Paul A.T. Kelly

Environmental enrichment selectively increases 5-HT1A receptor mRNA expression and binding in the rat hippocampus

Environmental enrichment augments neuronal plasticity and cognitive function and possible mediators of these changes are of considerable interest. In this study, male rats were exposed to environmental enrichment or single housing for 30 days. Rats from the enriched group had significantly higher 5-HT1A receptor mRNA expression in the dorsal hippocampus (62%, 59% and 44% increase in the CA1, CA2 and CA3 subfields, respectively). This was associated with significantly higher [3H]8-OH-DPAT binding in the inferior part of CA1. No changes were seen for 5-HT2A or 5-HT2C receptor mRNAs. The neuronal plasticity detected after environmental change may be mediated, in part, through 5-HT1A receptors.

Differential regulation of corticosteroid receptors by monoamine neurotransmitters and antidepressant drugs in primary hippocampal culture

Hyperactivity of the hypothalamic-pituitary-adrenal axis is a characteristic feature of depressive illness. The centrally located corticosteroid receptors, the glucocorticoid and mineralocorticoid receptors, are thought to be important modulators of this axis and changes in the levels of these receptors, particularly in the hippocampus, may underlie the hyperactivity observed. Various antidepressant drugs increase hippocampal mineralocorticoid and glucocorticoid receptor levels in vivo. These effects are thought to be mediated via alterations in monoaminergic neurotransmission. We examined whether serotonin (5HT) and noradrenaline (NA) have direct effects on glucocorticoid receptor and mineralocorticoid receptor expression in primary hippocampal neurones, and whether antidepressants also exert direct effects on target neurones. Exposure of hippocampal cells to 5HT for 4 days increased both glucocorticoid and mineralocorticoid receptor mRNA and protein expression. The induction of mineralocorticoid receptor mRNA was completely blocked by the 5HT(7) receptor antagonist SB 269970. In contrast glucocorticoid receptor induction was insensitive to the 5HT(7) receptor, whilst studies with the 5HT(1A) receptor agonist 8-hydroxy-2-(di-n-proplamino) tetralin hydrochloride and the 5HT(1A) receptor antagonist N-[2-[4-2-[O-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexane carboxamide trihydrochloride (WAY 100635) suggest a partial role for 5HT(1A) receptors in hippocampal glucocorticoid receptor regulation. Treatment with NA for 4 days also increased glucocorticoid receptor expression but had no effect on mineralocorticoid receptor expression. This was blocked by propanolol suggesting action via beta-adrenergic receptors. Similarly to NA, fluoxetine and amitriptyline also selectively increased glucocorticoid receptor mRNA and protein levels over this time course. However, glucocorticoid receptor induction by fluoxetine or amitriptyline was not blocked by WAY 100635 or propanolol. These results show that 5HT, NA and antidepressants act directly but via distinct mechanisms on hippocampal neurones to regulate mineralocorticoid and glucocorticoid receptor expression. Thusly, manipulation of neurotransmitter or antidepressant levels in the brain may aid in reversing hypothalamic-pituitary-adrenal axis hyperactivity by restoring hippocampal corticosteroid receptor balance.

Alterations in hippocampal function following repeated exposure to the amphetamine derivative methylenedioxymethamphetamine (“Ecstasy”)

The effect of the psychomotor stimulant, 3,4-methylenedioxymethamphetamine (MDMA, “Ecstasy”), upon integrated cerebral function was measured in rats using the quantitative [14C]deoxyglucose autoradiographic technique. Animals were injected with MDMA (20 mg/kg sc) twice daily for 4 days. Fourteen days after the final administration, [3H]-paroxetine binding to 5HT uptake sites was reduced by 89% in membranes prepared from tissue samples of frontal cortex. In the same rats [3H]-paroxetine binding autoradiography revealed heterogeneity in the regional distribution of 5-HT uptake site depletion within neocortex (0–92%) and hippocampus (30–95%). Despite these profound reductions in 5-HT uptake sites no significant alterations were found in glucose utilisation in any area of neocortex examined. However, significant increases in glucose use were found in subregions of the hippocampus, most notably within the pyramidal cell layer of CA2 and CA3 (25–35%). This study provides direct evidence that the loss of 5-HT innervation caused by exposure to MDMA results in lasting functional changes in hippocampus.

Functional consequences of perinatal exposure to3,4-methylenedioxymethamphetamine in rat brain

In this study we have examined methylenedioxymethamphetamine (MDMA)‐induced toxicity in perinatal rat brain, related this to normal development of serotonin transporter sites (SERT), and determined whether early exposure to MDMA subsequently alters cerebral function in adults. Perinatal development of SERT was visualized and quantified using [3H]‐paroxetine binding autoradiography in embryonic and neonatal rat brain from embryonic day 15 (E15) to postnatal day 30 (P30). Cerebral glucose utilization (lCMRglu) was measured by 2‐deoxyglucose autoradiography in adult rats. [3H]‐Paroxetine binding was observed in forebrain from E18. From birth (P0), binding was organized into neocortical columns (75% higher at P10 than in adult) which declined toward adult levels between P20 and P25. MDMA treatment (20 mg kg−1 s.c. twice daily for four days) commencing at developmental stages from E15 (treatment given to dams) to P20, had no effect upon [3H]‐paroxetine binding measured at P40. Treatments started on P25 or later resulted in significant decreases in [3H]‐paroxetine binding (46%). This was coincident with the development of adult patterns of binding in forebrain. Despite the lack of MDMA‐induced neurotoxicity, rats treated in utero (E15) showed increased lCMRglu in locus coeruleus (+37%), and in areas receiving ascending noradrenergic innervation, such as anterior thalamus (+44%) and septal nucleus (+24%). These studies confirm that the susceptibility of serotonergic terminals to the neurotoxic properties of MDMA is absent in the immediate perinatal period, but also suggests that in utero MDMA exposure produces significant long‐term effects on cerebral function by a mechanism as yet unknown.

Chronic 3,4-methylenedioxymethamphetamine administration decreases glucocorticoid and mineralocorticoid receptor, but increases 5-hydroxytryptamine1C receptor gene expression in the rat hippocampus.

Both glucocorticoids and serotonin have been implicated in the regulation of mood and neuroendocrine control. In this study we have examined the effects of the psychomotor stimulant, 3,4-methylenedioxymethamphetamine on corticosteroid and 5-hydroxytryptamine receptor subtype gene expression within the hippocampal formation using in situ hybridization histochemistry. Animals were injected subcutaneously with 3,4-methylenedioxymethamphetamine (20 mg/kg) twice daily for four days. Two weeks following this dosage regimen, shown to markedly reduce 5-hydroxytryptamine terminals, both glucocorticoid receptor and mineralocorticoid receptor messenger RNA expression were significantly decreased (30-47% fall) in the granule cells of the dentate gyrus and pyramidal cells of CA1-CA4 fields of Ammons horn, but not in parietal cortex neurons. In the same rats, 5-hydroxytryptamine1C receptor messenger RNA expression was significantly increased in CA3 pyramidal neurons (133% rise), but neither 5-hydroxytryptamine1A or 5-hydroxytryptamine2 receptor messenger RNA levels were altered in any dorsal hippocampal subfield. 3,4-Methylenedioxymethamphetamine treatment was associated with modest hypersecretion of coricosterone during the diurnal nadir, without other peripheral evidence of chronic glucocorticoid excess (unchanged thymic and adrenal weights and corticosterone-binding globulin levels). These results emphasize the importance of the serotonergic innervation in maintaining hippocampal corticosteroid receptor gene expression. It is suggested that 5-hydroxytryptamine1C receptors may be involved in mediating the effects of serotonin on hippocampal glucocorticoid receptor and mineralocorticoid receptor expression and perhaps mood.

Neuropeptide Y and the cerebral circulation.

The significance of neuropeptide Y (NPY) in the cerebral circulation has been examined in the rat using immunocytochemistry, isolated cerebral artery preparations, and quantitative autoradiographic techniques for determining local CBF and glucose utilisation. In the rat the middle cerebral artery and the lenticulostriate artery from which blood is supplied to the caudate nucleus were found to be invested with numerous perivascular NPY immunoreactive nerve fibres. NPY (3–300 nM) contracted rat middle cerebral artery segments in a concentration-dependent manner. Intracerebral microinjections of NPY (200 pmol) or vehicle (1 μl) were performed in rats after full recovery from anaesthesia via previously implanted guide cannulae. Following injection of NPY into the striatum, local blood flow was markedly decreased by 36% throughout the ipsilateral caudate nucleus (e.g., from 104 ± 25 to 67 ± 15 ml 100 g−1 min −1; mean ± SD), whereas glucose use in this region was not altered significantly (e.g., 73 ± 8 and 74 ± 10 μmol 100 g−1 min−1 with vehicle and NPY, respectively). Intrastriatal NPY did not alter CBF or glucose use in the majority of other brain areas, including all of the 40 contralateral regions examined and almost all regions within the ipsilateral hemisphere. In a small number of highly discrete brain areas remote from the injection site (e.g., amygdala), there were significant reductions in blood flow with minimal changes in glucose use. Since NPY is present around rat cerebral blood vessels, is capable of evoking their contraction, and has the ability to produce reductions in blood flow independently of oxidative metabolism, this neuropeptide may be of major importance in cerebrovascular regulation.

Acute cocaine administation: effects on local cerebral blood flow and metabolic demand in the rat

Local cerebral blood flow and glucose utilisation were measured in both saline (n = 10) and cocaine (10 mg/kg; n = 10) treated rats using [14C]iodoantipyrine and [14C]2-deoxyglucose quantitative autoradiography respectively. In control animals, the ratio of flow to metabolism was 1.40 (r = 0.92) for the 40 brain regions examined. Cocaine treatment altered neither the correlation (r = 0.83) nor the ratio (1.49). Thus, the fundamental relationship between CBF and metabolism remains intact following acute cocaine exposure.

Partial lesion of the serotonergic system by a single dose of MDMA results in behavioural disinhibition and enhances acute MDMA-induced social behaviour on the social interaction test

The acute effects of 3,4-methylenedioxymethamphetamine (MDMA, ecstasy) on anxiety-related behaviours were studied using indices of social interaction in Dark Agouti (DA) both drug naive rats and those pretreated with MDMA (15 mg/kg i.p.) 3 weeks earlier. The functional neuroanatomy of these MDMA effects was visualised using 2-deoxyglucose imaging of local cerebral glucose use (LCMRglu), whilst MDMA-induced serotonergic neurotoxicity was measured by radioligand binding with [3H]paroxetine. Acute MDMA alone markedly decreased most typical elements of social interaction but increased adjacent lying, a behaviour that also contains social elements. In animals pre-exposed to MDMA, decreased [3H]paroxetine binding indicated serotonergic terminal depletion, and in these animals significant increases in locomotor activity, exploratory behaviour and aggressive behaviour were found. Both behavioural effects and also the metabolic activation induced by acute MDMA were potentiated in rats previously exposed to the drug. In conclusion, a single dose of MDMA caused marked changes in social behaviour acutely that might be interpreted either as a decrease or increase in anxiety. Three weeks after MDMA a behavioural disinhibition similar to psychomotor agitation, a symptom connected to depression or mania, and a sensitization to the acute effects of MDMA are apparent in both the behavioural and brain metabolic effects of the drug.

Cerebrovascular autoregulation in response to hypertension induced by NG-nitro-l-arginine methyl ester

Local neocortical blood flow and glucose utilization were measured in conscious rats using [14C]iodoantipyrine and [14C]2-deoxyglucose quantitative autoradiography, respectively, following intravenous injection of the nitric oxide synthase inhibitor NG-nitro-L-arginine methyl ester (30 mg/kg). The dose of NG-nitro-L-arginine methyl ester was chosen so as to produce a level of hypertension equivalent to that produced in a parallel group of rats by the infusion of angiotensin-II (5 micrograms/ml at 0.5-2.0 ml/h). In those animals in which angiotensin-induced hypertension did not exceed 150 mmHg (mean arterial blood pressure), there were no significant effects upon cortical blood flow when compared to controls, but at higher pressures (157 +/- 1 mmHg), blood flow was significantly increased in circumscribed areas of cortex, most notably in parietal (from 204 +/- 10 to 780 +/- 44 ml/100 g per min) and occipital cortex (from 175 +/- 5 to 600 +/- 46 ml/100 g per min), whilst other cortical areas (e.g. temporal and frontal areas) were unchanged. Despite the fact that NG-nitro-L-arginine methyl ester increased blood pressure to levels (164 +/- 1 mmHg) which were in excess of the highest produced by angiotensin, there was no evidence of focal hyperaemia; indeed blood flow was significantly reduced in every cortical region except parietal area 1. No significant differences in glucose use were evident between any of the groups.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebrovascular and functional consequences of 5-HT1A receptor activation

Cerebral glucose utilization and blood flow were measured in rats using 2-deoxy-D-[14C]glucose and [14C]iodoantipyrine quantitative autoradiography, respectively, following treatment with the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT). In control and 8-OH-DPAT-treated animals blood flow and glucose use were similarly correlated, but the ratio was increased following 8-OH-DPAT treatment. Since 5-HT1A receptor activation is known to reduce neuronal 5-HT release, these results are consistent with a vasoconstrictor role for endogenous serotonin.