Methylenedioxyamphetamine: neurotoxic effects on serotonergic projections to brainstem nuclei in the rat.

This study employed immunocytochemistry to visualize the neurotoxic effects of methylenedioxyamphetamine hydrochloride (MDA) on serotonergic projections to brainstem structures. Separate groups of animals were injected twice a day, for 4 consecutive days, with: saline; MDA (40 mg/kg/day); or fluoxetine hydrochloride (10 mg/kg) prior to each injection of MDA. In agreement with previous reports, MDA produced a pronounced loss of 5-HT immunoreactivity in the forebrain, most notably in neocortex and hippocampus. However, our results revealed that MDA also produced a loss of 5-HT fibers in brainstem that was as severe as that seen in any region of forebrain. Regions most severely affected included: superior colliculus; superior olivary complex; trigeminal sensory complex and vestibular nuclei. The brains of animals treated with MDA demonstrated a relative absence of fine 5-HT axon terminals within these forebrain and brainstem regions, while thicker axonal elements were still present. The neurotoxic effects of MDA on serotonergic projections to forebrain and brainstem were completely blocked by the prior administration of the 5-HT reuptake inhibitor, fluoxetine. It was suggested that the denervation of the superior colliculus, superior olive and vestibular nuclei could alter visually guided eye movements and the vestibulo-ocular reflex while the loss of serotonergic inputs to the trigeminal sensory complex might be expected to alter tactual reflexes.

A study of proteins in the auditory system of rabbits using two-dimensional gels: identification of glial fibrillary acidic protein and vitamin D-dependent calcium binding protein.

Two-dimensional gel electrophoresis and computerized optical densitometry were employed to compare the relative content of proteins across major auditory brain regions in rabbits. Areas examined included the dorsal and ventral cochlear nuclei which receive the primary afferents from the organ of Corti, the lateral superior olivary nucleus which has strong reciprocal relationships with the cochlear nucleus, and the successively more rostral projections of the auditory pathways to inferior colliculus, medial geniculate and auditory cortex. Twelve proteins demonstrated significant decreases and 5 proteins significant increases in content at successively more rostral levels of the auditory system, including 2 proteins which were highly localized to the cochlear nuclei and 2 proteins greatest in amounts in the auditory cortex. One protein which was localized to the cochlear nuclei and lateral superior olive (molecular weight (MW) = 50.3, isoelectric point (pI) = 5.7) was identified as the glial fibrillary acidic protein by reaction of specific antisera on blots. Antisera to the vitamin D-dependent calcium binding protein reacted specifically with one protein (MW = 27.2, pI = 4.8) which was greatest in amount in the lateral superior olive (LSO) versus other auditory regions examined. The significance of these findings rests in the potential for identifying specific markers for cellular elements that are important in auditory function and which might be lost as a consequence of developmental abnormalities or other traumas.

Asymmetric uptake of 2-deoxy-D-[14C]glucose in the dorsal cochlear nucleus during Pavlovian conditioning in the rabbit.

Uptake of 2-deoxy-D-[14C]glucose was measured during Pavlovian conditioning of the rabbit's nictitating membrane response by both qualitative autoradiography and by quantitative measurement of radioactivity in samples of brain tissue. Conditioning was accomplished by pairing a tone stimulus delivered to both ears with an air-puff stimulus delivered to the right eye. Infusion of 2-deoxy-D-[14C]glucose during the first day of conditioning when there was no evidence of acquisition or during the 7th day of conditioning when animals demonstrated 68% conditioned responses resulted in a significantly greater uptake of radioactivity by the caudal portions of the left as compared with the right dorsal cochlear nucleus. Similar changes were not observed in other auditory and non-auditory nuclei. Rabbits that had acquired conditioned responses across 6 days of training and were exposed only to the tone-conditioned stimulus on the 7th day of testing exhibited 69% conditioned responses but no asymmetry in the uptake of 2-deoxy-D-[14C]glucose. Control animals receiving unpaired presentations of tone and air puff or no stimulation did not acquire conditioned responses and did not demonstrate asymmetric uptake of radioactivity in the dorsal cochlear nucleus. These results indicate that the asymmetric uptake of radioactivity by the dorsal cochlear nucleus did not result from the effects of stimulation per se or the prior occurrence of learning but was due to the explicit pairing of the tone stimulus with the asymmetric delivery of the air puff. It would appear that the caudal dorsal cochlear nucleus not only serves as a signal transducer for auditory stimuli but also receives inputs from other sensory systems thus allowing it to both recognize when an auditory stimulus is followed by a biologically significant event and to transmit such information to other brain regions that are, in turn, responsible for learning.

Anatomical study of the rabbit's corneal-VIth nerve reflex: connections between cornea, trigeminal sensory complex, and the abducens and accessory abducens nuclei.

The corneal-VIth nerve reflex of the rabbit, involving retraction of the eyeball by the retractor bulbi muscle and the correlated extension of the nictitating membrane, has been suggested to be mediated by retractor bulbi motoneurons in the accessory abducens-(ACC) nucleus but not by those in the abducens (ABD) nucleus, and to consist of both a fast, disynaptic, component and a slower component mediated by the reticular formation (RF). We, therefore, employed the anterograde and retrograde transport of horseradish peroxidase (HRP) to examine the neural connections between anatomical structures proposed to be involved in the afferent limb of the corneal VIth nerve reflex. The transganglionic transport of HRP from cornea indicated a primary projection to the ventral half of pars oralis of the trigeminal sensory complex. The retrograde transport of HRP infused into ACC resulted in a bilateral labeling of cells in ventral pars oralis with 75% of the labeled cells being ipsilateral to the side of infusion. In contrast, there was no retrograde labeling of cells in the trigeminal sensory complex after HRP infusions into ABD. Infusion of HRP into ACC and ABD also revealed retrogradely labeled cells in the RF caudal to these two nuclei and infusion of HRP into this area of the RF resulted in both the retrograde labeling of cells in ventral pars oralis and anterograde-like labeling in both ACC and ABD. These data provide anatomical support for a direct relationship of the ACC, but not ABD, to the trigeminal sensory system and for the suggested existence of two components of the corneal-VIth nerve reflex: a disynaptic component from cornea to ventral pars oralis which in turn projects only to the ACC nucleus; and a multisynaptic component consisting of projections from the ventral pars oralis to RF cells which, in turn, are premotor to the ACC and ABD nuclei.

The role of the accessory abducens nucleus in the rabbit nictitating membrane response.

Electrolytic and knife-cut lesions were employed in the rabbit to examine the role of the VIth cranial nerve, and of the motoneurons in the abducens (ABD) and accessory abducens (ACC) nuclei that supply the VIth nerve, in the reflex extension of the nictitating membrane. The nictitating membrane response (NMR) was elicited by tactual stimulation of the cornea with a puff of air or by electric shock delivered to the skin over the paraorbital region of the head. Total destruction of the VIth nerve or interruption of all ACC inputs to the VIth nerve (while leaving ABD inputs intact) produced a large and comparable reduction in the magnitude of the NMR elicited by air puff, although a small residual NMR of less than 1 mm could still be detected. In contrast, the magnitude of the NMR elicited by shock was not affected by ACC isolation and only reduced by 50% after VIth nerve lesions. Total isolation of ABD inputs to the VIth nerve (while leaving ACC inputs intact) had no effect on NMR magnitude elicited by either air puff or shock. The small residual NMRs to air puff and the larger NMRs to shock remaining after total destruction of the VIth nerve were not eliminated by the removal of all extraocular muscles (while leaving the retractor bulbi muscle intact). However, knife cut lesions that interrupted all ACC inputs to the VIth nerve and transected the VIIth (facial) nerve completely eliminated NMRs elicited by both air puff and shock. The results of this study indicate that NMRs elicited by tactual stimulation of the cornea are primarily mediated by retractor bulbi motoneurons in the ACC nucleus via the VIth nerve. In contrast, NMRs elicited by electric shock delivered to the skin over the paraorbital region of the head are produced by contraction of the retractor bulbi muscle via the VIth nerve and by contraction of the orbicularis oculi muscle via the VIIth nerve which then squeezes the nictitating membrane over the cornea.

Localization of retractor bulbi motoneurons in the rabbit.

Motoneurons innervating the rabbit retractor bulbi muscle have been identified by retrograde transport of horseradish peroxidase (HRP). Following injection of HRP into single slips or all 4 slips of the retractor bulbi muscle, labeled motoneurons were consistently observed in the abducens (ABD) nucleus and in the accessory abducens (ACC) nucleus located ventral, lateral and rostral to the ABD. Axons from the ACC motoneurons could be seen to enter the VIth nerve. Injection of HRP into the lateral rectus muscle produced consistent labeling of motoneurons in the ABD nucleus overlapping the distribution of retractor bulbi motoneurons, but labeling was never observed in the ACC nucleus. The number of labeled ABD neurons after lateral rectus injections was far less (36%) than after injection into all 4 slips of the retractor bulbi muscle (72%). Injection of HRP into the superior oblique, superior rectus or medial rectus muscle produced labeling of motoneurons in the corresponding subdivisions of the oculomotor nucleus or trochlear nucleus but no labeled motoneurons were observed in either the ABD or ACC nuclei. Some highly inconsistent labeling of oculomotor nucleus was observed after retractor bulbi or lateral rectus muscle injections and this was judged to be due to intraorbital diffusion of the HRP. It was concluded that the retractor bulbi muscle is innervated by motoneurons located in both the ABD and ACC nuclei.

Olfactory system damage and brain catecholamines in the rat.

Although a number of studies have reported that bulbectomy results in a fall in telencephalic norepinephrine (NE) content, the present study is the first to examine the relationship between the locus and extent of olfactory system damage and the depletion of telencephalic catecholamines after olfactory system surgery. Our findings indicate that the often reported depletion of telencephalic NE after olfactory bulb ablation is not due to removal of the olfactory bulbs per se, but instead is the result of incidentally produced destruction of tissue, caudal to the bulbs, through which noradrenergic fibers ascend on their way to various regions of the telencephalon.

P-Chloramphetamine: Selective neurotoxic action in brain.

Injection of 2.5,5, 10, or 20 milligrams of p-chloroamphetamine per kilogram of body weight into rats produced evidence of cytopathological changes in sections of brain stained by a Nissl or silver method. As early as 1 day after drug injection cells demonstrated an intense Nissl staining, intense argyrophilia, cellular shrinkage, and perineuronal spaces. At 30 days after injection both stains revealed cellular debris and glial reactions characteristic of cellular dissolution. The neurotoxic effects of 2.5, 5, or 10 milligrams of p-chloroamphetamine per kilogram were primarily restricted to an area of the ventral midbrain tegmentum corresponding to the distribution of the B-9 serotonergic cell group. After 20 milligrams of p-chloroamphetamine per kilogram there was also evidence of neurotoxic effects on cells within the substantia nigra. These results confirm previous suggestions that the long-term reduction in serotonin content of brain, tryptophan-5-hydroxylase activity, and uptake of serotonin after injection of p-chloroamphetamine is due to a neurotoxic effect of the drug or some metabolite on serotonergic cell bodies.

Fenfluramine: evidence for a neurotoxic action on midbrain and a long-term depletion of serotonin.

A single injection of fenfluramine (100 mumol/kg) produced evidence of neurotoxicity in cresyl violet or silver stained sections of rat brain which was restricted to the serotonergic (B-9) cell group located in the ventromedial midbrain tegmentum. Reacting cells throughout this region exhibited an irregular shape and an intense staining of the cytoplasm, while in the caudal 1/4 of this region the reacting cells also exhibited a perineuronal space. These effects were greatly reduced in the rostral 3/4 of B-9 at 14 and 30 days after fenfluramine. In the caudal 1/4 of B-9 the neurotoxic actions remained prominent and included signs of cellular dissolution. These signs of an irreversible degenerative effect of fenfluramine on cells in the caudal 1/4 of the B-9 region were identical to those seen after p-CA, while the effects in the rostral 3/4 of B-9 were not as prominent. The differential neurotoxic effects of fenfluramine and p-CA on cells in the rostral 3/4 of B-9 were associated with a differential effect on serotonin content of hippocampus and amygdala.