Organization of the basal forebrain in the cat: localization of L-enkephalin, substance P, and choline acetyltransferase immunoreactivity

The present study uses immunocytochemical techniques to determine whether cholinergic basal forebrain neurons in the cat are in a position to receive a homogeneous pattern of inputs, or if specific immunocytochemically defined afferent systems are localized to only selected regions of the basal forebrain. Monoclonal antibodies against choline acetyltransferase (ChAT) were used to identify the location of putative cholinergic neurons which are known to project to the cerebral cortex. In addition, polyclonal antibodies against substance P (SP) or enkephalin (Enk) were used on either adjacent or on the same histological sections reacted for ChAT to identify the neuropeptide plexuses that provide input to the basal forebrain. ChAT-immunoreactive (ChAT-IR) perikarya were located throughout the vertical limb, genu and horizontal limb of the diagonal band of Broca. ChAT-IR neurons also were located within the substantia innominata (SI), within the peripallidal zone around the globus pallidus, and were intercalated within the internal capsule. Enk-IR and SP-IR were used to determine the distribution of putative peptidergic terminals within the basal forebrain. Extensive Enk-IR and SP-IR terminal label was localized within the globus pallidus and the surrounding peripallidal zones, as well as within the SI, whereas the components of the diagonal band of Broca demonstrated negligible Enk-IR and SP-IR label. These data predict that the subdivisions of the cholinergic basal forebrain in the cat do not share a uniform afferent system, and only selective portions of this cholinergic system are in an anatomical position to receive a major direct input from the identified subcortical peptidergic afferents. The segregation of afferents has important consequences in the selective control of cortical function by the cholinergic basalocortical pathway.

Effects of chronic ethanol administration on acetylcholinesterase activity in the somatosensory cortex and basal forebrain of the rat

A chronic diet of ethanol has detrimental effects on the cholinergic system in adult humans and rats. This study examined the effects of chronic exposure to dietary ethanol on the anatomical organization of true acetylcholinesterase (AChE) active elements in rat cerebral cortex. We focused on the somatosensory cortex because of its highly organized chemical and cellular structure. Following 42 days of exposure to an ethanol diet (6.7% v/v), there were marked changes in the cortical plexus of AChE-positive fibers. The AChE-positive plexus in ethanol-treated rats was reduced in all cortical layers, in comparison to age-matched pair-fed control and chow-fed rats. The most marked reduction was evident in layers II/III, IV, and VIa. Moreover, the density of AChE-positive cell bodies was significantly reduced in the cortices of ethanol-fed rats, particularly in the deep laminae. These alterations in the chemoarchitecture of somatosensory cortex occurred in the absence of changes in the cytoarchitectonic organization of neocortex. There was no detectable ethanol-induced change in the density of Cresyl violet-stained neurons either in the horizontal limb of the diagonal band of Broca or in the nucleus basalis. The density of AChE-positive neurons in the nucleus basalis, however, was significantly lower in ethanol-fed rats than in controls. Thus, it appears that a mere 6 weeks of ethanol exposure is sufficient to alter the cholinergic innervation of the cerebral cortex. These cortical alterations occur despite the lack of an ethanol-induced death of neurons in the basal forebrain. Such changes may contribute to the memory loss associated with alcohol dementia.

Synaptic connections of central carotid sinus afferents in the nucleus of the tractus solitarius of the rat. I. An electron microscopic study

A transganglionic transport technique was used to study the synaptic connections of the central carotid sinus afferents in the nucleus of the tractus solitarius of the rat by electron microscopy. The caudal part of the nucleus was profusely innervated. Labelled fibres extended to the contralateral nucleus, and to the ipsilateral dorsal motor nucleus of the vagus nerve, nucleus ambiguus, spinal nucleus of the trigeminal nerve and the area postrema. The labelled terminals were densely packed with clear, predominantly spherical vesicles about 50 nm in diameter and a few often swollen mitochondria. The terminals synapsed on dendrites of various calibres, spindle- or pear-shaped somal profiles with short axes lesser than 8 microns, and axon terminals. In axo-axonal synapses, most labelled terminals appeared to be presynaptic. Frequently, profiles of labelled terminals were in direct apposition with one another. The latter may represent the morphological substrate of the interaction between baro- and chemoreceptor inputs in the nucleus of the tractus solitarius and warrants further study. The present results indicate that in addition to direct inputs, the carotid sinus afferents are able to influence second-order neurons in the nucleus of the tractus solitarius indirectly through presynaptic modulation.

Pulmonary vascular responses to forebrain stimulation in the cat

The effects of forebrain stimulation on the pulmonary vascular bed were investigated in the intact-chest cat under conditions of controlled blood flow and constant left atrial pressure. When pulmonary vascular tone was raised to a high steady level, direct electrical stimulation of the forebrain elicited a biphasic change in lobar arterial pressure. The response was characterized by an initial transient increase in lobar arterial pressure that was followed by a prolonged secondary decrease in pressure. When a delay coil was added to the extracorporeal perfusion circuit, the secondary vasodilator response was separated into initial brief and delayed prolonged components, suggesting that it was mediated in part by the release of a humoral factor. The entire response to forebrain stimulation was abolished by cervical cord section or freezing. The initial constrictor response and early brief dilator response were not blocked by classic pharmacological blocking agents. The delayed humorally mediated vasodilator response was blocked by propranolol or ICI 118551, indicating that it was mediated by a circulating factor with beta 2-stimulating properties. The delayed vasodilator response was associated with a large increase in arterial epinephrine levels, and this rise in plasma epinephrine was not altered by propranolol. The present data suggest that electrical stimulation of the forebrain causes a prolonged pulmonary vasodilator response that is mediated by way of a descending pathway, which results in a large rise in arterial epinephrine levels.

Topographic projections to the visual cortex from the basal forebrain in the rat

Recent studies have shown that cholinergic neurons within the basal forebrain give rise to a projection that terminates throughout the neocortex. The purpose of the present study is to determine the topographic organization of the basal forebrain projection to a single cortical region: the visual cortex. Injections of wheat germ agglutinin-horseradish peroxidase were placed within the infragranular layers of areas 17, 18a or 18b in Long-Evans hooded rats. Following injections placed within area 18a, retrogradely labeled neurons were located primarily within the caudal components of the basal forebrain including the basal nucleus of Meynert. Injections placed within area 18b, on the other hand, resulted in retrograde labeling of numerous neurons within rostral basal forebrain nuclei, including the horizontal limb of the diagonal band of Broca, whereas only a few labeled cells were located within the basal nucleus. Two patterns of labeled cells were evident following injections placed within area 17 and they resembled the results from injections within the adjacent extrastriate area; i.e. either area 18a or area 18b. Thus, injections restricted to the lateral portions of area 17 resulted in retrograde labeling of neurons located within the caudal levels of the basal forebrain, whereas injections within the medial portions of area 17 were followed by retrograde labeling of neurons within the rostral levels of the basal forebrain. Based on these results we suggest that the cholinergic corticopetal projection terminates within the visual cortex in a medial to lateral pattern.

Interhemispheric connections in the visual cortex of the squirrel monkey (Saimiri sciureus)

The callosal connections within the posterior parietal and occipital cortices were studied in the squirrel monkey with horseradish peroxidase tracing techniques. The data were evaluated with particular emphasis on the relationship of major callosal connections along the 17-18 border. The overall pattern of callosal connections in the squirrel monkey also was compared with callosal patterns in other New World simians. Our results show that the dense band of callosal connections along the 17-18 border in the squirrel monkey differs from the connections observed in other New World monkeys in that it is virtually confined to area 18 and avoids area 17. In addition to a continuous band of callosal connections in area 18 that parallels the 17-18 border, rostral extensions of the band are oriented perpendicular to the 17-18 border and present an obvious periodicity. The remaining parieto-occipital cortex contains a complex pattern of callosal connections that is strikingly similar to patterns reported for other New World monkeys. Thus, it is likely that the dorsolateral extrastriate visual cortex in the squirrel monkey is organized in a manner similar to that found within other New World monkeys.

Hypothalamic and ventral thalamic projections to the superior colliculus in the cat

The present report describes the organization of collicular afferents that arise within either the hypothalamus or the ventral thalamus. Following the placement of large injections of WGA-HRP into the superior colliculus of the cat, retrogradely labeled neurons are located within the reticular nucleus of the thalamus, the zona incerta, the fields of Forel, and throughout the hypothalamus. Although the dorsal hypothalamic area contains the largest number of labeled hypothalamic neurons, labeled cells are also found within the periventricular, paraventricular, dorsomedial, ventromedial, posterior, lateral, and anterior hypothalamic nuclei. A strikingly similar pattern of distribution of labeled neurons is also observed following placement of small injections of WGA-HRP that are restricted within the stratum griseum intermedium (SGI). In contrast, hypothalamic and ventral thalamic labeling is not seen after placement of injections within the stratum griseum superficiale. Following the placement of injections of tritiated anterograde tracers within the dorsal hypothalamic area, transported label is organized in two bands of clusters over the SGI. When injections of tritiated tracers are placed within the zona incerta, terminal label is also located over the SGI; however, the distribution of silver grains does not appear as clusters or distinct puffs. On the basis of the comparison of the cellular types that give rise to these projections and the differences in terminal distribution, we suggest that the hypothalamic and ventral thalamic projections to the superior colliculus are totally separate and unrelated pathways. The functional implications of the hypothalamotectal pathway are also discussed.

Organization of the rostral thalamus in the rat: evidence for connections to layer I of visual cortex

The present study demonstrates the organization of a thalamocortical projecting system which terminates within layer I of the visual cortex in the hooded rat. Horseradish peroxidase (HRP) injections restricted to layer I resulted in retrograde labeling of large and medium-sized multipolar and fusiform neurons that are located within the ventromedial (VM) nucleus and a dorsomedial subunit of the ventral anterolateral nucleus (VAL). Retrograde cellular labeling also occurs within the anteromedial nucleus (AM) following these injections. After restriction of HRP injections to layer I, peroxidase labeling was not found within neurons of the classically defined intralaminar system, i.e., central medial, paracentral, and central lateral nuclei, or within the rostral continuations of the intralaminar system. Since the VM, dorsomedial VAL, and AM nuclei are directly adjacent to portions of the internal medullary lamina, we refer to this amalgam of rostral thalamic nuclei that project to layer I as the "paralaminar" system. We also provide cytoarchitectonic criteria that can be used to distinguish three separate subdivisions within the VAL complex, including that portion of the VAL which is part of the "paralaminar" system. In contrast, when control injections of WGA-HRP are placed within either the cellular supragranular or infragranular layers of the visual cortex, no appreciable number of neurons are labeled within the VM, VAL, or AM.

Interhemispheric and subcortical collaterals of single cortical neurons in the adult cat

The results of this study demonstrate the existence of single neocortical neurons that send axon collaterals into the corpus callosum, to terminate within the contralateral hemisphere, and subcortically, to terminate within the ipsilateral superior colliculus.