Edgar Garcia-Rill

The pedunculopontine nucleus.

In an effort to account for a large number of reported functions mediated by a small portion of the midbrain, a hypothesis is advanced as a basis for discussion and not as established fact and is guided by reports from a large number of laboratories working on the same region but using widely disparate preparations. Overall, the hypothesized model suggests an underlying mechanism of action for what is essentially the ascending reticular activating system. The model proposed will hopefully be tested stringently in order to arrive at a better understanding of brain stem mechanisms modulating a host of rhythmic functions.

The mesencephalic locomotor region (MLR) in the rat

These studies demonstrate the presence of the MLR in the rat brain. Controlled locomotion on a treadmill could be induced by low level stimulation (less than 50 microA) of an area in the posterior midbrain following a precollicular-prenigral brainstem transection. This area included the lateral part of the cuneiform nucleus and anterior as well as posterior portions of the pedunculopontine nucleus. In addition, the presence of a subthalamic locomotor region in the fields of Forel was determined in rats after prethalamic transections.

The pedunculopontine nucleus—Auditory input, arousal and pathophysiology

This review describes the role of the pedunculopontine nucleus (PPN) in various functions, including sleep-wake mechanisms, arousal, locomotion and in several pathological conditions. Special emphasis is placed on the auditory input to the PPN and the possible role of this nucleus in the manifestation of the P1 middle latency auditory evoked response. The importance of these considerations is evident because the PPN is part of the cholinergic arm of the reticular activating system. As such, the auditory input to this region may modulate the level of arousal of the CNS and, consequently, abnormalities in the processing of this input can be expected to have serious consequences on the level of excitability of the CNS. The involvement of the PPN in such disorders as schizophrenia, anxiety disorder and narcolepsy is discussed.

The mesencephalic locomotor region. I. Activation of a medullary projection site

Previous anatomical studies from our laboratories have demonstrated descending projections of the physiologically identified mesencephalic locomotor region (MLR) to the medioventral medulla. In the present study, this area was activated electrically and chemically to determine the nature of locomotor events elicited in the precollicular-postmammillary transected cat. Low-amplitude (less than 70 microA), high-frequency (5-60 Hz) stimulation of a specific region in the medioventral medulla elicited controlled episodes of locomotion on a treadmill. Injections of cholinergic agonists into this area also were found to elicit short episodes of locomotion. Injections of cholinergic antagonists were found to block locomotion elicited by: electrical stimulation of the same area; injections of cholinergic agonists; or electrical stimulation of the MLR. Injections of substance P into the medioventral medulla were found to induce locomotion for longer periods of time than injections of cholinergic agonists. These findings demonstrate physiologically that a primary caudal termination site of the MLR is located in the medioventral medulla, and suggest that at least a portion of descending MLR projections employ acetylcholine and substance P as neurotransmitters.

The mesencephalic locomotor region. II. Projections to reticulospinal neurons

Single neurons were recorded extracellularly in an area of the medioventral medulla known to receive descending mesencephalic locomotor region (MLR) input. A large number (47%) of these cells were found to receive short-latency orthodromic input following stimulation of the physiologically identified MLR. Of the medioventral medulla neurons studied, 34% were found to project to the spinal cord (determined by antidromic activation following stimulation of the ventrolateral funiculus). Approximately one-half (17% of the total population) of these reticulospinal cells were found to receive short-latency orthodromic input from the MLR. The regional distribution of this group of reticulospinal cells corresponded with the area found to receive descending projections from the MLR in previous anatomical studies. In addition, electrical and chemical activation of the same region was found to elicit locomotion on a treadmill in a companion study. The present findings demonstrate that descending MLR projections influence a large number of medioventral medulla cells, some of which have direct spinal projections, and suggest that this area is a primary relay in the manifestation of MLR function.

Chemical activation of the mesecephalic locomotor region

Abstract Electrical stimulation of the mesencephalic locomotor region (MLR) in the precollicular-postmammillary transected cat is known to induce controlled locomotion on a treadmill. We have been able to induce and block locomotion in this preparation by using localized infusions of transmitters and their agonists and antagonists. Infusions of the GABA antagonists bicuculline and picrotoxin into the MLR elicit locomotion at low concentration (5 mM). Applications of muscimol (5 mM) or GABA (0.5 M) were found to block chemically-induced locomotion, as well as electrically-elicited and spontaneous walking. Priming infusions of Diazepam amplified the blockage of locomotion by GABA. On the other hand, applications of strychnine (10 mM) were ineffective in inducing stepping, as were infusions of the excitatory agents glutamic acid, acetylcholine and norepinephrine. These findings suggest that the MLR is under inhibitory GABAergic input. The substantia nigra is the only known afferent to the MLR located posterior to the brainstem transection, and is a likely source for this input. A model is proposed to account for our results, as well as those of others, and it provides a working hypothesis for the neurochemical events occuring in brainstem centers which modulate locomotor events.

Connections of the mesencephalic locomotor region (MLR) II. Afferents and efferents

Injections of a tritiated amino acid-fluorescent dye mixture were made unilaterally into the area of the mesencephalic locomotor region (MLR). After allowing for retrograde and anterograde transport, the same site was electrically stimulated to induce locomotion on a treadmill following a precollicular-postmamillary transection. The tritiated amino acid transported anterogradely primarily was found autoradiographically to descend in the area of Probsts tract and to ascend to the centremedian nucleus (CM) of the thalamus. Neurons labeled retrogradely by the fluorescent dye in the same injection-stimulation site were observed in the substantia nigra, entopeduncular nucleus, sub- and hypothalamus and amygdala. In subsequent experiments, injections of fluorescent tracers were made into the area of Probsts tract and CM. Neurons in the mesencephalic trigeminal root, cuneiform nucleus, nucleus tegmenti pedunculopontinus (NTPP), dorsal locus coeruleus and lateral central gray were labeled from Probsts tract injections. Neurons in medial and lateral central gray, as well as NTPP, were labeled from CM injections.

Mesopontine neurons in schizophrenia

Findings reported here show that there is a significant increase in the number of neurons in the pedunculopontine nucleus in most schizophrenic patients compared to age-matched controls. Nicotinamide adenine dinucleotide phosphate diaphorase histochemistry was used to label putative cholinergic neurons in the pedunculopontine nucleus and laterodorsal tegmental nucleus, while noradrenergic locus coeruleus neurons were labeled immunocytochemically using an antibody to tryosine hydroxylase. Cell counts of these neuronal groups were carried out using a Biographics image analysis system. We found significantly increased cell numbers in the pedunculopontine nucleus of schizophrenic patients compared to controls. The number of laterodorsal tegmental nucleus neurons was increased but this was not statistically significant. However, the total cell counts for pedunculopontine and laterodorsal tegmental nuclei were significantly higher in schizophrenic subjects. The number of locus coeruleus noradrenergic neurons was similar in both groups. These results implicate the brainstem reticular formation as a pathophysiological site in at least some patients with schizophrenia. In addition, these findings suggest a developmental etiology for the disease and account for some, but not all, of the symptoms of schizophrenia, including sensory gating abnormalities, sleep-wake disturbances and, perhaps, hallucinations. Overdriving of thalamic and substantia nigra function by cholinergic afferents from the midbrain may account for some of the symptoms seen in schizophrenia. These findings suggest that, at least in some schizophrenic patients, there is an increased number of neurons in the cholinergic arm of the reticular activating system. This may explain some of the symptoms of schizophrenia and points to a prenatal disturbance as one of the possible causes of the disease.

The brain stem reticular formation in schizophrenia.

Post-mortem brain tissue was obtained from four patients with schizophrenia and five controls to study cell groups in the brain stem reticular formation. Cholinergic neurons in the pedunculopontine nucleus (PPN) and lateral dorsal tegmental nucleus (LDT) were labeled using nicotinamide adenosine dinucleotide phosphate (NADPH)-diaphorase histochemistry, while catecholaminergic neurons of the locus ceruleus (LC) were labeled immunocytochemically using an antibody to tyrosine hydroxylase. In schizophrenic patients, there were increased numbers of neurons in the PPN labeled by NADPH-diaphorase and reduced cell size in the LC. These results implicate the reticular formation as a possible pathophysiological site for at least some patients with schizophrenia. This also suggests that some of the deficits observed may be based on faulty neurodevelopment.

Spinal cord stimulation-induced locomotion in the adult cat

Epidural and subdural stimulation of the dorsal surface of the spinal cord was found to elicit locomotion in the decerebrate cat in the active and fictive locomotion preparations. Stimulation of the cervical enlargement induced stepping in all four limbs, while stimulation of the lumbosacral enlargement induced hindlimb stepping. Lumbosacral enlargement stimulation induced hindlimb locomotion starting four hours following an acute midthoracic spinal cord transection. The preservation of the overall locomotor pattern and relationships between muscle groups, and of coordination between hindlimbs following transection, suggests that lumbar enlargement stimulation may be activating an intrinsically organized system. These findings suggest a method which may be applied clinically for the induction of limb alternation following epidural stimulation of the spinal cord.