Jeffrey K. Saelens

Antagonism by mianserin and classical α-adrenoceptor blocking drugs of some cardiovascular and behavioral effects of clonidine

Antagonism of pressor responses to sympathetic outflow stimulation and alpha-adrenoceptor agonists in pithed spontaneously hypertensive rats was used to estimate postsynaptic alpha-adrenoceptor blocking activity of mianserin, phentolamine, phenoxybenzamine, piperoxan and yohimbine. Estimation of presynaptic alpha-adrenoceptor blocking activity of these drugs was obtained by studying their ability to antagonize clonidine-induced suppression of positive chronotropic responses to sympathetic outflow stimulation. In this manner, evidence was obtained that mianserin causes selective presynaptic alpha-adrenoceptor blockade. Mianserin, piperoxan and yohimbine antagonized clonidine-induced avoidance blockade or hypotension in spontaneously hypertensive rats, but methysergide, phenoxybenzamine and phentolamine were ineffective. These results suggest that mianserin may antagonize the central effects of clonidine by blockade of noradrenergic presynaptic or autoreceptors and possibly explain the antidepressant effect of mianserin as due to indirect activation of central noradrenergic neurons.

Evidence for a cholinergic fiber tract connecting the thalamus with the head of the striatum of the rat.

Placement of electrolytic lesions in the zona incerta or parafascicular nucleus of the rat forebrain resulted in a marked reduction of choline acetyltransferase (ChAc) activity in the head of the striatum 2-4 weeks later. Lesions of the habenula did not cause this effect implying that concomitant destruction of the fasciculus retroflexus with the parafascicular nucleus was not responsible for the effects observed. The data suggest that there is a cholinergic fiber tract connection between the parafascicular nucleus of the thalamus and the head of the striatum in the rat forebrain.

Further evidence for cholinergic thalamo-striatal neurons.

RECENTLY, evidence has been presented for the existence of a cholinergic tract connecting the thalamus with the striatum (nucleus caudatus putamen) in the cat (WAGNER et al., 1975) and rat (SIMKE & SAELENS, 1977) forebrain. Both studies used lesion-induced changes in the level of enzymes associated with acetylcholine metabolism. i.e. choline acetyltransferase (WAGNER et al., 1975; SIMKE & SAELENS, 1977) and acetylcholinesterase (WAGNER et al., 1975), to demonstrate the anatomical location of the tract. Studies have now been completed, in which the acetylcholine (ACh) content of the striatum has been measured in rats with lesions of the parafascicular nucleus of the thalamus. These studies further confirm the existence of a thalamo-striatal cholinergic tract. Unilateral electrolytic lesions of the parafascicular nucleus were placed in metafane-anesthetized male SpragueDawley rats (150 5 g) as previously described (SIMKE & SAELENS, 1977). The coordinates, according to the atlas of KONIG & KLIPPEL (1974) were A3500, V-0.4, L-1.0. Rats were killed 4 weeks after lesion placement using the ‘nearfreezing’ technique (TAKAHASHI & APRISON, 1964). The microwave method of killing was not used in order to maintain parallelism between this and the ChAc study, and because the increased fragility of brain tissue after microwave treatment would be incompatable with the slicing and microdissection procedure. Recent studies suggest that both types of technique yield qualitatively comparable data under a variety of treatments where changes in ACh levels were examined (GUYENET et al., 1977). The preparation of tissue samples, lesion locations and tissue weight estimates were performed as previously described (SIMKE & SAELENS, 1977). Briefly, frontal slices were made at 1 mm intervals with razor blades reproducibly guided through a preformed polyester resin block which rigidly held the brain in place. Further microdissection was carried out on a thermoelectric cold plate. The weights of the brain areas were estimated planimetrically from enlarged photographs taken of the frontal slices before and after microdissection. Both the resin block and the microdissection stage of the cold plate were ice-chilled in a solution designed to minimize post-mortem changes in the ACh levels of the tissue samples during microdissection. The solution contained 145 mM-NaCI, 1 mM-MgCI,, 0.1 mw-physostigmine sulfate (a cholinesterase inhibitor) and 0.5 m~-phenylacetaldehyde sodium bisulfite addition product (a choline acetyltransferase inhibitor) in a 1 mu-sodium phosphate buffer (pH = 7.4 at 0°C). The previously designated striatal areas 6, 13 and 19 (SIMKE & SAELENS, 1977) were analyzed for ACh content on both the lesioned and non-lesioned side of the forebrain. Each piece of brain tissue was transferred to a numbered glass centrifuge tube, to which 400pI of ice-chilled homogenizing fluid was added. The tissue was homogenized for 30s with a hairpin shaped piece of 0.01 in. diameter platinum-iridium wire attached to the microshaft of a Virtis homogenizer with PE 260 polyethylene tubing. The homogenizing fluid consisted of 1 N-formic acid-acetone (15:85) as suggested by TORU & APRISON (1966). The samples were centrifuged for 5min at 5°C in an MSE, 4L centrifuge at 2000 rev/min. The supernatant was subjected to the radioenzymatic assay for ACh described by SAELENS et a!. (1970). The data in Table 1 was derived from six rats with unilateral electrolytic lesions of the parafascicular nucleus. There was a marked reduction in the ACh levels in the head of the striatum (area 6) on the lesioned side. All 6 rats showed this reduction which averaged 70%. There was no significant effect on the ACh levels in other areas (13 and 19) containing striatal tissue. These data correlate exactly with previously reported (SIMKE & SAELENS, 1977) reductions in the ChAc content of area 6 (but not 13 and 19) following lesions of the parafascicular nucleus in the thalamus of the rat and are viewed as further evidence for the existence of a cholinergic thalamo-striatal tract.

The effects of haloperidol and clozapine on circling induced by electrical stimulation of the substantia nigra and the ventromedial tegmentum

Abstract A procedure is described in which circling behaviour in rats was produced by electrical stimulation of the zona compacta of the substantia nigra and the ventromedial tegmentum. These regions contain the A9 and A10 dopaminergic cell bodies whose terminals end in the corpus striatum and mesolimbic area, respectively. The behavioural pattern of circling differed depending upon which area was stimulated. Although differing in potency, systemically administered haloperidol and clozapine attenuated electrically-induced circling of both areas equally. The similar sensitivities of the two areas to these antipsychotic agents suggest that this model does not distinguish between agents with high (haloperidol) and low (clozapine) extrapyramidal syndrome (EPS) liability in man.

Baclofen as an analgesic

Abstract An antinociceptive effect of systemically administered baclofen (BF) was demonstrated in two rodent models, the phenylquinone-induced writhing and tail-flick procedures, at doses which also cause neurological impairment. These results suggests an antinociceptive effect of BF coinciding with a spinal muscle relaxant effect. The neuronal substrates acted upon by BF appear to be different from the opiates in that BFs effects were unaltered by naloxone whereas those of morphine were markedly attenuated. Also, rats trained to discriminate morphine from saline failed to generalize BF to morphine in discriminative stimulus studies. However, interactions between BF and opiates were observed in other experiments. Subthreshold doses of BF increased the antinociceptive potency of acutely administered morphine but failed to alter the development of tolerance to repeatedly administered morphine in mice. Further, BF markedly reduced naloxone precipitated jumping in morphine-dependent mice.

Antagonism of intrastriatal and intravenous kainic acid by 1-nuciferine: Comparison with various anticonvulsants and gabamimetics

Abstract 1-Nuciferine has been proposed as an antagonist of kainic acid (KA) and/or glutamate on the basis of iontophoretic experimental results. Its effectiveness against KA-induced destruction of rat striatal cholinergic neurons was therefore evaluated and compared with that of diazepam, phenobarbital, baclofen, haloperidol, and related substances. Drugs were administered intraperitoneally before and after intrastriatal microinjection of KA (0.5–1.5 μg), and choline acetyltransferase activity in striatum was assessed 24 hr later. Among the substances tested, only 1-nuciferine attenuated KA-induced depletions of striatal choline acetyltransferase. This effect was not secondary to anticonvulsant activity, because (a) 1-nuciferine did not block metrazol-, maximal electroshock-, or intravenous KA-induced seizures, and (b) anticonvulsants such as phenobarbital and diazepam, which are effective in these procedures, failed to modify KA-induced striatal neurotoxicity. 1-Nuciferine antagonized certain other neurological effects of intravenous KA, but antagonism was also seen with some of the other drugs tested. Intrastriatal microinjection of KA and/or glutamate may offer a means to detect selective antagonism of KA and/or glutamate, as distinguished from simple anticonvulsant activity.

Chapter 4. Agents Affecting GABA in the CNS

Publisher Summary A considerable body of evidence points to the fact that γ-aminobutyric acid (GABA) is a major inhibitory transmitter in the central nervous system of animals and man. It is well established that GABA itself, GABA-mimetics, and certain other agents diminish the firing rate of dopaminergic neurons in two areas of the brain: those in the substantia nigra that project to the striatum, and those in the ventral tegmentum that project to the mesolimbic areas and cortex. As antipsychotic agents are known to block dopamine receptors in these brain areas, the use of GABA-mimetics, that also attenuate dopaminergic functions alone or as an adjunct to neuroleptic therapy, has received increased attention. There are several other disease states, in which a disruption of a GABA neuronal system has been implicated. In one of such states a clear loss of GABA function occurs, presumably because of the destruction of the striatal-nigral GABAergic pathway. The fact that there is no loss of GABA receptors in the striatum, further illustrates that the GABA pathway is not intrinsic to this extra-pyramidal area. Recently, the similarity between the cellular destruction caused by intrastriatal administration of kainic acid and the observed cellular losses in some aforementioned states has been noted. Because of its function as a major inhibitory transmitter in the central nervous system, GABA, or more specifically, the lack thereof, is suspected to play a role in epilepsy.

CORRELATION OF BEHAVIORAL AND NEUROCHEMICAL EVENTS FOLLOWING ACUTE AND REPEATED ADMINISTRATION OF HALOPERIDOL

ABSTRACT Acute administration of haloperidol to rats induced catalepsy, reduced motor activity, disrupted avoidance behavior, increased the formation of 3H-homovanillic acid from 3H-tyrosine and decreased regional binding of 3H-spiroperidol, in vivo. After repeated treatment tolerance developed to many but not all of the acute effects of haloperidol. Results are discussed in relation to alterations of the dopamine receptor induced by repeated haloperidol treatment.

Effects of baclofen on acetylcholine metabolism in the striatum of rats

Abstract Haloperidol decreased acetylcholine (ACh) levels in rat striatum, an effect believed to reflect removal of dopamine inhibition of ACh neurons. The resulting hyperactive cholinergic neuron released ACh at a rate in excess of its synthetic replacement. Baclofen blocked the haloperidol-induced decrease in ACh levels at a dose which did not appreciably influence nigral-striatal dopamine metabolism. This suggested a direct inhibitory effect of baclofen on striatal cholinergic neurons. The ability of repeated administration of haloperidol to reduce striatal ACh levels was restored by a single low dose of baclofen. Repeated administration of haloperidol leads to depolarization blockade of nigral-striatal DA neurons which can be reversed by hyperpolarizing agents such as GABA or baclofen (Grace and Bunney, this symposium). If the inability of repeated haloperidol to decrease striatal ACh was a reflection of an analogous depolarization blockade of striatal cholinergic neurons, its abrupt reversal would be explained by the repolarizing action of baclofen.