Evidence for intrinsic expression of enkephalin-like immunoreactivity and opioid binding sites in cat superior colliculus

Excitotoxic lesions of the superior colliculus preferentially impact multisensory neurons and multisensory integration

The superior colliculus (SC) plays an important role in integrating visual, auditory and somatosensory information, and in guiding the orientation of the eyes, ears and head. Previously we have shown that cats with unilateral SC lesions showed a preferential loss of multisensory orientation behaviors for stimuli contralateral to the lesion. Surprisingly, this behavioral loss was seen even under circumstances where the SC lesion was far from complete. To assess the physiological changes induced by these lesions, we employed single unit electrophysiological methods to record from individual neurons in both the intact and damaged SC following behavioral testing in two animals. In the damaged SC of these animals, multisensory neurons were preferentially reduced in incidence, comprising less than 25% of the sensory-responsive population (as compared with 49% on the control side). In those multisensory neurons that remained following the lesion, receptive fields were nearly twofold larger, and less than 25% showed normal patterns of multisensory integration, with those that did being found in areas outside of the lesion. These results strongly suggest that the multisensory behavioral deficits seen following SC lesions are the combined result of a loss of multisensory neurons and a loss of multisensory integration in those neurons that remain.

The superior colliculus of the camel: a neuronal-specific nuclear protein (NeuN) and neuropeptide study

In this study we examined the superior colliculus of the midbrain of the one-humped (dromedary) camel, Camelus dromedarius, using Nissl staining and anti-neuronal-specific nuclear protein (NeuN) immunohistochemistry for total neuronal population as well as for the enkephalins, somatostatin (SOM) and substance P (SP). It was found that, unlike in most mammals, the superior colliculus is much larger than the inferior colliculus. The superior colliculus is concerned with visual reflexes and the co-ordination of head, neck and eye movements, which are certainly of importance to this animal with large eyes, head and neck, and apparently good vision. The basic neuronal architecture and lamination of the superior colliculus are similar to that in other mammals. However, we describe for the first time an unusually large content of neurons in the superior colliculus with strong immunoreactivity for met-enkephalin, an endogenous opioid. We classified the majority of these neurons as small (perimeters of 40-50 microm), and localized diffusely throughout the superficial grey and stratum opticum. In addition, large pyramidal-like neurons with perimeters of 100 microm and above were present in the intermediate grey layer. Large unipolar cells were located immediately dorsal to the deep grey layer. By contrast, small neurons (perimeters of 40-50 microm) immunopositive to SOM and SP were located exclusively in the superficial grey layer. We propose that this system may be associated with a pain-inhibiting pathway that has been described from the periaqueductal grey matter, juxtaposing the deep layers of the superior colliculus, to the lower brainstem and spinal cord. Such pain inhibition could be important in relation to the camel's life in the harsh environment of its native deserts, often living in very high temperatures with no shade and a diet consisting largely of thorny branches.

Expression of messenger RNAs for glutamic acid decarboxylase, preprotachykinin, cholecystokinin, somatostatin, proenkephalin and neuropeptide Y in the adult rat superior colliculus

The mammalian superior colliculus is an important subcortical integrator of sensorimotor behaviours. It is multi-layered, each layer containing specific neuronal types and possessing distinct input/output relationships. Here we use in situ hybridisation methods to map the distribution of seven neurotransmitters/neuromodulator systems in adult rat superior colliculus. Coronal sections were probed for preprotachykinin, cholecystokinin, somatostatin, proenkephalin, neuropeptide Y and the enzymes glutamic acid decarboxylase and choline acetyltransferase, markers for GABA and acetylcholine respectively. Cells expressing glutamic acid decarboxylase messenger RNA were the most abundant, the highest density being found in the superficial layers. Many cells containing proprotachykinin messenger RNA were found in stratum zonale and the upper two-thirds of stratum griseum superficiale; cells were also located in deeper tectal laminae, particularly caudomedially. Most cholecystokinin messenger RNA expressing cells were located in the superficial layers with a prominent band in the middle third of stratum griseum superficiale. Cells expressing moderate to high levels of somatostatin messenger RNA formed a dense band in the lower third of stratum griseum superficiale/upper stratum opticum; two less distinct tiers of labelling were seen in deeper layers. These in situ hybridisation data reveal three distinct sub-laminae in rat stratum griseum superficiale. Cells expressing moderate to low levels of proenkephalin messenger RNA were located in lower stratum griseum superficiale/upper stratum opticum and intermediate laminae. A cluster of enkephalinergic cells was located medially in the deep tectal laminae. Expression of neuropeptide Y messenger RNA was relatively low and mostly confined to cells in stratum griseum superficiale and stratum opticum. No choline acetyltransferase messenger RNA was detected. This in situ analysis of seven different neurotransmitters/neuromodulator systems sheds new light on the neurochemical organisation of the rat superior colliculus. The data are related to what is known anatomically and physiologically about intrinsic and extrinsic tectal circuitry, and the potential involvement of different neuropeptides in these circuits is discussed. The work forms the basis for future developmental studies examining the effects of transplantation and visual deprivation/deafferentation on tectal neurochemistry and function.

Distribution of met-enkephalin immunoreactivity in the diencephalon and the brainstem of the dog

The endogenous opioid system, in particular the enkephalins, has been implicated in a vast array of neurological functions. The dog could be a suitable model for the study of complex interactions between behavioral state and regulatory physiology in which the opioid system appeared to be implicated. Moreover, opiate derivatives are currently used in veterinary clinic and sometimes pharmacologically tested in the dog. However, there are no anatomical data regarding the organization of the opioid system in this species. The present work represents the first attempt to map the distribution of Met(5)-enkephalin-like-immunoreactive (Met-enk-li) cell bodies and fibers in the diencephalon and the brainstem of the dog. In the diencephalon, labeled cells were present in all the mid-line and intralaminar thalamic nuclei; the lateral posterior, pulvinar and suprageniculate nuclei; the ventral nucleus of the lateral geniculate body and the medial geniculate body. Additionally, Met-enk-li cells were seen in every hypothalamic nucleus except in the supraoptic. Variable densities of labeled fibers were also seen in all these nuclei except in the medial geniculate body and in most areas of the lateral posterior and pulvinar nuclei. In the mesencephalon, positive cells were found in the periaqueductal gray, the Edinger-Westphal and interpeduncular nuclei, delimited areas of the superior and inferior colliculi and the ventral tegmental area. In the rhombencephalon, labeled cells were seen in the majority of the nuclei in the latero-dorsal pontine tegmentum, the nuclei of the lateral lemniscus, the trapezoid, vestibular medial, vestibular inferior and cochlear nuclei, the prepositus hypoglossal, the nucleus of the solitary tract and the dorsal motor nucleus of the vagus, the infratrigeminal nucleus and the caudal part of the spinal trigeminal nucleus and in the rhombencephalic reticular formation. The distribution of fibers included additionally the substantia nigra, all the trigeminal nerve nuclei, the facial nucleus and a restricted portion of the inferior olive. These results are discussed with regard to previous reports on the distribution of Met-enk in other species.

Neurochemical microcircuitry underlying visual and oculomotor function in the cat superior colliculus

The cat superior colliculus (SC) plays an important role in visual and oculomotor functions, including the initiation of saccadic eye movements. We have studied the organization of neurochemical specific circuits in SC that underly these functions. In this chapter we have reviewed three microcircuits that can be identified by cell type, chemical content, and synaptic input from specific afferents. The first is located within the upper sgl and is related to the W retinal pathway to this region of SC. This circuit includes relay and interneurons that contain the calcium binding protein calbindin (CB), GABA containing presynaptic dendrites, and retinal terminals that have a distribution and size typical of W retinal terminals in the cat SC. This circuit is a typical synaptic triad that mediates feedforward inhibition, possibly to regulate outflow of the W pathway to the lateral geniculate nucleus. CB neurons in SC and other structures may be uniquely related to low threshold calcium currents in these neurons. The second microcircuit consists of neurons that contain parvalbumin (PV), another calcium binding protein. These neurons are located in a dense tier with the deep sgl and upper ol and they receive input from retinal terminals that are likely from 'Y' retinal ganglion cells. Some of these neurons also project to the lateral posterior nucleus and some colocalize glutamate. We speculate that these neurons also receive cortical 'Y' input although we have yet to prove this experimentally. The role of PV in these cells is unknown, but PV has been shown to be contained in fast spiking, non-accomodating neurons in visual cortex which have very rapid spike discharges that are also characteristic of SC neurons innervated by 'Y' input. The third microcircuit consists of a group of clustered neurons within the igl of the cat SC that overlaps the patch-like innervation of afferents to this region that come from the pedunculopontine tegmental and lateral dorsal tegmental nuclie, the substantia nigra, and the cortical frontal eye fields. These clustered neurons project through the tectopontobulbar pathway and terminate within the cuneiform region (CFR) of the midbrain tegmentum. They transiently express NOS during development. Ongoing studies in our laboratory suggest that these cells receive synaptic inputs directly from the PPTN and SN and may represent functional modules involved in the initiation of saccadic eye movements.

Pattern formation in the developing superior colliculus: ontogeny of the periodic architecture in the intermediate layers

The superior colliculus of mammals contains a striking neurochemical architecture in which histochemically identifiable compartments are distributed in an iterative arrangement in the intermediate layers. We used stains for acetylcholinesterase activity as a compartment marker to trace ontogenesis of this architecture during pre- and postnatal development in the domestic cat. We found that compartmentation in the intermediate collicular layers is virtually absent at birth, and only gradually emerges during the first weeks of postnatal life. Over the same postnatal period, acetylcholinesterase activity shifts from a predominantly perikaryal expression pattern immediately postnatally to a nearly exclusive localization in the neuropil at maturity. Remarkably, a striking compartmentation of the superior colliculus was readily apparent with acetylcholinesterase histochemistry prenatally. The first appearance of a periodic architecture in the superior colliculus was observed at embryonic day 34, a time at which the collicular plate had not yet become laminated. The compartments characterized by high levels of acetylcholinesterase activity then gained in prominence until late in the prenatal period, when they receded and disappeared. The loss of the acetylcholinesterase-positive compartments in the perinatal period did not reflect a loss of compartmentation altogether. Neonatally, there was a distinct compartmental architecture visible with enkephalin immunohistochemistry. The virtual absence of acetylcholinesterase-positive compartments in the superior colliculus at birth therefore reflects developmental regulation of enzyme expression in the compartments, not regulation of the compartments as structural entities. We conclude that the periodic architecture, which characterizes the intermediate collicular layers in the adult cat, arises early in ontogenesis. These observations raise the possibility that the histochemical compartments are ontogenetic units that undergo remodeling as the superior colliculus matures.

Enkephalin-positive and acetylcholinesterase-positive patch systems in the superior colliculus have matching distributions but distinct developmental histories

Histochemical stains for acetylcholinesterase activity and enkephalin-like immunoreactivity both demonstrate a high degree of patterning in the superior colliculus, particularly in the intermediate and deep layers. Both markers occur predominantly in the neuropil of these layers, and both are principally distributed in distinct macroscopic compartments. We report here that patches of heightened acetylcholinesterase activity correspond to patches of high enkephalin-like immunoreactivity. The two markers thus delineate largely the same domain in the intermediate and deep layers. The most prominent zones of staining for enkephalin-like peptide and for acetylcholinesterase also coincided in the dorsolateral periaqueductal gray matter. These findings suggest a close interlocking of one or more acetylcholinesterase-containing systems with one or more pathways related to endogenous opioids in the superior colliculus. As the acetylcholinesterase expression in the patches is known to match in detail choline acetyltransferase expression, our results also suggest the possibility of local cholinergic-opiatergic interactions. In some sections, blood vessels associated with enkephalin-rich and acetylcholinesterase-rich patches extended beyond the colliculus into the periaqueductal gray matter, where they again became surrounded by dense fibrous labeling. This pattern suggests that neurohumoral signal exchange might occur through blood vessels even in a sensory-motor structure such as the colliculus. In a postnatal developmental series of kitten brains we found that enkephalin-like immunoreactivity was already distinctly compartmental in the intermediate layers at birth and continued to show this distribution throughout postnatal development. By contrast, acetylcholinesterase staining was nearly homogeneous at birth and became compartmental gradually during the first postnatal weeks. Thus, despite the eventual near coincidence of the enkephalin-rich and acetylcholinesterase-rich compartments of the superior colliculus, they mark systems that follow distinct programs of neurochemical development.

The organization of GABAergic neurons in the mammalian superior colliculus

GABA is an important inhibitory neurotransmitter in the mammalian superior colliculus. As in the lateral geniculate nucleus, GABA immunoreactive neurons in SC are almost all small and are distributed throughout the structure in all mammalian species studied to date. Unlike the LGN, GABA-labeled neurons in SC have a variety of morphologies. These cells have been best characterized in cat, where horizontal and two granule cell morphologies have been identified. Horizontal cells give rise to one class of presynaptic dendrite while granule C cells give rise to another class of spine-like presynaptic dendrite. Granule A cells may be the origin of some GABAergic axon terminals. GABA containing synaptic profiles form serial synapses, providing a possible substrate for disinhibition. The distribution of GABAA and GABAB receptor subtypes appears similar to that of GABA neurons, with the densest distribution found within the superficial gray layer. However, antibody immunocytochemistry of the beta 2 and beta 3 subunits of the GABAA receptor reveals that it is located at both synaptic and non-synaptic sites, and may be associated with membrane adjacent to terminals with either flattened or round vesicles. A few GABA containing neurons in SC colocalize the pentapeptide leucine enkephalin or the calcium binding protein calbindin. However, none appear to co-localize parvalbumin, a situation different from GABA containing interneurons in the LGN and visual cortex. The diversity of GABA neurons in SC rivals that found in visual cortex, although unlike visual cortex, the pattern of co-occurrence does not distinguish GABA cell types in SC. The superior colliculus also differs from both LGN and visual cortex in that GABA and calbindin immunoreactivity is not altered by either long-term occlusion and/or short-term enucleation in adult Rhesus monkeys. No consistent differences have been found in the optical density of GABA labeling in either cells or neuropil. To conclude, GABA neurons in the superior colliculus share some properties like those in LGN and others like those in visual cortex. In other properties, they differ from GABA neurons in both the LGN and visual cortex. The GABA systems in the superior colliculus are similar in all mammalian species studied, suggesting that they are phylogenetically conserved systems which are not amenable to plastic alterations, a situation different to that in the geniculostriate system.