Ronald H. Baisden

Cholinergic and non-cholinergic septo-hippocampal projections: a double-label horseradish peroxidase-acetylcholinesterase study in the rabbit

The existence of a massive cholinergic projection from cells in the medial septal nucleus (MS) and nucleus of the diagonal band (DB) to the hippocampal formation has been recognized for some time. However, the actual percentages of cholinergic and non-cholinergic neurons in the MS and DB which project to the hippocampus have not been reported. A procedure which combines horseradish peroxidase (HRP) and acetylcholinesterase (AChE) histochemistry in the same tissue was used to determine these percentages in the rabbit. Less than 50% of the neurons in the MS and DB which were labeled with reaction product following an HRP injection into the dorsal hippocampus also stained for AChE. Moreover, 70% of all neurons containing HRP reaction product were located in the DB, but neurons in the DB could not be differentiated from those in the MS on the basis of size or morphology. These data are taken to indicate that much of the MS-DB hippocampal projection is not cholinergic. Substance P is suggested as another possible transmitter within this anatomical system.

Embryonic hippocampal grafts ameliorate the deficit in DRL acquisition produced by hippocampectomy.

Transplants of fetal neural tissue survive and develop in lesion cavities produced in adult rats. The present experiment tested the effect of grafting fetal hippocampal or brainstem tissue on the ability of rats with hippocampal lesions to perform on a differential reinforcement of low response rate (DRL) operant schedule. The DRL interval was 20s. Eighty-six percent of the hippocampal grafts and 69% of the brainstem grafts developed to maturity. Inspection of sections from rats in which the mature transplant had been injected with Fast blue, indicated that these grafts formed connections with the host brain. Consistent with previous reports, rats with hippocampal lesions were impaired in performance of the DRL task. Rats given fetal grafts of hippocampal tissue into the hippocampal lesion site on the day of lesion production were significantly better in performance of the DRL requirement than were lesion-only rats or rats receiving grafts of fetal brainstem tissue. The results of this study confirm that grafts of fetal brain tissue can both develop in a lesion site in an adult brain and ameliorate lesion-induced behavioral deficits.

Localization of putative cholinergic neurons innervating the anteroventral thalamus

The brainstem localization of acetylcholinesterase (AChE)-containing neurons projecting to the anteroventral thalamic nucleus (AVN) was studied in rats. The AVN is one of several forebrain regions innervated by the AChE-containing dorsal tegmental pathway described by Shute and Lewis. In the present study, horseradish peroxidase (HRP) was injected into the region of the AVN to determine the brainstem origin of afferent projections. Alternate sections of tissue were stained for HRP or AChE. HRP-labeled neurons were found in the laterodorsal tegmental nucleus (LTN) and the locus coeruleus. Examination of adjacent sections revealed AChE-containing neurons in both of these nuclear regions. Combined HRP/AChE histochemistry demonstrated that transported HRP and AChE were in the same cells. In further experiments, unilateral lesions of the LTN were found to cause a decrease in AChE staining of the ipsilateral AVN. Destruction of the locus coeruleus had no effect. In combination with available evidence, the present findings suggest that cholinergic neurons in the LTN innervate the AVN.

Retrograde demonstration of hippocampal afferents from the interpeduncular and renuiens nuclei

The origin of hippocampal afferent projections was studied after horseradish peroxidase injections into the hippocampal formation. Labeled cells were found in the dorsal and ventrolateral aspects of the interpeduncular nucleus and in the ipsilateral portion of the nucleus reuniens thalami. In addition, neurons containing HRP were observed in the cortical, hypothalamic and brain stem areas reported by previous investigators.

Cerebellar nuclear lesions in rats: subsequent avoidance behavior and ascending anatomical connections.

Bilateral lesions of the rat cerebellar dentate and lateral interposed nuclei produced transient deficits in movement and posture, and facilitated acquisition of two-way active avoidance. Bilateral lesions of the fastigial and medial interposed nuclei of the rat cerebellum also produced transient deficits in movement and posture, but impaired acquisition of the avoidance task. Analysis of degeneration patterns after unilateral lesions to either the lateral or medial nuclear region indicated that the lateral area has a denser rostral projection than the medial area, while the medial nuclear region has a heavier caudal projection. It is suggested that these differences in anatomic connections may be related to the observed differences in lesion effect on two-way active avoidance.

Exposure to trimethyltin significantly enhances acetylcholinesterase staining in the rat dentate gyrus

Trimethyltin (TMT) is known to produce substantial damage to the hippocampal formation. It also destroys neurons within the entorhinal cortex, thereby causing degeneration of perforant path afferents that terminate in the outer molecular layer (OML) of the dentate gyrus. Surgical destruction of the entorhinal cortex also causes the perforant path to degenerate. This leads to reactive synaptogenesis (axonal sprouting) of septal afferents to the dentate gyrus. The purpose of the present study was to determine whether administration of 6 mg/kg of TMT by gavage to rats would cause axonal sprouting within the septo-dentate projection. A histochemical stain for acetylcholinesterase (AChE) was used. Compared to control subjects rats given TMT exhibited significantly denser AChE staining in the dentate OML. This is putative indication of reactive synaptogenesis within the cholinergic projection to this layer of the dentate and is somewhat surprising because other neurotoxins, such as lead and ethanol, that affect neurons within the hippocampal formation reduce the capacity for reactive synaptogenesis in response to lesions of the entorhinal cortex.

Effects of electrical stimulation of the pontine A5 cell group on blood pressure and heart rate in the rabbit

The effects of electrical stimulation of the A5 noradrenergic cell group of the ventrolateral pons was assessed in rabbits. Stimulation administered through either concentric bipolar or monopolar electrodes produced current-intensity related increases in mean arterial pressure (MAP). Decreases in heart rate (HR) accompanied the increases in MAP, but were essentially eliminated by bilateral vagotomy or destruction of the nucleus and tractus solitarii (NTS), thereby indicating that the HR decelerations were secondary to activation of baroreceptor reflexes. Neither vagotomy nor midcollicular section of the brainstem altered the MAP response to A5 stimulation. Bilateral destruction of the NTS slightly enhanced the response. Several rabbits received microinjections of 6-hydroxydopamine (6-OHDA) into the A5 region 2 weeks before the experiment. Other rabbits received vehicle injections and served as control subjects for the non-specific effects of the 6-OHDA injections. 6-OHDA injections, but not vehicle injections, prevented the vasopressor effects of A5 stimulation. However, stimulation of the A1 noradrenergic nucleus of the ventrolateral medulla produced decreases in MAP in rabbits given prior microinjections of 6-OHDA into A5. These observations are interpreted to indicate that the 6-OHDA injections produced neurotoxic effects which were relatively restricted to the A5 region. Furthermore, the data from all of these experiments are interpreted as indicating that the vasopressor effects observed as a consequence of electrical stimulation of A5 are due to excitation of the noradrenaline-containing neuron cell bodies of this region and that this effect is mediated via pathways arising from this region and terminating in the intermediolateral cell column of the spinal cord.

Grafts containing fetal hippocampal tissue reduce activity and improve passive avoidance in hippocampectomized or trimethyltin-exposed rats.

Embryonic Day 16 or 17 rat tissue containing either hippocampus with some medial pallial anlage or cerebellar/alar plate anlage was transplanted to the site of the ablated hippocampus of otherwise normal adult rats or adult rats previously exposed to the neurotoxin trimethyltin. Ninety to one hundred five days later these rats were compared to control rats in acquisition of passive avoidance and in open field activity. Transplantation of both types of tissue produced behavioral recovery on both tasks in rats with hippocampal lesions that had not been exposed to trimethyltin. Only hippocampal transplants produced recovery of function in rats given trimethyltin. Although transplants of hippocampal tissue had an organotypic structure that was easily differentiated with cell and fiber stains from that of the cerebellar transplants, neither of these routine histological procedures nor immunocytochemical analysis revealed differences between transplants made into normal rats or toxicant-exposed rats. Either of two mechanisms may account for the ability of the transplants to produce behavioral recovery. These are reconstruction of damaged circuitry by the transplant and neurotrophic action of the developing transplant on the host brain. The second mechanism alone may be sufficient to restore function in brain-lesioned but otherwise normal rats. Therefore, either type of transplant is effective. Both mechanisms may be necessary for recovery in brain-lesioned, toxicant-exposed rats. Therefore, only transplants of tissue homotypic to the tissue removed from the brain are effective.

Trimethyltin Neurotoxicity in the Rat as an Analogous Model of Alzheimer’s Disease

Animal models of diseases have been used in experiments to study the cause of a disease, the mechanisms underlying the cellular, organ, or systemic pathology of a disease, and to compare the efficacy of proposed therapeutic interventions to existing clinical treatments. A model may be useful for one of these purposes but not for the others. For example, a particular behavior, such as activity in an open field, may be changed in a dose-related fashion by drugs that have a recognized clinical effect in psychiatric patients, but the rat behavior may not have any clear relationship to the human psychopathology. This model is not necessarily useful for study of the cause and progression of the disease or of its pathophysiology. Rather, its validity relates to a consistent parametric relationship between the effect of a drug on this behavioral measure and the clinical efficacy of the same drug. Newly designed drugs that produce greater effects on the animal model without significant detrimental side effects would then be likely candidates for clinical trials.

Forebrain catecholamine concentrations in the rat after neocortical and hippocampal lesions

Abstract Spectrofluorometric procedures were used to measure forebrain content of dopamine and norepinephrine in rats after either dorsolateral neocortical ablation, or combined neocortical-hippocampal ablations. The concentration of both of these catecholamines was significantly higher in the ablated rats than in uninjured control rats. However, there was no difference between the rats receiving only neocortical lesions and the rats with combined lesions on this measure.