GABA-ergic influence on the crossed extensor reflex

The lateral vestibular nucleus is a site for the depressant action of benzodiazepines on the crossed extensor reflex

The effects of diazepam microinjected into the subcerebellar nuclei, the lateral vestibular nucleus (LVN), the red nucleus and the substantia nigra were investigated using the crossed extensor reflex (CER) model in chloralose-anesthetized rats. Microinjections of diazepam at doses of 1 and 25 ng/animal into the left LVN, ipsilateral to the testing muscle, produced a dose-related CER depression, while injections of diazepam at a dose of up to 100 ng/animal into the right LVN and into other regions tested produced little effect. The CER depression produced by diazepam could be clearly reversed by a subsequent intravenous administration of Ro 15-1788, a benzodiazepine antagonist. Electrical lesions of the left LVN decreased the amplitude of CER in otherwise untreated rats. Intravenously administered diazepam at a dose of 0.01 or 0.05 mg/kg decreased spontaneous firing of LVN neurons. Iontophoretically applied GABA or flurazepam, a water-soluble benzodiazepine, decreased the spontaneous firing of LVN neurons. Depressant actions on neuronal activity in the LVN produced by both diazepam and flurazepam were clearly antagonized by intravenously administered Ro 15-1788, while it failed to reverse the depression produced by GABA. These findings indicate that the LVN is a major site of the CER depressant action of benzodiazepines and a possible mechanism of action is discussed.

GABAergic modulation of a substance P-mediated reflex of slow time course in the isolated rat spinal cord

The effects of gamma-aminobutyric acid (GABA) and other drugs which interact with GABA receptors were studied on a reflex of slow time course in the spinal cord preparation isolated from the neonatal rat. A single shock to a dorsal root (L3-L5) elicited a stereotyped series of reflexes, consisting of fast and slow components, recorded from the contralateral ventral root of the corresponding segment. The slow component, i.e. the contralateral slow ventral root potential (v.r.p.) had a time-to-peak of 2-5 s and lasted 20-30 s. Bath-application of GABA (5-20 microM) or muscimol (0.05-0.5 microM) caused a decrease in the amplitude of the contralateral slow v.r.p. without producing any change in the d.c. potential recorded from the ventral root. The monosynaptic reflex recorded from the ipsilateral ventral root was not changed by the drugs at these concentrations. Diazepam (0.1-1 microM) potentiated the depolarizing response of the dorsal root to GABA and markedly depressed the contralateral slow v.r.p. Neither the d.c. potential of the ventral root nor the dorsal root was changed by diazepam. The monosynaptic reflex was also unaffected by the drug. Bicuculline (1 microM) suppressed the GABA-induced depolarization recorded from the dorsal root whilst it markedly potentiated the contralateral slow v.r.p. Baclofen at concentrations from 0.01 to 0.1 microM reduced the contralateral slow v.r.p. The inhibitory action of baclofen on the contralateral slow v.r.p. was more marked than on the monosynaptic reflex. 7 The depolarization of the ventral root induced by a brief application of substance P (SP) was depressed by muscimol, diazepam and baclofen, whereas the depolarization was potentiated by bicuculline. 8 The present results suggest that an intraspinal GABAergic inhibitory mechanism plays a role in the modulation of certain slow spinal reflexes. They also support the hypothesis that SP released from certain primary afferent fibres is a neurotransmitter involved in the contralateral slow v.r.p.

The development and recovery of motor function in spinal cats. II. Pharmacological enhancement of recovery

The effects of the gamma-aminobutyric acid (GABA) antagonist bicuculline (BCC) on hindlimb motor performance was examined in mature spinal cats with spinal cord transections made either on the day of birth or at approximately two weeks postpartum and in chronic adult cats with spinal transections made in adulthood. In those adult operates, whose pre-drug performance was poor compared to neonatal operates, treadmill locomotion and weight support were improved dramatically by BCC administration. In neonatal operates (examined as adults), BCC administration increased the force of extension to abnormal levels and this increase appeared to disrupt locomotion. Immunocytochemical localization of GABA's synthetic enzyme, glutamic acid decarboxylase (GAD) within the spinal cords of these animals revealed an abnormal distribution of GAD reaction product only in newborn operates. The behavioral results indicate that the full extent of recovery in adult operates is prevented by inhibitory influences and this may contribute to the comparatively superior performance of neonatal operates i.e., the infant lesion effect. The anatomical results suggest that one requirement for the normal development of some intrinsic spinal circuitry is transneuronal regulation mediated by the maturation of descending systems.

Supraspinal depressant action of diazepam on the crossed extensor reflex

The effects of diazepam on the crossed extensor reflex (CER) were compared between intact and spinally transected rats and also between IVth and lateral ventricular administration. The CER of the chronic spinal rat was not influenced by 0.05-2 mg/kg doses of diazepam, which was 5-200 times larger than the ED50 of the intact preparation. The IVth ventricular injection required only half as much diazepam compared to the lateral ventricular injection and the elapsed time between drug administration to the peak effect was significantly shorter. According to these findings, benzodiazepines seem to act primarily on the supraspinal structure near the IVth ventricle and thus depress the CER.

Interfering with inhibition may improve motor function

Motor behavior not seen in newborn cats can be revealed by spinal transection and is therefore normally suppressed. Motor performance of these spinal kittens after reaching adulthood surpasses that of chronic adult-operated spinal cats but the latter display a significant improvement when GABAergic inhibition is blocked pharmacologically. Thus, inhibitory processes influence normal motor development as well as recovery of function after neurological damage.

The mesencephalic locomotor region in cat: effects of local applications of diazepam and gamma-aminobutyric acid

In decorticate unanaesthetized cats, displaying sequences of stereotyped locomotor movements in response to electrical stimulation of their footpad, local injections were made into the mesencephalic locomotor region (MLR). Diazepam (at 10 micrograms) and GABA (at 500 micrograms) induced complete suppression for 5-10 min, followed by partial amplitude recovery. It is suggested that: (i) MLR contains both diazepam and GABA receptors, and (ii) diazepam (and GABA) blockade is only short lasting because of a substitution (vicariancy) process through other structures also involved in locomotion.

Caerulein, a cholecystokinin-related peptide, depresses somatic function via the vagal afferent system

Intravenous doses of 0.5-8 micrograms/kg (0.3-4.9 nmol/kg) of caerulein, a cholecystokinin-related peptide, depressed the crossed extensor reflex response of chloralose-anesthetized rats in a dose-dependent manner. Intraventricularly administered caerulein was effective only at high doses (1-2 micrograms/animal, i.e., 0.6-1.2 nmol/animal). Cervical vagotomy completely abolished the inhibitory effect of peripherally injected caerulein. Electrical stimulation of vagal afferent nerves could simulate the caerulein effect to some extent. These results suggest that the primary site of action of peripherally administered caerulein is located in the gastrointestinal tract and thus the generated afferent vagal impulses mediate the reflex depression via the nucleus tractus solitarius and other as yet unclarified brain stem structures.