experiments, the femoral artery was cannulated for pulsatile arterial pressure (PAP), mean arterial pressure (MAP) and heart rate (HR) measurements. In the first experiment, unanesthetized rats received microinjections of 2.5, 5.0 or 10.0 nmol/100 nL of L-glu in the LH. Dose-dependent hypotensive responses were observed, without significant concomitant changes in heart rate. In a second group of experiments, 5.0 nmol of L-glu was microinjected into the LH before and 10 min after pretreatment with glutamatergic antagonists. Pretreatments with the non-selective ionotropic glutamatergic-receptor antagonist kynurenic acid or the selective NMDA receptor antagonists AP-7 and LY235959 significantly reduced the hypotensive response to microinjection of L-glu in the LH. Pretreatment with the selective AMPA-receptor antagonist NBQX or with vehicle did not affect the hypotensive response. The present results suggest that the hypotensive response to the injection of L-glu into the LH is mediated by NMDA receptors.