Expression of the cannabinoid 1 (CB1) receptor and its regulation were studied in the different nociceptive and non-nociceptive sub-populations of cultured primary sensory neurones of adult rats. Bandairaea simplicifolia isolectin B4 (IB4) binding and calcitonin gene-related peptide (CGRP) immunostaining were used to distinguish between the glial cell-derived neurotrophic factor (GDNF)- and nerve growth factor (NGF)-responsive nociceptive and the non-nociceptive primary sensory neurones while a specific CB1 receptor antibody was used to study the expression of the CB1 receptor protein. About half of the total number of primary sensory neurones (47+/-3.2%) cultured for 1 day in the presence of both neurotrophic factors (50 ng/ml each) showed CB1 receptor-like immunostaining, whereas 21.8+/-3.3% and 32.7+/-5.6% of the neurones showed CGRP-like immunopositivity and IB4 binding, respectively. A proportion of the CB1 receptor-like immunopositive neurones was immunostained for CGRP (31.7+/-5%) and IB4 (48.2%+/-7.5), with a minimal (1%) co-expression of CGRP and IB4 binding. About a fifth of the CB1 receptor-like immunopositive neurones did not show either CGRP-like immunostaining or IB4 binding. To find out whether CB1 receptor expression in nociceptive primary sensory neurones is regulated by GDNF or NGF, cultures were grown in the presence or absence of the neurotrophic factors for 7 days. Vanilloid receptor 1 (VR1) immunostaining was used as a control marker to monitor the effect of the neurotrophins. In cultures maintained in the presence of both factors (50 ng/ml each) 51+/-2.6% and 42.4+/-1.2% of the cells showed CB1 receptor-like and VR1-like immunostaining, respectively. In cultures grown for 7 days in the absence of either of the neurotrophic factors the relative number of VR1-like immunopositive cells decreased to 13.4+/-2.7%, whereas the relative number of CB1 receptor-like immunopositive neurones was unchanged (50.6+/-1.1%). Our data suggest that the CB1 receptor is expressed in all of the three major sub-populations of primary sensory neurones and that the CB1 receptor expression is not regulated by either NGF or GDNF.